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Applications and Real-Time
HTTP Working GroupHypertext Transfer ProtocolHTTPHTTP semanticsHTTP payloadHTTP contentHTTP methodHTTP status code
The Hypertext Transfer Protocol (HTTP) is a stateless application-level
protocol for distributed, collaborative, hypertext information systems.
This document defines the semantics of HTTP: its architecture,
terminology, the "http" and "https" Uniform Resource Identifier (URI)
schemes, core request methods, request header fields, response status
codes, response header fields, and content negotiation.
This document obsoletes RFC 2818, RFC 7231, RFC 7232, RFC 7233,
RFC 7235, RFC 7538, RFC 7615, RFC 7694, and portions of RFC 7230.
This note is to be removed before publishing as an RFC.
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
.
Working Group information can be found at ;
source code and issues list for this draft can be found at
.
The changes in this draft are summarized in .
The Hypertext Transfer Protocol (HTTP) is a stateless application-level
request/response protocol that uses extensible semantics and
self-descriptive messages for flexible interaction with network-based
hypertext information systems. HTTP is defined by a series of documents
that collectively form the HTTP/1.1 specification:
"HTTP Semantics" (this document)"HTTP Caching" "HTTP/1.1 Messaging"
HTTP is a generic interface protocol for information systems. It is
designed to hide the details of how a service is implemented by presenting
a uniform interface to clients that is independent of the types of
resources provided. Likewise, servers do not need to be aware of each
client's purpose: an HTTP request can be considered in isolation rather
than being associated with a specific type of client or a predetermined
sequence of application steps. The result is a protocol that can be used
effectively in many different contexts and for which implementations can
evolve independently over time.
HTTP is also designed for use as an intermediation protocol for translating
communication to and from non-HTTP information systems.
HTTP proxies and gateways can provide access to alternative information
services by translating their diverse protocols into a hypertext
format that can be viewed and manipulated by clients in the same way
as HTTP services.
One consequence of this flexibility is that the protocol cannot be
defined in terms of what occurs behind the interface. Instead, we
are limited to defining the syntax of communication, the intent
of received communication, and the expected behavior of recipients.
If the communication is considered in isolation, then successful
actions ought to be reflected in corresponding changes to the
observable interface provided by servers. However, since multiple
clients might act in parallel and perhaps at cross-purposes, we
cannot require that such changes be observable beyond the scope
of a single response.
Each HTTP message is either a request or a
response. A server listens on a connection for a request, parses each
message received, interprets the message semantics in relation to the
identified target resource, and responds to that request with one or more
response messages. A client constructs request messages to communicate
specific intentions, examines received responses to see if the
intentions were carried out, and determines how to interpret the results.
HTTP provides a uniform interface for interacting with a resource
(), regardless of its type, nature, or
implementation, via the manipulation and transfer of representations
().
This document defines semantics that are common to all versions of HTTP.
HTTP semantics include the intentions defined by each request method
(), extensions to those semantics that might be
described in request header fields (),
the meaning of status codes to indicate a machine-readable response
(), and the meaning of other control data
and resource metadata that might be given in response header fields
().
This document also defines representation metadata that describe how a
payload is intended to be interpreted by a recipient, the request header
fields that might influence content selection, and the various selection
algorithms that are collectively referred to as
"content negotiation" ().
This document defines HTTP range requests, partial responses, and the
multipart/byteranges media type.
This document obsoletes the portions of
RFC 7230 that are independent of
the HTTP/1.1 messaging syntax and connection management, with the changes
being summarized in .
The other parts of RFC 7230 are
obsoleted by "HTTP/1.1 Messaging" .
This document also obsoletes
RFC 2818
(see ),
RFC 7231
(see ),
RFC 7232
(see ),
RFC 7233
(see ),
RFC 7235
(see ),
RFC 7538
(see ),
RFC 7615
(see ), and
RFC 7694
(see ).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in BCP 14 when, and only when, they
appear in all capitals, as shown here.
Conformance criteria and considerations regarding error handling
are defined in .
This specification uses the Augmented Backus-Naur Form (ABNF) notation of
, extended with the notation for case-sensitivity
in strings defined in .
It also uses a list extension, defined in ,
that allows for compact definition of comma-separated lists using a '#'
operator (similar to how the '*' operator indicates repetition). shows the collected grammar with all list
operators expanded to standard ABNF notation.
As a convention, ABNF rule names prefixed with "obs-" denote
"obsolete" grammar rules that appear for historical reasons.
The following core rules are included by
reference, as defined in Appendix B.1 of :
ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
DIGIT (decimal 0-9), DQUOTE (double quote),
HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
OCTET (any 8-bit sequence of data), SP (space), and
VCHAR (any visible US-ASCII character).
defines some generic syntactic
components for field values.
The rules below are defined in :
This specification uses the terms
"character",
"character encoding scheme",
"charset", and
"protocol element"
as they are defined in .
This specification uses three rules to denote the use of linear
whitespace: OWS (optional whitespace), RWS (required whitespace), and
BWS ("bad" whitespace).
The OWS rule is used where zero or more linear whitespace octets might
appear. For protocol elements where optional whitespace is preferred to
improve readability, a sender SHOULD generate the optional whitespace
as a single SP; otherwise, a sender SHOULD NOT generate optional
whitespace except as needed to white out invalid or unwanted protocol
elements during in-place message filtering.
The RWS rule is used when at least one linear whitespace octet is required
to separate field tokens. A sender SHOULD generate RWS as a single SP.
OWS and RWS have the same semantics as a single SP. Any content known to
be defined as OWS or RWS MAY be replaced with a single SP before
interpreting it or forwarding the message downstream.
The BWS rule is used where the grammar allows optional whitespace only for
historical reasons. A sender MUST NOT generate BWS in messages.
A recipient MUST parse for such bad whitespace and remove it before
interpreting the protocol element.
BWS has no semantics. Any content known to be
defined as BWS MAY be removed before interpreting it or forwarding the
message downstream.
HTTP was created for the World Wide Web (WWW) architecture
and has evolved over time to support the scalability needs of a worldwide
hypertext system. Much of that architecture is reflected in the terminology
and syntax productions used to define HTTP.
HTTP is a stateless request/response protocol that operates by exchanging
messages across a reliable transport- or session-layer
"connection".
An HTTP "client" is a program that establishes a connection
to a server for the purpose of sending one or more HTTP requests.
An HTTP "server" is a program that accepts connections
in order to service HTTP requests by sending HTTP responses.
The terms "client" and "server" refer only to the roles that
these programs perform for a particular connection. The same program
might act as a client on some connections and a server on others.
The term "user agent" refers to any of the various
client programs that initiate a request, including (but not limited to)
browsers, spiders (web-based robots), command-line tools, custom
applications, and mobile apps.
The term "origin server" refers to the program that can
originate authoritative responses for a given target resource.
The terms "sender" and "recipient" refer to
any implementation that sends or receives a given message, respectively.
HTTP relies upon the Uniform Resource Identifier (URI)
standard to indicate the target resource
() and relationships between resources.
Most HTTP communication consists of a retrieval request (GET) for
a representation of some resource identified by a URI. In the
simplest case, this might be accomplished via a single bidirectional
connection (===) between the user agent (UA) and the origin server (O).
Each major version of HTTP defines its own syntax for the inclusion of
information in messages. Nevertheless, a common abstraction is that a
message includes some form of envelope/framing, a potential set of named
fields up front (a header section), a potential body, and a
potential following set of named fields (a trailer section).
A client sends an HTTP request to a server in the form of a
request message with a method () and request target. The request might also
contain header fields for request modifiers, client information, and
representation metadata (),
a payload body () to be processed
in accordance with the method, and trailer fields for metadata
collected while sending the payload.
A server responds to a client's request by sending one or more HTTP
response messages, each including a success or error
code (). The response might also contain
header fields for server information, resource metadata, and representation
metadata (),
a payload body () to be interpreted
in accordance with the status code, and trailer fields for metadata
collected while sending the payload.
One of the functions of the message framing mechanism is to assure that
messages are complete. A message is considered complete
when all of the octets indicated by its framing are available. Note that,
when no explicit framing is used, a response message that is ended
by the transport connection's close is considered complete even though it
might be indistinguishable from an incomplete response, unless a
transport-level error indicates that it is not complete.
A connection might be used for multiple request/response exchanges. The
mechanism used to correlate between request and response messages is
version dependent; some versions of HTTP use implicit ordering of
messages, while others use an explicit identifier.
Responses (both final and interim) can be sent at any time after a
request is received, even if it is not yet complete. However, clients
(including intermediaries) might abandon a request if the response is not
forthcoming within a reasonable period of time.
The following example illustrates a typical message exchange for a
GET request () on the URI "http://www.example.com/hello.txt":
Client request:
Server response:
HTTP enables the use of intermediaries to satisfy requests through
a chain of connections. There are three common forms of HTTP
intermediary: proxy, gateway, and tunnel. In some cases,
a single intermediary might act as an origin server, proxy, gateway,
or tunnel, switching behavior based on the nature of each request.
The figure above shows three intermediaries (A, B, and C) between the
user agent and origin server. A request or response message that
travels the whole chain will pass through four separate connections.
Some HTTP communication options
might apply only to the connection with the nearest, non-tunnel
neighbor, only to the endpoints of the chain, or to all connections
along the chain. Although the diagram is linear, each participant might
be engaged in multiple, simultaneous communications. For example, B
might be receiving requests from many clients other than A, and/or
forwarding requests to servers other than C, at the same time that it
is handling A's request. Likewise, later requests might be sent through a
different path of connections, often based on dynamic configuration for
load balancing.
The terms "upstream" and "downstream" are
used to describe directional requirements in relation to the message flow:
all messages flow from upstream to downstream.
The terms "inbound" and "outbound" are used to describe directional
requirements in relation to the request route:
"inbound" means toward the origin server and
"outbound" means toward the user agent.
A "proxy" is a message-forwarding agent that is selected by the
client, usually via local configuration rules, to receive requests
for some type(s) of absolute URI and attempt to satisfy those
requests via translation through the HTTP interface. Some translations
are minimal, such as for proxy requests for "http" URIs, whereas
other requests might require translation to and from entirely different
application-level protocols. Proxies are often used to group an
organization's HTTP requests through a common intermediary for the
sake of security, annotation services, or shared caching. Some proxies
are designed to apply transformations to selected messages or payloads
while they are being forwarded, as described in
.
A "gateway" (a.k.a. "reverse proxy") is an
intermediary that acts as an origin server for the outbound connection but
translates received requests and forwards them inbound to another server or
servers. Gateways are often used to encapsulate legacy or untrusted
information services, to improve server performance through
"accelerator" caching, and to enable partitioning or load
balancing of HTTP services across multiple machines.
All HTTP requirements applicable to an origin server
also apply to the outbound communication of a gateway.
A gateway communicates with inbound servers using any protocol that
it desires, including private extensions to HTTP that are outside
the scope of this specification. However, an HTTP-to-HTTP gateway
that wishes to interoperate with third-party HTTP servers ought to conform
to user agent requirements on the gateway's inbound connection.
A "tunnel" acts as a blind relay between two connections
without changing the messages. Once active, a tunnel is not
considered a party to the HTTP communication, though the tunnel might
have been initiated by an HTTP request. A tunnel ceases to exist when
both ends of the relayed connection are closed. Tunnels are used to
extend a virtual connection through an intermediary, such as when
Transport Layer Security (TLS, ) is used to
establish confidential communication through a shared firewall proxy.
The above categories for intermediary only consider those acting as
participants in the HTTP communication. There are also intermediaries
that can act on lower layers of the network protocol stack, filtering or
redirecting HTTP traffic without the knowledge or permission of message
senders. Network intermediaries are indistinguishable (at a protocol level)
from a man-in-the-middle attack, often introducing security flaws or
interoperability problems due to mistakenly violating HTTP semantics.
For example, an
"interception proxy" (also commonly
known as a "transparent proxy" or
"captive portal")
differs from an HTTP proxy because it is not selected by the client.
Instead, an interception proxy filters or redirects outgoing TCP port 80
packets (and occasionally other common port traffic).
Interception proxies are commonly found on public network access points,
as a means of enforcing account subscription prior to allowing use of
non-local Internet services, and within corporate firewalls to enforce
network usage policies.
HTTP is defined as a stateless protocol, meaning that each request message
can be understood in isolation. Many implementations depend on HTTP's
stateless design in order to reuse proxied connections or dynamically
load balance requests across multiple servers. Hence, a server MUST NOT
assume that two requests on the same connection are from the same user
agent unless the connection is secured and specific to that agent.
Some non-standard HTTP extensions (e.g., ) have
been known to violate this requirement, resulting in security and
interoperability problems.
A "cache" is a local store of previous response messages and the
subsystem that controls its message storage, retrieval, and deletion.
A cache stores cacheable responses in order to reduce the response
time and network bandwidth consumption on future, equivalent
requests. Any client or server MAY employ a cache, though a cache
cannot be used by a server while it is acting as a tunnel.
The effect of a cache is that the request/response chain is shortened
if one of the participants along the chain has a cached response
applicable to that request. The following illustrates the resulting
chain if B has a cached copy of an earlier response from O (via C)
for a request that has not been cached by UA or A.
A response is "cacheable" if a cache is allowed to store a copy of
the response message for use in answering subsequent requests.
Even when a response is cacheable, there might be additional
constraints placed by the client or by the origin server on when
that cached response can be used for a particular request. HTTP
requirements for cache behavior and cacheable responses are
defined in Section 2 of .
There is a wide variety of architectures and configurations
of caches deployed across the World Wide Web and
inside large organizations. These include national hierarchies
of proxy caches to save transoceanic bandwidth, collaborative systems that
broadcast or multicast cache entries, archives of pre-fetched cache
entries for use in off-line or high-latency environments, and so on.
Uniform Resource Identifiers (URIs) are used
throughout HTTP as the means for identifying resources ().
URI references are used to target requests, indicate redirects, and define
relationships.
The definitions of "URI-reference",
"absolute-URI", "relative-part", "authority", "port", "host",
"path-abempty", "segment", and "query" are adopted from the
URI generic syntax.
An "absolute-path" rule is defined for protocol elements that can contain a
non-empty path component. (This rule differs slightly from the path-abempty
rule of RFC 3986, which allows for an empty path to be used in references,
and path-absolute rule, which does not allow paths that begin with "//".)
A "partial-URI" rule is defined for protocol elements
that can contain a relative URI but not a fragment component.
Each protocol element in HTTP that allows a URI reference will indicate
in its ABNF production whether the element allows any form of reference
(URI-reference), only a URI in absolute form (absolute-URI), only the
path and optional query components, or some combination of the above.
Unless otherwise indicated, URI references are parsed
relative to the target URI ().
It is RECOMMENDED that all senders and recipients support, at a minimum,
URIs with lengths of 8000 octets in protocol elements. Note that this
implies some structures and on-wire representations (for example, the
request line in HTTP/1.1) will necessarily be larger in some cases.
The target of an HTTP request is called a "resource".
HTTP does not limit the nature of a resource; it merely
defines an interface that might be used to interact with resources.
Most resources are identified by a Uniform Resource Identifier (URI), as
described in .
One design goal of HTTP is to separate resource identification from
request semantics, which is made possible by vesting the request
semantics in the request method () and a few
request-modifying header fields ().
If there is a conflict between the method semantics and any semantic
implied by the URI itself, as described in ,
the method semantics take precedence.
IANA maintains the registry of URI Schemes at
.
Although requests might target any URI scheme, the following schemes are
inherent to HTTP servers:
URI SchemeDescriptionReferencehttpHypertext Transfer ProtocolhttpsHypertext Transfer Protocol Secure
Note that the presence of an "http" or "https" URI does not imply that
there is always an HTTP server at the identified origin listening for
connections. Anyone can mint a URI, whether or not a server exists and
whether or not that server currently maps that identifier to a resource.
The delegated nature of registered names and IP addresses creates a
federated namespace whether or not an HTTP server is present.
The "http" URI scheme is hereby defined for minting identifiers within the
hierarchical namespace governed by a potential HTTP origin server
listening for TCP () connections on a given port.
The origin server for an "http" URI is identified by the
authority component, which includes a host identifier
and optional port number (, Section 3.2.2).
If the port subcomponent is empty or not given, TCP port 80 (the
reserved port for WWW services) is the default.
The origin determines who has the right to respond authoritatively to
requests that target the identified resource, as defined in
.
A sender MUST NOT generate an "http" URI with an empty host identifier.
A recipient that processes such a URI reference MUST reject it as invalid.
The hierarchical path component and optional query component identify the
target resource within that origin server's name space.
The "https" URI scheme is hereby defined for minting identifiers within the
hierarchical namespace governed by a potential origin server listening for
TCP connections on a given port and capable of establishing a TLS
() connection that has been secured for HTTP
communication. In this context, "secured" specifically
means that the server has been authenticated as acting on behalf of the
identified authority and all HTTP communication with that server has been
protected for confidentiality and integrity through the use of strong
encryption.
The origin server for an "https" URI is identified by the
authority component, which includes a host identifier
and optional port number (, Section 3.2.2).
If the port subcomponent is empty or not given, TCP port 443
(the reserved port for HTTP over TLS) is the default.
The origin determines who has the right to respond authoritatively to
requests that target the identified resource, as defined in
.
A sender MUST NOT generate an "https" URI with an empty host identifier.
A recipient that processes such a URI reference MUST reject it as invalid.
The hierarchical path component and optional query component identify the
target resource within that origin server's name space.
A client MUST ensure that its HTTP requests for an "https" resource are
secured, prior to being communicated, and that it only accepts secured
responses to those requests.
Resources made available via the "https" scheme have no shared identity
with the "http" scheme. They are distinct origins with separate namespaces.
However, an extension to HTTP that is defined to apply to all origins with
the same host, such as the Cookie protocol , can
allow information set by one service to impact communication with other
services within a matching group of host domains.
Since the "http" and "https" schemes conform to the URI generic syntax,
such URIs are normalized and compared according to the algorithm defined
in Section 6 of , using the defaults
described above for each scheme.
If the port is equal to the default port for a scheme, the normal form is
to omit the port subcomponent. When not being used as the
target of an OPTIONS request, an empty path component is equivalent
to an absolute path of "/", so the normal form is to provide a path of "/"
instead. The scheme and host are case-insensitive and normally provided in
lowercase; all other components are compared in a case-sensitive manner.
Characters other than those in the "reserved" set are equivalent to their
percent-encoded octets: the normal form is to not encode them
(see Sections 2.1 and
2.2 of
).
For example, the following three URIs are equivalent:
The URI generic syntax for authority also includes a userinfo subcomponent
(, Section 3.2.1) for including user
authentication information in the URI. In that subcomponent, the
use of the format "user:password" is deprecated.
Some implementations make use of the userinfo component for internal
configuration of authentication information, such as within command
invocation options, configuration files, or bookmark lists, even
though such usage might expose a user identifier or password.
A sender MUST NOT generate the userinfo subcomponent (and its "@"
delimiter) when an "http" or "https" URI reference is generated within a
message as a target URI or field value.
Before making use of an "http" or "https" URI reference received from an untrusted
source, a recipient SHOULD parse for userinfo and treat its presence as
an error; it is likely being used to obscure the authority for the sake of
phishing attacks.
Fragment identifiers allow for indirect identification
of a secondary resource, independent of the URI scheme, as defined in
Section 3.5 of .
Some protocol elements that refer to a URI allow inclusion of a fragment,
while others do not. They are distinguished by use of the ABNF rule for
elements where fragment is allowed; otherwise, a specific rule that excludes
fragments is used (see ).
Note: the fragment identifier component is not part of the actual scheme
definition for a URI scheme (see Section 4.3 of ),
thus does not appear in the ABNF definitions for the "http" and "https"
URI schemes above.
When considering the design of HTTP, it is easy to fall into a trap of
thinking that all user agents are general-purpose browsers and all origin
servers are large public websites. That is not the case in practice.
Common HTTP user agents include household appliances, stereos, scales,
firmware update scripts, command-line programs, mobile apps,
and communication devices in a multitude of shapes and sizes. Likewise,
common HTTP origin servers include home automation units, configurable
networking components, office machines, autonomous robots, news feeds,
traffic cameras, ad selectors, and video-delivery platforms.
The term "user agent" does not imply that there is a human user directly
interacting with the software agent at the time of a request. In many
cases, a user agent is installed or configured to run in the background
and save its results for later inspection (or save only a subset of those
results that might be interesting or erroneous). Spiders, for example, are
typically given a start URI and configured to follow certain behavior while
crawling the Web as a hypertext graph.
The implementation diversity of HTTP means that not all user agents can
make interactive suggestions to their user or provide adequate warning for
security or privacy concerns. In the few cases where this
specification requires reporting of errors to the user, it is acceptable
for such reporting to only be observable in an error console or log file.
Likewise, requirements that an automated action be confirmed by the user
before proceeding might be met via advance configuration choices,
run-time options, or simple avoidance of the unsafe action; confirmation
does not imply any specific user interface or interruption of normal
processing if the user has already made that choice.
This specification targets conformance criteria according to the role of
a participant in HTTP communication. Hence, HTTP requirements are placed
on senders, recipients, clients, servers, user agents, intermediaries,
origin servers, proxies, gateways, or caches, depending on what behavior
is being constrained by the requirement. Additional (social) requirements
are placed on implementations, resource owners, and protocol element
registrations when they apply beyond the scope of a single communication.
The verb "generate" is used instead of "send" where a requirement
differentiates between creating a protocol element and merely forwarding a
received element downstream.
An implementation is considered conformant if it complies with all of the
requirements associated with the roles it partakes in HTTP.
Conformance includes both the syntax and semantics of protocol
elements. A sender MUST NOT generate protocol elements that convey a
meaning that is known by that sender to be false. A sender MUST NOT
generate protocol elements that do not match the grammar defined by the
corresponding ABNF rules. Within a given message, a sender MUST NOT
generate protocol elements or syntax alternatives that are only allowed to
be generated by participants in other roles (i.e., a role that the sender
does not have for that message).
When a received protocol element is parsed, the recipient MUST be able to
parse any value of reasonable length that is applicable to the recipient's
role and that matches the grammar defined by the corresponding ABNF rules.
Note, however, that some received protocol elements might not be parsed.
For example, an intermediary forwarding a message might parse a
field into generic field name and field value components, but then
forward the field without further parsing inside the field value.
HTTP does not have specific length limitations for many of its protocol
elements because the lengths that might be appropriate will vary widely,
depending on the deployment context and purpose of the implementation.
Hence, interoperability between senders and recipients depends on shared
expectations regarding what is a reasonable length for each protocol
element. Furthermore, what is commonly understood to be a reasonable length
for some protocol elements has changed over the course of the past two
decades of HTTP use and is expected to continue changing in the future.
At a minimum, a recipient MUST be able to parse and process protocol
element lengths that are at least as long as the values that it generates
for those same protocol elements in other messages. For example, an origin
server that publishes very long URI references to its own resources needs
to be able to parse and process those same references when received as a
target URI.
A recipient MUST interpret a received protocol element according to the
semantics defined for it by this specification, including extensions to
this specification, unless the recipient has determined (through experience
or configuration) that the sender incorrectly implements what is implied by
those semantics.
For example, an origin server might disregard the contents of a received
Accept-Encoding header field if inspection of the
User-Agent header field indicates a specific implementation
version that is known to fail on receipt of certain content codings.
Unless noted otherwise, a recipient MAY attempt to recover a usable
protocol element from an invalid construct. HTTP does not define
specific error handling mechanisms except when they have a direct impact
on security, since different applications of the protocol require
different error handling strategies. For example, a Web browser might
wish to transparently recover from a response where the
Location header field doesn't parse according to the ABNF,
whereas a systems control client might consider any form of error recovery
to be dangerous.
Some requests can be automatically retried by a client in the event of
an underlying connection failure, as described in
.
While HTTP's core semantics don't change between protocol versions, the
expression of them "on the wire" can change, and so the
HTTP version number changes when incompatible changes are made to the wire
format. Additionally, HTTP allows incremental, backwards-compatible
changes to be made to the protocol without changing its version through
the use of defined extension points.
HTTP defines a number of generic extension points that can be used to
introduce capabilities to the protocol without introducing a new version,
including methods (), status codes
(), header and trailer fields
(), and further
extensibility points within defined fields (such as Cache-Control in Section 5.2.3 of ). Because the semantics of HTTP are
not versioned, these extension points are persistent; the version of the
protocol in use does not affect their semantics.
Version-independent extensions are discouraged from depending on or
interacting with the specific version of the protocol in use. When this is
unavoidable, careful consideration needs to be given to how the extension
can interoperate across versions.
Additionally, specific versions of HTTP might have their own extensibility
points, such as transfer-codings in HTTP/1.1 (Section 6.1 of ) and HTTP/2 ()
SETTINGS or frame types. These extension points are specific to the
version of the protocol they occur within.
Version-specific extensions cannot override or modify the semantics of
a version-independent mechanism or extension point (like a method or
header field) without explicitly being allowed by that protocol element. For
example, the CONNECT method () allows this.
These guidelines assure that the protocol operates correctly and
predictably, even when parts of the path implement different versions of
HTTP.
The HTTP version number consists of two decimal digits separated by a "."
(period or decimal point). The first digit ("major version") indicates the
HTTP messaging syntax, whereas the second digit ("minor version")
indicates the highest minor version within that major version to which the
sender is conformant and able to understand for future communication.
The protocol version as a whole indicates the sender's conformance with
the set of requirements laid out in that version's corresponding
specification of HTTP.
For example, the version "HTTP/1.1" is defined by the combined
specifications of this document, "HTTP Caching" ,
and "HTTP/1.1 Messaging" .
The minor version advertises the sender's communication capabilities even
when the sender is only using a backwards-compatible subset of the
protocol, thereby letting the recipient know that more advanced features
can be used in response (by servers) or in future requests (by clients).
A client SHOULD send a request version equal to the highest
version to which the client is conformant and
whose major version is no higher than the highest version supported
by the server, if this is known. A client MUST NOT send a
version to which it is not conformant.
A client MAY send a lower request version if it is known that
the server incorrectly implements the HTTP specification, but only
after the client has attempted at least one normal request and determined
from the response status code or header fields (e.g., Server) that
the server improperly handles higher request versions.
A server SHOULD send a response version equal to the highest version to
which the server is conformant that has a major version less than or equal
to the one received in the request.
A server MUST NOT send a version to which it is not conformant.
A server can send a 505 (HTTP Version Not Supported)
response if it wishes, for any reason, to refuse service of the client's
major protocol version.
HTTP's major version number is incremented when an incompatible message
syntax is introduced. The minor number is incremented when changes made to
the protocol have the effect of adding to the message semantics or
implying additional capabilities of the sender.
When an HTTP message is received with a major version number that the
recipient implements, but a higher minor version number than what the
recipient implements, the recipient SHOULD process the message as if it
were in the highest minor version within that major version to which the
recipient is conformant. A recipient can assume that a message with a
higher minor version, when sent to a recipient that has not yet indicated
support for that higher version, is sufficiently backwards-compatible to be
safely processed by any implementation of the same major version.
When a major version of HTTP does not define any minor versions, the minor
version "0" is implied and is used when referring to that protocol within a
protocol element that requires sending a minor version.
HTTP messages use key/value pairs to convey data about the
message, its payload, the target resource, or the connection (i.e.,
control data). They are called "HTTP fields" or just "fields".
Every message can have two separate areas that such fields can occur
within; the "header field section" (or just "header section") preceding
the message body and containing "header fields" (or just "headers",
colloquially) and the "trailer field section" (or just "trailer
section") after the message body containing "trailer fields" (or just
"trailers" colloquially). Header fields are more common; see for discussion of the applicability and
limitations of trailer fields.
Both sections are composed of any number of "field lines", each with a
"field name" (see ) identifying the field,
and a "field line value" that conveys data for the field.
Each field name present in a section has a corresponding "field value"
for that section, composed from all field line values with that given
field name in that section, concatenated together and separated with
commas. See for further discussion of the
semantics of field ordering and combination in messages, and for more discussion of field values.
For example, this section:
contains two field lines, both with the field name "Example-Field". The
first field line has a field line value of "Foo, Bar", while the second
field line value is "Baz". The field value for "Example-Field" is a list
with three members: "Foo", "Bar", and "Baz".
The interpretation of a field does not change between minor
versions of the same major HTTP version, though the default behavior of a
recipient in the absence of such a field can change. Unless specified
otherwise, fields are defined for all versions of HTTP.
In particular, the Host and Connection
fields ought to be implemented by all HTTP/1.x implementations
whether or not they advertise conformance with HTTP/1.1.
New fields can be introduced without changing the protocol version if
their defined semantics allow them to be safely ignored by recipients
that do not recognize them; see .
The order in which field lines with differing names are
received in a message is not significant. However, it is good practice to send
header fields that contain control data first, such as Host
on requests and Date on responses, so that implementations
can decide when not to handle a message as early as possible.
A server MUST NOT apply a request to the target resource until the entire
request header section is received, since later header field lines might include
conditionals, authentication credentials, or deliberately misleading
duplicate header fields that would impact request processing.
A recipient MAY combine multiple field lines with the same field name
into one field line, without changing the semantics of the message, by
appending each subsequent field line value to the initial field line value
in order, separated by a comma and OWS (optional whitespace).
For consistency, use comma SP.
The order in which field lines with the
same name are received is therefore significant to the interpretation of
the field value; a proxy MUST NOT change the order of these field line
values when forwarding a message.
This means that, aside from the well-known exception noted below, a sender
MUST NOT generate multiple field lines with the same name in a message
(whether in the headers or trailers), or append a field line when a field
line of the same name already exists in the message, unless that field's
definition allows multiple field line values to be recombined as a
comma-separated list [i.e., at least one alternative of the field's
definition allows a comma-separated list, such as an ABNF rule of
#(values) defined in ].
Note: In practice, the "Set-Cookie" header field ()
often appears in a response message across multiple field lines and does not
use the list syntax, violating the above requirements on multiple field lines
with the same field name. Since it cannot be combined into a single field
value, recipients ought to handle "Set-Cookie" as a special case while
processing fields. (See Appendix A.2.3 of for
details.)
HTTP does not place a predefined limit on the length of each field line, field value,
or on the length of the header or trailer section as a whole, as described in
. Various ad hoc limitations on individual
lengths are found in practice, often depending on the specific
field's semantics.
A server that receives a request header field line, field value, or set of
fields larger than it wishes to process MUST respond with an appropriate
4xx (Client Error) status code. Ignoring such header fields
would increase the server's vulnerability to request smuggling attacks
(Section 11.2 of ).
A client MAY discard or truncate received field lines that are larger
than the client wishes to process if the field semantics are such that the
dropped value(s) can be safely ignored without changing the
message framing or response semantics.
The field-name token labels the corresponding field value as having the
semantics defined by that field. For example, the Date
header field is defined in as containing the origination
timestamp for the message in which it appears.
Field names are case-insensitive and ought to be registered within the
"Hypertext Transfer Protocol (HTTP) Field Name Registry"; see .
Authors of specifications defining new fields are advised to choose a short
but descriptive field name. Short names avoid needless data transmission;
descriptive names avoid confusion and "squatting" on names that might have
broader uses.
To that end, limited-use fields (such as a header confined to a single
application or use case) are encouraged to use a name that includes its name
(or an abbreviation) as a prefix; for example, if the Foo Application needs
a Description field, it might use "Foo-Desc"; "Description" is too generic,
and "Foo-Description" is needlessly long.
While the field-name syntax is defined to allow any token character, in
practice some implementations place limits on the characters they accept
in field-names. To be interoperable, new field names SHOULD constrain
themselves to alphanumeric characters, "-", and ".", and SHOULD
begin with an alphanumeric character.
Field names ought not be prefixed with "X-"; see
for further information.
Other prefixes are sometimes used in HTTP field names; for example,
"Accept-" is used in many content negotiation headers. These prefixes are
only an aid to recognizing the purpose of a field, and do not
trigger automatic processing.
There is no limit on the introduction of new field names, each presumably
defining new semantics.
New fields can be defined such that, when they are understood by a
recipient, they might override or enhance the interpretation of previously
defined fields, define preconditions on request evaluation, or
refine the meaning of responses.
A proxy MUST forward unrecognized header fields unless the
field name is listed in the Connection header field
(Section 9.1 of ) or the proxy is specifically
configured to block, or otherwise transform, such fields.
Other recipients SHOULD ignore unrecognized header and trailer fields.
These requirements allow HTTP's functionality to be enhanced without
requiring prior update of deployed intermediaries.
The "Hypertext Transfer Protocol (HTTP) Field Name Registry" defines the
namespace for HTTP field names.
Any party can request registration of a HTTP field. See for considerations to take
into account when creating a new HTTP field.
The "Hypertext Transfer Protocol (HTTP) Field Name Registry" is located at
.
Registration requests can be made by following the instructions located
there or by sending an email to the "ietf-http-wg@ietf.org" mailing list.
Field names are registered on the advice of a Designated Expert
(appointed by the IESG or their delegate). Fields with the status
'permanent' are Specification Required
(, Section 4.6).
Registration requests consist of at least the following information:
The requested field name. It MUST conform to the
field-name syntax defined in , and SHOULD be
restricted to just letters, digits, hyphen ('-') and underscore ('_')
characters, with the first character being a letter.
"permanent" or "provisional".
Reference to the document that specifies
the field, preferably including a URI that can be used to retrieve
a copy of the document. An indication of the relevant section(s) can also
be included, but is not required.
And, optionally:
Additional information, such as about reserved entries.
The Expert(s) can define additional fields to be collected in the
registry, in consultation with the community.
Standards-defined names have a status of "permanent". Other names can also
be registered as permanent, if the Expert(s) find that they are in use, in
consultation with the community. Other names should be registered as
"provisional".
Provisional entries can be removed by the Expert(s) if — in consultation
with the community — the Expert(s) find that they are not in use. The
Experts can change a provisional entry's status to permanent at any time.
Note that names can be registered by third parties (including the
Expert(s)), if the Expert(s) determines that an unregistered name is widely
deployed and not likely to be registered in a timely manner otherwise.
HTTP field values typically have their syntax defined using ABNF
(), using the extension defined in
as necessary, and are usually constrained to the range of US-ASCII characters.
Fields needing a greater range of characters can use an encoding
such as the one defined in .
Historically, HTTP allowed field content with text in the ISO‑8859‑1
charset , supporting other charsets only
through use of encoding.
In practice, most HTTP field values use only a subset of the
US-ASCII charset . Newly defined
fields SHOULD limit their values to US‑ASCII octets.
A recipient SHOULD treat other octets in field content (obs‑text) as
opaque data.
Field values containing control (CTL) characters such as
CR or LF are invalid; recipients MUST either reject a field value
containing control characters, or convert them to SP before processing or
forwarding the message.
Leading and trailing whitespace in raw field values is removed upon field
parsing (Section 5.1 of ). Field definitions where leading or trailing
whitespace in values is significant will have to use a container syntax such
as quoted-string ().
Because commas (",") are used as a generic delimiter between members of a
list-based field value, they need to be treated with care if they are
allowed as data within those members. Typically, list members that might contain a
comma are enclosed in a quoted-string.
For example, a textual date and a URI (either of which might contain a comma)
could be safely carried in list-based field values like these:
Note that double-quote delimiters almost always are used with the
quoted-string production; using a different syntax inside double-quotes
will likely cause unnecessary confusion.
Many fields (such as Content-Type, defined in
) use a common syntax for parameters
that allows both unquoted (token) and quoted (quoted-string) syntax for
a parameter value (). Use of common syntax
allows recipients to reuse existing parser components. When allowing both
forms, the meaning of a parameter value ought to be the same whether it
was received as a token or a quoted string.
Historically, HTTP field values could be extended over multiple
lines by preceding each extra line with at least one space or horizontal
tab (obs-fold).
This document assumes that any such obsolete line folding has been replaced with one or more
SP octets prior to interpreting the field value,
as described in Section 5.2 of .
Note: This specification does not use ABNF rules to define each "Field
Name: Field Value" pair, as was done in earlier editions (published
before ). Instead, ABNF rules are named according
to each registered field name, wherein the rule defines the valid grammar
for that field's corresponding field values (i.e., after the field value
has been extracted by a generic field parser).
Many HTTP field values are defined using common syntax
components, separated by whitespace or specific delimiting characters.
Delimiters are chosen from the set of US-ASCII visual characters not
allowed in a token (DQUOTE and "(),/:;<=>?@[\]{}").
Tokens are short textual identifiers that do not include whitespace or
delimiters.
A string of text is parsed as a single value if it is quoted using
double-quote marks.
The backslash octet ("\") can be used as a single-octet
quoting mechanism within quoted-string and comment constructs.
Recipients that process the value of a quoted-string MUST handle a
quoted-pair as if it were replaced by the octet following the backslash.
A sender SHOULD NOT generate a quoted-pair in a quoted-string except
where necessary to quote DQUOTE and backslash octets occurring within that
string.
A sender SHOULD NOT generate a quoted-pair in a comment except
where necessary to quote parentheses ["(" and ")"] and backslash octets
occurring within that comment.
Comments can be included in some HTTP fields by surrounding
the comment text with parentheses. Comments are only allowed in
fields containing "comment" as part of their field value definition.
A parameter is a name=value pair that is often defined within field
values as a common syntax for appending auxiliary information to an item.
Each parameter is usually delimited by an immediately preceding semicolon.
Parameter names are case-insensitive. Parameter values might or might
not be case-sensitive, depending on the semantics of the parameter
name. Examples of parameters and some equivalent forms can be seen in
media types () and the Accept header field
().
A parameter value that matches the token production can be
transmitted either as a token or within a quoted-string. The quoted and
unquoted values are equivalent.
Note: Parameters do not allow whitespace (not even "bad" whitespace)
around the "=" character.
Prior to 1995, there were three different formats commonly used by servers
to communicate timestamps. For compatibility with old implementations, all
three are defined here. The preferred format is a fixed-length and
single-zone subset of the date and time specification used by the
Internet Message Format .
An example of the preferred format is
Examples of the two obsolete formats are
A recipient that parses a timestamp value in an HTTP field MUST
accept all three HTTP-date formats. When a sender generates a field
that contains one or more timestamps defined as HTTP-date,
the sender MUST generate those timestamps in the IMF-fixdate format.
An HTTP-date value represents time as an instance of Coordinated Universal
Time (UTC). The first two formats indicate UTC by the three-letter
abbreviation for Greenwich Mean Time, "GMT", a predecessor of the UTC name;
values in the asctime format are assumed to be in UTC.
A sender that generates HTTP-date values from a local clock ought to use
NTP () or some similar protocol to synchronize its
clock to UTC.
Preferred format:
Obsolete formats:
HTTP-date is case sensitive.
A sender MUST NOT generate additional whitespace in an HTTP-date beyond
that specifically included as SP in the grammar.
The semantics of day-name, day,
month, year, and time-of-day
are the same as those defined for the Internet Message Format constructs
with the corresponding name (, Section 3.3).
Recipients of a timestamp value in rfc850-date format, which uses a
two-digit year, MUST interpret a timestamp that appears to be more
than 50 years in the future as representing the most recent year in the
past that had the same last two digits.
Recipients of timestamp values are encouraged to be robust in parsing
timestamps unless otherwise restricted by the field definition.
For example, messages are occasionally forwarded over HTTP from a non-HTTP
source that might generate any of the date and time specifications defined
by the Internet Message Format.
Note: HTTP requirements for the date/time stamp format apply only
to their usage within the protocol stream. Implementations are
not required to use these formats for user presentation, request
logging, etc.
A #rule extension to the ABNF rules of is used to
improve readability in the definitions of some list-based field values.
A construct "#" is defined, similar to "*", for defining comma-delimited
lists of elements. The full form is "<n>#<m>element" indicating
at least <n> and at most <m> elements, each separated by a single
comma (",") and optional whitespace (OWS).
In any production that uses the list construct, a sender MUST NOT
generate empty list elements. In other words, a sender MUST generate
lists that satisfy the following syntax:
and:
and for n >= 1 and m > 1:
shows the collected ABNF for senders
after the list constructs have been expanded.
Empty elements do not contribute to the count of elements present.
A recipient MUST parse and ignore
a reasonable number of empty list elements: enough to handle common mistakes
by senders that merge values, but not so much that they could be used as a
denial-of-service mechanism. In other words, a recipient MUST accept lists
that satisfy the following syntax:
Note that because of the potential presence of empty list elements, the
RFC 5234 ABNF cannot enforce the cardinality of list elements, and
consequently all cases are mapped is if there was no cardinality specified.
For example, given these ABNF productions:
Then the following are valid values for example-list (not including the
double quotes, which are present for delimitation only):
In contrast, the following values would be invalid, since at least one
non-empty element is required by the example-list production:
In some HTTP versions, additional
metadata can be sent after the initial header section has been completed
(during or after transmission of the payload body), such as a message
integrity check, digital signature, or post-processing status.
For example, the chunked coding in HTTP/1.1 allows a trailer section after
the payload body (Section 7.1.2 of ) which can contain
trailer fields: field names and values that share the same syntax and
namespace as header fields but that are received after the header section.
Trailer fields ought to be processed and stored separately from the fields
in the header section to avoid contradicting message semantics known at
the time the header section was complete. The presence or absence of
certain header fields might impact choices made for the routing or
processing of the message as a whole before the trailers are received;
those choices cannot be unmade by the later discovery of trailer fields.
Many fields cannot be processed outside the header section because
their evaluation is necessary prior to receiving the message body, such as
those that describe message framing, routing, authentication,
request modifiers, response controls, or payload format.
A sender MUST NOT generate a trailer field unless the sender knows the
corresponding header field name's definition permits the field to be sent
in trailers.
Trailer fields can be difficult to process by intermediaries that forward
messages from one protocol version to another. If the entire message can be
buffered in transit, some intermediaries could merge trailer fields into
the header section (as appropriate) before it is forwarded. However, in
most cases, the trailers are simply discarded.
A recipient MUST NOT merge a trailer field into a header section unless
the recipient understands the corresponding header field definition and
that definition explicitly permits and defines how trailer field values
can be safely merged.
The presence of the keyword "trailers" in the TE header field (Section 7.4 of ) indicates that the client is willing to accept
trailer fields, on behalf of itself and any downstream clients. For
requests from an intermediary, this implies that all
downstream clients are willing to accept trailer fields in the forwarded
response. Note that the presence of "trailers" does not mean that the
client(s) will process any particular trailer field in the response; only
that the trailer section as a whole will not be dropped by any of the
clients.
Because of the potential for trailer fields to be discarded in transit, a
server SHOULD NOT generate trailer fields that it believes are necessary
for the user agent to receive.
The "Trailer" header field provides a list of field names that the sender
anticipates sending as trailer fields within that message. This allows a
recipient to prepare for receipt of the indicated metadata before it starts
processing the body.
For example, a sender might indicate that a message integrity check will
be computed as the payload is being streamed and provide the final
signature as a trailer field. This allows a recipient to perform the same
check on the fly as the payload data is received.
A sender that intends to generate one or more trailer fields in a message
SHOULD generate a Trailer header field in the header
section of that message to indicate which fields might be present in the
trailers.
See for a general requirements for field names,
and for a discussion of field values.
Authors of specifications defining new fields are advised to consider
documenting:
Whether the field is a single value or whether it can be a list
(delimited by commas; see ).
If it is not a list, document how to treat messages
where the field occurs multiple times (a sensible default would
be to ignore the field, but this might not always be the right
choice).
Note that intermediaries and software libraries might combine
multiple field instances into a single one, despite the
field's definition not allowing the list syntax. A robust format enables
recipients to discover these situations (good example: "Content-Type",
as the comma can only appear inside quoted strings;
bad example: "Location", as a comma can occur inside a URI).Under what conditions the field can be used; e.g., only in
responses or requests, in all messages, only on responses to a
particular request method, etc.What the scope of applicability for the information conveyed in
the field is. By default, fields apply only to the message they are
associated with, but some response fields are designed to apply to all
representations of a resource, the resource itself, or an even broader
scope. Specifications that expand the scope of a response field will
need to carefully consider issues such as content negotiation, the time
period of applicability, and (in some cases) multi-tenant server
deployments.Whether the field should be stored by origin servers that
understand it upon a PUT request.Whether the field semantics are further refined by the context,
such as by existing request methods or status codes.Whether it is appropriate to list the field name in the
Connection header field (i.e., if the field is to
be hop-by-hop; see Section 9.1 of ).Under what conditions intermediaries are allowed to insert,
delete, or modify the field's value.Whether it is appropriate to list the field name in a
Vary response header field (e.g., when the request header
field is used by an origin server's content selection algorithm; see
).Whether the field is allowable in trailers (see
).Whether the field ought to be preserved across redirects.Whether it introduces any additional security considerations, such
as disclosure of privacy-related data.
The following fields are defined by this document:
Field NameStatusReferenceAcceptstandardAccept-CharsetdeprecatedAccept-EncodingstandardAccept-LanguagestandardAccept-RangesstandardAllowstandardAuthentication-InfostandardAuthorizationstandardContent-EncodingstandardContent-LanguagestandardContent-LengthstandardContent-LocationstandardContent-RangestandardContent-TypestandardDatestandardETagstandardExpectstandardFromstandardHoststandardIf-MatchstandardIf-Modified-SincestandardIf-None-MatchstandardIf-RangestandardIf-Unmodified-SincestandardLast-ModifiedstandardLocationstandardMax-ForwardsstandardProxy-AuthenticatestandardProxy-Authentication-InfostandardProxy-AuthorizationstandardRangestandardRefererstandardRetry-AfterstandardServerstandardTrailerstandardUser-AgentstandardVarystandardViastandardWWW-Authenticatestandard
Furthermore, the field name "*" is reserved, since using that name as
an HTTP header field might conflict with its special semantics in the
Vary header field ().
Field NameStatusReferenceComments*standard
(reserved)
HTTP request message routing is determined by each client based on the
target resource, the client's proxy configuration, and
establishment or reuse of an inbound connection. The corresponding
response routing follows the same connection chain back to the client.
HTTP is used in a wide variety of applications, ranging from
general-purpose computers to home appliances. In some cases,
communication options are hard-coded in a client's configuration.
However, most HTTP clients rely on the same resource identification
mechanism and configuration techniques as general-purpose Web browsers.
HTTP communication is initiated by a user agent for some purpose. The
purpose is a combination of request semantics and a target resource upon
which to apply those semantics. The "request target" is
the protocol element that identifies the "target resource".
Typically, the request target is a URI reference ()
which a user agent would resolve to its absolute form in order to obtain
the "target URI". The target URI excludes the reference's
fragment component, if any, since fragment identifiers are reserved for
client-side processing (, Section 3.5).
However, there are two special, method-specific forms allowed for the
request target in specific circumstances:
For CONNECT (), the request target is the host
name and port number of the tunnel destination, separated by a colon.
For OPTIONS (), the request target can be a
single asterisk ("*").
See the respective method definitions for details. These forms MUST NOT
be used with other methods.
The "origin" for a given URI is the triple of scheme, host,
and port after normalizing the scheme and host to lowercase and
normalizing the port to remove any leading zeros. If port is elided from
the URI, the default port for that scheme is used. For example, the URI
would have the origin
which can also be described as the normalized URI prefix with port always
present:
Each origin defines its own namespace and controls how identifiers
within that namespace are mapped to resources. In turn, how the origin
responds to valid requests, consistently over time, determines the
semantics that users will associate with a URI, and the usefulness of
those semantics is what ultimately transforms these mechanisms into a
"resource" for users to reference and access in the future.
Two origins are distinct if they differ in scheme, host, or port. Even
when it can be verified that the same entity controls two distinct origins,
the two namespaces under those origins are distinct unless explicitly
aliased by a server authoritative for that origin.
Origin is also used within HTML and related Web protocols, beyond the
scope of this document, as described in .
Once the target URI and its origin are determined, a client decides whether
a network request is necessary to accomplish the desired semantics and,
if so, where that request is to be directed.
If the client has a cache and the request can be
satisfied by it, then the request is
usually directed there first.
If the request is not satisfied by a cache, then a typical client will
check its configuration to determine whether a proxy is to be used to
satisfy the request. Proxy configuration is implementation-dependent,
but is often based on URI prefix matching, selective authority matching,
or both, and the proxy itself is usually identified by an "http" or
"https" URI. If a proxy is applicable, the client connects inbound by
establishing (or reusing) a connection to that proxy.
If no proxy is applicable, a typical client will invoke a handler routine,
usually specific to the target URI's scheme, to connect directly
to an origin for the target resource. How that is accomplished is
dependent on the target URI scheme and defined by its associated
specification, similar to how this specification defines origin server
access for resolution of the "http" () and
"https" () schemes.
Although HTTP is independent of the transport protocol, the "http" scheme
is specific to associating authority with whomever controls the origin
server listening for TCP connections on the indicated port of whatever
host is identified within the authority component. This is a very weak
sense of authority because it depends on both client-specific name
resolution mechanisms and communication that might not be secured from
man-in-the-middle attacks. Nevertheless, it is a sufficient minimum for
binding "http" identifiers to an origin server for consistent resolution
within a trusted environment.
If the host identifier is provided as an IP address, the origin server is
the listener (if any) on the indicated TCP port at that IP address.
If host is a registered name, the registered name is an indirect identifier
for use with a name resolution service, such as DNS, to find an address for
an appropriate origin server.
When an "http" URI is used within a context that calls for access to the
indicated resource, a client MAY attempt access by resolving the host
identifier to an IP address, establishing a TCP connection to that address
on the indicated port, and sending an HTTP request message to the server
containing the URI's identifying data (Section 2 of ).
If the server responds to such a request with a non-interim HTTP response
message, as described in , then that response
is considered an authoritative answer to the client's request.
Note, however, that the above is not the only means for obtaining an
authoritative response, nor does it imply that an authoritative response
is always necessary (see ).
For example, the Alt-Svc header field allows an
origin server to identify other services that are also authoritative for
that origin. Access to "http" identified resources might also be provided
by protocols outside the scope of this document.
See for security considerations
related to establishing authority.
The "https" scheme associates authority based on the ability
of a server to use a private key associated with a certificate that
the client considers to be trustworthy for the identified host.
If a server presents a certificate that verifiably applies to the host,
along with proof that it controls the corresponding private key, then a
client will accept a secured connection to that server as being
authoritative for all origins with the same scheme and host.
A client is therefore relying upon a chain of trust, conveyed from some
trust anchor (which is usually prearranged or configured), through a
chain of certificates (e.g., ) to a final
certificate that binds a private key to the host name of the origin.
The handshake and certificate validation in
describe how that final certificate can
be used to initiate a secured connection.
Note that the "https" scheme does not rely on TCP and the connected port
number for associating authority, since both are outside the secured
communication and thus cannot be trusted as definitive. Hence, the HTTP
communication might take place over any channel that has been secured,
as defined in , including protocols that don't
use TCP. It is the origin's responsibility to ensure that any services
provided with control over its certificate's private key are equally
responsible for managing the corresponding "https" namespaces, or at least
prepared to reject requests that appear to have been misdirected.
Regardless, the origin's host and port value are passed within each HTTP
request, identifying the target resource and distinguishing it from other
namespaces that might be controlled by the same server.
In HTTP/1.1 and earlier, the only URIs for which a client will attribute
authority to a server are those for which a TLS connection was
specifically established toward the origin's host. Only the
characteristics of the connection establishment and certificate are used.
For a host that is a domain name, the client MUST include that name
in the TLS server_name extension (if used) and MUST verify that the
name also appears as either the CN field of the certificate subject or
as a dNSName in the subjectAltName field of the certificate
(see ).
For a host that is an IP address, the client MUST verify that the address
appears in the subjectAltName of the certificate.
In HTTP/2, a client will associate authority to all names that are present
in the certificate. However, a client will only do that if it concludes
that it could open a connection to the origin for that URI. In practice, a
client will make a DNS query and see that it contains the same server IP
address. A server sending the ORIGIN frame removes this restriction
.
In addition to the client's verification, an origin server is responsible
for verifying that requests it receives over a connection correspond
to resources for which it actually wants to be the origin. If a network
attacker causes connections for port N to be received at port Q, checking
the target URI is necessary to ensure that the attacker can't cause
"https://example.com:N/foo" to be replaced by "https://example.com:Q/foo"
without consent. Likewise, a server might be unwilling to serve as the
origin for some hosts even when they have the authority to do so.
When an "https" URI is used within a context that calls for access to the
indicated resource, a client MAY attempt access by resolving the host
identifier to an IP address, establishing a TCP connection to that address
on the indicated port, securing the connection end-to-end by successfully
initiating TLS over TCP with confidentiality and integrity protection, and
sending an HTTP request message to the server over that secured connection
containing the URI's identifying data (Section 2 of ).
If the server responds to such a request with a non-interim HTTP response
message, as described in , then that response
is considered an authoritative answer to the client's request.
Note, however, that the above is not the only means for obtaining an
authoritative response, nor does it imply that an authoritative response
is always necessary (see ).
Conceptually, HTTP/TLS is very simple. Simply use HTTP over TLS
precisely as you would use HTTP over TCP.
The agent acting as the HTTP client should also act as the TLS
client. It should initiate a connection to the server on the
appropriate port and then send the TLS ClientHello to begin the TLS
handshake. When the TLS handshake has finished. The client may then
initiate the first HTTP request. All HTTP data MUST be sent as TLS
"application data". Normal HTTP behavior, including retained
connections should be followed.
In general, HTTP/TLS requests are generated by dereferencing a URI.
As a consequence, the hostname for the server is known to the client.
If the hostname is available, the client MUST check it against the
server's identity as presented in the server's Certificate message,
in order to prevent man-in-the-middle attacks.
If the client has external information as to the expected identity of
the server, the hostname check MAY be omitted. (For instance, a
client may be connecting to a machine whose address and hostname are
dynamic but the client knows the certificate that the server will
present.) In such cases, it is important to narrow the scope of
acceptable certificates as much as possible in order to prevent man
in the middle attacks. In special cases, it may be appropriate for
the client to simply ignore the server's identity, but it must be
understood that this leaves the connection open to active attack.
If a subjectAltName extension of type dNSName is present, that MUST
be used as the identity. Otherwise, the (most specific) Common Name
field in the Subject field of the certificate MUST be used. Although
the use of the Common Name is existing practice, it is deprecated and
Certification Authorities are encouraged to use the dNSName instead.
Matching is performed using the matching rules specified by
. If more than one identity of a given type is present in
the certificate (e.g., more than one dNSName name, a match in any one
of the set is considered acceptable.) Names may contain the wildcard
character * which is considered to match any single domain name
component or component fragment. E.g., *.a.com matches foo.a.com but
not bar.foo.a.com. f*.com matches foo.com but not bar.com.
In some cases, the URI is specified as an IP address rather than a
hostname. In this case, the iPAddress subjectAltName must be present
in the certificate and must exactly match the IP in the URI.
If the hostname does not match the identity in the certificate, user
oriented clients MUST either notify the user (clients MAY give the
user the opportunity to continue with the connection in any case) or
terminate the connection with a bad certificate error. Automated
clients MUST log the error to an appropriate audit log (if available)
and SHOULD terminate the connection (with a bad certificate error).
Automated clients MAY provide a configuration setting that disables
this check, but MUST provide a setting which enables it.
Note that in many cases the URI itself comes from an untrusted
source. The above-described check provides no protection against
attacks where this source is compromised. For example, if the URI was
obtained by clicking on an HTML page which was itself obtained
without using HTTP/TLS, a man in the middle could have replaced the
URI. In order to prevent this form of attack, users should carefully
examine the certificate presented by the server to determine if it
meets their expectations.
Typically, the server has no external knowledge of what the client's
identity ought to be and so checks (other than that the client has a
certificate chain rooted in an appropriate CA) are not possible. If a
server has such knowledge (typically from some source external to
HTTP or TLS) it SHOULD check the identity as described above.
Once an inbound connection is obtained,
the client sends an HTTP request message (Section 2 of ).
Depending on the nature of the request, the client's target URI might be
split into components and transmitted (or implied) within various parts of
a request message. These parts are recombined by each recipient, in
accordance with their local configuration and incoming connection context,
to determine the target URI.
Appendix of defines how a server
determines the target URI for an HTTP/1.1 request.
Once the target URI has been reconstructed, an origin server needs
to decide whether or not to provide service for that URI via the connection
in which the request was received. For example, the request might have been
misdirected, deliberately or accidentally, such that the information within
a received Host header
field differs from the host or port upon which the connection has been
made. If the connection is from a trusted gateway, that inconsistency might
be expected; otherwise, it might indicate an attempt to bypass security
filters, trick the server into delivering non-public content, or poison a
cache. See for security
considerations regarding message routing.
Note: previous specifications defined the recomposed target URI as a
distinct concept, the effective request URI.
The "Host" header field in a request provides the host and port
information from the target URI, enabling the origin
server to distinguish among resources while servicing requests
for multiple host names on a single IP address.
A client MUST send a Host header field in all HTTP/1.1 request messages.
If the target URI includes an authority component, then a client MUST
send a field value for Host that is identical to that authority
component, excluding any userinfo subcomponent and its "@" delimiter
().
If the authority component is missing or undefined for the target URI,
then a client MUST send a Host header field with an empty field value.
Since the Host field value is critical information for handling a request,
a user agent SHOULD generate Host as the first header field following the
request-line.
For example, a GET request to the origin server for
<http://www.example.org/pub/WWW/> would begin with:
Since the Host header field acts as an application-level routing
mechanism, it is a frequent target for malware seeking to poison
a shared cache or redirect a request to an unintended server.
An interception proxy is particularly vulnerable if it relies on
the Host field value for redirecting requests to internal
servers, or for use as a cache key in a shared cache, without
first verifying that the intercepted connection is targeting a
valid IP address for that host.
A server MUST respond with a 400 (Bad Request) status code
to any HTTP/1.1 request message that lacks a Host header field and
to any request message that contains more than one Host header field
or a Host header field with an invalid field value.
As described in , intermediaries can serve
a variety of roles in the processing of HTTP requests and responses.
Some intermediaries are used to improve performance or availability.
Others are used for access control or to filter content.
Since an HTTP stream has characteristics similar to a pipe-and-filter
architecture, there are no inherent limits to the extent an intermediary
can enhance (or interfere) with either direction of the stream.
An intermediary not acting as a tunnel MUST implement the
Connection header field, as specified in
Section 9.1 of , and exclude fields from being forwarded
that are only intended for the incoming connection.
An intermediary MUST NOT forward a message to itself unless it is
protected from an infinite request loop. In general, an intermediary ought
to recognize its own server names, including any aliases, local variations,
or literal IP addresses, and respond to such requests directly.
An HTTP message can be parsed as a stream for incremental processing or
forwarding downstream. However, recipients cannot rely on incremental
delivery of partial messages, since some implementations will buffer or
delay message forwarding for the sake of network efficiency, security
checks, or payload transformations.
The "Via" header field indicates the presence of intermediate protocols and
recipients between the user agent and the server (on requests) or between
the origin server and the client (on responses), similar to the
"Received" header field in email
(Section 3.6.7 of ).
Via can be used for tracking message forwards,
avoiding request loops, and identifying the protocol capabilities of
senders along the request/response chain.
Each member of the Via field value represents a proxy or gateway that has
forwarded the message. Each intermediary appends its own information
about how the message was received, such that the end result is ordered
according to the sequence of forwarding recipients.
A proxy MUST send an appropriate Via header field, as described below, in
each message that it forwards.
An HTTP-to-HTTP gateway MUST send an appropriate Via header field in
each inbound request message and MAY send a Via header field in
forwarded response messages.
For each intermediary, the received-protocol indicates the protocol and
protocol version used by the upstream sender of the message. Hence, the
Via field value records the advertised protocol capabilities of the
request/response chain such that they remain visible to downstream
recipients; this can be useful for determining what backwards-incompatible
features might be safe to use in response, or within a later request, as
described in . For brevity, the protocol-name
is omitted when the received protocol is HTTP.
The received-by portion is normally the host and optional
port number of a recipient server or client that subsequently forwarded the
message.
However, if the real host is considered to be sensitive information, a
sender MAY replace it with a pseudonym. If a port is not provided,
a recipient MAY interpret that as meaning it was received on the default
TCP port, if any, for the received-protocol.
A sender MAY generate comments to identify the
software of each recipient, analogous to the User-Agent and
Server header fields. However, comments in Via
are optional, and a recipient MAY remove them prior to forwarding the
message.
For example, a request message could be sent from an HTTP/1.0 user
agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
forward the request to a public proxy at p.example.net, which completes
the request by forwarding it to the origin server at www.example.com.
The request received by www.example.com would then have the following
Via header field:
An intermediary used as a portal through a network firewall
SHOULD NOT forward the names and ports of hosts within the firewall
region unless it is explicitly enabled to do so. If not enabled, such an
intermediary SHOULD replace each received-by host of any host behind the
firewall by an appropriate pseudonym for that host.
An intermediary MAY combine an ordered subsequence of Via header
field list members into a single member if the entries have identical
received-protocol values. For example,
could be collapsed to
A sender SHOULD NOT combine multiple list members unless they are all
under the same organizational control and the hosts have already been
replaced by pseudonyms. A sender MUST NOT combine members that
have different received-protocol values.
Some intermediaries include features for transforming messages and their
payloads. A proxy might, for example, convert between image formats in
order to save cache space or to reduce the amount of traffic on a slow
link. However, operational problems might occur when these transformations
are applied to payloads intended for critical applications, such as medical
imaging or scientific data analysis, particularly when integrity checks or
digital signatures are used to ensure that the payload received is
identical to the original.
An HTTP-to-HTTP proxy is called a "transforming proxy"
if it is designed or configured to modify messages in a semantically
meaningful way (i.e., modifications, beyond those required by normal
HTTP processing, that change the message in a way that would be
significant to the original sender or potentially significant to
downstream recipients). For example, a transforming proxy might be
acting as a shared annotation server (modifying responses to include
references to a local annotation database), a malware filter, a
format transcoder, or a privacy filter. Such transformations are presumed
to be desired by whichever client (or client organization) selected the
proxy.
If a proxy receives a target URI with a host name that is not a
fully qualified domain name, it MAY add its own domain to the host name
it received when forwarding the request. A proxy MUST NOT change the
host name if the target URI contains a fully qualified domain name.
A proxy MUST NOT modify the "absolute-path" and "query" parts of the
received target URI when forwarding it to the next inbound server,
except as noted above to replace an empty path with "/" or "*".
A proxy MAY modify the message body through application
or removal of a transfer coding (Section 7 of ).
A proxy MUST NOT transform the payload () of a message that
contains a no-transform cache-control response directive (Section 5.2 of ).
A proxy MAY transform the payload of a message
that does not contain a no-transform cache-control directive.
A proxy that transforms the payload of a 200 (OK) response
can inform downstream recipients that a transformation has been
applied by changing the response status code to
203 (Non-Authoritative Information) ().
A proxy SHOULD NOT modify header fields that provide information about
the endpoints of the communication chain, the resource state, or the
selected representation (other than the payload) unless the field's
definition specifically allows such modification or the modification is
deemed necessary for privacy or security.
Considering that a resource could be anything, and that the uniform
interface provided by HTTP is similar to a window through which one can
observe and act upon such a thing only through the communication of
messages to some independent actor on the other side, an abstraction is
needed to represent ("take the place of") the current or desired state of
that thing in our communications. That abstraction is called a
representation .
For the purposes of HTTP, a "representation" is information
that is intended to reflect a past, current, or desired state of a given
resource, in a format that can be readily communicated via the protocol,
and that consists of a set of representation metadata and a potentially
unbounded stream of representation data.
An origin server might be provided with, or be capable of generating, multiple
representations that are each intended to reflect the current state of a
target resource. In such cases, some algorithm is used by
the origin server to select one of those representations as most applicable
to a given request, usually based on content negotiation.
This "selected representation" is used to provide the data
and metadata for evaluating conditional requests ()
and constructing the payload for 200 (OK),
206 (Partial Content), and
304 (Not Modified) responses to GET ().
The representation data associated with an HTTP message is
either provided as the payload body of the message or
referred to by the message semantics and the target
URI. The representation data is in a format and encoding defined by
the representation metadata header fields.
The data type of the representation data is determined via the header fields
Content-Type and Content-Encoding.
These define a two-layer, ordered encoding model:
HTTP uses media types in the
Content-Type ()
and Accept () header fields in
order to provide open and extensible data typing and type negotiation.
Media types define both a data format and various processing models:
how to process that data in accordance with each context in which it
is received.
The type and subtype tokens are case-insensitive.
The type/subtype MAY be followed by semicolon-delimited parameters
() in the form of name=value pairs.
The presence or absence of a parameter might be significant to the
processing of a media type, depending on its definition within the media
type registry.
Parameter values might or might not be case-sensitive, depending on the
semantics of the parameter name.
For example, the following media types are equivalent in describing HTML
text data encoded in the UTF-8 character encoding scheme, but the first is
preferred for consistency (the "charset" parameter value is defined as
being case-insensitive in , Section 4.1.2):
Media types ought to be registered with IANA according to the
procedures defined in .
HTTP uses charset names to indicate or negotiate the
character encoding scheme of a textual representation
.
A charset is identified by a case-insensitive token.
Charset names ought to be registered in the IANA "Character Sets" registry
()
according to the procedures defined in Section 2 of .
Note: In theory, charset names are defined by the "mime-charset" ABNF
rule defined in Section 2.3 of (as
corrected in ). That rule allows two characters
that are not included in "token" ("{" and "}"), but no charset name
registered at the time of this writing includes braces
(see ).
Media types are registered with a canonical form in order to be
interoperable among systems with varying native encoding formats.
Representations selected or transferred via HTTP ought to be in canonical
form, for many of the same reasons described by the Multipurpose Internet
Mail Extensions (MIME) .
However, the performance characteristics of email deployments (i.e., store
and forward messages to peers) are significantly different from those
common to HTTP and the Web (server-based information services).
Furthermore, MIME's constraints for the sake of compatibility with older
mail transfer protocols do not apply to HTTP
(see Appendix B of ).
MIME's canonical form requires that media subtypes of the "text"
type use CRLF as the text line break. HTTP allows the
transfer of text media with plain CR or LF alone representing a line
break, when such line breaks are consistent for an entire representation.
An HTTP sender MAY generate, and a recipient MUST be able to parse,
line breaks in text media that consist of CRLF, bare CR, or bare LF.
In addition, text media in HTTP is not limited to charsets that
use octets 13 and 10 for CR and LF, respectively.
This flexibility regarding line breaks applies only to text within a
representation that has been assigned a "text" media type; it does not
apply to "multipart" types or HTTP elements outside the payload body
(e.g., header fields).
If a representation is encoded with a content-coding, the underlying
data ought to be in a form defined above prior to being encoded.
MIME provides for a number of "multipart" types — encapsulations of
one or more representations within a single message body. All multipart
types share a common syntax, as defined in Section 5.1.1 of ,
and include a boundary parameter as part of the media type
value. The message body is itself a protocol element; a sender MUST
generate only CRLF to represent line breaks between body parts.
HTTP message framing does not use the multipart boundary as an indicator
of message body length, though it might be used by implementations that
generate or process the payload. For example, the "multipart/form-data"
type is often used for carrying form data in a request, as described in
, and the "multipart/byteranges" type is defined
by this specification for use in some 206 (Partial Content)
responses (see ).
Content coding values indicate an encoding transformation that has
been or can be applied to a representation. Content codings are primarily
used to allow a representation to be compressed or otherwise usefully
transformed without losing the identity of its underlying media type
and without loss of information. Frequently, the representation is stored
in coded form, transmitted directly, and only decoded by the final recipient.
All content codings are case-insensitive and ought to be registered
within the "HTTP Content Coding Registry", as defined in
Content-coding values are used in the
Accept-Encoding ()
and Content-Encoding ()
header fields.
The following content-coding values are defined by this specification:
NameDescriptionReferencecompressUNIX "compress" data format deflate"deflate" compressed data () inside
the "zlib" data format ()gzipGZIP file format identityReservedx-compressDeprecated (alias for compress)x-gzipDeprecated (alias for gzip)
The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
that is commonly produced by the UNIX file
compression program "compress".
A recipient SHOULD consider "x-compress" to be equivalent to "compress".
The "deflate" coding is a "zlib" data format
containing a "deflate" compressed data stream
that uses a combination of the Lempel-Ziv (LZ77) compression algorithm and
Huffman coding.
Note: Some non-conformant implementations send the "deflate"
compressed data without the zlib wrapper.
The "gzip" coding is an LZ77 coding with a 32-bit Cyclic Redundancy Check
(CRC) that is commonly
produced by the gzip file compression program .
A recipient SHOULD consider "x-gzip" to be equivalent to "gzip".
The "HTTP Content Coding Registry", maintained by
IANA at ,
registers content-coding names.
Content coding registrations MUST include the following fields:
NameDescriptionPointer to specification text
Names of content codings MUST NOT overlap with names of transfer codings
(Section 7 of ), unless the encoding transformation is identical (as
is the case for the compression codings defined in
).
Values to be added to this namespace require IETF Review
(see Section 4.8 of ) and MUST
conform to the purpose of content coding defined in
.
A language tag, as defined in , identifies a
natural language spoken, written, or otherwise conveyed by human beings for
communication of information to other human beings. Computer languages are
explicitly excluded.
HTTP uses language tags within the Accept-Language and
Content-Language header fields.
Accept-Language uses the broader language-range production
defined in , whereas
Content-Language uses the language-tag production defined
below.
A language tag is a sequence of one or more case-insensitive subtags, each
separated by a hyphen character ("-", %x2D). In most cases, a language tag
consists of a primary language subtag that identifies a broad family of
related languages (e.g., "en" = English), which is optionally followed by a
series of subtags that refine or narrow that language's range (e.g.,
"en-CA" = the variety of English as communicated in Canada).
Whitespace is not allowed within a language tag.
Example tags include:
See for further information.
Representation data can be partitioned into subranges when there are
addressable structural units inherent to that data's content coding or
media type. For example, octet (a.k.a., byte) boundaries are a structural
unit common to all representation data, allowing partitions of the data to
be identified as a range of bytes at some offset from the start or end of
that data.
This general notion of a "range unit" is used
in the Accept-Ranges ()
response header field to advertise support for range requests, the
Range () request header field
to delineate the parts of a representation that are requested, and the
Content-Range ()
payload header field to describe which part of a representation is being
transferred.
All range unit names are case-insensitive and ought to be registered
within the "HTTP Range Unit Registry", as defined in
The following range unit names are defined by this document:
Range Unit NameDescriptionReferencebytesa range of octetsnonereserved as keyword to indicate range requests are not supported
Ranges are expressed in terms of a range unit paired with a set of range
specifiers. The range unit name determines what kinds of range-spec
are applicable to its own specifiers. Hence, the following gramar is
generic: each range unit is expected to specify requirements on when
int-range, suffix-range, and
other-range are allowed.
A range request can specify a single range or a set
of ranges within a single representation.
An int-range is a range expressed as two non-negative
integers or as one non-negative integer through to the end of the
representation data.
The range unit specifies what the integers mean (e.g., they might indicate
unit offsets from the beginning, inclusive numbered parts, etc.).
An int-range is invalid if the
last-pos value is present and less than the
first-pos.
A suffix-range is a range expressed as a suffix of the
representation data with the provided non-negative integer maximum length
(in range units). In other words, the last N units of the representation
data.
To provide for extensibility, the other-range rule is a
mostly unconstrained grammar that allows application-specific or future
range units to define additional range specifiers.
The "bytes" range unit is used to express subranges of a representation
data's octet sequence.
Each byte range is expressed as an integer range at some offset, relative
to either the beginning (int-range) or end
(suffix-range) of the representation data.
Byte ranges do not use the other-range specifier.
The first-pos value in a bytes int-range
gives the offset of the first byte in a range.
The last-pos value gives the offset of the last
byte in the range; that is, the byte positions specified are inclusive.
Byte offsets start at zero.
If the representation data has a content coding applied, each byte range is
calculated with respect to the encoded sequence of bytes, not the sequence
of underlying bytes that would be obtained after decoding.
Examples of bytes range specifiers:
The first 500 bytes (byte offsets 0-499, inclusive):The second 500 bytes (byte offsets 500-999, inclusive):
A client can limit the number of bytes requested without knowing the size
of the selected representation.
If the last-pos value is absent, or if the value is
greater than or equal to the current length of the representation data, the
byte range is interpreted as the remainder of the representation (i.e., the
server replaces the value of last-pos with a value that
is one less than the current length of the selected representation).
A client can request the last N bytes (N > 0) of the selected
representation using a suffix-range.
If the selected representation is shorter than the specified
suffix-length, the entire representation is used.
Additional examples, assuming a representation of length 10000:
The final 500 bytes (byte offsets 9500-9999, inclusive):
Or:The first and last bytes only (bytes 0 and 9999):The first, middle, and last 1000 bytes:Other valid (but not canonical) specifications of the second 500
bytes (byte offsets 500-999, inclusive):
If a valid bytes range-set includes at least one
range-spec with a first-pos that is
less than the current length of the representation, or at least one
suffix-range with a non-zero suffix-length,
then the bytes range-set is satisfiable. Otherwise,
the bytes range-set is unsatisfiable.
If the selected representation has zero length, the only satisfiable form of
range-spec is a suffix-range with a non-zero
suffix-length.
In the byte-range syntax, first-pos,
last-pos, and suffix-length are
expressed as decimal number of octets. Since there is no predefined limit
to the length of a payload, recipients MUST anticipate potentially
large decimal numerals and prevent parsing errors due to integer conversion
overflows.
Other range units, such as format-specific boundaries like pages,
sections, records, rows, or time, are potentially usable in HTTP for
application-specific purposes, but are not commonly used in practice.
Implementors of alternative range units ought to consider how they would
work with content codings and general-purpose intermediaries.
Range units are intended to be extensible. New range units ought to be
registered with IANA, as defined in .
The "HTTP Range Unit Registry" defines the namespace for the range
unit names and refers to their corresponding specifications.
It is maintained at
.
Registration of an HTTP Range Unit MUST include the following fields:
NameDescriptionPointer to specification text
Values to be added to this namespace require IETF Review
(see , Section 4.8).
Representation header fields provide metadata about the representation.
When a message includes a payload body, the representation header fields
describe how to interpret the representation data enclosed in the payload
body. In a response to a HEAD request, the representation header fields
describe the representation data that would have been enclosed in the
payload body if the same request had been a GET.
The following header fields convey representation metadata:
Field NameDefined in...Content-TypeContent-EncodingContent-LanguageContent-LengthContent-Location
The "Content-Type" header field indicates the media type of the
associated representation: either the representation enclosed in
the message payload or the selected representation, as determined by the
message semantics. The indicated media type defines both the data format
and how that data is intended to be processed by a recipient, within the
scope of the received message semantics, after any content codings
indicated by Content-Encoding are decoded.
Media types are defined in . An example of the
field is
A sender that generates a message containing a payload body SHOULD
generate a Content-Type header field in that message unless the intended
media type of the enclosed representation is unknown to the sender.
If a Content-Type header field is not present, the recipient MAY either
assume a media type of
"application/octet-stream" (, Section 4.5.1)
or examine the data to determine its type.
In practice, resource owners do not always properly configure their origin
server to provide the correct Content-Type for a given representation.
Some user agents examine a payload's content and, in certain cases,
override the received type (for example, see ).
This "MIME sniffing" risks drawing incorrect conclusions about the data,
which might expose the user to additional security risks (e.g., "privilege
escalation"). Furthermore, it is impossible to determine the sender's
intended processing model by examining the data format: many data formats
match multiple media types that differ only in processing semantics.
Implementers are encouraged to provide a means to disable such sniffing.
The "Content-Encoding" header field indicates what content codings
have been applied to the representation, beyond those inherent in the media
type, and thus what decoding mechanisms have to be applied in order to
obtain data in the media type referenced by the Content-Type
header field.
Content-Encoding is primarily used to allow a representation's data to be
compressed without losing the identity of its underlying media type.
An example of its use is
If one or more encodings have been applied to a representation, the sender
that applied the encodings MUST generate a Content-Encoding header field
that lists the content codings in the order in which they were applied.
Note that the coding named "identity" is reserved for its special role
in Accept-Encoding, and thus SHOULD NOT be included.
Additional information about the encoding parameters can be provided
by other header fields not defined by this specification.
Unlike Transfer-Encoding (Section 6.1 of ), the codings listed
in Content-Encoding are a characteristic of the representation; the
representation is defined in terms of the coded form, and all other
metadata about the representation is about the coded form unless otherwise
noted in the metadata definition. Typically, the representation is only
decoded just prior to rendering or analogous usage.
If the media type includes an inherent encoding, such as a data format
that is always compressed, then that encoding would not be restated in
Content-Encoding even if it happens to be the same algorithm as one
of the content codings. Such a content coding would only be listed if,
for some bizarre reason, it is applied a second time to form the
representation. Likewise, an origin server might choose to publish the
same data as multiple representations that differ only in whether
the coding is defined as part of Content-Type or
Content-Encoding, since some user agents will behave differently in their
handling of each response (e.g., open a "Save as ..." dialog instead of
automatic decompression and rendering of content).
An origin server MAY respond with a status code of
415 (Unsupported Media Type) if a representation in the
request message has a content coding that is not acceptable.
The "Content-Language" header field describes the natural
language(s) of the intended audience for the representation. Note that this might
not be equivalent to all the languages used within the representation.
Language tags are defined in . The primary purpose of
Content-Language is to allow a user to identify and differentiate
representations according to the users' own preferred language. Thus, if the
content is intended only for a Danish-literate audience, the
appropriate field is
If no Content-Language is specified, the default is that the content
is intended for all language audiences. This might mean that the
sender does not consider it to be specific to any natural language,
or that the sender does not know for which language it is intended.
Multiple languages MAY be listed for content that is intended for
multiple audiences. For example, a rendition of the "Treaty of
Waitangi", presented simultaneously in the original Maori and English
versions, would call for
However, just because multiple languages are present within a representation
does not mean that it is intended for multiple linguistic audiences.
An example would be a beginner's language primer, such as "A First
Lesson in Latin", which is clearly intended to be used by an
English-literate audience. In this case, the Content-Language would
properly only include "en".
Content-Language MAY be applied to any media type — it is not
limited to textual documents.
The "Content-Length" header field indicates the number of data octets
(body length) for the representation. In some cases, Content-Length is
used to define or estimate message framing.
An example is
A sender MUST NOT send a Content-Length header field in any message that
contains a Transfer-Encoding header field.
A user agent SHOULD send a Content-Length in a request message when no
Transfer-Encoding is sent and the request method defines
a meaning for an enclosed payload body. For example, a Content-Length
header field is normally sent in a POST request even when the value is
0 (indicating an empty payload body). A user agent SHOULD NOT send a
Content-Length header field when the request message does not contain a
payload body and the method semantics do not anticipate such a body.
A server MAY send a Content-Length header field in a response to a HEAD
request (); a server MUST NOT send Content-Length in such a
response unless its field value equals the decimal number of octets that
would have been sent in the payload body of a response if the same
request had used the GET method.
A server MAY send a Content-Length header field in a
304 (Not Modified) response to a conditional GET request
(); a server MUST NOT send Content-Length in such a
response unless its field value equals the decimal number of octets that
would have been sent in the payload body of a 200 (OK)
response to the same request.
A server MUST NOT send a Content-Length header field in any response
with a status code of
1xx (Informational) or 204 (No Content).
A server MUST NOT send a Content-Length header field in any
2xx (Successful) response to a CONNECT request ().
Aside from the cases defined above, in the absence of Transfer-Encoding,
an origin server SHOULD send a Content-Length header field when the
payload body size is known prior to sending the complete header section.
This will allow downstream recipients to measure transfer progress,
know when a received message is complete, and potentially reuse the
connection for additional requests.
Any Content-Length field value greater than or equal to zero is valid.
Since there is no predefined limit to the length of a payload, a
recipient MUST anticipate potentially large decimal numerals and
prevent parsing errors due to integer conversion overflows
().
If a message is received that has a Content-Length header field value
consisting of the same decimal value as a comma-separated list () — for example, "Content-Length: 42, 42" —
indicating that duplicate Content-Length header fields have been generated
or combined by an upstream message processor, then the recipient MUST
either reject the message as invalid or replace the duplicated field
values with a single valid Content-Length field containing that decimal
value prior to determining the message body length or forwarding the
message.
The "Content-Location" header field references a URI that can be used
as an identifier for a specific resource corresponding to the
representation in this message's payload.
In other words, if one were to perform a GET request on this URI at the time
of this message's generation, then a 200 (OK) response would
contain the same representation that is enclosed as payload in this message.
The field value is either an absolute-URI or a
partial-URI. In the latter case (),
the referenced URI is relative to the target URI
(, Section 5).
The Content-Location value is not a replacement for the target URI
(). It is representation metadata.
It has the same syntax and semantics as the header field of the same name
defined for MIME body parts in Section 4 of .
However, its appearance in an HTTP message has some special implications
for HTTP recipients.
If Content-Location is included in a 2xx (Successful)
response message and its value refers (after conversion to absolute form)
to a URI that is the same as the target URI, then
the recipient MAY consider the payload to be a current representation of
that resource at the time indicated by the message origination date.
For a GET () or HEAD () request,
this is the same as the default semantics when no Content-Location is
provided by the server.
For a state-changing request like PUT () or
POST (), it implies that the server's response
contains the new representation of that resource, thereby distinguishing it
from representations that might only report about the action
(e.g., "It worked!").
This allows authoring applications to update their local copies without
the need for a subsequent GET request.
If Content-Location is included in a 2xx (Successful)
response message and its field value refers to a URI that differs from the
target URI, then the origin server claims that the URI
is an identifier for a different resource corresponding to the enclosed
representation. Such a claim can only be trusted if both identifiers share
the same resource owner, which cannot be programmatically determined via
HTTP.
For a response to a GET or HEAD request, this is an indication that the
target URI refers to a resource that is subject to content
negotiation and the Content-Location field value is a more specific
identifier for the selected representation.For a 201 (Created) response to a state-changing method,
a Content-Location field value that is identical to the
Location field value indicates that this payload is a
current representation of the newly created resource.Otherwise, such a Content-Location indicates that this payload is a
representation reporting on the requested action's status and that the
same report is available (for future access with GET) at the given URI.
For example, a purchase transaction made via a POST request might
include a receipt document as the payload of the 200 (OK)
response; the Content-Location field value provides an identifier for
retrieving a copy of that same receipt in the future.
A user agent that sends Content-Location in a request message is stating
that its value refers to where the user agent originally obtained the
content of the enclosed representation (prior to any modifications made by
that user agent). In other words, the user agent is providing a back link
to the source of the original representation.
An origin server that receives a Content-Location field in a request
message MUST treat the information as transitory request context rather
than as metadata to be saved verbatim as part of the representation.
An origin server MAY use that context to guide in processing the
request or to save it for other uses, such as within source links or
versioning metadata. However, an origin server MUST NOT use such context
information to alter the request semantics.
For example, if a client makes a PUT request on a negotiated resource and
the origin server accepts that PUT (without redirection), then the new
state of that resource is expected to be consistent with the one
representation supplied in that PUT; the Content-Location cannot be used as
a form of reverse content selection identifier to update only one of the
negotiated representations. If the user agent had wanted the latter
semantics, it would have applied the PUT directly to the Content-Location
URI.
Some HTTP messages transfer a complete or partial representation as the
message "payload". In some cases, a payload might contain
only the associated representation's header fields (e.g., responses to
HEAD) or only some part(s) of the representation data
(e.g., the 206 (Partial Content) status code).
Header fields that specifically describe the payload, rather than the
associated representation, are referred to as "payload header fields".
Payload header fields are defined in other parts of this specification,
due to their impact on message parsing.
Field NameDefined in...Content-RangeTrailerTransfer-EncodingSection 6.1 of
The purpose of a payload in a request is defined by the method semantics.
For example, a representation in the payload of a PUT request
() represents the desired state of the
target resource if the request is successfully applied,
whereas a representation in the payload of a POST request
() represents information to be processed by the
target resource.
In a response, the payload's purpose is defined by both the request method
and the response status code.
For example, the payload of a 200 (OK) response to GET
() represents the current state of the
target resource, as observed at the time of the message
origination date (), whereas the payload of
the same status code in a response to POST might represent either the
processing result or the new state of the target resource after applying
the processing. Response messages with an error status code usually contain
a payload that represents the error condition, such that it describes the
error state and what next steps are suggested for resolving it.
When a complete or partial representation is transferred in a message
payload, it is often desirable for the sender to supply, or the recipient
to determine, an identifier for a resource corresponding to that
representation.
For a request message:
If the request has a Content-Location header field,
then the sender asserts that the payload is a representation of the
resource identified by the Content-Location field value. However,
such an assertion cannot be trusted unless it can be verified by
other means (not defined by this specification). The information
might still be useful for revision history links.Otherwise, the payload is unidentified.
For a response message, the following rules are applied in order until a
match is found:
If the request method is GET or HEAD and the response status code is
200 (OK),
204 (No Content),
206 (Partial Content), or
304 (Not Modified),
the payload is a representation of the resource identified by the
target URI ().If the request method is GET or HEAD and the response status code is
203 (Non-Authoritative Information), the payload is
a potentially modified or enhanced representation of the
target resource as provided by an intermediary.If the response has a Content-Location header field
and its field value is a reference to the same URI as the target URI,
the payload is a representation of the target resource.If the response has a Content-Location header field
and its field value is a reference to a URI different from the
target URI, then the sender asserts that the payload is a
representation of the resource identified by the Content-Location
field value. However, such an assertion cannot be trusted unless
it can be verified by other means (not defined by this specification).Otherwise, the payload is unidentified.
The payload body contains the data of a request or response. This is
distinct from the message body (e.g., Section 6 of ),
which is how the payload body is transferred "on the wire", and might be
encoded, depending on the HTTP version in use.
It is also distinct from a request or response's representation data
(), which can be inferred from
protocol operation, rather than necessarily appearing "on the wire."
The presence of a payload body in a request depends on whether the request
method used defines semantics for it.
The presence of a payload body in a response depends on both the request
method to which it is responding and the response status code ().
Responses to the HEAD request method () never include
a payload body because the associated response header fields indicate only
what their values would have been if the request method had been GET
().
2xx (Successful) responses to a CONNECT request method
() switch the connection to tunnel mode instead of
having a payload body.
All 1xx (Informational), 204 (No Content), and
304 (Not Modified) responses do not include a payload body.
All other responses do include a payload body, although that body
might be of zero length.
The "Content-Range" header field is sent in a single part
206 (Partial Content) response to indicate the partial range
of the selected representation enclosed as the message payload, sent in
each part of a multipart 206 response to indicate the range enclosed within
each body part, and sent in 416 (Range Not Satisfiable)
responses to provide information about the selected representation.
If a 206 (Partial Content) response contains a
Content-Range header field with a range unit
() that the recipient does not understand, the
recipient MUST NOT attempt to recombine it with a stored representation.
A proxy that receives such a message SHOULD forward it downstream.
For byte ranges, a sender SHOULD indicate the complete length of the
representation from which the range has been extracted, unless the complete
length is unknown or difficult to determine. An asterisk character ("*") in
place of the complete-length indicates that the representation length was
unknown when the header field was generated.
The following example illustrates when the complete length of the selected
representation is known by the sender to be 1234 bytes:
and this second example illustrates when the complete length is unknown:
A Content-Range field value is invalid if it contains a
range-resp that has a last-pos
value less than its first-pos value, or a
complete-length value less than or equal to its
last-pos value. The recipient of an invalid
Content-Range MUST NOT attempt to recombine the received
content with a stored representation.
A server generating a 416 (Range Not Satisfiable) response
to a byte-range request SHOULD send a Content-Range header field with an
unsatisfied-range value, as in the following example:
The complete-length in a 416 response indicates the current length of the
selected representation.
The Content-Range header field has no meaning for status codes that do
not explicitly describe its semantic. For this specification, only the
206 (Partial Content) and
416 (Range Not Satisfiable) status codes describe a meaning
for Content-Range.
The following are examples of Content-Range values in which the
selected representation contains a total of 1234 bytes:
The first 500 bytes:The second 500 bytes:All except for the first 500 bytes:The last 500 bytes:
When a 206 (Partial Content) response message includes the
content of multiple ranges, they are transmitted as body parts in a
multipart message body (, Section 5.1)
with the media type of "multipart/byteranges".
The multipart/byteranges media type includes one or more body parts, each
with its own Content-Type and Content-Range
fields. The required boundary parameter specifies the boundary string used
to separate each body part.
Implementation Notes:
Additional CRLFs might precede the first boundary string in the body.Although permits the boundary string to be
quoted, some existing implementations handle a quoted boundary
string incorrectly.A number of clients and servers were coded to an early draft
of the byteranges specification that used a media type of
multipart/x-byteranges,
which is almost (but not quite) compatible with this type.
Despite the name, the "multipart/byteranges" media type is not limited to
byte ranges. The following example uses an "exampleunit" range unit:
The following information serves as the registration form for the
multipart/byteranges media type.
multipartbyterangesboundaryN/Aonly "7bit", "8bit", or "binary" are permittedsee N/AThis specification (see ).HTTP components supporting multiple ranges in a single request.N/AN/AN/AN/AN/ASee Authors' Addresses section.COMMONN/ASee Authors' Addresses section.IESG
When responses convey payload information, whether indicating a success or
an error, the origin server often has different ways of representing that
information; for example, in different formats, languages, or encodings.
Likewise, different users or user agents might have differing capabilities,
characteristics, or preferences that could influence which representation,
among those available, would be best to deliver. For this reason, HTTP
provides mechanisms for content negotiation.
This specification defines three patterns of content negotiation that can
be made visible within the protocol:
"proactive" negotiation, where the server selects the representation based
upon the user agent's stated preferences, "reactive" negotiation,
where the server provides a list of representations for the user agent to
choose from, and "request payload" negotiation, where the user agent
selects the representation for a future request based upon the server's
stated preferences in past responses. Other patterns of content negotiation include
"conditional content", where the representation consists of multiple
parts that are selectively rendered based on user agent parameters,
"active content", where the representation contains a script that
makes additional (more specific) requests based on the user agent
characteristics, and "Transparent Content Negotiation"
(), where content selection is performed by
an intermediary. These patterns are not mutually exclusive, and each has
trade-offs in applicability and practicality.
Note that, in all cases, HTTP is not aware of the resource semantics.
The consistency with which an origin server responds to requests, over time
and over the varying dimensions of content negotiation, and thus the
"sameness" of a resource's observed representations over time, is
determined entirely by whatever entity or algorithm selects or generates
those responses.
When content negotiation preferences are sent by the user agent in a
request to encourage an algorithm located at the server to
select the preferred representation, it is called
proactive negotiation
(a.k.a., server-driven negotiation). Selection is based on
the available representations for a response (the dimensions over which it
might vary, such as language, content-coding, etc.) compared to various
information supplied in the request, including both the explicit
negotiation fields of and implicit
characteristics, such as the client's network address or parts of the
User-Agent field.
Proactive negotiation is advantageous when the algorithm for
selecting from among the available representations is difficult to
describe to a user agent, or when the server desires to send its
"best guess" to the user agent along with the first response (hoping to
avoid the round trip delay of a subsequent request if the "best
guess" is good enough for the user). In order to improve the server's
guess, a user agent MAY send request header fields that describe
its preferences.
Proactive negotiation has serious disadvantages:
It is impossible for the server to accurately determine what
might be "best" for any given user, since that would require
complete knowledge of both the capabilities of the user agent
and the intended use for the response (e.g., does the user want
to view it on screen or print it on paper?);
Having the user agent describe its capabilities in every
request can be both very inefficient (given that only a small
percentage of responses have multiple representations) and a
potential risk to the user's privacy;
It complicates the implementation of an origin server and the
algorithms for generating responses to a request; and,
It limits the reusability of responses for shared caching.
A user agent cannot rely on proactive negotiation preferences being
consistently honored, since the origin server might not implement proactive
negotiation for the requested resource or might decide that sending a
response that doesn't conform to the user agent's preferences is better
than sending a 406 (Not Acceptable) response.
A Vary header field () is
often sent in a response subject to proactive negotiation to indicate what
parts of the request information were used in the selection algorithm.
With reactive negotiation
(a.k.a., agent-driven negotiation), selection of the best
response representation (regardless of the status code) is performed by the
user agent after receiving an initial response from the origin server that
contains a list of resources for alternative representations.
If the user agent is not satisfied by the initial response representation,
it can perform a GET request on one or more of the alternative resources,
selected based on metadata included in the list, to obtain a different form
of representation for that response. Selection of alternatives might be
performed automatically by the user agent or manually by the user selecting
from a generated (possibly hypertext) menu.
Note that the above refers to representations of the response, in general,
not representations of the resource. The alternative representations are
only considered representations of the target resource if the response in
which those alternatives are provided has the semantics of being a
representation of the target resource (e.g., a 200 (OK)
response to a GET request) or has the semantics of providing links to
alternative representations for the target resource
(e.g., a 300 (Multiple Choices) response to a GET request).
A server might choose not to send an initial representation, other than
the list of alternatives, and thereby indicate that reactive
negotiation by the user agent is preferred. For example, the alternatives
listed in responses with the 300 (Multiple Choices) and
406 (Not Acceptable) status codes include information about
the available representations so that the user or user agent can react by
making a selection.
Reactive negotiation is advantageous when the response would vary
over commonly used dimensions (such as type, language, or encoding),
when the origin server is unable to determine a user agent's
capabilities from examining the request, and generally when public
caches are used to distribute server load and reduce network usage.
Reactive negotiation suffers from the disadvantages of transmitting
a list of alternatives to the user agent, which degrades user-perceived
latency if transmitted in the header section, and needing a second request
to obtain an alternate representation. Furthermore, this specification
does not define a mechanism for supporting automatic selection, though it
does not prevent such a mechanism from being developed as an extension.
When content negotiation preferences are sent in a server's response, the
listed preferences are called request payload negotiation
because they intend to influence selection of an appropriate payload for
subsequent requests to that resource. For example,
the Accept-Encoding field
() can be sent in a response to
indicate preferred content codings for subsequent requests to that
resource .
Similarly, Section 3.1 of defines
the "Accept-Patch" response header field which allows discovery of
which content types are accepted in PATCH requests.
The content negotiation fields defined by this specification
use a common parameter, named "q" (case-insensitive), to assign a relative
"weight" to the preference for that associated kind of content.
This weight is referred to as a "quality value" (or "qvalue") because
the same parameter name is often used within server configurations to
assign a weight to the relative quality of the various representations
that can be selected for a resource.
The weight is normalized to a real number in the range 0 through 1,
where 0.001 is the least preferred and 1 is the most preferred;
a value of 0 means "not acceptable". If no "q" parameter is present,
the default weight is 1.
A sender of qvalue MUST NOT generate more than three digits after the
decimal point. User configuration of these values ought to be limited in
the same fashion.
The request method token is the primary source of request semantics;
it indicates the purpose for which the client has made this request
and what is expected by the client as a successful result.
The request method's semantics might be further specialized by the
semantics of some header fields when present in a request
() if those additional semantics do
not conflict with the method.
For example, a client can send conditional request header fields
() to make the requested
action conditional on the current state of the target resource.
HTTP was originally designed to be usable as an interface to distributed
object systems. The request method was envisioned as applying semantics to
a target resource in much the same way as invoking a
defined method on an identified object would apply semantics.
The method token is case-sensitive because it might be used as a gateway
to object-based systems with case-sensitive method names. By convention,
standardized methods are defined in all-uppercase US-ASCII letters.
Unlike distributed objects, the standardized request methods in HTTP are
not resource-specific, since uniform interfaces provide for better
visibility and reuse in network-based systems .
Once defined, a standardized method ought to have the same semantics when
applied to any resource, though each resource determines for itself
whether those semantics are implemented or allowed.
This specification defines a number of standardized methods that are
commonly used in HTTP, as outlined by the following table.
MethodDescriptionSec.GETTransfer a current representation of the target resource.HEADSame as GET, but do not transfer the response body.POSTPerform resource-specific processing on the request payload.PUTReplace all current representations of the target resource with
the request payload.DELETERemove all current representations of the target resource.CONNECTEstablish a tunnel to the server identified by the target resource.OPTIONSDescribe the communication options for the target resource.TRACEPerform a message loop-back test along the path to the target resource.
All general-purpose servers MUST support the methods GET and HEAD.
All other methods are OPTIONAL.
The set of methods allowed by a target resource can be listed in an
Allow header field ().
However, the set of allowed methods can change dynamically.
When a request method is received that is unrecognized or not implemented
by an origin server, the origin server SHOULD respond with the
501 (Not Implemented) status code.
When a request method is received that is known by an origin server but
not allowed for the target resource, the origin server SHOULD respond
with the 405 (Method Not Allowed) status code.
MethodSafeIdempotentReferenceCONNECTnonoDELETEnoyesGETyesyesHEADyesyesOPTIONSyesyesPOSTnonoPUTnoyesTRACEyesyes
Request methods are considered "safe" if
their defined semantics are essentially read-only; i.e., the client does
not request, and does not expect, any state change on the origin server
as a result of applying a safe method to a target resource. Likewise,
reasonable use of a safe method is not expected to cause any harm,
loss of property, or unusual burden on the origin server.
This definition of safe methods does not prevent an implementation from
including behavior that is potentially harmful, that is not entirely read-only,
or that causes side effects while invoking a safe method. What is
important, however, is that the client did not request that additional
behavior and cannot be held accountable for it. For example,
most servers append request information to access log files at the
completion of every response, regardless of the method, and that is
considered safe even though the log storage might become full and crash
the server. Likewise, a safe request initiated by selecting an
advertisement on the Web will often have the side effect of charging an
advertising account.
Of the request methods defined by this specification, the
GET, HEAD, OPTIONS, and TRACE methods are defined to be safe.
The purpose of distinguishing between safe and unsafe methods is to
allow automated retrieval processes (spiders) and cache performance
optimization (pre-fetching) to work without fear of causing harm.
In addition, it allows a user agent to apply appropriate constraints
on the automated use of unsafe methods when processing potentially
untrusted content.
A user agent SHOULD distinguish between safe and unsafe methods when
presenting potential actions to a user, such that the user can be made
aware of an unsafe action before it is requested.
When a resource is constructed such that parameters within the target URI
have the effect of selecting an action, it is the resource
owner's responsibility to ensure that the action is consistent with the
request method semantics.
For example, it is common for Web-based content editing software
to use actions within query parameters, such as "page?do=delete".
If the purpose of such a resource is to perform an unsafe action, then
the resource owner MUST disable or disallow that action when it is
accessed using a safe request method. Failure to do so will result in
unfortunate side effects when automated processes perform a GET on
every URI reference for the sake of link maintenance, pre-fetching,
building a search index, etc.
A request method is considered
"idempotent"
if the intended effect on the server of multiple identical requests with
that method is the same as the effect for a single such request.
Of the request methods defined by this
specification, PUT, DELETE, and safe request methods are idempotent.
Like the definition of safe, the idempotent property only applies to
what has been requested by the user; a server is free to log each request
separately, retain a revision control history, or implement other
non-idempotent side effects for each idempotent request.
Idempotent methods are distinguished because the request can be repeated
automatically if a communication failure occurs before the client is
able to read the server's response. For example, if a client sends a PUT
request and the underlying connection is closed before any response is
received, then the client can establish a new connection and retry the
idempotent request. It knows that repeating the request will have
the same intended effect, even if the original request succeeded, though
the response might differ.
A client SHOULD NOT automatically retry a request with a non-idempotent
method unless it has some means to know that the request semantics are
actually idempotent, regardless of the method, or some means to detect that
the original request was never applied.
For example, a user agent that knows (through design or configuration)
that a POST request to a given resource is safe can repeat that request
automatically. Likewise, a user agent designed specifically to operate on
a version control repository might be able to recover from partial failure
conditions by checking the target resource revision(s) after a failed
connection, reverting or fixing any changes that were partially applied,
and then automatically retrying the requests that failed.
Some clients use weaker signals to initiate automatic retries. For
example, when a POST request is sent, but the underlying transport
connection is closed before any part of the response is received. Although
this is commonly implemented, it is not recommended.
A proxy MUST NOT automatically retry non-idempotent requests.
A client SHOULD NOT automatically retry a failed automatic retry.
For a cache to store and use a response, the associated method needs to
explicitly allow caching, and detail under what conditions a response can
be used to satisfy subsequent requests; a method definition which does not
do so cannot be cached. For additional requirements see .
This specification defines caching semantics for GET, HEAD, and POST,
although the overwhelming majority of cache implementations only support
GET and HEAD.
The GET method requests transfer of a current selected representation for
the target resource. GET is the primary mechanism of
information retrieval and the focus of almost all performance
optimizations. Hence, when people speak of retrieving some identifiable
information via HTTP, they are generally referring to making a GET request.
The GET method is specifically intended to reflect the quality of
"sameness" identified by the request URI as if it were referenced as an
ordinary hypertext link.
It is tempting to think of resource identifiers as remote file system
pathnames and of representations as being a copy of the contents of such
files. In fact, that is how many resources are implemented (see
for related security considerations).
However, there are no such limitations in practice. The HTTP interface for
a resource is just as likely to be implemented as a tree of content
objects, a programmatic view on various database records, or a gateway to
other information systems. Even when the URI mapping mechanism is tied to a
file system, an origin server might be configured to execute the files with
the request as input and send the output as the representation rather than
transfer the files directly. Regardless, only the origin server needs to
know how each of its resource identifiers corresponds to an implementation
and how each implementation manages to select and send a current
representation of the target resource in a response to GET.
A client can alter the semantics of GET to be a "range request", requesting
transfer of only some part(s) of the selected representation, by sending a
Range header field in the request ().
A client SHOULD NOT generate a body in a GET
request. A payload received in a GET request has no defined semantics,
cannot alter the meaning or target of the request, and might lead some
implementations to reject the request and close the connection because of
its potential as a request smuggling attack
(Section 11.2 of ).
The response to a GET request is cacheable; a cache MAY use it to satisfy
subsequent GET and HEAD requests unless otherwise indicated by the
Cache-Control header field (Section 5.2 of ).
A cache that receives a payload in a GET request is likely to ignore that
payload and cache regardless of the payload contents.
The HEAD method is identical to GET except that the server MUST NOT
send a message body in the response (i.e., the response terminates at the
end of the header section). The server SHOULD send the same header fields
in response to a HEAD request as it would have sent if the request had been
a GET, except that the payload header fields ()
MAY be omitted. This method can be used for obtaining metadata about the
selected representation without transferring the representation data and is
often used for testing hypertext links for validity, accessibility, and
recent modification.
A payload within a HEAD request message has no defined semantics;
sending a payload body on a HEAD request might cause some existing
implementations to reject the request.
The response to a HEAD request is cacheable; a cache MAY use it to
satisfy subsequent HEAD requests unless otherwise indicated by the
Cache-Control header field (Section 5.2 of ). A HEAD response might
also have an effect on previously cached responses to GET; see Section 4.3.5 of .
The POST method requests that the target resource process
the representation enclosed in the request according to the resource's own
specific semantics. For example, POST is used for the following functions
(among others):
Providing a block of data, such as the fields entered into an HTML
form, to a data-handling process;Posting a message to a bulletin board, newsgroup, mailing list, blog,
or similar group of articles;Creating a new resource that has yet to be identified by the origin
server; andAppending data to a resource's existing representation(s).
An origin server indicates response semantics by choosing an appropriate
status code depending on the result of processing the POST request;
almost all of the status codes defined by this specification might be
received in a response to POST (the exceptions being 206 (Partial Content),
304 (Not Modified), and 416 (Range Not Satisfiable)).
If one or more resources has been created on the origin server as a result
of successfully processing a POST request, the origin server SHOULD send
a 201 (Created) response containing a Location
header field that provides an identifier for the primary resource created
() and a representation that describes the
status of the request while referring to the new resource(s).
Responses to POST requests are only cacheable when they include explicit
freshness information (see Section 4.2.1 of ) and a
Content-Location header field that has the same value as
the POST's target URI (). A cached POST response can be reused
to satisfy a later GET or HEAD request, but not a POST request, since POST
is required to be written through to the origin server, because it is
unsafe; see Section 4 of .
If the result of processing a POST would be equivalent to a representation
of an existing resource, an origin server MAY redirect the user agent to
that resource by sending a 303 (See Other) response with the
existing resource's identifier in the Location field.
This has the benefits of providing the user agent a resource identifier
and transferring the representation via a method more amenable to shared
caching, though at the cost of an extra request if the user agent does not
already have the representation cached.
The PUT method requests that the state of the target resource
be created or replaced with the state defined by the representation
enclosed in the request message payload. A successful PUT of a given
representation would suggest that a subsequent GET on that same target
resource will result in an equivalent representation being sent in
a 200 (OK) response. However, there is no guarantee that
such a state change will be observable, since the target resource might be
acted upon by other user agents in parallel, or might be subject to dynamic
processing by the origin server, before any subsequent GET is received.
A successful response only implies that the user agent's intent was
achieved at the time of its processing by the origin server.
If the target resource does not have a current representation and
the PUT successfully creates one, then the origin server MUST inform
the user agent by sending a 201 (Created) response. If the
target resource does have a current representation and that representation is
successfully modified in accordance with the state of the enclosed
representation, then the origin server MUST send either a
200 (OK) or a 204 (No Content) response to
indicate successful completion of the request.
An origin server SHOULD ignore unrecognized header and trailer fields
received in a PUT request (i.e., do not save them as part of the resource
state).
An origin server SHOULD verify that the PUT representation is
consistent with any constraints the server has for the target
resource that cannot or will not be changed by the PUT. This is
particularly important when the origin server uses internal
configuration information related to the URI in order to set the
values for representation metadata on GET responses. When a PUT
representation is inconsistent with the target resource, the origin
server SHOULD either make them consistent, by transforming the
representation or changing the resource configuration, or respond
with an appropriate error message containing sufficient information
to explain why the representation is unsuitable. The
409 (Conflict) or 415 (Unsupported Media Type)
status codes are suggested, with the latter being specific to constraints on
Content-Type values.
For example, if the target resource is configured to always have a
Content-Type of "text/html" and the representation being PUT
has a Content-Type of "image/jpeg", the origin server ought to do one of:
reconfigure the target resource to reflect the new media type;transform the PUT representation to a format consistent with that
of the resource before saving it as the new resource state; or,reject the request with a 415 (Unsupported Media Type)
response indicating that the target resource is limited to "text/html",
perhaps including a link to a different resource that would be a
suitable target for the new representation.
HTTP does not define exactly how a PUT method affects the state
of an origin server beyond what can be expressed by the intent of
the user agent request and the semantics of the origin server response.
It does not define what a resource might be, in any sense of that
word, beyond the interface provided via HTTP. It does not define
how resource state is "stored", nor how such storage might change
as a result of a change in resource state, nor how the origin server
translates resource state into representations. Generally speaking,
all implementation details behind the resource interface are
intentionally hidden by the server.
An origin server MUST NOT send a validator header field
(), such as an ETag or
Last-Modified field, in a successful response to PUT unless
the request's representation data was saved without any transformation
applied to the body (i.e., the resource's new representation data is
identical to the representation data received in the PUT request) and the
validator field value reflects the new representation.
This requirement allows a user agent to know when the representation body
it has in memory remains current as a result of the PUT, thus not in need
of being retrieved again from the origin server, and that the new validator(s)
received in the response can be used for future conditional requests in
order to prevent accidental overwrites ().
The fundamental difference between the POST and PUT methods is
highlighted by the different intent for the enclosed representation.
The target resource in a POST request is intended to handle the
enclosed representation according to the resource's own semantics,
whereas the enclosed representation in a PUT request is defined as
replacing the state of the target resource. Hence, the intent of PUT is
idempotent and visible to intermediaries, even though the exact effect is
only known by the origin server.
Proper interpretation of a PUT request presumes that the user agent knows
which target resource is desired. A service that selects a proper URI on
behalf of the client, after receiving a state-changing request, SHOULD be
implemented using the POST method rather than PUT. If the origin server
will not make the requested PUT state change to the target resource and
instead wishes to have it applied to a different resource, such as when the
resource has been moved to a different URI, then the origin server MUST
send an appropriate 3xx (Redirection) response; the
user agent MAY then make its own decision regarding whether or not to
redirect the request.
A PUT request applied to the target resource can have side effects
on other resources. For example, an article might have a URI for
identifying "the current version" (a resource) that is separate
from the URIs identifying each particular version (different
resources that at one point shared the same state as the current version
resource). A successful PUT request on "the current version" URI might
therefore create a new version resource in addition to changing the
state of the target resource, and might also cause links to be added
between the related resources.
An origin server that allows PUT on a given target resource MUST send a
400 (Bad Request) response to a PUT request that contains a
Content-Range header field (), since
the payload is likely to be partial content that has been mistakenly PUT as
a full representation.
Partial content updates are possible by targeting a separately identified
resource with state that overlaps a portion of the larger resource, or by
using a different method that has been specifically defined for partial
updates (for example, the PATCH method defined in
).
Responses to the PUT method are not cacheable. If a successful PUT request
passes through a cache that has one or more stored responses for the
target URI, those stored responses will be invalidated
(see Section 4.4 of ).
The DELETE method requests that the origin server remove the association
between the target resource and its current functionality.
In effect, this method is similar to the rm command in UNIX: it expresses a
deletion operation on the URI mapping of the origin server rather than an
expectation that the previously associated information be deleted.
If the target resource has one or more current representations, they might
or might not be destroyed by the origin server, and the associated storage
might or might not be reclaimed, depending entirely on the nature of the
resource and its implementation by the origin server (which are beyond the
scope of this specification). Likewise, other implementation aspects of a
resource might need to be deactivated or archived as a result of a DELETE,
such as database or gateway connections. In general, it is assumed that the
origin server will only allow DELETE on resources for which it has a
prescribed mechanism for accomplishing the deletion.
Relatively few resources allow the DELETE method — its primary use
is for remote authoring environments, where the user has some direction
regarding its effect. For example, a resource that was previously created
using a PUT request, or identified via the Location header field after a
201 (Created) response to a POST request, might allow a
corresponding DELETE request to undo those actions. Similarly, custom
user agent implementations that implement an authoring function, such as
revision control clients using HTTP for remote operations, might use
DELETE based on an assumption that the server's URI space has been crafted
to correspond to a version repository.
If a DELETE method is successfully applied, the origin server SHOULD send
a 202 (Accepted) status code if the action will likely succeed but
has not yet been enacted,a 204 (No Content) status code if the action has been
enacted and no further information is to be supplied, ora 200 (OK) status code if the action has been enacted and
the response message includes a representation describing the status.
A client SHOULD NOT generate a body in a DELETE request. A payload
received in a DELETE request has no defined semantics, cannot alter the
meaning or target of the request, and might lead some implementations to
reject the request.
Responses to the DELETE method are not cacheable. If a successful DELETE
request passes through a cache that has one or more stored responses for
the target URI, those stored responses will be invalidated (see
Section 4.4 of ).
The CONNECT method requests that the recipient establish a tunnel to the
destination origin server identified by the request target and, if
successful, thereafter restrict its behavior to blind forwarding of
data, in both directions, until the tunnel is closed.
Tunnels are commonly used to create an end-to-end virtual connection,
through one or more proxies, which can then be secured using TLS
(Transport Layer Security, ).
Because CONNECT changes the request/response nature of an HTTP connection,
specific HTTP versions might have different ways of mapping its semantics
into the protocol's wire format.
CONNECT is intended only for use in requests to a proxy.
An origin server that receives a CONNECT request for itself MAY
respond with a 2xx (Successful) status code to indicate that a connection
is established. However, most origin servers do not implement CONNECT.
A client sending a CONNECT request MUST send the authority component
(described in Section 3.2 of ) as the request target;
i.e., the request target consists of only the host name and port number of the
tunnel destination, separated by a colon. For example,
The recipient proxy can establish a tunnel either by directly connecting to
the request target or, if configured to use another proxy, by forwarding
the CONNECT request to the next inbound proxy.
Any 2xx (Successful) response indicates
that the sender (and all inbound proxies) will switch to tunnel mode
immediately after the blank line that concludes the successful response's
header section; data received after that blank line is from the server
identified by the request target.
Any response other than a successful response indicates that the tunnel
has not yet been formed and that the connection remains governed by HTTP.
A tunnel is closed when a tunnel intermediary detects that either side
has closed its connection: the intermediary MUST attempt to send any
outstanding data that came from the closed side to the other side, close
both connections, and then discard any remaining data left undelivered.
Proxy authentication might be used to establish the
authority to create a tunnel. For example,
There are significant risks in establishing a tunnel to arbitrary servers,
particularly when the destination is a well-known or reserved TCP port that
is not intended for Web traffic. For example, a CONNECT to
"example.com:25" would suggest that the proxy connect to the reserved
port for SMTP traffic; if allowed, that could trick the proxy into
relaying spam email. Proxies that support CONNECT SHOULD restrict its
use to a limited set of known ports or a configurable whitelist of safe
request targets.
A server MUST NOT send any Transfer-Encoding or
Content-Length header fields in a
2xx (Successful) response to CONNECT.
A client MUST ignore any Content-Length or Transfer-Encoding header
fields received in a successful response to CONNECT.
A payload within a CONNECT request message has no defined semantics;
sending a payload body on a CONNECT request might cause some existing
implementations to reject the request.
Responses to the CONNECT method are not cacheable.
The OPTIONS method requests information about the communication options
available for the target resource, at either the origin server or an
intervening intermediary. This method allows a client to determine the
options and/or requirements associated with a resource, or the capabilities
of a server, without implying a resource action.
An OPTIONS request with an asterisk ("*") as the request target
() applies to the server in general rather than to a
specific resource. Since a server's communication options typically depend
on the resource, the "*" request is only useful as a "ping" or "no-op"
type of method; it does nothing beyond allowing the client to test
the capabilities of the server. For example, this can be used to test
a proxy for HTTP/1.1 conformance (or lack thereof).
If the request target is not an asterisk, the OPTIONS request applies
to the options that are available when communicating with the target
resource.
A server generating a successful response to OPTIONS SHOULD send any
header that might indicate optional features implemented by the
server and applicable to the target resource (e.g., Allow),
including potential extensions not defined by this specification.
The response payload, if any, might also describe the communication options
in a machine or human-readable representation. A standard format for such a
representation is not defined by this specification, but might be defined by
future extensions to HTTP.
A client MAY send a Max-Forwards header field in an
OPTIONS request to target a specific recipient in the request chain (see
). A proxy MUST NOT generate a
Max-Forwards header field while forwarding a request unless that request
was received with a Max-Forwards field.
A client that generates an OPTIONS request containing a payload body
MUST send a valid Content-Type header field describing
the representation media type. Note that this specification does not define
any use for such a payload.
Responses to the OPTIONS method are not cacheable.
The TRACE method requests a remote, application-level loop-back of the
request message. The final recipient of the request SHOULD reflect the
message received, excluding some fields described below, back to the client
as the message body of a 200 (OK) response with a
Content-Type of "message/http" (Section 10.1 of ).
The final recipient is either the origin server or the first server to
receive a Max-Forwards value of zero (0) in the request
().
A client MUST NOT generate fields in a TRACE request containing
sensitive data that might be disclosed by the response. For example, it
would be foolish for a user agent to send stored user credentials
or cookies in a TRACE
request. The final recipient of the request SHOULD exclude any request
fields that are likely to contain sensitive data when that recipient
generates the response body.
TRACE allows the client to see what is being received at the other
end of the request chain and use that data for testing or diagnostic
information. The value of the Via header field ()
is of particular interest, since it acts as a trace of the request chain.
Use of the Max-Forwards header field allows the client to
limit the length of the request chain, which is useful for testing a chain
of proxies forwarding messages in an infinite loop.
A client MUST NOT send a message body in a TRACE request.
Responses to the TRACE method are not cacheable.
Additional methods, outside the scope of this specification, have been
specified for use in HTTP. All such methods ought to be registered
within the "Hypertext Transfer Protocol (HTTP) Method Registry".
The "Hypertext Transfer Protocol (HTTP) Method Registry", maintained by
IANA at ,
registers method names.
HTTP method registrations MUST include the following fields:
Method Name (see )Safe ("yes" or "no", see )Idempotent ("yes" or "no", see )Pointer to specification text
Values to be added to this namespace require IETF Review
(see , Section 4.8).
Standardized methods are generic; that is, they are potentially
applicable to any resource, not just one particular media type, kind of
resource, or application. As such, it is preferred that new methods
be registered in a document that isn't specific to a single application or
data format, since orthogonal technologies deserve orthogonal specification.
Since message parsing (Section 6 of ) needs to be independent of method
semantics (aside from responses to HEAD), definitions of new methods
cannot change the parsing algorithm or prohibit the presence of a message
body on either the request or the response message.
Definitions of new methods can specify that only a zero-length message body
is allowed by requiring a Content-Length header field with a value of "0".
A new method definition needs to indicate whether it is safe (), idempotent (),
cacheable (), what
semantics are to be associated with the payload body if any is present
in the request and what refinements the method makes to header field
or status code semantics.
If the new method is cacheable, its definition ought to describe how, and
under what conditions, a cache can store a response and use it to satisfy a
subsequent request.
The new method ought to describe whether it can be made conditional
() and, if so, how a server responds
when the condition is false.
Likewise, if the new method might have some use for partial response
semantics (), it ought to document this, too.
Note: Avoid defining a method name that starts with "M-", since that
prefix might be misinterpreted as having the semantics assigned to it
by .
A client sends request header fields to provide more information about
the request context, make the request conditional based on the target
resource state, suggest preferred formats for the response, supply
authentication credentials, or modify the expected request processing.
These fields act as request modifiers, similar to the parameters on a
programming language method invocation.
Controls are request header fields that direct specific handling of the
request.
Field NameDefined in...Cache-ControlSection 5.2 of ExpectHostMax-ForwardsPragmaSection 5.4 of TESection 7.4 of
The "Expect" header field in a request indicates a certain set of
behaviors (expectations) that need to be supported by the server in
order to properly handle this request. The only such expectation defined
by this specification is 100-continue.
The Expect field value is case-insensitive.
A server that receives an Expect field value other than
100-continue MAY respond with a
417 (Expectation Failed) status code to indicate that the
unexpected expectation cannot be met.
A 100-continue expectation informs recipients that the
client is about to send a (presumably large) message body in this request
and wishes to receive a 100 (Continue) interim response if
the method, target URI, and header fields are not sufficient to cause an immediate
success, redirect, or error response. This allows the client to wait for an
indication that it is worthwhile to send the message body before actually
doing so, which can improve efficiency when the message body is huge or
when the client anticipates that an error is likely (e.g., when sending a
state-changing method, for the first time, without previously verified
authentication credentials).
For example, a request that begins with
allows the origin server to immediately respond with an error message, such
as 401 (Unauthorized) or 405 (Method Not Allowed),
before the client starts filling the pipes with an unnecessary data
transfer.
Requirements for clients:
A client MUST NOT generate a 100-continue expectation in a request that
does not include a message body.
A client that will wait for a 100 (Continue) response
before sending the request message body MUST send an
Expect header field containing a 100-continue expectation.
A client that sends a 100-continue expectation is not required to wait
for any specific length of time; such a client MAY proceed to send the
message body even if it has not yet received a response. Furthermore,
since 100 (Continue) responses cannot be sent through an
HTTP/1.0 intermediary, such a client SHOULD NOT wait for an indefinite
period before sending the message body.
A client that receives a 417 (Expectation Failed) status
code in response to a request containing a 100-continue expectation
SHOULD repeat that request without a 100-continue expectation, since
the 417 response merely indicates that the response chain does not
support expectations (e.g., it passes through an HTTP/1.0 server).
Requirements for servers:
A server that receives a 100-continue expectation in an HTTP/1.0 request
MUST ignore that expectation.
A server MAY omit sending a 100 (Continue) response if
it has already received some or all of the message body for the
corresponding request, or if the framing indicates that there is no
message body.
A server that sends a 100 (Continue) response MUST
ultimately send a final status code, once the message body is received
and processed, unless the connection is closed prematurely.
A server that responds with a final status code before reading the
entire request payload body SHOULD indicate whether it intends to
close the connection (see Section 9.7 of ) or
continue reading the payload body.
An origin server MUST, upon receiving an HTTP/1.1 (or later) request that has a method, target URI,
and complete header section that contains a 100-continue expectation and
indicates a request message body will follow, either send an immediate
response with a final status code, if that status can be determined by
examining just the method, target URI, and header fields, or send an immediate
100 (Continue) response to encourage the client to send the
request's message body. The origin server MUST NOT wait for the message
body before sending the 100 (Continue) response.
A proxy MUST, upon receiving an HTTP/1.1 (or later) request that has a method, target URI,
and complete header section that contains a 100-continue expectation and
indicates a request message body will follow, either send an immediate
response with a final status code, if that status can be determined by
examining just the method, target URI, and header fields, or begin forwarding the
request toward the origin server by sending a corresponding request-line
and header section to the next inbound server. If the proxy believes (from
configuration or past interaction) that the next inbound server only
supports HTTP/1.0, the proxy MAY generate an immediate
100 (Continue) response to encourage the client to begin
sending the message body.
Note: The Expect header field was added after the original publication of
HTTP/1.1 as both the means to request an interim
100 (Continue) response and the general mechanism for indicating must-understand
extensions. However, the extension mechanism has not been used by clients
and the must-understand requirements have not been implemented by many
servers, rendering the extension mechanism useless. This specification has
removed the extension mechanism in order to simplify the definition and
processing of 100-continue.
The "Max-Forwards" header field provides a mechanism with the
TRACE () and OPTIONS ()
request methods to limit the number of times that the request is forwarded by
proxies. This can be useful when the client is attempting to
trace a request that appears to be failing or looping mid-chain.
The Max-Forwards value is a decimal integer indicating the remaining
number of times this request message can be forwarded.
Each intermediary that receives a TRACE or OPTIONS request containing a
Max-Forwards header field MUST check and update its value prior to
forwarding the request. If the received value is zero (0), the intermediary
MUST NOT forward the request; instead, the intermediary MUST respond as
the final recipient. If the received Max-Forwards value is greater than
zero, the intermediary MUST generate an updated Max-Forwards field in the
forwarded message with a field value that is the lesser of a) the received
value decremented by one (1) or b) the recipient's maximum supported value
for Max-Forwards.
A recipient MAY ignore a Max-Forwards header field received with any
other request methods.
A conditional request is an HTTP request with one or more request header
fields that indicate a precondition to be tested before
applying the request method to the target resource.
defines when preconditions are applied.
defines the order of evaluation when more than
one precondition is present.
Conditional GET requests are the most efficient mechanism for HTTP
cache updates . Conditionals can also be
applied to state-changing methods, such as PUT and DELETE, to prevent
the "lost update" problem: one client accidentally overwriting
the work of another client that has been acting in parallel.
Conditional request preconditions are based on the state of the target
resource as a whole (its current value set) or the state as observed
in a previously obtained representation (one value in that set).
A resource might have multiple current representations, each with its
own observable state. The conditional request mechanisms assume that
the mapping of requests to a selected representation ()
will be consistent over time if the server intends to take advantage of
conditionals. Regardless, if the mapping is inconsistent and the server is
unable to select the appropriate representation, then no harm will result
when the precondition evaluates to false.
The following request header fields allow a
client to place a precondition on the state of the target resource, so that
the action corresponding to the method semantics will not be applied if the
precondition evaluates to false. Each precondition defined by this
specification consists of a comparison between a set of validators obtained
from prior representations of the target resource to the current state of
validators for the selected representation
(). Hence, these preconditions evaluate
whether the state of the target resource has changed since a given state
known by the client. The effect of such an evaluation depends on the method
semantics and choice of conditional, as defined in .
Field NameDefined in...If-MatchIf-None-MatchIf-Modified-SinceIf-Unmodified-SinceIf-Range
Except when excluded below, a recipient cache or origin server MUST
evaluate received request preconditions after it has successfully performed
its normal request checks and just before it would perform the action
associated with the request method.
A server MUST ignore all received preconditions if its response to the
same request without those conditions would have been a status code other
than a 2xx (Successful) or 412 (Precondition Failed).
In other words, redirects and failures take precedence over the evaluation
of preconditions in conditional requests.
A server that is not the origin server for the target resource and cannot
act as a cache for requests on the target resource MUST NOT evaluate the
conditional request header fields defined by this specification, and it
MUST forward them if the request is forwarded, since the generating
client intends that they be evaluated by a server that can provide a
current representation.
Likewise, a server MUST ignore the conditional request header fields
defined by this specification when received with a request method that does
not involve the selection or modification of a
selected representation, such as CONNECT, OPTIONS, or TRACE.
Note that protocol extensions can modify the conditions under which
revalidation is triggered. For example, the "immutable" cache directive
(defined by ) instructs caches to forgo
revalidation of fresh responses even when requested by the client.
Conditional request header fields that are defined by extensions to HTTP
might place conditions on all recipients, on the state of the target
resource in general, or on a group of resources. For instance, the "If"
header field in WebDAV can make a request conditional on various aspects
of multiple resources, such as locks, if the recipient understands and
implements that field (, Section 10.4).
Although conditional request header fields are defined as being usable with
the HEAD method (to keep HEAD's semantics consistent with those of GET),
there is no point in sending a conditional HEAD because a successful
response is around the same size as a 304 (Not Modified)
response and more useful than a 412 (Precondition Failed)
response.
When more than one conditional request header field is present in a request,
the order in which the fields are evaluated becomes important. In practice,
the fields defined in this document are consistently implemented in a
single, logical order, since "lost update" preconditions have more strict
requirements than cache validation, a validated cache is more efficient
than a partial response, and entity tags are presumed to be more accurate
than date validators.
A recipient cache or origin server MUST evaluate the request
preconditions defined by this specification in the following order:
When recipient is the origin server and
If-Match is present,
evaluate the If-Match precondition:if true, continue to step if false, respond 412 (Precondition Failed) unless
it can be determined that the state-changing request has already
succeeded (see )When recipient is the origin server,
If-Match is not present, and
If-Unmodified-Since is present,
evaluate the If-Unmodified-Since precondition:if true, continue to step if false, respond 412 (Precondition Failed) unless
it can be determined that the state-changing request has already
succeeded (see )When If-None-Match is present,
evaluate the If-None-Match precondition:if true, continue to step if false for GET/HEAD, respond 304 (Not Modified)if false for other methods, respond 412 (Precondition Failed)When the method is GET or HEAD,
If-None-Match is not present, and
If-Modified-Since is present,
evaluate the If-Modified-Since precondition:if true, continue to step if false, respond 304 (Not Modified)When the method is GET and both
Range and If-Range are present,
evaluate the If-Range precondition:if the validator matches and the Range specification is
applicable to the selected representation, respond
206 (Partial Content)Otherwise,all conditions are met, so perform the requested action and
respond according to its success or failure.
Any extension to HTTP that defines additional conditional request
header fields ought to define its own expectations regarding the order
for evaluating such fields in relation to those defined in this document
and other conditionals that might be found in practice.
The "If-Match" header field makes the request method conditional on the
recipient origin server either having at least one current
representation of the target resource, when the field value is "*", or
having a current representation of the target resource that has an
entity-tag matching a member of the list of entity-tags provided in the
field value.
An origin server MUST use the strong comparison function when comparing
entity-tags for If-Match (), since
the client intends this precondition to prevent the method from being
applied if there have been any changes to the representation data.
Examples:
If-Match is most often used with state-changing methods (e.g., POST, PUT,
DELETE) to prevent accidental overwrites when multiple user agents might be
acting in parallel on the same resource (i.e., to prevent the "lost update"
problem). It can also be used with safe methods to abort a request if the
selected representation does not match one already stored
(or partially stored) from a prior request.
An origin server that receives an If-Match header field MUST evaluate the
condition as per prior to performing the method.
To evaluate a received If-Match header field:
If the field value is "*", the condition is true if the origin server
has a current representation for the target resource.
If the field value is a list of entity-tags, the condition is true if
any of the listed tags match the entity-tag of the selected representation.
Otherwise, the condition is false.
An origin server MUST NOT perform the requested method if a received
If-Match condition evaluates to false; instead, the origin server MUST
respond with either
a) the 412 (Precondition Failed) status code or
b) one of the 2xx (Successful) status codes if the origin
server has verified that a state change is being requested and the final
state is already reflected in the current state of the target resource
(i.e., the change requested by the user agent has already succeeded, but
the user agent might not be aware of it, perhaps because the prior response
was lost or a compatible change was made by some other user agent).
In the latter case, the origin server MUST NOT send a validator header
field in the response unless it can verify that the request is a duplicate
of an immediately prior change made by the same user agent.
The If-Match header field can be ignored by caches and intermediaries
because it is not applicable to a stored response.
Note that an If-Match header field with a list value containing "*" and
other values (including other instances of "*") is unlikely to be interoperable.
The "If-None-Match" header field makes the request method conditional on
a recipient cache or origin server either not having any current
representation of the target resource, when the field value is "*", or
having a selected representation with an entity-tag that does not match any
of those listed in the field value.
A recipient MUST use the weak comparison function when comparing
entity-tags for If-None-Match (),
since weak entity-tags can be used for cache validation even if there have
been changes to the representation data.
Examples:
If-None-Match is primarily used in conditional GET requests to enable
efficient updates of cached information with a minimum amount of
transaction overhead. When a client desires to update one or more stored
responses that have entity-tags, the client SHOULD generate an
If-None-Match header field containing a list of those entity-tags when
making a GET request; this allows recipient servers to send a
304 (Not Modified) response to indicate when one of those
stored responses matches the selected representation.
If-None-Match can also be used with a value of "*" to prevent an unsafe
request method (e.g., PUT) from inadvertently modifying an existing
representation of the target resource when the client believes that
the resource does not have a current representation ().
This is a variation on the "lost update" problem that might arise if more
than one client attempts to create an initial representation for the target
resource.
An origin server that receives an If-None-Match header field MUST
evaluate the condition as per prior to performing the method.
To evaluate a received If-None-Match header field:
If the field value is "*", the condition is false if the origin server
has a current representation for the target resource.
If the field value is a list of entity-tags, the condition is false if
one of the listed tags matches the entity-tag of the selected representation.
Otherwise, the condition is true.
An origin server MUST NOT perform the requested method if the condition
evaluates to false; instead, the origin server MUST respond with either
a) the 304 (Not Modified) status code if the request method
is GET or HEAD or b) the 412 (Precondition Failed) status
code for all other request methods.
Requirements on cache handling of a received If-None-Match header field
are defined in Section 4.3.2 of .
Note that an If-None-Match header field with a list value containing "*" and
other values (including other instances of "*") is unlikely to be interoperable.
The "If-Modified-Since" header field makes a GET or HEAD request method
conditional on the selected representation's modification date being more
recent than the date provided in the field value. Transfer of the selected
representation's data is avoided if that data has not changed.
An example of the field is:
A recipient MUST ignore If-Modified-Since if the request contains an
If-None-Match header field; the condition in
If-None-Match is considered to be a more accurate
replacement for the condition in If-Modified-Since, and the two are only
combined for the sake of interoperating with older intermediaries that
might not implement If-None-Match.
A recipient MUST ignore the If-Modified-Since header field if the
received field value is not a valid HTTP-date, or if the request method
is neither GET nor HEAD.
A recipient MUST interpret an If-Modified-Since field value's timestamp
in terms of the origin server's clock.
If-Modified-Since is typically used for two distinct purposes:
1) to allow efficient updates of a cached representation that does not
have an entity-tag and 2) to limit the scope of a web traversal to resources
that have recently changed.
When used for cache updates, a cache will typically use the value of the
cached message's Last-Modified field to generate the field
value of If-Modified-Since. This behavior is most interoperable for cases
where clocks are poorly synchronized or when the server has chosen to only
honor exact timestamp matches (due to a problem with Last-Modified dates
that appear to go "back in time" when the origin server's clock is
corrected or a representation is restored from an archived backup).
However, caches occasionally generate the field value based on other data,
such as the Date header field of the cached message or the
local clock time that the message was received, particularly when the
cached message does not contain a Last-Modified field.
When used for limiting the scope of retrieval to a recent time window, a
user agent will generate an If-Modified-Since field value based on either
its own local clock or a Date header field received from the
server in a prior response. Origin servers that choose an exact timestamp
match based on the selected representation's Last-Modified
field will not be able to help the user agent limit its data transfers to
only those changed during the specified window.
An origin server that receives an If-Modified-Since header field SHOULD
evaluate the condition as per prior to performing the method.
The origin server SHOULD NOT perform the requested method if the selected
representation's last modification date is earlier than or equal to the
date provided in the field value; instead, the origin server SHOULD
generate a 304 (Not Modified) response, including only those
metadata that are useful for identifying or updating a previously cached
response.
Requirements on cache handling of a received If-Modified-Since header field
are defined in Section 4.3.2 of .
The "If-Unmodified-Since" header field makes the request method conditional
on the selected representation's last modification date being earlier than or
equal to the date provided in the field value. This field accomplishes the
same purpose as If-Match for cases where the user agent does
not have an entity-tag for the representation.
An example of the field is:
A recipient MUST ignore If-Unmodified-Since if the request contains an
If-Match header field; the condition in
If-Match is considered to be a more accurate replacement for
the condition in If-Unmodified-Since, and the two are only combined for the
sake of interoperating with older intermediaries that might not implement
If-Match.
A recipient MUST ignore the If-Unmodified-Since header field if the
received field value is not a valid HTTP-date.
A recipient MUST interpret an If-Unmodified-Since field value's timestamp
in terms of the origin server's clock.
If-Unmodified-Since is most often used with state-changing methods
(e.g., POST, PUT, DELETE) to prevent accidental overwrites when multiple
user agents might be acting in parallel on a resource that does
not supply entity-tags with its representations (i.e., to prevent the
"lost update" problem). It can also be used with safe methods to abort a
request if the selected representation does not match one
already stored (or partially stored) from a prior request.
An origin server that receives an If-Unmodified-Since header field MUST
evaluate the condition as per prior to performing
the method.
If the selected representation has a last modification date, the origin server
MUST NOT perform the requested method if that date is more recent than the date
provided in the field value. Instead, the origin server MUST respond with either
a) the 412 (Precondition Failed) status code or
b) one of the 2xx (Successful) status codes if the origin
server has verified that a state change is being requested and the final
state is already reflected in the current state of the target resource
(i.e., the change requested by the user agent has already succeeded, but
the user agent might not be aware of that because the prior response message
was lost or a compatible change was made by some other user agent).
In the latter case, the origin server MUST NOT send a validator header
field in the response unless it can verify that the request is a duplicate
of an immediately prior change made by the same user agent.
The If-Unmodified-Since header field can be ignored by caches and
intermediaries because it is not applicable to a stored response.
The "If-Range" header field provides a special conditional request
mechanism that is similar to the If-Match and
If-Unmodified-Since header fields but that instructs the
recipient to ignore the Range header field if the validator
doesn't match, resulting in transfer of the new selected representation
instead of a 412 (Precondition Failed) response.
If a client has a partial copy of a representation and wishes
to have an up-to-date copy of the entire representation, it could use the
Range header field with a conditional GET (using
either or both of If-Unmodified-Since and
If-Match.) However, if the precondition fails because the
representation has been modified, the client would then have to make a
second request to obtain the entire current representation.
The "If-Range" header field allows a client to "short-circuit" the second
request. Informally, its meaning is as follows: if the representation is unchanged,
send me the part(s) that I am requesting in Range; otherwise, send me the
entire representation.
A client MUST NOT generate an If-Range header field in a request that
does not contain a Range header field.
A server MUST ignore an If-Range header field received in a request that
does not contain a Range header field.
An origin server MUST ignore an If-Range header field received in a
request for a target resource that does not support Range requests.
A client MUST NOT generate an If-Range header field containing an
entity-tag that is marked as weak.
A client MUST NOT generate an If-Range header field containing an
HTTP-date unless the client has no entity-tag for
the corresponding representation and the date is a strong validator
in the sense defined by .
A server that evaluates an If-Range precondition MUST use the strong
comparison function when comparing entity-tags ()
and MUST evaluate the condition as false if an HTTP-date
validator is provided that is not a strong validator in the sense defined
by .
A valid entity-tag can be distinguished from a valid
HTTP-date by examining the first two characters for a
DQUOTE.
If the validator given in the If-Range header field matches the current
validator for the selected representation of the target resource, then
the server SHOULD process the Range header field as
requested. If the validator does not match, the server MUST ignore the
Range header field. Note that this comparison by exact
match, including when the validator is an HTTP-date, differs
from the "earlier than or equal to" comparison used when evaluating an
If-Unmodified-Since conditional.
The "Range" header field on a GET request modifies the method semantics to
request transfer of only one or more subranges of the
selected representation data (),
rather than the entire selected representation.
Clients often encounter interrupted data
transfers as a result of canceled requests or dropped connections. When a
client has stored a partial representation, it is desirable to request the
remainder of that representation in a subsequent request rather than
transfer the entire representation. Likewise, devices with limited local
storage might benefit from being able to request only a subset of a larger
representation, such as a single page of a very large document, or the
dimensions of an embedded image.
Range requests are an OPTIONAL feature
of HTTP, designed so that recipients not implementing this feature (or not
supporting it for the target resource) can respond as if it is a normal
GET request without impacting interoperability. Partial responses are
indicated by a distinct status code to not be mistaken for full responses
by caches that might not implement the feature.
A server MAY ignore the Range header field. However, origin servers and
intermediate caches ought to support byte ranges when possible, since they
support efficient recovery from partially failed transfers and partial
retrieval of large representations. A server MUST ignore a Range header
field received with a request method other than GET.
Although the range request mechanism is designed to allow for
extensible range types, this specification only defines requests for
byte ranges.
An origin server MUST ignore a Range header field that contains a range
unit it does not understand. A proxy MAY discard a Range header
field that contains a range unit it does not understand.
A server that supports range requests MAY ignore or reject a
Range header field that consists of more than two
overlapping ranges, or a set of many small ranges that are not listed
in ascending order, since both are indications of either a broken client or
a deliberate denial-of-service attack ().
A client SHOULD NOT request multiple ranges that are inherently less
efficient to process and transfer than a single range that encompasses the
same data.
A server that supports range requests MAY ignore a Range
header field when the selected representation has no body
(i.e., the selected representation data is of zero length).
A client that is requesting multiple ranges SHOULD list those ranges in
ascending order (the order in which they would typically be received in a
complete representation) unless there is a specific need to request a later
part earlier. For example, a user agent processing a large representation
with an internal catalog of parts might need to request later parts first,
particularly if the representation consists of pages stored in reverse
order and the user agent wishes to transfer one page at a time.
The Range header field is evaluated after evaluating the precondition header
fields defined in , and only if the result in absence
of the Range header field would be a 200 (OK) response. In
other words, Range is ignored when a conditional GET would result in a
304 (Not Modified) response.
The If-Range header field () can be used as
a precondition to applying the Range header field.
If all of the preconditions are true, the server supports the Range header
field for the target resource, and the specified range(s) are valid and
satisfiable (as defined in ), the
server SHOULD send a 206 (Partial Content) response with a
payload containing one or more partial representations that correspond to
the satisfiable ranges requested.
If all of the preconditions are true, the server supports the Range header
field for the target resource, and the specified range(s) are invalid or
unsatisfiable, the server SHOULD send a
416 (Range Not Satisfiable) response.
The following request header fields can be sent by a user agent to engage in
proactive negotiation of the response content, as defined in
. The preferences sent in these
fields apply to any content in the response, including representations of
the target resource, representations of error or processing status, and
potentially even the miscellaneous text strings that might appear within
the protocol.
Field NameDefined in...AcceptAccept-CharsetAccept-EncodingAccept-Language
For each of these header fields, a request that does not contain it
implies that the user agent has no preference on that axis of negotiation.
If the header field is present in a request and none of the available
representations for the response can be considered acceptable according to
it, the origin server can either honor the header field by sending a
406 (Not Acceptable) response or disregard the header field
by treating the response as if it is not subject to content negotiation
for that request header field. This does not imply, however, that the
client will be able to use the representation.
Note: Sending these header fields makes it easier for a server to
identify an individual by virtue of the user agent's request
characteristics ().
Each of these header fields defines a wildcard value (often, "*") to
select unspecified values. If no wildcard is present, all values not
explicitly mentioned in the field are considered "not acceptable" to the
client.
Note: In practice, using wildcards in content negotiation has limited
practical value, because it is seldom useful to say, for example, "I
prefer image/* more or less than (some other specific value)". Clients can
explicitly request a 406 (Not Acceptable) response if a
more preferred format is not available by sending Accept: */*;q=0, but
they still need to be able to handle a different response, since the
server is allowed to ignore their preference.
The "Accept" header field can be used by user agents to specify their
preferences regarding response media types. For example, Accept header
fields can be used to indicate that the request is specifically limited to
a small set of desired types, as in the case of a request for an in-line
image.
When sent by a server in a response, Accept provides information
about what content types are preferred in the payload of a subsequent
request to the same resource.
The asterisk "*" character is used to group media types into ranges,
with "*/*" indicating all media types and "type/*" indicating all
subtypes of that type. The media-range can include media type
parameters that are applicable to that range.
Each media-range might be followed by zero or more applicable media type
parameters (e.g., charset), an optional "q" parameter for
indicating a relative weight (), and then zero or more extension
parameters. The "q" parameter is necessary if any extensions (accept-ext) are present,
since it acts as a separator between the two parameter sets.
Note: Use of the "q" parameter name to separate media type
parameters from Accept extension parameters is due to historical
practice. Although this prevents any media type parameter named
"q" from being used with a media range, such an event is believed
to be unlikely given the lack of any "q" parameters in the IANA
media type registry and the rare usage of any media type
parameters in Accept. Future media types are discouraged from
registering any parameter named "q".
The example
is interpreted as "I prefer audio/basic, but send me any audio
type if it is the best available after an 80% markdown in quality".
A more elaborate example is
Verbally, this would be interpreted as "text/html and text/x-c are
the equally preferred media types, but if they do not exist, then send the
text/x-dvi representation, and if that does not exist, send the text/plain
representation".
Media ranges can be overridden by more specific media ranges or
specific media types. If more than one media range applies to a given
type, the most specific reference has precedence. For example,
have the following precedence:
text/plain;format=flowedtext/plaintext/**/*
The media type quality factor associated with a given type is
determined by finding the media range with the highest precedence
that matches the type. For example,
would cause the following values to be associated:
Media TypeQuality Valuetext/plain;format=flowed1text/plain0.7text/html0.3image/jpeg0.5text/plain;format=fixed0.4text/html;level=30.7
Note: A user agent might be provided with a default set of quality
values for certain media ranges. However, unless the user agent is
a closed system that cannot interact with other rendering agents,
this default set ought to be configurable by the user.
The "Accept-Charset" header field can be sent by a user agent to indicate
its preferences for charsets in textual response content. For example,
this field allows user agents capable of understanding more comprehensive
or special-purpose charsets to signal that capability to an origin server
that is capable of representing information in those charsets.
Charset names are defined in .
A user agent MAY associate a quality value with each charset to indicate
the user's relative preference for that charset, as defined in .
An example is
The special value "*", if present in the Accept-Charset field,
matches every charset that is not mentioned elsewhere in the
Accept-Charset field.
Note: Accept-Charset is deprecated because UTF-8 has become nearly
ubiquitous and sending a detailed list of user-preferred charsets wastes
bandwidth, increases latency, and makes passive fingerprinting far too
easy (). Most general-purpose user agents
do not send Accept-Charset, unless specifically configured to do so.
The "Accept-Encoding" header field can be used to indicate preferences
regarding the use of content codings ().
When sent by a user agent in a request, Accept-Encoding indicates the
content codings acceptable in a response.
When sent by a server in a response, Accept-Encoding provides information
about what content codings are preferred in the payload of a subsequent
request to the same resource.
An "identity" token is used as a synonym for
"no encoding" in order to communicate when no encoding is preferred.
Each codings value MAY be given an associated quality value
representing the preference for that encoding, as defined in .
The asterisk "*" symbol in an Accept-Encoding field matches any available
content-coding not explicitly listed in the header field.
For example,
A server tests whether a content-coding for a given representation is
acceptable using these rules:
If no Accept-Encoding field is in the request, any content-coding is
considered acceptable by the user agent.If the representation has no content-coding, then it is acceptable
by default unless specifically excluded by the Accept-Encoding field
stating either "identity;q=0" or "*;q=0" without a more specific
entry for "identity".If the representation's content-coding is one of the content-codings
listed in the Accept-Encoding field value, then it is acceptable unless
it is accompanied by a qvalue of 0. (As defined in , a
qvalue of 0 means "not acceptable".)If multiple content-codings are acceptable, then the acceptable
content-coding with the highest non-zero qvalue is preferred.
An Accept-Encoding header field with a field value that is empty
implies that the user agent does not want any content-coding in response.
If an Accept-Encoding header field is present in a request and none of the
available representations for the response have a content-coding that
is listed as acceptable, the origin server SHOULD send a response
without any content-coding.
When the Accept-Encoding header field is present in a response, it indicates
what content codings the resource was willing to accept in the associated
request. The field value is evaluated the same way as in a request.
Note that this information is specific to the associated request; the set of
supported encodings might be different for other resources on the same
server and could change over time or depend on other aspects of the request
(such as the request method).
Servers that fail a request due to an unsupported content coding ought to
respond with a 415 (Unsupported Media Type) status and
include an Accept-Encoding header field in that response, allowing
clients to distinguish between issues related to content codings and media
types. In order to avoid confusion with issues related to media types,
servers that fail a request with a 415 status for reasons unrelated to
content codings MUST NOT include the Accept-Encoding header
field.
The most common use of Accept-Encoding is in responses with a
415 (Unsupported Media Type) status code, in response to
optimistic use of a content coding by clients. However, the header field
can also be used to indicate to clients that content codings are supported,
to optimize future interactions. For example, a resource might include it
in a 2xx (Successful) response when the request payload was
big enough to justify use of a compression coding but the client failed do
so.
Note: Most HTTP/1.0 applications do not recognize or obey qvalues
associated with content-codings. This means that qvalues might not
work and are not permitted with x-gzip or x-compress.
The "Accept-Language" header field can be used by user agents to
indicate the set of natural languages that are preferred in the response.
Language tags are defined in .
Each language-range can be given an associated quality value
representing an estimate of the user's preference for the languages
specified by that range, as defined in . For example,
would mean: "I prefer Danish, but will accept British English and
other types of English".
Note that some recipients treat the order in which language tags are listed
as an indication of descending priority, particularly for tags that are
assigned equal quality values (no value is the same as q=1). However, this
behavior cannot be relied upon. For consistency and to maximize
interoperability, many user agents assign each language tag a unique
quality value while also listing them in order of decreasing quality.
Additional discussion of language priority lists can be found in
Section 2.3 of .
For matching, Section 3 of defines
several matching schemes. Implementations can offer the most appropriate
matching scheme for their requirements. The "Basic Filtering" scheme
(, Section 3.3.1) is identical to the
matching scheme that was previously defined for HTTP in
Section 14.4 of .
It might be contrary to the privacy expectations of the user to send
an Accept-Language header field with the complete linguistic preferences of
the user in every request ().
Since intelligibility is highly dependent on the individual user, user
agents need to allow user control over the linguistic preference (either
through configuration of the user agent itself or by defaulting to a user
controllable system setting).
A user agent that does not provide such control to the user MUST NOT
send an Accept-Language header field.
Note: User agents ought to provide guidance to users when setting a
preference, since users are rarely familiar with the details of language
matching as described above. For example, users might assume that on
selecting "en-gb", they will be served any kind of English document if
British English is not available. A user agent might suggest, in such a
case, to add "en" to the list for better matching behavior.
HTTP provides a general framework for access control and authentication,
via an extensible set of challenge-response authentication schemes, which
can be used by a server to challenge a client request and by a client to
provide authentication information.
Two header fields are used for carrying authentication credentials.
Note that various custom mechanisms for
user authentication use the Cookie header field for this purpose, as
defined in .
Field NameDefined in...AuthorizationProxy-Authorization
HTTP provides a simple challenge-response authentication framework
that can be used by a server to challenge a client request and by a
client to provide authentication information. It uses a case-insensitive
token as a means to identify the authentication scheme, followed
by additional information necessary for achieving authentication via that
scheme. The latter can be either a comma-separated list of parameters or a
single sequence of characters capable of holding base64-encoded
information.
Authentication parameters are name=value pairs, where the name token is
matched case-insensitively,
and each parameter name MUST only occur once per challenge.
The token68 syntax allows the 66 unreserved URI characters
(), plus a few others, so that it can hold a
base64, base64url (URL and filename safe alphabet), base32, or base16 (hex)
encoding, with or without padding, but excluding whitespace
().
A 401 (Unauthorized) response message is used by an origin
server to challenge the authorization of a user agent, including a
WWW-Authenticate header field containing at least one
challenge applicable to the requested resource.
A 407 (Proxy Authentication Required) response message is
used by a proxy to challenge the authorization of a client, including a
Proxy-Authenticate header field containing at least one
challenge applicable to the proxy for the requested resource.
Note: Many clients fail to parse a challenge that contains an unknown
scheme. A workaround for this problem is to list well-supported schemes
(such as "basic") first.
A user agent that wishes to authenticate itself with an origin server
— usually, but not necessarily, after receiving a
401 (Unauthorized) — can do so by including an
Authorization header field with the request.
A client that wishes to authenticate itself with a proxy — usually,
but not necessarily, after receiving a
407 (Proxy Authentication Required) — can do so by
including a Proxy-Authorization header field with the
request.
Both the Authorization field value and the
Proxy-Authorization field value contain the client's
credentials for the realm of the resource being requested, based upon a
challenge received in a response (possibly at some point in the past).
When creating their values, the user agent ought to do so by selecting the
challenge with what it considers to be the most secure auth-scheme that it
understands, obtaining credentials from the user as appropriate.
Transmission of credentials within header field values implies significant
security considerations regarding the confidentiality of the underlying
connection, as described in
.
Upon receipt of a request for a protected resource that omits credentials,
contains invalid credentials (e.g., a bad password) or partial credentials
(e.g., when the authentication scheme requires more than one round trip),
an origin server SHOULD send a 401 (Unauthorized) response
that contains a WWW-Authenticate header field with at least
one (possibly new) challenge applicable to the requested resource.
Likewise, upon receipt of a request that omits proxy credentials or
contains invalid or partial proxy credentials, a proxy that requires
authentication SHOULD generate a
407 (Proxy Authentication Required) response that contains
a Proxy-Authenticate header field with at least one
(possibly new) challenge applicable to the proxy.
A server that receives valid credentials that are not adequate to gain
access ought to respond with the 403 (Forbidden) status
code ().
HTTP does not restrict applications to this simple challenge-response
framework for access authentication. Additional mechanisms can be used,
such as authentication at the transport level or via message encapsulation,
and with additional header fields specifying authentication information.
However, such additional mechanisms are not defined by this specification.
The "realm" authentication parameter is reserved for use by
authentication schemes that wish to indicate a scope of protection.
A protection space is defined by the canonical root URI (the
scheme and authority components of the target URI; see
) of the
server being accessed, in combination with the realm value if present.
These realms allow the protected resources on a server to be
partitioned into a set of protection spaces, each with its own
authentication scheme and/or authorization database. The realm value
is a string, generally assigned by the origin server, that can have
additional semantics specific to the authentication scheme. Note that a
response can have multiple challenges with the same auth-scheme but
with different realms.
The protection space determines the domain over which credentials can
be automatically applied. If a prior request has been authorized, the
user agent MAY reuse the same credentials for all other requests within
that protection space for a period of time determined by the authentication
scheme, parameters, and/or user preferences (such as a configurable
inactivity timeout). Unless specifically allowed by the authentication
scheme, a single protection space cannot extend outside the scope of its
server.
For historical reasons, a sender MUST only generate the quoted-string syntax.
Recipients might have to support both token and quoted-string syntax for
maximum interoperability with existing clients that have been accepting both
notations for a long time.
The "Authorization" header field allows a user agent to authenticate itself
with an origin server — usually, but not necessarily, after receiving
a 401 (Unauthorized) response. Its value consists of
credentials containing the authentication information of the user agent for
the realm of the resource being requested.
If a request is authenticated and a realm specified, the same credentials
are presumed to be valid for all other requests within this realm (assuming
that the authentication scheme itself does not require otherwise, such as
credentials that vary according to a challenge value or using synchronized
clocks).
A proxy forwarding a request MUST NOT modify any
Authorization fields in that request.
See Section 3.3 of for details of and requirements
pertaining to handling of the Authorization field by HTTP caches.
The "Proxy-Authorization" header field allows the client to
identify itself (or its user) to a proxy that requires
authentication. Its value consists of credentials containing the
authentication information of the client for the proxy and/or realm of the
resource being requested.
Unlike Authorization, the Proxy-Authorization header field
applies only to the next inbound proxy that demanded authentication using
the Proxy-Authenticate field. When multiple proxies are used
in a chain, the Proxy-Authorization header field is consumed by the first
inbound proxy that was expecting to receive credentials. A proxy MAY
relay the credentials from the client request to the next proxy if that is
the mechanism by which the proxies cooperatively authenticate a given
request.
Aside from the general framework, this document does not specify any
authentication schemes. New and existing authentication schemes are
specified independently and ought to be registered within the
"Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry".
For example, the "basic" and "digest" authentication schemes are defined by
RFC 7617 and
RFC 7616, respectively.
The "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry"
defines the namespace for the authentication schemes in challenges and
credentials. It is maintained
at .
Registrations MUST include the following fields:
Authentication Scheme NamePointer to specification textNotes (optional)
Values to be added to this namespace require IETF Review
(see , Section 4.8).
There are certain aspects of the HTTP Authentication framework that put
constraints on how new authentication schemes can work:
HTTP authentication is presumed to be stateless: all of the information
necessary to authenticate a request MUST be provided in the request,
rather than be dependent on the server remembering prior requests.
Authentication based on, or bound to, the underlying connection is
outside the scope of this specification and inherently flawed unless
steps are taken to ensure that the connection cannot be used by any
party other than the authenticated user
(see ).
The authentication parameter "realm" is reserved for defining protection
spaces as described in . New schemes
MUST NOT use it in a way incompatible with that definition.
The "token68" notation was introduced for compatibility with existing
authentication schemes and can only be used once per challenge or credential.
Thus, new schemes ought to use the auth-param syntax instead, because
otherwise future extensions will be impossible.
The parsing of challenges and credentials is defined by this specification
and cannot be modified by new authentication schemes. When the auth-param
syntax is used, all parameters ought to support both token and
quoted-string syntax, and syntactical constraints ought to be defined on
the field value after parsing (i.e., quoted-string processing). This is
necessary so that recipients can use a generic parser that applies to
all authentication schemes.
Note: The fact that the value syntax for the "realm" parameter
is restricted to quoted-string was a bad design choice not to be repeated
for new parameters.
Definitions of new schemes ought to define the treatment of unknown
extension parameters. In general, a "must-ignore" rule is preferable
to a "must-understand" rule, because otherwise it will be hard to introduce
new parameters in the presence of legacy recipients. Furthermore,
it's good to describe the policy for defining new parameters (such
as "update the specification" or "use this registry").
Authentication schemes need to document whether they are usable in
origin-server authentication (i.e., using WWW-Authenticate),
and/or proxy authentication (i.e., using Proxy-Authenticate).
The credentials carried in an Authorization header field are specific to
the user agent and, therefore, have the same effect on HTTP caches as the
"private" Cache-Control response directive (Section 5.2.2.7 of ),
within the scope of the request in which they appear.
Therefore, new authentication schemes that choose not to carry
credentials in the Authorization header field (e.g., using a newly defined
header field) will need to explicitly disallow caching, by mandating the use of
Cache-Control response directives (e.g., "private").
Schemes using Authentication-Info, Proxy-Authentication-Info,
or any other authentication related response header field need to
consider and document the related security considerations (see
).
The following request header fields provide additional information about the
request context, including information about the user, user agent, and
resource behind the request.
Field NameDefined in...FromRefererUser-Agent
The "From" header field contains an Internet email address for a human
user who controls the requesting user agent. The address ought to be
machine-usable, as defined by "mailbox"
in Section 3.4 of :
An example is:
The From header field is rarely sent by non-robotic user agents.
A user agent SHOULD NOT send a From header field without explicit
configuration by the user, since that might conflict with the user's
privacy interests or their site's security policy.
A robotic user agent SHOULD send a valid From header field so that the
person responsible for running the robot can be contacted if problems
occur on servers, such as if the robot is sending excessive, unwanted,
or invalid requests.
A server SHOULD NOT use the From header field for access control or
authentication, since most recipients will assume that the field value is
public information.
The "Referer" [sic] header field allows the user agent to specify a URI
reference for the resource from which the target URI was
obtained (i.e., the "referrer", though the field name is misspelled).
A user agent MUST NOT include the fragment and userinfo components
of the URI reference , if any, when generating the
Referer field value.
The field value is either an absolute-URI or a
partial-URI. In the latter case (),
the referenced URI is relative to the target URI
(, Section 5).
The Referer header field allows servers to generate back-links to other
resources for simple analytics, logging, optimized caching, etc. It also
allows obsolete or mistyped links to be found for maintenance. Some servers
use the Referer header field as a means of denying links from other sites
(so-called "deep linking") or restricting cross-site request forgery (CSRF),
but not all requests contain it.
Example:
If the target URI was obtained from a source that does not have its own
URI (e.g., input from the user keyboard, or an entry within the user's
bookmarks/favorites), the user agent MUST either exclude the Referer field
or send it with a value of "about:blank".
The Referer field has the potential to reveal information about the request
context or browsing history of the user, which is a privacy concern if the
referring resource's identifier reveals personal information (such as an
account name) or a resource that is supposed to be confidential (such as
behind a firewall or internal to a secured service). Most general-purpose
user agents do not send the Referer header field when the referring
resource is a local "file" or "data" URI. A user agent MUST NOT send a
Referer header field in an unsecured HTTP request if the
referring page was received with a secure protocol.
See for additional
security considerations.
Some intermediaries have been known to indiscriminately remove Referer
header fields from outgoing requests. This has the unfortunate side effect
of interfering with protection against CSRF attacks, which can be far
more harmful to their users. Intermediaries and user agent extensions that
wish to limit information disclosure in Referer ought to restrict their
changes to specific edits, such as replacing internal domain names with
pseudonyms or truncating the query and/or path components.
An intermediary SHOULD NOT modify or delete the Referer header field when
the field value shares the same scheme and host as the target URI.
The "User-Agent" header field contains information about the user agent
originating the request, which is often used by servers to help identify
the scope of reported interoperability problems, to work around or tailor
responses to avoid particular user agent limitations, and for analytics
regarding browser or operating system use. A user agent SHOULD send
a User-Agent field in each request unless specifically configured not
to do so.
The User-Agent field value consists of one or more product identifiers,
each followed by zero or more comments (), which together
identify the user agent software and its significant subproducts.
By convention, the product identifiers are listed in decreasing order of
their significance for identifying the user agent software. Each product
identifier consists of a name and optional version.
A sender SHOULD limit generated product identifiers to what is necessary
to identify the product; a sender MUST NOT generate advertising or other
nonessential information within the product identifier.
A sender SHOULD NOT generate information in product-version
that is not a version identifier (i.e., successive versions of the same
product name ought to differ only in the product-version portion of the
product identifier).
Example:
A user agent SHOULD NOT generate a User-Agent field containing needlessly
fine-grained detail and SHOULD limit the addition of subproducts by third
parties. Overly long and detailed User-Agent field values increase request
latency and the risk of a user being identified against their wishes
("fingerprinting").
Likewise, implementations are encouraged not to use the product tokens of
other implementations in order to declare compatibility with them, as this
circumvents the purpose of the field. If a user agent masquerades as a
different user agent, recipients can assume that the user intentionally
desires to see responses tailored for that identified user agent, even
if they might not work as well for the actual user agent being used.
The (response) status code is a three-digit integer code giving the result of the
attempt to understand and satisfy the request.
HTTP status codes are extensible. HTTP clients are not required
to understand the meaning of all registered status codes, though such
understanding is obviously desirable. However, a client MUST
understand the class of any status code, as indicated by the first
digit, and treat an unrecognized status code as being equivalent to the
x00 status code of that class.
For example, if an unrecognized status code of 471 is received by a client,
the client can assume that there was something wrong with its request and
treat the response as if it had received a 400 (Bad Request) status code. The response
message will usually contain a representation that explains the status.
The first digit of the status code defines the class of response. The
last two digits do not have any categorization role. There are five
values for the first digit:
1xx (Informational): The request was received, continuing
process
2xx (Successful): The request was successfully received,
understood, and accepted
3xx (Redirection): Further action needs to be taken in order to
complete the request
4xx (Client Error): The request contains bad syntax or cannot
be fulfilled
5xx (Server Error): The server failed to fulfill an apparently
valid request
A single request can have multiple associated responses: zero or more
interim (non-final) responses with status codes in the "informational"
(1xx) range, followed by exactly one final
response with a status code in one of the other ranges.
The status codes listed below are defined in this specification.
The reason phrases listed here are only recommendations — they can be
replaced by local equivalents without affecting the protocol.
Responses with status codes that are defined as heuristically cacheable
(e.g., 200, 203, 204, 206, 300, 301, 308, 404, 405, 410, 414, and 501 in this
specification) can be reused by a cache with heuristic expiration unless
otherwise indicated by the method definition or explicit cache controls
; all other status codes are not heuristically cacheable.
ValueDescriptionReference100Continue101Switching Protocols200OK201Created202Accepted203Non-Authoritative Information204No Content205Reset Content206Partial Content300Multiple Choices301Moved Permanently302Found303See Other304Not Modified305Use Proxy306(Unused)307Temporary Redirect308Permanent Redirect400Bad Request401Unauthorized402Payment Required403Forbidden404Not Found405Method Not Allowed406Not Acceptable407Proxy Authentication Required408Request Timeout409Conflict410Gone411Length Required412Precondition Failed413Payload Too Large414URI Too Long415Unsupported Media Type416Range Not Satisfiable417Expectation Failed418(Unused)422Unprocessable Payload426Upgrade Required500Internal Server Error501Not Implemented502Bad Gateway503Service Unavailable504Gateway Timeout505HTTP Version Not Supported
Note that this list is not exhaustive — it does not include
extension status codes defined in other specifications
().
The 1xx (Informational) class of status code indicates an
interim response for communicating connection status or request progress
prior to completing the requested action and sending a final response.
1xx responses are terminated by the end of the header section.
Since HTTP/1.0 did not define any 1xx status codes, a server MUST NOT send
a 1xx response to an HTTP/1.0 client.
A client MUST be able to parse one or more 1xx responses received
prior to a final response, even if the client does not expect one.
A user agent MAY ignore unexpected 1xx responses.
A proxy MUST forward 1xx responses unless the proxy itself
requested the generation of the 1xx response. For example, if a
proxy adds an "Expect: 100-continue" field when it forwards a request,
then it need not forward the corresponding 100 (Continue)
response(s).
The 100 (Continue) status code indicates that the initial
part of a request has been received and has not yet been rejected by the
server. The server intends to send a final response after the request has
been fully received and acted upon.
When the request contains an Expect header field that
includes a 100-continue expectation, the 100 response
indicates that the server wishes to receive the request payload body,
as described in . The client
ought to continue sending the request and discard the 100 response.
If the request did not contain an Expect header field
containing the 100-continue expectation,
the client can simply discard this interim response.
The 101 (Switching Protocols) status code indicates that the
server understands and is willing to comply with the client's request,
via the Upgrade header field (Section 9.9 of ), for
a change in the application protocol being used on this connection.
The server MUST generate an Upgrade header field in the response that
indicates which protocol(s) will be switched to immediately after the empty
line that terminates the 101 response.
It is assumed that the server will only agree to switch protocols when
it is advantageous to do so. For example, switching to a newer version of
HTTP might be advantageous over older versions, and switching to a
real-time, synchronous protocol might be advantageous when delivering
resources that use such features.
The 2xx (Successful) class of status code indicates that
the client's request was successfully received, understood, and accepted.
The 200 (OK) status code indicates that the request has
succeeded. The payload sent in a 200 response depends on the request
method. For the methods defined by this specification, the intended meaning
of the payload can be summarized as:
a representation of the target resource;
the same representation as GET, but without the representation data;
a representation of the status of, or results obtained from, the action;
a representation of the status of the action;
a representation of the communications options;
a representation of the request message as received by the
end server.
Aside from responses to CONNECT, a 200 response always has a payload,
though an origin server MAY generate a payload body of zero length.
If no payload is desired, an origin server ought to send
204 (No Content) instead.
For CONNECT, no payload is allowed because the successful result is a
tunnel, which begins immediately after the 200 response header section.
A 200 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of ).
The 201 (Created) status code indicates that the request has
been fulfilled and has resulted in one or more new resources being created.
The primary resource created by the request is identified by either a
Location header field in the response or, if no
Location field is received, by the target URI.
The 201 response payload typically describes and links to the resource(s)
created. See for a discussion of the
meaning and purpose of validator header fields, such as
ETag and Last-Modified, in a 201 response.
The 202 (Accepted) status code indicates that the request
has been accepted for processing, but the processing has not been
completed. The request might or might not eventually be acted upon, as it
might be disallowed when processing actually takes place. There is no
facility in HTTP for re-sending a status code from an asynchronous
operation.
The 202 response is intentionally noncommittal. Its purpose is to
allow a server to accept a request for some other process (perhaps a
batch-oriented process that is only run once per day) without
requiring that the user agent's connection to the server persist
until the process is completed. The representation sent with this
response ought to describe the request's current status and point to
(or embed) a status monitor that can provide the user with an estimate of
when the request will be fulfilled.
The 203 (Non-Authoritative Information) status code
indicates that the request was successful but the enclosed payload has been
modified from that of the origin server's 200 (OK) response
by a transforming proxy (). This status code allows the
proxy to notify recipients when a transformation has been applied, since
that knowledge might impact later decisions regarding the content. For
example, future cache validation requests for the content might only be
applicable along the same request path (through the same proxies).
The 203 response is similar to the Warning code of 214 Transformation
Applied (Section 5.5 of ), which has the advantage of being applicable
to responses with any status code.
A 203 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of ).
The 204 (No Content) status code indicates that the server
has successfully fulfilled the request and that there is no additional
content to send in the response payload body. Metadata in the response
header fields refer to the target resource and its
selected representation after the requested action was applied.
For example, if a 204 status code is received in response to a PUT
request and the response contains an ETag field, then
the PUT was successful and the ETag field value contains the entity-tag for
the new representation of that target resource.
The 204 response allows a server to indicate that the action has been
successfully applied to the target resource, while implying that the
user agent does not need to traverse away from its current "document view"
(if any). The server assumes that the user agent will provide some
indication of the success to its user, in accord with its own interface,
and apply any new or updated metadata in the response to its active
representation.
For example, a 204 status code is commonly used with document editing
interfaces corresponding to a "save" action, such that the document
being saved remains available to the user for editing. It is also
frequently used with interfaces that expect automated data transfers
to be prevalent, such as within distributed version control systems.
A 204 response is terminated by the first empty line after the header
fields because it cannot contain a message body.
A 204 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of ).
The 205 (Reset Content) status code indicates that the
server has fulfilled the request and desires that the user agent reset the
"document view", which caused the request to be sent, to its original state
as received from the origin server.
This response is intended to support a common data entry use case where
the user receives content that supports data entry (a form, notepad,
canvas, etc.), enters or manipulates data in that space, causes the entered
data to be submitted in a request, and then the data entry mechanism is
reset for the next entry so that the user can easily initiate another
input action.
Since the 205 status code implies that no additional content will be
provided, a server MUST NOT generate a payload in a 205 response.
The 206 (Partial Content) status code indicates that the
server is successfully fulfilling a range request for the target resource
by transferring one or more parts of the selected representation.
When a 206 response is generated, the server MUST generate the following
header fields, in addition to those required in the subsections below, if the field would
have been sent in a 200 (OK) response to the same request:
Date, Cache-Control, ETag,
Expires, Content-Location, and
Vary.
If a 206 is generated in response to a request with an If-Range
header field, the sender SHOULD NOT generate other representation header
fields beyond those required, because the client is understood to
already have a prior response containing those header fields.
Otherwise, the sender MUST generate all of the representation header
fields that would have been sent in a 200 (OK) response
to the same request.
A 206 response is heuristically cacheable; i.e., unless otherwise indicated by
explicit cache controls (see Section 4.2.2 of ).
If a single part is being transferred, the server generating the 206
response MUST generate a Content-Range header field,
describing what range of the selected representation is enclosed, and a
payload consisting of the range. For example:
If multiple parts are being transferred, the server generating the 206
response MUST generate a "multipart/byteranges" payload, as defined
in , and a
Content-Type header field containing the
multipart/byteranges media type and its required boundary parameter.
To avoid confusion with single-part responses, a server MUST NOT generate
a Content-Range header field in the HTTP header section of a
multiple part response (this field will be sent in each part instead).
Within the header area of each body part in the multipart payload, the
server MUST generate a Content-Range header field
corresponding to the range being enclosed in that body part.
If the selected representation would have had a Content-Type
header field in a 200 (OK) response, the server SHOULD
generate that same Content-Type field in the header area of
each body part. For example:
When multiple ranges are requested, a server MAY coalesce any of the
ranges that overlap, or that are separated by a gap that is smaller than the
overhead of sending multiple parts, regardless of the order in which the
corresponding range-spec appeared in the received Range
header field. Since the typical overhead between parts of a
multipart/byteranges payload is around 80 bytes, depending on the selected
representation's media type and the chosen boundary parameter length, it
can be less efficient to transfer many small disjoint parts than it is to
transfer the entire selected representation.
A server MUST NOT generate a multipart response to a request for a single
range, since a client that does not request multiple parts might not
support multipart responses. However, a server MAY generate a
multipart/byteranges payload with only a single body part if multiple
ranges were requested and only one range was found to be satisfiable or
only one range remained after coalescing.
A client that cannot process a multipart/byteranges response MUST NOT
generate a request that asks for multiple ranges.
When a multipart response payload is generated, the server SHOULD send
the parts in the same order that the corresponding range-spec appeared
in the received Range header field, excluding those ranges
that were deemed unsatisfiable or that were coalesced into other ranges.
A client that receives a multipart response MUST inspect the
Content-Range header field present in each body part in
order to determine which range is contained in that body part; a client
cannot rely on receiving the same ranges that it requested, nor the same
order that it requested.
A response might transfer only a subrange of a representation if the
connection closed prematurely or if the request used one or more Range
specifications. After several such transfers, a client might have
received several ranges of the same representation. These ranges can only
be safely combined if they all have in common the same strong validator
().
A client that has received multiple partial responses to GET requests on a
target resource MAY combine those responses into a larger continuous
range if they share the same strong validator.
If the most recent response is an incomplete 200 (OK)
response, then the header fields of that response are used for any
combined response and replace those of the matching stored responses.
If the most recent response is a 206 (Partial Content)
response and at least one of the matching stored responses is a
200 (OK), then the combined response header fields consist
of the most recent 200 response's header fields. If all of the matching
stored responses are 206 responses, then the stored response with the most
recent header fields is used as the source of header fields for the
combined response, except that the client MUST use other header fields
provided in the new response, aside from Content-Range, to
replace all instances of the corresponding header fields in the stored
response.
The combined response message body consists of the union of partial
content ranges in the new response and each of the selected responses.
If the union consists of the entire range of the representation, then the
client MUST process the combined response as if it were a complete
200 (OK) response, including a Content-Length
header field that reflects the complete length.
Otherwise, the client MUST process the set of continuous ranges as one of
the following:
an incomplete 200 (OK) response if the combined response is
a prefix of the representation,
a single 206 (Partial Content) response containing a
multipart/byteranges body, or
multiple 206 (Partial Content) responses, each with one
continuous range that is indicated by a Content-Range header
field.
The 3xx (Redirection) class of status code indicates that
further action needs to be taken by the user agent in order to fulfill the
request. If a Location header field
() is provided, the user agent MAY
automatically redirect its request to the URI referenced by the Location
field value, even if the specific status code is not understood.
Automatic redirection needs to be done with care for methods not known to be
safe, as defined in , since
the user might not wish to redirect an unsafe request.
There are several types of redirects:
Redirects that indicate the resource might be available at a
different URI, as provided by the Location field,
as in the status codes 301 (Moved Permanently),
302 (Found), 307 (Temporary Redirect), and
308 (Permanent Redirect).
Redirection that offers a choice of matching resources, each capable
of representing the original target resource, as in the
300 (Multiple Choices) status code.
Redirection to a different resource, identified by the
Location field, that can represent an indirect
response to the request, as in the 303 (See Other)
status code.
Redirection to a previously cached result, as in the
304 (Not Modified) status code.
Note: In HTTP/1.0, the status codes 301 (Moved Permanently)
and 302 (Found) were defined for the first type of redirect
(, Section 9.3). Early user agents split
on whether the method applied to the redirect target would be the same as
the original request or would be rewritten as GET. Although HTTP
originally defined the former semantics for 301 and
302 (to match its original implementation at CERN), and
defined 303 (See Other) to match the latter semantics,
prevailing practice gradually converged on the latter semantics for
301 and 302 as well. The first revision of
HTTP/1.1 added 307 (Temporary Redirect) to indicate the
former semantics of 302 without being impacted by divergent practice.
For the same reason, 308 (Permanent Redirect) was later on
added in to match 301.
Over 10 years later, most user agents still do method rewriting for
301 and 302; therefore,
made that behavior conformant when the original request is POST.
A client SHOULD detect and intervene in cyclical redirections (i.e.,
"infinite" redirection loops).
Note: An earlier version of this specification recommended a
maximum of five redirections (, Section 10.3).
Content developers need to be aware that some clients might
implement such a fixed limitation.
The 300 (Multiple Choices) status code indicates that the
target resource has more than one representation, each with
its own more specific identifier, and information about the alternatives is
being provided so that the user (or user agent) can select a preferred
representation by redirecting its request to one or more of those
identifiers. In other words, the server desires that the user agent engage
in reactive negotiation to select the most appropriate representation(s)
for its needs ().
If the server has a preferred choice, the server SHOULD generate a
Location header field containing a preferred choice's URI
reference. The user agent MAY use the Location field value for automatic
redirection.
For request methods other than HEAD, the server SHOULD generate a payload
in the 300 response containing a list of representation metadata and URI
reference(s) from which the user or user agent can choose the one most
preferred. The user agent MAY make a selection from that list
automatically if it understands the provided media type. A specific format
for automatic selection is not defined by this specification because HTTP
tries to remain orthogonal to the definition of its payloads.
In practice, the representation is provided in some easily parsed format
believed to be acceptable to the user agent, as determined by shared design
or content negotiation, or in some commonly accepted hypertext format.
A 300 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of ).
Note: The original proposal for the 300 status code defined the URI header field as
providing a list of alternative representations, such that it would be
usable for 200, 300, and 406 responses and be transferred in responses to
the HEAD method. However, lack of deployment and disagreement over syntax
led to both URI and Alternates (a subsequent proposal) being dropped from
this specification. It is possible to communicate the list as a
Link header field value whose members have a relationship of
"alternate", though deployment is a chicken-and-egg problem.
The 301 (Moved Permanently) status code indicates that the
target resource has been assigned a new permanent URI and
any future references to this resource ought to use one of the enclosed
URIs. Clients with link-editing capabilities ought to automatically re-link
references to the target URI to one or more of the new
references sent by the server, where possible.
The server SHOULD generate a Location header field in the
response containing a preferred URI reference for the new permanent URI.
The user agent MAY use the Location field value for automatic redirection.
The server's response payload usually contains a short hypertext note with
a hyperlink to the new URI(s).
Note: For historical reasons, a user agent MAY change the
request method from POST to GET for the subsequent request. If this
behavior is undesired, the 308 (Permanent Redirect)
status code can be used instead.
A 301 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of ).
The 302 (Found) status code indicates that the target
resource resides temporarily under a different URI. Since the redirection
might be altered on occasion, the client ought to continue to use the
target URI for future requests.
The server SHOULD generate a Location header field in the
response containing a URI reference for the different URI.
The user agent MAY use the Location field value for automatic redirection.
The server's response payload usually contains a short hypertext note with
a hyperlink to the different URI(s).
Note: For historical reasons, a user agent MAY change the
request method from POST to GET for the subsequent request. If this
behavior is undesired, the 307 (Temporary Redirect)
status code can be used instead.
The 303 (See Other) status code indicates that the server is
redirecting the user agent to a different resource, as indicated by a URI
in the Location header field, which is intended to provide
an indirect response to the original request. A user agent can perform a
retrieval request targeting that URI (a GET or HEAD request if using HTTP),
which might also be redirected, and present the eventual result as an
answer to the original request. Note that the new URI in the Location
header field is not considered equivalent to the target URI.
This status code is applicable to any HTTP method. It is
primarily used to allow the output of a POST action to redirect
the user agent to a selected resource, since doing so provides the
information corresponding to the POST response in a form that
can be separately identified, bookmarked, and cached, independent
of the original request.
A 303 response to a GET request indicates that the origin server does not
have a representation of the target resource that can be
transferred by the server over HTTP. However, the
Location field value refers to a resource that is
descriptive of the target resource, such that making a retrieval request
on that other resource might result in a representation that is useful to
recipients without implying that it represents the original target resource.
Note that answers to the questions of what can be represented, what
representations are adequate, and what might be a useful description are
outside the scope of HTTP.
Except for responses to a HEAD request, the representation of a 303
response ought to contain a short hypertext note with a hyperlink to the
same URI reference provided in the Location header field.
The 304 (Not Modified) status code indicates that a
conditional GET or HEAD request has been
received and would have resulted in a 200 (OK) response
if it were not for the fact that the condition evaluated to false.
In other words, there is no need for the server to transfer a
representation of the target resource because the request indicates that
the client, which made the request conditional, already has a valid
representation; the server is therefore redirecting the client to make
use of that stored representation as if it were the payload of a
200 (OK) response.
The server generating a 304 response MUST generate any of the following
header fields that would have been sent in a 200 (OK)
response to the same request:
Cache-Control,
Content-Location,
Date,
ETag,
Expires, and
Vary.
Since the goal of a 304 response is to minimize information transfer
when the recipient already has one or more cached representations,
a sender SHOULD NOT generate representation metadata other
than the above listed fields unless said metadata exists for the
purpose of guiding cache updates (e.g., Last-Modified might
be useful if the response does not have an ETag field).
Requirements on a cache that receives a 304 response are defined in
Section 4.3.4 of . If the conditional request originated with an
outbound client, such as a user agent with its own cache sending a
conditional GET to a shared proxy, then the proxy SHOULD forward the
304 response to that client.
A 304 response cannot contain a message-body; it is always
terminated by the first empty line after the header fields.
The 305 (Use Proxy) status code was defined in a previous
version of this specification and is now deprecated (Appendix B of ).
The 306 status code was defined in a previous version of this
specification, is no longer used, and the code is reserved.
The 307 (Temporary Redirect) status code indicates that the
target resource resides temporarily under a different URI
and the user agent MUST NOT change the request method if it performs an
automatic redirection to that URI.
Since the redirection can change over time, the client ought to continue
using the original target URI for future requests.
The server SHOULD generate a Location header field in the
response containing a URI reference for the different URI.
The user agent MAY use the Location field value for automatic redirection.
The server's response payload usually contains a short hypertext note with
a hyperlink to the different URI(s).
The 308 (Permanent Redirect) status code indicates that the
target resource has been assigned a new permanent URI and
any future references to this resource ought to use one of the enclosed
URIs. Clients with link editing capabilities ought to automatically re-link
references to the target URI
to one or more of the new references sent by the server, where possible.
The server SHOULD generate a Location header field in the
response containing a preferred URI reference for the new permanent URI.
The user agent MAY use the Location field value for automatic redirection.
The server's response payload usually contains a short hypertext note with
a hyperlink to the new URI(s).
A 308 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of ).
Note: This status code is much younger (June 2014) than its sibling codes, and thus
might not be recognized everywhere. See Section 4 of
for deployment considerations.
The 4xx (Client Error) class of status code indicates that
the client seems to have erred. Except when responding to a HEAD request,
the server SHOULD send a representation containing an explanation of
the error situation, and whether it is a temporary or permanent condition.
These status codes are applicable to any request method. User agents
SHOULD display any included representation to the user.
The 400 (Bad Request) status code indicates that the server
cannot or will not process the request due to something that is perceived
to be a client error (e.g., malformed request syntax, invalid request
message framing, or deceptive request routing).
The 401 (Unauthorized) status code indicates that the
request has not been applied because it lacks valid authentication
credentials for the target resource.
The server generating a 401 response MUST send a
WWW-Authenticate header field
()
containing at least one challenge applicable to the target resource.
If the request included authentication credentials, then the 401 response
indicates that authorization has been refused for those credentials.
The user agent MAY repeat the request with a new or replaced
Authorization header field ().
If the 401 response contains the same challenge as the prior response, and
the user agent has already attempted authentication at least once, then the
user agent SHOULD present the enclosed representation to the user, since
it usually contains relevant diagnostic information.
The 402 (Payment Required) status code is reserved for
future use.
The 403 (Forbidden) status code indicates that the
server understood the request but refuses to fulfill it.
A server that wishes to make public why the request has been forbidden
can describe that reason in the response payload (if any).
If authentication credentials were provided in the request, the
server considers them insufficient to grant access.
The client SHOULD NOT automatically repeat the request with the same
credentials.
The client MAY repeat the request with new or different credentials.
However, a request might be forbidden for reasons unrelated to the
credentials.
An origin server that wishes to "hide" the current existence of a forbidden
target resource MAY instead respond with a status
code of 404 (Not Found).
The 404 (Not Found) status code indicates that the origin
server did not find a current representation for the
target resource or is not willing to disclose that one
exists. A 404 status code does not indicate whether this lack of representation
is temporary or permanent; the 410 (Gone) status code is
preferred over 404 if the origin server knows, presumably through some
configurable means, that the condition is likely to be permanent.
A 404 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of ).
The 405 (Method Not Allowed) status code indicates that the
method received in the request-line is known by the origin server but
not supported by the target resource.
The origin server MUST generate an Allow header field in
a 405 response containing a list of the target resource's currently
supported methods.
A 405 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of ).
The 406 (Not Acceptable) status code indicates that the
target resource does not have a current representation that
would be acceptable to the user agent, according to the
proactive negotiation header fields received in the request
(), and the server is unwilling to supply a
default representation.
The server SHOULD generate a payload containing a list of available
representation characteristics and corresponding resource identifiers from
which the user or user agent can choose the one most appropriate.
A user agent MAY automatically select the most appropriate choice from
that list. However, this specification does not define any standard for
such automatic selection, as described in .
The 407 (Proxy Authentication Required) status code is
similar to 401 (Unauthorized), but it indicates that the client
needs to authenticate itself in order to use a proxy for this request.
The proxy MUST send a Proxy-Authenticate header field
() containing a challenge
applicable to that proxy for the request. The client MAY repeat
the request with a new or replaced Proxy-Authorization
header field ().
The 408 (Request Timeout) status code indicates
that the server did not receive a complete request message within the time
that it was prepared to wait.
If the client has an outstanding request in transit,
the client MAY repeat that request on a new connection.
The 409 (Conflict) status code indicates that the request
could not be completed due to a conflict with the current state of the target
resource. This code is used in situations where the user might be able to
resolve the conflict and resubmit the request. The server SHOULD generate
a payload that includes enough information for a user to recognize the
source of the conflict.
Conflicts are most likely to occur in response to a PUT request. For
example, if versioning were being used and the representation being PUT
included changes to a resource that conflict with those made by an
earlier (third-party) request, the origin server might use a 409 response
to indicate that it can't complete the request. In this case, the response
representation would likely contain information useful for merging the
differences based on the revision history.
The 410 (Gone) status code indicates that access to the
target resource is no longer available at the origin
server and that this condition is likely to be permanent. If the origin
server does not know, or has no facility to determine, whether or not the
condition is permanent, the status code 404 (Not Found)
ought to be used instead.
The 410 response is primarily intended to assist the task of web
maintenance by notifying the recipient that the resource is
intentionally unavailable and that the server owners desire that
remote links to that resource be removed. Such an event is common for
limited-time, promotional services and for resources belonging to
individuals no longer associated with the origin server's site. It is not
necessary to mark all permanently unavailable resources as "gone" or
to keep the mark for any length of time — that is left to the
discretion of the server owner.
A 410 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of ).
The 411 (Length Required) status code indicates that the
server refuses to accept the request without a defined
Content-Length ().
The client MAY repeat the request if it adds a valid Content-Length
header field containing the length of the message body in the request
message.
The 412 (Precondition Failed) status code indicates that one
or more conditions given in the request header fields evaluated to false
when tested on the server. This response status code allows the client to place
preconditions on the current resource state (its current representations
and metadata) and, thus, prevent the request method from being applied if the
target resource is in an unexpected state.
The 413 (Payload Too Large) status code indicates
that the server is refusing to process a request because the request
payload is larger than the server is willing or able to process.
The server MAY terminate the request, if the protocol version in use
allows it; otherwise, the server MAY close the connection.
If the condition is temporary, the server SHOULD generate a
Retry-After header field to indicate that it is temporary
and after what time the client MAY try again.
The 414 (URI Too Long) status code indicates that the server
is refusing to service the request because the
target URI is longer than the server is willing to
interpret. This rare condition is only likely to occur when a client has
improperly converted a POST request to a GET request with long query
information, when the client has descended into a "black hole" of
redirection (e.g., a redirected URI prefix that points to a suffix of
itself) or when the server is under attack by a client attempting to
exploit potential security holes.
A 414 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of ).
The 415 (Unsupported Media Type) status code indicates that
the origin server is refusing to service the request because the payload is
in a format not supported by this method on the target resource.
The format problem might be due to the request's indicated
Content-Type or Content-Encoding, or as a
result of inspecting the data directly.
If the problem was caused by an unsupported content coding, the
Accept-Encoding response header field
() ought to be
used to indicate what (if any) content codings would have been accepted
in the request.
On the other hand, if the cause was an unsupported media type, the
Accept response header field ()
can be used to indicate what media types would have been accepted
in the request.
The 416 (Range Not Satisfiable) status code indicates that
the set of ranges in the request's Range header field
() has been rejected either because none of
the requested ranges are satisfiable or because the client has requested
an excessive number of small or overlapping ranges (a potential denial of
service attack).
Each range unit defines what is required for its own range sets to be
satisfiable. For example, defines what makes
a bytes range set satisfiable.
When this status code is generated in response to a byte-range request, the
sender SHOULD generate a Content-Range header field
specifying the current length of the selected representation
().
For example:
Note: Because servers are free to ignore Range, many
implementations will respond with the entire selected representation
in a 200 (OK) response. That is partly because
most clients are prepared to receive a 200 (OK) to
complete the task (albeit less efficiently) and partly because clients
might not stop making an invalid partial request until they have received
a complete representation. Thus, clients cannot depend on receiving a
416 (Range Not Satisfiable) response even when it is most
appropriate.
The 417 (Expectation Failed) status code indicates that the
expectation given in the request's Expect header field
() could not be met by at least one of the
inbound servers.
was an April 1 RFC that lampooned the various
ways HTTP was abused; one such abuse was the definition of an
application-specific 418 status code. In the intervening years, this
status code has been widely implemented as an "Easter Egg", and therefore
is effectively consumed by this use.
Therefore, the 418 status code is reserved in the IANA HTTP Status Code
Registry. This indicates that the status code cannot be assigned to other
applications currently. If future circumstances require its use (e.g.,
exhaustion of 4NN status codes), it can be re-assigned to another use.
The 422 (Unprocessable Payload) status code indicates that the server
understands the content type of the request payload (hence a
415 (Unsupported Media Type) status code is inappropriate),
and the syntax of the request payload is correct, but was unable to process
the contained instructions. For example, this status code can be sent if
an XML request payload contains well-formed (i.e., syntactically correct), but
semantically erroneous XML instructions.
The 426 (Upgrade Required) status code indicates that the
server refuses to perform the request using the current protocol but might
be willing to do so after the client upgrades to a different protocol.
The server MUST send an Upgrade header field in a 426
response to indicate the required protocol(s) (Section 9.9 of ).
Example:
The 5xx (Server Error) class of status code indicates that
the server is aware that it has erred or is incapable of performing the
requested method.
Except when responding to a HEAD request, the server SHOULD send a
representation containing an explanation of the error situation, and
whether it is a temporary or permanent condition.
A user agent SHOULD display any included representation to the user.
These response codes are applicable to any request method.
The 500 (Internal Server Error) status code indicates that
the server encountered an unexpected condition that prevented it from
fulfilling the request.
The 501 (Not Implemented) status code indicates that the
server does not support the functionality required to fulfill the request.
This is the appropriate response when the server does not recognize the
request method and is not capable of supporting it for any resource.
A 501 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of ).
The 502 (Bad Gateway) status code indicates that the server,
while acting as a gateway or proxy, received an invalid response from an
inbound server it accessed while attempting to fulfill the request.
The 503 (Service Unavailable) status code indicates that the
server is currently unable to handle the request due to a temporary overload
or scheduled maintenance, which will likely be alleviated after some delay.
The server MAY send a Retry-After header field
() to suggest an appropriate
amount of time for the client to wait before retrying the request.
Note: The existence of the 503 status code does not imply that a
server has to use it when becoming overloaded. Some servers might
simply refuse the connection.
The 504 (Gateway Timeout) status code indicates that the
server, while acting as a gateway or proxy, did not receive a timely
response from an upstream server it needed to access in order to
complete the request.
The 505 (HTTP Version Not Supported) status code indicates
that the server does not support, or refuses to support, the major version
of HTTP that was used in the request message. The server is indicating that
it is unable or unwilling to complete the request using the same major
version as the client, as described in , other than with this
error message. The server SHOULD generate a representation for the 505
response that describes why that version is not supported and what other
protocols are supported by that server.
Additional status codes, outside the scope of this specification, have been
specified for use in HTTP. All such status codes ought to be registered
within the "Hypertext Transfer Protocol (HTTP) Status Code Registry".
The "Hypertext Transfer Protocol (HTTP) Status Code Registry", maintained
by IANA at ,
registers status code numbers.
A registration MUST include the following fields:
Status Code (3 digits)Short DescriptionPointer to specification text
Values to be added to the HTTP status code namespace require IETF Review
(see , Section 4.8).
When it is necessary to express semantics for a response that are not
defined by current status codes, a new status code can be registered.
Status codes are generic; they are potentially applicable to any resource,
not just one particular media type, kind of resource, or application of
HTTP. As such, it is preferred that new status codes be registered in a
document that isn't specific to a single application.
New status codes are required to fall under one of the categories
defined in . To allow existing parsers to
process the response message, new status codes cannot disallow a payload,
although they can mandate a zero-length payload body.
Proposals for new status codes that are not yet widely deployed ought to
avoid allocating a specific number for the code until there is clear
consensus that it will be registered; instead, early drafts can use a
notation such as "4NN", or "3N0" .. "3N9", to indicate the class
of the proposed status code(s) without consuming a number prematurely.
The definition of a new status code ought to explain the request
conditions that would cause a response containing that status code (e.g.,
combinations of request header fields and/or method(s)) along with any
dependencies on response header fields (e.g., what fields are required,
what fields can modify the semantics, and what field semantics are
further refined when used with the new status code).
By default, a status code applies only to the request corresponding to the
response it occurs within. If a status code applies to a larger scope of
applicability — for example, all requests to the resource in question, or
all requests to a server — this must be explicitly specified. When doing
so, it should be noted that not all clients can be expected to
consistently apply a larger scope, because they might not understand the
new status code.
The definition of a new status code ought to specify whether or not it is
cacheable. Note that all status codes can be cached if the response they
occur in has explicit freshness information; however, status codes that are
defined as being cacheable are allowed to be cached without explicit
freshness information. Likewise, the definition of a status code can place
constraints upon cache behavior. See for more information.
Finally, the definition of a new status code ought to indicate whether the
payload has any implied association with an identified resource ().
The response header fields allow the server to pass additional
information about the response beyond the status code.
These header fields give information about the server, about
further access to the target resource, or about related
resources.
Although each response header field has a defined meaning, in general,
the precise semantics might be further refined by the semantics of the
request method and/or response status code.
Response header fields can supply control data that supplements the
status code, directs caching, or instructs the client where to go next.
Field NameDefined in...AgeSection 5.1 of Cache-ControlSection 5.2 of ExpiresSection 5.3 of DateLocationRetry-AfterVaryWarningSection 5.5 of
The "Date" header field represents the date and time at which
the message was originated, having the same semantics as the Origination
Date Field (orig-date) defined in Section 3.6.1 of .
The field value is an HTTP-date, as defined in .
An example is
When a Date header field is generated, the sender SHOULD generate its
field value as the best available approximation of the date and time of
message generation. In theory, the date ought to represent the moment just
before the payload is generated. In practice, the date can be generated at
any time during message origination.
An origin server MUST NOT send a Date header field if it does not have a
clock capable of providing a reasonable approximation of the current
instance in Coordinated Universal Time.
An origin server MAY send a Date header field if the response is in the
1xx (Informational) or 5xx (Server Error)
class of status codes.
An origin server MUST send a Date header field in all other cases.
A recipient with a clock that receives a response message without a Date
header field MUST record the time it was received and append a
corresponding Date header field to the message's header section if it is
cached or forwarded downstream.
A user agent MAY send a Date header field in a request, though generally
will not do so unless it is believed to convey useful information to the
server. For example, custom applications of HTTP might convey a Date if
the server is expected to adjust its interpretation of the user's request
based on differences between the user agent and server clocks.
The "Location" header field is used in some responses to refer to a
specific resource in relation to the response. The type of relationship is
defined by the combination of request method and status code semantics.
The field value consists of a single URI-reference. When it has the form
of a relative reference (, Section 4.2),
the final value is computed by resolving it against the target
URI (, Section 5).
For 201 (Created) responses, the Location value refers to
the primary resource created by the request.
For 3xx (Redirection) responses, the Location value refers
to the preferred target resource for automatically redirecting the request.
If the Location value provided in a 3xx (Redirection)
response does not have a fragment component, a user agent MUST process the
redirection as if the value inherits the fragment component of the URI
reference used to generate the target URI (i.e., the redirection
inherits the original reference's fragment, if any).
For example, a GET request generated for the URI reference
"http://www.example.org/~tim" might result in a
303 (See Other) response containing the header field:
which suggests that the user agent redirect to
"http://www.example.org/People.html#tim"
Likewise, a GET request generated for the URI reference
"http://www.example.org/index.html#larry" might result in a
301 (Moved Permanently) response containing the header
field:
which suggests that the user agent redirect to
"http://www.example.net/index.html#larry", preserving the original fragment
identifier.
There are circumstances in which a fragment identifier in a Location
value would not be appropriate. For example, the Location header field in a
201 (Created) response is supposed to provide a URI that is
specific to the created resource.
Note: Some recipients attempt to recover from Location fields
that are not valid URI references. This specification does not mandate or
define such processing, but does allow it for the sake of robustness.
Note: The Content-Location header field
() differs from Location in that the
Content-Location refers to the most specific resource corresponding to the
enclosed representation. It is therefore possible for a response to contain
both the Location and Content-Location header fields.
Servers send the "Retry-After" header field to indicate how long the user
agent ought to wait before making a follow-up request. When sent with a
503 (Service Unavailable) response, Retry-After indicates
how long the service is expected to be unavailable to the client.
When sent with any 3xx (Redirection) response, Retry-After
indicates the minimum time that the user agent is asked to wait before
issuing the redirected request.
The value of this field can be either an HTTP-date or a number
of seconds to delay after the response is received.
A delay-seconds value is a non-negative decimal integer, representing time
in seconds.
Two examples of its use are
In the latter example, the delay is 2 minutes.
The "Vary" header field in a response describes what parts of a request
message, aside from the method, Host header field, and target URI,
might influence the origin server's process for selecting and representing
this response.
A Vary field value is a list of request field names, known as the selecting header fields,
that might have a role in selecting the representation for this response. Potential selecting
header fields are not limited to those defined by this specification.
If the list contains "*", it signals that other aspects of the request might
play a role in selecting the response representation, possibly including
elements outside the message syntax (e.g., the client's network address).
A recipient will not be able to determine whether this response is
appropriate for a later request without forwarding the request to the
origin server. A proxy MUST NOT generate "*" in a Vary field value.
For example, a response that contains
indicates that the origin server might have used the
request's Accept-Encoding and Accept-Language
fields (or lack thereof) as determining factors while choosing the content
for this response.
An origin server might send Vary with a list of fields for two purposes:
To inform cache recipients that they MUST NOT use this response
to satisfy a later request unless the later request has the
same values for the listed fields as the original request
(Section 4.1 of ). In other words, Vary expands the cache key
required to match a new request to the stored cache entry.
To inform user agent recipients that this response is subject to
content negotiation () and that a
different representation might be sent in a subsequent request if
additional parameters are provided in the listed header fields
(proactive negotiation).
An origin server SHOULD send a Vary header field when its algorithm for
selecting a representation varies based on aspects of the request message
other than the method and target URI, unless the variance cannot be
crossed or the origin server has been deliberately configured to prevent
cache transparency. For example, there is no need to send the Authorization
field name in Vary because reuse across users is constrained by the field
definition (). Likewise, an origin server might use
Cache-Control response directives (Section 5.2 of ) to supplant Vary if it
considers the variance less significant than the performance cost of Vary's
impact on caching.
Validator header fields convey metadata about the
selected representation ().
In responses to safe requests, validator fields describe the selected
representation chosen by the origin server while handling the response.
Note that, depending on the status code semantics, the
selected representation for a given response is not
necessarily the same as the representation enclosed as response payload.
In a successful response to a state-changing request, validator fields
describe the new representation that has replaced the prior
selected representation as a result of processing the
request.
For example, an ETag field in a 201 (Created) response communicates the
entity-tag of the newly created resource's representation, so that it can
be used in later conditional requests to prevent the "lost update"
problem .
Field NameDefined in...ETagLast-Modified
This specification defines two forms of metadata that are commonly used
to observe resource state and test for preconditions: modification dates
() and opaque entity tags
(). Additional metadata that reflects resource state
has been defined by various extensions of HTTP, such as Web Distributed
Authoring and Versioning (WebDAV, ), that are beyond the scope of this specification.
A resource metadata value is referred to as a "validator"
when it is used within a precondition.
Validators come in two flavors: strong or weak. Weak validators are easy
to generate but are far less useful for comparisons. Strong validators
are ideal for comparisons but can be very difficult (and occasionally
impossible) to generate efficiently. Rather than impose that all forms
of resource adhere to the same strength of validator, HTTP exposes the
type of validator in use and imposes restrictions on when weak validators
can be used as preconditions.
A "strong validator" is representation metadata that changes value whenever
a change occurs to the representation data that would be observable in the
payload body of a 200 (OK) response to GET.
A strong validator might change for reasons other than a change to the
representation data, such as when a
semantically significant part of the representation metadata is changed
(e.g., Content-Type), but it is in the best interests of the
origin server to only change the value when it is necessary to invalidate
the stored responses held by remote caches and authoring tools.
Cache entries might persist for arbitrarily long periods, regardless
of expiration times. Thus, a cache might attempt to validate an
entry using a validator that it obtained in the distant past.
A strong validator is unique across all versions of all
representations associated with a particular resource over time.
However, there is no implication of uniqueness across representations
of different resources (i.e., the same strong validator might be
in use for representations of multiple resources at the same time
and does not imply that those representations are equivalent).
There are a variety of strong validators used in practice. The best are
based on strict revision control, wherein each change to a representation
always results in a unique node name and revision identifier being assigned
before the representation is made accessible to GET. A collision-resistant hash
function applied to the representation data is also sufficient if the data
is available prior to the response header fields being sent and the digest
does not need to be recalculated every time a validation request is
received. However, if a resource has distinct representations that differ
only in their metadata, such as might occur with content negotiation over
media types that happen to share the same data format, then the origin
server needs to incorporate additional information in the validator to
distinguish those representations.
In contrast, a "weak validator" is representation metadata that
might not change for every change to the representation data. This
weakness might be due to limitations in how the value is calculated, such
as clock resolution, an inability to ensure uniqueness for all possible
representations of the resource, or a desire of the resource owner
to group representations by some self-determined set of equivalency
rather than unique sequences of data. An origin server SHOULD change a
weak entity-tag whenever it considers prior representations to be
unacceptable as a substitute for the current representation. In other words,
a weak entity-tag ought to change whenever the origin server wants caches to
invalidate old responses.
For example, the representation of a weather report that changes in
content every second, based on dynamic measurements, might be grouped
into sets of equivalent representations (from the origin server's
perspective) with the same weak validator in order to allow cached
representations to be valid for a reasonable period of time (perhaps
adjusted dynamically based on server load or weather quality).
Likewise, a representation's modification time, if defined with only
one-second resolution, might be a weak validator if it is possible
for the representation to be modified twice during a single second and
retrieved between those modifications.
Likewise, a validator is weak if it is shared by two or more
representations of a given resource at the same time, unless those
representations have identical representation data. For example, if the
origin server sends the same validator for a representation with a gzip
content coding applied as it does for a representation with no content
coding, then that validator is weak. However, two simultaneous
representations might share the same strong validator if they differ only
in the representation metadata, such as when two different media types are
available for the same representation data.
Strong validators are usable for all conditional requests, including cache
validation, partial content ranges, and "lost update" avoidance.
Weak validators are only usable when the client does not require exact
equality with previously obtained representation data, such as when
validating a cache entry or limiting a web traversal to recent changes.
The "Last-Modified" header field in a response provides a timestamp
indicating the date and time at which the origin server believes the
selected representation was last modified, as determined at the conclusion
of handling the request.
An example of its use is
An origin server SHOULD send Last-Modified for any selected
representation for which a last modification date can be reasonably
and consistently determined, since its use in conditional requests
and evaluating cache freshness () results in a substantial
reduction of HTTP traffic on the Internet and can be a significant
factor in improving service scalability and reliability.
A representation is typically the sum of many parts behind the
resource interface. The last-modified time would usually be
the most recent time that any of those parts were changed.
How that value is determined for any given resource is an
implementation detail beyond the scope of this specification.
What matters to HTTP is how recipients of the Last-Modified
header field can use its value to make conditional requests
and test the validity of locally cached responses.
An origin server SHOULD obtain the Last-Modified value of the
representation as close as possible to the time that it generates the
Date field value for its response. This allows a recipient to
make an accurate assessment of the representation's modification time,
especially if the representation changes near the time that the
response is generated.
An origin server with a clock MUST NOT send a Last-Modified date
that is later than the server's time of message origination (Date).
If the last modification time is derived from implementation-specific
metadata that evaluates to some time in the future, according to the
origin server's clock, then the origin server MUST replace that
value with the message origination date. This prevents a future
modification date from having an adverse impact on cache validation.
An origin server without a clock MUST NOT assign Last-Modified
values to a response unless these values were associated
with the resource by some other system or user with a reliable clock.
A Last-Modified time, when used as a validator in a request, is
implicitly weak unless it is possible to deduce that it is strong,
using the following rules:
The validator is being compared by an origin server to the
actual current validator for the representation and,That origin server reliably knows that the associated representation did
not change twice during the second covered by the presented
validator.
or
The validator is about to be used by a client in an If-Modified-Since,
If-Unmodified-Since, or If-Range header
field, because the client has a cache entry for the associated
representation, andThat cache entry includes a Date value, which gives the
time when the origin server sent the original response, andThe presented Last-Modified time is at least 60 seconds before
the Date value.
or
The validator is being compared by an intermediate cache to the
validator stored in its cache entry for the representation, andThat cache entry includes a Date value, which gives the
time when the origin server sent the original response, andThe presented Last-Modified time is at least 60 seconds before
the Date value.
This method relies on the fact that if two different responses were
sent by the origin server during the same second, but both had the
same Last-Modified time, then at least one of those responses would
have a Date value equal to its Last-Modified time. The
arbitrary 60-second limit guards against the possibility that the Date and
Last-Modified values are generated from different clocks or at somewhat
different times during the preparation of the response. An
implementation MAY use a value larger than 60 seconds, if it is
believed that 60 seconds is too short.
The "ETag" field in a response provides the current entity-tag for
the selected representation, as determined at the conclusion of handling
the request.
An entity-tag is an opaque validator for differentiating between
multiple representations of the same resource, regardless of whether
those multiple representations are due to resource state changes over
time, content negotiation resulting in multiple representations being
valid at the same time, or both. An entity-tag consists of an opaque
quoted string, possibly prefixed by a weakness indicator.
Note: Previously, opaque-tag was defined to be a quoted-string
(, Section 3.11); thus, some recipients
might perform backslash unescaping. Servers therefore ought to avoid
backslash characters in entity tags.
An entity-tag can be more reliable for validation than a modification
date in situations where it is inconvenient to store modification
dates, where the one-second resolution of HTTP date values is not
sufficient, or where modification dates are not consistently maintained.
Examples:
An entity-tag can be either a weak or strong validator, with
strong being the default. If an origin server provides an entity-tag
for a representation and the generation of that entity-tag does not satisfy
all of the characteristics of a strong validator
(), then the origin server
MUST mark the entity-tag as weak by prefixing its opaque value
with "W/" (case-sensitive).
A sender MAY send the Etag field in a trailer section (see
). However, since trailers are often
ignored, it is preferable to send Etag as a header field unless the
entity-tag is generated while sending the message body.
The principle behind entity-tags is that only the service author
knows the implementation of a resource well enough to select the
most accurate and efficient validation mechanism for that resource,
and that any such mechanism can be mapped to a simple sequence of
octets for easy comparison. Since the value is opaque, there is no
need for the client to be aware of how each entity-tag is constructed.
For example, a resource that has implementation-specific versioning
applied to all changes might use an internal revision number, perhaps
combined with a variance identifier for content negotiation, to
accurately differentiate between representations.
Other implementations might use a collision-resistant hash of
representation content, a combination of various file attributes, or
a modification timestamp that has sub-second resolution.
An origin server SHOULD send an ETag for any selected representation
for which detection of changes can be reasonably and consistently
determined, since the entity-tag's use in conditional requests and
evaluating cache freshness () can result in a substantial
reduction of HTTP network traffic and can be a significant factor in
improving service scalability and reliability.
There are two entity-tag comparison functions, depending on whether or not
the comparison context allows the use of weak validators:
Strong comparison: two entity-tags are equivalent if both
are not weak and their opaque-tags match character-by-character.Weak comparison: two entity-tags are equivalent if their opaque-tags
match character-by-character, regardless of either or both
being tagged as "weak".
The example below shows the results for a set of entity-tag pairs and both
the weak and strong comparison function results:
ETag 1ETag 2Strong ComparisonWeak ComparisonW/"1"W/"1"no matchmatchW/"1"W/"2"no matchno matchW/"1""1"no matchmatch"1""1"matchmatch
Consider a resource that is subject to content negotiation
(), and where the representations sent in response to
a GET request vary based on the Accept-Encoding request
header field ():
>> Request:
In this case, the response might or might not use the gzip content coding.
If it does not, the response might look like:
>> Response:
An alternative representation that does use gzip content coding would be:
>> Response:
Note: Content codings are a property of the representation data,
so a strong entity-tag for a content-encoded representation has to be
distinct from the entity tag of an unencoded representation to prevent
potential conflicts during cache updates and range requests. In contrast,
transfer codings (Section 7 of ) apply only during message transfer
and do not result in distinct entity-tags.
In 200 (OK) responses to GET or HEAD, an origin server:
SHOULD send an entity-tag validator unless it is not feasible to
generate one.MAY send a weak entity-tag instead of a strong entity-tag, if
performance considerations support the use of weak entity-tags,
or if it is unfeasible to send a strong entity-tag.SHOULD send a Last-Modified value if it is feasible to
send one.
In other words, the preferred behavior for an origin server
is to send both a strong entity-tag and a Last-Modified
value in successful responses to a retrieval request.
A client:
MUST send that entity-tag in any cache validation request (using
If-Match or If-None-Match) if an
entity-tag has been provided by the origin server.SHOULD send the Last-Modified value in non-subrange
cache validation requests (using If-Modified-Since)
if only a Last-Modified value has been provided by the origin server.MAY send the Last-Modified value in subrange
cache validation requests (using If-Unmodified-Since)
if only a Last-Modified value has been provided by an HTTP/1.0 origin
server. The user agent SHOULD provide a way to disable this, in case
of difficulty.SHOULD send both validators in cache validation requests if both an
entity-tag and a Last-Modified value have been provided
by the origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to
respond appropriately.
Authentication challenges indicate what mechanisms are available for the
client to provide authentication credentials in future requests.
Field NameDefined in...WWW-AuthenticateProxy-Authenticate
Furthermore, the "Authentication-Info" and
"Proxy-Authentication-Info" response header fields are defined
for use in authentication schemes that need to return
information once the client's authentication credentials have been accepted.
Field NameDefined in...Authentication-InfoProxy-Authentication-Info
The "WWW-Authenticate" header field indicates the authentication scheme(s)
and parameters applicable to the target resource.
A server generating a 401 (Unauthorized) response
MUST send a WWW-Authenticate header field containing at least one
challenge. A server MAY generate a WWW-Authenticate header field
in other response messages to indicate that supplying credentials
(or different credentials) might affect the response.
A proxy forwarding a response MUST NOT modify any
WWW-Authenticate fields in that response.
User agents are advised to take special care in parsing the field value, as
it might contain more than one challenge, and each challenge can contain a
comma-separated list of authentication parameters. Furthermore, the header
field itself can occur multiple times.
For instance:
This header field contains two challenges; one for the "Newauth" scheme
with a realm value of "apps", and two additional parameters "type" and
"title", and another one for the "Basic" scheme with a realm value of
"simple".
Note: The challenge grammar production uses the list syntax as
well. Therefore, a sequence of comma, whitespace, and comma can be
considered either as applying to the preceding challenge, or to be an
empty entry in the list of challenges. In practice, this ambiguity
does not affect the semantics of the header field value and thus is
harmless.
The "Proxy-Authenticate" header field consists of at least one
challenge that indicates the authentication scheme(s) and parameters
applicable to the proxy for this request.
A proxy MUST send at least one Proxy-Authenticate header field in
each 407 (Proxy Authentication Required) response that it
generates.
Unlike WWW-Authenticate, the Proxy-Authenticate header field
applies only to the next outbound client on the response chain.
This is because only the client that chose a given proxy is likely to have
the credentials necessary for authentication. However, when multiple
proxies are used within the same administrative domain, such as office and
regional caching proxies within a large corporate network, it is common
for credentials to be generated by the user agent and passed through the
hierarchy until consumed. Hence, in such a configuration, it will appear
as if Proxy-Authenticate is being forwarded because each proxy will send
the same challenge set.
Note that the parsing considerations for WWW-Authenticate
apply to this header field as well; see
for details.
HTTP authentication schemes can use the Authentication-Info response header
field to communicate information after the client's authentication credentials have been accepted.
This information can include a finalization message from the server (e.g., it can contain the
server authentication).
The field value is a list of parameters (name/value pairs), using the "auth-param"
syntax defined in .
This specification only describes the generic format; authentication schemes
using Authentication-Info will define the individual parameters. The "Digest"
Authentication Scheme, for instance, defines multiple parameters in
Section 3.5 of .
The Authentication-Info header field can be used in any HTTP response,
independently of request method and status code. Its semantics are defined
by the authentication scheme indicated by the Authorization header field
() of the corresponding request.
A proxy forwarding a response is not allowed to modify the field value in any
way.
Authentication-Info can be used inside trailers ()
when the authentication scheme explicitly allows this.
Parameter values can be expressed either as "token" or as "quoted-string"
().
Authentication scheme definitions need to allow both notations, both for
senders and recipients. This allows recipients to use generic parsing
components, independent of the authentication scheme in use.
For backwards compatibility, authentication scheme definitions can restrict
the format for senders to one of the two variants. This can be important
when it is known that deployed implementations will fail when encountering
one of the two formats.
The Proxy-Authentication-Info response header field is equivalent to
Authentication-Info, except that it applies to proxy authentication ()
and its semantics are defined by the
authentication scheme indicated by the Proxy-Authorization header field
()
of the corresponding request:
However, unlike Authentication-Info, the Proxy-Authentication-Info header
field applies only to the next outbound client on the response chain. This is
because only the client that chose a given proxy is likely to have the
credentials necessary for authentication. However, when multiple proxies are
used within the same administrative domain, such as office and regional
caching proxies within a large corporate network, it is common for
credentials to be generated by the user agent and passed through the
hierarchy until consumed. Hence, in such a configuration, it will appear as
if Proxy-Authentication-Info is being forwarded because each proxy will send
the same field value.
The remaining response header fields provide more information about
the target resource for potential use in later requests.
Field NameDefined in...Accept-RangesAllowServer
The "Accept-Ranges" header field allows a server to indicate that it
supports range requests for the target resource.
An origin server that supports byte-range requests for a given target
resource MAY send
to indicate what range units are supported. A client MAY generate range
requests without having received this header field for the resource
involved. Range units are defined in .
A server that does not support any kind of range request for the target
resource MAY send
to advise the client not to attempt a range request.
The "Allow" header field lists the set of methods advertised as
supported by the target resource. The purpose of this field
is strictly to inform the recipient of valid request methods associated
with the resource.
Example of use:
The actual set of allowed methods is defined by the origin server at the
time of each request. An origin server MUST generate an Allow field in a
405 (Method Not Allowed) response and MAY do so in any
other response. An empty Allow field value indicates that the resource
allows no methods, which might occur in a 405 response if the resource has
been temporarily disabled by configuration.
A proxy MUST NOT modify the Allow header field — it does not need
to understand all of the indicated methods in order to handle them
according to the generic message handling rules.
The "Server" header field contains information about the
software used by the origin server to handle the request, which is often
used by clients to help identify the scope of reported interoperability
problems, to work around or tailor requests to avoid particular server
limitations, and for analytics regarding server or operating system use.
An origin server MAY generate a Server field in its responses.
The Server field value consists of one or more product identifiers, each
followed by zero or more comments (), which together
identify the origin server software and its significant subproducts.
By convention, the product identifiers are listed in decreasing order of
their significance for identifying the origin server software. Each product
identifier consists of a name and optional version, as defined in
.
Example:
An origin server SHOULD NOT generate a Server field containing needlessly
fine-grained detail and SHOULD limit the addition of subproducts by third
parties. Overly long and detailed Server field values increase response
latency and potentially reveal internal implementation details that might
make it (slightly) easier for attackers to find and exploit known security
holes.
This section is meant to inform developers, information providers, and
users of known security concerns relevant to HTTP semantics and its
use for transferring information over the Internet. Considerations related
to message syntax, parsing, and routing are discussed in
Section 11 of .
The list of considerations below is not exhaustive. Most security concerns
related to HTTP semantics are about securing server-side applications (code
behind the HTTP interface), securing user agent processing of payloads
received via HTTP, or secure use of the Internet in general, rather than
security of the protocol. Various organizations maintain topical
information and links to current research on Web application security
(e.g., ).
HTTP relies on the notion of an authoritative response: a
response that has been determined by (or at the direction of) the origin
server identified within the target URI to be the most appropriate response
for that request given the state of the target resource at the time of
response message origination.
When a registered name is used in the authority component, the "http" URI
scheme () relies on the user's local name
resolution service to determine where it can find authoritative responses.
This means that any attack on a user's network host table, cached names,
or name resolution libraries becomes an avenue for attack on establishing
authority for "http" URIs. Likewise, the user's choice of server for
Domain Name Service (DNS), and the hierarchy of servers from which it
obtains resolution results, could impact the authenticity of address
mappings; DNS Security Extensions (DNSSEC, ) are
one way to improve authenticity.
Furthermore, after an IP address is obtained, establishing authority for
an "http" URI is vulnerable to attacks on Internet Protocol routing.
The "https" scheme () is intended to prevent
(or at least reveal) many of these potential attacks on establishing
authority, provided that the negotiated TLS connection is secured and
the client properly verifies that the communicating server's identity
matches the target URI's authority component
(). Correctly implementing such verification
can be difficult (see ).
Authority for a given origin server can be delegated through protocol
extensions; for example, . Likewise, the set of
servers that a connection is considered authoritative for can be changed
with a protocol extension like .
Providing a response from a non-authoritative source, such as a shared
proxy cache, is often useful to improve performance and availability, but
only to the extent that the source can be trusted or the distrusted
response can be safely used.
Unfortunately, communicating authority to users can be difficult.
For example, phishing is an attack on the user's perception
of authority, where that perception can be misled by presenting similar
branding in hypertext, possibly aided by userinfo obfuscating the authority
component (see ).
User agents can reduce the impact of phishing attacks by enabling users to
easily inspect a target URI prior to making an action, by prominently
distinguishing (or rejecting) userinfo when present, and by not sending
stored credentials and cookies when the referring document is from an
unknown or untrusted source.
By their very nature, HTTP intermediaries are men-in-the-middle and, thus,
represent an opportunity for man-in-the-middle attacks. Compromise of
the systems on which the intermediaries run can result in serious security
and privacy problems. Intermediaries might have access to security-related
information, personal information about individual users and
organizations, and proprietary information belonging to users and
content providers. A compromised intermediary, or an intermediary
implemented or configured without regard to security and privacy
considerations, might be used in the commission of a wide range of
potential attacks.
Intermediaries that contain a shared cache are especially vulnerable
to cache poisoning attacks, as described in Section 7 of .
Implementers need to consider the privacy and security
implications of their design and coding decisions, and of the
configuration options they provide to operators (especially the
default configuration).
Users need to be aware that intermediaries are no more trustworthy than
the people who run them; HTTP itself cannot solve this problem.
Origin servers frequently make use of their local file system to manage the
mapping from target URI to resource representations.
Most file systems are not designed to protect against malicious file
or path names. Therefore, an origin server needs to avoid accessing
names that have a special significance to the system when mapping the
target resource to files, folders, or directories.
For example, UNIX, Microsoft Windows, and other operating systems use ".."
as a path component to indicate a directory level above the current one,
and they use specially named paths or file names to send data to system devices.
Similar naming conventions might exist within other types of storage
systems. Likewise, local storage systems have an annoying tendency to
prefer user-friendliness over security when handling invalid or unexpected
characters, recomposition of decomposed characters, and case-normalization
of case-insensitive names.
Attacks based on such special names tend to focus on either denial-of-service
(e.g., telling the server to read from a COM port) or disclosure
of configuration and source files that are not meant to be served.
Origin servers often use parameters within the URI as a
means of identifying system services, selecting database entries, or
choosing a data source. However, data received in a request cannot be
trusted. An attacker could construct any of the request data elements
(method, target URI, header fields, or body) to contain data that might
be misinterpreted as a command, code, or query when passed through a
command invocation, language interpreter, or database interface.
For example, SQL injection is a common attack wherein additional query
language is inserted within some part of the target URI or header
fields (e.g., Host, Referer, etc.).
If the received data is used directly within a SELECT statement, the
query language might be interpreted as a database command instead of a
simple string value. This type of implementation vulnerability is extremely
common, in spite of being easy to prevent.
In general, resource implementations ought to avoid use of request data
in contexts that are processed or interpreted as instructions. Parameters
ought to be compared to fixed strings and acted upon as a result of that
comparison, rather than passed through an interface that is not prepared
for untrusted data. Received data that isn't based on fixed parameters
ought to be carefully filtered or encoded to avoid being misinterpreted.
Similar considerations apply to request data when it is stored and later
processed, such as within log files, monitoring tools, or when included
within a data format that allows embedded scripts.
Because HTTP uses mostly textual, character-delimited fields, parsers are
often vulnerable to attacks based on sending very long (or very slow)
streams of data, particularly where an implementation is expecting a
protocol element with no predefined length
().
To promote interoperability, specific recommendations are made for minimum
size limits on request-line (Section 3 of )
and fields (). These are
minimum recommendations, chosen to be supportable even by implementations
with limited resources; it is expected that most implementations will
choose substantially higher limits.
A server can reject a message that
has a target URI that is too long () or a request payload
that is too large (). Additional status codes related to
capacity limits have been defined by extensions to HTTP
.
Recipients ought to carefully limit the extent to which they process other
protocol elements, including (but not limited to) request methods, response
status phrases, field names, numeric values, and body chunks.
Failure to limit such processing can result in buffer overflows, arithmetic
overflows, or increased vulnerability to denial-of-service attacks.
Clients are often privy to large amounts of personal information,
including both information provided by the user to interact with resources
(e.g., the user's name, location, mail address, passwords, encryption
keys, etc.) and information about the user's browsing activity over
time (e.g., history, bookmarks, etc.). Implementations need to
prevent unintentional disclosure of personal information.
A server is in the position to save personal data about a user's requests
over time, which might identify their reading patterns or subjects of
interest. In particular, log information gathered at an intermediary
often contains a history of user agent interaction, across a multitude
of sites, that can be traced to individual users.
HTTP log information is confidential in nature; its handling is often
constrained by laws and regulations. Log information needs to be securely
stored and appropriate guidelines followed for its analysis.
Anonymization of personal information within individual entries helps,
but it is generally not sufficient to prevent real log traces from being
re-identified based on correlation with other access characteristics.
As such, access traces that are keyed to a specific client are unsafe to
publish even if the key is pseudonymous.
To minimize the risk of theft or accidental publication, log information
ought to be purged of personally identifiable information, including
user identifiers, IP addresses, and user-provided query parameters,
as soon as that information is no longer necessary to support operational
needs for security, auditing, or fraud control.
URIs are intended to be shared, not secured, even when they identify secure
resources. URIs are often shown on displays, added to templates when a page
is printed, and stored in a variety of unprotected bookmark lists.
It is therefore unwise to include information within a URI that
is sensitive, personally identifiable, or a risk to disclose.
Authors of services ought to avoid GET-based forms for the submission of
sensitive data because that data will be placed in the target URI. Many
existing servers, proxies, and user agents log or display the target URI
in places where it might be visible to third parties. Such services ought
to use POST-based form submission instead.
Since the Referer header field tells a target site about the
context that resulted in a request, it has the potential to reveal
information about the user's immediate browsing history and any personal
information that might be found in the referring resource's URI.
Limitations on the Referer header field are described in to
address some of its security considerations.
Although fragment identifiers used within URI references are not sent
in requests, implementers ought to be aware that they will be visible to
the user agent and any extensions or scripts running as a result of the
response. In particular, when a redirect occurs and the original request's
fragment identifier is inherited by the new reference in
Location (), this might
have the effect of disclosing one site's fragment to another site.
If the first site uses personal information in fragments, it ought to
ensure that redirects to other sites include a (possibly empty) fragment
component in order to block that inheritance.
The User-Agent (),
Via (), and
Server () header fields often
reveal information about the respective sender's software systems.
In theory, this can make it easier for an attacker to exploit known
security holes; in practice, attackers tend to try all potential holes
regardless of the apparent software versions being used.
Proxies that serve as a portal through a network firewall ought to take
special precautions regarding the transfer of header information that might
identify hosts behind the firewall. The Via header field
allows intermediaries to replace sensitive machine names with pseudonyms.
Browser fingerprinting is a set of techniques for identifying a specific
user agent over time through its unique set of characteristics. These
characteristics might include information related to its TCP behavior,
feature capabilities, and scripting environment, though of particular
interest here is the set of unique characteristics that might be
communicated via HTTP. Fingerprinting is considered a privacy concern
because it enables tracking of a user agent's behavior over time
() without
the corresponding controls that the user might have over other forms of
data collection (e.g., cookies). Many general-purpose user agents
(i.e., Web browsers) have taken steps to reduce their fingerprints.
There are a number of request header fields that might reveal information
to servers that is sufficiently unique to enable fingerprinting.
The From header field is the most obvious, though it is
expected that From will only be sent when self-identification is desired by
the user. Likewise, Cookie header fields are deliberately designed to
enable re-identification, so fingerprinting concerns only apply to
situations where cookies are disabled or restricted by the user agent's
configuration.
The User-Agent header field might contain enough information
to uniquely identify a specific device, usually when combined with other
characteristics, particularly if the user agent sends excessive details
about the user's system or extensions. However, the source of unique
information that is least expected by users is
proactive negotiation (),
including the Accept, Accept-Charset,
Accept-Encoding, and Accept-Language
header fields.
In addition to the fingerprinting concern, detailed use of the
Accept-Language header field can reveal information the
user might consider to be of a private nature. For example, understanding
a given language set might be strongly correlated to membership in a
particular ethnic group.
An approach that limits such loss of privacy would be for a user agent
to omit the sending of Accept-Language except for sites that have been
whitelisted, perhaps via interaction after detecting a Vary
header field that indicates language negotiation might be useful.
In environments where proxies are used to enhance privacy, user agents
ought to be conservative in sending proactive negotiation header fields.
General-purpose user agents that provide a high degree of header field
configurability ought to inform users about the loss of privacy that might
result if too much detail is provided. As an extreme privacy measure,
proxies could filter the proactive negotiation header fields in relayed
requests.
The validators defined by this specification are not intended to ensure
the validity of a representation, guard against malicious changes, or
detect man-in-the-middle attacks. At best, they enable more efficient cache
updates and optimistic concurrent writes when all participants are behaving
nicely. At worst, the conditions will fail and the client will receive a
response that is no more harmful than an HTTP exchange without conditional
requests.
An entity-tag can be abused in ways that create privacy risks. For example,
a site might deliberately construct a semantically invalid entity-tag that
is unique to the user or user agent, send it in a cacheable response with a
long freshness time, and then read that entity-tag in later conditional
requests as a means of re-identifying that user or user agent. Such an
identifying tag would become a persistent identifier for as long as the
user agent retained the original cache entry. User agents that cache
representations ought to ensure that the cache is cleared or replaced
whenever the user performs privacy-maintaining actions, such as clearing
stored cookies or changing to a private browsing mode.
Unconstrained multiple range requests are susceptible to denial-of-service
attacks because the effort required to request many overlapping ranges of
the same data is tiny compared to the time, memory, and bandwidth consumed
by attempting to serve the requested data in many parts.
Servers ought to ignore, coalesce, or reject egregious range requests, such
as requests for more than two overlapping ranges or for many small ranges
in a single set, particularly when the ranges are requested out of order
for no apparent reason. Multipart range requests are not designed to
support random access.
Everything about the topic of HTTP authentication is a security
consideration, so the list of considerations below is not exhaustive.
Furthermore, it is limited to security considerations regarding the
authentication framework, in general, rather than discussing all of the
potential considerations for specific authentication schemes (which ought
to be documented in the specifications that define those schemes).
Various organizations maintain topical information and links to current
research on Web application security (e.g., ),
including common pitfalls for implementing and using the authentication
schemes found in practice.
The HTTP authentication framework does not define a single mechanism for
maintaining the confidentiality of credentials; instead, each
authentication scheme defines how the credentials are encoded prior to
transmission. While this provides flexibility for the development of future
authentication schemes, it is inadequate for the protection of existing
schemes that provide no confidentiality on their own, or that do not
sufficiently protect against replay attacks. Furthermore, if the server
expects credentials that are specific to each individual user, the exchange
of those credentials will have the effect of identifying that user even if
the content within credentials remains confidential.
HTTP depends on the security properties of the underlying transport- or
session-level connection to provide confidential transmission of
fields. In other words, if a server limits access to authenticated users
using this framework, the server needs to ensure that the connection is
properly secured in accordance with the nature of the authentication
scheme used. For example, services that depend on individual user
authentication often require a connection to be secured with TLS
("Transport Layer Security", ) prior to exchanging
any credentials.
Existing HTTP clients and user agents typically retain authentication
information indefinitely. HTTP does not provide a mechanism for the
origin server to direct clients to discard these cached credentials, since
the protocol has no awareness of how credentials are obtained or managed
by the user agent. The mechanisms for expiring or revoking credentials can
be specified as part of an authentication scheme definition.
Circumstances under which credential caching can interfere with the
application's security model include but are not limited to:
Clients that have been idle for an extended period, following
which the server might wish to cause the client to re-prompt the
user for credentials.Applications that include a session termination indication
(such as a "logout" or "commit" button on a page) after which
the server side of the application "knows" that there is no
further reason for the client to retain the credentials.
User agents that cache credentials are encouraged to provide a readily
accessible mechanism for discarding cached credentials under user control.
Authentication schemes that solely rely on the "realm" mechanism for
establishing a protection space will expose credentials to all resources on
an origin server. Clients that have successfully made authenticated requests
with a resource can use the same authentication credentials for other
resources on the same origin server. This makes it possible for a different
resource to harvest authentication credentials for other resources.
This is of particular concern when an origin server hosts resources for multiple
parties under the same canonical root URI ().
Possible mitigation strategies include restricting direct access to
authentication credentials (i.e., not making the content of the
Authorization request header field available), and separating protection
spaces by using a different host name (or port number) for each party.
Adding information to responses that are sent over an unencrypted
channel can affect security and privacy. The presence of the
Authentication-Info and Proxy-Authentication-Info
header fields alone indicates that HTTP authentication is in use. Additional
information could be exposed by the contents of the authentication-scheme
specific parameters; this will have to be considered in the definitions of these
schemes.
The change controller for the following registrations is:
"IETF (iesg@ietf.org) - Internet Engineering Task Force".
Please update the registry of URI Schemes at
with the
permanent schemes listed in the first table of .
Please update the "Hypertext Transfer Protocol (HTTP) Method Registry" at
with the
registration procedure of and the method
names summarized in the table of .
Furthermore, the method name "*" is reserved, since using that name as
HTTP method name might conflict with special semantics in fields such
as "Access-Control-Request-Method". Thus, please add the entry below to
the registry:
*nono
Please update the "Hypertext Transfer Protocol (HTTP) Status Code Registry"
at with
the registration procedure of and the
status code values summarized in the table of
.
Additionally, please update the following entry in the Hypertext Transfer
Protocol (HTTP) Status Code Registry:
418(Unused)
Please create a new registry as outlined in .
After creating the registry, all entries in the Permanent and Provisional
Message Header Registries with the protocol 'http' are to be moved to it,
with the following changes applied:
The 'Applicable Protocol' field is to be omitted.Entries with a status of 'standard', 'experimental', 'reserved', or
'informational' are to have a status of 'permanent'.Provisional entries without a status are to have a status of
'provisional'.Permanent entries without a status (after confirmation that the
registration document did not define one) will have a status of
'provisional'. The Expert(s) can choose to update their status if there is
evidence that another is more appropriate.
Please annotate the Permanent and Provisional Message Header registries to
indicate that HTTP field name registrations have moved, with an
appropriate link.
After that is complete, please update the new registry with the
field names listed in the table of .
Finally, please update the "Content-MD5" entry in the new registry to have
a status of 'obsoleted' with references to
Section 14.15 of (for the definition
of the header field) and
Appendix B of (which removed the field
definition from the updated specification).
Please update the
"Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry"
at with
the registration procedure of .
No authentication schemes are defined in this document.
Please update the "HTTP Content Coding Registry" at
with the registration procedure of
and the content coding names summarized in the table of
.
Please update the "HTTP Range Unit Registry" at
with the registration procedure of
and the range unit names summarized in the table of
.
Please update the "Media Types" registry at
with the registration information in
for the media type "multipart/byteranges".
Please update the "Service Name and Transport Protocol Port Number"
registry at
for the services on ports 80 and 443 that use UDP or TCP to:
use this document as "Reference", andwhen currently unspecified, set "Assignee" to "IESG" and "Contact" to
"IETF_Chair".HTTP/1.1 MessagingAdobe345 Park AveSan JoseCA95110United States of Americafielding@gbiv.comhttps://roy.gbiv.com/Fastlymnot@mnot.nethttps://www.mnot.net/greenbytes GmbHHafenweg 16Muenster48155Germanyjulian.reschke@greenbytes.dehttps://greenbytes.de/tech/webdav/HTTP CachingAdobe345 Park AveSan JoseCA95110United States of Americafielding@gbiv.comhttps://roy.gbiv.com/Fastlymnot@mnot.nethttps://www.mnot.net/greenbytes GmbHHafenweg 16Muenster48155Germanyjulian.reschke@greenbytes.dehttps://greenbytes.de/tech/webdav/Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message BodiesInnosoft International, Inc.ned@innosoft.comFirst Virtual Holdingsnsb@nsb.fv.comMultipurpose Internet Mail Extensions (MIME) Part Two: Media TypesInnosoft International, Inc.ned@innosoft.comFirst Virtual Holdingsnsb@nsb.fv.comKey words for use in RFCs to Indicate Requirement LevelsAmbiguity of Uppercase vs Lowercase in RFC 2119 Key WordsUniform Resource Identifier (URI): Generic SyntaxWorld Wide Web Consortiumtimbl@w3.orghttp://www.w3.org/People/Berners-Lee/Day Softwarefielding@gbiv.comhttp://roy.gbiv.com/AdobeLMM@acm.orghttp://larry.masinter.net/Transmission Control ProtocolUniversity of Southern California (USC)/Information Sciences InstituteMatching of Language TagsYahoo! Inc.addison@inter-locale.comGooglemark.davis@macchiato.comThe Base16, Base32, and Base64 Data EncodingsAugmented BNF for Syntax Specifications: ABNFBrandenburg InternetWorkingdcrocker@bbiw.netTHUS plc.paul.overell@thus.netInternet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) ProfileTags for Identifying LanguagesLab126addison@inter-locale.comGooglemark.davis@google.comTerminology Used in Internationalization in the IETFCase-Sensitive String Support in ABNFpkyzivat@alum.mit.eduCoded Character Set -- 7-bit American Standard Code for Information InterchangeAmerican National Standards InstituteZLIB Compressed Data Format Specification version 3.3Aladdin Enterprisesghost@aladdin.comDEFLATE Compressed Data Format Specification version 1.3Aladdin Enterprisesghost@aladdin.comGZIP file format specification version 4.3Aladdin Enterprisesghost@aladdin.comgzip@prep.ai.mit.edumadler@alumni.caltech.edughost@aladdin.comrandeg@alumni.rpi.eduA Technique for High-Performance Data CompressionErratum ID 1912, RFC 2978RFC ErrataErratum ID 5433, RFC 2978RFC ErrataA Survey on Web Tracking: Mechanisms, Implications, and DefensesThe Most Dangerous Code in the World: Validating SSL Certificates in Non-browser SoftwareInformation technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1International Organization for StandardizationHTTP Cookies: Standards, Privacy, and PoliticsMIME SniffingWHATWGArchitectural Styles and the Design of Network-based Software ArchitecturesClassical versus Transparent IP Proxiesmchatel@pax.eunet.chHypertext Transfer Protocol -- HTTP/1.0MIT, Laboratory for Computer Sciencetimbl@w3.orgUniversity of California, Irvine, Department of Information and Computer Sciencefielding@ics.uci.eduW3 Consortium, MIT Laboratory for Computer Sciencefrystyk@w3.orgMIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII TextUniversity of Tennesseemoore@cs.utk.eduHypertext Transfer Protocol -- HTTP/1.1University of California, Irvine, Department of Information and Computer Sciencefielding@ics.uci.eduMIT Laboratory for Computer Sciencejg@w3.orgDigital Equipment Corporation, Western Research Laboratorymogul@wrl.dec.comMIT Laboratory for Computer Sciencefrystyk@w3.orgMIT Laboratory for Computer Sciencetimbl@w3.orgUse and Interpretation of HTTP Version NumbersWestern Research Laboratorymogul@wrl.dec.comDepartment of Information and Computer Sciencefielding@ics.uci.eduMIT Laboratory for Computer Sciencejg@w3.orgW3 Consortiumfrystyk@w3.orgTransparent Content Negotiation in HTTPTechnische Universiteit Eindhovenkoen@win.tue.nlHewlett-Packard Companymutz@hpl.hp.comHyper Text Coffee Pot Control Protocol (HTCPCP/1.0)MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)Stockholm University and KTHjpalme@dsv.su.seMicrosoft Corporationalexhop@microsoft.comLotus Development CorporationShelness@lotus.comstef@nma.comHypertext Transfer Protocol -- HTTP/1.1University of California, Irvinefielding@ics.uci.eduW3Cjg@w3.orgCompaq Computer Corporationmogul@wrl.dec.comMIT Laboratory for Computer Sciencefrystyk@w3.orgXerox Corporationmasinter@parc.xerox.comMicrosoft Corporationpaulle@microsoft.comW3Ctimbl@w3.orgHTTP Authentication: Basic and Digest Access AuthenticationNorthwestern University, Department of Mathematicsjohn@math.nwu.eduVerisign Inc.pbaker@verisign.comAbiSource, Inc.jeff@AbiSource.comAgranat Systems, Inc.lawrence@agranat.comMicrosoft Corporationpaulle@microsoft.comNetscape Communications CorporationOpen Market, Inc.stewart@OpenMarket.comAn HTTP Extension Frameworkfrystyk@w3.orgpaulle@microsoft.comlawrence@agranat.comHTTP Over TLSRTFM, Inc.ekr@rtfm.comIANA Charset Registration ProceduresInternet Web Replication and Caching TaxonomyEquinix, Inc.UNINETTCacheFlow Inc.DNS Security Introduction and RequirementsSPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft WindowsHTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)CommerceNetldusseault@commerce.netHypertext Transfer Protocol Version 2 (HTTP/2)The Transport Layer Security (TLS) Protocol Version 1.3Network Time Protocol Version 4: Protocol and Algorithms SpecificationRepresentation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)The Web Origin ConceptHypertext Transfer Protocol (HTTP/1.1): Message Syntax and RoutingAdobefielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP/1.1): Semantics and ContentAdobefielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP/1.1): Conditional RequestsAdobefielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP): Range RequestsAdobefielding@gbiv.comWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP/1.1): AuthenticationAdobefielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deReturning Values from Forms: multipart/form-dataHTTP Authentication-Info and Proxy-Authentication-Info Response Header Fieldsgreenbytes GmbHjulian.reschke@greenbytes.deHTTP Alternative ServicesThe ORIGIN HTTP/2 FrameMedia Type Specifications and Registration ProceduresOraclened+ietf@mrochek.comjohn+ietf@jck.comAT&T Laboratoriestony+mtsuffix@maillennium.att.comGuidelines and Registration Procedures for URI SchemesDeprecating the "X-" Prefix and Similar Constructs in Application ProtocolsInternet Message FormatQualcomm IncorporatedPATCH Method for HTTPLinden LabHTTP State Management Mechanism
University of California, Berkeley
abarth@eecs.berkeley.eduAdditional HTTP Status CodesRackspaceAdobeThe Hypertext Transfer Protocol Status Code 308 (Permanent Redirect)HTTP Digest Access AuthenticationAvayaNetzkonformThe 'Basic' HTTP Authentication Schemegreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP) Client-Initiated Content-Encodinggreenbytes GmbHjulian.reschke@greenbytes.deGuidelines for Writing an IANA Considerations Section in RFCsIndicating Character Encoding and Language for HTTP Header Field ParametersHTTP Immutable ResponsesWeb LinkingA Guide to Building Secure Web Applications and Web ServicesIn the collected ABNF below, list rules are expanded as per .
None yet.
The sections introducing HTTP's design goals, history, architecture,
conformance criteria, protocol versioning, URIs, message routing, and
header fields have been moved here (without substantive change).
"Field value" now refers to the value after multiple instances are combined
with commas — by far the most common use. To refer to a single header
line's value, use "field line value".
()
Trailer field semantics now transcend the specifics of chunked encoding.
Use of trailer fields has been further limited to only allow generation
as a trailer field when the sender knows the field defines that usage and
to only allow merging into the header section if the recipient knows the
corresponding field definition permits and defines how to merge. In all
other cases, implementations are encouraged to either store the trailer
fields separately or discard them instead of merging.
()
Made the priority of the absolute form of the request URI over the Host
header by origin servers explicit, to align with proxy handling.
()
The grammar definition for the Via field's "received-by" was
expanded in 7230 due to changes in the URI grammar for host
that are not desirable for Via. For simplicity,
we have removed uri-host from the received-by production because it can
be encompassed by the existing grammar for pseudonym. In particular, this
change removed comma from the allowed set of charaters for a host name in
received-by.
()
Added status code 308 (previously defined in )
so that it's defined closer to status codes 301, 302, and 307.
()
Added status code 422 (previously defined in
Section 11.2 of ) because of its general
applicability.
()
The description of an origin and authoritative access to origin servers has
been extended for both "http" and "https" URIs to account for alternative
services and secured connections that are not necessarily based on TCP.
(, ,
, )
Minimum URI lengths to be supported by implementations are now recommended.
()
Clarify that control characters in field values are to be rejected or
mapped to SP.
()
The term "effective request URI" has been replaced with "target URI".
()
Range units are compared in a case insensitive fashion.
()
Restrictions on client retries have been loosened, to reflect implementation
behavior.
()
Clarified that request bodies on GET and DELETE are not interoperable.
(, )
Removed a superfluous requirement about setting Content-Length
from the description of the OPTIONS method.
()
Allow Accept and Accept-Encoding in response
messages; the latter was introduced by .
()
Clarify that If-Unmodified-Since doesn't apply to a resource without a
concept of modification time.
()
Refactored the range-unit and ranges-specifier grammars to simplify
and reduce artificial distinctions between bytes and other
(extension) range units, removing the overlapping grammar of
other-range-unit by defining range units generically as a token and
placing extensions within the scope of a range-spec (other-range).
This disambiguates the role of list syntax (commas) in all range sets,
including extension range units, for indicating a range-set of more than
one range. Moving the extension grammar into range specifiers also allows
protocol specific to byte ranges to be specified separately.
None yet.
None yet.
None yet.
This specification includes the extension defined in ,
but leaves out examples and deployment considerations.
This section is to be removed before publishing as an RFC.
The changes were purely editorial:
Change boilerplate and abstract to indicate the "draft" status, and update references to ancestor specifications.Remove version "1.1" from document title, indicating that this specification applies to all HTTP versions.Adjust historical notes.Update links to sibling specifications.Replace sections listing changes from RFC 2616 by new empty sections referring to RFC 723x.Remove acknowledgements specific to RFC 723x.Move "Acknowledgements" to the very end and make them unnumbered.
The changes in this draft are editorial, with respect to HTTP as a whole,
to merge core HTTP semantics into this document:
Merged introduction, architecture, conformance, and ABNF extensions from
RFC 7230 (Messaging).Rearranged architecture to extract conformance, http(s) schemes, and
protocol versioning into a separate major section.Moved discussion of MIME differences to since
that is primarily concerned with transforming 1.1 messages.Merged entire content of RFC 7232 (Conditional Requests).Merged entire content of RFC 7233 (Range Requests).Merged entire content of RFC 7235 (Auth Framework).Moved all extensibility tips, registration procedures, and registry
tables from the IANA considerations to normative sections, reducing the
IANA considerations to just instructions that will be removed prior to
publication as an RFC.Improve [Welch] citation ()Remove HTTP/1.1-ism about Range Requests ()Cite RFC 8126 instead of RFC 5226 ()Cite RFC 7538 instead of RFC 7238 ()Cite RFC 8288 instead of RFC 5988 ()Cite RFC 8187 instead of RFC 5987 ()Cite RFC 7578 instead of RFC 2388 ()Cite RFC 7595 instead of RFC 4395 ()improve ABNF readability for qdtext (, )Clarify "resource" vs "representation" in definition of status code 416 (, )Resolved erratum 4072, no change needed here (, )Clarify DELETE status code suggestions (, )In , fix ABNF for "other-range-resp" to use VCHAR instead of CHAR (, )Resolved erratum 5162, no change needed here (, )Replace "response code" with "response status code" and "status-code" (the ABNF production name from the HTTP/1.1 message format) by "status code" (, )Added a missing word in (, )In , fixed an example that had trailing whitespace where it shouldn't (, )In , remove words that were potentially misleading with respect to the relation to the requested ranges (, )Included (Proxy-)Auth-Info header field definition from RFC 7615 ()In , clarify POST caching ()Add to reserve the 418 status code ()In and , clarified when a response can be sent ()In , explain the difference between the "token" production, the RFC 2978 ABNF for charset names, and the actual registration practice (, )In , removed the fragment component in the URI scheme definitions as per Section 4.3 of ,
furthermore moved fragment discussion into a separate section
(, , )In , add language about minor HTTP version number defaulting ()Added for status code 422, previously defined in Section 11.2 of ()In , fixed prose about byte range comparison (, )In , explain that request/response correlation is version specific ()In , include status code 308 from RFC 7538 ()In , clarify that the charset parameter value is case-insensitive due to the definition in RFC 2046 ()Define a separate registry for HTTP header field names ()In , refactor and clarify description of wildcard ("*") handling ()Deprecate Accept-Charset ()In , mention Cache-Control: immutable ()In , clarify when header field combination is allowed ()In , instruct IANA to mark Content-MD5 as obsolete ()Use RFC 7405 ABNF notation for case-sensitive string constants ()Rework to be more version-independent ()In , clarify that DELETE needs to be successful to invalidate cache (, )In , fix field-content ABNF (, )Move into its own section ()In , reference MIME Sniffing ()In , simplify the #rule mapping for recipients (, )In , remove misleading text about "extension" of HTTP is needed to define method payloads ()Fix editorial issue in ()In , rephrase language not to use "entity" anymore, and also avoid lowercase "may" ()Move discussion of retries from into ()Moved transport-independent part of the description of trailers into ()Loosen requirements on retries based upon implementation behavior ()In , update IANA port registry for TCP/UDP on ports 80 and 443 ()In , revise guidelines for new header field names ()In , remove concept of "cacheable methods" in favor of prose (, )In , mention that the concept of authority can be modified by protocol extensions ()Create new subsection on payload body in , taken from portions of message body ()Moved definition of "Whitespace" into new container "Generic Syntax" ()In , recommend minimum URI size support for implementations ()In , refactored the range-unit and ranges-specifier grammars (, )In , caution against a request body more strongly ()Reorganized text in ()In , replace "authorize" with "fulfill" ()In , removed a misleading statement about Content-Length (, )In , add text from RFC 2818 ()Changed "cacheable by default" to "heuristically cacheable" throughout ()In , simplify received-by grammar (and disallow comma character) ()In , give guidance on interoperable field names ()In , define the semantics and possible replacement of whitespace when it is known to occur (, )In , introduce field terminology and distinguish between field line values and field values; use terminology consistently throughout ()Moved #rule definition into and whitespace into ()In , explicitly call out range unit names as case-insensitive, and encourage registration ()In , explicitly call out content codings as case-insensitive, and encourage registration ()In , explicitly call out field names as case-insensitive ()In , cite ()In , formally define "final" and "interim" status codes ()In , caution against a request body more strongly ()In , note that Etag can be used in trailers ()In , consider reserved fields as well ()In , be more correct about what was deprecated by RFC 3986 (, )In , recommend comma SP when combining field lines ()In , make explicit requirements on origin server to use authority from absolute-form when available ()In , , , and , refactored schemes to define origin and authoritative access to an origin server for both "http" and "https" URIs to account for alternative services and secured connections that are not necessarily based on TCP ()In , reference RFC 8174 as well ()In , explicitly reference the definition of representation data as including any content codings ()Move TE: trailers from into ()In , adjust requirements for handling multiple content-length values ()In and , clarified condition evaluation ()In , remove concept of obs-fold, as that is HTTP/1-specific ()In , introduce the concept of request payload negotiation () and define for Accept-Encoding ()In , , and , remove HTTP/1-specific, connection-related requirements ()In , correct language about what is forwarded ()Throughout, replace "effective request URI", "request-target" and similar with "target URI" ()In and , describe how extensions should consider scope of applicability ()In , don't rely on the HTTP/1.1 Messaging specification to define "message" ()In and , note that URL resolution is necessary ()In , explicitly reference 206 as one of the status codes that provide representation data ()In , refine requirements so that they don't apply to resources without a concept of modification time ()In , specify the scope as a request, not a target resource ()In , introduce concept of "complete" messages ()In , , and , refine use of "request target" ()Throughout, remove "status-line" and "request-line", as these are HTTP/1.1-specific ()In , remove duplicate definition of what makes a range satisfiable and refer instead to each range unit's definition ()In and , clarify that a selected representation of zero length can only be satisfiable as a suffix range and that a server can still ignore Range for that case ()In and , allow "Accept" as response field () now uses the sender variant of the "#" list expansion ()In , make the field list-based even when "*" is present ()In , add optional "Comments" entry ()In , reserve "*" as field name ()In , reserve "*" as method name ()In and , state that multiple "*" is unlikely to be interoperable ()In , avoid use of obsolete media type parameter on text/html (, )Rephrase prose in to become version-agnostic ()In , instruct recipients how to deal with control characters in field values ()In , update note about field ABNF ()Add about Extending and Versioning HTTP ()In , include status 308 in list of heuristically cacheable status codes ()In , make it clearer that "identity" is not to be included ()
This edition of the HTTP specification builds on the many contributions that went into
RFC 1945,
RFC 2068,
RFC 2145,
RFC 2616, and
RFC 2818, including
substantial contributions made by the previous authors, editors, and
Working Group Chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
Paul J. Leach, Eric Rescorla, and Yves Lafon.
See Section 10 of for further
acknowledgements from prior revisions.
In addition, this document has reincorporated the HTTP Authentication
Framework, previously defined in
RFC 7235 and
RFC 2617.
We thank John Franks, Phillip M. Hallam-Baker, Jeffery L. Hostetler,
Scott D. Lawrence, Paul J. Leach, Ari Luotonen, and Lawrence C. Stewart
for their work on that specification.
See Section 6 of
for further acknowledgements.
New acks to be added here.