HTTP                                                              K. Oku
Internet-Draft                                                    Fastly
Intended status: Standards Track                               L. Pardue
Expires: 23 May 2022                                          Cloudflare
                                                        19 November 2021


               Extensible Prioritization Scheme for HTTP
                     draft-ietf-httpbis-priority-10

Abstract

   This document describes a scheme that allows an HTTP client to
   communicate its preferences for how the upstream server prioritizes
   responses to its requests, and also allows a server to hint to a
   downstream intermediary how its responses should be prioritized when
   they are forwarded.  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
   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/
   (https://lists.w3.org/Archives/Public/ietf-http-wg/).

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

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.





Oku & Pardue               Expires 23 May 2022                  [Page 1]


Internet-Draft               HTTP Priorities               November 2021


   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 23 May 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   5
   2.  Motivation for Replacing RFC 7540 Priorities  . . . . . . . .   5
     2.1.  Disabling RFC 7540 Priorities . . . . . . . . . . . . . .   6
       2.1.1.  Advice when Using Extensible Priorities as the
               Alternative . . . . . . . . . . . . . . . . . . . . .   7
   3.  Applicability of the Extensible Priority Scheme . . . . . . .   7
   4.  Priority Parameters . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Urgency . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Incremental . . . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Defining New Parameters . . . . . . . . . . . . . . . . .  10
       4.3.1.  Registration  . . . . . . . . . . . . . . . . . . . .  10
   5.  The Priority HTTP Header Field  . . . . . . . . . . . . . . .  11
   6.  Reprioritization  . . . . . . . . . . . . . . . . . . . . . .  12
   7.  The PRIORITY_UPDATE Frame . . . . . . . . . . . . . . . . . .  12
     7.1.  HTTP/2 PRIORITY_UPDATE Frame  . . . . . . . . . . . . . .  13
     7.2.  HTTP/3 PRIORITY_UPDATE Frame  . . . . . . . . . . . . . .  14
   8.  Merging Client- and Server-Driven Parameters  . . . . . . . .  15
   9.  Client Scheduling . . . . . . . . . . . . . . . . . . . . . .  16
   10. Server Scheduling . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Intermediaries with Multiple Backend Connections . . . .  18
   11. Scheduling and the CONNECT Method . . . . . . . . . . . . . .  19
   12. Retransmission Scheduling . . . . . . . . . . . . . . . . . .  19
   13. Fairness  . . . . . . . . . . . . . . . . . . . . . . . . . .  20
     13.1.  Coalescing Intermediaries  . . . . . . . . . . . . . . .  20



Oku & Pardue               Expires 23 May 2022                  [Page 2]


Internet-Draft               HTTP Priorities               November 2021


     13.2.  HTTP/1.x Back Ends . . . . . . . . . . . . . . . . . . .  21
     13.3.  Intentional Introduction of Unfairness . . . . . . . . .  21
   14. Why use an End-to-End Header Field? . . . . . . . . . . . . .  21
   15. Security Considerations . . . . . . . . . . . . . . . . . . .  22
   16. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     17.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  25
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  25
     B.1.  Since draft-ietf-httpbis-priority-09  . . . . . . . . . .  25
     B.2.  Since draft-ietf-httpbis-priority-08  . . . . . . . . . .  25
     B.3.  Since draft-ietf-httpbis-priority-07  . . . . . . . . . .  26
     B.4.  Since draft-ietf-httpbis-priority-06  . . . . . . . . . .  26
     B.5.  Since draft-ietf-httpbis-priority-05  . . . . . . . . . .  26
     B.6.  Since draft-ietf-httpbis-priority-04  . . . . . . . . . .  26
     B.7.  Since draft-ietf-httpbis-priority-03  . . . . . . . . . .  26
     B.8.  Since draft-ietf-httpbis-priority-02  . . . . . . . . . .  27
     B.9.  Since draft-ietf-httpbis-priority-01  . . . . . . . . . .  27
     B.10. Since draft-ietf-httpbis-priority-00  . . . . . . . . . .  27
     B.11. Since draft-kazuho-httpbis-priority-04  . . . . . . . . .  27
     B.12. Since draft-kazuho-httpbis-priority-03  . . . . . . . . .  28
     B.13. Since draft-kazuho-httpbis-priority-02  . . . . . . . . .  28
     B.14. Since draft-kazuho-httpbis-priority-01  . . . . . . . . .  28
     B.15. Since draft-kazuho-httpbis-priority-00  . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

1.  Introduction

   It is common for representations of an HTTP [HTTP] resource to have
   relationships to one or more other resources.  Clients will often
   discover these relationships while processing a retrieved
   representation, which may lead to further retrieval requests.
   Meanwhile, the nature of the relationship determines whether the
   client is blocked from continuing to process locally available
   resources.  An example of this is visual rendering of an HTML
   document, which 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.











Oku & Pardue               Expires 23 May 2022                  [Page 3]


Internet-Draft               HTTP Priorities               November 2021


   HTTP/2 [HTTP2] and HTTP/3 [HTTP3] support multiplexing of requests
   and responses in a single connection.  An important feature of any
   implementation of a protocol that provides multiplexing is the
   ability to prioritize the sending of information.  For example, to
   provide meaningful presentation of an HTML 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 to a
   client.

   A server that operates in ignorance of how clients issue requests and
   consume responses can cause suboptimal client application
   performance.  Priority signals allow clients to communicate their
   view of request priority.  Servers have their own needs that are
   independent from client needs, so they often combine priority signals
   with other available information in order to inform scheduling of
   response data.

   RFC 7540 [RFC7540] stream priority allowed a client to send a series
   of priority signals that communicate to the server a "priority tree";
   the structure of this tree represents the client's preferred relative
   ordering and weighted distribution of the bandwidth among HTTP
   responses.  Servers could use these priority signals as input into
   prioritization decision making.

   The design and implementation of RFC 7540 stream priority was
   observed to have shortcomings, explained in Section 2.  HTTP/2
   [HTTP2] has consequently deprecated the use of these stream priority
   signals.

   This document describes an extensible scheme for prioritizing HTTP
   responses that uses absolute values.  Section 4 defines priority
   parameters, which are a standardized and extensible format of
   priority information.  Section 5 defines the Priority HTTP header
   field, a protocol-version-independent and end-to-end priority signal.
   Clients can use this header to signal priority to servers in order to
   specify the precedence of HTTP responses.  Similarly, servers behind
   an intermediary can use it to signal priority to the intermediary.
   Section 7.1 and Section 7.2 define version-specific frames that carry
   parameters, which clients can use for reprioritization.

   Header field and frame priority signals are input to a server's
   response prioritization process.  They are only a suggestion and do
   not guarantee any particular processing or transmission order for one
   response relative to any other response.  Section 10 and Section 12
   provide consideration and guidance about how servers might act upon
   signals.





Oku & Pardue               Expires 23 May 2022                  [Page 4]


Internet-Draft               HTTP Priorities               November 2021


   The prioritization scheme and priority signals defined herein can act
   as a substitute for RFC 7540 stream priority.

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The terms Dictionary, sf-boolean, sf-dictionary, and sf-integer are
   imported from [STRUCTURED-FIELDS].

   Example HTTP requests and responses use the HTTP/2-style formatting
   from [HTTP2].

   This document uses the variable-length integer encoding from [QUIC].

   The term control stream is used to describe the HTTP/2 stream with
   identifier 0x0, and HTTP/3 control stream; see Section 6.2.1 of
   [HTTP3].

   The term HTTP/2 priority signal is used to describe the priority
   information sent from clients to servers in HTTP/2 frames; see
   Section 5.3.2 of [HTTP2].

2.  Motivation for Replacing RFC 7540 Priorities

   RFC 7540 stream priority (see Section 5.3 of [RFC7540]) is a complex
   system where clients signal stream dependencies and weights to
   describe an unbalanced tree.  It suffered from limited deployment and
   interoperability and was deprecated in a revision of HTTP/2 [HTTP2].
   HTTP/2 retains these protocol elements in order to maintain wire
   compatibility (see Section 5.3.2 of [HTTP2]), which means that they
   might still be used in the absence of alternative signaling, such as
   the scheme this document describes.

   Many RFC 7540 server implementations do not act on HTTP/2 priority
   signals.

   Prioritization can use information that servers have about resources
   or the order in which requests are generated.  For example, a server,
   with knowledge of an HTML document structure, might want to
   prioritize the delivery of images that are critical to user
   experience above other images.  With RFC 7540 it is difficult for
   servers to interpret signals from clients for prioritization as the
   same conditions could result in very different signaling from



Oku & Pardue               Expires 23 May 2022                  [Page 5]


Internet-Draft               HTTP Priorities               November 2021


   different clients.  This document describes signaling that is simpler
   and more constrained, requiring less interpretation and allowing less
   variation.

   RFC 7540 does not define a method that can be used by a server to
   provide a priority signal for intermediaries.

   RFC 7540 priority is expressed relative to other requests on the same
   connection.  Many requests are generated without knowledge of how
   other requests might share a connection, which makes this difficult
   to use reliably, especially in protocols that do not have strong
   ordering guarantees, like HTTP/3 [HTTP3].

   Multiple experiments from independent research have shown that
   simpler schemes can reach at least equivalent performance
   characteristics compared to the more complex RFC 7540 setups seen in
   practice, at least for the web use case.

2.1.  Disabling RFC 7540 Priorities

   The problems and insights set out above provided the motivation for
   an alternative to RFC 7540 stream priority (see Section 5.3 of
   [HTTP2]).

   The SETTINGS_NO_RFC7540_PRIORITIES HTTP/2 setting is defined by this
   document in order to allow endpoints to omit or ignore HTTP/2
   priority signals (see Section 5.3.2 of [HTTP2]), as described below.
   The value of SETTINGS_NO_RFC7540_PRIORITIES 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 [HTTP2]) of type PROTOCOL_ERROR.  The initial value
   is 0.

   If endpoints use SETTINGS_NO_RFC7540_PRIORITIES they MUST send it in
   the first SETTINGS frame.  Senders MUST NOT change the
   SETTINGS_NO_RFC7540_PRIORITIES value after the first SETTINGS frame.
   Receivers that detect a change MAY treat it as a connection error of
   type PROTOCOL_ERROR.

   Clients can send SETTINGS_NO_RFC7540_PRIORITIES with a value of 1 to
   indicate that they are not using HTTP/2 priority signals.  The
   SETTINGS frame precedes any HTTP/2 priority signal sent from clients,
   so servers can determine whether they need to allocate any resources
   to signal handling before signals arrive.  A server that receives
   SETTINGS_NO_RFC7540_PRIORITIES with a value of 1 MUST ignore HTTP/2
   priority signals.






Oku & Pardue               Expires 23 May 2022                  [Page 6]


Internet-Draft               HTTP Priorities               November 2021


   Servers can send SETTINGS_NO_RFC7540_PRIORITIES with a value of 1 to
   indicate that they will ignore HTTP/2 priority signals sent by
   clients.

   Endpoints that send SETTINGS_NO_RFC7540_PRIORITIES are encouraged to
   use alternative priority signals (for example, Section 5 or
   Section 7.1) but there is no requirement to use a specific signal
   type.

2.1.1.  Advice when Using Extensible Priorities as the Alternative

   Before receiving a SETTINGS frame from a server, a client does not
   know if the server is ignoring HTTP/2 priority signals.  Therefore,
   until the client receives the SETTINGS frame from the server, the
   client SHOULD send both the HTTP/2 priority signals and the signals
   of this prioritization scheme (see Section 5 and Section 7.1).

   Once the client receives the first SETTINGS frame that contains the
   SETTINGS_NO_RFC7540_PRIORITIES parameter with value of 1, it SHOULD
   stop sending the HTTP/2 priority signals.  This avoids sending
   redundant signals that are known to be ignored.

   Similarly, if the client receives SETTINGS_NO_RFC7540_PRIORITIES with
   value of 0 or if the settings parameter was absent, it SHOULD stop
   sending PRIORITY_UPDATE frames (Section 7.1), since those frames are
   likely to be ignored.  However, the client MAY continue sending the
   Priority header field (Section 5), as it is an end-to-end signal that
   might be useful to nodes behind the server that the client is
   directly connected to.

3.  Applicability of the Extensible Priority Scheme

   The priority scheme defined by this document considers only the
   prioritization of HTTP messages and tunnels, see Section 9,
   Section 10, and Section 11.

   Where HTTP extensions change stream behavior or define new data
   carriage mechanisms, they can also define how this priority scheme
   can be applied.

4.  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.






Oku & Pardue               Expires 23 May 2022                  [Page 7]


Internet-Draft               HTTP Priorities               November 2021


   The Priority HTTP header field (Section 5) 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
   PRIORITY_UPDATE frames (Section 7.1 and Section 7.2) are used by
   clients to transmit the same information on a single hop.

   Intermediaries can consume and produce priority signals in a
   PRIORITY_UPDATE frame or Priority header field.  Sending a
   PRIORITY_UPDATE frame preserves the signal from the client, but
   provides a signal that overrides that for the next hop; see
   Section 14.  Replacing or adding a Priority header field overrides
   any signal from a client and can affect prioritization for all
   subsequent recipients.

   For both the Priority header field and the PRIORITY_UPDATE frame, the
   set of priority parameters is encoded as a Structured Fields
   Dictionary (see Section 3.2 of [STRUCTURED-FIELDS]).

   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 8.

   Receivers parse the Dictionary as defined in Section 4.2 of
   [STRUCTURED-FIELDS].  Where the Dictionary is successfully parsed,
   this document places the additional requirement that unknown priority
   parameters, parameters with out-of-range values, or values of
   unexpected types MUST be ignored.

4.1.  Urgency

   The urgency parameter (u) takes an integer between 0 and 7, in
   descending order of priority.

   The value is encoded as an sf-integer.  The default value is 3.

   Endpoints use this parameter to communicate their view of the
   precedence of HTTP responses.  The chosen value of urgency can be
   based on the expectation that servers might use this information to
   transmit HTTP responses in the order of their urgency.  The smaller
   the value, the higher the precedence.

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






Oku & Pardue               Expires 23 May 2022                  [Page 8]


Internet-Draft               HTTP Priorities               November 2021


   :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 8).

   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.

4.2.  Incremental

   The incremental parameter (i) takes an sf-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).

   If a client makes concurrent requests with the incremental parameter
   set to false, there is no benefit serving responses with the same
   urgency concurrently because the client is not going to process those
   responses incrementally.  Serving non-incremental responses with the
   same urgency one by one, in the order in which those requests were
   generated is considered to be the best strategy.

   If a client makes concurrent requests with the incremental parameter
   set to true, serving requests with the same urgency concurrently
   might be beneficial.  Doing this distributes the connection
   bandwidth, meaning that responses take longer to complete.
   Incremental delivery is most useful where multiple partial responses
   might provide some value to clients ahead of a complete response
   being available.

   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





Oku & Pardue               Expires 23 May 2022                  [Page 9]


Internet-Draft               HTTP Priorities               November 2021


4.3.  Defining New Parameters

   When attempting to define new parameters, care must be taken so that
   they do not adversely interfere with prioritization performed by
   existing endpoints or intermediaries that do not understand the newly
   defined parameter.  Since unknown parameters are ignored, new
   parameters should not change the interpretation of, or modify, the
   urgency (see Section 4.1) or incremental (see Section 4.2) parameters
   in a way that is not backwards compatible or fallback safe.

   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.

   Generic parameters are preferred over vendor-specific, application-
   specific or deployment-specific values.  If a generic value cannot be
   agreed upon in the community, the parameter's name should be
   correspondingly specific (e.g., with a prefix that identifies the
   vendor, application or deployment).

4.3.1.  Registration

   New Priority parameters can be defined by registering them in the
   HTTP Priority Parameters Registry.  The registry governs the keys
   (short textual strings) used in Structured Fields Dictionary (see
   Section 3.2 of [STRUCTURED-FIELDS]).  Since each HTTP request can
   have associated priority signals, there is value in having short key
   lengths, especially single-character strings.  In order to encourage
   extension while avoiding unintended conflict among attractive key
   values, the HTTP Priority Parameters Registry operates two
   registration policies depending on key length.

   *  Registration requests for parameters with a key length of one use
      the Specification Required policy, as per Section 4.6 of
      [RFC8126].

   *  Registration requests for parameters with a key length greater
      than one use the Expert Review policy, as per Section 4.5 of
      [RFC8126].  A specification document is appreciated, but not
      required.





Oku & Pardue               Expires 23 May 2022                 [Page 10]


Internet-Draft               HTTP Priorities               November 2021


   When reviewing registration requests, the designated expert(s) can
   consider the additional guidance provided in Section 4.3 but cannot
   use it as a basis for rejection.

   Registration requests should use the following template:

   Name:  [a name for the Priority Parameter that matches key]

   Description:  [a description of the parameter semantics and value]

   Reference:  [to a specification defining this parameter]

   See the registry at https://iana.org/assignments/http-priority
   (https://iana.org/assignments/http-priority) for details on where to
   send registration requests.

5.  The Priority HTTP Header Field

   The Priority HTTP header field carries priority parameters Section 4.
   It can appear in requests and responses.  It is an end-to-end signal
   of the request priority from the client or the response priority from
   the server.  Section 8 describes how intermediaries can combine the
   priority information from client requests and server responses to
   correct or amend the precedence.  Clients cannot interpret the
   appearance or omission of a Priority response header as
   acknowledgement that any prioritization has occurred.  Guidance for
   how endpoints can act on Priority header values is given in
   Section 10 and Section 9.

   Priority is a Dictionary (Section 3.2 of [STRUCTURED-FIELDS]):

   Priority   = sf-dictionary

   As is the ordinary case for HTTP caching [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).











Oku & Pardue               Expires 23 May 2022                 [Page 11]


Internet-Draft               HTTP Priorities               November 2021


6.  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 signal with the priority field value set to u=0.
   The PRIORITY_UPDATE frame (Section 7) can be used for such
   reprioritization.

7.  The PRIORITY_UPDATE Frame

   This document specifies a new PRIORITY_UPDATE frame for HTTP/2
   [HTTP2] and HTTP/3 [HTTP3].  It carries priority parameters and
   references the target of the prioritization based on a version-
   specific identifier.  In HTTP/2, this identifier is the Stream ID; in
   HTTP/3, the identifier is either the Stream ID or Push ID.  Unlike
   the Priority header field, the PRIORITY_UPDATE frame is a hop-by-hop
   signal.

   PRIORITY_UPDATE frames are sent by clients on the control stream,
   allowing them to be sent independent from the stream that carries the
   response.  This means they can be used to reprioritize a response or
   a push stream; or signal the initial priority of a response instead
   of the Priority header field.

   A PRIORITY_UPDATE frame communicates a complete set of all parameters
   in the Priority Field Value field.  Omitting a parameter is a signal
   to use the parameter's default value.  Failure to parse the Priority
   Field Value MAY be treated as a connection error.  In HTTP/2 the
   error is of type PROTOCOL_ERROR; in HTTP/3 the error is of type
   H3_GENERAL_PROTOCOL_ERROR.

   A client MAY send a PRIORITY_UPDATE frame before the stream that it
   references is open (except for HTTP/2 push streams; see Section 7.1).
   Furthermore, HTTP/3 offers no guaranteed ordering across streams,
   which could cause the frame to be received earlier than intended.
   Either case leads to a race condition where a server receives a
   PRIORITY_UPDATE frame that references a request stream that is yet to
   be opened.  To solve this condition, for the purposes of scheduling,
   the most recently received PRIORITY_UPDATE frame can be considered as
   the most up-to-date information that overrides any other signal.
   Servers SHOULD buffer the most recently received PRIORITY_UPDATE
   frame and apply it once the referenced stream is opened.  Holding
   PRIORITY_UPDATE frames for each stream requires server resources,
   which can can be bound by local implementation policy.  Although



Oku & Pardue               Expires 23 May 2022                 [Page 12]


Internet-Draft               HTTP Priorities               November 2021


   there is no limit to the number of PRIORITY_UPDATES that can be sent,
   storing only the most recently received frame limits resource
   commitment.

7.1.  HTTP/2 PRIORITY_UPDATE Frame

   The HTTP/2 PRIORITY_UPDATE frame (type=0x10) is used by clients to
   signal the initial priority of a response, or to reprioritize a
   response or push stream.  It carries the stream ID of the response
   and the priority in ASCII text, using the same representation as the
   Priority header field value.

   The Stream Identifier field (see Section 5.1.1 of [HTTP2]) in the
   PRIORITY_UPDATE frame header MUST be zero (0x0).  Receiving a
   PRIORITY_UPDATE frame with a field of any other value MUST be treated
   as a connection error of type PROTOCOL_ERROR.

   HTTP/2 PRIORITY_UPDATE Frame {
     Length (24),
     Type (i) = 10,

     Unused Flags (8).

     Reserved (1),
     Stream Identifier (31),

     Reserved (1),
     Prioritized Stream ID (31),
     Priority Field Value (..),
   }

               Figure 1: HTTP/2 PRIORITY_UPDATE Frame Payload

   The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
   fields are described in Section 4 of [HTTP2].  The frame payload of
   PRIORITY_UPDATE frame payload contains the following additional
   fields:

   Reserved:  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.

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

   Priority Field Value:  The priority update value in ASCII text,
      encoded using Structured Fields.  This is the same representation
      as the Priority header field value.



Oku & Pardue               Expires 23 May 2022                 [Page 13]


Internet-Draft               HTTP Priorities               November 2021


   When the PRIORITY_UPDATE frame applies to a request stream, clients
   SHOULD provide a Prioritized Stream ID that refers to a stream in the
   "open", "half-closed (local)", or "idle" state.  Servers can discard
   frames where the Prioritized Stream ID refers to a stream in the
   "half-closed (local)" or "closed" state.  The number of streams which
   have been prioritized but remain in the "idle" state plus the number
   of active streams (those in the "open" or either "half-closed" state;
   see Section 5.1.2 of [HTTP2]) MUST NOT exceed the value of the
   SETTINGS_MAX_CONCURRENT_STREAMS parameter.  Servers that receive such
   a PRIORITY_UPDATE MUST respond with a connection error of type
   PROTOCOL_ERROR.

   When the PRIORITY_UPDATE frame applies to a push stream, clients
   SHOULD provide a Prioritized Stream ID that refers to a stream in the
   "reserved (remote)" or "half-closed (local)" state.  Servers can
   discard frames where the Prioritized Stream ID refers to a stream in
   the "closed" state.  Clients MUST NOT provide a Prioritized Stream ID
   that refers to a push stream in the "idle" state.  Servers that
   receive a PRIORITY_UPDATE for a push stream in the "idle" state MUST
   respond with a connection error of type PROTOCOL_ERROR.

   If a PRIORITY_UPDATE frame is received with a Prioritized Stream ID
   of 0x0, the recipient MUST respond with a connection error of type
   PROTOCOL_ERROR.

   If a client receives a PRIORITY_UPDATE frame, it MUST respond with a
   connection error of type PROTOCOL_ERROR.

7.2.  HTTP/3 PRIORITY_UPDATE Frame

   The HTTP/3 PRIORITY_UPDATE frame (type=0xF0700 or 0xF0701) is used by
   clients to signal the initial priority of a response, or to
   reprioritize a response or push stream.  It carries the identifier of
   the element that is being prioritized, and the updated priority in
   ASCII text, using the same representation as that of the Priority
   header field value.  PRIORITY_UPDATE with a frame type of 0xF0700 is
   used for request streams, while PRIORITY_UPDATE with a frame type of
   0xF0701 is used for push streams.

   The PRIORITY_UPDATE frame MUST be sent on the client control stream
   (see Section 6.2.1 of [HTTP3]).  Receiving a PRIORITY_UPDATE frame on
   a stream other than the client control stream MUST be treated as a
   connection error of type H3_FRAME_UNEXPECTED.








Oku & Pardue               Expires 23 May 2022                 [Page 14]


Internet-Draft               HTTP Priorities               November 2021


   HTTP/3 PRIORITY_UPDATE Frame {
     Type (i) = 0xF0700..0xF0701,
     Length (i),
     Prioritized Element ID (i),
     Priority Field Value (..),
   }

                   Figure 2: HTTP/3 PRIORITY_UPDATE Frame

   The PRIORITY_UPDATE frame payload has the following fields:

   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 Fields.  This is the same representation
      as the Priority header field value.

   The request-stream variant of PRIORITY_UPDATE (type=0xF0700) MUST
   reference a request stream.  If a server receives a PRIORITY_UPDATE
   (type=0xF0700) for a Stream ID that is not a request stream, this
   MUST be treated as a connection error of type H3_ID_ERROR.  The
   Stream ID MUST be within the client-initiated bidirectional stream
   limit.  If a server receives a PRIORITY_UPDATE (type=0xF0700) with a
   Stream ID that is beyond the stream limits, this SHOULD be treated as
   a connection error of type H3_ID_ERROR.  Generating an error is not
   mandatory because HTTP/3 implementations might have practical
   barriers to determining the active stream concurrency limit that is
   applied by the QUIC layer.

   The push-stream variant PRIORITY_UPDATE (type=0xF0701) MUST reference
   a promised push stream.  If a server receives a PRIORITY_UPDATE
   (type=0xF0701) with a Push ID that is greater than the maximum Push
   ID or which has not yet been promised, this MUST be treated as a
   connection error of type H3_ID_ERROR.

   PRIORITY_UPDATE frames of either type are only sent by clients.  If a
   client receives a PRIORITY_UPDATE frame, this MUST be treated as a
   connection error of type H3_FRAME_UNEXPECTED.

8.  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



Oku & Pardue               Expires 23 May 2022                 [Page 15]


Internet-Draft               HTTP Priorities               November 2021


   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 4).

   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.

9.  Client Scheduling

   A client MAY use priority values to make local processing or
   scheduling choices about the requests it initiates.






Oku & Pardue               Expires 23 May 2022                 [Page 16]


Internet-Draft               HTTP Priorities               November 2021


10.  Server Scheduling

   Priority signals are input to a prioritization process.  They do not
   guarantee any particular processing or transmission order for one
   response relative to any other response.  An endpoint cannot force a
   peer to process concurrent request in a particular order using
   priority.  Expressing priority is therefore only a suggestion.

   A server can use priority signals along with other inputs to make
   scheduling decisions.  No guidance is provided about how this can or
   should be done.  Factors such as implementation choices or deployment
   environment also play a role.  Any given connection is likely to have
   many dynamic permutations.  For these reasons, there is no unilateral
   perfect scheduler and this document only provides some basic
   recommendations for implementations.

   Clients cannot depend on particular treatment based on priority
   signals.  Servers can use other information to prioritize responses.

   It is RECOMMENDED that, when possible, servers respect the urgency
   parameter (Section 4.1), sending higher urgency responses before
   lower urgency responses.

   The incremental parameter indicates how a client processes response
   bytes as they arrive.  It is RECOMMENDED that, when possible, servers
   respect the incremental parameter (Section 4.2).  Non-incremental
   resources can only be used when all of the response payload has been
   received.  Therefore, non-incremental responses of the same urgency
   SHOULD be served in their entirety, one-by-one, based on the stream
   ID, which corresponds to the order in which clients make requests.
   Doing so ensures that clients can use request ordering to influence
   response order.

   Incremental responses of the same urgency SHOULD be served by sharing
   bandwidth amongst them.  Incremental resources are used as parts, or
   chunks, of the response payload are received.  A client might benefit
   more from receiving a portion of all these resources rather than the
   entirety of a single resource.  How large a portion of the resource
   is needed to be useful in improving performance varies.  Some
   resource types place critical elements early, others can use
   information progressively.  This scheme provides no explicit mandate
   about how a server should use size, type or any other input to decide
   how to prioritize.

   There can be scenarios where a server will need to schedule multiple
   incremental and non-incremental responses at the same urgency level.
   Strictly abiding the scheduling guidance based on urgency and request
   generation order might lead to sub-optimal results at the client, as



Oku & Pardue               Expires 23 May 2022                 [Page 17]


Internet-Draft               HTTP Priorities               November 2021


   early non-incremental responses might prevent serving of incremental
   responses issued later.  The following are examples of such
   challenges.

   1.  At the same urgency level, a non-incremental request for a large
       resource followed by an incremental request for a small resource.

   2.  At the same urgency level, an incremental request of
       indeterminate length followed by a non-incremental large
       resource.

   It is RECOMMENDED that servers avoid such starvation where possible.
   The method to do so is an implementation decision.  For example, a
   server might pre-emptively send responses of a particular incremental
   type based on other information such as content size.

   Optimal scheduling of server push is difficult, especially when
   pushed resources contend with active concurrent requests.  Servers
   can consider many factors when scheduling, such as the type or size
   of resource being pushed, the priority of the request that triggered
   the push, the count of active concurrent responses, the priority of
   other active concurrent responses, etc.  There is no general guidance
   on the best way to apply these.  A server that is too simple could
   easily push at too high a priority and block client requests, or push
   at too low a priority and delay the response, negating intended goals
   of server push.

   Priority signals are a factor for server push scheduling.  The
   concept of parameter value defaults applies slightly differently
   because there is no explicit client-signalled initial priority.  A
   server can apply priority signals provided in an origin response; see
   the merging guidance given in Section 8.  In the absence of origin
   signals, applying default parameter values could be suboptimal.  By
   whatever means a server decides to schedule a pushed response, it can
   signal the intended priority to the client by including the Priority
   field in a PUSH_PROMISE or HEADERS frame.

10.1.  Intermediaries with Multiple Backend Connections

   An intermediary serving an HTTP connection might split requests over
   multiple backend connections.  When it applies prioritization rules
   strictly, low priority requests cannot make progress while requests
   with higher priorities are inflight.  This blocking can propagate to
   backend connections, which the peer might interpret as a connection
   stall.  Endpoints often implement protections against stalls, such as
   abruptly closing connections after a certain time period.  To reduce
   the possibility of this occurring, intermediaries can avoid strictly
   following prioritization and instead allocate small amounts of



Oku & Pardue               Expires 23 May 2022                 [Page 18]


Internet-Draft               HTTP Priorities               November 2021


   bandwidth for all the requests that they are forwarding, so that
   every request can make some progress over time.

   Similarly, servers SHOULD allocate some amount of bandwidths to
   streams acting as tunnels.

11.  Scheduling and the CONNECT Method

   When a request stream carries the CONNECT method, the scheduling
   guidance in this document applies to the frames on the stream.  A
   client that issues multiple CONNECT requests can set the incremental
   parameter to true, servers that implement the recommendation in
   Section 10 will schedule these fairly.

12.  Retransmission Scheduling

   Transport protocols such as TCP and QUIC provide reliability by
   detecting packet losses and retransmitting lost information.  While
   this document specifies HTTP-layer prioritization, its effectiveness
   can be further enhanced if the transport layer factors priority into
   scheduling both new data and retransmission data.  The remainder of
   this section discusses considerations when using QUIC.

   Section 13.3 of [QUIC] states "Endpoints SHOULD prioritize
   retransmission of data over sending new data, unless priorities
   specified by the application indicate otherwise".  When an HTTP/3
   application uses the priority scheme defined in this document and the
   QUIC transport implementation supports application indicated stream
   priority, a transport that considers the relative priority of streams
   when scheduling both new data and retransmission data might better
   match the expectations of the application.  However, there are no
   requirements on how a transport chooses to schedule based on this
   information because the decision depends on several factors and
   trade-offs.  It could prioritize new data for a higher urgency stream
   over retransmission data for a lower priority stream, or it could
   prioritize retransmission data over new data irrespective of
   urgencies.

   Section 6.2.4 of [QUIC-RECOVERY], also highlights consideration of
   application priorities when sending probe packets after Probe Timeout
   timer expiration.  A QUIC implementation supporting application-
   indicated priorities might use the relative priority of streams when
   choosing probe data.








Oku & Pardue               Expires 23 May 2022                 [Page 19]


Internet-Draft               HTTP Priorities               November 2021


13.  Fairness

   As a general guideline, a server SHOULD NOT use priority information
   for making scheduling 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.

13.1.  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.

   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 user agent to be delayed totally after those
   going to another.

   In order to mitigate this fairness problem, a server could use
   knowledge about the intermediary as another signal in its
   prioritization decisions.  For instance, if a server knows the
   intermediary is coalescing requests, then it could avoid serving the
   responses in their entirety and instead distribute bandwidth (for
   example, in a round-robin manner).  This can work if the constrained
   resource is network capacity between the intermediary and the user
   agent, as the intermediary buffers responses and forwards the chunks
   based on the prioritization scheme it implements.

   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 [FORWARDED], X-Forwarded-For

   *  Via (see Section 7.6.3 of [HTTP])






Oku & Pardue               Expires 23 May 2022                 [Page 20]


Internet-Draft               HTTP Priorities               November 2021


13.2.  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.  HTTP/1.1 and older do not support response
   multiplexing in a single connection, so there is not a fairness
   problem.  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.

13.3.  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.

14.  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.




Oku & Pardue               Expires 23 May 2022                 [Page 21]


Internet-Draft               HTTP Priorities               November 2021


   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.

15.  Security Considerations

   [RFC7540] stream prioritization relies on dependencies.
   Considerations are presented to implementations, describing how
   limiting state or work commitments can avoid some types of problems.
   In addition, [CVE-2019-9513] aka "Resource Loop", is an example of a
   DoS attack that abuses stream dependencies.  Extensible priorities
   does not use dependencies, which avoids these issues.

   Section 7 describes considerations for server buffering of
   PRIORITY_UPDATE frames.

   Section 10 presents examples where servers that prioritize responses
   in a certain way might be starved of the ability to transmit payload.

   The security considerations from [STRUCTURED-FIELDS] apply to
   processing of priority parameters defined in Section 4.

16.  IANA Considerations

   This specification registers the following entry in the the Hypertext
   Transfer Protocol (HTTP) Field Name Registry established by [HTTP]:

   Field name:  Priority

   Status:  permanent

   Specification document(s):  This document

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

   Name:  SETTINGS_NO_RFC7540_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



Oku & Pardue               Expires 23 May 2022                 [Page 22]


Internet-Draft               HTTP Priorities               November 2021


   Code:  0x10

   Specification:  This document

   This specification registers the following entries in the HTTP/3
   Frame Type registry established by [HTTP3]:

   Frame Type:  PRIORITY_UPDATE

   Code:  0xF0700 and 0xF0701

   Specification:  This document

   Upon publication, please create the HTTP Priority Parameters registry
   at https://iana.org/assignments/http-priority
   (https://iana.org/assignments/http-priority) and populate it with the
   types defined in Section 4; see Section 4.3.1 for its associated
   procedures.

17.  References

17.1.  Normative References

   [HTTP]     Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              semantics-19>.

   [HTTP2]    Thomson, M. and C. Benfield, "Hypertext Transfer Protocol
              Version 2 (HTTP/2)", Work in Progress, Internet-Draft,
              draft-ietf-httpbis-http2bis-05, 26 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              http2bis-05>.

   [HTTP3]    Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-34, 2 February 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              http-34>.

   [QUIC]     Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9000>.






Oku & Pardue               Expires 23 May 2022                 [Page 23]


Internet-Draft               HTTP Priorities               November 2021


   [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/rfc/rfc2119>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [STRUCTURED-FIELDS]
              Nottingham, M. and P-H. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
              <https://www.rfc-editor.org/rfc/rfc8941>.

17.2.  Informative References

   [CACHING]  Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Caching", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-cache-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              cache-19>.

   [CVE-2019-9513]
              Common Vulnerabilities and Exposures, "CVE-2019-9513", 1
              March 2019, <https://cve.mitre.org/cgi-bin/
              cvename.cgi?name=CVE-2019-9513>.

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

   [I-D.lassey-priority-setting]
              Lassey, B. and L. Pardue, "Declaring Support for HTTP/2
              Priorities", Work in Progress, Internet-Draft, draft-
              lassey-priority-setting-00, 25 July 2019,
              <https://datatracker.ietf.org/doc/html/draft-lassey-
              priority-setting-00>.

   [QUIC-RECOVERY]
              Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
              May 2021, <https://www.rfc-editor.org/rfc/rfc9002>.



Oku & Pardue               Expires 23 May 2022                 [Page 24]


Internet-Draft               HTTP Priorities               November 2021


   [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/rfc/rfc7540>.

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

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 (http://tools.ietf.org/agenda/83/slides/
   slides-83-httpbis-5.pdf).  In https://github.com/pmeenan/http3-
   prioritization-proposal (https://github.com/pmeenan/http3-
   prioritization-proposal), Patrick Meenan advocated for representing
   the priorities using a tuple of urgency and concurrency.  The ability
   to disable HTTP/2 prioritization is inspired by
   [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.

   Yang Chi contributed the section on retransmission scheduling.

Appendix B.  Change Log

   _RFC EDITOR: please remove this section before publication_

B.1.  Since draft-ietf-httpbis-priority-09

   *  Editorial changes

B.2.  Since draft-ietf-httpbis-priority-08

   *  Changelog fixups




Oku & Pardue               Expires 23 May 2022                 [Page 25]


Internet-Draft               HTTP Priorities               November 2021


B.3.  Since draft-ietf-httpbis-priority-07

   *  Relax requirements of receiving SETTINGS_NO_RFC7540_PRIORITIES
      that changes value (#1714, #1725)

   *  Clarify how intermediaries might use frames vs. headers (#1715,
      #1735)

   *  Relax requirement when receiving a PRIORITY_UPDATE with an invalid
      structured field value (#1741, #1756)

B.4.  Since draft-ietf-httpbis-priority-06

   *  Focus on editorial changes

   *  Clarify rules about Sf-Dictionary handling in headers

   *  Split policy for parameter IANA registry into two sections based
      on key length

B.5.  Since draft-ietf-httpbis-priority-05

   *  Renamed SETTINGS_DEPRECATE_RFC7540_PRIORITIES to
      SETTINGS_NO_RFC7540_PRIORITIES

   *  Clarify that senders of the HTTP/2 setting can use any alternative
      (#1679, #1705)

B.6.  Since draft-ietf-httpbis-priority-04

   *  Renamed SETTINGS_DEPRECATE_HTTP2_PRIORITIES to
      SETTINGS_DEPRECATE_RFC7540_PRIORITIES (#1601)

   *  Reoriented text towards RFC7540bis (#1561, #1601)

   *  Clarify intermediary behavior (#1562)

B.7.  Since draft-ietf-httpbis-priority-03

   *  Add statement about what this scheme applies to.  Clarify
      extensions can use it but must define how themselves (#1550,
      #1559)

   *  Describe scheduling considerations for the CONNECT method (#1495,
      #1544)

   *  Describe scheduling considerations for retransmitted data (#1429,
      #1504)



Oku & Pardue               Expires 23 May 2022                 [Page 26]


Internet-Draft               HTTP Priorities               November 2021


   *  Suggest intermediaries might avoid strict prioritization (#1562)

B.8.  Since draft-ietf-httpbis-priority-02

   *  Describe considerations for server push prioritization (#1056,
      #1345)

   *  Define HTTP/2 PRIORITY_UPDATE ID limits in HTTP/2 terms (#1261,
      #1344)

   *  Add a Parameters registry (#1371)

B.9.  Since draft-ietf-httpbis-priority-01

   *  PRIORITY_UPDATE frame changes (#1096, #1079, #1167, #1262, #1267,
      #1271)

   *  Add section to describe server scheduling considerations (#1215,
      #1232, #1266)

   *  Remove specific instructions related to intermediary fairness
      (#1022, #1264)

B.10.  Since draft-ietf-httpbis-priority-00

   *  Move text around (#1217, #1218)

   *  Editorial change to the default urgency.  The value is 3, which
      was always the intent of previous changes.

B.11.  Since draft-kazuho-httpbis-priority-04

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

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

   *  Remove mention of CDN-Loop (#1062)

   *  Editorial changes

   *  Make changes due to WG adoption

   *  Removed outdated Consideration (#118)







Oku & Pardue               Expires 23 May 2022                 [Page 27]


Internet-Draft               HTTP Priorities               November 2021


B.12.  Since draft-kazuho-httpbis-priority-03

   *  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)

B.13.  Since draft-kazuho-httpbis-priority-02

   *  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)

B.14.  Since draft-kazuho-httpbis-priority-01

   *  Explain how reprioritization might be supported.

B.15.  Since draft-kazuho-httpbis-priority-00

   *  Expand urgency levels from 3 to 8.

Authors' Addresses

   Kazuho Oku
   Fastly

   Email: kazuhooku@gmail.com


   Lucas Pardue
   Cloudflare

   Email: lucaspardue.24.7@gmail.com










Oku & Pardue               Expires 23 May 2022                 [Page 28]