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/
Applications and Real-Time
HTTP Working GroupHypertext Transfer ProtocolHTTPHTTP message format
The Hypertext Transfer Protocol (HTTP) is a stateless application-level
protocol for distributed, collaborative, hypertext information systems.
This document specifies the HTTP/1.1 message syntax, message parsing,
connection management, and related security concerns.
This document obsoletes 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" "HTTP Caching" "HTTP/1.1 Messaging" (this document)
This document defines HTTP/1.1 message syntax and framing requirements
and their associated connection management.
Our goal is to define all of the mechanisms necessary for HTTP/1.1 message
handling that are independent of message semantics, thereby defining the
complete set of requirements for message parsers and
message-forwarding intermediaries.
This document obsoletes the portions of
RFC 7230 related to HTTP/1.1
messaging and connection management, with the changes being summarized in
. The other parts of
RFC 7230 are obsoleted by
"HTTP Semantics" .
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in .
Conformance criteria and considerations regarding error handling
are defined in Section 3 of .
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 Section 12 of ,
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:
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 character).
The rules below are defined in :
An HTTP/1.1 message consists of a start-line followed by a CRLF and a
sequence of
octets in a format similar to the Internet Message Format
: zero or more header fields (collectively
referred to as the "headers" or the "header section"), an empty line
indicating the end of the header section, and an optional message body.
A message can be either a request from client to server or a
response from server to client. Syntactically, the two types of message
differ only in the start-line, which is either a request-line (for requests)
or a status-line (for responses), and in the algorithm for determining
the length of the message body ().
In theory, a client could receive requests and a server could receive
responses, distinguishing them by their different start-line formats.
In practice, servers are implemented to only expect a request
(a response is interpreted as an unknown or invalid request method)
and clients are implemented to only expect a response.
Although HTTP makes use of some protocol elements similar to
the Multipurpose Internet Mail Extensions (MIME) ,
see for the differences
between HTTP and MIME messages.
The normal procedure for parsing an HTTP message is to read the
start-line into a structure, read each header field into a hash
table by field name until the empty line, and then use the parsed
data to determine if a message body is expected. If a message body
has been indicated, then it is read as a stream until an amount
of octets equal to the message body length is read or the connection
is closed.
A recipient MUST parse an HTTP message as a sequence of octets in an
encoding that is a superset of US-ASCII .
Parsing an HTTP message as a stream of Unicode characters, without regard
for the specific encoding, creates security vulnerabilities due to the
varying ways that string processing libraries handle invalid multibyte
character sequences that contain the octet LF (%x0A). String-based
parsers can only be safely used within protocol elements after the element
has been extracted from the message, such as within a header field-value
after message parsing has delineated the individual fields.
Although the line terminator for the start-line and header
fields is the sequence CRLF, a recipient MAY recognize a
single LF as a line terminator and ignore any preceding CR.
Older HTTP/1.0 user agent implementations might send an extra CRLF
after a POST request as a workaround for some early server
applications that failed to read message body content that was
not terminated by a line-ending. An HTTP/1.1 user agent MUST NOT
preface or follow a request with an extra CRLF. If terminating
the request message body with a line-ending is desired, then the
user agent MUST count the terminating CRLF octets as part of the
message body length.
In the interest of robustness, a server that is expecting to receive and
parse a request-line SHOULD ignore at least one empty line (CRLF)
received prior to the request-line.
A sender MUST NOT send whitespace between the start-line and
the first header field.
A recipient that receives whitespace between the start-line and
the first header field MUST either reject the message as invalid or
consume each whitespace-preceded line without further processing of it
(i.e., ignore the entire line, along with any subsequent lines preceded
by whitespace, until a properly formed header field is received or the
header section is terminated).
The presence of such whitespace in a request
might be an attempt to trick a server into ignoring that field or
processing the line after it as a new request, either of which might
result in a security vulnerability if other implementations within
the request chain interpret the same message differently.
Likewise, the presence of such whitespace in a response might be
ignored by some clients or cause others to cease parsing.
When a server listening only for HTTP request messages, or processing
what appears from the start-line to be an HTTP request message,
receives a sequence of octets that does not match the HTTP-message
grammar aside from the robustness exceptions listed above, the
server SHOULD respond with a 400 (Bad Request) response.
HTTP uses a "<major>.<minor>" numbering scheme to indicate
versions of the protocol. This specification defines version "1.1".
Section 3.5 of specifies the semantics of HTTP version
numbers.
The version of an HTTP/1.x message is indicated by an HTTP-version field
in the start-line. HTTP-version is case-sensitive.
When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
or a recipient whose version is unknown,
the HTTP/1.1 message is constructed such that it can be interpreted
as a valid HTTP/1.0 message if all of the newer features are ignored.
This specification places recipient-version requirements on some
new features so that a conformant sender will only use compatible
features until it has determined, through configuration or the
receipt of a message, that the recipient supports HTTP/1.1.
Intermediaries that process HTTP messages (i.e., all intermediaries
other than those acting as tunnels) MUST send their own HTTP-version
in forwarded messages. In other words, they are not allowed to blindly
forward the start-line without ensuring that the
protocol version in that message matches a version to which that
intermediary is conformant for both the receiving and
sending of messages. Forwarding an HTTP message without rewriting
the HTTP-version might result in communication errors when downstream
recipients use the message sender's version to determine what features
are safe to use for later communication with that sender.
A server MAY send an HTTP/1.0 response to an HTTP/1.1 request
if it is known or suspected that the client incorrectly implements the
HTTP specification and is incapable of correctly processing later
version responses, such as when a client fails to parse the version
number correctly or when an intermediary is known to blindly forward
the HTTP-version even when it doesn't conform to the given minor
version of the protocol. Such protocol downgrades SHOULD NOT be
performed unless triggered by specific client attributes, such as when
one or more of the request header fields (e.g., User-Agent)
uniquely match the values sent by a client known to be in error.
A request-line begins with a method token, followed by a single
space (SP), the request-target, another single space (SP), and ends
with the protocol version.
Although the request-line grammar rule requires that each of the component
elements be separated by a single SP octet, recipients MAY instead parse
on whitespace-delimited word boundaries and, aside from the CRLF
terminator, treat any form of whitespace as the SP separator while
ignoring preceding or trailing whitespace; such whitespace includes one or
more of the following octets: SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
However, lenient parsing can result in request smuggling security
vulnerabilities if there are multiple recipients of the message and each
has its own unique interpretation of robustness
(see ).
HTTP does not place a predefined limit on the length of a request-line,
as described in Section 3 of .
A server that receives a method longer than any that it implements
SHOULD respond with a 501 (Not Implemented) status code.
A server that receives a request-target longer than any URI it wishes to
parse MUST respond with a
414 (URI Too Long) status code (see Section 9.5.15 of ).
Various ad hoc limitations on request-line length are found in practice.
It is RECOMMENDED that all HTTP senders and recipients support, at a
minimum, request-line lengths of 8000 octets.
The method token indicates the request method to be performed on the
target resource. The request method is case-sensitive.
The request methods defined by this specification can be found in
Section 7 of , along with information regarding the HTTP method
registry and considerations for defining new methods.
The request-target identifies the target resource upon which to apply the
request. The client derives a request-target from its desired target URI.
There are four distinct formats for the request-target, depending on both
the method being requested and whether the request is to a proxy.
No whitespace is allowed in the request-target.
Unfortunately, some user agents fail to properly encode or exclude
whitespace found in hypertext references, resulting in those disallowed
characters being sent as the request-target in a malformed request-line.
Recipients of an invalid request-line SHOULD respond with either a
400 (Bad Request) error or a 301 (Moved Permanently)
redirect with the request-target properly encoded. A recipient SHOULD NOT
attempt to autocorrect and then process the request without a redirect,
since the invalid request-line might be deliberately crafted to bypass
security filters along the request chain.
The most common form of request-target is the origin-form.
When making a request directly to an origin server, other than a CONNECT
or server-wide OPTIONS request (as detailed below),
a client MUST send only the absolute path and query components of
the target URI as the request-target.
If the target URI's path component is empty, the client MUST send
"/" as the path within the origin-form of request-target.
A Host header field is also sent, as defined in
Section 5.4 of .
For example, a client wishing to retrieve a representation of the resource
identified as
directly from the origin server would open (or reuse) a TCP connection
to port 80 of the host "www.example.org" and send the lines:
followed by the remainder of the request message.
When making a request to a proxy, other than a CONNECT or server-wide
OPTIONS request (as detailed below), a client MUST send the target URI
in absolute-form as the request-target.
The proxy is requested to either service that request from a valid cache,
if possible, or make the same request on the client's behalf to either
the next inbound proxy server or directly to the origin server indicated
by the request-target. Requirements on such "forwarding" of messages are
defined in Section 5.5 of .
An example absolute-form of request-line would be:
To allow for transition to the absolute-form for all requests in some
future version of HTTP, a server MUST accept the absolute-form
in requests, even though HTTP/1.1 clients will only send them in requests
to proxies.
The authority-form of request-target is only used for
CONNECT requests (Section 7.3.6 of ).
When making a CONNECT request to establish a
tunnel through one or more proxies, a client MUST send only the target
URI's authority component (excluding any userinfo and its "@" delimiter) as
the request-target. For example,
The asterisk-form of request-target is only used for a server-wide
OPTIONS request (Section 7.3.7 of ).
When a client wishes to request OPTIONS
for the server as a whole, as opposed to a specific named resource of
that server, the client MUST send only "*" (%x2A) as the request-target.
For example,
If a proxy receives an OPTIONS request with an absolute-form of
request-target in which the URI has an empty path and no query component,
then the last proxy on the request chain MUST send a request-target
of "*" when it forwards the request to the indicated origin server.
For example, the request
would be forwarded by the final proxy as
after connecting to port 8001 of host "www.example.org".
Since the request-target often contains only part of the user agent's
target URI, a server reconstructs the intended target as an
effective request URI to properly service the request
(Section 5.3 of ).
If the request-target is in absolute-form,
the effective request URI is the same as the request-target. Otherwise, the
effective request URI is constructed as follows:
If the server's configuration (or outbound gateway) provides a fixed URI
scheme, that scheme is used for the effective request URI.
Otherwise, if the request is received over a TLS-secured TCP connection,
the effective request URI's scheme is "https"; if not, the scheme is "http".
If the server's configuration (or outbound gateway) provides a fixed URI
authority component, that authority is used for the
effective request URI. If not, then if the request-target is in
authority-form, the effective request URI's authority
component is the same as the request-target.
If not, then if a Host header field is supplied with a
non-empty field-value, the authority component is the same as the
Host field-value. Otherwise, the authority component is assigned
the default name configured for the server and, if the connection's
incoming TCP port number differs from the default port for the effective
request URI's scheme, then a colon (":") and the incoming port number (in
decimal form) are appended to the authority component.
If the request-target is in authority-form or
asterisk-form, the effective request URI's combined
path and query component is empty. Otherwise,
the combined path and query component is the
same as the request-target.
The components of the effective request URI, once determined as above, can
be combined into absolute-URI form by concatenating the
scheme, "://", authority, and combined path and query component.
Example 1: the following message received over an insecure TCP connection
has an effective request URI of
Example 2: the following message received over a TLS-secured TCP connection
has an effective request URI of
Recipients of an HTTP/1.0 request that lacks a Host header
field might need to use heuristics (e.g., examination of the URI path for
something unique to a particular host) in order to guess the
effective request URI's authority component.
The first line of a response message is the status-line, consisting
of the protocol version, a space (SP), the status code, another space,
and ending with an OPTIONAL textual phrase describing the status code.
Although the status-line grammar rule requires that each of the component
elements be separated by a single SP octet, recipients MAY instead parse
on whitespace-delimited word boundaries and, aside from the line
terminator, treat any form of whitespace as the SP separator while
ignoring preceding or trailing whitespace; such whitespace includes one or
more of the following octets: SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
However, lenient parsing can result in response splitting security
vulnerabilities if there are multiple recipients of the message and each
has its own unique interpretation of robustness
(see ).
The status-code element is a 3-digit integer code describing the
result of the server's attempt to understand and satisfy the client's
corresponding request. The rest of the response message is to be
interpreted in light of the semantics defined for that status code.
See Section 9 of for information about the semantics of status codes,
including the classes of status code (indicated by the first digit),
the status codes defined by this specification, considerations for the
definition of new status codes, and the IANA registry.
The reason-phrase element exists for the sole purpose of providing a
textual description associated with the numeric status code, mostly out of
deference to earlier Internet application protocols that were more
frequently used with interactive text clients.
A client SHOULD ignore the reason-phrase content because it is not a
reliable channel for information (it might be translated for a given locale,
overwritten by intermediaries, or discarded when the message is forwarded
via other versions of HTTP).
A server MUST send the space that separates status-code from the
reason-phrase even when the reason-phrase is absent (i.e., the status-line
would end with the three octets SP CR LF).
Each header field consists of a case-insensitive field name
followed by a colon (":"), optional leading whitespace, the field value,
and optional trailing whitespace.
Most HTTP field names and the rules for parsing within field values are
defined in Section 4 of . This section covers the
generic syntax for header field inclusion within, and extraction from,
HTTP/1.1 messages. In addition, the following header fields are defined by
this document because they are specific to HTTP/1.1 message processing:
Header Field NameStatusReferenceConnectionstandardMIME-VersionstandardTEstandardTransfer-EncodingstandardUpgradestandard
Furthermore, the field name "Close" is reserved, since using that name as
an HTTP header field might conflict with the "close" connection option of
the Connection header field
().
Header Field NameProtocolStatusReferenceClosehttpreserved
Messages are parsed using a generic algorithm, independent of the
individual header field names. The contents within a given field value are
not parsed until a later stage of message interpretation (usually after the
message's entire header section has been processed).
No whitespace is allowed between the header field-name and colon.
In the past, differences in the handling of such whitespace have led to
security vulnerabilities in request routing and response handling.
A server MUST reject any received request message that contains
whitespace between a header field-name and colon with a response status code of
400 (Bad Request). A proxy MUST remove any such whitespace
from a response message before forwarding the message downstream.
A field value might be preceded and/or followed by optional whitespace
(OWS); a single SP preceding the field-value is preferred for consistent
readability by humans.
The field value does not include any leading or trailing whitespace: OWS
occurring before the first non-whitespace octet of the field value or after
the last non-whitespace octet of the field value ought to be excluded by
parsers when extracting the field value from a header field.
Historically, HTTP header field values could be extended over multiple
lines by preceding each extra line with at least one space or horizontal
tab (obs-fold). This specification deprecates such line folding except
within the message/http media type
().
A sender MUST NOT generate a message that includes line folding
(i.e., that has any field-value that contains a match to the
obs-fold rule) unless the message is intended for packaging
within the message/http media type.
A server that receives an obs-fold in a request message that
is not within a message/http container MUST either reject the message by
sending a 400 (Bad Request), preferably with a
representation explaining that obsolete line folding is unacceptable, or
replace each received obs-fold with one or more
SP octets prior to interpreting the field value or
forwarding the message downstream.
A proxy or gateway that receives an obs-fold in a response
message that is not within a message/http container MUST either discard
the message and replace it with a 502 (Bad Gateway)
response, preferably with a representation explaining that unacceptable
line folding was received, or replace each received obs-fold
with one or more SP octets prior to interpreting the field
value or forwarding the message downstream.
A user agent that receives an obs-fold in a response message
that is not within a message/http container MUST replace each received
obs-fold with one or more SP octets prior to
interpreting the field value.
The message body (if any) of an HTTP message is used to carry the payload
body (Section 6.3.3 of ) of that request or response. The
message body is identical to the payload body unless a transfer coding has
been applied, as described in .
The rules for determining when a message body is present in an HTTP/1.1
message differ for requests and responses.
The presence of a message body in a request is signaled by a
Content-Length or Transfer-Encoding header
field. Request message framing is independent of method semantics,
even if the method does not define any use for a message body.
The presence of a message body in a response depends on both the request
method to which it is responding and the response status code (), and corresponds to when a payload body is
allowed; see Section 6.3.3 of .
The Transfer-Encoding header field lists the transfer coding names
corresponding to the sequence of transfer codings that have been
(or will be) applied to the payload body in order to form the message body.
Transfer codings are defined in .
Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
MIME, which was designed to enable safe transport of binary data over a
7-bit transport service (, Section 6).
However, safe transport has a different focus for an 8bit-clean transfer
protocol. In HTTP's case, Transfer-Encoding is primarily intended to
accurately delimit a dynamically generated payload and to distinguish
payload encodings that are only applied for transport efficiency or
security from those that are characteristics of the selected resource.
A recipient MUST be able to parse the chunked transfer coding
() because it plays a crucial role in
framing messages when the payload body size is not known in advance.
A sender MUST NOT apply chunked more than once to a message body
(i.e., chunking an already chunked message is not allowed).
If any transfer coding other than chunked is applied to a request payload
body, the sender MUST apply chunked as the final transfer coding to
ensure that the message is properly framed.
If any transfer coding other than chunked is applied to a response payload
body, the sender MUST either apply chunked as the final transfer coding
or terminate the message by closing the connection.
For example,
indicates that the payload body has been compressed using the gzip
coding and then chunked using the chunked coding while forming the
message body.
Unlike Content-Encoding (Section 6.1.2 of ),
Transfer-Encoding is a property of the message, not of the representation, and
any recipient along the request/response chain MAY decode the received
transfer coding(s) or apply additional transfer coding(s) to the message
body, assuming that corresponding changes are made to the Transfer-Encoding
field-value. Additional information about the encoding parameters can be
provided by other header fields not defined by this specification.
Transfer-Encoding MAY be sent in a response to a HEAD request or in a
304 (Not Modified) response (Section 9.4.5 of ) to a GET request,
neither of which includes a message body,
to indicate that the origin server would have applied a transfer coding
to the message body if the request had been an unconditional GET.
This indication is not required, however, because any recipient on
the response chain (including the origin server) can remove transfer
codings when they are not needed.
A server MUST NOT send a Transfer-Encoding header field in any response
with a status code of
1xx (Informational) or 204 (No Content).
A server MUST NOT send a Transfer-Encoding header field in any
2xx (Successful) response to a CONNECT request (Section 7.3.6 of ).
Transfer-Encoding was added in HTTP/1.1. It is generally assumed that
implementations advertising only HTTP/1.0 support will not understand
how to process a transfer-encoded payload.
A client MUST NOT send a request containing Transfer-Encoding unless it
knows the server will handle HTTP/1.1 (or later) requests; such knowledge
might be in the form of specific user configuration or by remembering the
version of a prior received response.
A server MUST NOT send a response containing Transfer-Encoding unless
the corresponding request indicates HTTP/1.1 (or later).
A server that receives a request message with a transfer coding it does
not understand SHOULD respond with 501 (Not Implemented).
When a message does not have a Transfer-Encoding header
field, a Content-Length header field can provide the anticipated size,
as a decimal number of octets, for a potential payload body.
For messages that do include a payload body, the Content-Length field-value
provides the framing information necessary for determining where the body
(and message) ends. For messages that do not include a payload body, the
Content-Length indicates the size of the selected representation
(Section 6.2.4 of ).
Note: HTTP's use of Content-Length for message framing differs
significantly from the same field's use in MIME, where it is an optional
field used only within the "message/external-body" media-type.
The length of a message body is determined by one of the following
(in order of precedence):
Any response to a HEAD request and any response with a
1xx (Informational), 204 (No Content), or
304 (Not Modified) status code is always
terminated by the first empty line after the header fields, regardless of
the header fields present in the message, and thus cannot contain a
message body.
Any 2xx (Successful) response to a CONNECT request implies that the
connection will become a tunnel immediately after the empty line that
concludes the header fields. A client MUST ignore any
Content-Length or Transfer-Encoding header
fields received in such a message.
If a Transfer-Encoding header field is present
and the chunked transfer coding ()
is the final encoding, the message body length is determined by reading
and decoding the chunked data until the transfer coding indicates the
data is complete.
If a Transfer-Encoding header field is present in a
response and the chunked transfer coding is not the final encoding, the
message body length is determined by reading the connection until it is
closed by the server.
If a Transfer-Encoding header field is present in a request and the
chunked transfer coding is not the final encoding, the message body
length cannot be determined reliably; the server MUST respond with
the 400 (Bad Request) status code and then close the connection.
If a message is received with both a Transfer-Encoding
and a Content-Length header field, the Transfer-Encoding
overrides the Content-Length. Such a message might indicate an attempt to
perform request smuggling () or
response splitting () and ought to be
handled as an error. A sender MUST remove the received Content-Length
field prior to forwarding such a message downstream.
If a message is received without Transfer-Encoding and with
either multiple Content-Length header fields having
differing field-values or a single Content-Length header field having an
invalid value, then the message framing is invalid and
the recipient MUST treat it as an unrecoverable error.
If this is a request message, the server MUST respond with
a 400 (Bad Request) status code and then close the connection.
If this is a response message received by a proxy,
the proxy MUST close the connection to the server, discard the received
response, and send a 502 (Bad Gateway) response to the
client.
If this is a response message received by a user agent,
the user agent MUST close the connection to the server and discard the
received response.
If a valid Content-Length header field is present without
Transfer-Encoding, its decimal value defines the
expected message body length in octets.
If the sender closes the connection or the recipient times out before the
indicated number of octets are received, the recipient MUST consider
the message to be incomplete and close the connection.
If this is a request message and none of the above are true, then the
message body length is zero (no message body is present).
Otherwise, this is a response message without a declared message body
length, so the message body length is determined by the number of octets
received prior to the server closing the connection.
Since there is no way to distinguish a successfully completed,
close-delimited message from a partially received message interrupted
by network failure, a server SHOULD generate encoding or
length-delimited messages whenever possible. The close-delimiting
feature exists primarily for backwards compatibility with HTTP/1.0.
A server MAY reject a request that contains a message body but
not a Content-Length by responding with
411 (Length Required).
Unless a transfer coding other than chunked has been applied,
a client that sends a request containing a message body SHOULD
use a valid Content-Length header field if the message body
length is known in advance, rather than the chunked transfer coding, since some
existing services respond to chunked with a 411 (Length Required)
status code even though they understand the chunked transfer coding. This
is typically because such services are implemented via a gateway that
requires a content-length in advance of being called and the server
is unable or unwilling to buffer the entire request before processing.
A user agent that sends a request containing a message body MUST send a
valid Content-Length header field if it does not know the
server will handle HTTP/1.1 (or later) requests; such knowledge can be in
the form of specific user configuration or by remembering the version of a
prior received response.
If the final response to the last request on a connection has been
completely received and there remains additional data to read, a user agent
MAY discard the remaining data or attempt to determine if that data
belongs as part of the prior response body, which might be the case if the
prior message's Content-Length value is incorrect. A client MUST NOT
process, cache, or forward such extra data as a separate response, since
such behavior would be vulnerable to cache poisoning.
Transfer coding names are used to indicate an encoding
transformation that has been, can be, or might need to be applied to a
payload body in order to ensure "safe transport" through the network.
This differs from a content coding in that the transfer coding is a
property of the message rather than a property of the representation
that is being transferred.
Parameters are in the form of a name=value pair.
All transfer-coding names are case-insensitive and ought to be registered
within the HTTP Transfer Coding registry, as defined in
.
They are used in the TE () and
Transfer-Encoding ()
header fields.
NameDescriptionReferencechunkedTransfer in a series of chunkscompressUNIX "compress" data format deflate"deflate" compressed data () inside
the "zlib" data format ()gzipGZIP file format trailers(reserved)x-compressDeprecated (alias for compress)x-gzipDeprecated (alias for gzip)
Note: the coding name "trailers" is reserved because its use would
conflict with the keyword "trailers" in the TE
header field ().
The chunked transfer coding wraps the payload body in order to transfer it
as a series of chunks, each with its own size indicator, followed by an
OPTIONAL trailer section containing trailer fields. Chunked enables content
streams of unknown size to be transferred as a sequence of length-delimited
buffers, which enables the sender to retain connection persistence and the
recipient to know when it has received the entire message.
The chunk-size field is a string of hex digits indicating the size of
the chunk-data in octets. The chunked transfer coding is complete when a
chunk with a chunk-size of zero is received, possibly followed by a
trailer section, and finally terminated by an empty line.
A recipient MUST be able to parse and decode the chunked transfer coding.
The chunked encoding does not define any parameters. Their presence
SHOULD be treated as an error.
The chunked encoding allows each chunk to include zero or more chunk
extensions, immediately following the chunk-size, for the
sake of supplying per-chunk metadata (such as a signature or hash),
mid-message control information, or randomization of message body size.
The chunked encoding is specific to each connection and is likely to be
removed or recoded by each recipient (including intermediaries) before any
higher-level application would have a chance to inspect the extensions.
Hence, use of chunk extensions is generally limited to specialized HTTP
services such as "long polling" (where client and server can have shared
expectations regarding the use of chunk extensions) or for padding within
an end-to-end secured connection.
A recipient MUST ignore unrecognized chunk extensions.
A server ought to limit the total length of chunk extensions received in a
request to an amount reasonable for the services provided, in the same way
that it applies length limitations and timeouts for other parts of a
message, and generate an appropriate 4xx (Client Error)
response if that amount is exceeded.
A trailer section allows the sender to include additional fields at the end
of a chunked message in order to supply metadata that might be dynamically
generated while the message body is sent, such as a message integrity
check, digital signature, or post-processing status. The proper use and
limitations of trailer fields are defined in Section 4.3 of .
A recipient that decodes and removes the chunked encoding from a message
(e.g., for storage or forwarding to a non-HTTP/1.1 peer) MUST discard
any received trailer fields, store/forward them separately from the header
fields, or selectively merge into the header section only those trailer
fields corresponding to header field definitions that are understood by
the recipient to explicitly permit and define how their corresponding
trailer field value can be safely merged.
A process for decoding the chunked transfer coding
can be represented in pseudo-code as:
The following transfer coding names for compression are defined by
the same algorithm as their corresponding content coding:
See Section 6.1.2.1 of .See Section 6.1.2.2 of .See Section 6.1.2.3 of .
The compression codings do not define any parameters. Their presence
SHOULD be treated as an error.
The "HTTP Transfer Coding Registry" defines the namespace for transfer
coding names. It is maintained at .
Registrations MUST include the following fields:
NameDescriptionPointer to specification text
Names of transfer codings MUST NOT overlap with names of content codings
(Section 6.1.2 of ) unless the encoding transformation is identical, as
is the case for the compression codings defined in
.
The TE header field () uses a
pseudo parameter named "q" as rank value when multiple transfer codings
are acceptable. Future registrations of transfer codings SHOULD NOT
define parameters called "q" (case-insensitively) in order to avoid
ambiguities.
Values to be added to this namespace require IETF Review (see
Section 4.8 of ), and MUST
conform to the purpose of transfer coding defined in this specification.
Use of program names for the identification of encoding formats
is not desirable and is discouraged for future encodings.
The "TE" header field in a request indicates what transfer codings,
besides chunked, the client is willing to accept in response, and
whether or not the client is willing to accept trailer fields in a
chunked transfer coding.
The TE field-value consists of a comma-separated list of transfer coding
names, each allowing for optional parameters (as described in
), and/or the keyword "trailers".
A client MUST NOT send the chunked transfer coding name in TE;
chunked is always acceptable for HTTP/1.1 recipients.
Three examples of TE use are below.
The presence of the keyword "trailers" indicates that the client is willing
to accept trailer fields in a chunked transfer coding, as defined in
, on behalf of itself and any downstream
clients. For requests from an intermediary, this implies that either:
(a) all downstream clients are willing to accept trailer fields in the
forwarded response; or,
(b) the intermediary will attempt to buffer the response on behalf of
downstream recipients.
Note that HTTP/1.1 does not define any means to limit the size of a
chunked response such that an intermediary can be assured of buffering the
entire response.
When multiple transfer codings are acceptable, the client MAY rank the
codings by preference using a case-insensitive "q" parameter (similar to
the qvalues used in content negotiation fields, Section 8.4.1 of ). The rank value
is 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 the TE field-value is empty or if no TE field is present, the only
acceptable transfer coding is chunked. A message with no transfer coding
is always acceptable.
Since the TE header field only applies to the immediate connection,
a sender of TE MUST also send a "TE" connection option within the
Connection header field ()
in order to prevent the TE field from being forwarded by intermediaries
that do not support its semantics.
A server that receives an incomplete request message, usually due to a
canceled request or a triggered timeout exception, MAY send an error
response prior to closing the connection.
A client that receives an incomplete response message, which can occur
when a connection is closed prematurely or when decoding a supposedly
chunked transfer coding fails, MUST record the message as incomplete.
Cache requirements for incomplete responses are defined in
Section 3 of .
If a response terminates in the middle of the header section (before the
empty line is received) and the status code might rely on header fields to
convey the full meaning of the response, then the client cannot assume
that meaning has been conveyed; the client might need to repeat the
request in order to determine what action to take next.
A message body that uses the chunked transfer coding is
incomplete if the zero-sized chunk that terminates the encoding has not
been received. A message that uses a valid Content-Length is
incomplete if the size of the message body received (in octets) is less than
the value given by Content-Length. A response that has neither chunked
transfer coding nor Content-Length is terminated by closure of the
connection and, thus, is considered complete regardless of the number of
message body octets received, provided that the header section was received
intact.
HTTP messaging is independent of the underlying transport- or
session-layer connection protocol(s). HTTP only presumes a reliable
transport with in-order delivery of requests and the corresponding
in-order delivery of responses. The mapping of HTTP request and
response structures onto the data units of an underlying transport
protocol is outside the scope of this specification.
As described in Section 5.2 of , the specific
connection protocols to be used for an HTTP interaction are determined by
client configuration and the target URI.
For example, the "http" URI scheme
(Section 2.5.1 of ) indicates a default connection of TCP
over IP, with a default TCP port of 80, but the client might be
configured to use a proxy via some other connection, port, or protocol.
HTTP implementations are expected to engage in connection management,
which includes maintaining the state of current connections,
establishing a new connection or reusing an existing connection,
processing messages received on a connection, detecting connection
failures, and closing each connection.
Most clients maintain multiple connections in parallel, including
more than one connection per server endpoint.
Most servers are designed to maintain thousands of concurrent connections,
while controlling request queues to enable fair use and detect
denial-of-service attacks.
The "Connection" header field allows the sender to indicate desired
control options for the current connection. In order to avoid confusing
downstream recipients, a proxy or gateway MUST remove or replace any
received connection options before forwarding the message.
When a header field aside from Connection is used to supply control
information for or about the current connection, the sender MUST list
the corresponding field-name within the Connection header field.
A proxy or gateway MUST parse a received Connection
header field before a message is forwarded and, for each
connection-option in this field, remove any header field(s) from
the message with the same name as the connection-option, and then
remove the Connection header field itself (or replace it with the
intermediary's own connection options for the forwarded message).
Hence, the Connection header field provides a declarative way of
distinguishing header fields that are only intended for the
immediate recipient ("hop-by-hop") from those fields that are
intended for all recipients on the chain ("end-to-end"), enabling the
message to be self-descriptive and allowing future connection-specific
extensions to be deployed without fear that they will be blindly
forwarded by older intermediaries.
The Connection header field's value has the following grammar:
Connection options are case-insensitive.
A sender MUST NOT send a connection option corresponding to a header
field that is intended for all recipients of the payload.
For example, Cache-Control is never appropriate as a
connection option (Section 5.2 of ).
The connection options do not always correspond to a header field
present in the message, since a connection-specific header field
might not be needed if there are no parameters associated with a
connection option. In contrast, a connection-specific header field that
is received without a corresponding connection option usually indicates
that the field has been improperly forwarded by an intermediary and
ought to be ignored by the recipient.
When defining new connection options, specification authors ought to survey
existing header field names and ensure that the new connection option does
not share the same name as an already deployed header field.
Defining a new connection option essentially reserves that potential
field-name for carrying additional information related to the
connection option, since it would be unwise for senders to use
that field-name for anything else.
The "close" connection option is defined for a
sender to signal that this connection will be closed after completion of
the response. For example,
in either the request or the response header fields indicates that the
sender is going to close the connection after the current request/response
is complete ().
A client that does not support persistent connections MUST
send the "close" connection option in every request message.
A server that does not support persistent connections MUST
send the "close" connection option in every response message that
does not have a 1xx (Informational) status code.
It is beyond the scope of this specification to describe how connections
are established via various transport- or session-layer protocols.
Each connection applies to only one transport link.
HTTP/1.1 does not include a request identifier for associating a given
request message with its corresponding one or more response messages.
Hence, it relies on the order of response arrival to correspond exactly
to the order in which requests are made on the same connection.
More than one response message per request only occurs when one or more
informational responses (1xx, see Section 9.2 of ) precede a
final response to the same request.
A client that has more than one outstanding request on a connection MUST
maintain a list of outstanding requests in the order sent and MUST
associate each received response message on that connection to the highest
ordered request that has not yet received a final (non-1xx)
response.
If an HTTP/1.1 client receives data on a connection that doesn't have any
outstanding requests, it MUST NOT consider them to be a response to a
not-yet-issued request; it SHOULD close the connection, since message
delimitation is now ambiguous, unless the data consists only of one or
more CRLF (which can be discarded, as per ).
HTTP/1.1 defaults to the use of "persistent connections",
allowing multiple requests and responses to be carried over a single
connection. The "close" connection option is used to signal
that a connection will not persist after the current request/response.
HTTP implementations SHOULD support persistent connections.
A recipient determines whether a connection is persistent or not based on
the most recently received message's protocol version and
Connection header field (if any):
If the "close" connection option is present, the
connection will not persist after the current response; else,If the received protocol is HTTP/1.1 (or later), the connection will
persist after the current response; else,If the received protocol is HTTP/1.0, the "keep-alive" connection
option is present, either the recipient is not a proxy or the
message is a response, and the recipient wishes to honor the
HTTP/1.0 "keep-alive" mechanism, the connection will persist after
the current response; otherwise,The connection will close after the current response.
A client MAY send additional requests on a persistent connection until it
sends or receives a "close" connection option or receives an
HTTP/1.0 response without a "keep-alive" connection option.
In order to remain persistent, all messages on a connection need to
have a self-defined message length (i.e., one not defined by closure
of the connection), as described in .
A server MUST read the entire request message body or close
the connection after sending its response, since otherwise the
remaining data on a persistent connection would be misinterpreted
as the next request. Likewise,
a client MUST read the entire response message body if it intends
to reuse the same connection for a subsequent request.
A proxy server MUST NOT maintain a persistent connection with an
HTTP/1.0 client (see Section 19.7.1 of for
information and discussion of the problems with the Keep-Alive header field
implemented by many HTTP/1.0 clients).
See
for more information on backwards compatibility with HTTP/1.0 clients.
Connections can be closed at any time, with or without intention.
Implementations ought to anticipate the need to recover
from asynchronous close events. The conditions under which a client can
automatically retry a sequence of outstanding requests are defined in
Section 7.2.2 of .
A client that supports persistent connections MAY "pipeline"
its requests (i.e., send multiple requests without waiting for each
response). A server MAY process a sequence of pipelined requests in
parallel if they all have safe methods (Section 7.2.1 of ), but it MUST send
the corresponding responses in the same order that the requests were
received.
A client that pipelines requests SHOULD retry unanswered requests if the
connection closes before it receives all of the corresponding responses.
When retrying pipelined requests after a failed connection (a connection
not explicitly closed by the server in its last complete response), a
client MUST NOT pipeline immediately after connection establishment,
since the first remaining request in the prior pipeline might have caused
an error response that can be lost again if multiple requests are sent on a
prematurely closed connection (see the TCP reset problem described in
).
Idempotent methods (Section 7.2.2 of ) are significant to pipelining
because they can be automatically retried after a connection failure.
A user agent SHOULD NOT pipeline requests after a non-idempotent method,
until the final response status code for that method has been received,
unless the user agent has a means to detect and recover from partial
failure conditions involving the pipelined sequence.
An intermediary that receives pipelined requests MAY pipeline those
requests when forwarding them inbound, since it can rely on the outbound
user agent(s) to determine what requests can be safely pipelined. If the
inbound connection fails before receiving a response, the pipelining
intermediary MAY attempt to retry a sequence of requests that have yet
to receive a response if the requests all have idempotent methods;
otherwise, the pipelining intermediary SHOULD forward any received
responses and then close the corresponding outbound connection(s) so that
the outbound user agent(s) can recover accordingly.
A client ought to limit the number of simultaneous open
connections that it maintains to a given server.
Previous revisions of HTTP gave a specific number of connections as a
ceiling, but this was found to be impractical for many applications. As a
result, this specification does not mandate a particular maximum number of
connections but, instead, encourages clients to be conservative when opening
multiple connections.
Multiple connections are typically used to avoid the "head-of-line
blocking" problem, wherein a request that takes significant server-side
processing and/or has a large payload blocks subsequent requests on the
same connection. However, each connection consumes server resources.
Furthermore, using multiple connections can cause undesirable side effects
in congested networks.
Note that a server might reject traffic that it deems abusive or
characteristic of a denial-of-service attack, such as an excessive number
of open connections from a single client.
Servers will usually have some timeout value beyond which they will
no longer maintain an inactive connection. Proxy servers might make
this a higher value since it is likely that the client will be making
more connections through the same proxy server. The use of persistent
connections places no requirements on the length (or existence) of
this timeout for either the client or the server.
A client or server that wishes to time out SHOULD issue a graceful close
on the connection. Implementations SHOULD constantly monitor open
connections for a received closure signal and respond to it as appropriate,
since prompt closure of both sides of a connection enables allocated system
resources to be reclaimed.
A client, server, or proxy MAY close the transport connection at any
time. For example, a client might have started to send a new request
at the same time that the server has decided to close the "idle"
connection. From the server's point of view, the connection is being
closed while it was idle, but from the client's point of view, a
request is in progress.
A server SHOULD sustain persistent connections, when possible, and allow
the underlying transport's flow-control mechanisms to resolve temporary overloads, rather
than terminate connections with the expectation that clients will retry.
The latter technique can exacerbate network congestion.
A client sending a message body SHOULD monitor
the network connection for an error response while it is transmitting
the request. If the client sees a response that indicates the server does
not wish to receive the message body and is closing the connection, the
client SHOULD immediately cease transmitting the body and close its side
of the connection.
The Connection header field
() provides a "close"
connection option that a sender SHOULD send when it wishes to close
the connection after the current request/response pair.
A client that sends a "close" connection option MUST NOT
send further requests on that connection (after the one containing
"close") and MUST close the connection after reading the
final response message corresponding to this request.
A server that receives a "close" connection option MUST
initiate a close of the connection (see below) after it sends the
final response to the request that contained "close".
The server SHOULD send a "close" connection option
in its final response on that connection. The server MUST NOT process
any further requests received on that connection.
A server that sends a "close" connection option MUST
initiate a close of the connection (see below) after it sends the
response containing "close". The server MUST NOT process
any further requests received on that connection.
A client that receives a "close" connection option MUST
cease sending requests on that connection and close the connection
after reading the response message containing the "close"; if additional
pipelined requests had been sent on the connection, the client SHOULD NOT
assume that they will be processed by the server.
If a server performs an immediate close of a TCP connection, there is a
significant risk that the client will not be able to read the last HTTP
response. If the server receives additional data from the client on a
fully closed connection, such as another request that was sent by the
client before receiving the server's response, the server's TCP stack will
send a reset packet to the client; unfortunately, the reset packet might
erase the client's unacknowledged input buffers before they can be read
and interpreted by the client's HTTP parser.
To avoid the TCP reset problem, servers typically close a connection in
stages. First, the server performs a half-close by closing only the write
side of the read/write connection. The server then continues to read from
the connection until it receives a corresponding close by the client, or
until the server is reasonably certain that its own TCP stack has received
the client's acknowledgement of the packet(s) containing the server's last
response. Finally, the server fully closes the connection.
It is unknown whether the reset problem is exclusive to TCP or might also
be found in other transport connection protocols.
TLS provides a facility for secure connection closure. When a valid
closure alert is received, an implementation can be assured that no
further data will be received on that connection. TLS
implementations MUST initiate an exchange of closure alerts before
closing a connection. A TLS implementation MAY, after sending a
closure alert, close the connection without waiting for the peer to
send its closure alert, generating an "incomplete close". Note that
an implementation which does this MAY choose to reuse the session.
This SHOULD only be done when the application knows (typically
through detecting HTTP message boundaries) that it has received all
the message data that it cares about.
As specified in , any implementation which receives a
connection close without first receiving a valid closure alert (a
"premature close") MUST NOT reuse that session. Note that a
premature close does not call into question the security of the data
already received, but simply indicates that subsequent data might
have been truncated. Because TLS is oblivious to HTTP
request/response boundaries, it is necessary to examine the HTTP data
itself (specifically the Content-Length header) to determine whether
the truncation occurred inside a message or between messages.
When encountering a premature close, a client SHOULD treat as completed
all requests for which it has received as much data as specified in the
Content-Length header.
A client detecting an incomplete close SHOULD recover gracefully. It
MAY resume a TLS session closed in this fashion.
Clients MUST send a closure alert before closing the connection.
Clients which are unprepared to receive any more data MAY choose not
to wait for the server's closure alert and simply close the
connection, thus generating an incomplete close on the server side.
Servers SHOULD be prepared to receive an incomplete close from the client,
since the client can often determine when the end of server data is.
Servers SHOULD be willing to resume TLS sessions closed in this
fashion.
Servers MUST attempt to initiate an exchange of closure alerts with
the client before closing the connection. Servers MAY close the
connection after sending the closure alert, thus generating an
incomplete close on the client side.
The "Upgrade" header field is intended to provide a simple mechanism
for transitioning from HTTP/1.1 to some other protocol on the same
connection.
A client MAY send a list of protocol names in the Upgrade header field
of a request to invite the server to switch to one or more of the named
protocols, in order of descending preference, before sending
the final response. A server MAY ignore a received Upgrade header field
if it wishes to continue using the current protocol on that connection.
Upgrade cannot be used to insist on a protocol change.
Although protocol names are registered with a preferred case,
recipients SHOULD use case-insensitive comparison when matching each
protocol-name to supported protocols.
A server that sends a 101 (Switching Protocols) response
MUST send an Upgrade header field to indicate the new protocol(s) to
which the connection is being switched; if multiple protocol layers are
being switched, the sender MUST list the protocols in layer-ascending
order. A server MUST NOT switch to a protocol that was not indicated by
the client in the corresponding request's Upgrade header field.
A server MAY choose to ignore the order of preference indicated by the
client and select the new protocol(s) based on other factors, such as the
nature of the request or the current load on the server.
A server that sends a 426 (Upgrade Required) response
MUST send an Upgrade header field to indicate the acceptable protocols,
in order of descending preference.
A server MAY send an Upgrade header field in any other response to
advertise that it implements support for upgrading to the listed protocols,
in order of descending preference, when appropriate for a future request.
The following is a hypothetical example sent by a client:
The capabilities and nature of the
application-level communication after the protocol change is entirely
dependent upon the new protocol(s) chosen. However, immediately after
sending the 101 (Switching Protocols) response, the server is expected to continue responding to
the original request as if it had received its equivalent within the new
protocol (i.e., the server still has an outstanding request to satisfy
after the protocol has been changed, and is expected to do so without
requiring the request to be repeated).
For example, if the Upgrade header field is received in a GET request
and the server decides to switch protocols, it first responds
with a 101 (Switching Protocols) message in HTTP/1.1 and
then immediately follows that with the new protocol's equivalent of a
response to a GET on the target resource. This allows a connection to be
upgraded to protocols with the same semantics as HTTP without the
latency cost of an additional round trip. A server MUST NOT switch
protocols unless the received message semantics can be honored by the new
protocol; an OPTIONS request can be honored by any protocol.
The following is an example response to the above hypothetical request:
When Upgrade is sent, the sender MUST also send a
Connection header field ()
that contains an "upgrade" connection option, in order to prevent Upgrade
from being accidentally forwarded by intermediaries that might not implement
the listed protocols. A server MUST ignore an Upgrade header field that
is received in an HTTP/1.0 request.
A client cannot begin using an upgraded protocol on the connection until
it has completely sent the request message (i.e., the client can't change
the protocol it is sending in the middle of a message).
If a server receives both an Upgrade and an Expect header field
with the "100-continue" expectation (Section 8.1.1 of ), the
server MUST send a 100 (Continue) response before sending
a 101 (Switching Protocols) response.
The Upgrade header field only applies to switching protocols on top of the
existing connection; it cannot be used to switch the underlying connection
(transport) protocol, nor to switch the existing communication to a
different connection. For those purposes, it is more appropriate to use a
3xx (Redirection) response (Section 9.4 of ).
This specification only defines the protocol name "HTTP" for use by
the family of Hypertext Transfer Protocols, as defined by the HTTP
version rules of Section 3.5 of and future updates to this
specification. Additional protocol names ought to be registered using the
registration procedure defined in .
NameDescriptionExpected Version TokensReferenceHTTPHypertext Transfer Protocolany DIGIT.DIGIT (e.g, "2.0")Section 3.5 of
The "Hypertext Transfer Protocol (HTTP) Upgrade Token Registry" defines
the namespace for protocol-name tokens used to identify protocols in the
Upgrade header field. The registry is maintained at
.
Each registered protocol name is associated with contact information
and an optional set of specifications that details how the connection
will be processed after it has been upgraded.
Registrations happen on a "First Come First Served" basis (see
Section 4.4 of ) and are subject to the
following rules:
A protocol-name token, once registered, stays registered forever.A protocol-name token is case-insensitive and registered with the
preferred case to be generated by senders.The registration MUST name a responsible party for the
registration.The registration MUST name a point of contact.The registration MAY name a set of specifications associated with
that token. Such specifications need not be publicly available.The registration SHOULD name a set of expected "protocol-version"
tokens associated with that token at the time of registration.The responsible party MAY change the registration at any time.
The IANA will keep a record of all such changes, and make them
available upon request.The IESG MAY reassign responsibility for a protocol token.
This will normally only be used in the case when a
responsible party cannot be contacted.
The message/http media type can be used to enclose a single HTTP request or
response message, provided that it obeys the MIME restrictions for all
"message" types regarding line length and encodings.
messagehttpN/Aversion, msgtype
The HTTP-version number of the enclosed message
(e.g., "1.1"). If not present, the version can be
determined from the first line of the body.
The message type — "request" or "response". If not
present, the type can be determined from the first
line of the body.
only "7bit", "8bit", or "binary" are permittedsee N/AThis specification (see ).N/AN/AN/AN/AN/AN/ASee Authors' Addresses section.COMMONN/ASee Authors' Addresses section.IESG
The application/http media type can be used to enclose a pipeline of one or more
HTTP request or response messages (not intermixed).
applicationhttpN/A
version, msgtype
The HTTP-version number of the enclosed messages
(e.g., "1.1"). If not present, the version can be
determined from the first line of the body.
The message type — "request" or "response". If not
present, the type can be determined from the first
line of the body.
HTTP messages enclosed by this type
are in "binary" format; use of an appropriate
Content-Transfer-Encoding is required when
transmitted via email.
see N/A
This specification (see ).
N/AN/AN/AN/AN/AN/ASee Authors' Addresses section.COMMONN/ASee Authors' Addresses section.IESG
This section is meant to inform developers, information providers, and
users of known security considerations relevant to HTTP message syntax,
parsing, and routing. Security considerations about HTTP semantics and
payloads are addressed in .
Response splitting (a.k.a, CRLF injection) is a common technique, used in
various attacks on Web usage, that exploits the line-based nature of HTTP
message framing and the ordered association of requests to responses on
persistent connections . This technique can be
particularly damaging when the requests pass through a shared cache.
Response splitting exploits a vulnerability in servers (usually within an
application server) where an attacker can send encoded data within some
parameter of the request that is later decoded and echoed within any of the
response header fields of the response. If the decoded data is crafted to
look like the response has ended and a subsequent response has begun, the
response has been split and the content within the apparent second response
is controlled by the attacker. The attacker can then make any other request
on the same persistent connection and trick the recipients (including
intermediaries) into believing that the second half of the split is an
authoritative answer to the second request.
For example, a parameter within the request-target might be read by an
application server and reused within a redirect, resulting in the same
parameter being echoed in the Location header field of the
response. If the parameter is decoded by the application and not properly
encoded when placed in the response field, the attacker can send encoded
CRLF octets and other content that will make the application's single
response look like two or more responses.
A common defense against response splitting is to filter requests for data
that looks like encoded CR and LF (e.g., "%0D" and "%0A"). However, that
assumes the application server is only performing URI decoding, rather
than more obscure data transformations like charset transcoding, XML entity
translation, base64 decoding, sprintf reformatting, etc. A more effective
mitigation is to prevent anything other than the server's core protocol
libraries from sending a CR or LF within the header section, which means
restricting the output of header fields to APIs that filter for bad octets
and not allowing application servers to write directly to the protocol
stream.
Request smuggling () is a technique that exploits
differences in protocol parsing among various recipients to hide additional
requests (which might otherwise be blocked or disabled by policy) within an
apparently harmless request. Like response splitting, request smuggling
can lead to a variety of attacks on HTTP usage.
This specification has introduced new requirements on request parsing,
particularly with regard to message framing in
, to reduce the effectiveness of
request smuggling.
HTTP does not define a specific mechanism for ensuring message integrity,
instead relying on the error-detection ability of underlying transport
protocols and the use of length or chunk-delimited framing to detect
completeness. Additional integrity mechanisms, such as hash functions or
digital signatures applied to the content, can be selectively added to
messages via extensible metadata header fields. Historically, the lack of
a single integrity mechanism has been justified by the informal nature of
most HTTP communication. However, the prevalence of HTTP as an information
access mechanism has resulted in its increasing use within environments
where verification of message integrity is crucial.
User agents are encouraged to implement configurable means for detecting
and reporting failures of message integrity such that those means can be
enabled within environments for which integrity is necessary. For example,
a browser being used to view medical history or drug interaction
information needs to indicate to the user when such information is detected
by the protocol to be incomplete, expired, or corrupted during transfer.
Such mechanisms might be selectively enabled via user agent extensions or
the presence of message integrity metadata in a response.
At a minimum, user agents ought to provide some indication that allows a
user to distinguish between a complete and incomplete response message
() when such verification is desired.
HTTP relies on underlying transport protocols to provide message
confidentiality when that is desired. HTTP has been specifically designed
to be independent of the transport protocol, such that it can be used
over many different forms of encrypted connection, with the selection of
such transports being identified by the choice of URI scheme or within
user agent configuration.
The "https" scheme can be used to identify resources that require a
confidential connection, as described in Section 2.5.2 of .
The change controller for the following registrations is:
"IETF (iesg@ietf.org) - Internet Engineering Task Force".
Please update the "Hypertext Transfer Protocol (HTTP) Header Field
Registry" registry at with the
header field names listed in the two tables of .
Please update the "Media Types" registry at
with the registration information in
and
for the media types
"message/http" and "application/http", respectively.
Please update the "HTTP Transfer Coding Registry" at
with the registration procedure of
and the content coding names summarized in the table of
.
Please update the
"Hypertext Transfer Protocol (HTTP) Upgrade Token Registry" at
with the registration procedure of
and the upgrade token names summarized in the table of
.
HTTP SemanticsAdobe345 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/Uniform 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/Augmented BNF for Syntax Specifications: ABNFBrandenburg InternetWorkingdcrocker@bbiw.netTHUS plc.paul.overell@thus.netCase-Sensitive String Support in ABNFpkyzivat@alum.mit.eduKey words for use in RFCs to Indicate Requirement LevelsHarvard Universitysob@harvard.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.eduThe Transport Layer Security (TLS) Protocol Version 1.3A Technique for High-Performance Data CompressionHypertext 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.orgMultipurpose 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.comMultipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and ExamplesInnosoft International, Inc.ned@innosoft.comFirst Virtual Holdingsnsb@nsb.fv.comHypertext 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.orgMIME Encapsulation of Aggregate Documents, such as HTML (MHTML)Stockholm University and KTHjpalme@dsv.su.seMicrosoft Corporationalexhop@microsoft.comLotus Development CorporationShelness@lotus.comstef@nma.comInternet Message FormatQualcomm IncorporatedHypertext 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.deGuidelines for Writing an IANA Considerations Section in RFCsDivide and Conquer - HTTP Response Splitting, Web Cache Poisoning Attacks, and Related TopicsSanctum, Inc.HTTP Request SmugglingErratum ID 4667RFC ErrataIn the collected ABNF below, list rules are expanded as per Section 12 of .
HTTP/1.1 uses many of the constructs defined for the
Internet Message Format and the Multipurpose
Internet Mail Extensions (MIME) to
allow a message body to be transmitted in an open variety of
representations and with extensible header fields. However, RFC 2045
is focused only on email; applications of HTTP have many characteristics
that differ from email; hence, HTTP has features that differ from MIME.
These differences were carefully chosen to optimize performance over binary
connections, to allow greater freedom in the use of new media types, to
make date comparisons easier, and to acknowledge the practice of some early
HTTP servers and clients.
This appendix describes specific areas where HTTP differs from MIME.
Proxies and gateways to and from strict MIME environments need to be
aware of these differences and provide the appropriate conversions
where necessary.
HTTP is not a MIME-compliant protocol. However, messages can
include a single MIME-Version header field to indicate what
version of the MIME protocol was used to construct the message. Use
of the MIME-Version header field indicates that the message is in
full conformance with the MIME protocol (as defined in ).
Senders are responsible for ensuring full conformance (where
possible) when exporting HTTP messages to strict MIME environments.
MIME requires that an Internet mail body part be converted to canonical
form prior to being transferred, as described in Section 4 of . Section 6.1.1.2 of
describes the forms allowed for subtypes of the "text"
media type when transmitted over HTTP. requires
that content with a type of "text" represent line breaks as CRLF and
forbids the use of CR or LF outside of line break sequences. HTTP allows
CRLF, bare CR, and bare LF to indicate a line break within text content.
A proxy or gateway from HTTP to a strict MIME
environment ought to translate all line breaks within text media
types to the RFC 2049 canonical form of CRLF. Note, however,
this might be complicated by the presence of a Content-Encoding
and by the fact that HTTP allows the use of some charsets
that do not use octets 13 and 10 to represent CR and LF, respectively.
Conversion will break any cryptographic
checksums applied to the original content unless the original content
is already in canonical form. Therefore, the canonical form is
recommended for any content that uses such checksums in HTTP.
HTTP/1.1 uses a restricted set of date formats (Section 10.1.1.1 of ) to
simplify the process of date comparison. Proxies and gateways from
other protocols ought to ensure that any Date header field
present in a message conforms to one of the HTTP/1.1 formats and rewrite
the date if necessary.
MIME does not include any concept equivalent to HTTP/1.1's
Content-Encoding header field. Since this acts as a modifier
on the media type, proxies and gateways from HTTP to MIME-compliant
protocols ought to either change the value of the Content-Type
header field or decode the representation before forwarding the message.
(Some experimental applications of Content-Type for Internet mail have used
a media-type parameter of ";conversions=<content-coding>" to perform
a function equivalent to Content-Encoding. However, this parameter is
not part of the MIME standards).
HTTP does not use the Content-Transfer-Encoding field of MIME.
Proxies and gateways from MIME-compliant protocols to HTTP need to remove
any Content-Transfer-Encoding prior to delivering the response message to
an HTTP client.
Proxies and gateways from HTTP to MIME-compliant protocols are
responsible for ensuring that the message is in the correct format
and encoding for safe transport on that protocol, where "safe
transport" is defined by the limitations of the protocol being used.
Such a proxy or gateway ought to transform and label the data with an
appropriate Content-Transfer-Encoding if doing so will improve the
likelihood of safe transport over the destination protocol.
HTTP implementations that share code with MHTML
implementations need to be aware of MIME line length limitations.
Since HTTP does not have this limitation, HTTP does not fold long lines.
MHTML messages being transported by HTTP follow all conventions of MHTML,
including line length limitations and folding, canonicalization, etc.,
since HTTP transfers message-bodies as payload and, aside from the
"multipart/byteranges" type (Section 6.3.5 of ), does not interpret
the content or any MIME header lines that might be contained therein.
HTTP has been in use since 1990. The first version, later referred to as
HTTP/0.9, was a simple protocol for hypertext data transfer across the
Internet, using only a single request method (GET) and no metadata.
HTTP/1.0, as defined by , added a range of request
methods and MIME-like messaging, allowing for metadata to be transferred
and modifiers placed on the request/response semantics. However,
HTTP/1.0 did not sufficiently take into consideration the effects of
hierarchical proxies, caching, the need for persistent connections, or
name-based virtual hosts. The proliferation of incompletely implemented
applications calling themselves "HTTP/1.0" further necessitated a
protocol version change in order for two communicating applications
to determine each other's true capabilities.
HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
requirements that enable reliable implementations, adding only
those features that can either be safely ignored by an HTTP/1.0
recipient or only be sent when communicating with a party advertising
conformance with HTTP/1.1.
HTTP/1.1 has been designed to make supporting previous versions easy.
A general-purpose HTTP/1.1 server ought to be able to understand any valid
request in the format of HTTP/1.0, responding appropriately with an
HTTP/1.1 message that only uses features understood (or safely ignored) by
HTTP/1.0 clients. Likewise, an HTTP/1.1 client can be expected to
understand any valid HTTP/1.0 response.
Since HTTP/0.9 did not support header fields in a request, there is no
mechanism for it to support name-based virtual hosts (selection of resource
by inspection of the Host header field).
Any server that implements name-based virtual hosts ought to disable
support for HTTP/0.9. Most requests that appear to be HTTP/0.9 are, in
fact, badly constructed HTTP/1.x requests caused by a client failing to
properly encode the request-target.
This section summarizes major differences between versions HTTP/1.0
and HTTP/1.1.
The requirements that clients and servers support the Host
header field (Section 5.4 of ), report an error if it is
missing from an HTTP/1.1 request, and accept absolute URIs
()
are among the most important changes defined by HTTP/1.1.
Older HTTP/1.0 clients assumed a one-to-one relationship of IP
addresses and servers; there was no other established mechanism for
distinguishing the intended server of a request than the IP address
to which that request was directed. The Host header field was
introduced during the development of HTTP/1.1 and, though it was
quickly implemented by most HTTP/1.0 browsers, additional requirements
were placed on all HTTP/1.1 requests in order to ensure complete
adoption. At the time of this writing, most HTTP-based services
are dependent upon the Host header field for targeting requests.
In HTTP/1.0, each connection is established by the client prior to the
request and closed by the server after sending the response. However, some
implementations implement the explicitly negotiated ("Keep-Alive") version
of persistent connections described in Section 19.7.1 of .
Some clients and servers might wish to be compatible with these previous
approaches to persistent connections, by explicitly negotiating for them
with a "Connection: keep-alive" request header field. However, some
experimental implementations of HTTP/1.0 persistent connections are faulty;
for example, if an HTTP/1.0 proxy server doesn't understand
Connection, it will erroneously forward that header field
to the next inbound server, which would result in a hung connection.
One attempted solution was the introduction of a Proxy-Connection header
field, targeted specifically at proxies. In practice, this was also
unworkable, because proxies are often deployed in multiple layers, bringing
about the same problem discussed above.
As a result, clients are encouraged not to send the Proxy-Connection header
field in any requests.
Clients are also encouraged to consider the use of Connection: keep-alive
in requests carefully; while they can enable persistent connections with
HTTP/1.0 servers, clients using them will need to monitor the
connection for "hung" requests (which indicate that the client ought stop
sending the header field), and this mechanism ought not be used by clients
at all when a proxy is being used.
HTTP/1.1 introduces the Transfer-Encoding header field
().
Transfer codings need to be decoded prior to forwarding an HTTP message
over a MIME-compliant protocol.
Most of the sections introducing HTTP's design goals, history, architecture,
conformance criteria, protocol versioning, URIs, message routing, and
header fields have been moved to .
This document has been reduced to just the messaging syntax and
connection management requirements specific to HTTP/1.1.
Trailer field semantics now transcend the specifics of chunked encoding.
The decoding algorithm for chunked () has
been updated to encourage storage/forwarding of trailer fields separately
from the header section, to only allow merging into the header section if
the recipient knows the corresponding field definition permits and defines
how to merge, and otherwise to discard the trailer fields instead of
merging. The trailer part is now called the trailer section to be more
consistent with the header section and more distinct from a body part
().
In the ABNF for chunked extensions, re-introduced (bad) whitespace around
";" and "=" (). Whitespace was removed
in , but that change was found to break existing
implementations (see ).
Disallowed transfer coding parameters called "q" in order to avoid
conflicts with the use of ranks in the TE header field
().
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.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 move all core HTTP semantics into :
Moved introduction, architecture, conformance, and ABNF extensions from
RFC 7230 (Messaging) to
semantics .Moved discussion of MIME differences from
RFC 7231 (Semantics) to
since they mostly cover transforming 1.1 messages.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.Cite RFC 8126 instead of RFC 5226 ()Resolved erratum 4779, no change needed here (, )In , fixed prose claiming transfer parameters allow bare names (, )Resolved erratum 4225, no change needed here (, )Replace "response code" with "response status code" (, )In , clarify statement about HTTP/1.0 keep-alive (, )In , re-introduce (bad) whitespace around ";" and "=" (,
, )In , state that transfer codings should not use parameters named "q" (, )In , mark coding name "trailers" as reserved in the IANA registry ()In , explain why the reason phrase should be ignored by clients ().Add to explain how request/response correlation is performed ()In , caution against treating data on a connection as part of a not-yet-issued request ()In , remove the predefined codings from the ABNF and make it generic instead ()Use RFC 7405 ABNF notation for case-sensitive string constants ()In , clarify that protocol-name is to be matched case-insensitively ()In , add leading optional whitespace to obs-fold ABNF (, )In , add clarifications about empty reason phrases ()Move discussion of retries from into ()In , the trailer part has been renamed the trailer section (for consistency with the header section) and trailers are no longer merged as header fields by default, but rather can be discarded, kept separate from header fields, or merged with header fields only if understood and defined as being mergeable ()In and related Sections, move the trailing CRLF from the line grammars into the message format ()Moved down ()In , use 'websocket' instead of 'HTTP/2.0' in examples ()Move version non-specific text from into semantics as "payload body" ()In , add text from RFC 2818 ()
See Appendix "Acknowledgments" of .