draft-ietf-httpbis-priority-10.txt   draft-ietf-httpbis-priority-11.txt 
HTTP K. Oku HTTP K. Oku
Internet-Draft Fastly Internet-Draft Fastly
Intended status: Standards Track L. Pardue Intended status: Standards Track L. Pardue
Expires: 23 May 2022 Cloudflare Expires: 12 June 2022 Cloudflare
19 November 2021 9 December 2021
Extensible Prioritization Scheme for HTTP Extensible Prioritization Scheme for HTTP
draft-ietf-httpbis-priority-10 draft-ietf-httpbis-priority-11
Abstract Abstract
This document describes a scheme that allows an HTTP client to This document describes a scheme that allows an HTTP client to
communicate its preferences for how the upstream server prioritizes communicate its preferences for how the upstream server prioritizes
responses to its requests, and also allows a server to hint to a responses to its requests, and also allows a server to hint to a
downstream intermediary how its responses should be prioritized when downstream intermediary how its responses should be prioritized when
they are forwarded. This document defines the Priority header field they are forwarded. This document defines the Priority header field
for communicating the initial priority in an HTTP version-independent for communicating the initial priority in an HTTP version-independent
manner, as well as HTTP/2 and HTTP/3 frames for reprioritizing manner, as well as HTTP/2 and HTTP/3 frames for reprioritizing
responses. These share a common format structure that is designed to responses. These share a common format structure that is designed to
provide future extensibility. provide future extensibility.
Note to Readers About This Document
_RFC EDITOR: please remove this section before publication_ This note is to be removed before publishing as an RFC.
Discussion of this draft takes place on the HTTP working group Status information for this document may be found at
mailing list (ietf-http-wg@w3.org), which is archived at https://datatracker.ietf.org/doc/draft-ietf-httpbis-priority/.
https://lists.w3.org/Archives/Public/ietf-http-wg/
(https://lists.w3.org/Archives/Public/ietf-http-wg/).
Working Group information can be found at https://httpwg.org/ Discussion of this document takes place on the HTTP Working Group
(https://httpwg.org/); source code and issues list for this draft can mailing list (mailto:ietf-http-wg@w3.org), which is archived at
be found at https://github.com/httpwg/http-extensions/labels/ https://lists.w3.org/Archives/Public/ietf-http-wg/. Working Group
priorities (https://github.com/httpwg/http-extensions/labels/ information can be found at https://httpwg.org/.
priorities).
Source for this draft and an issue tracker can be found at
https://github.com/httpwg/http-extensions/labels/priorities.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on 23 May 2022. This Internet-Draft will expire on 12 June 2022.
Copyright Notice Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/ Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document. license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
skipping to change at page 2, line 35 skipping to change at page 2, line 35
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 5 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 5
2. Motivation for Replacing RFC 7540 Priorities . . . . . . . . 5 2. Motivation for Replacing RFC 7540 Priorities . . . . . . . . 5
2.1. Disabling RFC 7540 Priorities . . . . . . . . . . . . . . 6 2.1. Disabling RFC 7540 Priorities . . . . . . . . . . . . . . 6
2.1.1. Advice when Using Extensible Priorities as the 2.1.1. Advice when Using Extensible Priorities as the
Alternative . . . . . . . . . . . . . . . . . . . . . 7 Alternative . . . . . . . . . . . . . . . . . . . . . 7
3. Applicability of the Extensible Priority Scheme . . . . . . . 7 3. Applicability of the Extensible Priority Scheme . . . . . . . 7
4. Priority Parameters . . . . . . . . . . . . . . . . . . . . . 7 4. Priority Parameters . . . . . . . . . . . . . . . . . . . . . 8
4.1. Urgency . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.1. Urgency . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Incremental . . . . . . . . . . . . . . . . . . . . . . . 9 4.2. Incremental . . . . . . . . . . . . . . . . . . . . . . . 9
4.3. Defining New Parameters . . . . . . . . . . . . . . . . . 10 4.3. Defining New Priority Parameters . . . . . . . . . . . . 10
4.3.1. Registration . . . . . . . . . . . . . . . . . . . . 10 4.3.1. Registration . . . . . . . . . . . . . . . . . . . . 10
5. The Priority HTTP Header Field . . . . . . . . . . . . . . . 11 5. The Priority HTTP Header Field . . . . . . . . . . . . . . . 11
6. Reprioritization . . . . . . . . . . . . . . . . . . . . . . 12 6. Reprioritization . . . . . . . . . . . . . . . . . . . . . . 12
7. The PRIORITY_UPDATE Frame . . . . . . . . . . . . . . . . . . 12 7. The PRIORITY_UPDATE Frame . . . . . . . . . . . . . . . . . . 12
7.1. HTTP/2 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 13 7.1. HTTP/2 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 13
7.2. HTTP/3 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 14 7.2. HTTP/3 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 14
8. Merging Client- and Server-Driven Parameters . . . . . . . . 15 8. Merging Client- and Server-Driven Priority Parameters . . . . 16
9. Client Scheduling . . . . . . . . . . . . . . . . . . . . . . 16 9. Client Scheduling . . . . . . . . . . . . . . . . . . . . . . 17
10. Server Scheduling . . . . . . . . . . . . . . . . . . . . . . 17 10. Server Scheduling . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Intermediaries with Multiple Backend Connections . . . . 18 10.1. Intermediaries with Multiple Backend Connections . . . . 19
11. Scheduling and the CONNECT Method . . . . . . . . . . . . . . 19 11. Scheduling and the CONNECT Method . . . . . . . . . . . . . . 19
12. Retransmission Scheduling . . . . . . . . . . . . . . . . . . 19 12. Retransmission Scheduling . . . . . . . . . . . . . . . . . . 19
13. Fairness . . . . . . . . . . . . . . . . . . . . . . . . . . 20 13. Fairness . . . . . . . . . . . . . . . . . . . . . . . . . . 20
13.1. Coalescing Intermediaries . . . . . . . . . . . . . . . 20 13.1. Coalescing Intermediaries . . . . . . . . . . . . . . . 20
13.2. HTTP/1.x Back Ends . . . . . . . . . . . . . . . . . . . 21 13.2. HTTP/1.x Back Ends . . . . . . . . . . . . . . . . . . . 21
13.3. Intentional Introduction of Unfairness . . . . . . . . . 21 13.3. Intentional Introduction of Unfairness . . . . . . . . . 21
14. Why use an End-to-End Header Field? . . . . . . . . . . . . . 21 14. Why use an End-to-End Header Field? . . . . . . . . . . . . . 21
15. Security Considerations . . . . . . . . . . . . . . . . . . . 22 15. Security Considerations . . . . . . . . . . . . . . . . . . . 22
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
17.1. Normative References . . . . . . . . . . . . . . . . . . 23 17.1. Normative References . . . . . . . . . . . . . . . . . . 23
17.2. Informative References . . . . . . . . . . . . . . . . . 24 17.2. Informative References . . . . . . . . . . . . . . . . . 24
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 25 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 25
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 25 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 26
B.1. Since draft-ietf-httpbis-priority-09 . . . . . . . . . . 25 B.1. Since draft-ietf-httpbis-priority-10 . . . . . . . . . . 26
B.2. Since draft-ietf-httpbis-priority-08 . . . . . . . . . . 25 B.2. Since draft-ietf-httpbis-priority-09 . . . . . . . . . . 26
B.3. Since draft-ietf-httpbis-priority-07 . . . . . . . . . . 26 B.3. Since draft-ietf-httpbis-priority-08 . . . . . . . . . . 26
B.4. Since draft-ietf-httpbis-priority-06 . . . . . . . . . . 26 B.4. Since draft-ietf-httpbis-priority-07 . . . . . . . . . . 26
B.5. Since draft-ietf-httpbis-priority-05 . . . . . . . . . . 26 B.5. Since draft-ietf-httpbis-priority-06 . . . . . . . . . . 26
B.6. Since draft-ietf-httpbis-priority-04 . . . . . . . . . . 26 B.6. Since draft-ietf-httpbis-priority-05 . . . . . . . . . . 27
B.7. Since draft-ietf-httpbis-priority-03 . . . . . . . . . . 26 B.7. Since draft-ietf-httpbis-priority-04 . . . . . . . . . . 27
B.8. Since draft-ietf-httpbis-priority-02 . . . . . . . . . . 27 B.8. Since draft-ietf-httpbis-priority-03 . . . . . . . . . . 27
B.9. Since draft-ietf-httpbis-priority-01 . . . . . . . . . . 27 B.9. Since draft-ietf-httpbis-priority-02 . . . . . . . . . . 27
B.10. Since draft-ietf-httpbis-priority-00 . . . . . . . . . . 27 B.10. Since draft-ietf-httpbis-priority-01 . . . . . . . . . . 27
B.11. Since draft-kazuho-httpbis-priority-04 . . . . . . . . . 27 B.11. Since draft-ietf-httpbis-priority-00 . . . . . . . . . . 28
B.12. Since draft-kazuho-httpbis-priority-03 . . . . . . . . . 28 B.12. Since draft-kazuho-httpbis-priority-04 . . . . . . . . . 28
B.13. Since draft-kazuho-httpbis-priority-02 . . . . . . . . . 28 B.13. Since draft-kazuho-httpbis-priority-03 . . . . . . . . . 28
B.14. Since draft-kazuho-httpbis-priority-01 . . . . . . . . . 28 B.14. Since draft-kazuho-httpbis-priority-02 . . . . . . . . . 28
B.15. Since draft-kazuho-httpbis-priority-00 . . . . . . . . . 28 B.15. Since draft-kazuho-httpbis-priority-01 . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 B.16. Since draft-kazuho-httpbis-priority-00 . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction 1. Introduction
It is common for representations of an HTTP [HTTP] resource to have It is common for representations of an HTTP [HTTP] resource to have
relationships to one or more other resources. Clients will often relationships to one or more other resources. Clients will often
discover these relationships while processing a retrieved discover these relationships while processing a retrieved
representation, which may lead to further retrieval requests. representation, which may lead to further retrieval requests.
Meanwhile, the nature of the relationship determines whether the Meanwhile, the nature of the relationship determines whether the
client is blocked from continuing to process locally available client is blocked from continuing to process locally available
resources. An example of this is visual rendering of an HTML resources. An example of this is visual rendering of an HTML
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RFC 7540 [RFC7540] stream priority allowed a client to send a series RFC 7540 [RFC7540] stream priority allowed a client to send a series
of priority signals that communicate to the server a "priority tree"; of priority signals that communicate to the server a "priority tree";
the structure of this tree represents the client's preferred relative the structure of this tree represents the client's preferred relative
ordering and weighted distribution of the bandwidth among HTTP ordering and weighted distribution of the bandwidth among HTTP
responses. Servers could use these priority signals as input into responses. Servers could use these priority signals as input into
prioritization decision making. prioritization decision making.
The design and implementation of RFC 7540 stream priority was The design and implementation of RFC 7540 stream priority was
observed to have shortcomings, explained in Section 2. HTTP/2 observed to have shortcomings, explained in Section 2. HTTP/2
[HTTP2] has consequently deprecated the use of these stream priority [HTTP2] has consequently deprecated the use of these stream priority
signals. 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 This document describes an extensible scheme for prioritizing HTTP
responses that uses absolute values. Section 4 defines priority responses that uses absolute values. Section 4 defines priority
parameters, which are a standardized and extensible format of parameters, which are a standardized and extensible format of
priority information. Section 5 defines the Priority HTTP header priority information. Section 5 defines the Priority HTTP header
field, a protocol-version-independent and end-to-end priority signal. field, a protocol-version-independent and end-to-end priority signal.
Clients can use this header to signal priority to servers in order to Clients can send this header field to signal their view of how
specify the precedence of HTTP responses. Similarly, servers behind responses should be prioritized. Similarly, servers behind an
an intermediary can use it to signal priority to the intermediary. intermediary can use it to signal priority to the intermediary.
Section 7.1 and Section 7.2 define version-specific frames that carry After sending a request, a client can change the priority of the
parameters, which clients can use for reprioritization. response (see Section 6) using 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 Header field and frame priority signals are input to a server's
response prioritization process. They are only a suggestion and do response prioritization process. They are only a suggestion and do
not guarantee any particular processing or transmission order for one not guarantee any particular processing or transmission order for one
response relative to any other response. Section 10 and Section 12 response relative to any other response. Section 10 and Section 12
provide consideration and guidance about how servers might act upon provide consideration and guidance about how servers might act upon
signals. signals.
The prioritization scheme and priority signals defined herein can act
as a substitute for RFC 7540 stream priority.
1.1. Notational Conventions 1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
The terms Dictionary, sf-boolean, sf-dictionary, and sf-integer are The terms Dictionary, sf-boolean, sf-dictionary, and sf-integer are
imported from [STRUCTURED-FIELDS]. imported from [STRUCTURED-FIELDS].
Example HTTP requests and responses use the HTTP/2-style formatting Example HTTP requests and responses use the HTTP/2-style formatting
from [HTTP2]. from [HTTP2].
This document uses the variable-length integer encoding from [QUIC]. This document uses the variable-length integer encoding from [QUIC].
The term control stream is used to describe the HTTP/2 stream with The term control stream is used to describe both the HTTP/2 stream
identifier 0x0, and HTTP/3 control stream; see Section 6.2.1 of with identifier 0x0, and the HTTP/3 control stream; see Section 6.2.1
[HTTP3]. of [HTTP3].
The term HTTP/2 priority signal is used to describe the priority The term HTTP/2 priority signal is used to describe the priority
information sent from clients to servers in HTTP/2 frames; see information sent from clients to servers in HTTP/2 frames; see
Section 5.3.2 of [HTTP2]. Section 5.3.2 of [HTTP2].
2. Motivation for Replacing RFC 7540 Priorities 2. Motivation for Replacing RFC 7540 Priorities
RFC 7540 stream priority (see Section 5.3 of [RFC7540]) is a complex RFC 7540 stream priority (see Section 5.3 of [RFC7540]) is a complex
system where clients signal stream dependencies and weights to system where clients signal stream dependencies and weights to
describe an unbalanced tree. It suffered from limited deployment and describe an unbalanced tree. It suffered from limited deployment and
interoperability and was deprecated in a revision of HTTP/2 [HTTP2]. interoperability and was deprecated in a revision of HTTP/2 [HTTP2].
HTTP/2 retains these protocol elements in order to maintain wire HTTP/2 retains these protocol elements in order to maintain wire
compatibility (see Section 5.3.2 of [HTTP2]), which means that they compatibility (see Section 5.3.2 of [HTTP2]), which means that they
might still be used in the absence of alternative signaling, such as might still be used even in the presence of alternative signaling,
the scheme this document describes. such as the scheme this document describes.
Many RFC 7540 server implementations do not act on HTTP/2 priority Many RFC 7540 server implementations do not act on HTTP/2 priority
signals. signals.
Prioritization can use information that servers have about resources Prioritization can use information that servers have about resources
or the order in which requests are generated. For example, a server, or the order in which requests are generated. For example, a server,
with knowledge of an HTML document structure, might want to with knowledge of an HTML document structure, might want to
prioritize the delivery of images that are critical to user prioritize the delivery of images that are critical to user
experience above other images. With RFC 7540 it is difficult for experience above other images. With RFC 7540 it is difficult for
servers to interpret signals from clients for prioritization as the servers to interpret signals from clients for prioritization as the
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RFC 7540 does not define a method that can be used by a server to RFC 7540 does not define a method that can be used by a server to
provide a priority signal for intermediaries. provide a priority signal for intermediaries.
RFC 7540 priority is expressed relative to other requests on the same RFC 7540 priority is expressed relative to other requests on the same
connection. Many requests are generated without knowledge of how connection. Many requests are generated without knowledge of how
other requests might share a connection, which makes this difficult other requests might share a connection, which makes this difficult
to use reliably, especially in protocols that do not have strong to use reliably, especially in protocols that do not have strong
ordering guarantees, like HTTP/3 [HTTP3]. ordering guarantees, like HTTP/3 [HTTP3].
Multiple experiments from independent research have shown that Multiple experiments from independent research ([MARX], [MEENAN])
simpler schemes can reach at least equivalent performance have shown that simpler schemes can reach at least equivalent
characteristics compared to the more complex RFC 7540 setups seen in performance characteristics compared to the more complex RFC 7540
practice, at least for the web use case. setups seen in practice, at least for the web use case.
2.1. Disabling RFC 7540 Priorities 2.1. Disabling RFC 7540 Priorities
The problems and insights set out above provided the motivation for The problems and insights set out above provided the motivation for
an alternative to RFC 7540 stream priority (see Section 5.3 of an alternative to RFC 7540 stream priority (see Section 5.3 of
[HTTP2]). [HTTP2]).
The SETTINGS_NO_RFC7540_PRIORITIES HTTP/2 setting is defined by this The SETTINGS_NO_RFC7540_PRIORITIES HTTP/2 setting is defined by this
document in order to allow endpoints to omit or ignore HTTP/2 document in order to allow endpoints to omit or ignore HTTP/2
priority signals (see Section 5.3.2 of [HTTP2]), as described below. priority signals (see Section 5.3.2 of [HTTP2]), as described below.
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Similarly, if the client receives SETTINGS_NO_RFC7540_PRIORITIES with Similarly, if the client receives SETTINGS_NO_RFC7540_PRIORITIES with
value of 0 or if the settings parameter was absent, it SHOULD stop value of 0 or if the settings parameter was absent, it SHOULD stop
sending PRIORITY_UPDATE frames (Section 7.1), since those frames are sending PRIORITY_UPDATE frames (Section 7.1), since those frames are
likely to be ignored. However, the client MAY continue sending the 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 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 might be useful to nodes behind the server that the client is
directly connected to. directly connected to.
3. Applicability of the Extensible Priority Scheme 3. Applicability of the Extensible Priority Scheme
The priority scheme defined by this document considers only the The priority scheme defined by this document is primarily focused on
prioritization of HTTP messages and tunnels, see Section 9, the prioritization of HTTP response messages (see Section 3.4 of
Section 10, and Section 11. [HTTP]). It defines new priority parameters (Section 4) and their
conveyors (Section 5 and Section 7) 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.
Where HTTP extensions change stream behavior or define new data The CONNECT method (see Section 9.3.6 of [HTTP]) can be used to
carriage mechanisms, they can also define how this priority scheme establish tunnels. Signaling applies similarly to tunnels;
can be applied. 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 4. Priority Parameters
The priority information is a sequence of key-value pairs, providing The priority information is a sequence of key-value pairs, providing
room for future extensions. Each key-value pair represents a room for future extensions. Each key-value pair represents a
priority parameter. priority parameter.
The Priority HTTP header field (Section 5) is an end-to-end way to The Priority HTTP header field (Section 5) is an end-to-end way to
transmit this set of parameters when a request or a response is transmit this set of priority parameters when a request or a response
issued. In order to reprioritize a request, HTTP-version-specific is issued. In order to reprioritize a request (Section 6), HTTP-
PRIORITY_UPDATE frames (Section 7.1 and Section 7.2) are used by version-specific PRIORITY_UPDATE frames (Section 7.1 and Section 7.2)
clients to transmit the same information on a single hop. are used by clients to transmit the same information on a single hop.
Intermediaries can consume and produce priority signals in a Intermediaries can consume and produce priority signals in a
PRIORITY_UPDATE frame or Priority header field. Sending a PRIORITY_UPDATE frame or Priority header field. Sending a
PRIORITY_UPDATE frame preserves the signal from the client, but PRIORITY_UPDATE frame preserves the signal from the client, but
provides a signal that overrides that for the next hop; see provides a signal that overrides that for the next hop; see
Section 14. Replacing or adding a Priority header field overrides Section 14. Replacing or adding a Priority header field overrides
any signal from a client and can affect prioritization for all any signal from a client and can affect prioritization for all
subsequent recipients. subsequent recipients.
For both the Priority header field and the PRIORITY_UPDATE frame, the For both the Priority header field and the PRIORITY_UPDATE frame, the
set of priority parameters is encoded as a Structured Fields set of priority parameters is encoded as a Structured Fields
Dictionary (see Section 3.2 of [STRUCTURED-FIELDS]). Dictionary (see Section 3.2 of [STRUCTURED-FIELDS]).
This document defines the urgency(u) and incremental(i) parameters. This document defines the urgency(u) and incremental(i) priority
When receiving an HTTP request that does not carry these priority parameters. When receiving an HTTP request that does not carry these
parameters, a server SHOULD act as if their default values were priority parameters, a server SHOULD act as if their default values
specified. Note that handling of omitted parameters is different were specified.
when processing an HTTP response; see Section 8.
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 Receivers parse the Dictionary as defined in Section 4.2 of
[STRUCTURED-FIELDS]. Where the Dictionary is successfully parsed, [STRUCTURED-FIELDS]. Where the Dictionary is successfully parsed,
this document places the additional requirement that unknown priority this document places the additional requirement that unknown priority
parameters, parameters with out-of-range values, or values of parameters, priority parameters with out-of-range values, or values
unexpected types MUST be ignored. of unexpected types MUST be ignored.
4.1. Urgency 4.1. Urgency
The urgency parameter (u) takes an integer between 0 and 7, in The urgency parameter (u) takes an integer between 0 and 7, in
descending order of priority. descending order of priority.
The value is encoded as an sf-integer. The default value is 3. The value is encoded as an sf-integer. The default value is 3.
Endpoints use this parameter to communicate their view of the Endpoints use this parameter to communicate their view of the
precedence of HTTP responses. The chosen value of urgency can be precedence of HTTP responses. The chosen value of urgency can be
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The following example shows a request for a JPEG file with the The following example shows a request for a JPEG file with the
urgency parameter set to 5 and the incremental parameter set to true. urgency parameter set to 5 and the incremental parameter set to true.
:method = GET :method = GET
:scheme = https :scheme = https
:authority = example.net :authority = example.net
:path = /image.jpg :path = /image.jpg
priority = u=5, i priority = u=5, i
4.3. Defining New Parameters 4.3. Defining New Priority Parameters
When attempting to define new parameters, care must be taken so that When attempting to define new priority parameters, care must be taken
they do not adversely interfere with prioritization performed by so that they do not adversely interfere with prioritization performed
existing endpoints or intermediaries that do not understand the newly by existing endpoints or intermediaries that do not understand the
defined parameter. Since unknown parameters are ignored, new newly defined priority parameter. Since unknown priority parameters
parameters should not change the interpretation of, or modify, the are ignored, new priority parameters should not change the
urgency (see Section 4.1) or incremental (see Section 4.2) parameters interpretation of, or modify, the urgency (see Section 4.1) or
in a way that is not backwards compatible or fallback safe. 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 For example, if there is a need to provide more granularity than
eight urgency levels, it would be possible to subdivide the range eight urgency levels, it would be possible to subdivide the range
using an additional parameter. Implementations that do not recognize using an additional priority parameter. Implementations that do not
the parameter can safely continue to use the less granular eight recognize the parameter can safely continue to use the less granular
levels. eight levels.
Alternatively, the urgency can be augmented. For example, a Alternatively, the urgency can be augmented. For example, a
graphical user agent could send a visible parameter to indicate if graphical user agent could send a visible priority parameter to
the resource being requested is within the viewport. indicate if the resource being requested is within the viewport.
Generic parameters are preferred over vendor-specific, application- Generic priority parameters are preferred over vendor-specific,
specific or deployment-specific values. If a generic value cannot be application-specific or deployment-specific values. If a generic
agreed upon in the community, the parameter's name should be value cannot be agreed upon in the community, the parameter's name
correspondingly specific (e.g., with a prefix that identifies the should be correspondingly specific (e.g., with a prefix that
vendor, application or deployment). identifies the vendor, application or deployment).
4.3.1. Registration 4.3.1. Registration
New Priority parameters can be defined by registering them in the New priority parameters can be defined by registering them in the
HTTP Priority Parameters Registry. The registry governs the keys HTTP Priority Parameters Registry. The registry governs the keys
(short textual strings) used in Structured Fields Dictionary (see (short textual strings) used in the Structured Fields Dictionary (see
Section 3.2 of [STRUCTURED-FIELDS]). Since each HTTP request can Section 3.2 of [STRUCTURED-FIELDS]). Since each HTTP request can
have associated priority signals, there is value in having short key have associated priority signals, there is value in having short key
lengths, especially single-character strings. In order to encourage lengths, especially single-character strings. In order to encourage
extension while avoiding unintended conflict among attractive key extension while avoiding unintended conflict among attractive key
values, the HTTP Priority Parameters Registry operates two values, the HTTP Priority Parameters Registry operates two
registration policies depending on key length. registration policies depending on key length.
* Registration requests for parameters with a key length of one use * Registration requests for priority parameters with a key length of
the Specification Required policy, as per Section 4.6 of one use the Specification Required policy, as per Section 4.6 of
[RFC8126]. [RFC8126].
* Registration requests for parameters with a key length greater * Registration requests for priority parameters with a key length
than one use the Expert Review policy, as per Section 4.5 of greater than one use the Expert Review policy, as per Section 4.5
[RFC8126]. A specification document is appreciated, but not of [RFC8126]. A specification document is appreciated, but not
required. required.
When reviewing registration requests, the designated expert(s) can When reviewing registration requests, the designated expert(s) can
consider the additional guidance provided in Section 4.3 but cannot consider the additional guidance provided in Section 4.3 but cannot
use it as a basis for rejection. use it as a basis for rejection.
Registration requests should use the following template: Registration requests should use the following template:
Name: [a name for the Priority Parameter that matches key] Name: [a name for the Priority Parameter that matches key]
Description: [a description of the parameter semantics and value] Description: [a description of the priority parameter semantics and
value]
Reference: [to a specification defining this parameter] Reference: [to a specification defining this priority parameter]
See the registry at https://iana.org/assignments/http-priority See the registry at https://iana.org/assignments/http-priority
(https://iana.org/assignments/http-priority) for details on where to (https://iana.org/assignments/http-priority) for details on where to
send registration requests. send registration requests.
5. The Priority HTTP Header Field 5. The Priority HTTP Header Field
The Priority HTTP header field carries priority parameters Section 4. The Priority HTTP header field carries priority parameters (see
It can appear in requests and responses. It is an end-to-end signal Section 4). It can appear in requests and responses. It is an end-
of the request priority from the client or the response priority from to-end signal of the request priority from the client or the response
the server. Section 8 describes how intermediaries can combine the priority from the server. Section 8 describes how intermediaries can
priority information from client requests and server responses to combine the priority information sent from clients and servers.
correct or amend the precedence. Clients cannot interpret the Clients cannot interpret the appearance or omission of a Priority
appearance or omission of a Priority response header as response header field as acknowledgement that any prioritization has
acknowledgement that any prioritization has occurred. Guidance for occurred. Guidance for how endpoints can act on Priority header
how endpoints can act on Priority header values is given in values is given in Section 10 and Section 9.
Section 10 and Section 9.
Priority is a Dictionary (Section 3.2 of [STRUCTURED-FIELDS]): Priority is a Dictionary (Section 3.2 of [STRUCTURED-FIELDS]):
Priority = sf-dictionary Priority = sf-dictionary
An HTTP request with a Priority header field might be cached and re-
As is the ordinary case for HTTP caching [CACHING], a response with a used for subsequent requests; see [CACHING]. When an origin server
Priority header field might be cached and re-used for subsequent generates the Priority response header field based on properties of
requests. When an origin server generates the Priority response an HTTP request it receives, the server is expected to control the
header field based on properties of an HTTP request it receives, the cacheability or the applicability of the cached response, by using
server is expected to control the cacheability or the applicability header fields that control the caching behavior (e.g., Cache-Control,
of the cached response, by using header fields that control the Vary).
caching behavior (e.g., Cache-Control, Vary).
6. Reprioritization 6. Reprioritization
After a client sends a request, it may be beneficial to change the 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 priority of the response. As an example, a web browser might issue a
prefetch request for a JavaScript file with the urgency parameter of prefetch request for a JavaScript file with the urgency parameter of
the Priority request header field set to u=7 (background). Then, the Priority request header field set to u=7 (background). Then,
when the user navigates to a page which references the new JavaScript when the user navigates to a page which references the new JavaScript
file, while the prefetch is in progress, the browser would send a file, while the prefetch is in progress, the browser would send a
reprioritization signal with the priority field value set to u=0. reprioritization signal with the priority field value set to u=0.
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HTTP/3, the identifier is either the Stream ID or Push ID. Unlike 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 the Priority header field, the PRIORITY_UPDATE frame is a hop-by-hop
signal. signal.
PRIORITY_UPDATE frames are sent by clients on the control stream, PRIORITY_UPDATE frames are sent by clients on the control stream,
allowing them to be sent independent from the stream that carries the allowing them to be sent independent from the stream that carries the
response. This means they can be used to reprioritize a response or response. This means they can be used to reprioritize a response or
a push stream; or signal the initial priority of a response instead a push stream; or signal the initial priority of a response instead
of the Priority header field. of the Priority header field.
A PRIORITY_UPDATE frame communicates a complete set of all parameters A PRIORITY_UPDATE frame communicates a complete set of all priority
in the Priority Field Value field. Omitting a parameter is a signal parameters in the Priority Field Value field. Omitting a priority
to use the parameter's default value. Failure to parse the Priority parameter is a signal to use its default value. Failure to parse the
Field Value MAY be treated as a connection error. In HTTP/2 the Priority Field Value MAY be treated as a connection error. In HTTP/2
error is of type PROTOCOL_ERROR; in HTTP/3 the error is of type the error is of type PROTOCOL_ERROR; in HTTP/3 the error is of type
H3_GENERAL_PROTOCOL_ERROR. H3_GENERAL_PROTOCOL_ERROR.
A client MAY send a PRIORITY_UPDATE frame before the stream that it 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). references is open (except for HTTP/2 push streams; see Section 7.1).
Furthermore, HTTP/3 offers no guaranteed ordering across streams, Furthermore, HTTP/3 offers no guaranteed ordering across streams,
which could cause the frame to be received earlier than intended. which could cause the frame to be received earlier than intended.
Either case leads to a race condition where a server receives a Either case leads to a race condition where a server receives a
PRIORITY_UPDATE frame that references a request stream that is yet to PRIORITY_UPDATE frame that references a request stream that is yet to
be opened. To solve this condition, for the purposes of scheduling, be opened. To solve this condition, for the purposes of scheduling,
the most recently received PRIORITY_UPDATE frame can be considered as the most recently received PRIORITY_UPDATE frame can be considered as
the most up-to-date information that overrides any other signal. the most up-to-date information that overrides any other signal.
Servers SHOULD buffer the most recently received PRIORITY_UPDATE Servers SHOULD buffer the most recently received PRIORITY_UPDATE
frame and apply it once the referenced stream is opened. Holding frame and apply it once the referenced stream is opened. Holding
PRIORITY_UPDATE frames for each stream requires server resources, PRIORITY_UPDATE frames for each stream requires server resources,
which can can be bound by local implementation policy. Although which can can be bounded by local implementation policy. Although
there is no limit to the number of PRIORITY_UPDATES that can be sent, there is no limit to the number of PRIORITY_UPDATES that can be sent,
storing only the most recently received frame limits resource storing only the most recently received frame limits resource
commitment. commitment.
7.1. HTTP/2 PRIORITY_UPDATE Frame 7.1. HTTP/2 PRIORITY_UPDATE Frame
The HTTP/2 PRIORITY_UPDATE frame (type=0x10) is used by clients to The HTTP/2 PRIORITY_UPDATE frame (type=0x10) is used by clients to
signal the initial priority of a response, or to reprioritize a signal the initial priority of a response, or to reprioritize a
response or push stream. It carries the stream ID of the response response or push stream. It carries the stream ID of the response
and the priority in ASCII text, using the same representation as the and the priority in ASCII text, using the same representation as the
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The push-stream variant PRIORITY_UPDATE (type=0xF0701) MUST reference The push-stream variant PRIORITY_UPDATE (type=0xF0701) MUST reference
a promised push stream. If a server receives a PRIORITY_UPDATE a promised push stream. If a server receives a PRIORITY_UPDATE
(type=0xF0701) with a Push ID that is greater than the maximum Push (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 ID or which has not yet been promised, this MUST be treated as a
connection error of type H3_ID_ERROR. connection error of type H3_ID_ERROR.
PRIORITY_UPDATE frames of either type are only sent by clients. If a 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 client receives a PRIORITY_UPDATE frame, this MUST be treated as a
connection error of type H3_FRAME_UNEXPECTED. connection error of type H3_FRAME_UNEXPECTED.
8. Merging Client- and Server-Driven Parameters 8. Merging Client- and Server-Driven Priority Parameters
It is not always the case that the client has the best understanding It is not always the case that the client has the best understanding
of how the HTTP responses deserve to be prioritized. The server of how the HTTP responses deserve to be prioritized. The server
might have additional information that can be combined with the might have additional information that can be combined with the
client's indicated priority in order to improve the prioritization of client's indicated priority in order to improve the prioritization of
the response. For example, use of an HTML document might depend the response. For example, use of an HTML document might depend
heavily on one of the inline images; existence of such dependencies heavily on one of the inline images; existence of such dependencies
is typically best known to the server. Or, a server that receives is typically best known to the server. Or, a server that receives
requests for a font [RFC8081] and images with the same urgency might requests for a font [RFC8081] and images with the same urgency might
give higher precedence to the font, so that a visual client can give higher precedence to the font, so that a visual client can
render textual information at an early moment. render textual information at an early moment.
An origin can use the Priority response header field to indicate its An origin can use the Priority response header field to indicate its
view on how an HTTP response should be prioritized. An intermediary view on how an HTTP response should be prioritized. An intermediary
that forwards an HTTP response can use the parameters found in the that forwards an HTTP response can use the priority parameters found
Priority response header field, in combination with the client in the Priority response header field, in combination with the client
Priority request header field, as input to its prioritization Priority request header field, as input to its prioritization
process. No guidance is provided for merging priorities, this is process. No guidance is provided for merging priorities, this is
left as an implementation decision. left as an implementation decision.
Absence of a priority parameter in an HTTP response indicates the Absence of a priority parameter in an HTTP response indicates the
server's disinterest in changing the client-provided value. This is server's disinterest in changing the client-provided value. This is
different from the logic being defined for the request header field, different from the the request header field, in which omission of a
in which omission of a priority parameter implies the use of their priority parameter implies the use of their default values (see
default values (see Section 4). Section 4).
As a non-normative example, when the client sends an HTTP request As a non-normative example, when the client sends an HTTP request
with the urgency parameter set to 5 and the incremental parameter set with the urgency parameter set to 5 and the incremental parameter set
to true to true
:method = GET :method = GET
:scheme = https :scheme = https
:authority = example.net :authority = example.net
:path = /menu.png :path = /menu.png
priority = u=5, i priority = u=5, i
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the client, as the server did not specify the incremental(i) the client, as the server did not specify the incremental(i)
parameter. parameter.
9. Client Scheduling 9. Client Scheduling
A client MAY use priority values to make local processing or A client MAY use priority values to make local processing or
scheduling choices about the requests it initiates. scheduling choices about the requests it initiates.
10. Server Scheduling 10. Server Scheduling
Priority signals are input to a prioritization process. They do not It is generally beneficial for an HTTP server to send all responses
guarantee any particular processing or transmission order for one as early as possible. However, when serving multiple requests on a
response relative to any other response. An endpoint cannot force a single connection, there could be competition between the requests
peer to process concurrent request in a particular order using for resources such as connection bandwidth. This section describes
priority. Expressing priority is therefore only a suggestion. considerations regarding how servers can schedule the order in which
the competing responses will be sent, when such competition exists.
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 Server scheduling is a prioritization process based on many inputs,
signals. Servers can use other information to prioritize responses. 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 is no unilateral perfect scheduler. This
document provides some basic, 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 It is RECOMMENDED that, when possible, servers respect the urgency
parameter (Section 4.1), sending higher urgency responses before parameter (Section 4.1), sending higher urgency responses before
lower urgency responses. lower urgency responses.
The incremental parameter indicates how a client processes response The incremental parameter indicates how a client processes response
bytes as they arrive. It is RECOMMENDED that, when possible, servers bytes as they arrive. It is RECOMMENDED that, when possible, servers
respect the incremental parameter (Section 4.2). Non-incremental respect the incremental parameter (Section 4.2).
resources can only be used when all of the response payload has been
received. Therefore, non-incremental responses of the same urgency Non-incremental responses of the same urgency SHOULD be served by
SHOULD be served in their entirety, one-by-one, based on the stream prioritizing bandwidth allocation in ascending order of the stream
ID, which corresponds to the order in which clients make requests. ID, which corresponds to the order in which clients make requests.
Doing so ensures that clients can use request ordering to influence Doing so ensures that clients can use request ordering to influence
response order. response order.
Incremental responses of the same urgency SHOULD be served by sharing Incremental responses of the same urgency SHOULD be served by sharing
bandwidth amongst them. Incremental resources are used as parts, or bandwidth amongst them. Incremental resources are used as parts, or
chunks, of the response payload are received. A client might benefit chunks, of the response payload are received. A client might benefit
more from receiving a portion of all these resources rather than the more from receiving a portion of all these resources rather than the
entirety of a single resource. How large a portion of the resource entirety of a single resource. How large a portion of the resource
is needed to be useful in improving performance varies. Some is needed to be useful in improving performance varies. Some
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signals, applying default parameter values could be suboptimal. By signals, applying default parameter values could be suboptimal. By
whatever means a server decides to schedule a pushed response, it can whatever means a server decides to schedule a pushed response, it can
signal the intended priority to the client by including the Priority signal the intended priority to the client by including the Priority
field in a PUSH_PROMISE or HEADERS frame. field in a PUSH_PROMISE or HEADERS frame.
10.1. Intermediaries with Multiple Backend Connections 10.1. Intermediaries with Multiple Backend Connections
An intermediary serving an HTTP connection might split requests over An intermediary serving an HTTP connection might split requests over
multiple backend connections. When it applies prioritization rules multiple backend connections. When it applies prioritization rules
strictly, low priority requests cannot make progress while requests strictly, low priority requests cannot make progress while requests
with higher priorities are inflight. This blocking can propagate to with higher priorities are in flight. This blocking can propagate to
backend connections, which the peer might interpret as a connection backend connections, which the peer might interpret as a connection
stall. Endpoints often implement protections against stalls, such as stall. Endpoints often implement protections against stalls, such as
abruptly closing connections after a certain time period. To reduce abruptly closing connections after a certain time period. To reduce
the possibility of this occurring, intermediaries can avoid strictly the possibility of this occurring, intermediaries can avoid strictly
following prioritization and instead allocate small amounts of following prioritization and instead allocate small amounts of
bandwidth for all the requests that they are forwarding, so that bandwidth for all the requests that they are forwarding, so that
every request can make some progress over time. every request can make some progress over time.
Similarly, servers SHOULD allocate some amount of bandwidths to Similarly, servers SHOULD allocate some amount of bandwidths to
streams acting as tunnels. streams acting as tunnels.
11. Scheduling and the CONNECT Method 11. Scheduling and the CONNECT Method
When a request stream carries the CONNECT method, the scheduling When a request stream carries the CONNECT method, the scheduling
guidance in this document applies to the frames on the stream. A guidance in this document applies to the frames on the stream. A
client that issues multiple CONNECT requests can set the incremental client that issues multiple CONNECT requests can set the incremental
parameter to true, servers that implement the recommendation in parameter to true. Servers that implement the recommendations for
Section 10 will schedule these fairly. handling of the incremental parameter in Section 10 are likely to
schedule these fairly, avoiding one CONNECT stream from blocking
others.
12. Retransmission Scheduling 12. Retransmission Scheduling
Transport protocols such as TCP and QUIC provide reliability by Transport protocols such as TCP and QUIC provide reliability by
detecting packet losses and retransmitting lost information. While detecting packet losses and retransmitting lost information. In
this document specifies HTTP-layer prioritization, its effectiveness addition to the considerations in Section 10, scheduling of
can be further enhanced if the transport layer factors priority into retransmission data could compete with new data. The remainder of
scheduling both new data and retransmission data. The remainder of
this section discusses considerations when using QUIC. this section discusses considerations when using QUIC.
Section 13.3 of [QUIC] states "Endpoints SHOULD prioritize Section 13.3 of [QUIC] states "Endpoints SHOULD prioritize
retransmission of data over sending new data, unless priorities retransmission of data over sending new data, unless priorities
specified by the application indicate otherwise". When an HTTP/3 specified by the application indicate otherwise". When an HTTP/3
application uses the priority scheme defined in this document and the application uses the priority scheme defined in this document and the
QUIC transport implementation supports application indicated stream QUIC transport implementation supports application indicated stream
priority, a transport that considers the relative priority of streams priority, a transport that considers the relative priority of streams
when scheduling both new data and retransmission data might better when scheduling both new data and retransmission data might better
match the expectations of the application. However, there are no match the expectations of the application. However, there are no
requirements on how a transport chooses to schedule based on this requirements on how a transport chooses to schedule based on this
information because the decision depends on several factors and information because the decision depends on several factors and
trade-offs. It could prioritize new data for a higher urgency stream trade-offs. It could prioritize new data for a higher urgency stream
over retransmission data for a lower priority stream, or it could over retransmission data for a lower priority stream, or it could
prioritize retransmission data over new data irrespective of prioritize retransmission data over new data irrespective of
urgencies. urgencies.
Section 6.2.4 of [QUIC-RECOVERY], also highlights consideration of Section 6.2.4 of [QUIC-RECOVERY] also highlights consideration of
application priorities when sending probe packets after Probe Timeout application priorities when sending probe packets after Probe Timeout
timer expiration. A QUIC implementation supporting application- timer expiration. A QUIC implementation supporting application-
indicated priorities might use the relative priority of streams when indicated priorities might use the relative priority of streams when
choosing probe data. choosing probe data.
13. Fairness 13. Fairness
As a general guideline, a server SHOULD NOT use priority information Typically, HTTP implementations depend on the underlying transport to
for making scheduling decisions across multiple connections, unless maintain fairness between connections competing for bandwidth. When
it knows that those connections originate from the same client. Due HTTP requests are forwarded through intermediaries, progress made by
to this, priority information conveyed over a non-coalesced HTTP each connection originating from end clients can become different
connection (e.g., HTTP/1.1) might go unused. 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.
The remainder of this section discusses scenarios where unfairness is Conversely, Section 13.3 discusses how servers might intentionally
problematic and presents possible mitigations, or where unfairness is allocate unequal bandwidth to some connections depending on the
desirable. priority signals.
13.1. Coalescing Intermediaries 13.1. Coalescing Intermediaries
When an intermediary coalesces HTTP requests coming from multiple When an intermediary coalesces HTTP requests coming from multiple
clients into one HTTP/2 or HTTP/3 connection going to the backend clients into one HTTP/2 or HTTP/3 connection going to the backend
server, requests that originate from one client might have higher server, requests that originate from one client might carry signals
precedence than those coming from others. indicating higher priority than those coming from others.
It is sometimes beneficial for the server running behind an It is sometimes beneficial for the server running behind an
intermediary to obey to the value of the Priority header field. As intermediary to obey Priority header field values. As an example, a
an example, a resource-constrained server might defer the resource-constrained server might defer the transmission of software
transmission of software update files that would have the background update files that would have the background urgency being associated.
urgency being associated. However, in the worst case, the asymmetry However, in the worst case, the asymmetry between the priority
between the precedence declared by multiple clients might cause declared by multiple clients might cause responses going to one user
responses going to one user agent to be delayed totally after those agent to be delayed totally after those going to another.
going to another.
In order to mitigate this fairness problem, a server could use In order to mitigate this fairness problem, a server could use
knowledge about the intermediary as another signal in its knowledge about the intermediary as another input in its
prioritization decisions. For instance, if a server knows the prioritization decisions. For instance, if a server knows the
intermediary is coalescing requests, then it could avoid serving the intermediary is coalescing requests, then it could avoid serving the
responses in their entirety and instead distribute bandwidth (for responses in their entirety and instead distribute bandwidth (for
example, in a round-robin manner). This can work if the constrained example, in a round-robin manner). This can work if the constrained
resource is network capacity between the intermediary and the user resource is network capacity between the intermediary and the user
agent, as the intermediary buffers responses and forwards the chunks agent, as the intermediary buffers responses and forwards the chunks
based on the prioritization scheme it implements. based on the prioritization scheme it implements.
A server can determine if a request came from an intermediary through A server can determine if a request came from an intermediary through
configuration, or by consulting if that request contains one of the configuration, or by consulting if that request contains one of the
skipping to change at page 21, line 34 skipping to change at page 21, line 45
amount of unfairness between the connections and therefore between amount of unfairness between the connections and therefore between
the requests served on those connections. the requests served on those connections.
For example, a server might use a scavenging congestion controller on For example, a server might use a scavenging congestion controller on
connections that only convey background priority responses such as connections that only convey background priority responses such as
software update images. Doing so improves responsiveness of other software update images. Doing so improves responsiveness of other
connections at the cost of delaying the delivery of updates. connections at the cost of delaying the delivery of updates.
14. Why use an End-to-End Header Field? 14. Why use an End-to-End Header Field?
Contrary to the prioritization scheme of HTTP/2 that uses a hop-by- In contrast to the prioritization scheme of HTTP/2 that uses a hop-
hop frame, the Priority header field is defined as end-to-end. by-hop frame, the Priority header field is defined as end-to-end.
The rationale is that the Priority header field transmits how each The way that a client processes a response is a property associated
response affects the client's processing of those responses, rather with the client generating that request. Not that of an
than how relatively urgent each response is to others. The way a intermediary. Therefore, it is an end-to-end property. How these
client processes a response is a property associated to that client end-to-end properties carried by the Priority header field affect the
generating that request. Not that of an intermediary. Therefore, it prioritization between the responses that share a connection is a
is an end-to-end property. How these end-to-end properties carried hop-by-hop issue.
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 Having the Priority header field defined as end-to-end is important
for caching intermediaries. Such intermediaries can cache the value for caching intermediaries. Such intermediaries can cache the value
of the Priority header field along with the response, and utilize the of the Priority header field along with the response, and utilize the
value of the cached header field when serving the cached response, value of the cached header field when serving the cached response,
only because the header field is defined as end-to-end rather than only because the header field is defined as end-to-end rather than
hop-by-hop. hop-by-hop.
It should also be noted that the use of a header field carrying a
textual value makes the prioritization scheme extensible; see the
discussion below.
15. Security Considerations 15. Security Considerations
[RFC7540] stream prioritization relies on dependencies.
Considerations are presented to implementations, describing how
limiting state or work commitments can avoid some types of problems.
In addition, [CVE-2019-9513] aka "Resource Loop", is an example of a
DoS attack that abuses stream dependencies. Extensible priorities
does not use dependencies, which avoids these issues.
Section 7 describes considerations for server buffering of Section 7 describes considerations for server buffering of
PRIORITY_UPDATE frames. PRIORITY_UPDATE frames.
Section 10 presents examples where servers that prioritize responses Section 10 presents examples where servers that prioritize responses
in a certain way might be starved of the ability to transmit payload. in a certain way might be starved of the ability to transmit payload.
The security considerations from [STRUCTURED-FIELDS] apply to The security considerations from [STRUCTURED-FIELDS] apply to
processing of priority parameters defined in Section 4. processing of priority parameters defined in Section 4.
16. IANA Considerations 16. IANA Considerations
skipping to change at page 23, line 20 skipping to change at page 23, line 25
Frame Type: PRIORITY_UPDATE Frame Type: PRIORITY_UPDATE
Code: 0xF0700 and 0xF0701 Code: 0xF0700 and 0xF0701
Specification: This document Specification: This document
Upon publication, please create the HTTP Priority Parameters registry Upon publication, please create the HTTP Priority Parameters registry
at https://iana.org/assignments/http-priority at https://iana.org/assignments/http-priority
(https://iana.org/assignments/http-priority) and populate it with the (https://iana.org/assignments/http-priority) and populate it with the
types defined in Section 4; see Section 4.3.1 for its associated entries in Table 1; see Section 4.3.1 for its associated procedures.
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. References
17.1. Normative References 17.1. Normative References
[HTTP] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP [HTTP] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Semantics", Work in Progress, Internet-Draft, draft-ietf- Semantics", Work in Progress, Internet-Draft, draft-ietf-
httpbis-semantics-19, 12 September 2021, httpbis-semantics-19, 12 September 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis- <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
semantics-19>. semantics-19>.
[HTTP2] Thomson, M. and C. Benfield, "Hypertext Transfer Protocol [HTTP2] Thomson, M. and C. Benfield, "Hypertext Transfer Protocol
Version 2 (HTTP/2)", Work in Progress, Internet-Draft, Version 2 (HTTP/2)", Work in Progress, Internet-Draft,
draft-ietf-httpbis-http2bis-05, 26 September 2021, draft-ietf-httpbis-http2bis-06, 18 November 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis- <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
http2bis-05>. http2bis-06>.
[HTTP3] Bishop, M., "Hypertext Transfer Protocol Version 3 [HTTP3] Bishop, M., "Hypertext Transfer Protocol Version 3
(HTTP/3)", Work in Progress, Internet-Draft, draft-ietf- (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
quic-http-34, 2 February 2021, quic-http-34, 2 February 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic- <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
http-34>. http-34>.
[QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based [QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000, Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021, DOI 10.17487/RFC9000, May 2021,
skipping to change at page 24, line 32 skipping to change at page 24, line 49
<https://www.rfc-editor.org/rfc/rfc8941>. <https://www.rfc-editor.org/rfc/rfc8941>.
17.2. Informative References 17.2. Informative References
[CACHING] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP [CACHING] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Caching", Work in Progress, Internet-Draft, draft-ietf- Caching", Work in Progress, Internet-Draft, draft-ietf-
httpbis-cache-19, 12 September 2021, httpbis-cache-19, 12 September 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis- <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
cache-19>. cache-19>.
[CVE-2019-9513]
Common Vulnerabilities and Exposures, "CVE-2019-9513", 1
March 2019, <https://cve.mitre.org/cgi-bin/
cvename.cgi?name=CVE-2019-9513>.
[FORWARDED] [FORWARDED]
Petersson, A. and M. Nilsson, "Forwarded HTTP Extension", Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
RFC 7239, DOI 10.17487/RFC7239, June 2014, RFC 7239, DOI 10.17487/RFC7239, June 2014,
<https://www.rfc-editor.org/rfc/rfc7239>. <https://www.rfc-editor.org/rfc/rfc7239>.
[I-D.lassey-priority-setting] [I-D.lassey-priority-setting]
Lassey, B. and L. Pardue, "Declaring Support for HTTP/2 Lassey, B. and L. Pardue, "Declaring Support for HTTP/2
Priorities", Work in Progress, Internet-Draft, draft- Priorities", Work in Progress, Internet-Draft, draft-
lassey-priority-setting-00, 25 July 2019, lassey-priority-setting-00, 25 July 2019,
<https://datatracker.ietf.org/doc/html/draft-lassey- <https://datatracker.ietf.org/doc/html/draft-lassey-
priority-setting-00>. priority-setting-00>.
[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,
<https://www.doi.org/10.5220/0008191701300143>.
[MEENAN] Meenan, P., "Better HTTP/2 Prioritization for a Faster
Web", 14 May 2019, <https://blog.cloudflare.com/better-
http-2-prioritization-for-a-faster-web/>.
[QUIC-RECOVERY] [QUIC-RECOVERY]
Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
and Congestion Control", RFC 9002, DOI 10.17487/RFC9002, and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
May 2021, <https://www.rfc-editor.org/rfc/rfc9002>. May 2021, <https://www.rfc-editor.org/rfc/rfc9002>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540, Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015, DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/rfc/rfc7540>. <https://www.rfc-editor.org/rfc/rfc7540>.
skipping to change at page 25, line 45 skipping to change at page 26, line 22
Alan Frindell, Andrew Galloni, Craig Taylor, Ian Swett, Kazuho Oku, Alan Frindell, Andrew Galloni, Craig Taylor, Ian Swett, Kazuho Oku,
Lucas Pardue, Matthew Cox, Mike Bishop, Roberto Peon, Robin Marx, Roy Lucas Pardue, Matthew Cox, Mike Bishop, Roberto Peon, Robin Marx, Roy
Fielding. Fielding.
Yang Chi contributed the section on retransmission scheduling. Yang Chi contributed the section on retransmission scheduling.
Appendix B. Change Log Appendix B. Change Log
_RFC EDITOR: please remove this section before publication_ _RFC EDITOR: please remove this section before publication_
B.1. Since draft-ietf-httpbis-priority-09 B.1. Since draft-ietf-httpbis-priority-10
* Editorial changes * Editorial changes
B.2. Since draft-ietf-httpbis-priority-08 * Add clearer IANA instructions for Priority Parameter initial
population
B.2. Since draft-ietf-httpbis-priority-09
* Editorial changes
B.3. Since draft-ietf-httpbis-priority-08
* Changelog fixups * Changelog fixups
B.3. Since draft-ietf-httpbis-priority-07 B.4. Since draft-ietf-httpbis-priority-07
* Relax requirements of receiving SETTINGS_NO_RFC7540_PRIORITIES * Relax requirements of receiving SETTINGS_NO_RFC7540_PRIORITIES
that changes value (#1714, #1725) that changes value (#1714, #1725)
* Clarify how intermediaries might use frames vs. headers (#1715, * Clarify how intermediaries might use frames vs. headers (#1715,
#1735) #1735)
* Relax requirement when receiving a PRIORITY_UPDATE with an invalid * Relax requirement when receiving a PRIORITY_UPDATE with an invalid
structured field value (#1741, #1756) structured field value (#1741, #1756)
B.4. Since draft-ietf-httpbis-priority-06 B.5. Since draft-ietf-httpbis-priority-06
* Focus on editorial changes * Focus on editorial changes
* Clarify rules about Sf-Dictionary handling in headers * Clarify rules about Sf-Dictionary handling in headers
* Split policy for parameter IANA registry into two sections based * Split policy for parameter IANA registry into two sections based
on key length on key length
B.5. Since draft-ietf-httpbis-priority-05 B.6. Since draft-ietf-httpbis-priority-05
* Renamed SETTINGS_DEPRECATE_RFC7540_PRIORITIES to * Renamed SETTINGS_DEPRECATE_RFC7540_PRIORITIES to
SETTINGS_NO_RFC7540_PRIORITIES SETTINGS_NO_RFC7540_PRIORITIES
* Clarify that senders of the HTTP/2 setting can use any alternative * Clarify that senders of the HTTP/2 setting can use any alternative
(#1679, #1705) (#1679, #1705)
B.6. Since draft-ietf-httpbis-priority-04 B.7. Since draft-ietf-httpbis-priority-04
* Renamed SETTINGS_DEPRECATE_HTTP2_PRIORITIES to * Renamed SETTINGS_DEPRECATE_HTTP2_PRIORITIES to
SETTINGS_DEPRECATE_RFC7540_PRIORITIES (#1601) SETTINGS_DEPRECATE_RFC7540_PRIORITIES (#1601)
* Reoriented text towards RFC7540bis (#1561, #1601) * Reoriented text towards RFC7540bis (#1561, #1601)
* Clarify intermediary behavior (#1562) * Clarify intermediary behavior (#1562)
B.7. Since draft-ietf-httpbis-priority-03 B.8. Since draft-ietf-httpbis-priority-03
* Add statement about what this scheme applies to. Clarify * Add statement about what this scheme applies to. Clarify
extensions can use it but must define how themselves (#1550, extensions can use it but must define how themselves (#1550,
#1559) #1559)
* Describe scheduling considerations for the CONNECT method (#1495, * Describe scheduling considerations for the CONNECT method (#1495,
#1544) #1544)
* Describe scheduling considerations for retransmitted data (#1429, * Describe scheduling considerations for retransmitted data (#1429,
#1504) #1504)
* Suggest intermediaries might avoid strict prioritization (#1562) * Suggest intermediaries might avoid strict prioritization (#1562)
B.8. Since draft-ietf-httpbis-priority-02 B.9. Since draft-ietf-httpbis-priority-02
* Describe considerations for server push prioritization (#1056, * Describe considerations for server push prioritization (#1056,
#1345) #1345)
* Define HTTP/2 PRIORITY_UPDATE ID limits in HTTP/2 terms (#1261, * Define HTTP/2 PRIORITY_UPDATE ID limits in HTTP/2 terms (#1261,
#1344) #1344)
* Add a Parameters registry (#1371) * Add a Priority Parameters registry (#1371)
B.9. Since draft-ietf-httpbis-priority-01 B.10. Since draft-ietf-httpbis-priority-01
* PRIORITY_UPDATE frame changes (#1096, #1079, #1167, #1262, #1267, * PRIORITY_UPDATE frame changes (#1096, #1079, #1167, #1262, #1267,
#1271) #1271)
* Add section to describe server scheduling considerations (#1215, * Add section to describe server scheduling considerations (#1215,
#1232, #1266) #1232, #1266)
* Remove specific instructions related to intermediary fairness * Remove specific instructions related to intermediary fairness
(#1022, #1264) (#1022, #1264)
B.10. Since draft-ietf-httpbis-priority-00 B.11. Since draft-ietf-httpbis-priority-00
* Move text around (#1217, #1218) * Move text around (#1217, #1218)
* Editorial change to the default urgency. The value is 3, which * Editorial change to the default urgency. The value is 3, which
was always the intent of previous changes. was always the intent of previous changes.
B.11. Since draft-kazuho-httpbis-priority-04 B.12. Since draft-kazuho-httpbis-priority-04
* Minimize semantics of Urgency levels (#1023, #1026) * Minimize semantics of Urgency levels (#1023, #1026)
* Reduce guidance about how intermediary implements merging priority * Reduce guidance about how intermediary implements merging priority
signals (#1026) signals (#1026)
* Remove mention of CDN-Loop (#1062) * Remove mention of CDN-Loop (#1062)
* Editorial changes * Editorial changes
* Make changes due to WG adoption * Make changes due to WG adoption
* Removed outdated Consideration (#118) * Removed outdated Consideration (#118)
B.12. Since draft-kazuho-httpbis-priority-03 B.13. Since draft-kazuho-httpbis-priority-03
* Changed numbering from [-1,6] to [0,7] (#78) * Changed numbering from [-1,6] to [0,7] (#78)
* Replaced priority scheme negotiation with HTTP/2 priority * Replaced priority scheme negotiation with HTTP/2 priority
deprecation (#100) deprecation (#100)
* Shorten parameter names (#108) * Shorten parameter names (#108)
* Expand on considerations (#105, #107, #109, #110, #111, #113) * Expand on considerations (#105, #107, #109, #110, #111, #113)
B.13. Since draft-kazuho-httpbis-priority-02 B.14. Since draft-kazuho-httpbis-priority-02
* Consolidation of the problem statement (#61, #73) * Consolidation of the problem statement (#61, #73)
* Define SETTINGS_PRIORITIES for negotiation (#58, #69) * Define SETTINGS_PRIORITIES for negotiation (#58, #69)
* Define PRIORITY_UPDATE frame for HTTP/2 and HTTP/3 (#51) * Define PRIORITY_UPDATE frame for HTTP/2 and HTTP/3 (#51)
* Explain fairness issue and mitigations (#56) * Explain fairness issue and mitigations (#56)
B.14. Since draft-kazuho-httpbis-priority-01 B.15. Since draft-kazuho-httpbis-priority-01
* Explain how reprioritization might be supported. * Explain how reprioritization might be supported.
B.15. Since draft-kazuho-httpbis-priority-00 B.16. Since draft-kazuho-httpbis-priority-00
* Expand urgency levels from 3 to 8. * Expand urgency levels from 3 to 8.
Authors' Addresses Authors' Addresses
Kazuho Oku Kazuho Oku
Fastly Fastly
Email: kazuhooku@gmail.com Email: kazuhooku@gmail.com
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