Extensible Prioritization Scheme for HTTPFastlykazuhooku@gmail.comCloudflarelucaspardue.24.7@gmail.com
Applications and Real-Time
HTTPInternet-DraftThis document describes a scheme for prioritizing HTTP responses. This scheme
expresses the priority of each HTTP response using absolute values, rather than
as a relative relationship between a group of HTTP responses.This document defines the Priority header field for communicating the initial
priority in an HTTP version-independent manner, as well as HTTP/2 and HTTP/3
frames for reprioritizing the responses. These share a common format structure
that is designed to provide future extensibility.RFC EDITOR: please remove this section before publicationDiscussion of this draft takes place on the HTTP working group mailing list
(ietf-http-wg@w3.org), which is archived at https://lists.w3.org/Archives/Public/ietf-http-wg/.Working Group information can be found at https://httpwg.org/; source
code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions/labels/priorities.It is common for an HTTP () resource representation to have
relationships to one or more other resources. Clients will often discover these
relationships while processing a retrieved representation, leading to further
retrieval requests. Meanwhile, the nature of the relationship determines
whether the client is blocked from continuing to process locally available
resources. For example, visual rendering of an HTML document could be blocked
by the retrieval of a CSS file that the document refers to. In contrast, inline
images do not block rendering and get drawn incrementally as the chunks of the
images arrive.To provide meaningful presentation of a document at the earliest moment, it is
important for an HTTP server to prioritize the HTTP responses, or the chunks of
those HTTP responses, that it sends.HTTP/2 () provides such a prioritization scheme. A client sends a
series of PRIORITY frames to communicate to the server a “priority tree”; this
represents the client’s preferred ordering and weighted distribution of the
bandwidth among the HTTP responses. However, the design and implementation of
this scheme has been observed to have shortcomings, explained in .This document defines the Priority HTTP header field that can be used by both
client and server to specify the precedence of HTTP responses in a standardized,
extensible, protocol-version-independent, end-to-end format. Along with the
protocol-version-specific frame for reprioritization, this prioritization scheme
acts as a substitute for the original prioritization scheme of HTTP/2.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 .The terms sh-token and sh-boolean are imported from
.Example HTTP requests and responses use the HTTP/2-style formatting from
.This document uses the variable-length integer encoding from
.An important feature of any implementation of a protocol that provides
multiplexing is the ability to prioritize the sending of information. This was
an important realization in the design of HTTP/2. Prioritization is a
difficult problem, so it will always be suboptimal, particularly if one endpoint
operates in ignorance of the needs of its peer.HTTP/2 introduced a complex prioritization signaling scheme that used a
combination of dependencies and weights, formed into an unbalanced tree. This
scheme has suffered from poor deployment and interoperability.The rich flexibility of client-driven HTTP/2 prioritization tree building is
rarely exercised. Experience has shown that clients tend to choose a single
model optimized for a web use case and experiment within the model constraints,
or do nothing at all. Furthermore, many clients build their prioritization tree
in a unique way, which makes it difficult for servers to understand their intent
and act or intervene accordingly.Many HTTP/2 server implementations do not include support for the priority
scheme, some favoring instead bespoke server-driven schemes based on heuristics
and other hints, like the content type of resources and the request generation
order. For example, a server, with knowledge of the document structure, might
want to prioritize the delivery of images that are critical to user experience
above other images, but below the CSS files. Since client trees vary, it is
impossible for the server to determine how such images should be prioritized
against other responses.The HTTP/2 scheme allows intermediaries to coalesce multiple client trees into a
single tree that is used for a single upstream HTTP/2 connection. However, most
intermediaries do not support this. The scheme does not define a method that can
be used by a server to express the priority of a response. Without such a
method, intermediaries cannot coordinate client-driven and server-driven
priorities.HTTP/2 describes denial-of-service considerations for implementations. On
2019-08-13 Netflix issued an advisory notice about the discovery of several
resource exhaustion vectors affecting multiple HTTP/2 implementations. One
attack, aka “Resource Loop”, is based on manipulation of the
priority tree.The HTTP/2 scheme depends on in-order delivery of signals, leading to challenges
in porting the scheme to protocols that do not provide global ordering. For
example, the scheme cannot be used in HTTP/3 without
changing the signal and its processing.Considering the problems with deployment and adaptability to HTTP/3, retaining
the HTTP/2 priority scheme increases the complexity of the entire system without
any evidence that the value it provides offsets that complexity. In fact,
multiple experiments from independent research have shown that simpler schemes
can reach at least equivalent performance characteristics compared to the more
complex HTTP/2 setups seen in practice, at least for the web use case.The problems and insights set out above are motivation for allowing endpoints to
opt out of using the HTTP/2 priority scheme, in favor of using an alternative
such as the scheme defined in this specification. The
SETTINGS_DEPRECATE_HTTP2_PRIORITIES setting described below enables endpoints to
understand their peer’s intention. The value of the parameter MUST
be 0 or 1. Any value other than 0 or 1 MUST be treated as a connection error
(see , Section 5.4.1) of type PROTOCOL_ERROR.Endpoints MUST send this SETTINGS parameter as part of the first SETTINGS frame.
When the peer receives the first SETTINGS frame, it learns the sender has
deprecated the HTTP/2 priority scheme if it receives the
SETTINGS_DEPRECATE_HTTP2_PRIORITIES parameter with the value of 1.A sender MUST NOT change the SETTINGS_DEPRECATE_HTTP2_PRIORITIES parameter value
after the first SETTINGS frame. Detection of a change by a receiver MUST be
treated as a connection error of type PROTOCOL_ERROR.Until the client receives the SETTINGS frame from the server, the client SHOULD
send both the priority signal defined in the HTTP/2 priority scheme and also
that of this prioritization scheme. Once the client learns that the HTTP/2
priority scheme is deprecated, it SHOULD stop sending the HTTP/2 priority
signals. If the client learns that the HTTP/2 priority scheme is not deprecated,
it SHOULD stop sending PRIORITY_UPDATE frames (), but MAY
continue sending the Priority header field (), as it is an
end-to-end signal that might be useful to nodes behind the server that the
client is directly connected to.The SETTINGS frame precedes any priority signal sent from a client in HTTP/2,
so a server can determine if it should respect the HTTP/2 scheme before
building state.The priority information is a sequence of key-value pairs, providing room for
future extensions. Each key-value pair represents a priority parameter.The Priority HTTP header field () is an end-to-end way to
transmit this set of parameters when a request or a response is issued. In order
to reprioritize a request, HTTP-version-specific frames ( and
) are used by clients to transmit the same information on a
single hop. If intermediaries want to specify prioritization on a multiplexed
HTTP connection, they SHOULD use a PRIORITY_UPDATE frame and SHOULD NOT change
the Priority header field.In both cases, the set of priority parameters is encoded as a Structured Headers
Dictionary ().This document defines the urgency(u) and incremental(i) parameters. When
receiving an HTTP request that does not carry these priority parameters, a
server SHOULD act as if their default values were specified. Note that handling
of omitted parameters is different when processing an HTTP response; see
.Unknown parameters, parameters with out-of-range values or values of unexpected
types MUST be ignored.The urgency parameter (u) takes an integer between 0 and 7, in descending
order of priority. This range provides sufficient granularity for prioritizing
responses for ordinary web browsing, at minimal complexity.The value is encoded as an sh-integer. The default value is 3.This parameter indicates the sender’s recommendation, based on the expectation
that the server would transmit HTTP responses in the order of their urgency
values if possible. The smaller the value, the higher the precedence.The following example shows a request for a CSS file with the urgency set to
0:A client that fetches a document that likely consists of multiple HTTP resources
(e.g., HTML) SHOULD assign the default urgency level to the main resource. This
convention allows servers to refine the urgency using
knowledge specific to the web-site (see ).The lowest urgency level (7) is reserved for background tasks such as delivery
of software updates. This urgency level SHOULD NOT be used for fetching
responses that have impact on user interaction.The incremental parameter (i) takes an sh-boolean as the value that indicates
if an HTTP response can be processed incrementally, i.e. provide some
meaningful output as chunks of the response arrive.The default value of the incremental parameter is false (0).A server might distribute the bandwidth of a connection between incremental
responses that share the same urgency, hoping that providing those responses in
parallel would be more helpful to the client than delivering the responses one
by one.If a client makes concurrent requests with the incremental parameter set to
false, there is no benefit serving responses in parallel because the client is
not going to process those responses incrementally. Serving non-incremental
responses one by one, in the order in which those requests were generated is
considered to be the best strategy.The following example shows a request for a JPEG file with the urgency parameter
set to 5 and the incremental parameter set to true.When attempting to extend priorities, care must be taken to ensure any use of
existing parameters are either unchanged or modified in a way that is backwards
compatible for peers that are unaware of the extended meaning.For example, if there is a need to provide more granularity than eight urgency
levels, it would be possible to subdivide the range using an additional
parameter. Implementations that do not recognize the parameter can safely
continue to use the less granular eight levels.Alternatively, the urgency can be augmented. For example, a graphical user agent
could send a visible parameter to indicate if the resource being requested is
within the viewport.The Priority HTTP header field can appear in requests and responses. A client
uses it to specify the priority of the response. A server uses it to inform
the client that the priority was overwritten. An intermediary can use the
Priority information from client requests and server responses to correct or
amend the precedence to suit it (see ).The Priority header field is an end-to-end signal of the request priority from
the client or the response priority from the server.As is the ordinary case for HTTP caching (), a response with a
Priority header field might be cached and re-used for subsequent requests.
When an origin server generates the Priority response header field based on
properties of an HTTP request it receives, the server is expected to control the
cacheability or the applicability of the cached response, by using header fields
that control the caching behavior (e.g., Cache-Control, Vary).After a client sends a request, it may be beneficial to change the priority of
the response. As an example, a web browser might issue a prefetch request for
a JavaScript file with the urgency parameter of the Priority request header
field set to u=7 (background). Then, when the user navigates to a page which
references the new JavaScript file, while the prefetch is in progress, the
browser would send a reprioritization frame with the priority field value
set to u=0.In HTTP/2 and HTTP/3, after a request message is sent on a stream, the stream
transitions to a state that prevents the client from sending additional
frames on the stream. Therefore, a client cannot reprioritize a response by
using the Priority header field. Modifying this behavior would require a
semantic change to the protocol, but this is avoided by restricting the
stream on which a PRIORITY_UPDATE frame can be sent. In HTTP/2 the frame
is on stream zero and in HTTP/3 it is sent on the control stream
(, Section 6.2.1).This document specifies a new PRIORITY_UPDATE frame type for HTTP/2
() and HTTP/3 () which enables
reprioritization. It carries updated priority parameters and references the
target of the reprioritization based on a version-specific identifier; in
HTTP/2 this is the Stream ID, in HTTP/3 this is either the Stream ID or Push ID.Unlike the header field, the reprioritization frame is a hop-by-hop signal.The HTTP/2 PRIORITY_UPDATE frame (type=0xF) carries the stream ID of the
response that is being reprioritized, and the updated priority in ASCII text,
using the same representation as that of the Priority header field value.The Stream Identifier field (, Section 4.1) in the PRIORITY_UPDATE
frame header MUST be zero (0x0).The PRIORITY_UPDATE frame payload has the following fields:
A reserved 1-bit field. The semantics of this bit are undefined, and the bit
MUST remain unset (0x0) when sending and MUST be ignored when receiving.
A 31-bit stream identifier for the stream that is the target of the priority
update.
The priority update value in ASCII text, encoded using Structured Headers.The HTTP/2 PRIORITY_UPDATE frame MUST NOT be sent prior to opening the
stream. If a PRIORITY_UPDATE is received prior to the stream being opened,
it MAY be treated as a connection error of type PROTOCOL_ERROR.TODO: add more description of how to handle things like receiving
PRIORITY_UPDATE on wrong stream, a PRIORITY_UPDATE with an invalid ID, etc.The HTTP/3 PRIORITY_UPDATE frame (type=0xF) carries the identifier of the
element that is being reprioritized, and the updated priority in ASCII text,
using the same representation as that of the Priority header field value.The PRIORITY_UPDATE frame MUST be sent on the control stream
(, Section 6.2.1).The PRIORITY_UPDATE frame payload has the following fields:
A one-bit field indicating the type of element
being prioritized. A value of 0 indicates a reprioritization for a Request
Stream, so the Prioritized Element ID is interpreted as a Stream ID. A
value of 1 indicates a reprioritization for a Push stream, so the Prioritized
Element ID is interpreted as a Push ID.
A seven-bit field that has no semantic value.
The stream ID or push ID that is the target of the priority update.
The priority update value in ASCII text, encoded using Structured Headers.The HTTP/3 PRIORITY_UPDATE frame MUST NOT be sent with an invalid identifier,
including before the request stream has been opened or before a promised
request has been received. If a server receives a PRIORITY_UPDATE specifying
a push ID that has not been promised, it SHOULD be treated as a connection
error of type H3_ID_ERROR.Because the HTTP/3 PRIORITY_UPDATE frame is sent on the control stream and
there are no ordering guarantees between streams, a client that reprioritizes
a request before receiving the response data might cause the server to receive
a PRIORITY_UPDATE for an unknown request. If the request stream ID is within
bidirectional stream limits, the PRIORITY_UPDATE frame SHOULD be buffered
until the stream is opened and applied immediately after the request message
has been processed. Holding PRIORITY_UPDATES consumes extra state on the peer,
although the size of the state is bounded by bidirectional stream limits. There
is no bound on the number of PRIORITY_UPDATES that can be sent, so an
endpoint SHOULD store only the most recently received frame.TODO: add more description of how to handle things like receiving
PRIORITY_UPDATE on wrong stream, a PRIORITY_UPDATE with an invalid ID, etc.It is not always the case that the client has the best understanding of how the
HTTP responses deserve to be prioritized. The server might have additional
information that can be combined with the client’s indicated priority in order
to improve the prioritization of the response. For example, use of an HTML
document might depend heavily on one of the inline images; existence of such
dependencies is typically best known to the server. Or, a server that receives
requests for a font and images with the same urgency might give
higher precedence to the font, so that a visual client can render textual
information at an early moment.An origin can use the Priority response header field to indicate its view on how
an HTTP response should be prioritized. An intermediary that forwards an HTTP
response can use the parameters found in the Priority response header field, in
combination with the client Priority request header field, as input to its
prioritization process. No guidance is provided for merging priorities, this is
left as an implementation decision.Absence of a priority parameter in an HTTP response indicates the server’s
disinterest in changing the client-provided value. This is different from the
logic being defined for the request header field, in which omission of a
priority parameter implies the use of their default values (see ).As a non-normative example, when the client sends an HTTP request with the
urgency parameter set to 5 and the incremental parameter set to trueand the origin responds withthe intermediary might alter its understanding of the urgency from 5 to 1,
because it prefers the server-provided value over the client’s. The incremental
value continues to be true, the value specified by the client, as the server did
not specify the incremental(i) parameter.A client MAY use priority values to make local scheduling choices about the
requests it initiates.As a general guideline, a server SHOULD NOT use priority information for making
schedule decisions across multiple connections, unless it knows that those
connections originate from the same client. Due to this, priority information
conveyed over a non-coalesced HTTP connection (e.g., HTTP/1.1) might go unused.The remainder of this section discusses scenarios where unfairness is
problematic and presents possible mitigations, or where unfairness is desirable.TODO: Discuss if we should add a signal that mitigates this issue. For example,
we might add a SETTINGS parameter that indicates the next hop that the
connection is NOT coalesced (see https://github.com/kazuho/draft-kazuho-httpbis-priority/issues/99).When an intermediary coalesces HTTP requests coming from multiple clients into
one HTTP/2 or HTTP/3 connection going to the backend server, requests that
originate from one client might have higher precedence than those coming from
others.It is sometimes beneficial for the server running behind an intermediary to obey
to the value of the Priority header field. As an example, a resource-constrained
server might defer the transmission of software update files that would have the
background urgency being associated. However, in the worst case, the asymmetry
between the precedence declared by multiple clients might cause responses going
to one end client to be delayed totally after those going to another.In order to mitigate this fairness problem, when a server responds to a request
that is known to have come through an intermediary, the server SHOULD prioritize
the response as if it was assigned the priority of u=1, i
(i.e. round-robin) regardless of the value of the Priority header field being
transmitted, unless the server knows the intermediary is not
coalescing requests from multiple clients.A server can determine if a request came from an intermediary through
configuration, or by consulting if that request contains one of the following
header fields:Forwarded, X-Forwarded-For ()Via (, Section 5.7.1)Responding to requests coming through an intermediary in a round-robin manner
works well when the network bottleneck exists between the intermediary and the
end client, as the intermediary would be buffering the responses and then be
forwarding the chunks of those buffered responses based on the prioritization
scheme it implements. A sophisticated server MAY use a weighted round-robin
reflecting the urgencies expressed in the requests, so that less urgent
responses would receive less bandwidth in case the bottleneck exists between the
server and the intermediary.It is common for CDN infrastructure to support different HTTP versions on the
front end and back end. For instance, the client-facing edge might support
HTTP/2 and HTTP/3 while communication to back end servers is done using
HTTP/1.1. Unlike with connection coalescing, the CDN will “de-mux” requests into
discrete connections to the back end. As HTTP/1.1 and older do not provide a way
to concurrently transmit multiple responses, there is no immediate fairness
issue in protocol. However, back end servers MAY still use client headers for
request scheduling. Back end servers SHOULD only schedule based on client
priority information where that information can be scoped to individual end
clients. Authentication and other session information might provide this
linkability.It is sometimes beneficial to deprioritize the transmission of one connection
over others, knowing that doing so introduces a certain amount of unfairness
between the connections and therefore between the requests served on those
connections.For example, a server might use a scavenging congestion controller on
connections that only convey background priority responses such as software
update images. Doing so improves responsiveness of other connections at the cost
of delaying the delivery of updates.Contrary to the prioritization scheme of HTTP/2 that uses a hop-by-hop frame,
the Priority header field is defined as end-to-end.The rationale is that the Priority header field transmits how each response
affects the client’s processing of those responses, rather than how relatively
urgent each response is to others. The way a client processes a response is a
property associated to that client generating that request. Not that of an
intermediary. Therefore, it is an end-to-end property. How these end-to-end
properties carried by the Priority header field affect the prioritization
between the responses that share a connection is a hop-by-hop issue.Having the Priority header field defined as end-to-end is important for caching
intermediaries. Such intermediaries can cache the value of the Priority header
field along with the response, and utilize the value of the cached header field
when serving the cached response, only because the header field is defined as
end-to-end rather than hop-by-hop.It should also be noted that the use of a header field carrying a textual value
makes the prioritization scheme extensible; see the discussion below. aka “Resource Loop”, is a DoS attack based on manipulation of
the HTTP/2 priority tree. Extensible priorities does not use stream
dependencies, which mitigates this vulnerability.TBD: depending on the outcome of reprioritization discussions, following
paragraphs may change or be removed., Section 5.3.4 describes a scenario where closure of streams in the
priority tree could cause suboptimal prioritization. To avoid this,
states that “an endpoint SHOULD retain stream prioritization state for a period
after streams become closed”. Retaining state for streams no longer counted
towards stream concurrency consumes server resources. Furthermore,
identifies that reprioritization of a closed stream could affect dependents; it
recommends updating the priority tree if sufficient state is stored, which will
also consume server resources. To limit this commitment, it is stated that “The
amount of prioritization state that is retained MAY be limited” and “If a limit
is applied, endpoints SHOULD maintain state for at least as many streams as
allowed by their setting for SETTINGS_MAX_CONCURRENT_STREAMS.”. Extensible
priorities does not use stream dependencies, which minimizes most of the
resource concerns related to this scenario., Section 5.3.4 also presents considerations about the state required
to store priority information about streams in an “idle” state. This state can
be limited by adopting the guidance about concurrency limits described above.
Extensible priorities is subject to a similar consideration because
PRIORITY_UPDATE frames may arrive before the request that they reference. A
server is required to store the information in order to apply the most
up-to-date signal to the request. However, HTTP/3 implementations might have
practical barriers to determining reasonable stream concurrency limits depending
on the information that is available to them from the QUIC transport layer.
TODO: so what can we suggest?This specification registers the following entry in the Permanent Message Header
Field Names registry established by :
Priority
http
standard
IETF
This document
n/aThis specification registers the following entry in the HTTP/2 Settings registry
established by :
SETTINGS_DEPRECATE_HTTP2_PRIORITIES
0x9
0
This documentThis specification registers the following entry in the HTTP/2 Frame Type
registry established by :
PRIORITY_UPDATE
0xF
This documentThis specification registers the following entries in the HTTP/3 Frame Type
registry established by :
PRIORITY_UPDATE
0xF
This documentHypertext Transfer Protocol (HTTP/1.1): Message Syntax and RoutingThe Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document provides an overview of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes related security concerns for implementations.Hypertext Transfer Protocol Version 2 (HTTP/2)This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP), referred to as HTTP version 2 (HTTP/2). HTTP/2 enables a more efficient use of network resources and a reduced perception of latency by introducing header field compression and allowing multiple concurrent exchanges on the same connection. It also introduces unsolicited push of representations from servers to clients.This specification is an alternative to, but does not obsolete, the HTTP/1.1 message syntax. HTTP's existing semantics remain unchanged.Key words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Structured Field Values for HTTPThis document describes a set of data types and associated algorithms that are intended to make it easier and safer to define and handle HTTP header and trailer fields, known as "Structured Fields", "Structured Headers", or "Structured Trailers". It is intended for use by specifications of new HTTP fields that wish to use a common syntax that is more restrictive than traditional HTTP field values.QUIC: A UDP-Based Multiplexed and Secure TransportThis document defines the core of the QUIC transport protocol. Accompanying documents describe QUIC's loss detection and congestion control and the use of TLS for key negotiation. Note to Readers Discussion of this draft takes place on the QUIC working group mailing list (quic@ietf.org (mailto:quic@ietf.org)), which is archived at https://mailarchive.ietf.org/arch/search/?email_list=quic Working Group information can be found at https://github.com/quicwg; source code and issues list for this draft can be found at https://github.com/quicwg/base-drafts/labels/-transport.Hypertext Transfer Protocol Version 3 (HTTP/3)The QUIC transport protocol has several features that are desirable in a transport for HTTP, such as stream multiplexing, per-stream flow control, and low-latency connection establishment. This document describes a mapping of HTTP semantics over QUIC. This document also identifies HTTP/2 features that are subsumed by QUIC, and describes how HTTP/2 extensions can be ported to HTTP/3.CVE-2019-9513Common Vulnerabilities and ExposuresHypertext Transfer Protocol (HTTP/1.1): CachingThe Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.The "font" Top-Level Media TypeThis memo serves to register and document the "font" top-level media type, under which subtypes for representation formats for fonts may be registered. This document also serves as a registration application for a set of intended subtypes, which are representative of some existing subtypes already in use, and currently registered under the "application" tree by their separate registrations.Forwarded HTTP ExtensionThis document defines an HTTP extension header field that allows proxy components to disclose information lost in the proxying process, for example, the originating IP address of a request or IP address of the proxy on the user-agent-facing interface. In a path of proxying components, this makes it possible to arrange it so that each subsequent component will have access to, for example, all IP addresses used in the chain of proxied HTTP requests.This document also specifies guidelines for a proxy administrator to anonymize the origin of a request.Registration Procedures for Message Header FieldsThis specification defines registration procedures for the message header fields used by Internet mail, HTTP, Netnews and other applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Declaring Support for HTTP/2 PrioritiesHTTP/2 provides a prioritization scheme but experience has shown that implementation support varies. This document defines an HTTP/2 setting that endpoints can use as an affirmative signal to indicate their support for HTTP/2 Priorities.Roy Fielding presented the idea of using a header field for representing
priorities in http://tools.ietf.org/agenda/83/slides/slides-83-httpbis-5.pdf.
In https://github.com/pmeenan/http3-prioritization-proposal, Patrick Meenan
advocates for representing the priorities using a tuple of urgency and
concurrency. The ability to deprecate HTTP/2 prioritization is based on
, authored by Brad Lassey and Lucas Pardue, with
modifications based on feedback that was not incorporated into an update to that
document.The motivation for defining an alternative to HTTP/2 priorities is drawn from
discussion within the broad HTTP community. Special thanks to Roberto Peon,
Martin Thomson and Netflix for text that was incorporated explicitly in this
document.In addition to the people above, this document owes a lot to the extensive
discussion in the HTTP priority design team, consisting of Alan Frindell,
Andrew Galloni, Craig Taylor, Ian Swett, Kazuho Oku, Lucas Pardue, Matthew Cox,
Mike Bishop, Roberto Peon, Robin Marx, Roy Fielding.Move text around (#1217, #1218)Editorial change to the default urgency. The value is 3, which was always the
intent of previous changes.Minimize semantics of Urgency levels (#1023, #1026)Reduce guidance about how intermediary implements merging priority signals
(#1026)Remove mention of CDN-Loop (#1062)Editorial changesMake changes due to WG adoptionRemoved outdated Consideration (#118)Changed numbering from [-1,6] to [0,7] (#78)Replaced priority scheme negotiation with HTTP/2 priority deprecation (#100)Shorten parameter names (#108)Expand on considerations (#105, #107, #109, #110, #111, #113)Consolidation of the problem statement (#61, #73)Define SETTINGS_PRIORITIES for negotiation (#58, #69)Define PRIORITY_UPDATE frame for HTTP/2 and HTTP/3 (#51)Explain fairness issue and mitigations (#56)Explain how reprioritization might be supported.Expand urgency levels from 3 to 8.