draft-ietf-httpbis-priority-01.txt   draft-ietf-httpbis-priority-02.txt 
HTTP K. Oku HTTP K. Oku
Internet-Draft Fastly Internet-Draft Fastly
Intended status: Standards Track L. Pardue Intended status: Standards Track L. Pardue
Expires: January 14, 2021 Cloudflare Expires: April 4, 2021 Cloudflare
July 13, 2020 October 01, 2020
Extensible Prioritization Scheme for HTTP Extensible Prioritization Scheme for HTTP
draft-ietf-httpbis-priority-01 draft-ietf-httpbis-priority-02
Abstract Abstract
This document describes a scheme for prioritizing HTTP responses. This document describes a scheme for prioritizing HTTP responses.
This scheme expresses the priority of each HTTP response using This scheme expresses the priority of each HTTP response using
absolute values, rather than as a relative relationship between a absolute values, rather than as a relative relationship between a
group of HTTP responses. group of HTTP responses.
This document defines the Priority header field for communicating the This document defines the Priority header field for communicating the
initial priority in an HTTP version-independent manner, as well as initial priority in an HTTP version-independent manner, as well as
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 14, 2021. This Internet-Draft will expire on April 4, 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. Motivation for Replacing HTTP/2 Priorities . . . . . . . . . 4 2. Motivation for Replacing HTTP/2 Priorities . . . . . . . . . 4
2.1. Disabling HTTP/2 Priorities . . . . . . . . . . . . . . . 5 2.1. Disabling HTTP/2 Priorities . . . . . . . . . . . . . . . 5
3. Priority Parameters . . . . . . . . . . . . . . . . . . . . . 6 3. Priority Parameters . . . . . . . . . . . . . . . . . . . . . 6
3.1. Urgency . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Urgency . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Incremental . . . . . . . . . . . . . . . . . . . . . . . 7 3.2. Incremental . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Defining New Parameters . . . . . . . . . . . . . . . . . 8 3.3. Defining New Parameters . . . . . . . . . . . . . . . . . 8
4. The Priority HTTP Header Field . . . . . . . . . . . . . . . 8 4. The Priority HTTP Header Field . . . . . . . . . . . . . . . 8
5. Reprioritization . . . . . . . . . . . . . . . . . . . . . . 8 5. Reprioritization . . . . . . . . . . . . . . . . . . . . . . 9
5.1. HTTP/2 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 9 6. The PRIORITY_UPDATE Frame . . . . . . . . . . . . . . . . . . 9
5.2. HTTP/3 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 10 6.1. HTTP/2 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 10
6. Merging Client- and Server-Driven Parameters . . . . . . . . 11 6.2. HTTP/3 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 11
7. Client Scheduling . . . . . . . . . . . . . . . . . . . . . . 12 7. Merging Client- and Server-Driven Parameters . . . . . . . . 12
8. Fairness . . . . . . . . . . . . . . . . . . . . . . . . . . 12 8. Client Scheduling . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Coalescing Intermediaries . . . . . . . . . . . . . . . . 13 9. Server Scheduling . . . . . . . . . . . . . . . . . . . . . . 13
8.2. HTTP/1.x Back Ends . . . . . . . . . . . . . . . . . . . 13 10. Fairness . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.3. Intentional Introduction of Unfairness . . . . . . . . . 14 10.1. Coalescing Intermediaries . . . . . . . . . . . . . . . 15
9. Why use an End-to-End Header Field? . . . . . . . . . . . . . 14 10.2. HTTP/1.x Back Ends . . . . . . . . . . . . . . . . . . . 15
10. Security Considerations . . . . . . . . . . . . . . . . . . . 15 10.3. Intentional Introduction of Unfairness . . . . . . . . . 16
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 11. Why use an End-to-End Header Field? . . . . . . . . . . . . . 16
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 12. Security Considerations . . . . . . . . . . . . . . . . . . . 16
12.1. Normative References . . . . . . . . . . . . . . . . . . 16 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
12.2. Informative References . . . . . . . . . . . . . . . . . 17 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
12.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 18 14.1. Normative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 18 14.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 18 14.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 19
B.1. Since draft-ietf-httpbis-priority-00 . . . . . . . . . . 18 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 20
B.2. Since draft-kazuho-httpbis-priority-04 . . . . . . . . . 19 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 20
B.3. Since draft-kazuho-httpbis-priority-03 . . . . . . . . . 19 B.1. Since draft-ietf-httpbis-priority-01 . . . . . . . . . . 20
B.4. Since draft-kazuho-httpbis-priority-02 . . . . . . . . . 19 B.2. Since draft-ietf-httpbis-priority-00 . . . . . . . . . . 20
B.5. Since draft-kazuho-httpbis-priority-01 . . . . . . . . . 19 B.3. Since draft-kazuho-httpbis-priority-04 . . . . . . . . . 21
B.6. Since draft-kazuho-httpbis-priority-00 . . . . . . . . . 19 B.4. Since draft-kazuho-httpbis-priority-03 . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 B.5. Since draft-kazuho-httpbis-priority-02 . . . . . . . . . 21
B.6. Since draft-kazuho-httpbis-priority-01 . . . . . . . . . 21
B.7. Since draft-kazuho-httpbis-priority-00 . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction 1. Introduction
It is common for an HTTP ([RFC7230]) resource representation to have It is common for an HTTP ([RFC7230]) resource representation 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, leading to further retrieval requests. Meanwhile, representation, leading to further retrieval requests. Meanwhile,
the nature of the relationship determines whether the client is the nature of the relationship determines whether the client is
blocked from continuing to process locally available resources. For blocked from continuing to process locally available resources. For
example, visual rendering of an HTML document could be blocked by the example, visual rendering of an HTML document could be blocked by the
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end format. Along with the protocol-version-specific frame for end format. Along with the protocol-version-specific frame for
reprioritization, this prioritization scheme acts as a substitute for reprioritization, this prioritization scheme acts as a substitute for
the original prioritization scheme of HTTP/2. the original prioritization scheme of HTTP/2.
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", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
The terms sh-token and sh-boolean are imported from The terms sf-token and sf-boolean are imported from
[STRUCTURED-HEADERS]. [STRUCTURED-FIELDS].
Example HTTP requests and responses use the HTTP/2-style formatting Example HTTP requests and responses use the HTTP/2-style formatting
from [RFC7540]. from [RFC7540].
This document uses the variable-length integer encoding from This document uses the variable-length integer encoding from
[I-D.ietf-quic-transport]. [I-D.ietf-quic-transport].
The term control stream is used to describe the HTTP/2 stream with
identifier 0x0, and HTTP/3 control stream; see [I-D.ietf-quic-http],
Section 6.2.1.
2. Motivation for Replacing HTTP/2 Priorities 2. Motivation for Replacing HTTP/2 Priorities
An important feature of any implementation of a protocol that An important feature of any implementation of a protocol that
provides multiplexing is the ability to prioritize the sending of provides multiplexing is the ability to prioritize the sending of
information. This was an important realization in the design of information. This was an important realization in the design of
HTTP/2. Prioritization is a difficult problem, so it will always be HTTP/2. Prioritization is a difficult problem, so it will always be
suboptimal, particularly if one endpoint operates in ignorance of the suboptimal, particularly if one endpoint operates in ignorance of the
needs of its peer. needs of its peer.
HTTP/2 introduced a complex prioritization signaling scheme that used HTTP/2 introduced a complex prioritization signaling scheme that used
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The rich flexibility of client-driven HTTP/2 prioritization tree The rich flexibility of client-driven HTTP/2 prioritization tree
building is rarely exercised. Experience has shown that clients tend building is rarely exercised. Experience has shown that clients tend
to choose a single model optimized for a web use case and experiment to choose a single model optimized for a web use case and experiment
within the model constraints, or do nothing at all. Furthermore, within the model constraints, or do nothing at all. Furthermore,
many clients build their prioritization tree in a unique way, which many clients build their prioritization tree in a unique way, which
makes it difficult for servers to understand their intent and act or makes it difficult for servers to understand their intent and act or
intervene accordingly. intervene accordingly.
Many HTTP/2 server implementations do not include support for the Many HTTP/2 server implementations do not include support for the
priority scheme, some favoring instead bespoke server-driven schemes priority scheme. Some instead favor custom server-driven schemes
based on heuristics and other hints, like the content type of based on heuristics or other hints, such as resource content type or
resources and the request generation order. For example, a server, request generation order. For example, a server, with knowledge of
with knowledge of the document structure, might want to prioritize the document structure, might want to prioritize the delivery of
the delivery of images that are critical to user experience above images that are critical to user experience above other images, but
other images, but below the CSS files. Since client trees vary, it below the CSS files. Since client trees vary, it is impossible for
is impossible for the server to determine how such images should be the server to determine how such images should be prioritized against
prioritized against other responses. other responses.
The HTTP/2 scheme allows intermediaries to coalesce multiple client The HTTP/2 scheme allows intermediaries to coalesce multiple client
trees into a single tree that is used for a single upstream HTTP/2 trees into a single tree that is used for a single upstream HTTP/2
connection. However, most intermediaries do not support this. The connection. However, most intermediaries do not support this. The
scheme does not define a method that can be used by a server to scheme does not define a method that can be used by a server to
express the priority of a response. Without such a method, express the priority of a response. Without such a method,
intermediaries cannot coordinate client-driven and server-driven intermediaries cannot coordinate client-driven and server-driven
priorities. priorities.
HTTP/2 describes denial-of-service considerations for HTTP/2 describes denial-of-service considerations for
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parameter value after the first SETTINGS frame. Detection of a parameter value after the first SETTINGS frame. Detection of a
change by a receiver MUST be treated as a connection error of type change by a receiver MUST be treated as a connection error of type
PROTOCOL_ERROR. PROTOCOL_ERROR.
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 priority signal defined in the HTTP/2 client SHOULD send both the priority signal defined in the HTTP/2
priority scheme and also that of this prioritization scheme. Once priority scheme and also that of this prioritization scheme. Once
the client learns that the HTTP/2 priority scheme is deprecated, it the client learns that the HTTP/2 priority scheme is deprecated, it
SHOULD stop sending the HTTP/2 priority signals. If the client SHOULD stop sending the HTTP/2 priority signals. If the client
learns that the HTTP/2 priority scheme is not deprecated, it SHOULD learns that the HTTP/2 priority scheme is not deprecated, it SHOULD
stop sending PRIORITY_UPDATE frames (Section 5.1), but MAY continue stop sending PRIORITY_UPDATE frames (Section 6.1), but MAY continue
sending the Priority header field (Section 4), as it is an end-to-end sending the Priority header field (Section 4), as it is an end-to-end
signal that might be useful to nodes behind the server that the signal that might be useful to nodes behind the server that the
client is directly connected to. client is directly connected to.
The SETTINGS frame precedes any priority signal sent from a client in The SETTINGS frame precedes any priority signal sent from a client in
HTTP/2, so a server can determine if it should respect the HTTP/2 HTTP/2, so a server can determine if it should respect the HTTP/2
scheme before building state. scheme before building state. A server that receives
SETTINGS_DEPRECATE_HTTP2_PRIORITIES MUST ignore HTTP/2 priority
signals.
Where both endpoints disable HTTP/2 priorities, the client is
expected to send this scheme's priority signal. Handling of omitted
signals is described in Section 3.
3. Priority Parameters 3. 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 4) is an end-to-end way to The Priority HTTP header field (Section 4) is an end-to-end way to
transmit this set of parameters when a request or a response is transmit this set of parameters when a request or a response is
issued. In order to reprioritize a request, HTTP-version-specific issued. In order to reprioritize a request, HTTP-version-specific
frames (Section 5.1 and Section 5.2) are used by clients to transmit frames (Section 6.1 and Section 6.2) are used by clients to transmit
the same information on a single hop. If intermediaries want to the same information on a single hop. If intermediaries want to
specify prioritization on a multiplexed HTTP connection, they SHOULD specify prioritization on a multiplexed HTTP connection, they SHOULD
use a PRIORITY_UPDATE frame and SHOULD NOT change the Priority header use a PRIORITY_UPDATE frame and SHOULD NOT change the Priority header
field. field.
In both cases, the set of priority parameters is encoded as a In both cases, the set of priority parameters is encoded as a
Structured Headers Dictionary ([STRUCTURED-HEADERS]). Structured Fields Dictionary ([STRUCTURED-FIELDS]).
This document defines the urgency("u") and incremental("i") This document defines the urgency("u") and incremental("i")
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. Note that handling of omitted parameters is were specified. Note that handling of omitted parameters is
different when processing an HTTP response; see Section 6. different when processing an HTTP response; see Section 7.
Unknown parameters, parameters with out-of-range values or values of Unknown parameters, parameters with out-of-range values or values of
unexpected types MUST be ignored. unexpected types MUST be ignored.
3.1. Urgency 3.1. Urgency
The urgency parameter ("u") takes an integer between 0 and 7, in The urgency parameter ("u") takes an integer between 0 and 7, in
descending order of priority. This range provides sufficient descending order of priority. This range provides sufficient
granularity for prioritizing responses for ordinary web browsing, at granularity for prioritizing responses for ordinary web browsing, at
minimal complexity. minimal complexity.
The value is encoded as an sh-integer. The default value is 3. The value is encoded as an sf-integer. The default value is 3.
This parameter indicates the sender's recommendation, based on the This parameter indicates the sender's recommendation, based on the
expectation that the server would transmit HTTP responses in the expectation that the server would transmit HTTP responses in the
order of their urgency values if possible. The smaller the value, order of their urgency values if possible. The smaller the value,
the higher the precedence. 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 6). urgency using knowledge specific to the web-site (see Section 7).
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 impact on user interaction.
3.2. Incremental 3.2. Incremental
The incremental parameter ("i") takes an sh-boolean as the value that The incremental parameter ("i") takes an sf-boolean as the value that
indicates if an HTTP response can be processed incrementally, i.e. indicates if an HTTP response can be processed incrementally, i.e.
provide some meaningful output as chunks of the response arrive. 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").
A server might distribute the bandwidth of a connection between A server might distribute the bandwidth of a connection between
incremental responses that share the same urgency, hoping that incremental responses that share the same urgency, hoping that
providing those responses in parallel would be more helpful to the providing those responses in parallel would be more helpful to the
client than delivering the responses one by one. client than delivering the responses one by one.
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:method = GET :method = GET
:scheme = https :scheme = https
:authority = example.net :authority = example.net
:path = /image.jpg :path = /image.jpg
priority = u=5, i priority = u=5, i
3.3. Defining New Parameters 3.3. Defining New Parameters
When attempting to extend priorities, care must be taken to ensure When attempting to extend priorities, care must be taken to ensure
any use of existing parameters are either unchanged or modified in a any use of existing parameters leaves them either unchanged or
way that is backwards compatible for peers that are unaware of the modified in a way that is backwards compatible for peers that are
extended meaning. unaware of the extended meaning.
For example, if there is a need to provide more granularity than For example, if there is a need to provide more granularity than
eight urgency levels, it would be possible to subdivide the range eight urgency levels, it would be possible to subdivide the range
using an additional parameter. Implementations that do not recognize using an additional parameter. Implementations that do not recognize
the parameter can safely continue to use the less granular eight the parameter can safely continue to use the less granular eight
levels. levels.
Alternatively, the urgency can be augmented. For example, a Alternatively, the urgency can be augmented. For example, a
graphical user agent could send a "visible" parameter to indicate if graphical user agent could send a "visible" parameter to indicate if
the resource being requested is within the viewport. the resource being requested is within the viewport.
4. The Priority HTTP Header Field 4. The Priority HTTP Header Field
The Priority HTTP header field can appear in requests and responses. The Priority HTTP header field can appear in requests and responses.
A client uses it to specify the priority of the response. A server A client uses it to specify the priority of the response. A server
uses it to inform the client that the priority was overwritten. An uses it to inform the client that the priority was overwritten. An
intermediary can use the Priority information from client requests intermediary can use the Priority information from client requests
and server responses to correct or amend the precedence to suit it and server responses to correct or amend the precedence to suit it
(see Section 6). (see Section 7).
The Priority header field is an end-to-end signal of the request The Priority header field is an end-to-end signal of the request
priority from the client or the response priority from the server. priority from the client or the response priority from the server.
As is the ordinary case for HTTP caching ([RFC7234]), a response with As is the ordinary case for HTTP caching ([RFC7234]), a response with
a Priority header field might be cached and re-used for subsequent a Priority header field might be cached and re-used for subsequent
requests. When an origin server generates the Priority response requests. When an origin server generates the Priority response
header field based on properties of an HTTP request it receives, the header field based on properties of an HTTP request it receives, the
server is expected to control the cacheability or the applicability server is expected to control the cacheability or the applicability
of the cached response, by using header fields that control the of the cached response, by using header fields that control the
caching behavior (e.g., Cache-Control, Vary). caching behavior (e.g., Cache-Control, Vary).
An endpoint that fails to parse the Priority header field SHOULD use
default parameter values.
5. Reprioritization 5. Reprioritization
After a client sends a request, it may be beneficial to change the After a client sends a request, it may be beneficial to change the
priority of the response. As an example, a web browser might issue a priority of the response. As an example, a web browser might issue a
prefetch request for a JavaScript file with the urgency parameter of prefetch request for a JavaScript file with the urgency parameter of
the Priority request header field set to "u=7" (background). Then, the Priority request header field set to "u=7" (background). Then,
when the user navigates to a page which references the new JavaScript when the user navigates to a page which references the new JavaScript
file, while the prefetch is in progress, the browser would send a file, while the prefetch is in progress, the browser would send a
reprioritization frame 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 6) can be used for such
reprioritization.
In HTTP/2 and HTTP/3, after a request message is sent on a stream, 6. The PRIORITY_UPDATE Frame
the stream transitions to a state that prevents the client from
sending additional frames on the stream. Therefore, a client cannot
reprioritize a response by using the Priority header field.
Modifying this behavior would require a semantic change to the
protocol, but this is avoided by restricting the stream on which a
PRIORITY_UPDATE frame can be sent. In HTTP/2 the frame is on stream
zero and in HTTP/3 it is sent on the control stream
([I-D.ietf-quic-http], Section 6.2.1).
This document specifies a new PRIORITY_UPDATE frame type for HTTP/2 This document specifies a new PRIORITY_UPDATE frame for HTTP/2
([RFC7540]) and HTTP/3 ([I-D.ietf-quic-http]) which enables ([RFC7540]) and HTTP/3 ([I-D.ietf-quic-http]). It carries priority
reprioritization. It carries updated priority parameters and parameters and references the target of the prioritization based on a
references the target of the reprioritization based on a version- version-specific identifier. In HTTP/2, this identifier is the
specific identifier; in HTTP/2 this is the Stream ID, in HTTP/3 this Stream ID; in HTTP/3, the identifier is either the Stream ID or Push
is either the Stream ID or Push ID. ID. Unlike the Priority header field, the PRIORITY_UPDATE frame is a
hop-by-hop signal.
Unlike the header field, the reprioritization frame is a hop-by-hop PRIORITY_UPDATE frames are sent by clients on the control stream,
signal. allowing them to be sent independent from the stream that carries the
response. This means they can be used to reprioritize a response or
a push stream; or signal the initial priority of a response instead
of the Priority header field.
5.1. HTTP/2 PRIORITY_UPDATE Frame A PRIORITY_UPDATE frame communicates a complete set of all parameters
in the Priority Field Value field. Omitting a parameter is a signal
to use the parameter's default value. Failure to parse the Priority
Field Value MUST be treated as a connection error. In HTTP/2 the
error is of type PROTOCOL_ERROR; in HTTP/3 the error is of type
H3_FRAME_ERROR.
The HTTP/2 PRIORITY_UPDATE frame (type=0xF) carries the stream ID of A client MAY send a PRIORITY_UPDATE frame before the stream that it
the response that is being reprioritized, and the updated priority in references is open. Furthermore, HTTP/3 offers no guaranteed
ASCII text, using the same representation as that of the Priority ordering across streams, which could cause the frame to be received
header field value. earlier than intended. Either case leads to a race condition where a
server receives a PRIORITY_UPDATE frame that references a request
stream that is yet to be opened. To solve this condition, for the
purposes of scheduling, the most recently received PRIORITY_UPDATE
frame can be considered as the most up-to-date information that
overrides any other signal. Servers SHOULD buffer the most recently
received PRIORITY_UPDATE frame and apply it once the referenced
stream is opened. Holding PRIORITY_UPDATE frames for each stream
requires server resources, which can can be bound by local
implementation policy. (TODO: consider resolving #1261, and adding
more text about bounds). Although there is no limit to the number
PRIORITY_UPDATES that can be sent, storing only the most recently
received frame limits resource commitment.
6.1. HTTP/2 PRIORITY_UPDATE Frame
The HTTP/2 PRIORITY_UPDATE frame (type=0x10) is used by clients to
signal the initial priority of a response, or to reprioritize a
response or push stream. It carries the stream ID of the response
and the priority in ASCII text, using the same representation as the
Priority header field value.
The Stream Identifier field ([RFC7540], Section 4.1) in the The Stream Identifier field ([RFC7540], Section 4.1) in the
PRIORITY_UPDATE frame header MUST be zero (0x0). PRIORITY_UPDATE frame header MUST be zero (0x0). Receiving a
PRIORITY_UPDATE frame with a field of any other value MUST be treated
as a connection error of type PROTOCOL_ERROR.
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+ +---------------------------------------------------------------+
|R| Stream ID (31) | |R| 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 Payload
The PRIORITY_UPDATE frame payload has the following fields: The PRIORITY_UPDATE frame payload has the following fields:
R: A reserved 1-bit field. The semantics of this bit are undefined, R: A reserved 1-bit field. The semantics of this bit are undefined,
and the bit MUST remain unset (0x0) when sending and MUST be and the bit MUST remain unset (0x0) when sending and MUST be
ignored when receiving. ignored when receiving.
Stream ID: A 31-bit stream identifier for the stream that is the Prioritized Stream ID: A 31-bit stream identifier for the stream
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 Headers. encoded using Structured Fields.
The HTTP/2 PRIORITY_UPDATE frame MUST NOT be sent prior to opening The Prioritized Stream ID MUST be within the stream limit. If a
the stream. If a PRIORITY_UPDATE is received prior to the stream server receives a PRIORITY_UPDATE with a Prioritized Stream ID that
being opened, it MAY be treated as a connection error of type is beyond the stream limits, this SHOULD be treated as a connection
error of type PROTOCOL_ERROR.
If a PRIORITY_UPDATE frame is received with a Prioritized Stream ID
of 0x0, the recipient MUST respond with a connection error of type
PROTOCOL_ERROR. PROTOCOL_ERROR.
TODO: add more description of how to handle things like receiving If a client receives a PRIORITY_UPDATE frame, it MUST respond with a
PRIORITY_UPDATE on wrong stream, a PRIORITY_UPDATE with an invalid connection error of type PROTOCOL_ERROR.
ID, etc.
5.2. HTTP/3 PRIORITY_UPDATE Frame 6.2. HTTP/3 PRIORITY_UPDATE Frame
The HTTP/3 PRIORITY_UPDATE frame (type=0xF) carries the identifier of The HTTP/3 PRIORITY_UPDATE frame (type=0xF0700 or 0xF0701) is used by
the element that is being reprioritized, and the updated priority in clients to signal the initial priority of a response, or to
reprioritize a response or push stream. It carries the identifier of
the element that is being prioritized, and the updated priority in
ASCII text, using the same representation as that of the Priority ASCII text, using the same representation as that of the Priority
header field value. header field value. PRIORITY_UPDATE with a frame type of 0xF0700 is
used for request streams, while PRIORITY_UPDATE with a frame type of
0xF0701 is used for push streams.
The PRIORITY_UPDATE frame MUST be sent on the control stream The PRIORITY_UPDATE frame MUST be sent on the client control stream
([I-D.ietf-quic-http], Section 6.2.1). ([I-D.ietf-quic-http], Section 6.2.1). Receiving a PRIORITY_UPDATE
frame on a stream other than the client control stream MUST be
treated as a connection error of type H3_FRAME_UNEXPECTED.
0 1 2 3 HTTP/3 PRIORITY_UPDATE Frame {
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 Type (i) = 0xF0700..0xF0701,
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Length (i),
|T| Empty | Prioritized Element ID (i) ... Prioritized Element ID (i),
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Priority Field Value (..),
| Priority Field Value (*) ... }
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: HTTP/3 PRIORITY_UPDATE Frame Payload Figure 2: HTTP/3 PRIORITY_UPDATE Frame
The PRIORITY_UPDATE frame payload has the following fields: The PRIORITY_UPDATE frame payload has the following fields:
T (Prioritized Element Type): A one-bit field indicating the type of
element being prioritized. A value of 0 indicates a
reprioritization for a Request Stream, so the Prioritized Element
ID is interpreted as a Stream ID. A value of 1 indicates a
reprioritization for a Push stream, so the Prioritized Element ID
is interpreted as a Push ID.
Empty: A seven-bit field that has no semantic value.
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 Headers. encoded using Structured Fields.
The HTTP/3 PRIORITY_UPDATE frame MUST NOT be sent with an invalid The request-stream variant of PRIORITY_UPDATE (type=0xF0700) MUST
identifier, including before the request stream has been opened or reference a request stream. If a server receives a PRIORITY_UPDATE
before a promised request has been received. If a server receives a (type=0xF0700) for a Stream ID that is not a request stream, this
PRIORITY_UPDATE specifying a push ID that has not been promised, it MUST be treated as a connection error of type H3_ID_ERROR. The
SHOULD be treated as a connection error of type H3_ID_ERROR. Stream ID MUST be within the client-initiated bidirectional stream
limit. If a server receives a PRIORITY_UPDATE (type=0xF0700) with a
Stream ID that is beyond the stream limits, this SHOULD be treated as
a connection error of type H3_ID_ERROR.
Because the HTTP/3 PRIORITY_UPDATE frame is sent on the control The push-stream variant PRIORITY_UPDATE (type=0xF0701) MUST reference
stream and there are no ordering guarantees between streams, a client a promised push stream. If a server receives a PRIORITY_UPDATE
that reprioritizes a request before receiving the response data might (type=0xF0701) with a Push ID that is greater than the maximum Push
cause the server to receive a PRIORITY_UPDATE for an unknown request. ID or which has not yet been promised, this MUST be treated as a
If the request stream ID is within bidirectional stream limits, the connection error of type H3_ID_ERROR.
PRIORITY_UPDATE frame SHOULD be buffered until the stream is opened
and applied immediately after the request message has been processed.
Holding PRIORITY_UPDATES consumes extra state on the peer, although
the size of the state is bounded by bidirectional stream limits.
There is no bound on the number of PRIORITY_UPDATES that can be sent,
so an endpoint SHOULD store only the most recently received frame.
TODO: add more description of how to handle things like receiving PRIORITY_UPDATE frames of either type are only sent by clients. If a
PRIORITY_UPDATE on wrong stream, a PRIORITY_UPDATE with an invalid client receives a PRIORITY_UPDATE frame, this MUST be treated as a
ID, etc. connection error of type H3_FRAME_UNEXPECTED.
6. Merging Client- and Server-Driven Parameters 7. Merging Client- and Server-Driven Parameters
It is not always the case that the client has the best understanding It is not always the case that the client has the best understanding
of how the HTTP responses deserve to be prioritized. The server of how the HTTP responses deserve to be prioritized. The server
might have additional information that can be combined with the might have additional information that can be combined with the
client's indicated priority in order to improve the prioritization of client's indicated priority in order to improve the prioritization of
the response. For example, use of an HTML document might depend the response. For example, use of an HTML document might depend
heavily on one of the inline images; existence of such dependencies heavily on one of the inline images; existence of such dependencies
is typically best known to the server. Or, a server that receives is typically best known to the server. Or, a server that receives
requests for a font [RFC8081] and images with the same urgency might requests for a font [RFC8081] and images with the same urgency might
give higher precedence to the font, so that a visual client can give higher precedence to the font, so that a visual client can
skipping to change at page 12, line 8 skipping to change at page 13, line 4
Absence of a priority parameter in an HTTP response indicates the Absence of a priority parameter in an HTTP response indicates the
server's disinterest in changing the client-provided value. This is server's disinterest in changing the client-provided value. This is
different from the logic being defined for the request header field, different from the logic being defined for the request header field,
in which omission of a priority parameter implies the use of their in which omission of a priority parameter implies the use of their
default values (see Section 3). default values (see Section 3).
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 with the urgency parameter set to "5" and the incremental parameter
set to "true" set 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 the intermediary might alter its understanding of the urgency from
"5" to "1", because it prefers the server-provided value over the "5" to "1", because it prefers the server-provided value over the
client's. The incremental value continues to be "true", the value client's. The incremental value continues to be "true", the value
specified by the client, as the server did not specify the specified by the client, as the server did not specify the
incremental("i") parameter. incremental("i") parameter.
7. Client Scheduling 8. Client Scheduling
A client MAY use priority values to make local scheduling choices A client MAY use priority values to make local processing or
about the requests it initiates. scheduling choices about the requests it initiates.
8. Fairness 9. Server Scheduling
Priority signals are input to a prioritization process. They do not
guarantee any particular processing or transmission order for one
response relative to any other response. An endpoint cannot force a
peer to process concurrent request in a particular order using
priority. Expressing priority is therefore only a suggestion.
A server can use priority signals along with other inputs to make
scheduling decisions. No guidance is provided about how this can or
should be done. Factors such as implementation choices or deployment
environment also play a role. Any given connection is likely to have
many dynamic permutations. For these reasons, there is no unilateral
perfect scheduler and this document only provides some basic
recommendations for implementations.
Clients cannot depend on particular treatment based on priority
signals. Servers can use other information to prioritize responses.
It is RECOMMENDED that, when possible, servers respect the urgency
parameter (Section 3.1), sending higher urgency responses before
lower urgency responses.
It is RECOMMENDED that, when possible, servers respect the
incremental parameter (Section 3.2). Non-incremental responses of
the same urgency SHOULD be served one-by-one based on the Stream ID,
which corresponds to the order in which clients make requests. Doing
so ensures that clients can use request ordering to influence
response order. Incremental responses of the same urgency SHOULD be
served in round-robin manner.
The incremental parameter indicates how a client processes response
bytes as they arrive. Non-incremental resources are only used when
all of the response payload has been received. Incremental resources
are used as parts, or chunks, of the response payload are received.
Therefore, the timing of response data reception at the client, such
as the time to early bytes or the time to receive the entire payload,
plays an important role in perceived performance. Timings depend on
resource size but this scheme provides no explicit guidance about how
a server should use size as an input to prioritization. Instead, the
following examples demonstrate how a server that strictly abides the
scheduling guidance based on urgency and request generation order
could find that early requests prevent serving of later requests.
1. At the same urgency level, a non-incremental request for a large
resource followed by an incremental request for a small resource.
2. At the same urgency level, an incremental request of
indeterminate length followed by a non-incremental large
resource.
It is RECOMMENDED that servers avoid such starvation where possible.
The method to do so is an implementation decision. For example, a
server might pre-emptively send responses of a particular incremental
type based on other information such as content size.
10. Fairness
As a general guideline, a server SHOULD NOT use priority information As a general guideline, a server SHOULD NOT use priority information
for making schedule decisions across multiple connections, unless it for making schedule decisions across multiple connections, unless it
knows that those connections originate from the same client. Due to knows that those connections originate from the same client. Due to
this, priority information conveyed over a non-coalesced HTTP this, priority information conveyed over a non-coalesced HTTP
connection (e.g., HTTP/1.1) might go unused. connection (e.g., HTTP/1.1) might go unused.
The remainder of this section discusses scenarios where unfairness is The remainder of this section discusses scenarios where unfairness is
problematic and presents possible mitigations, or where unfairness is problematic and presents possible mitigations, or where unfairness is
desirable. desirable.
TODO: Discuss if we should add a signal that mitigates this issue. TODO: Discuss if we should add a signal that mitigates this issue.
For example, we might add a SETTINGS parameter that indicates the For example, we might add a SETTINGS parameter that indicates the
next hop that the connection is NOT coalesced (see next hop that the connection is NOT coalesced (see
https://github.com/kazuho/draft-kazuho-httpbis-priority/issues/99). https://github.com/kazuho/draft-kazuho-httpbis-priority/issues/99).
8.1. Coalescing Intermediaries 10.1. Coalescing Intermediaries
When an intermediary coalesces HTTP requests coming from multiple When an intermediary coalesces HTTP requests coming from multiple
clients into one HTTP/2 or HTTP/3 connection going to the backend clients into one HTTP/2 or HTTP/3 connection going to the backend
server, requests that originate from one client might have higher server, requests that originate from one client might have higher
precedence than those coming from others. precedence than those coming from others.
It is sometimes beneficial for the server running behind an It is sometimes beneficial for the server running behind an
intermediary to obey to the value of the Priority header field. As intermediary to obey to the value of the Priority header field. As
an example, a resource-constrained server might defer the an example, a resource-constrained server might defer the
transmission of software update files that would have the background transmission of software update files that would have the background
urgency being associated. However, in the worst case, the asymmetry urgency being associated. However, in the worst case, the asymmetry
between the precedence declared by multiple clients might cause between the precedence declared by multiple clients might cause
responses going to one end client to be delayed totally after those responses going to one user agent to be delayed totally after those
going to another. going to another.
In order to mitigate this fairness problem, when a server responds to In order to mitigate this fairness problem, a server could use
a request that is known to have come through an intermediary, the knowledge about the intermediary as another signal in its
server SHOULD prioritize the response as if it was assigned the prioritization decisions. For instance, if a server knows the
priority of "u=1, i" (i.e. round-robin) regardless of the value of intermediary is coalescing requests, then it could serve the
the Priority header field being transmitted, unless the server knows responses in round-robin manner. This can work if the constrained
the intermediary is not coalescing requests from multiple clients. resource is network capacity between the intermediary and the user
agent, as the intermediary buffers responses and forwards the chunks
based on the prioritization scheme it implements.
A server can determine if a request came from an intermediary through A server can determine if a request came from an intermediary through
configuration, or by consulting if that request contains one of the configuration, or by consulting if that request contains one of the
following header fields: following header fields:
o Forwarded, X-Forwarded-For ([RFC7239]) o Forwarded, X-Forwarded-For ([RFC7239])
o Via ([RFC7230], Section 5.7.1) o Via ([RFC7230], Section 5.7.1)
Responding to requests coming through an intermediary in a round- 10.2. HTTP/1.x Back Ends
robin manner works well when the network bottleneck exists between
the intermediary and the end client, as the intermediary would be
buffering the responses and then be forwarding the chunks of those
buffered responses based on the prioritization scheme it implements.
A sophisticated server MAY use a weighted round-robin reflecting the
urgencies expressed in the requests, so that less urgent responses
would receive less bandwidth in case the bottleneck exists between
the server and the intermediary.
8.2. HTTP/1.x Back Ends
It is common for CDN infrastructure to support different HTTP It is common for CDN infrastructure to support different HTTP
versions on the front end and back end. For instance, the client- versions on the front end and back end. For instance, the client-
facing edge might support HTTP/2 and HTTP/3 while communication to facing edge might support HTTP/2 and HTTP/3 while communication to
back end servers is done using HTTP/1.1. Unlike with connection back end servers is done using HTTP/1.1. Unlike with connection
coalescing, the CDN will "de-mux" requests into discrete connections coalescing, the CDN will "de-mux" requests into discrete connections
to the back end. As HTTP/1.1 and older do not provide a way to to the back end. As HTTP/1.1 and older do not provide a way to
concurrently transmit multiple responses, there is no immediate concurrently transmit multiple responses, there is no immediate
fairness issue in protocol. However, back end servers MAY still use fairness issue in protocol. However, back end servers MAY still use
client headers for request scheduling. Back end servers SHOULD only client headers for request scheduling. Back end servers SHOULD only
schedule based on client priority information where that information schedule based on client priority information where that information
can be scoped to individual end clients. Authentication and other can be scoped to individual end clients. Authentication and other
session information might provide this linkability. session information might provide this linkability.
8.3. Intentional Introduction of Unfairness 10.3. 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.
9. Why use an End-to-End Header Field? 11. Why use an End-to-End Header Field?
Contrary to the prioritization scheme of HTTP/2 that uses a hop-by- Contrary to the prioritization scheme of HTTP/2 that uses a hop-by-
hop frame, the Priority header field is defined as end-to-end. hop frame, the Priority header field is defined as end-to-end.
The rationale is that the Priority header field transmits how each The rationale is that the Priority header field transmits how each
response affects the client's processing of those responses, rather response affects the client's processing of those responses, rather
than how relatively urgent each response is to others. The way a than how relatively urgent each response is to others. The way a
client processes a response is a property associated to that client client processes a response is a property associated to that client
generating that request. Not that of an intermediary. Therefore, it generating that request. Not that of an intermediary. Therefore, it
is an end-to-end property. How these end-to-end properties carried is an end-to-end property. How these end-to-end properties carried
skipping to change at page 15, line 5 skipping to change at page 16, line 42
for caching intermediaries. Such intermediaries can cache the value for caching intermediaries. Such intermediaries can cache the value
of the Priority header field along with the response, and utilize the of the Priority header field along with the response, and utilize the
value of the cached header field when serving the cached response, value of the cached header field when serving the cached response,
only because the header field is defined as end-to-end rather than only because the header field is defined as end-to-end rather than
hop-by-hop. hop-by-hop.
It should also be noted that the use of a header field carrying a It should also be noted that the use of a header field carrying a
textual value makes the prioritization scheme extensible; see the textual value makes the prioritization scheme extensible; see the
discussion below. discussion below.
10. Security Considerations 12. Security Considerations
[CVE-2019-9513] aka "Resource Loop", is a DoS attack based on [CVE-2019-9513] aka "Resource Loop", is a DoS attack based on
manipulation of the HTTP/2 priority tree. Extensible priorities does manipulation of the HTTP/2 priority tree. Extensible priorities does
not use stream dependencies, which mitigates this vulnerability. not use stream dependencies, which mitigates this vulnerability.
TBD: depending on the outcome of reprioritization discussions, TBD: depending on the outcome of reprioritization discussions,
following paragraphs may change or be removed. following paragraphs may change or be removed.
[RFC7540], Section 5.3.4 describes a scenario where closure of [RFC7540], Section 5.3.4 describes a scenario where closure of
streams in the priority tree could cause suboptimal prioritization. streams in the priority tree could cause suboptimal prioritization.
skipping to change at page 15, line 42 skipping to change at page 17, line 31
state. This state can be limited by adopting the guidance about state. This state can be limited by adopting the guidance about
concurrency limits described above. Extensible priorities is subject concurrency limits described above. Extensible priorities is subject
to a similar consideration because PRIORITY_UPDATE frames may arrive to a similar consideration because PRIORITY_UPDATE frames may arrive
before the request that they reference. A server is required to before the request that they reference. A server is required to
store the information in order to apply the most up-to-date signal to store the information in order to apply the most up-to-date signal to
the request. However, HTTP/3 implementations might have practical the request. However, HTTP/3 implementations might have practical
barriers to determining reasonable stream concurrency limits barriers to determining reasonable stream concurrency limits
depending on the information that is available to them from the QUIC depending on the information that is available to them from the QUIC
transport layer. TODO: so what can we suggest? transport layer. TODO: so what can we suggest?
11. IANA Considerations 13. IANA Considerations
This specification registers the following entry in the Permanent This specification registers the following entry in the Permanent
Message Header Field Names registry established by [RFC3864]: Message Header Field Names registry established by [RFC3864]:
Header field name: Priority Header field name: Priority
Applicable protocol: http Applicable protocol: http
Status: standard Status: standard
Author/change controller: IETF Author/change controller: IETF
Specification document(s): This document Specification document(s): This document
Related information: n/a Related information: n/a
This specification registers the following entry in the HTTP/2 This specification registers the following entry in the HTTP/2
Settings registry established by [RFC7540]: Settings registry established by [RFC7540]:
Name: SETTINGS_DEPRECATE_HTTP2_PRIORITIES Name: SETTINGS_DEPRECATE_HTTP2_PRIORITIES
skipping to change at page 16, line 14 skipping to change at page 18, line 4
Author/change controller: IETF Author/change controller: IETF
Specification document(s): This document Specification document(s): This document
Related information: n/a Related information: n/a
This specification registers the following entry in the HTTP/2 This specification registers the following entry in the HTTP/2
Settings registry established by [RFC7540]: Settings registry established by [RFC7540]:
Name: SETTINGS_DEPRECATE_HTTP2_PRIORITIES Name: SETTINGS_DEPRECATE_HTTP2_PRIORITIES
Code: 0x9 Code: 0x9
Initial value: 0 Initial value: 0
Specification: This document Specification: This document
This specification registers the following entry in the HTTP/2 Frame This specification registers the following entry in the HTTP/2 Frame
Type registry established by [RFC7540]: Type registry established by [RFC7540]:
Frame Type: PRIORITY_UPDATE Frame Type: PRIORITY_UPDATE
Code: 0xF Code: 0x10
Specification: This document Specification: This document
This specification registers the following entries in the HTTP/3 This specification registers the following entries in the HTTP/3
Frame Type registry established by [I-D.ietf-quic-http]: Frame Type registry established by [I-D.ietf-quic-http]:
Frame Type: PRIORITY_UPDATE Frame Type: PRIORITY_UPDATE
Code: 0xF Code: 0xF0700 and 0xF0701
Specification: This document Specification: This document
12. References 14. References
12.1. Normative References 14.1. Normative References
[I-D.ietf-quic-http] [I-D.ietf-quic-http]
Bishop, M., "Hypertext Transfer Protocol Version 3 Bishop, M., "Hypertext Transfer Protocol Version 3
(HTTP/3)", draft-ietf-quic-http-29 (work in progress), (HTTP/3)", draft-ietf-quic-http-31 (work in progress),
June 2020. September 2020.
[I-D.ietf-quic-transport] [I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-29 (work and Secure Transport", draft-ietf-quic-transport-31 (work
in progress), June 2020. in progress), September 2020.
[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/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing", Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014, RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>. <https://www.rfc-editor.org/info/rfc7230>.
[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/info/rfc7540>. <https://www.rfc-editor.org/info/rfc7540>.
[STRUCTURED-HEADERS] [STRUCTURED-FIELDS]
Nottingham, M. and P. Kamp, "Structured Field Values for Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", draft-ietf-httpbis-header-structure-19 (work in HTTP", draft-ietf-httpbis-header-structure-19 (work in
progress), June 2020. progress), June 2020.
12.2. Informative References 14.2. Informative References
[CVE-2019-9513] [CVE-2019-9513]
Common Vulnerabilities and Exposures, "CVE-2019-9513", Common Vulnerabilities and Exposures, "CVE-2019-9513",
March 2019, <https://cve.mitre.org/cgi-bin/ March 2019, <https://cve.mitre.org/cgi-bin/
cvename.cgi?name=CVE-2019-9513>. cvename.cgi?name=CVE-2019-9513>.
[I-D.lassey-priority-setting] [I-D.lassey-priority-setting]
Lassey, B. and L. Pardue, "Declaring Support for HTTP/2 Lassey, B. and L. Pardue, "Declaring Support for HTTP/2
Priorities", draft-lassey-priority-setting-00 (work in Priorities", draft-lassey-priority-setting-00 (work in
progress), July 2019. progress), July 2019.
skipping to change at page 18, line 9 skipping to change at page 19, line 45
<https://www.rfc-editor.org/info/rfc7234>. <https://www.rfc-editor.org/info/rfc7234>.
[RFC7239] Petersson, A. and M. Nilsson, "Forwarded HTTP Extension", [RFC7239] 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/info/rfc7239>. <https://www.rfc-editor.org/info/rfc7239>.
[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/info/rfc8081>. <https://www.rfc-editor.org/info/rfc8081>.
12.3. URIs 14.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/ [1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] https://httpwg.org/ [2] https://httpwg.org/
[3] https://github.com/httpwg/http-extensions/labels/priorities [3] https://github.com/httpwg/http-extensions/labels/priorities
[4] http://tools.ietf.org/agenda/83/slides/slides-83-httpbis-5.pdf [4] http://tools.ietf.org/agenda/83/slides/slides-83-httpbis-5.pdf
[5] https://github.com/pmeenan/http3-prioritization-proposal [5] https://github.com/pmeenan/http3-prioritization-proposal
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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, Kazuho Oku,
Lucas Pardue, Matthew Cox, Mike Bishop, Roberto Peon, Robin Marx, Roy Lucas Pardue, Matthew Cox, Mike Bishop, Roberto Peon, Robin Marx, Roy
Fielding. Fielding.
Appendix B. Change Log Appendix B. Change Log
B.1. Since draft-ietf-httpbis-priority-00 B.1. Since draft-ietf-httpbis-priority-01
o PRIORITY_UPDATE frame changes (#1096, #1079, #1167, #1262, #1267,
#1271)
o Add section to describe server scheduling considerations (#1215,
#1232, #1266)
o Remove specific instructions related to intermediary fairness
(#1022, #1264)
B.2. Since draft-ietf-httpbis-priority-00
o Move text around (#1217, #1218) o Move text around (#1217, #1218)
o Editorial change to the default urgency. The value is 3, which o Editorial change to the default urgency. The value is 3, which
was always the intent of previous changes. was always the intent of previous changes.
B.2. Since draft-kazuho-httpbis-priority-04 B.3. Since draft-kazuho-httpbis-priority-04
o Minimize semantics of Urgency levels (#1023, #1026) o Minimize semantics of Urgency levels (#1023, #1026)
o Reduce guidance about how intermediary implements merging priority o Reduce guidance about how intermediary implements merging priority
signals (#1026) signals (#1026)
o Remove mention of CDN-Loop (#1062) o Remove mention of CDN-Loop (#1062)
o Editorial changes o Editorial changes
o Make changes due to WG adoption o Make changes due to WG adoption
o Removed outdated Consideration (#118) o Removed outdated Consideration (#118)
B.3. Since draft-kazuho-httpbis-priority-03 B.4. Since draft-kazuho-httpbis-priority-03
o Changed numbering from "[-1,6]" to "[0,7]" (#78) o Changed numbering from "[-1,6]" to "[0,7]" (#78)
o Replaced priority scheme negotiation with HTTP/2 priority o Replaced priority scheme negotiation with HTTP/2 priority
deprecation (#100) deprecation (#100)
o Shorten parameter names (#108) o Shorten parameter names (#108)
o Expand on considerations (#105, #107, #109, #110, #111, #113) o Expand on considerations (#105, #107, #109, #110, #111, #113)
B.4. Since draft-kazuho-httpbis-priority-02 B.5. Since draft-kazuho-httpbis-priority-02
o Consolidation of the problem statement (#61, #73) o Consolidation of the problem statement (#61, #73)
o Define SETTINGS_PRIORITIES for negotiation (#58, #69) o Define SETTINGS_PRIORITIES for negotiation (#58, #69)
o Define PRIORITY_UPDATE frame for HTTP/2 and HTTP/3 (#51) o Define PRIORITY_UPDATE frame for HTTP/2 and HTTP/3 (#51)
o Explain fairness issue and mitigations (#56) o Explain fairness issue and mitigations (#56)
B.5. Since draft-kazuho-httpbis-priority-01 B.6. Since draft-kazuho-httpbis-priority-01
o Explain how reprioritization might be supported. o Explain how reprioritization might be supported.
B.6. Since draft-kazuho-httpbis-priority-00 B.7. Since draft-kazuho-httpbis-priority-00
o Expand urgency levels from 3 to 8. o Expand urgency levels from 3 to 8.
Authors' Addresses Authors' Addresses
Kazuho Oku Kazuho Oku
Fastly Fastly
Email: kazuhooku@gmail.com Email: kazuhooku@gmail.com
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