rfc9218.original   rfc9218.txt 
HTTP K. Oku Internet Engineering Task Force (IETF) 奥 一穂 (K. Oku)
Internet-Draft Fastly Request for Comments: 9218 Fastly
Intended status: Standards Track L. Pardue Category: Standards Track L. Pardue
Expires: 22 July 2022 Cloudflare ISSN: 2070-1721 Cloudflare
18 January 2022 June 2022
Extensible Prioritization Scheme for HTTP Extensible Prioritization Scheme for HTTP
draft-ietf-httpbis-priority-12
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.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-httpbis-priority/.
Discussion of this document takes place on the HTTP Working Group
mailing list (mailto:ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/. Working Group
information can be found at https://httpwg.org/.
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
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 5 1.1. Notational Conventions
2. Motivation for Replacing RFC 7540 Priorities . . . . . . . . 5 2. Motivation for Replacing RFC 7540 Stream Priorities
2.1. Disabling RFC 7540 Priorities . . . . . . . . . . . . . . 6 2.1. Disabling RFC 7540 Stream Priorities
2.1.1. Advice when Using Extensible Priorities as the 2.1.1. Advice when Using Extensible Priorities as the
Alternative . . . . . . . . . . . . . . . . . . . . . 7 Alternative
3. Applicability of the Extensible Priority Scheme . . . . . . . 7 3. Applicability of the Extensible Priority Scheme
4. Priority Parameters . . . . . . . . . . . . . . . . . . . . . 8 4. Priority Parameters
4.1. Urgency . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.1. Urgency
4.2. Incremental . . . . . . . . . . . . . . . . . . . . . . . 9 4.2. Incremental
4.3. Defining New Priority Parameters . . . . . . . . . . . . 10 4.3. Defining New Priority Parameters
4.3.1. Registration . . . . . . . . . . . . . . . . . . . . 10 4.3.1. Registration
5. The Priority HTTP Header Field . . . . . . . . . . . . . . . 11 5. The Priority HTTP Header Field
6. Reprioritization . . . . . . . . . . . . . . . . . . . . . . 12 6. Reprioritization
7. The PRIORITY_UPDATE Frame . . . . . . . . . . . . . . . . . . 12 7. The PRIORITY_UPDATE Frame
7.1. HTTP/2 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 13 7.1. HTTP/2 PRIORITY_UPDATE Frame
7.2. HTTP/3 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 14 7.2. HTTP/3 PRIORITY_UPDATE Frame
8. Merging Client- and Server-Driven Priority Parameters . . . . 16 8. Merging Client- and Server-Driven Priority Parameters
9. Client Scheduling . . . . . . . . . . . . . . . . . . . . . . 17 9. Client Scheduling
10. Server Scheduling . . . . . . . . . . . . . . . . . . . . . . 17 10. Server Scheduling
10.1. Intermediaries with Multiple Backend Connections . . . . 19 10.1. Intermediaries with Multiple Backend Connections
11. Scheduling and the CONNECT Method . . . . . . . . . . . . . . 19 11. Scheduling and the CONNECT Method
12. Retransmission Scheduling . . . . . . . . . . . . . . . . . . 19 12. Retransmission Scheduling
13. Fairness . . . . . . . . . . . . . . . . . . . . . . . . . . 20 13. Fairness
13.1. Coalescing Intermediaries . . . . . . . . . . . . . . . 20 13.1. Coalescing Intermediaries
13.2. HTTP/1.x Back Ends . . . . . . . . . . . . . . . . . . . 21 13.2. HTTP/1.x Back Ends
13.3. Intentional Introduction of Unfairness . . . . . . . . . 21 13.3. Intentional Introduction of Unfairness
14. Why use an End-to-End Header Field? . . . . . . . . . . . . . 21 14. Why Use an End-to-End Header Field?
15. Security Considerations . . . . . . . . . . . . . . . . . . . 22 15. Security Considerations
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 16. IANA Considerations
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 17. References
17.1. Normative References . . . . . . . . . . . . . . . . . . 23 17.1. Normative References
17.2. Informative References . . . . . . . . . . . . . . . . . 24 17.2. Informative References
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 25 Acknowledgements
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 26 Authors' Addresses
B.1. Since draft-ietf-httpbis-priority-11 . . . . . . . . . . 26
B.2. Since draft-ietf-httpbis-priority-10 . . . . . . . . . . 26
B.3. Since draft-ietf-httpbis-priority-09 . . . . . . . . . . 26
B.4. Since draft-ietf-httpbis-priority-08 . . . . . . . . . . 26
B.5. Since draft-ietf-httpbis-priority-07 . . . . . . . . . . 26
B.6. Since draft-ietf-httpbis-priority-06 . . . . . . . . . . 26
B.7. Since draft-ietf-httpbis-priority-05 . . . . . . . . . . 27
B.8. Since draft-ietf-httpbis-priority-04 . . . . . . . . . . 27
B.9. Since draft-ietf-httpbis-priority-03 . . . . . . . . . . 27
B.10. Since draft-ietf-httpbis-priority-02 . . . . . . . . . . 27
B.11. Since draft-ietf-httpbis-priority-01 . . . . . . . . . . 27
B.12. Since draft-ietf-httpbis-priority-00 . . . . . . . . . . 28
B.13. Since draft-kazuho-httpbis-priority-04 . . . . . . . . . 28
B.14. Since draft-kazuho-httpbis-priority-03 . . . . . . . . . 28
B.15. Since draft-kazuho-httpbis-priority-02 . . . . . . . . . 28
B.16. Since draft-kazuho-httpbis-priority-01 . . . . . . . . . 29
B.17. Since draft-kazuho-httpbis-priority-00 . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction 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 relationships determines whether a
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 the visual rendering of an HTML
document, which could be blocked by the retrieval of a CSS file that document, which could be blocked by the retrieval of a Cascading
the document refers to. In contrast, inline images do not block Style Sheets (CSS) file that the document refers to. In contrast,
rendering and get drawn incrementally as the chunks of the images inline images do not block rendering and get drawn incrementally as
arrive. the chunks of the images arrive.
HTTP/2 [HTTP2] and HTTP/3 [HTTP3] support multiplexing of requests HTTP/2 [HTTP/2] and HTTP/3 [HTTP/3] support multiplexing of requests
and responses in a single connection. An important feature of any and responses in a single connection. An important feature of any
implementation of a protocol that provides multiplexing is the implementation of a protocol that provides multiplexing is the
ability to prioritize the sending of information. For example, to ability to prioritize the sending of information. For example, to
provide meaningful presentation of an HTML document at the earliest provide meaningful presentation of an HTML document at the earliest
moment, it is important for an HTTP server to prioritize the HTTP moment, it is important for an HTTP server to prioritize the HTTP
responses, or the chunks of those HTTP responses, that it sends to a responses, or the chunks of those HTTP responses, that it sends to a
client. client.
HTTP/2 and HTTP/3 servers can schedule transmission of concurrent HTTP/2 and HTTP/3 servers can schedule transmission of concurrent
response data by any means they choose. Servers can ignore client response data by any means they choose. Servers can ignore client
skipping to change at page 4, line 24 skipping to change at line 128
communicate their view of request priority. Servers have their own communicate their view of request priority. Servers have their own
needs that are independent of client needs, so they often combine needs that are independent of client needs, so they often combine
priority signals with other available information in order to inform priority signals with other available information in order to inform
scheduling of response data. scheduling of response data.
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 decisions.
The design and implementation of RFC 7540 stream priority was The design and implementation of RFC 7540 stream priority were
observed to have shortcomings, explained in Section 2. HTTP/2 observed to have shortcomings, as explained in Section 2. HTTP/2
[HTTP2] has consequently deprecated the use of these stream priority [HTTP/2] has consequently deprecated the use of these stream priority
signals. The prioritization scheme and priority signals defined signals. The prioritization scheme and priority signals defined
herein can act as a substitute for RFC 7540 stream priority. 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, which is an end-to-end priority signal that is independent of
Clients can send this header field to signal their view of how protocol version. Clients can send this header field to signal their
responses should be prioritized. Similarly, servers behind an view of how responses should be prioritized. Similarly, servers
intermediary can use it to signal priority to the intermediary. behind an intermediary can use it to signal priority to the
After sending a request, a client can change their view of response intermediary. After sending a request, a client can change their
priority (see Section 6) by sending HTTP-version-specific frames view of response priority (see Section 6) by sending HTTP-version-
defined in Section 7.1 and Section 7.2. specific frames as defined in Sections 7.1 and 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. Sections 10 and 12 provide
provide consideration and guidance about how servers might act upon considerations and guidance about how servers might act upon signals.
signals.
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 This document uses the following terminology from Section 3 of
imported from [STRUCTURED-FIELDS]. [STRUCTURED-FIELDS] to specify syntax and parsing: "Boolean",
"Dictionary", and "Integer".
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 [HTTP/2].
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 both the HTTP/2 stream The term "control stream" is used to describe both the HTTP/2 stream
with identifier 0x0 and the HTTP/3 control stream; see Section 6.2.1 with identifier 0x0 and the HTTP/3 control stream; see Section 6.2.1
of [HTTP3]. of [HTTP/3].
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 [HTTP/2].
2. Motivation for Replacing RFC 7540 Priorities 2. Motivation for Replacing RFC 7540 Stream 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 has been deprecated in a revision of HTTP/2
HTTP/2 retains these protocol elements in order to maintain wire [HTTP/2]. HTTP/2 retains these protocol elements in order to
compatibility (see Section 5.3.2 of [HTTP2]), which means that they maintain wire compatibility (see Section 5.3.2 of [HTTP/2]), which
might still be used even in the presence of alternative signaling, means that they might still be used even in the presence of
such as the scheme this document describes. alternative signaling, 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
same conditions could result in very different signaling from same conditions could result in very different signaling from
different clients. This document describes signaling that is simpler different clients. This document describes signaling that is simpler
and more constrained, requiring less interpretation and allowing less and more constrained, requiring less interpretation and allowing less
variation. variation.
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 sharing the RFC 7540 stream priority is expressed relative to other requests
same connection at the same time. It is difficult to incorporate sharing the same connection at the same time. It is difficult to
such design into applications that generate requests without incorporate such a design into applications that generate requests
knowledge of how other requests might share a connection, or into without knowledge of how other requests might share a connection, or
protocols that do not have strong ordering guarantees across streams, into protocols that do not have strong ordering guarantees across
like HTTP/3 [HTTP3]. streams, like HTTP/3 [HTTP/3].
Experiments from independent research ([MARX]) have shown that Experiments from independent research [MARX] have shown that simpler
simpler schemes can reach at least equivalent performance schemes can reach at least equivalent performance characteristics
characteristics compared to the more complex RFC 7540 setups seen in compared to the more complex RFC 7540 setups seen in practice, at
practice, at least for the web use case. least for the Web use case.
2.1. Disabling RFC 7540 Priorities 2.1. Disabling RFC 7540 Stream 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]). [HTTP/2]).
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 [HTTP/2]), as described below.
The value of SETTINGS_NO_RFC7540_PRIORITIES MUST be 0 or 1. Any The value of SETTINGS_NO_RFC7540_PRIORITIES MUST be 0 or 1. Any
value other than 0 or 1 MUST be treated as a connection error (see value other than 0 or 1 MUST be treated as a connection error (see
Section 5.4.1 of [HTTP2]) of type PROTOCOL_ERROR. The initial value Section 5.4.1 of [HTTP/2]) of type PROTOCOL_ERROR. The initial value
is 0. is 0.
If endpoints use SETTINGS_NO_RFC7540_PRIORITIES they MUST send it in If endpoints use SETTINGS_NO_RFC7540_PRIORITIES, they MUST send it in
the first SETTINGS frame. Senders MUST NOT change the the first SETTINGS frame. Senders MUST NOT change the
SETTINGS_NO_RFC7540_PRIORITIES value after the first SETTINGS frame. SETTINGS_NO_RFC7540_PRIORITIES value after the first SETTINGS frame.
Receivers that detect a change MAY treat it as a connection error of Receivers that detect a change MAY treat it as a connection error of
type PROTOCOL_ERROR. type PROTOCOL_ERROR.
Clients can send SETTINGS_NO_RFC7540_PRIORITIES with a value of 1 to Clients can send SETTINGS_NO_RFC7540_PRIORITIES with a value of 1 to
indicate that they are not using HTTP/2 priority signals. The indicate that they are not using HTTP/2 priority signals. The
SETTINGS frame precedes any HTTP/2 priority signal sent from clients, SETTINGS frame precedes any HTTP/2 priority signal sent from clients,
so servers can determine whether they need to allocate any resources so servers can determine whether they need to allocate any resources
to signal handling before signals arrive. A server that receives to signal handling before signals arrive. A server that receives
SETTINGS_NO_RFC7540_PRIORITIES with a value of 1 MUST ignore HTTP/2 SETTINGS_NO_RFC7540_PRIORITIES with a value of 1 MUST ignore HTTP/2
priority signals. priority signals.
Servers can send SETTINGS_NO_RFC7540_PRIORITIES with a value of 1 to Servers can send SETTINGS_NO_RFC7540_PRIORITIES with a value of 1 to
indicate that they will ignore HTTP/2 priority signals sent by indicate that they will ignore HTTP/2 priority signals sent by
clients. clients.
Endpoints that send SETTINGS_NO_RFC7540_PRIORITIES are encouraged to Endpoints that send SETTINGS_NO_RFC7540_PRIORITIES are encouraged to
use alternative priority signals (for example, Section 5 or use alternative priority signals (for example, see Section 5 or
Section 7.1) but there is no requirement to use a specific signal Section 7.1), but there is no requirement to use a specific signal
type. type.
2.1.1. Advice when Using Extensible Priorities as the Alternative 2.1.1. Advice when Using Extensible Priorities as the Alternative
Before receiving a SETTINGS frame from a server, a client does not Before receiving a SETTINGS frame from a server, a client does not
know if the server is ignoring HTTP/2 priority signals. Therefore, know if the server is ignoring HTTP/2 priority signals. Therefore,
until the client receives the SETTINGS frame from the server, the until the client receives the SETTINGS frame from the server, the
client SHOULD send both the HTTP/2 priority signals and the signals client SHOULD send both the HTTP/2 priority signals and the signals
of this prioritization scheme (see Section 5 and Section 7.1). of this prioritization scheme (see Sections 5 and 7.1).
Once the client receives the first SETTINGS frame that contains the Once the client receives the first SETTINGS frame that contains the
SETTINGS_NO_RFC7540_PRIORITIES parameter with value of 1, it SHOULD SETTINGS_NO_RFC7540_PRIORITIES parameter with a value of 1, it SHOULD
stop sending the HTTP/2 priority signals. This avoids sending stop sending the HTTP/2 priority signals. This avoids sending
redundant signals that are known to be ignored. redundant signals that are known to be ignored.
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 a 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 is primarily focused on The priority scheme defined by this document is primarily focused on
the prioritization of HTTP response messages (see Section 3.4 of the prioritization of HTTP response messages (see Section 3.4 of
[HTTP]). It defines new priority parameters (Section 4) and a means [HTTP]). It defines new priority parameters (Section 4) and a means
of conveying those parameters (Section 5 and Section 7), which is of conveying those parameters (Sections 5 and 7), which is intended
intended to communicate the priority of responses to a server that is to communicate the priority of responses to a server that is
responsible for prioritizing them. Section 10 provides responsible for prioritizing them. Section 10 provides
considerations for servers about acting on those signals in considerations for servers about acting on those signals in
combination with other inputs and factors. combination with other inputs and factors.
The CONNECT method (see Section 9.3.6 of [HTTP]) can be used to The CONNECT method (see Section 9.3.6 of [HTTP]) can be used to
establish tunnels. Signaling applies similarly to tunnels; establish tunnels. Signaling applies similarly to tunnels;
additional considerations for server prioritization are given in additional considerations for server prioritization are given in
Section 11. Section 11.
Section 9 describes how clients can optionally apply elements of this Section 9 describes how clients can optionally apply elements of this
scheme locally to the request messages that they generate. scheme locally to the request messages that they generate.
Some forms of HTTP extensions might change HTTP/2 or HTTP/3 stream Some forms of HTTP extensions might change HTTP/2 or HTTP/3 stream
behavior or define new data carriage mechanisms. Such extensions can behavior or define new data carriage mechanisms. Such extensions can
define themselves how this priority scheme is to be applied. themselves define 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 priority parameters when a request or a response transmit this set of priority parameters when a request or a response
is issued. After sending a request, a client can change their view is issued. After sending a request, a client can change their view
of response priority (Section 6) by sending HTTP-version-specific of response priority (Section 6) by sending HTTP-version-specific
PRIORITY_UPDATE frames defined in Section 7.1 and Section 7.2. PRIORITY_UPDATE frames as defined in Sections 7.1 and 7.2. Frames
Frames transmit priority parameters on a single hop only. transmit priority parameters on a single hop only.
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. An intermediary that
PRIORITY_UPDATE frame preserves the signal from the client carried by passes only the Priority request header field to the next hop
the Priority header field, but provides a signal that overrides that preserves the original end-to-end signal from the client; see
for the next hop; see Section 14. Replacing or adding a Priority Section 14. An intermediary could pass the Priority header field and
header field overrides any signal from a client and can affect additionally send a PRIORITY_UPDATE frame. This would have the
prioritization for all subsequent recipients. effect of preserving the original client end-to-end signal, while
instructing the next hop to use a different priority, per the
guidance in Section 7. An intermediary that replaces or adds a
Priority request header field overrides the original client end-to-
end signal, which can affect prioritization for all subsequent
recipients of the request.
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 Dictionary (see
Dictionary (see Section 3.2 of [STRUCTURED-FIELDS]). Section 3.2 of [STRUCTURED-FIELDS]).
This document defines the urgency(u) and incremental(i) priority This document defines the urgency (u) and incremental (i) priority
parameters. When receiving an HTTP request that does not carry these parameters. When receiving an HTTP request that does not carry these
priority parameters, a server SHOULD act as if their default values priority parameters, a server SHOULD act as if their default values
were specified. were specified.
An intermediary can combine signals from requests and responses that An intermediary can combine signals from requests and responses that
it forwards. Note that omission of priority parameters in responses it forwards. Note that omission of priority parameters in responses
is handled differently from omission in requests; see Section 8. 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 described 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, priority parameters with out-of-range values, or values parameters, priority parameters with out-of-range values, or values
of 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 (u) parameter value is Integer (see Section 3.3.1 of
descending order of priority. [STRUCTURED-FIELDS]), between 0 and 7 inclusive, in descending order
of priority. The default 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
based on the expectation that servers might use this information to based on the expectation that servers might use this information to
transmit HTTP responses in the order of their urgency. The smaller transmit HTTP responses in the order of their urgency. The smaller
the value, the higher the precedence. the value, the higher the precedence.
The following example shows a request for a CSS file with the urgency The following example shows a request for a CSS file with the urgency
set to 0: set to 0:
:method = GET :method = GET
:scheme = https :scheme = https
:authority = example.net :authority = example.net
:path = /style.css :path = /style.css
priority = u=0 priority = u=0
A client that fetches a document that likely consists of multiple A client that fetches a document that likely consists of multiple
HTTP resources (e.g., HTML) SHOULD assign the default urgency level HTTP resources (e.g., HTML) SHOULD assign the default urgency level
to the main resource. This convention allows servers to refine the to the main resource. This convention allows servers to refine the
urgency using knowledge specific to the web-site (see Section 8). urgency using knowledge specific to the website (see Section 8).
The lowest urgency level (7) is reserved for background tasks such as The lowest urgency level (7) is reserved for background tasks such as
delivery of software updates. This urgency level SHOULD NOT be used delivery of software updates. This urgency level SHOULD NOT be used
for fetching responses that have impact on user interaction. for fetching responses that have any impact on user interaction.
4.2. Incremental 4.2. Incremental
The incremental parameter (i) takes an sf-boolean as the value that The incremental (i) parameter value is Boolean (see Section 3.3.6 of
indicates if an HTTP response can be processed incrementally, i.e., [STRUCTURED-FIELDS]). It indicates if an HTTP response can be
provide some meaningful output as chunks of the response arrive. processed incrementally, i.e., provide some meaningful output as
chunks of the response arrive.
The default value of the incremental parameter is false (0). The default value of the incremental parameter is false (0).
If a client makes concurrent requests with the incremental parameter If a client makes concurrent requests with the incremental parameter
set to false, there is no benefit serving responses with the same set to false, there is no benefit in serving responses with the same
urgency concurrently because the client is not going to process those urgency concurrently because the client is not going to process those
responses incrementally. Serving non-incremental responses with the responses incrementally. Serving non-incremental responses with the
same urgency one by one, in the order in which those requests were same urgency one by one, in the order in which those requests were
generated is considered to be the best strategy. generated, is considered to be the best strategy.
If a client makes concurrent requests with the incremental parameter If a client makes concurrent requests with the incremental parameter
set to true, serving requests with the same urgency concurrently set to true, serving requests with the same urgency concurrently
might be beneficial. Doing this distributes the connection might be beneficial. Doing this distributes the connection
bandwidth, meaning that responses take longer to complete. bandwidth, meaning that responses take longer to complete.
Incremental delivery is most useful where multiple partial responses Incremental delivery is most useful where multiple partial responses
might provide some value to clients ahead of a complete response might provide some value to clients ahead of a complete response
being available. being available.
The following example shows a request for a JPEG file with the The following example shows a request for a JPEG file with the
skipping to change at page 10, line 19 skipping to change at line 413
: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 Priority Parameters 4.3. Defining New Priority Parameters
When attempting to define new priority parameters, care must be taken When attempting to define new priority parameters, care must be taken
so that they do not adversely interfere with prioritization performed so that they do not adversely interfere with prioritization performed
by existing endpoints or intermediaries that do not understand the by existing endpoints or intermediaries that do not understand the
newly defined priority parameter. Since unknown priority parameters newly defined priority parameters. Since unknown priority parameters
are ignored, new priority parameters should not change the are ignored, new priority parameters should not change the
interpretation of, or modify, the urgency (see Section 4.1) or interpretation of, or modify, the urgency (see Section 4.1) or
incremental (see Section 4.2) priority parameters in a way that is incremental (see Section 4.2) priority parameters in a way that is
not backwards compatible or fallback safe. 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 priority parameter. Implementations that do not using an additional priority parameter. Implementations that do not
recognize the parameter can safely continue to use the less granular recognize the parameter can safely continue to use the less granular
eight 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 priority parameter to graphical user agent could send a visible priority parameter to
indicate if the resource being requested is within the viewport. indicate if the resource being requested is within the viewport.
Generic priority parameters are preferred over vendor-specific, Generic priority parameters are preferred over vendor-specific,
application-specific or deployment-specific values. If a generic application-specific, or deployment-specific values. If a generic
value cannot be agreed upon in the community, the parameter's name value cannot be agreed upon in the community, the parameter's name
should be correspondingly specific (e.g., with a prefix that should be correspondingly specific (e.g., with a prefix that
identifies the 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" registry. This registry governs the keys (short
(short textual strings) used in the Structured Fields Dictionary (see textual strings) used in the Dictionary (see Section 3.2 of
Section 3.2 of [STRUCTURED-FIELDS]). Since each HTTP request can [STRUCTURED-FIELDS]). Since each HTTP request can have associated
have associated priority signals, there is value in having short key priority signals, there is value in having short key lengths,
lengths, especially single-character strings. In order to encourage especially single-character strings. In order to encourage
extension while avoiding unintended conflict among attractive key extensions while avoiding unintended conflict among attractive key
values, the HTTP Priority Parameters Registry operates two values, the "HTTP Priority" registry operates two registration
registration policies depending on key length. policies, depending on key length.
* Registration requests for priority parameters with a key length of * Registration requests for priority parameters with a key length of
one use the Specification Required policy, as per Section 4.6 of one use the Specification Required policy, per Section 4.6 of
[RFC8126]. [RFC8126].
* Registration requests for priority parameters with a key length * Registration requests for priority parameters with a key length
greater than one use the Expert Review policy, as per Section 4.5 greater than one use the Expert Review policy, per Section 4.5 of
of [RFC8126]. A specification document is appreciated, but not [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 the parameter
key]
Description: [a description of the priority parameter semantics and Description: [a description of the priority parameter semantics and
value] value]
Reference: [to a specification defining this priority parameter] Reference: [to a specification defining this priority parameter]
See the registry at https://iana.org/assignments/http-priority See the registry at <https://www.iana.org/assignments/http-priority>
(https://iana.org/assignments/http-priority) for details on where to 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 (see The Priority HTTP header field is a Dictionary that carries priority
Section 4). It can appear in requests and responses. It is an end- parameters (see Section 4). It can appear in requests and responses.
to-end signal that indicates the endpoint's view of how HTTP It is an end-to-end signal that indicates the endpoint's view of how
responses should be prioritized. Section 8 describes how HTTP responses should be prioritized. Section 8 describes how
intermediaries can combine the priority information sent from clients intermediaries can combine the priority information sent from clients
and servers. Clients cannot interpret the appearance or omission of and servers. Clients cannot interpret the appearance or omission of
a Priority response header field as acknowledgement that any a Priority response header field as acknowledgement that any
prioritization has occurred. Guidance for how endpoints can act on prioritization has occurred. Guidance for how endpoints can act on
Priority header values is given in Section 9 and Section 10. Priority header values is given in Sections 9 and 10.
Priority is a Dictionary (Section 3.2 of [STRUCTURED-FIELDS]):
Priority = sf-dictionary An HTTP request with a Priority header field might be cached and
An HTTP request with a Priority header field might be cached and re- reused for subsequent requests; see [CACHING]. When an origin server
used for subsequent requests; see [CACHING]. When an origin server
generates the Priority response header field based on properties of generates the Priority response header field based on properties of
an HTTP request it receives, the server is expected to control the an HTTP request it receives, the server is expected to control the
cacheability or the applicability of the cached response, by using cacheability or the applicability of the cached response by using
header fields that control the caching behavior (e.g., Cache-Control, header fields that control the caching behavior (e.g., Cache-Control,
Vary). 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 that 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.
The PRIORITY_UPDATE frame (Section 7) can be used for such The PRIORITY_UPDATE frame (Section 7) can be used for such
reprioritization. reprioritization.
7. The PRIORITY_UPDATE Frame 7. The PRIORITY_UPDATE Frame
This document specifies a new PRIORITY_UPDATE frame for HTTP/2 This document specifies a new PRIORITY_UPDATE frame for HTTP/2
[HTTP2] and HTTP/3 [HTTP3]. It carries priority parameters and [HTTP/2] and HTTP/3 [HTTP/3]. It carries priority parameters and
references the target of the prioritization based on a version- references the target of the prioritization based on a version-
specific identifier. In HTTP/2, this identifier is the Stream ID; in 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 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 of the stream that carries the allowing them to be sent independently of 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 to signal the initial priority of a response
of the Priority header field. instead of the Priority header field.
A PRIORITY_UPDATE frame communicates a complete set of all priority A PRIORITY_UPDATE frame communicates a complete set of all priority
parameters in the Priority Field Value field. Omitting a priority parameters in the Priority Field Value field. Omitting a priority
parameter is a signal to use its default value. Failure to parse the parameter is a signal to use its default value. Failure to parse the
Priority Field Value MAY be treated as a connection error. In HTTP/2 Priority Field Value MAY be treated as a connection error. In
the error is of type PROTOCOL_ERROR; in HTTP/3 the error is of type HTTP/2, the error is of type PROTOCOL_ERROR; in HTTP/3, the error is
H3_GENERAL_PROTOCOL_ERROR. of type 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.
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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
Priority header field value. Priority header field value.
The Stream Identifier field (see Section 5.1.1 of [HTTP2]) in the The Stream Identifier field (see Section 5.1.1 of [HTTP/2]) in the
PRIORITY_UPDATE frame header MUST be zero (0x0). Receiving a PRIORITY_UPDATE frame header MUST be zero (0x0). Receiving a
PRIORITY_UPDATE frame with a field of any other value MUST be treated PRIORITY_UPDATE frame with a field of any other value MUST be treated
as a connection error of type PROTOCOL_ERROR. as a connection error of type PROTOCOL_ERROR.
HTTP/2 PRIORITY_UPDATE Frame { HTTP/2 PRIORITY_UPDATE Frame {
Length (24), Length (24),
Type (8) = 0x10, Type (8) = 0x10,
Unused Flags (8). Unused Flags (8),
Reserved (1), Reserved (1),
Stream Identifier (31), Stream Identifier (31),
Reserved (1), Reserved (1),
Prioritized Stream ID (31), Prioritized Stream ID (31),
Priority Field Value (..), Priority Field Value (..),
} }
Figure 1: HTTP/2 PRIORITY_UPDATE Frame Payload Figure 1: HTTP/2 PRIORITY_UPDATE Frame Format
The Length, Type, Unused Flag(s), Reserved, and Stream Identifier The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
fields are described in Section 4 of [HTTP2]. The PRIORITY_UPDATE fields are described in Section 4 of [HTTP/2]. The PRIORITY_UPDATE
frame payload contains the following additional fields: frame payload contains the following additional fields:
Reserved: A reserved 1-bit field. The semantics of this bit are
undefined. It MUST remain unset (0x0) when sending and MUST be
ignored when receiving.
Prioritized Stream ID: A 31-bit stream identifier for the stream Prioritized Stream ID: A 31-bit stream identifier for the stream
that is the target of the priority update. that is the target of the priority update.
Priority Field Value: The priority update value in ASCII text, Priority Field Value: The priority update value in ASCII text,
encoded using Structured Fields. This is the same representation encoded using Structured Fields. This is the same representation
as the Priority header field value. as the Priority header field value.
When the PRIORITY_UPDATE frame applies to a request stream, clients When the PRIORITY_UPDATE frame applies to a request stream, clients
SHOULD provide a Prioritized Stream ID that refers to a stream in the SHOULD provide a prioritized stream ID that refers to a stream in the
"open", "half-closed (local)", or "idle" state. Servers can discard "open", "half-closed (local)", or "idle" state (i.e., streams where
frames where the Prioritized Stream ID refers to a stream in the data might still be received). Servers can discard frames where the
"half-closed (local)" or "closed" state. The number of streams which prioritized stream ID refers to a stream in the "half-closed (local)"
have been prioritized but remain in the "idle" state plus the number or "closed" state (i.e., streams where no further data will be sent).
of active streams (those in the "open" or either "half-closed" state; The number of streams that have been prioritized but remain in the
see Section 5.1.2 of [HTTP2]) MUST NOT exceed the value of the "idle" state plus the number of active streams (those in the "open"
state or in either of the "half-closed" states; see Section 5.1.2 of
[HTTP/2]) MUST NOT exceed the value of the
SETTINGS_MAX_CONCURRENT_STREAMS parameter. Servers that receive such SETTINGS_MAX_CONCURRENT_STREAMS parameter. Servers that receive such
a PRIORITY_UPDATE MUST respond with a connection error of type a PRIORITY_UPDATE MUST respond with a connection error of type
PROTOCOL_ERROR. PROTOCOL_ERROR.
When the PRIORITY_UPDATE frame applies to a push stream, clients When the PRIORITY_UPDATE frame applies to a push stream, clients
SHOULD provide a Prioritized Stream ID that refers to a stream in the SHOULD provide a prioritized stream ID that refers to a stream in the
"reserved (remote)" or "half-closed (local)" state. Servers can "reserved (remote)" or "half-closed (local)" state. Servers can
discard frames where the Prioritized Stream ID refers to a stream in discard frames where the prioritized stream ID refers to a stream in
the "closed" state. Clients MUST NOT provide a Prioritized Stream ID the "closed" state. Clients MUST NOT provide a prioritized stream ID
that refers to a push stream in the "idle" state. Servers that 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 receive a PRIORITY_UPDATE for a push stream in the "idle" state MUST
respond with a connection error of type PROTOCOL_ERROR. respond with a connection error of type PROTOCOL_ERROR.
If a PRIORITY_UPDATE frame is received with a Prioritized Stream ID If a PRIORITY_UPDATE frame is received with a prioritized stream ID
of 0x0, the recipient MUST respond with a connection error of type of 0x0, the recipient MUST respond with a connection error of type
PROTOCOL_ERROR. PROTOCOL_ERROR.
Servers MUST NOT send PRIORITY_UPDATE frames. If a client receives a Servers MUST NOT send PRIORITY_UPDATE frames. If a client receives a
PRIORITY_UPDATE frame, it MUST respond with a connection error of PRIORITY_UPDATE frame, it MUST respond with a connection error of
type PROTOCOL_ERROR. type PROTOCOL_ERROR.
7.2. HTTP/3 PRIORITY_UPDATE Frame 7.2. HTTP/3 PRIORITY_UPDATE Frame
The HTTP/3 PRIORITY_UPDATE frame (type=0xF0700 or 0xF0701) is used by The HTTP/3 PRIORITY_UPDATE frame (type=0xF0700 or 0xF0701) is used by
clients to signal the initial priority of a response, or to clients to signal the initial priority of a response, or to
reprioritize a response or push stream. It carries the identifier of reprioritize a response or push stream. It carries the identifier of
the element that is being prioritized and the updated priority in the element that is being prioritized and the updated priority in
ASCII text that uses the same representation as that of the Priority ASCII text that uses the same representation as that of the Priority
header field value. PRIORITY_UPDATE with a frame type of 0xF0700 is header field value. PRIORITY_UPDATE with a frame type of 0xF0700 is
used for request streams, while PRIORITY_UPDATE with a frame type of used for request streams, while PRIORITY_UPDATE with a frame type of
0xF0701 is used for push streams. 0xF0701 is used for push streams.
The PRIORITY_UPDATE frame MUST be sent on the client control stream The PRIORITY_UPDATE frame MUST be sent on the client control stream
(see Section 6.2.1 of [HTTP3]). Receiving a PRIORITY_UPDATE frame on (see Section 6.2.1 of [HTTP/3]). Receiving a PRIORITY_UPDATE frame
a stream other than the client control stream MUST be treated as a on a stream other than the client control stream MUST be treated as a
connection error of type H3_FRAME_UNEXPECTED. connection error of type H3_FRAME_UNEXPECTED.
HTTP/3 PRIORITY_UPDATE Frame { HTTP/3 PRIORITY_UPDATE Frame {
Type (i) = 0xF0700..0xF0701, Type (i) = 0xF0700..0xF0701,
Length (i), Length (i),
Prioritized Element ID (i), Prioritized Element ID (i),
Priority Field Value (..), Priority Field Value (..),
} }
Figure 2: HTTP/3 PRIORITY_UPDATE Frame Figure 2: HTTP/3 PRIORITY_UPDATE Frame
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Prioritized Element ID: The stream ID or push ID that is the target Prioritized Element ID: The stream ID or push ID that is the target
of the priority update. of the priority update.
Priority Field Value: The priority update value in ASCII text, Priority Field Value: The priority update value in ASCII text,
encoded using Structured Fields. This is the same representation encoded using Structured Fields. This is the same representation
as the Priority header field value. as the Priority header field value.
The request-stream variant of PRIORITY_UPDATE (type=0xF0700) MUST The request-stream variant of PRIORITY_UPDATE (type=0xF0700) MUST
reference a request stream. If a server receives a PRIORITY_UPDATE reference a request stream. If a server receives a PRIORITY_UPDATE
(type=0xF0700) for a Stream ID that is not a request stream, this (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 MUST be treated as a connection error of type H3_ID_ERROR. The
Stream ID MUST be within the client-initiated bidirectional stream stream ID MUST be within the client-initiated bidirectional stream
limit. If a server receives a PRIORITY_UPDATE (type=0xF0700) with a 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 stream ID that is beyond the stream limits, this SHOULD be treated as
a connection error of type H3_ID_ERROR. Generating an error is not a connection error of type H3_ID_ERROR. Generating an error is not
mandatory because HTTP/3 implementations might have practical mandatory because HTTP/3 implementations might have practical
barriers to determining the active stream concurrency limit that is barriers to determining the active stream concurrency limit that is
applied by the QUIC layer. applied by the QUIC layer.
The push-stream variant PRIORITY_UPDATE (type=0xF0701) MUST reference The push-stream variant of PRIORITY_UPDATE (type=0xF0701) MUST
a promised push stream. If a server receives a PRIORITY_UPDATE reference a promised push stream. If a server receives a
(type=0xF0701) with a Push ID that is greater than the maximum Push PRIORITY_UPDATE (type=0xF0701) with a push ID that is greater than
ID or which has not yet been promised, this MUST be treated as a the maximum push ID or that has not yet been promised, this MUST be
connection error of type H3_ID_ERROR. treated as a connection error of type H3_ID_ERROR.
Servers MUST NOT send PRIORITY_UPDATE frames of either type. If a Servers MUST NOT send PRIORITY_UPDATE frames of either type. 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 Priority 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; the existence of such
is typically best known to the server. Or, a server that receives dependencies is typically best known to the server. Or, a server
requests for a font [RFC8081] and images with the same urgency might that receives requests for a font [RFC8081] and images with the same
give higher precedence to the font, so that a visual client can urgency might give higher precedence to the font, so that a visual
render textual information at an early moment. client can 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 priority parameters found that forwards an HTTP response can use the priority parameters found
in the 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 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 request header field, in which omission of a different from the request header field, in which omission of a
priority parameter implies the use of their default values (see priority parameter implies the use of its default value (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
and the origin responds with and the origin responds with
:status = 200 :status = 200
content-type = image/png content-type = image/png
priority = u=1 priority = u=1
the intermediary might alter its understanding of the urgency from 5 the intermediary might alter its understanding of the urgency from 5
to 1, because it prefers the server-provided value over the client's. 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 incremental value continues to be true, i.e., the value specified
the client, as the server did not specify the incremental(i) by 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
It is generally beneficial for an HTTP server to send all responses It is generally beneficial for an HTTP server to send all responses
skipping to change at page 17, line 32 skipping to change at line 748
Any given connection is likely to have many dynamic permutations. Any given connection is likely to have many dynamic permutations.
For these reasons, it is not possible to describe a universal For these reasons, it is not possible to describe a universal
scheduling algorithm. This document provides some basic, non- scheduling algorithm. This document provides some basic, non-
exhaustive recommendations for how servers might act on priority exhaustive recommendations for how servers might act on priority
parameters. It does not describe in detail how servers might combine parameters. It does not describe in detail how servers might combine
priority signals with other factors. Endpoints cannot depend on priority signals with other factors. Endpoints cannot depend on
particular treatment based on priority signals. Expressing priority particular treatment based on priority signals. Expressing priority
is only a suggestion. 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). respect the incremental parameter (Section 4.2).
Non-incremental responses of the same urgency SHOULD be served by Non-incremental responses of the same urgency SHOULD be served by
prioritizing bandwidth allocation in ascending order of 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 among them. Payload of incremental responses are used in bandwidth among them. The message content of incremental responses
parts, or chunks, as they are received. A client might benefit more is used as parts, or chunks, are received. A client might benefit
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
resource types place critical elements early; others can use resource types place critical elements early; others can use
information progressively. This scheme provides no explicit mandate information progressively. This scheme provides no explicit mandate
about how a server should use size, type or any other input to decide about how a server should use size, type, or any other input to
how to prioritize. decide how to prioritize.
There can be scenarios where a server will need to schedule multiple There can be scenarios where a server will need to schedule multiple
incremental and non-incremental responses at the same urgency level. incremental and non-incremental responses at the same urgency level.
Strictly abiding the scheduling guidance based on urgency and request Strictly abiding by the scheduling guidance based on urgency and
generation order might lead to suboptimal results at the client, as request generation order might lead to suboptimal results at the
early non-incremental responses might prevent serving of incremental client, as early non-incremental responses might prevent the serving
responses issued later. The following are examples of such of incremental responses issued later. The following are examples of
challenges. such challenges:
1. At the same urgency level, a non-incremental request for a large 1. At the same urgency level, a non-incremental request for a large
resource followed by an incremental request for a small resource. resource followed by an incremental request for a small resource.
2. At the same urgency level, an incremental request of 2. At the same urgency level, an incremental request of
indeterminate length followed by a non-incremental large indeterminate length followed by a non-incremental large
resource. resource.
It is RECOMMENDED that servers avoid such starvation where possible. It is RECOMMENDED that servers avoid such starvation where possible.
The method to do so is an implementation decision. For example, a The method for doing so is an implementation decision. For example,
server might pre-emptively send responses of a particular incremental a server might preemptively send responses of a particular
type based on other information such as content size. incremental type based on other information such as content size.
Optimal scheduling of server push is difficult, especially when Optimal scheduling of server push is difficult, especially when
pushed resources contend with active concurrent requests. Servers pushed resources contend with active concurrent requests. Servers
can consider many factors when scheduling, such as the type or size can consider many factors when scheduling, such as the type or size
of resource being pushed, the priority of the request that triggered of resource being pushed, the priority of the request that triggered
the push, the count of active concurrent responses, the priority of the push, the count of active concurrent responses, the priority of
other active concurrent responses, etc. There is no general guidance other active concurrent responses, etc. There is no general guidance
on the best way to apply these. A server that is too simple could 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 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 at too low a priority and delay the response, negating intended goals
of server push. of server push.
Priority signals are a factor for server push scheduling. The Priority signals are a factor for server push scheduling. The
concept of parameter value defaults applies slightly differently concept of parameter value defaults applies slightly differently
because there is no explicit client-signalled initial priority. A because there is no explicit client-signaled initial priority. A
server can apply priority signals provided in an origin response; see server can apply priority signals provided in an origin response; see
the merging guidance given in Section 8. In the absence of origin the merging guidance given in Section 8. In the absence of origin
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 in flight. 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 stream carries a CONNECT request, the scheduling guidance in
guidance in this document applies to the frames on the stream. A this document applies to the frames on the stream. A client that
client that issues multiple CONNECT requests can set the incremental issues multiple CONNECT requests can set the incremental parameter to
parameter to true. Servers that implement the recommendations for true. Servers that implement the recommendations for handling of the
handling of the incremental parameter in Section 10 are likely to incremental parameter (Section 10) are likely to schedule these
schedule these fairly, avoiding one CONNECT stream from blocking fairly, preventing one CONNECT stream from blocking others.
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. In detecting packet losses and retransmitting lost information. In
addition to the considerations in Section 10, scheduling of addition to the considerations in Section 10, scheduling of
retransmission data could compete with new data. The remainder of retransmission data could compete with new 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 the following: "Endpoints SHOULD
retransmission of data over sending new data, unless priorities prioritize retransmission of data over sending new data, unless
specified by the application indicate otherwise". When an HTTP/3 priorities specified by the application indicate otherwise". When an
application uses the priority scheme defined in this document and the HTTP/3 application uses the priority scheme defined in this document
QUIC transport implementation supports application indicated stream and the QUIC transport implementation supports application-indicated
priority, a transport that considers the relative priority of streams stream priority, a transport that considers the relative priority of
when scheduling both new data and retransmission data might better streams when scheduling both new data and retransmission data might
match the expectations of the application. However, there are no better match the expectations of the application. However, there are
requirements on how a transport chooses to schedule based on this no 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 considerations
application priorities when sending probe packets after Probe Timeout regarding application priorities when sending probe packets after
timer expiration. A QUIC implementation supporting application- Probe Timeout timer expiration. A QUIC implementation supporting
indicated priorities might use the relative priority of streams when application-indicated priorities might use the relative priority of
choosing probe data. streams when choosing probe data.
13. Fairness 13. Fairness
Typically, HTTP implementations depend on the underlying transport to Typically, HTTP implementations depend on the underlying transport to
maintain fairness between connections competing for bandwidth. When maintain fairness between connections competing for bandwidth. When
HTTP requests are forwarded through intermediaries, progress made by an intermediary receives HTTP requests on client connections, it
each connection originating from end clients can become different forwards them to backend connections. Depending on how the
over time, depending on how intermediaries coalesce or split requests intermediary coalesces or splits requests across different backend
into backend connections. This unfairness can expand if priority connections, different clients might experience dissimilar
signals are used. Section 13.1 and Section 13.2 discuss mitigations performance. This dissimilarity might expand if the intermediary
against this expansion of unfairness. also uses priority signals when forwarding requests. Sections 13.1
and 13.2 discuss mitigations of this expansion of unfairness.
Conversely, Section 13.3 discusses how servers might intentionally Conversely, Section 13.3 discusses how servers might intentionally
allocate unequal bandwidth to some connections depending on the allocate unequal bandwidth to some connections, depending on the
priority signals. 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 carry signals server, requests that originate from one client might carry signals
indicating higher priority 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 Priority header field values. As an example, a intermediary to obey Priority header field values. As an example, a
resource-constrained server might defer the transmission of software resource-constrained server might defer the transmission of software
update files that have the background urgency. However, in the worst update files that have the background urgency level (7). However, in
case, the asymmetry between the priority declared by multiple clients the worst case, the asymmetry between the priority declared by
might cause responses going to one user agent to be delayed totally multiple clients might cause all responses going to one user agent to
after those going to another. be delayed until all responses going to another user agent have been
sent.
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 input 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 can check to see if the request contains one of the
following header fields: following header fields:
* Forwarded [FORWARDED], X-Forwarded-For * Forwarded [FORWARDED], X-Forwarded-For
* Via (see Section 7.6.3 of [HTTP]) * Via (see Section 7.6.3 of [HTTP])
13.2. HTTP/1.x Back Ends 13.2. HTTP/1.x Back Ends
It is common for CDN infrastructure to support different HTTP It is common for Content Delivery Network (CDN) infrastructure to
versions on the front end and back end. For instance, the client- support different HTTP versions on the front end and back end. For
facing edge might support HTTP/2 and HTTP/3 while communication to instance, the client-facing edge might support HTTP/2 and HTTP/3
back end servers is done using HTTP/1.1. Unlike with connection while communication to backend servers is done using HTTP/1.1.
coalescing, the CDN will "de-mux" requests into discrete connections Unlike connection coalescing, the CDN will "demux" requests into
to the back end. HTTP/1.1 and older do not support response discrete connections to the back end. Response multiplexing in a
multiplexing in a single connection, so there is not a fairness single connection is not supported by HTTP/1.1 (or older), so there
problem. However, back end servers MAY still use client headers for is not a fairness problem. However, backend servers MAY still use
request scheduling. Back end servers SHOULD only schedule based on client headers for request scheduling. Backend servers SHOULD only
client priority information where that information can be scoped to schedule based on client priority information where that information
individual end clients. Authentication and other session information can be scoped to individual end clients. Authentication and other
might provide this linkability. session information might provide this linkability.
13.3. Intentional Introduction of Unfairness 13.3. Intentional Introduction of Unfairness
It is sometimes beneficial to deprioritize the transmission of one It is sometimes beneficial to deprioritize the transmission of one
connection over others, knowing that doing so introduces a certain connection over others, knowing that doing so introduces a certain
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?
In contrast to the prioritization scheme of HTTP/2 that uses a hop- In contrast to the prioritization scheme of HTTP/2, which uses a hop-
by-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 way that a client processes a response is a property associated The way that a client processes a response is a property associated
with the client generating that request, not that of an intermediary. with the client generating that request, not that of an intermediary.
Therefore, it is an end-to-end property. How these end-to-end Therefore, it is an end-to-end property. How these end-to-end
properties carried by the Priority header field affect the properties carried by the Priority header field affect the
prioritization between the responses that share a connection is a prioritization between the responses that share a connection is a
hop-by-hop issue. 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
skipping to change at page 22, line 25 skipping to change at line 970
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.
15. Security Considerations 15. Security Considerations
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
responses.
The security considerations from [STRUCTURED-FIELDS] apply to The security considerations from [STRUCTURED-FIELDS] apply to the
processing of priority parameters defined in Section 4. processing of priority parameters defined in Section 4.
16. IANA Considerations 16. IANA Considerations
This specification registers the following entry in the Hypertext This specification registers the following entry in the "Hypertext
Transfer Protocol (HTTP) Field Name Registry established by [HTTP]: Transfer Protocol (HTTP) Field Name Registry" defined in [HTTP/2]:
Field name: Priority
Field Name: Priority
Status: permanent Status: permanent
Reference: This document
Specification document(s): This document This specification registers the following entry in the "HTTP/2
Settings" registry defined in [HTTP/2]:
This specification registers the following entry in the HTTP/2
Settings registry established by [RFC7540]:
Name: SETTINGS_NO_RFC7540_PRIORITIES
Code: 0x9 Code: 0x9
Name: SETTINGS_NO_RFC7540_PRIORITIES
Initial Value: 0
Reference: This document
Initial value: 0 This specification registers the following entry in the "HTTP/2 Frame
Type" registry defined in [HTTP/2]:
Specification: This document
This specification registers the following entry in the HTTP/2 Frame
Type registry established by [RFC7540]:
Frame Type: PRIORITY_UPDATE
Code: 0x10 Code: 0x10
Specification: This document
This specification registers the following entries in the HTTP/3
Frame Type registry established by [HTTP3]:
Frame Type: PRIORITY_UPDATE Frame Type: PRIORITY_UPDATE
Reference: This document
Code: 0xF0700 and 0xF0701 This specification registers the following entry in the "HTTP/3 Frame
Types" registry established by [HTTP/3]:
Specification: This document Value: 0xF0700-0xF0701
Frame Type: PRIORITY_UPDATE
Status: permanent
Reference: This document
Change Controller: IETF
Contact: ietf-http-wg@w3.org
Upon publication, please create the HTTP Priority Parameters registry IANA has created the "Hypertext Transfer Protocol (HTTP) Priority"
at https://iana.org/assignments/http-priority registry at <https://www.iana.org/assignments/http-priority> and has
(https://iana.org/assignments/http-priority) and populate it with the populated it with the entries in Table 1; see Section 4.3.1 for its
entries in Table 1; see Section 4.3.1 for its associated procedures. associated procedures.
+======+==================================+===============+ +======+==================================+=============+
| Name | Description | Specification | | Name | Description | Reference |
+======+==================================+===============+ +======+==================================+=============+
| u | The urgency of an HTTP response. | Section 4.1 | | u | The urgency of an HTTP response. | Section 4.1 |
+------+----------------------------------+---------------+ +------+----------------------------------+-------------+
| i | Whether an HTTP response can be | Section 4.2 | | i | Whether an HTTP response can be | Section 4.2 |
| | processed incrementally. | | | | processed incrementally. | |
+------+----------------------------------+---------------+ +------+----------------------------------+-------------+
Table 1: Initial Priority Parameters 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., Ed., Nottingham, M., Ed., and J. Reschke,
Semantics", Work in Progress, Internet-Draft, draft-ietf- Ed., "HTTP Semantics", STD 97, RFC 9110,
httpbis-semantics-19, 12 September 2021, DOI 10.17487/RFC9110, June 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis- <https://www.rfc-editor.org/info/rfc9110>.
semantics-19>.
[HTTP2] Thomson, M. and C. Benfield, "Hypertext Transfer Protocol [HTTP/2] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
Version 2 (HTTP/2)", Work in Progress, Internet-Draft, DOI 10.17487/RFC9113, June 2022,
draft-ietf-httpbis-http2bis-06, 18 November 2021, <https://www.rfc-editor.org/info/rfc9113>.
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
http2bis-06>.
[HTTP3] Bishop, M., "Hypertext Transfer Protocol Version 3 [HTTP/3] Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
(HTTP/3)", Work in Progress, Internet-Draft, draft-ietf- June 2022, <https://www.rfc-editor.org/info/rfc9114>.
quic-http-34, 2 February 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
http-34>.
[QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based [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,
<https://www.rfc-editor.org/rfc/rfc9000>. <https://www.rfc-editor.org/info/rfc9000>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26, Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017, RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/rfc8126>. <https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[STRUCTURED-FIELDS] [STRUCTURED-FIELDS]
Nottingham, M. and P-H. Kamp, "Structured Field Values for Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021, HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/rfc/rfc8941>. <https://www.rfc-editor.org/info/rfc8941>.
17.2. Informative References 17.2. Informative References
[CACHING] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP [CACHING] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Caching", Work in Progress, Internet-Draft, draft-ietf- Ed., "HTTP Caching", STD 98, RFC 9111,
httpbis-cache-19, 12 September 2021, DOI 10.17487/RFC9111, June 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis- <https://www.rfc-editor.org/info/rfc9111>.
cache-19>.
[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/info/rfc7239>.
[I-D.lassey-priority-setting] [MARX] Marx, R., De Decker, T., Quax, P., and W. Lamotte, "Of the
Utmost Importance: Resource Prioritization in HTTP/3 over
QUIC", SCITEPRESS Proceedings of the 15th International
Conference on Web Information Systems and Technologies
(pages 130-143), DOI 10.5220/0008191701300143, September
2019, <https://www.doi.org/10.5220/0008191701300143>.
[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>.
[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/info/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/info/rfc7540>.
[RFC8081] Lilley, C., "The "font" Top-Level Media Type", RFC 8081, [RFC8081] Lilley, C., "The "font" Top-Level Media Type", RFC 8081,
DOI 10.17487/RFC8081, February 2017, DOI 10.17487/RFC8081, February 2017,
<https://www.rfc-editor.org/rfc/rfc8081>. <https://www.rfc-editor.org/info/rfc8081>.
Appendix A. Acknowledgements Acknowledgements
Roy Fielding presented the idea of using a header field for Roy Fielding presented the idea of using a header field for
representing priorities in representing priorities in
https://www.ietf.org/proceedings/83/slides/slides-83-httpbis-5.pdf <https://www.ietf.org/proceedings/83/slides/slides-83-httpbis-5.pdf>.
(https://www.ietf.org/proceedings/83/slides/slides-83-httpbis-5.pdf). In <https://github.com/pmeenan/http3-prioritization-proposal>,
In https://github.com/pmeenan/http3-prioritization-proposal Patrick Meenan advocated for representing the priorities using a
(https://github.com/pmeenan/http3-prioritization-proposal), Patrick tuple of urgency and concurrency. The ability to disable HTTP/2
Meenan advocated for representing the priorities using a tuple of prioritization is inspired by [PRIORITY-SETTING], authored by Brad
urgency and concurrency. The ability to disable HTTP/2 Lassey and Lucas Pardue, with modifications based on feedback that
prioritization is inspired by [I-D.lassey-priority-setting], authored was not incorporated into an update to that document.
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 The motivation for defining an alternative to HTTP/2 priorities is
drawn from discussion within the broad HTTP community. Special drawn from discussion within the broad HTTP community. Special
thanks to Roberto Peon, Martin Thomson and Netflix for text that was thanks to Roberto Peon, Martin Thomson, and Netflix for text that was
incorporated explicitly in this document. incorporated explicitly in this document.
In addition to the people above, this document owes a lot to the In addition to the people above, this document owes a lot to the
extensive discussion in the HTTP priority design team, consisting of extensive discussion in the HTTP priority design team, consisting of
Alan Frindell, Andrew Galloni, Craig Taylor, Ian Swett, Kazuho Oku, Alan Frindell, Andrew Galloni, Craig Taylor, Ian Swett, Matthew Cox,
Lucas Pardue, Matthew Cox, Mike Bishop, Roberto Peon, Robin Marx, Roy Mike Bishop, Roberto Peon, Robin Marx, Roy Fielding, and the authors
Fielding. of this document.
Yang Chi contributed the section on retransmission scheduling. Yang Chi contributed the section on retransmission scheduling.
Appendix B. Change Log
_RFC EDITOR: please remove this section before publication_
B.1. Since draft-ietf-httpbis-priority-11
* Changes to address Last Call/IESG feedback
B.2. Since draft-ietf-httpbis-priority-10
* Editorial changes
* Add clearer IANA instructions for Priority Parameter initial
population
B.3. Since draft-ietf-httpbis-priority-09
* Editorial changes
B.4. Since draft-ietf-httpbis-priority-08
* Changelog fixups
B.5. Since draft-ietf-httpbis-priority-07
* Relax requirements of receiving SETTINGS_NO_RFC7540_PRIORITIES
that changes value (#1714, #1725)
* Clarify how intermediaries might use frames vs. headers (#1715,
#1735)
* Relax requirement when receiving a PRIORITY_UPDATE with an invalid
structured field value (#1741, #1756)
B.6. Since draft-ietf-httpbis-priority-06
* Focus on editorial changes
* Clarify rules about Sf-Dictionary handling in headers
* Split policy for parameter IANA registry into two sections based
on key length
B.7. Since draft-ietf-httpbis-priority-05
* Renamed SETTINGS_DEPRECATE_RFC7540_PRIORITIES to
SETTINGS_NO_RFC7540_PRIORITIES
* Clarify that senders of the HTTP/2 setting can use any alternative
(#1679, #1705)
B.8. Since draft-ietf-httpbis-priority-04
* Renamed SETTINGS_DEPRECATE_HTTP2_PRIORITIES to
SETTINGS_DEPRECATE_RFC7540_PRIORITIES (#1601)
* Reoriented text towards RFC7540bis (#1561, #1601)
* Clarify intermediary behavior (#1562)
B.9. Since draft-ietf-httpbis-priority-03
* Add statement about what this scheme applies to. Clarify
extensions can use it but must define how themselves (#1550,
#1559)
* Describe scheduling considerations for the CONNECT method (#1495,
#1544)
* Describe scheduling considerations for retransmitted data (#1429,
#1504)
* Suggest intermediaries might avoid strict prioritization (#1562)
B.10. Since draft-ietf-httpbis-priority-02
* Describe considerations for server push prioritization (#1056,
#1345)
* Define HTTP/2 PRIORITY_UPDATE ID limits in HTTP/2 terms (#1261,
#1344)
* Add a Priority Parameters registry (#1371)
B.11. Since draft-ietf-httpbis-priority-01
* PRIORITY_UPDATE frame changes (#1096, #1079, #1167, #1262, #1267,
#1271)
* Add section to describe server scheduling considerations (#1215,
#1232, #1266)
* Remove specific instructions related to intermediary fairness
(#1022, #1264)
B.12. Since draft-ietf-httpbis-priority-00
* Move text around (#1217, #1218)
* Editorial change to the default urgency. The value is 3, which
was always the intent of previous changes.
B.13. Since draft-kazuho-httpbis-priority-04
* Minimize semantics of Urgency levels (#1023, #1026)
* Reduce guidance about how intermediary implements merging priority
signals (#1026)
* Remove mention of CDN-Loop (#1062)
* Editorial changes
* Make changes due to WG adoption
* Removed outdated Consideration (#118)
B.14. Since draft-kazuho-httpbis-priority-03
* Changed numbering from [-1,6] to [0,7] (#78)
* Replaced priority scheme negotiation with HTTP/2 priority
deprecation (#100)
* Shorten parameter names (#108)
* Expand on considerations (#105, #107, #109, #110, #111, #113)
B.15. Since draft-kazuho-httpbis-priority-02
* Consolidation of the problem statement (#61, #73)
* Define SETTINGS_PRIORITIES for negotiation (#58, #69)
* Define PRIORITY_UPDATE frame for HTTP/2 and HTTP/3 (#51)
* Explain fairness issue and mitigations (#56)
B.16. Since draft-kazuho-httpbis-priority-01
* Explain how reprioritization might be supported.
B.17. Since draft-kazuho-httpbis-priority-00
* Expand urgency levels from 3 to 8.
Authors' Addresses Authors' Addresses
Kazuho Oku Kazuho Oku
Fastly Fastly
Email: kazuhooku@gmail.com Email: kazuhooku@gmail.com
Additional contact information:
奥 一穂
Fastly
Lucas Pardue Lucas Pardue
Cloudflare Cloudflare
Email: lucaspardue.24.7@gmail.com Email: lucaspardue.24.7@gmail.com
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