Extensible Prioritization Scheme for HTTP
draft-ietf-httpbis-priority-00

HTTP                                                              K. Oku
Internet-Draft                                                    Fastly
Intended status: Standards Track                               L. Pardue
Expires: September 6, 2020                                    Cloudflare
                                                          March 05, 2020

               Extensible Prioritization Scheme for HTTP
                     draft-ietf-httpbis-priority-00

Abstract

   This 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.

Note to Readers

   _RFC EDITOR: please remove this section before publication_

   Discussion 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/ [1].

   Working Group information can be found at https://httpwg.org/ [2];
   source code and issues list for this draft can be found at
   https://github.com/httpwg/http-extensions/labels/priorities [3].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any

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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 6, 2020.

Copyright Notice

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   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Motivation for Replacing HTTP/2 Priorities  . . . . . . . . .   4
     2.1.  Disabling HTTP/2 Priorities . . . . . . . . . . . . . . .   5
   3.  Priority Parameters . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Urgency . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Incremental . . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Defining New Parameters . . . . . . . . . . . . . . . . .   8
   4.  The Priority HTTP Header Field  . . . . . . . . . . . . . . .   8
   5.  Reprioritization  . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  HTTP/2 PRIORITY_UPDATE Frame  . . . . . . . . . . . . . .   9
     5.2.  HTTP/3 PRIORITY_UPDATE Frame  . . . . . . . . . . . . . .  10
   6.  Merging Client- and Server-Driven Parameters  . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
     7.1.  Fairness  . . . . . . . . . . . . . . . . . . . . . . . .  12
       7.1.1.  Coalescing Intermediaries . . . . . . . . . . . . . .  12
       7.1.2.  HTTP/1.x Back Ends  . . . . . . . . . . . . . . . . .  13
       7.1.3.  Intentional Introduction of Unfairness  . . . . . . .  14
   8.  Considerations  . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Why use an End-to-End Header Field? . . . . . . . . . . .  14
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     10.2.  Informative References . . . . . . . . . . . . . . . . .  16
     10.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  17

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   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  17
     B.1.  Since draft-kazuho-httpbis-priority-04  . . . . . . . . .  18
     B.2.  Since draft-kazuho-httpbis-priority-03  . . . . . . . . .  18
     B.3.  Since draft-kazuho-httpbis-priority-02  . . . . . . . . .  18
     B.4.  Since draft-kazuho-httpbis-priority-01  . . . . . . . . .  18
     B.5.  Since draft-kazuho-httpbis-priority-00  . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   It is common for an HTTP ([RFC7230]) 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 ([RFC7540]) 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 Section 2.

   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.

1.1.  Notational Conventions

   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 [RFC2119].

   The terms sh-token and sh-boolean are imported from
   [STRUCTURED-HEADERS].

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   Example HTTP requests and responses use the HTTP/2-style formatting
   from [RFC7540].

   This document uses the variable-length integer encoding from
   [I-D.ietf-quic-transport].

2.  Motivation for Replacing HTTP/2 Priorities

   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

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   about the discovery of several resource exhaustion vectors affecting
   multiple HTTP/2 implementations.  One attack, [CVE-2019-9513] 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
   [I-D.ietf-quic-http] 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.

2.1.  Disabling HTTP/2 Priorities

   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
   [RFC7540]; 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 (Section 5.1), but MAY continue
   sending the Priority header field (Section 4), as it is an end-to-end

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   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.

3.  Priority Parameters

   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 (Section 4) 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 (Section 5.1 and Section 5.2) 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 ([STRUCTURED-HEADERS]).

   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 Section 6.

   Unknown parameters, parameters with out-of-range values or values of
   unexpected types MUST be ignored.

3.1.  Urgency

   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 1.

   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.

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   The following example shows a request for a CSS file with the urgency
   set to "0":

   :method = GET
   :scheme = https
   :authority = example.net
   :path = /style.css
   priority = u=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 Section 6).

   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.

3.2.  Incremental

   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".

   :method = GET
   :scheme = https
   :authority = example.net
   :path = /image.jpg
   priority = u=5, i

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3.3.  Defining New Parameters

   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.

4.  The Priority HTTP Header Field

   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 Section 6).

   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 ([RFC7234]), 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).

5.  Reprioritization

   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".

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   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
   ([I-D.ietf-quic-http], Section 6.2.1).

   This document specifies a new PRIORITY_UPDATE frame type for HTTP/2
   ([RFC7540]) and HTTP/3 ([I-D.ietf-quic-http]) 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.

5.1.  HTTP/2 PRIORITY_UPDATE Frame

   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 ([RFC7540], Section 4.1) in the
   PRIORITY_UPDATE frame header MUST be zero (0x0).

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +---------------------------------------------------------------+
    |R|                        Stream ID (31)                       |
    +---------------------------------------------------------------+
    |                   Priority Field Value (*)                  ...
    +---------------------------------------------------------------+

              Figure 1: HTTP/2 PRIORITY_UPDATE Frame Payload

   The PRIORITY_UPDATE frame payload has the following fields:

   R: 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.

   Stream ID:  A 31-bit stream identifier for the stream that is the
      target of the priority update.

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   Priority Field Value:  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.

5.2.  HTTP/3 PRIORITY_UPDATE Frame

   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
   ([I-D.ietf-quic-http], Section 6.2.1).

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |T|    Empty    |   Prioritized Element ID (i)                ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   Priority Field Value (*)                  ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 2: HTTP/3 PRIORITY_UPDATE Frame Payload

   The PRIORITY_UPDATE frame payload has the following fields:

   T (Prioritized Element Type):  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.

   Empty:  A seven-bit field that has no semantic value.

   Prioritized Element ID:  The stream ID or push ID that is the target
      of the priority update.

   Priority Field Value:  The priority update value in ASCII text,
      encoded using Structured Headers.

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   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.

6.  Merging Client- and Server-Driven Parameters

   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 [RFC8081] 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 Section 3).

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   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 "true"

   :method = GET
   :scheme = https
   :authority = example.net
   :path = /menu.png
   priority = u=5, i

   and the origin responds with

   :status = 200
   content-type = image/png
   priority = u=1

   the 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.

7.  Security Considerations

7.1.  Fairness

   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).

7.1.1.  Coalescing Intermediaries

   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.

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   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:

   o  Forwarded, X-Forwarded-For ([RFC7239])

   o  Via ([RFC7230], 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.

7.1.2.  HTTP/1.x Back Ends

   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.

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7.1.3.  Intentional Introduction of Unfairness

   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.

   Also, a client MAY use the priority values for making local
   scheduling choices for the requests it initiates.

8.  Considerations

8.1.  Why use an End-to-End Header Field?

   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.

9.  IANA Considerations

   This specification registers the following entry in the Permanent
   Message Header Field Names registry established by [RFC3864]:

   Header field name:  Priority

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   Applicable protocol:  http

   Status:  standard

   Author/change controller:  IETF

   Specification document(s):  This document

   Related information:  n/a

   This specification registers the following entry in the HTTP/2
   Settings registry established by [RFC7540]:

   Name:  SETTINGS_DEPRECATE_HTTP2_PRIORITIES

   Code:  0x9

   Initial value:  0

   Specification:  This document

   This specification registers the following entry in the HTTP/2 Frame
   Type registry established by [RFC7540]:

   Frame Type:  PRIORITY_UPDATE

   Code:  0xF

   Specification:  This document

   This specification registers the following entries in the HTTP/3
   Frame Type registry established by [I-D.ietf-quic-http]:

   Frame Type:  PRIORITY_UPDATE

   Code:  0xF

   Specification:  This document

10.  References

10.1.  Normative References

   [I-D.ietf-quic-http]
              Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", draft-ietf-quic-http-27 (work in progress),
              February 2020.

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   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-27 (work
              in progress), February 2020.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [STRUCTURED-HEADERS]
              Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
              draft-ietf-httpbis-header-structure-15 (work in progress),
              January 2020.

10.2.  Informative References

   [CVE-2019-9513]
              Common Vulnerabilities and Exposures, "CVE-2019-9513",
              March 2019, <https://cve.mitre.org/cgi-bin/
              cvename.cgi?name=CVE-2019-9513>.

   [I-D.lassey-priority-setting]
              Lassey, B. and L. Pardue, "Declaring Support for HTTP/2
              Priorities", draft-lassey-priority-setting-00 (work in
              progress), July 2019.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,
              <https://www.rfc-editor.org/info/rfc3864>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <https://www.rfc-editor.org/info/rfc7234>.

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   [RFC7239]  Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
              RFC 7239, DOI 10.17487/RFC7239, June 2014,
              <https://www.rfc-editor.org/info/rfc7239>.

   [RFC8081]  Lilley, C., "The "font" Top-Level Media Type", RFC 8081,
              DOI 10.17487/RFC8081, February 2017,
              <https://www.rfc-editor.org/info/rfc8081>.

10.3.  URIs

   [1] https://lists.w3.org/Archives/Public/ietf-http-wg/

   [2] https://httpwg.org/

   [3] https://github.com/httpwg/http-extensions/labels/priorities

   [4] http://tools.ietf.org/agenda/83/slides/slides-83-httpbis-5.pdf

   [5] https://github.com/pmeenan/http3-prioritization-proposal

Appendix A.  Acknowledgements

   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 [4].  In https://github.com/pmeenan/http3-
   prioritization-proposal [5], Patrick Meenan advocates for
   representing the priorities using a tuple of urgency and concurrency.
   The ability to deprecate HTTP/2 prioritization is based on
   [I-D.lassey-priority-setting], 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.

Appendix B.  Change Log

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B.1.  Since draft-kazuho-httpbis-priority-04

   o  Minimize semantics of Urgency levels (#1023, #1026)

   o  Reduce guidance about how intermediary implements merging priority
      signals (#1026)

   o  Remove mention of CDN-Loop (#1062)

   o  Editorial changes

   o  Make changes due to WG adoption

   o  Removed outdated Consideration (#118)

B.2.  Since draft-kazuho-httpbis-priority-03

   o  Changed numbering from "[-1,6]" to "[0,7]" (#78)

   o  Replaced priority scheme negotiation with HTTP/2 priority
      deprecation (#100)

   o  Shorten parameter names (#108)

   o  Expand on considerations (#105, #107, #109, #110, #111, #113)

B.3.  Since draft-kazuho-httpbis-priority-02

   o  Consolidation of the problem statement (#61, #73)

   o  Define SETTINGS_PRIORITIES for negotiation (#58, #69)

   o  Define PRIORITY_UPDATE frame for HTTP/2 and HTTP/3 (#51)

   o  Explain fairness issue and mitigations (#56)

B.4.  Since draft-kazuho-httpbis-priority-01

   o  Explain how reprioritization might be supported.

B.5.  Since draft-kazuho-httpbis-priority-00

   o  Expand urgency levels from 3 to 8.

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Authors' Addresses

   Kazuho Oku
   Fastly

   Email: kazuhooku@gmail.com

   Lucas Pardue
   Cloudflare

   Email: lucaspardue.24.7@gmail.com

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