HTTP                                                              K. Oku
Internet-Draft                                                    Fastly
Intended status: Standards Track                               L. Pardue
Expires: April 4, 15 July 2021                                         Cloudflare
                                                        October 01, 2020
                                                         11 January 2021

               Extensible Prioritization Scheme for HTTP


   This document describes a scheme for prioritizing HTTP responses.
   This scheme expresses the priority of each HTTP response using
   absolute values, rather than as a relative relationship between a
   group of HTTP responses.

   This document defines the Priority header field for communicating the
   initial priority in an HTTP version-independent manner, as well as
   HTTP/2 and HTTP/3 frames for reprioritizing the responses.  These
   share a common format structure that is designed to provide future

Note to Readers

   _RFC EDITOR: please remove this section before publication_

   Discussion of this draft takes place on the HTTP working group
   mailing list (, which is archived at [1].

   Working Group information can be found at [2];
   (; source code and issues list for this draft can
   be found at [3].
   priorities (

Status of This Memo

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   This Internet-Draft will expire on April 4, 15 July 2021.

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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3   4
   2.  Motivation for Replacing HTTP/2 Priorities  . . . . . . . . .   4
     2.1.  Disabling HTTP/2 Priorities . . . . . . . . . . . . . . .   5
   3.  Priority Parameters . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Urgency . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Incremental . . . . . . . . . . . . . . . . . . . . . . .   7   8
     3.3.  Defining New Parameters . . . . . . . . . . . . . . . . .   8
       3.3.1.  Registration  . . . . . . . . . . . . . . . . . . . .   9
   4.  The Priority HTTP Header Field  . . . . . . . . . . . . . . .   8   9
   5.  Reprioritization  . . . . . . . . . . . . . . . . . . . . . .   9  10
   6.  The PRIORITY_UPDATE Frame . . . . . . . . . . . . . . . . . .   9  10
     6.1.  HTTP/2 PRIORITY_UPDATE Frame  . . . . . . . . . . . . . .  10  11
     6.2.  HTTP/3 PRIORITY_UPDATE Frame  . . . . . . . . . . . . . .  11  12
   7.  Merging Client- and Server-Driven Parameters  . . . . . . . .  12  13
   8.  Client Scheduling . . . . . . . . . . . . . . . . . . . . . .  13  14
   9.  Server Scheduling . . . . . . . . . . . . . . . . . . . . . .  13  14
   10. Fairness  . . . . . . . . . . . . . . . . . . . . . . . . . .  14  16
     10.1.  Coalescing Intermediaries  . . . . . . . . . . . . . . .  15  16
     10.2.  HTTP/1.x Back Ends . . . . . . . . . . . . . . . . . . .  15  17
     10.3.  Intentional Introduction of Unfairness . . . . . . . . .  16  17
   11. Why use an End-to-End Header Field? . . . . . . . . . . . . .  16  17
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  16  18
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17  19
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18  20
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  18  20
     14.2.  Informative References . . . . . . . . . . . . . . . . .  19
     14.3.  URIs . . . . . . . .  21
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  19  21
   Appendix A.  Acknowledgements . . . . . . . . B.  Change Log . . . . . . . . . .  20
   Appendix B.  Change Log . . . . . . . . . . .  22
     B.1.  Since draft-ietf-httpbis-priority-01  . . . . . . . . . .  20
     B.1.  22
     B.2.  Since draft-ietf-httpbis-priority-01  . . . . . . . . . .  20
     B.2.  22
     B.3.  Since draft-ietf-httpbis-priority-00  . . . . . . . . . .  20
     B.3.  22
     B.4.  Since draft-kazuho-httpbis-priority-04  . . . . . . . . .  21
     B.4.  22
     B.5.  Since draft-kazuho-httpbis-priority-03  . . . . . . . . .  21
     B.5.  23
     B.6.  Since draft-kazuho-httpbis-priority-02  . . . . . . . . .  21
     B.6.  23
     B.7.  Since draft-kazuho-httpbis-priority-01  . . . . . . . . .  21
     B.7.  23
     B.8.  Since draft-kazuho-httpbis-priority-00  . . . . . . . . .  21  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22  23

1.  Introduction

   It is common for an HTTP ([RFC7230]) resource representation to have
   relationships to one or more other resources.  Clients will often
   discover these relationships while processing a retrieved
   representation, leading to further retrieval requests.  Meanwhile,
   the nature of the relationship determines whether the client is
   blocked from continuing to process locally available resources.  For
   example, visual rendering of an HTML document could be blocked by the
   retrieval of a CSS file that the document refers to.  In contrast,
   inline images do not block rendering and get drawn incrementally as
   the chunks of the images arrive.

   To provide meaningful presentation of a document at the earliest
   moment, it is important for an HTTP server to prioritize the HTTP
   responses, or the chunks of those HTTP responses, that it sends.

   HTTP/2 ([RFC7540]) provides such a prioritization scheme.  A client
   sends a series of PRIORITY frames to communicate to the server a
   "priority tree"; this represents the client's preferred ordering and
   weighted distribution of the bandwidth among the HTTP responses.
   However, the design and implementation of this scheme has been
   observed to have shortcomings, explained in Section 2.

   This document defines the Priority HTTP header field that can be used
   by both client and server to specify the precedence of HTTP responses
   in a standardized, extensible, protocol-version-independent, end-to-
   end format.  Along with the protocol-version-specific frame for
   reprioritization, this prioritization scheme acts as a substitute for
   the original prioritization scheme of HTTP/2.

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   The terms sf-token and sf-boolean are imported from

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

   This document uses the variable-length integer encoding from

   The term control stream is used to describe the HTTP/2 stream with
   identifier 0x0, and HTTP/3 control stream; see [I-D.ietf-quic-http],
   Section 6.2.1.

2.  Motivation for Replacing HTTP/2 Priorities

   An important feature of any implementation of a protocol that
   provides multiplexing is the ability to prioritize the sending of
   information.  This was an important realization in the design of
   HTTP/2.  Prioritization is a difficult problem, so it will always be
   suboptimal, particularly if one endpoint operates in ignorance of the
   needs of its peer.

   HTTP/2 introduced a complex prioritization signaling scheme that used
   a combination of dependencies and weights, formed into an unbalanced
   tree.  This scheme has suffered from poor deployment and

   The rich flexibility of client-driven HTTP/2 prioritization tree
   building is rarely exercised.  Experience has shown that clients tend
   to choose a single model optimized for a web use case and experiment
   within the model constraints, or do nothing at all.  Furthermore,
   many clients build their prioritization tree in a unique way, which
   makes it difficult for servers to understand their intent and act or
   intervene accordingly.

   Many HTTP/2 server implementations do not include support for the
   priority scheme.  Some instead favor custom server-driven schemes
   based on heuristics or other hints, such as resource content type or
   request generation order.  For example, a server, with knowledge of
   the document structure, might want to prioritize the delivery of
   images that are critical to user experience above other images, but
   below the CSS files.  Since client trees vary, it is impossible for
   the server to determine how such images should be prioritized against
   other responses.

   The HTTP/2 scheme allows intermediaries to coalesce multiple client
   trees into a single tree that is used for a single upstream HTTP/2
   connection.  However, most intermediaries do not support this.  The
   scheme does not define a method that can be used by a server to
   express the priority of a response.  Without such a method,
   intermediaries cannot coordinate client-driven and server-driven

   HTTP/2 describes denial-of-service considerations for
   implementations.  On 2019-08-13 Netflix issued an advisory notice
   about the discovery of several resource exhaustion vectors affecting
   multiple HTTP/2 implementations.  One attack, [CVE-2019-9513] aka
   "Resource Loop", is based on manipulation of the priority tree.

   The HTTP/2 scheme depends on in-order delivery of signals, leading to
   challenges in porting the scheme to protocols that do not provide
   global ordering.  For example, the scheme cannot be used in HTTP/3
   [I-D.ietf-quic-http] without changing the signal and its processing.

   Considering the problems with deployment and adaptability to HTTP/3,
   retaining the HTTP/2 priority scheme increases the complexity of the
   entire system without any evidence that the value it provides offsets
   that complexity.  In fact, multiple experiments from independent
   research have shown that simpler schemes can reach at least
   equivalent performance characteristics compared to the more complex
   HTTP/2 setups seen in practice, at least for the web use case.

2.1.  Disabling HTTP/2 Priorities

   The problems and insights set out above are motivation for allowing
   endpoints to opt out of using the HTTP/2 priority scheme, in favor of
   using an alternative such as the scheme defined in this
   specification.  The SETTINGS_DEPRECATE_HTTP2_PRIORITIES setting
   described below enables endpoints to understand their peer's
   intention.  The value of the parameter MUST be 0 or 1.  Any value
   other than 0 or 1 MUST be treated as a connection error (see
   [RFC7540], Section 5.4.1) of type PROTOCOL_ERROR.

   Endpoints MUST send this SETTINGS parameter as part of the first
   SETTINGS frame.  When the peer receives the first SETTINGS frame, it
   learns the sender has deprecated the HTTP/2 priority scheme if it
   receives the SETTINGS_DEPRECATE_HTTP2_PRIORITIES parameter with the
   value of 1.

   parameter value after the first SETTINGS frame.  Detection of a
   change by a receiver MUST be treated as a connection error of type

   Until the client receives the SETTINGS frame from the server, the
   client SHOULD send both the priority signal defined in the HTTP/2
   priority scheme and also that of this prioritization scheme.  Once
   the client learns that the HTTP/2 priority scheme is deprecated, it
   SHOULD stop sending the HTTP/2 priority signals.  If the client
   learns that the HTTP/2 priority scheme is not deprecated, it SHOULD
   stop sending PRIORITY_UPDATE frames (Section 6.1), but MAY continue
   sending the Priority header field (Section 4), as it is an end-to-end
   signal that might be useful to nodes behind the server that the
   client is directly connected to.

   The SETTINGS frame precedes any priority signal sent from a client in
   HTTP/2, so a server can determine if it should respect the HTTP/2
   scheme before building state.  A server that receives

   Where both endpoints disable HTTP/2 priorities, the client is
   expected to send this scheme's priority signal.  Handling of omitted
   signals is described in Section 3.

3.  Priority Parameters

   The priority information is a sequence of key-value pairs, providing
   room for future extensions.  Each key-value pair represents a
   priority parameter.

   The Priority HTTP header field (Section 4) is an end-to-end way to
   transmit this set of parameters when a request or a response is
   issued.  In order to reprioritize a request, HTTP-version-specific
   frames (Section 6.1 and Section 6.2) are used by clients to transmit
   the same information on a single hop.  If intermediaries want to
   specify prioritization on a multiplexed HTTP connection, they SHOULD
   use a PRIORITY_UPDATE frame and SHOULD NOT change the Priority header

   In both cases, the set of priority parameters is encoded as a
   Structured Fields Dictionary ([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 7.

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

3.1.  Urgency

   The urgency parameter ("u") takes an integer between 0 and 7, in
   descending order of priority.  This range provides sufficient
   granularity for prioritizing responses for ordinary web browsing, at
   minimal complexity.

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

   This parameter indicates the sender's recommendation, based on the
   expectation that the server would transmit HTTP responses in the
   order of their urgency values if possible.  The smaller the value,
   the higher the precedence.

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

   :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 7).

   The lowest urgency level (7) is reserved for background tasks such as
   delivery of software updates.  This urgency level SHOULD NOT be used
   for fetching responses that have impact on user interaction.

3.2.  Incremental

   The incremental parameter ("i") takes an 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").

   A server might distribute the bandwidth of a connection between
   incremental responses that share the same urgency, hoping that
   providing those responses in parallel would be more helpful to the
   client than delivering the responses one by one.

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

   The following example shows a request for a JPEG file with the
   urgency parameter set to "5" and the incremental parameter set to

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

3.3.  Defining New Parameters

   When attempting to extend priorities, define new parameters, care must be taken to ensure
   any use of 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 leaves them either unchanged are ignored, new
   parameters should not change the interpretation of or
   modified modify the
   predefined parameters in a way that is not backwards compatible for peers that are
   unaware of the extended meaning. 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

   Alternatively, the urgency can be augmented.  For example, a
   graphical user agent could send a "visible" parameter to indicate if
   the resource being requested is within the viewport.

4.  The Priority HTTP Header Field

   The Priority HTTP header field can appear

   Generic parameters are preferred over vendor-specific, application-
   specific or deployment-specific values.  If a generic value cannot be
   agreed upon in requests and responses.
   A client uses it to specify the priority of the response.  A server
   uses it to inform community, the client parameter's name should be
   correspondingly specific (e.g., with a prefix that identifies the priority was overwritten.  An
   vendor, application or deployment).

3.3.1.  Registration

   New Priority parameters can use be defined by registering them in the
   HTTP Priority information from client Parameters Registry.

   Registration requests are reviewed and server responses to correct or amend approved by a Designated
   Expert, as per [RFC8126], Section 4.5.  A specification document is
   appreciated, but not required.

   The Expert(s) should consider the following factors when evaluating

   *  Community feedback

   *  If the parameters are sufficiently well-defined and adhere to the
      guidance provided in Section 3.3.

   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
   ( for details on where to
   send registration requests.

4.  The Priority HTTP Header Field

   The Priority HTTP header field can appear in requests and responses.
   A client uses it to specify the priority of the response.  A server
   uses it to inform the client that the priority was overwritten.  An
   intermediary can use the Priority information from client requests
   and server responses to correct or amend the precedence to suit it
   (see Section 7).

   The Priority header field is an end-to-end signal of the request
   priority from the client or the response priority from the server.

   As is the ordinary case for HTTP caching ([RFC7234]), a response with
   a Priority header field might be cached and re-used for subsequent
   requests.  When an origin server generates the Priority response
   header field based on properties of an HTTP request it receives, the
   server is expected to control the cacheability or the applicability
   of the cached response, by using header fields that control the
   caching behavior (e.g., Cache-Control, Vary).

   An endpoint that fails to parse the Priority header field SHOULD use
   default parameter values.

5.  Reprioritization

   After a client sends a request, it may be beneficial to change the
   priority of the response.  As an example, a web browser might issue a
   prefetch request for a JavaScript file with the urgency parameter of
   the Priority request header field set to "u=7" (background).  Then,
   when the user navigates to a page which references the new JavaScript
   file, while the prefetch is in progress, the browser would send a
   reprioritization signal with the priority field value set to "u=0".
   The PRIORITY_UPDATE frame (Section 6) can be used for such


   This document specifies a new PRIORITY_UPDATE frame for HTTP/2
   ([RFC7540]) and HTTP/3 ([I-D.ietf-quic-http]).  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 MUST 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. open (except for HTTP/2 push streams; see Section 6.1).
   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.  (TODO: consider resolving #1261, and adding
   more text about bounds).  Although
   there is no limit to the number of PRIORITY_UPDATES 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 ([RFC7540], Section 4.1) 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.

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

               Figure 1: HTTP/2 PRIORITY_UPDATE Frame Payload

   The PRIORITY_UPDATE frame payload has the following fields:

   R:  A reserved 1-bit field.  The semantics of this bit are undefined,
      and the bit MUST remain unset (0x0) when sending and MUST be
      ignored when receiving.

   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.


   When the PRIORITY_UPDATE frame applies to a request stream, clients
   SHOULD provide a Prioritized Stream ID MUST be within the that refers to a stream limit.  If in the
   "open", "half-closed (local)", or "idle" state.  Servers can discard
   frames where the Prioritized Stream ID refers to a
   server receives 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 [RFC7540]) 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
   is beyond 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 limits, this SHOULD be treated as 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

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


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

   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.

   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.

7.  Merging Client- and Server-Driven Parameters

   It is not always the case that the client has the best understanding
   of how the HTTP responses deserve to be prioritized.  The server
   might have additional information that can be combined with the
   client's indicated priority in order to improve the prioritization of
   the response.  For example, use of an HTML document might depend
   heavily on one of the inline images; existence of such dependencies
   is typically best known to the server.  Or, a server that receives
   requests for a font [RFC8081] and images with the same urgency might
   give higher precedence to the font, so that a visual client can
   render textual information at an early moment.

   An origin can use the Priority response header field to indicate its
   view on how an HTTP response should be prioritized.  An intermediary
   that forwards an HTTP response can use the parameters found in the
   Priority response header field, in combination with the client
   Priority request header field, as input to its prioritization
   process.  No guidance is provided for merging priorities, this is
   left as an implementation decision.

   Absence of a priority parameter in an HTTP response indicates the
   server's disinterest in changing the client-provided value.  This is
   different from the logic being defined for the request header field,
   in which omission of a priority parameter implies the use of their
   default values (see Section 3).

   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") parameter.

8.  Client Scheduling

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

9.  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 3.1), sending higher urgency responses before
   lower urgency responses.

   It is RECOMMENDED that, when possible, servers respect the
   incremental parameter (Section 3.2).  Non-incremental responses of
   the same urgency SHOULD be served 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 in round-robin manner.

   The incremental parameter indicates how a client processes response
   bytes as they arrive.  Non-incremental resources are only used when
   all of the response payload has been received.  Incremental resources
   are used as parts, or chunks, of the response payload are received.
   Therefore, the timing of response data reception at the client, such
   as the time to early bytes or the time to receive the entire payload,
   plays an important role in perceived performance.  Timings depend on
   resource size but this scheme provides no explicit guidance about how
   a server should use size as an input to prioritization.  Instead, the
   following examples demonstrate how a server that strictly abides the
   scheduling guidance based on urgency and request generation order
   could find that early requests prevent serving of later requests.

   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 7.  In the absence of origin
   signals, applying default parameter values could be suboptimal.  How
   ever 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.  Fairness

   As a general guideline, a server SHOULD NOT use priority information
   for making schedule decisions across multiple connections, unless it
   knows that those connections originate from the same client.  Due to
   this, priority information conveyed over a non-coalesced HTTP
   connection (e.g., HTTP/1.1) might go unused.

   The remainder of this section discusses scenarios where unfairness is
   problematic and presents possible mitigations, or where unfairness is

   TODO: Discuss if we should add a signal that mitigates this issue.
   For example, we might add a SETTINGS parameter that indicates the
   next hop that the connection is NOT coalesced (see

10.1.  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 serve the
   responses in 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, X-Forwarded-For ([RFC7239])


   *  Via ([RFC7230], Section 5.7.1)

10.2.  HTTP/1.x Back Ends

   It is common for CDN infrastructure to support different HTTP
   versions on the front end and back end.  For instance, the client-
   facing edge might support HTTP/2 and HTTP/3 while communication to
   back end servers is done using HTTP/1.1.  Unlike with connection
   coalescing, the CDN will "de-mux" requests into discrete connections
   to the back end.  As HTTP/1.1 and older do not provide a way to
   concurrently transmit multiple responses, there is no immediate
   fairness issue in protocol.  However, back end servers MAY still use
   client headers for request scheduling.  Back end servers SHOULD only
   schedule based on client priority information where that information
   can be scoped to individual end clients.  Authentication and other
   session information might provide this linkability.

10.3.  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.

11.  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

   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.

12.  Security Considerations

   [CVE-2019-9513] aka "Resource Loop", is a DoS attack based on
   manipulation of the HTTP/2 priority tree.  Extensible priorities does
   not use stream dependencies, which mitigates this vulnerability.

   TBD: depending on the outcome of reprioritization discussions,
   following paragraphs may change or be removed.

   [RFC7540], Section 5.3.4 describes a scenario where closure of
   streams in the priority tree could cause suboptimal prioritization.
   To avoid this, [RFC7540] states that "an endpoint SHOULD retain
   stream prioritization state for a period after streams become
   closed".  Retaining state for streams no longer counted towards
   stream concurrency consumes server resources.  Furthermore, [RFC7540]
   identifies that reprioritization of a closed stream could affect
   dependents; it recommends updating the priority tree if sufficient
   state is stored, which will also consume server resources.  To limit
   this commitment, it is stated that "The amount of prioritization
   state that is retained MAY be limited" and "If a limit is applied,
   endpoints SHOULD maintain state for at least as many streams as
   allowed by their setting for SETTINGS_MAX_CONCURRENT_STREAMS.".
   Extensible priorities does not use stream dependencies, which
   minimizes most of the resource concerns related to this scenario.

   [RFC7540], Section 5.3.4 also presents considerations about the state
   required to store priority information about streams in an "idle"
   state.  This state can be limited by adopting the guidance about
   concurrency limits described above.  Extensible priorities is subject
   to a similar consideration because PRIORITY_UPDATE frames may arrive
   before the request that they reference.  A server is required to
   store the information in order to apply the most up-to-date signal to
   the request.  However, HTTP/3 implementations might have practical
   barriers to determining reasonable stream concurrency limits
   depending on the information that is available to them from the QUIC
   transport layer.  TODO: so what can we suggest?

13.  IANA Considerations

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

   Header field name:  Priority

   Applicable protocol:  http

   Status:  standard

   Author/change controller:  IETF

   Specification document(s):  This document

   Related information:  n/a

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


   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 [I-D.ietf-quic-http]:


   Code:  0xF0700 and 0xF0701

   Specification:  This document

   Upon publication, please create the HTTP Priority Parameters registry
   ( and populate it with the
   types defined in Section 3; see Section 3.3.1 for its associated

14.  References

14.1.  Normative References

              Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", draft-ietf-quic-http-31 (work Work in progress),
              September 2020. Progress, Internet-Draft, draft-ietf-
              quic-http-33, 15 December 2020, <

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-31 (work Work in progress), September 2020. Progress, Internet-Draft,
              draft-ietf-quic-transport-33, 13 December 2020,

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

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [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,

              Nottingham, M. and P. Kamp, "Structured Field Values for
              HTTP", draft-ietf-httpbis-header-structure-19 (work Work in
              progress), Progress, Internet-Draft, draft-ietf-
              httpbis-header-structure-19, 3 June 2020. 2020,

14.2.  Informative References

              Common Vulnerabilities and Exposures, "CVE-2019-9513", 1
              March 2019, <

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

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,

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

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

14.3.  URIs






Appendix A.  Acknowledgements

   Roy Fielding presented the idea of using a header field for
   representing priorities in
   slides-83-httpbis-5.pdf [4]. (
   slides-83-httpbis-5.pdf).  In
   prioritization-proposal [5], (
   prioritization-proposal), Patrick Meenan advocates for representing
   the priorities using a tuple of urgency and concurrency.  The ability
   to deprecate HTTP/2 prioritization is based on
   [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

Appendix B.  Change Log

B.1.  Since draft-ietf-httpbis-priority-01


   *  Describe considerations for server push prioritisation (#1056,

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

   *  Add a Parameters registry (#1371)

B.2.  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.3.  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.4.  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.5.  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.6.  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.7.  Since draft-kazuho-httpbis-priority-01


   *  Explain how reprioritization might be supported.


B.8.  Since draft-kazuho-httpbis-priority-00


   *  Expand urgency levels from 3 to 8.

Authors' Addresses

   Kazuho Oku

   Lucas Pardue