HTTP                                                              K. Oku
Internet-Draft                                                    Fastly
Intended status: Standards Track                               L. Pardue
Expires: 12 June 22 July 2022                                         Cloudflare
                                                         9 December 2021
                                                         18 January 2022

               Extensible Prioritization Scheme for HTTP


   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.

About This Document

   This note is to be removed before publishing as an RFC.

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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 . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Urgency . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Incremental . . . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Defining New Priority 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 Priority Parameters . . . .  16
   9.  Client Scheduling . . . . . . . . . . . . . . . . . . . . . .  17
   10. Server Scheduling . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Intermediaries with Multiple Backend Connections . . . .  19
   11. Scheduling and the CONNECT Method . . . . . . . . . . . . . .  19
   12. Retransmission Scheduling . . . . . . . . . . . . . . . . . .  19
   13. Fairness  . . . . . . . . . . . . . . . . . . . . . . . . . .  20
     13.1.  Coalescing Intermediaries  . . . . . . . . . . . . . . .  20
     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 . . . . . . . . . . . . . . . . . . . . .  26
     B.1.  Since draft-ietf-httpbis-priority-10 draft-ietf-httpbis-priority-11  . . . . . . . . . .  26
     B.2.  Since draft-ietf-httpbis-priority-09 draft-ietf-httpbis-priority-10  . . . . . . . . . .  26
     B.3.  Since draft-ietf-httpbis-priority-08 draft-ietf-httpbis-priority-09  . . . . . . . . . .  26
     B.4.  Since draft-ietf-httpbis-priority-07 draft-ietf-httpbis-priority-08  . . . . . . . . . .  26
     B.5.  Since draft-ietf-httpbis-priority-06 draft-ietf-httpbis-priority-07  . . . . . . . . . .  26
     B.6.  Since draft-ietf-httpbis-priority-06  . . . . . . . . . .  26
     B.7.  Since draft-ietf-httpbis-priority-05  . . . . . . . . . .  27
     B.8.  Since draft-ietf-httpbis-priority-04  . . . . . . . . . .  27
     B.9.  Since draft-ietf-httpbis-priority-03  . . . . . . . . . .  27
     B.10. Since draft-ietf-httpbis-priority-02  . . . . . . . . . .  27
     B.11. Since draft-ietf-httpbis-priority-01  . . . . . . . . . .  27
     B.12. Since draft-ietf-httpbis-priority-00  . . . . . . . . . .  28
     B.13. Since draft-kazuho-httpbis-priority-04  . . . . . . . . .  28
     B.14. Since draft-kazuho-httpbis-priority-03  . . . . . . . . .  28
     B.15. Since draft-kazuho-httpbis-priority-02  . . . . . . . . .  28
     B.16. Since draft-kazuho-httpbis-priority-01  . . . . . . . . .  29
     B.17. Since draft-kazuho-httpbis-priority-00  . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

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

   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

   A server

   HTTP/2 and HTTP/3 servers can schedule transmission of concurrent
   response data by any means they choose.  Servers can ignore client
   priority signals and still successfully serve HTTP responses.
   However, servers that operates operate 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 of 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. 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.  The prioritization scheme and priority signals defined
   herein can act as a substitute for RFC 7540 stream priority.

   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 send this header field to signal their view of how
   responses should be prioritized.  Similarly, servers behind an
   intermediary can use it to signal priority to the intermediary.
   After sending a request, a client can change the priority their view of the response
   priority (see Section 6) using by sending HTTP-version-specific frames
   defined in Section 7.1 and Section 7.2.

   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

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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 both the HTTP/2 stream
   with identifier 0x0, 0x0 and the 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 even in the presence of alternative signaling,
   such as the scheme this document describes.

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

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

   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 sharing the
   connection.  Many connection at the same time.  It is difficult to incorporate
   such design into applications that generate requests are generated without
   knowledge of how other requests might share a connection, which makes this difficult
   to use reliably, especially in or into
   protocols that do not have strong ordering guarantees, guarantees across streams,
   like HTTP/3 [HTTP3].

   Multiple experiments

   Experiments from independent research ([MARX], [MEENAN]) ([MARX]) 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

   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

   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.

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

   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

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 is primarily focused on
   the prioritization of HTTP response messages (see Section 3.4 of
   [HTTP]).  It defines new priority parameters (Section 4) and their
   conveyors a means
   of conveying those parameters (Section 5 and Section 7) 7), which is
   intended to communicate the priority of responses to a server that is
   responsible for prioritizing them.  Section 10 provides
   considerations for servers about acting on those signals in
   combination with other inputs and factors.

   The CONNECT method (see Section 9.3.6 of [HTTP]) can be used to
   establish tunnels.  Signaling applies similarly to tunnels;
   additional considerations for server prioritization are given in
   Section 11.

   Section 9 describes how clients can optionally apply elements of this
   scheme locally to the request messages that they generate.

   Some forms of HTTP extensions might change HTTP/2 or HTTP/3 stream
   behavior or define new data carriage mechanisms.  Such extensions can
   define themselves how this priority scheme is to 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.

   The Priority HTTP header field (Section 5) is an end-to-end way to
   transmit this set of priority parameters when a request or a response
   is issued.  In order to reprioritize  After sending a request request, a client can change their view
   of response priority (Section 6), HTTP-
   version-specific 6) by sending HTTP-version-specific
   PRIORITY_UPDATE frames (Section defined in Section 7.1 and Section 7.2)
   are used by clients to 7.2.
   Frames transmit the same information priority parameters on a single hop. hop only.

   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, client carried by
   the Priority header field, 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) priority
   parameters.  When receiving an HTTP request that does not carry these
   priority parameters, a server SHOULD act as if their default values
   were specified.

   An intermediary can combine signals from requests and responses that
   it forwards.  Note that omission of priority parameters in responses
   is handled differently from omission in requests; 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, priority 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:

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

4.3.  Defining New Priority Parameters

   When attempting to define new priority 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 priority parameter.  Since unknown priority parameters
   are ignored, new priority parameters should not change the
   interpretation of, or modify, the urgency (see Section 4.1) or
   incremental (see Section 4.2) priority 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 priority 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 priority parameter to
   indicate if the resource being requested is within the viewport.

   Generic priority 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 the 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 priority parameters with a key length of
      one use the Specification Required policy, as per Section 4.6 of

   *  Registration requests for priority 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

   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 priority parameter semantics and

   Reference:  [to a specification defining this priority parameter]

   See the registry at
   ( for details on where to
   send registration requests.

5.  The Priority HTTP Header Field

   The Priority HTTP header field carries priority parameters (see
   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 that indicates the server. endpoint's view of how HTTP
   responses should be prioritized.  Section 8 describes how
   intermediaries can combine the priority information sent from clients
   and servers.  Clients cannot interpret the appearance or omission of
   a Priority response header field as acknowledgement that any
   prioritization has occurred.  Guidance for how endpoints can act on
   Priority header values is given in Section 10 9 and Section 9. 10.

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

   Priority   = sf-dictionary
   An HTTP request with a Priority header field might be cached and re-
   used for subsequent requests; see [CACHING].  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,

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


   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

   PRIORITY_UPDATE frames are sent by clients on the control stream,
   allowing them to be sent independent from of 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 priority
   parameters in the Priority Field Value field.  Omitting a priority
   parameter is a signal to use its 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

   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 bounded by local implementation policy.  Although there
   is no limit to the number of PRIORITY_UPDATES PRIORITY_UPDATE frames that can be sent,
   storing only the most recently received frame limits resource


   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.

     Length (24),
     Type (i) (8) = 10, 0x10,

     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
      undefined.  It 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.

   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

   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

   Servers MUST NOT send PRIORITY_UPDATE frames.  If a client receives a
   PRIORITY_UPDATE frame, it MUST respond with a connection error of


   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, prioritized and the updated priority in
   ASCII text, using text that uses 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.

     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.

   Servers MUST NOT send PRIORITY_UPDATE frames of either type are only sent by clients. type.  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 Priority 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 priority 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, 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 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 =
   :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)

9.  Client Scheduling

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

10.  Server Scheduling

   It is generally beneficial for an HTTP server to send all responses
   as early as possible.  However, when serving multiple requests on a
   single connection, there could be competition between the requests
   for resources such as connection bandwidth.  This section describes
   considerations regarding how servers can schedule the order in which
   the competing responses will be sent, sent when such competition exists.

   Server scheduling is a prioritization process based on many inputs,
   with priority signals being only one form of input.  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 it is no unilateral perfect scheduler. not possible to describe a universal
   scheduling algorithm.  This document provides some basic, non-exhaustive, non-
   exhaustive recommendations for how servers might act on priority
   parameters.  It does not describe in detail how servers might combine
   priority signals with other factors.  Endpoints cannot depend on
   particular treatment based on priority signals.  Expressing priority
   is only a suggestion.

   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 responses of the same urgency SHOULD be served by
   prioritizing bandwidth allocation in ascending order of 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 among them.  Incremental resources  Payload of incremental responses are used as in
   parts, or chunks, of the response payload as they 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, 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 suboptimal results at the client, as
   early non-incremental responses might prevent serving of incremental
   responses issued later.  The following are examples of such

   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

   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 in flight.  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
   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 recommendations for
   handling of the incremental parameter in Section 10 are likely to
   schedule these fairly, avoiding one CONNECT stream from blocking

12.  Retransmission Scheduling

   Transport protocols such as TCP and QUIC provide reliability by
   detecting packet losses and retransmitting lost information.  In
   addition to the considerations in Section 10, scheduling of
   retransmission data could compete with new 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

   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.

13.  Fairness

   Typically, HTTP implementations depend on the underlying transport to
   maintain fairness between connections competing for bandwidth.  When
   HTTP requests are forwarded through intermediaries, progress made by
   each connection originating from end clients can become different
   over time, depending on how intermediaries coalesce or split requests
   into backend connections.  This unfairness can expand if priority
   signals are used.  Section 13.1 and Section 13.2 discuss mitigations
   against this expansion of unfairness.

   Conversely, Section 13.3 discusses how servers might intentionally
   allocate unequal bandwidth to some connections depending on the
   priority signals.

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 carry signals
   indicating higher priority than those coming from others.

   It is sometimes beneficial for the server running behind an
   intermediary to obey Priority header field values.  As an example, a
   resource-constrained server might defer the transmission of software
   update files that would have the background urgency being associated. urgency.  However, in the worst
   case, the asymmetry between the priority 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 input 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])

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?

   In contrast 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 way that a client processes a response is a property associated
   with the client generating that request.  Not 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, 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

15.  Security Considerations

   Section 7 describes considerations for server buffering of

   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]:


   Code:  0x9

   Initial value:  0

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


   Code:  0x10

   Specification:  This document

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


   Code:  0xF0700 and 0xF0701

   Specification:  This document

   Upon publication, please create the HTTP Priority Parameters registry
   ( and populate it with the
   entries in Table 1; see Section 4.3.1 for its associated procedures.

        | Name |           Description            | Specification |
        | u    | The urgency of an HTTP response. | Section 4.1   |
        | i    | Whether an HTTP response can be  | Section 4.2   |
        |      |     processed incrementally.     |               |

                    Table 1: Initial Priority Parameters

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,

   [HTTP2]    Thomson, M. and C. Benfield, "Hypertext Transfer Protocol
              Version 2 (HTTP/2)", Work in Progress, Internet-Draft,
              draft-ietf-httpbis-http2bis-06, 18 November 2021,

   [HTTP3]    Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-34, 2 February 2021,

   [QUIC]     Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

              Nottingham, M. and P-H. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,

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,

              Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
              RFC 7239, DOI 10.17487/RFC7239, June 2014,

              Lassey, B. and L. Pardue, "Declaring Support for HTTP/2
              Priorities", Work in Progress, Internet-Draft, draft-
              lassey-priority-setting-00, 25 July 2019,

   [MARX]     Marx, R., Decker, T.D., Quax, P., and W. Lamotte, "Of the
              Utmost Importance: Resource Prioritization in HTTP/3 over
              QUIC", DOI 10.5220/0008191701300143,
              SCITEPRESS Proceedings of the 15th International
              Conference on Web Information Systems and Technologies
              (pages 130-143), September 2019,

   [MEENAN]   Meenan, P., "Better HTTP/2 Prioritization for a Faster
              Web", 14 May 2019, <

              Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
              May 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,

   [RFC8081]  Lilley, C., "The "font" Top-Level Media Type", RFC 8081,
              DOI 10.17487/RFC8081, February 2017,

Appendix A.  Acknowledgements

   Roy Fielding presented the idea of using a header field for
   representing priorities in
   slides-83-httpbis-5.pdf (
   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

   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-11

   *  Changes to address Last Call/IESG feedback

B.2.  Since draft-ietf-httpbis-priority-10

   *  Editorial changes

   *  Add clearer IANA instructions for Priority Parameter initial


B.3.  Since draft-ietf-httpbis-priority-09

   *  Editorial changes


B.4.  Since draft-ietf-httpbis-priority-08

   *  Changelog fixups


B.5.  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,

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


B.6.  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.7.  Since draft-ietf-httpbis-priority-05


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


B.8.  Since draft-ietf-httpbis-priority-04


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

   *  Clarify intermediary behavior (#1562)


B.9.  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,

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

   *  Describe scheduling considerations for retransmitted data (#1429,

   *  Suggest intermediaries might avoid strict prioritization (#1562)


B.10.  Since draft-ietf-httpbis-priority-02

   *  Describe considerations for server push prioritization (#1056,

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

   *  Add a Priority Parameters registry (#1371)


B.11.  Since draft-ietf-httpbis-priority-01

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

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

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


B.12.  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.13.  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)


B.14.  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.15.  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.16.  Since draft-kazuho-httpbis-priority-01

   *  Explain how reprioritization might be supported.


B.17.  Since draft-kazuho-httpbis-priority-00

   *  Expand urgency levels from 3 to 8.

Authors' Addresses

   Kazuho Oku


   Lucas Pardue