draft-ietf-httpbis-priority-03.txt   draft-ietf-httpbis-priority-04.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: 15 July 2021 Cloudflare Expires: 13 January 2022 Cloudflare
11 January 2021 12 July 2021
Extensible Prioritization Scheme for HTTP Extensible Prioritization Scheme for HTTP
draft-ietf-httpbis-priority-03 draft-ietf-httpbis-priority-04
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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This Internet-Draft will expire on 15 July 2021. This Internet-Draft will expire on 13 January 2022.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4
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 . . . . . . . . . . . . . . . 6
3. Priority Parameters . . . . . . . . . . . . . . . . . . . . . 6 3. Applicability of the Extensible Priority Scheme . . . . . . . 6
3.1. Urgency . . . . . . . . . . . . . . . . . . . . . . . . . 7 4. Priority Parameters . . . . . . . . . . . . . . . . . . . . . 7
3.2. Incremental . . . . . . . . . . . . . . . . . . . . . . . 8 4.1. Urgency . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Defining New Parameters . . . . . . . . . . . . . . . . . 8 4.2. Incremental . . . . . . . . . . . . . . . . . . . . . . . 8
3.3.1. Registration . . . . . . . . . . . . . . . . . . . . 9 4.3. Defining New Parameters . . . . . . . . . . . . . . . . . 9
4. The Priority HTTP Header Field . . . . . . . . . . . . . . . 9 4.3.1. Registration . . . . . . . . . . . . . . . . . . . . 9
5. Reprioritization . . . . . . . . . . . . . . . . . . . . . . 10 5. The Priority HTTP Header Field . . . . . . . . . . . . . . . 10
6. The PRIORITY_UPDATE Frame . . . . . . . . . . . . . . . . . . 10 6. Reprioritization . . . . . . . . . . . . . . . . . . . . . . 10
6.1. HTTP/2 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 11 7. The PRIORITY_UPDATE Frame . . . . . . . . . . . . . . . . . . 11
6.2. HTTP/3 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 12 7.1. HTTP/2 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 11
7. Merging Client- and Server-Driven Parameters . . . . . . . . 13 7.2. HTTP/3 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 13
8. Client Scheduling . . . . . . . . . . . . . . . . . . . . . . 14 8. Merging Client- and Server-Driven Parameters . . . . . . . . 14
9. Server Scheduling . . . . . . . . . . . . . . . . . . . . . . 14 9. Client Scheduling . . . . . . . . . . . . . . . . . . . . . . 15
10. Fairness . . . . . . . . . . . . . . . . . . . . . . . . . . 16 10. Server Scheduling . . . . . . . . . . . . . . . . . . . . . . 15
10.1. Coalescing Intermediaries . . . . . . . . . . . . . . . 16 10.1. Intermediaries with Multiple Backend Connections . . . . 17
10.2. HTTP/1.x Back Ends . . . . . . . . . . . . . . . . . . . 17 11. Scheduling and the CONNECT Method . . . . . . . . . . . . . . 17
10.3. Intentional Introduction of Unfairness . . . . . . . . . 17 12. Retransmission Scheduling . . . . . . . . . . . . . . . . . . 17
11. Why use an End-to-End Header Field? . . . . . . . . . . . . . 17 13. Fairness . . . . . . . . . . . . . . . . . . . . . . . . . . 18
12. Security Considerations . . . . . . . . . . . . . . . . . . . 18 13.1. Coalescing Intermediaries . . . . . . . . . . . . . . . 18
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 13.2. HTTP/1.x Back Ends . . . . . . . . . . . . . . . . . . . 19
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 13.3. Intentional Introduction of Unfairness . . . . . . . . . 19
14.1. Normative References . . . . . . . . . . . . . . . . . . 20
14.2. Informative References . . . . . . . . . . . . . . . . . 21 14. Why use an End-to-End Header Field? . . . . . . . . . . . . . 19
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 21 15. Security Considerations . . . . . . . . . . . . . . . . . . . 20
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 22 16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
B.1. Since draft-ietf-httpbis-priority-01 . . . . . . . . . . 22 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
B.2. Since draft-ietf-httpbis-priority-01 . . . . . . . . . . 22 17.1. Normative References . . . . . . . . . . . . . . . . . . 22
B.3. Since draft-ietf-httpbis-priority-00 . . . . . . . . . . 22 17.2. Informative References . . . . . . . . . . . . . . . . . 22
B.4. Since draft-kazuho-httpbis-priority-04 . . . . . . . . . 22 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 23
B.5. Since draft-kazuho-httpbis-priority-03 . . . . . . . . . 23 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 24
B.6. Since draft-kazuho-httpbis-priority-02 . . . . . . . . . 23 B.1. Since draft-ietf-httpbis-priority-03 . . . . . . . . . . 24
B.7. Since draft-kazuho-httpbis-priority-01 . . . . . . . . . 23 B.2. Since draft-ietf-httpbis-priority-02 . . . . . . . . . . 24
B.8. Since draft-kazuho-httpbis-priority-00 . . . . . . . . . 23 B.3. Since draft-ietf-httpbis-priority-01 . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 B.4. Since draft-ietf-httpbis-priority-00 . . . . . . . . . . 25
B.5. Since draft-kazuho-httpbis-priority-04 . . . . . . . . . 25
B.6. Since draft-kazuho-httpbis-priority-03 . . . . . . . . . 25
B.7. Since draft-kazuho-httpbis-priority-02 . . . . . . . . . 25
B.8. Since draft-kazuho-httpbis-priority-01 . . . . . . . . . 25
B.9. Since draft-kazuho-httpbis-priority-00 . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
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
retrieval of a CSS file that the document refers to. In contrast, retrieval of a CSS file that the document refers to. In contrast,
inline images do not block rendering and get drawn incrementally as inline images do not block rendering and get drawn incrementally as
the chunks of the images arrive. the chunks of the images arrive.
To provide meaningful presentation of a document at the earliest To provide meaningful presentation of a 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. responses, or the chunks of those HTTP responses, that it sends.
HTTP/2 ([RFC7540]) provides such a prioritization scheme. A client HTTP/2 ([HTTP2]) provides such a prioritization scheme. A client
sends a series of PRIORITY frames to communicate to the server a sends a series of PRIORITY frames to communicate to the server a
"priority tree"; this represents the client's preferred ordering and "priority tree"; this represents the client's preferred ordering and
weighted distribution of the bandwidth among the HTTP responses. weighted distribution of the bandwidth among the HTTP responses.
However, the design and implementation of this scheme has been However, the design and implementation of this scheme has been
observed to have shortcomings, explained in Section 2. observed to have shortcomings, explained in Section 2.
This document defines the Priority HTTP header field that can be used This document defines the Priority HTTP header field that can be used
by both client and server to specify the precedence of HTTP responses by both client and server to specify the precedence of HTTP responses
in a standardized, extensible, protocol-version-independent, end-to- in a standardized, extensible, protocol-version-independent, end-to-
end format. Along with the protocol-version-specific frame for end format. Along with the protocol-version-specific frame for
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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 sf-token and sf-boolean are imported from The terms sf-token and sf-boolean are imported from
[STRUCTURED-FIELDS]. [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 [HTTP2].
This document uses the variable-length integer encoding from This document uses the variable-length integer encoding from [QUIC].
[I-D.ietf-quic-transport].
The term control stream is used to describe the HTTP/2 stream with 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], identifier 0x0, and HTTP/3 control stream; see [HTTP3],
Section 6.2.1. 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 scheme that uses a
a combination of dependencies and weights, formed into an unbalanced combination of stream dependencies and weights to describe an
tree. This scheme has suffered from poor deployment and unbalanced tree. This scheme has suffered from poor deployment and
interoperability. interoperability.
The rich flexibility of client-driven HTTP/2 prioritization tree Clients build an HTTP/2 prioritization tree through a series of
building is rarely exercised. Experience has shown that clients tend individual stream relationships, which are transferred to the server
to choose a single model optimized for a web use case and experiment using HTTP/2 priority signals in either of three forms. First, a
within the model constraints, or do nothing at all. Furthermore, HEADERS frame with the PRIORITY flag set is an explicit signal that
many clients build their prioritization tree in a unique way, which includes an Exclusive flag, Stream Dependency field, and Weight
makes it difficult for servers to understand their intent and act or field. Second, a HEADERS frame with no PRIORITY flag is an implicit
intervene accordingly. signal to use the default priority. Third, the PRIORITY frame, which
is always explicit since it always contains an Exclusive flag, Stream
Dependency field, and Weight field.
The rich flexibility of tree building is rarely exercised.
Experience has shown that clients tend to choose a single model
optimized for a web use case and experiment within the model
constraints, or do nothing at all. Furthermore, many clients build
their prioritization tree in a unique way, which makes it difficult
for servers to understand their intent and act or intervene
accordingly.
Many HTTP/2 server implementations do not include support for the Many HTTP/2 server implementations do not include support for the
priority scheme. Some instead favor custom server-driven schemes priority scheme. Some instead favor custom server-driven schemes
based on heuristics or other hints, such as resource content type or based on heuristics or other hints, such as resource content type or
request generation order. For example, a server, with knowledge of request generation order. For example, a server, with knowledge of
the document structure, might want to prioritize the delivery of the document structure, might want to prioritize the delivery of
images that are critical to user experience above other images, but images that are critical to user experience above other images, but
below the CSS files. Since client trees vary, it is impossible for below the CSS files. Since client trees vary, it is impossible for
the server to determine how such images should be prioritized against the server to determine how such images should be prioritized against
other responses. other responses.
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HTTP/2 describes denial-of-service considerations for HTTP/2 describes denial-of-service considerations for
implementations. On 2019-08-13 Netflix issued an advisory notice implementations. On 2019-08-13 Netflix issued an advisory notice
about the discovery of several resource exhaustion vectors affecting about the discovery of several resource exhaustion vectors affecting
multiple HTTP/2 implementations. One attack, [CVE-2019-9513] aka multiple HTTP/2 implementations. One attack, [CVE-2019-9513] aka
"Resource Loop", is based on manipulation of the priority tree. "Resource Loop", is based on manipulation of the priority tree.
The HTTP/2 scheme depends on in-order delivery of signals, leading to The HTTP/2 scheme depends on in-order delivery of signals, leading to
challenges in porting the scheme to protocols that do not provide challenges in porting the scheme to protocols that do not provide
global ordering. For example, the scheme cannot be used in HTTP/3 global ordering. For example, the scheme cannot be used in HTTP/3
[I-D.ietf-quic-http] without changing the signal and its processing. [HTTP3] without changing the signal and its processing.
Considering the problems with deployment and adaptability to HTTP/3, Considering the problems with deployment and adaptability to HTTP/3,
retaining the HTTP/2 priority scheme increases the complexity of the retaining the HTTP/2 priority scheme increases the complexity of the
entire system without any evidence that the value it provides offsets entire system without any evidence that the value it provides offsets
that complexity. In fact, multiple experiments from independent that complexity. In fact, multiple experiments from independent
research have shown that simpler schemes can reach at least research have shown that simpler schemes can reach at least
equivalent performance characteristics compared to the more complex equivalent performance characteristics compared to the more complex
HTTP/2 setups seen in practice, at least for the web use case. HTTP/2 setups seen in practice, at least for the web use case.
2.1. Disabling HTTP/2 Priorities 2.1. Disabling HTTP/2 Priorities
The problems and insights set out above are motivation for allowing The problems and insights set out above are motivation for allowing
endpoints to opt out of using the HTTP/2 priority scheme, in favor of endpoints to opt out of using the HTTP/2 priority scheme, in favor of
using an alternative such as the scheme defined in this using an alternative such as the scheme defined in this
specification. The SETTINGS_DEPRECATE_HTTP2_PRIORITIES setting specification. The SETTINGS_DEPRECATE_HTTP2_PRIORITIES setting
described below enables endpoints to understand their peer's described below enables endpoints to understand their peer's
intention. The value of the parameter MUST be 0 or 1. Any value intention. The value of the parameter MUST be 0 or 1. Any value
other than 0 or 1 MUST be treated as a connection error (see other than 0 or 1 MUST be treated as a connection error (see [HTTP2],
[RFC7540], Section 5.4.1) of type PROTOCOL_ERROR. Section 5.4.1) of type PROTOCOL_ERROR.
Endpoints MUST send this SETTINGS parameter as part of the first Endpoints MUST send this SETTINGS parameter as part of the first
SETTINGS frame. When the peer receives the first SETTINGS frame, it SETTINGS frame. A sender MUST NOT change the
learns the sender has deprecated the HTTP/2 priority scheme if it SETTINGS_DEPRECATE_HTTP2_PRIORITIES parameter value after the first
receives the SETTINGS_DEPRECATE_HTTP2_PRIORITIES parameter with the SETTINGS frame. Detection of a change by a receiver MUST be treated
value of 1. as a connection error of type PROTOCOL_ERROR.
A sender MUST NOT change the SETTINGS_DEPRECATE_HTTP2_PRIORITIES
parameter value after the first SETTINGS frame. Detection of a
change by a receiver MUST be treated as a connection error of type
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 the signals of the HTTP/2 priority scheme (see
priority scheme and also that of this prioritization scheme. Once Section 2) and the signals of this prioritization scheme (see
the client learns that the HTTP/2 priority scheme is deprecated, it Section 5 and Section 7.1). When the client receives the first
SHOULD stop sending the HTTP/2 priority signals. If the client SETTINGS frame that contains the SETTINGS_DEPRECATE_HTTP2_PRIORITIES
learns that the HTTP/2 priority scheme is not deprecated, it SHOULD parameter with value of 1, it SHOULD stop sending the HTTP/2 priority
stop sending PRIORITY_UPDATE frames (Section 6.1), but MAY continue signals. If the value was 0 or if the settings parameter was absent,
sending the Priority header field (Section 4), as it is an end-to-end it SHOULD stop sending PRIORITY_UPDATE frames (Section 7.1), but MAY
signal that might be useful to nodes behind the server that the continue sending the Priority header field (Section 5), as it is an
client is directly connected to. end-to-end signal that might be useful to nodes behind the server
that the client is directly connected to.
The SETTINGS frame precedes any priority signal sent from a client in 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. A server that receives scheme before building state. A server that receives
SETTINGS_DEPRECATE_HTTP2_PRIORITIES MUST ignore HTTP/2 priority SETTINGS_DEPRECATE_HTTP2_PRIORITIES with value of 1 MUST ignore
signals. HTTP/2 priority signals.
Where both endpoints disable HTTP/2 priorities, the client is Where both endpoints disable HTTP/2 priorities, the client is
expected to send this scheme's priority signal. Handling of omitted expected to send this scheme's priority signal. Handling of omitted
signals is described in Section 3. signals is described in Section 4.
3. Priority Parameters 3. Applicability of the Extensible Priority Scheme
The priority scheme defined by this document considers only the
prioritization of HTTP messages and tunnels, see Section 9,
Section 10, and Section 11.
Where HTTP extensions change stream behavior or define new data
carriage mechanisms, they MAY also define how this priority scheme
can be applied.
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 4) is an end-to-end way to The Priority HTTP header field (Section 5) is an end-to-end way to
transmit this set of parameters when a request or a response is transmit this set of 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 6.1 and Section 6.2) are used by clients to transmit frames (Section 7.1 and Section 7.2) 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 Fields Dictionary ([STRUCTURED-FIELDS]). 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 7. different when processing an HTTP response; see Section 8.
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 4.1. Urgency
The urgency parameter ("u") takes an integer between 0 and 7, in The urgency parameter ("u") takes an integer between 0 and 7, in
descending order of priority. 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 sf-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
skipping to change at page 7, line 43 skipping to change at page 8, line 14
: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 7). urgency using knowledge specific to the web-site (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 impact on user interaction.
3.2. Incremental 4.2. Incremental
The incremental parameter ("i") takes an sf-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
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The following example shows a request for a JPEG file with the The following example shows a request for a JPEG file with the
urgency parameter set to "5" and the incremental parameter set to urgency parameter set to "5" and the incremental parameter set to
"true". "true".
:method = GET :method = GET
:scheme = https :scheme = https
:authority = example.net :authority = example.net
:path = /image.jpg :path = /image.jpg
priority = u=5, i priority = u=5, i
3.3. Defining New Parameters 4.3. Defining New Parameters
When attempting to define new parameters, care must be taken so that When attempting to define new parameters, care must be taken so that
they do not adversely interfere with prioritization performed by they do not adversely interfere with prioritization performed by
existing endpoints or intermediaries that do not understand the newly existing endpoints or intermediaries that do not understand the newly
defined parameter. Since unknown parameters are ignored, new defined parameter. Since unknown parameters are ignored, new
parameters should not change the interpretation of or modify the parameters should not change the interpretation of or modify the
predefined parameters in a way that is not backwards compatible or predefined parameters in a way that is not backwards compatible or
fallback safe. 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
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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.
Generic parameters are preferred over vendor-specific, application- Generic parameters are preferred over vendor-specific, application-
specific or deployment-specific values. If a generic value cannot be specific or deployment-specific values. If a generic value cannot be
agreed upon in the community, the parameter's name should be agreed upon in the community, the parameter's name should be
correspondingly specific (e.g., with a prefix that identifies the correspondingly specific (e.g., with a prefix that identifies the
vendor, application or deployment). vendor, application or deployment).
3.3.1. 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. HTTP Priority Parameters Registry.
Registration requests are reviewed and approved by a Designated Registration requests are reviewed and approved by a Designated
Expert, as per [RFC8126], Section 4.5. A specification document is Expert, as per [RFC8126], Section 4.5. A specification document is
appreciated, but not required. appreciated, but not required.
The Expert(s) should consider the following factors when evaluating The Expert(s) should consider the following factors when evaluating
requests: requests:
* Community feedback * Community feedback
* If the parameters are sufficiently well-defined and adhere to the * If the parameters are sufficiently well-defined and adhere to the
guidance provided in Section 3.3. guidance provided in Section 4.3.
Registration requests should use the following template: Registration requests should use the following template:
* Name: [a name for the Priority Parameter that matches key] * Name: [a name for the Priority Parameter that matches key]
* Description: [a description of the parameter semantics and value] * Description: [a description of the parameter semantics and value]
* Reference: [to a specification defining this parameter] * Reference: [to a specification defining this parameter]
See the registry at https://iana.org/assignments/http-priority See the registry at https://iana.org/assignments/http-priority
(https://iana.org/assignments/http-priority) for details on where to (https://iana.org/assignments/http-priority) for details on where to
send registration requests. send registration requests.
4. The Priority HTTP Header Field 5. 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 7). (see Section 8).
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 An endpoint that fails to parse the Priority header field SHOULD use
default parameter values. default parameter values.
5. Reprioritization 6. Reprioritization
After a client sends a request, it may be beneficial to change the After a client sends a request, it may be beneficial to change the
priority of the response. As an example, a web browser might issue a priority of the response. As an example, a web browser might issue a
prefetch request for a JavaScript file with the urgency parameter of prefetch request for a JavaScript file with the urgency parameter of
the Priority request header field set to "u=7" (background). Then, the Priority request header field set to "u=7" (background). Then,
when the user navigates to a page which references the new JavaScript when the user navigates to a page which references the new JavaScript
file, while the prefetch is in progress, the browser would send a file, while the prefetch is in progress, the browser would send a
reprioritization signal with the priority field value set to "u=0". reprioritization signal with the priority field value set to "u=0".
The PRIORITY_UPDATE frame (Section 6) can be used for such The PRIORITY_UPDATE frame (Section 7) can be used for such
reprioritization. reprioritization.
6. 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
([RFC7540]) and HTTP/3 ([I-D.ietf-quic-http]). It carries priority ([HTTP2]) and HTTP/3 ([HTTP3]). It carries priority parameters and
parameters and references the target of the prioritization based on a references the target of the prioritization based on a version-
version-specific identifier. In HTTP/2, this identifier is the specific identifier. In HTTP/2, this identifier is the Stream ID; in
Stream ID; in HTTP/3, the identifier is either the Stream ID or Push HTTP/3, the identifier is either the Stream ID or Push ID. Unlike
ID. Unlike the Priority header field, the PRIORITY_UPDATE frame is a the Priority header field, the PRIORITY_UPDATE frame is a hop-by-hop
hop-by-hop signal. signal.
PRIORITY_UPDATE frames are sent by clients on the control stream, PRIORITY_UPDATE frames are sent by clients on the control stream,
allowing them to be sent independent from the stream that carries the allowing them to be sent independent from the stream that carries the
response. This means they can be used to reprioritize a response or response. This means they can be used to reprioritize a response or
a push stream; or signal the initial priority of a response instead a push stream; or signal the initial priority of a response instead
of the Priority header field. of the Priority header field.
A PRIORITY_UPDATE frame communicates a complete set of all parameters A PRIORITY_UPDATE frame communicates a complete set of all parameters
in the Priority Field Value field. Omitting a parameter is a signal in the Priority Field Value field. Omitting a parameter is a signal
to use the parameter's default value. Failure to parse the Priority 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 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 error is of type PROTOCOL_ERROR; in HTTP/3 the error is of type
H3_FRAME_ERROR. H3_FRAME_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 6.1). references is open (except for HTTP/2 push streams; see Section 7.1).
Furthermore, HTTP/3 offers no guaranteed ordering across streams, Furthermore, HTTP/3 offers no guaranteed ordering across streams,
which could cause the frame to be received earlier than intended. which could cause the frame to be received earlier than intended.
Either case leads to a race condition where a server receives a Either case leads to a race condition where a server receives a
PRIORITY_UPDATE frame that references a request stream that is yet to PRIORITY_UPDATE frame that references a request stream that is yet to
be opened. To solve this condition, for the purposes of scheduling, be opened. To solve this condition, for the purposes of scheduling,
the most recently received PRIORITY_UPDATE frame can be considered as the most recently received PRIORITY_UPDATE frame can be considered as
the most up-to-date information that overrides any other signal. the most up-to-date information that overrides any other signal.
Servers SHOULD buffer the most recently received PRIORITY_UPDATE Servers SHOULD buffer the most recently received PRIORITY_UPDATE
frame and apply it once the referenced stream is opened. Holding frame and apply it once the referenced stream is opened. Holding
PRIORITY_UPDATE frames for each stream requires server resources, PRIORITY_UPDATE frames for each stream requires server resources,
which can can be bound by local implementation policy. Although which can can be bound by local implementation policy. Although
there is no limit to the number of PRIORITY_UPDATES that can be sent, there is no limit to the number of PRIORITY_UPDATES that can be sent,
storing only the most recently received frame limits resource storing only the most recently received frame limits resource
commitment. commitment.
6.1. 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 ([RFC7540], Section 4.1) in the The Stream Identifier field ([HTTP2], Section 4.1) 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.
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| Prioritized Stream ID (31) | |R| Prioritized Stream ID (31) |
+---------------------------------------------------------------+ +---------------------------------------------------------------+
| Priority Field Value (*) ... | Priority Field Value (*) ...
skipping to change at page 12, line 15 skipping to change at page 12, line 39
Priority Field Value: The priority update value in ASCII text, Priority Field Value: The priority update value in ASCII text,
encoded using Structured Fields. encoded using Structured Fields.
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. Servers can discard
frames where the Prioritized Stream ID refers to a stream in the frames where the Prioritized Stream ID refers to a stream in the
"half-closed (local)" or "closed" state. The number of streams which "half-closed (local)" or "closed" state. The number of streams which
have been prioritized but remain in the "idle" state plus the number have been prioritized but remain in the "idle" state plus the number
of active streams (those in the "open" or either "half-closed" state; of active streams (those in the "open" or either "half-closed" state;
see section 5.1.2 of [RFC7540]) MUST NOT exceed the value of the see section 5.1.2 of [HTTP2]) 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.
If a client receives a PRIORITY_UPDATE frame, it MUST respond with a If a client receives a PRIORITY_UPDATE frame, it MUST respond with a
connection error of type PROTOCOL_ERROR. connection error of type PROTOCOL_ERROR.
6.2. 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, using the same representation as that of the Priority ASCII text, using 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
([I-D.ietf-quic-http], Section 6.2.1). Receiving a PRIORITY_UPDATE ([HTTP3], Section 6.2.1). Receiving a PRIORITY_UPDATE frame on a
frame on a stream other than the client control stream MUST be stream other than the client control stream MUST be treated as a
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
skipping to change at page 13, line 41 skipping to change at page 14, line 15
The push-stream variant PRIORITY_UPDATE (type=0xF0701) MUST reference The push-stream variant PRIORITY_UPDATE (type=0xF0701) MUST reference
a promised push stream. If a server receives a PRIORITY_UPDATE a promised push stream. If a server receives a PRIORITY_UPDATE
(type=0xF0701) with a Push ID that is greater than the maximum Push (type=0xF0701) with a Push ID that is greater than the maximum Push
ID or which has not yet been promised, this MUST be treated as a ID or which has not yet been promised, this MUST be treated as a
connection error of type H3_ID_ERROR. connection error of type H3_ID_ERROR.
PRIORITY_UPDATE frames of either type are only sent by clients. If a PRIORITY_UPDATE frames of either type are only sent by clients. If a
client receives a PRIORITY_UPDATE frame, this MUST be treated as a client receives a PRIORITY_UPDATE frame, this MUST be treated as a
connection error of type H3_FRAME_UNEXPECTED. connection error of type H3_FRAME_UNEXPECTED.
7. Merging Client- and Server-Driven Parameters 8. 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 14, line 17 skipping to change at page 14, line 40
that forwards an HTTP response can use the parameters found in the that forwards an HTTP response can use the parameters found in the
Priority response header field, in combination with the client Priority response header field, in combination with the client
Priority request header field, as input to its prioritization Priority request header field, as input to its prioritization
process. No guidance is provided for merging priorities, this is process. No guidance is provided for merging priorities, this is
left as an implementation decision. left as an implementation decision.
Absence of a priority parameter in an HTTP response indicates the Absence of a priority parameter in an HTTP response indicates the
server's disinterest in changing the client-provided value. This is server's disinterest in changing the client-provided value. This is
different from the logic being defined for the request header field, different from the 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 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 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
skipping to change at page 14, line 41 skipping to change at page 15, line 14
: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.
8. 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.
9. Server Scheduling 10. Server Scheduling
Priority signals are input to a prioritization process. They do not Priority signals are input to a prioritization process. They do not
guarantee any particular processing or transmission order for one guarantee any particular processing or transmission order for one
response relative to any other response. An endpoint cannot force a response relative to any other response. An endpoint cannot force a
peer to process concurrent request in a particular order using peer to process concurrent request in a particular order using
priority. Expressing priority is therefore only a suggestion. priority. Expressing priority is therefore only a suggestion.
A server can use priority signals along with other inputs to make A server can use priority signals along with other inputs to make
scheduling decisions. No guidance is provided about how this can or scheduling decisions. No guidance is provided about how this can or
should be done. Factors such as implementation choices or deployment should be done. Factors such as implementation choices or deployment
environment also play a role. Any given connection is likely to have environment also play a role. Any given connection is likely to have
many dynamic permutations. For these reasons, there is no unilateral many dynamic permutations. For these reasons, there is no unilateral
perfect scheduler and this document only provides some basic perfect scheduler and this document only provides some basic
recommendations for implementations. recommendations for implementations.
Clients cannot depend on particular treatment based on priority Clients cannot depend on particular treatment based on priority
signals. Servers can use other information to prioritize responses. signals. Servers can use other information to prioritize responses.
It is RECOMMENDED that, when possible, servers respect the urgency It is RECOMMENDED that, when possible, servers respect the urgency
parameter (Section 3.1), sending higher urgency responses before parameter (Section 4.1), sending higher urgency responses before
lower urgency responses. lower urgency responses.
It is RECOMMENDED that, when possible, servers respect the It is RECOMMENDED that, when possible, servers respect the
incremental parameter (Section 3.2). Non-incremental responses of incremental parameter (Section 4.2). Non-incremental responses of
the same urgency SHOULD be served one-by-one based on the Stream ID, 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 which corresponds to the order in which clients make requests. Doing
so ensures that clients can use request ordering to influence so ensures that clients can use request ordering to influence
response order. Incremental responses of the same urgency SHOULD be response order. Incremental responses of the same urgency SHOULD be
served in round-robin manner. served in round-robin manner.
The incremental parameter indicates how a client processes response The incremental parameter indicates how a client processes response
bytes as they arrive. Non-incremental resources are only used when bytes as they arrive. Non-incremental resources are only used when
all of the response payload has been received. Incremental resources all of the response payload has been received. Incremental resources
are used as parts, or chunks, of the response payload are received. are used as parts, or chunks, of the response payload are received.
skipping to change at page 16, line 20 skipping to change at page 16, line 45
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-signalled 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 7. 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. How signals, applying default parameter values could be suboptimal. How
ever a server decides to schedule a pushed response, it can signal ever a server decides to schedule a pushed response, it can signal
the intended priority to the client by including the Priority field the intended priority to the client by including the Priority field
in a PUSH_PROMISE or HEADERS frame. in a PUSH_PROMISE or HEADERS frame.
10. Fairness 10.1. Intermediaries with Multiple Backend Connections
An intermediary serving an HTTP connection might split requests over
multiple backend connections. When it applies prioritization rules
strictly, low priority requests cannot make progress while requests
with higher priorities are inflight. This blocking can propagate to
backend connections, which the peer might interpret as a connection
stall. Endpoints often implement protections against stalls, such as
abruptly closing connections after a certain time period. To reduce
the possibility of this occurring, intermediaries can avoid strictly
following prioritization and instead allocate small amounts of
bandwidth for all the requests that they are forwarding, so that
every request can make some progress over time.
Similarly, servers SHOULD allocate some amount of bandwidths to
streams acting as tunnels.
11. Scheduling and the CONNECT Method
When a request stream carries the CONNECT method, the scheduling
guidance in this document applies to the frames on the stream. A
client that issues multiple CONNECT requests can set the incremental
parameter to "true", servers that implement the recommendation in
Section 10 will schedule these fairly.
12. Retransmission Scheduling
Transport protocols such as TCP and QUIC provide reliability by
detecting packet losses and retransmitting lost information. While
this document specifies HTTP-layer prioritization, its effectiveness
can be further enhanced if the transport layer factors priority into
scheduling both new data and retransmission data. The remainder of
this section discusses considerations when using QUIC.
[QUIC], Section 13.3 states "Endpoints SHOULD prioritize
retransmission of data over sending new data, unless priorities
specified by the application indicate otherwise". When an HTTP/3
application uses the priority scheme defined in this document and the
QUIC transport implementation supports application indicated stream
priority, a transport that considers the relative priority of streams
when scheduling both new data and retransmission data might better
match the expectations of the application. However, there are no
requirements on how a transport chooses to schedule based on this
information because the decision depends on several factors and
trade-offs. It could prioritize new data for a higher urgency stream
over retransmission data for a lower priority stream, or it could
prioritize retransmission data over new data irrespective of
urgencies.
[QUIC-RECOVERY], Section 6.2.4 also highlights consideration of
application priorities when sending probe packets after PTO timer
expiration. An QUIC implementation supporting application-indicated
priorities might use the relative priority of streams when choosing
probe data.
13. 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.
10.1. Coalescing Intermediaries 13.1. Coalescing Intermediaries
When an intermediary coalesces HTTP requests coming from multiple When an intermediary coalesces HTTP requests coming from multiple
clients into one HTTP/2 or HTTP/3 connection going to the backend clients into one HTTP/2 or HTTP/3 connection going to the backend
server, requests that originate from one client might have higher server, requests that originate from one client might 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
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responses in round-robin manner. This can work if the constrained responses in round-robin manner. This can work if the constrained
resource is network capacity between the intermediary and the user resource is network capacity between the intermediary and the user
agent, as the intermediary buffers responses and forwards the chunks agent, as the intermediary buffers responses and forwards the chunks
based on the prioritization scheme it implements. based on the prioritization scheme it implements.
A server can determine if a request came from an intermediary through A server can determine if a request came from an intermediary through
configuration, or by consulting if that request contains one of the configuration, or by consulting if that request contains one of the
following header fields: following header fields:
* Forwarded, X-Forwarded-For ([RFC7239]) * Forwarded, X-Forwarded-For ([RFC7239])
* Via ([RFC7230], Section 5.7.1) * Via ([RFC7230], Section 5.7.1)
10.2. 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 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.
10.3. 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.
11. Why use an End-to-End Header Field? 14. Why use an End-to-End Header Field?
Contrary to the prioritization scheme of HTTP/2 that uses a hop-by- 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 18, line 25 skipping to change at page 20, line 16
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.
12. Security Considerations 15. 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 [HTTP2], Section 5.3.4 describes a scenario where closure of streams
streams in the priority tree could cause suboptimal prioritization. in the priority tree could cause suboptimal prioritization. To avoid
To avoid this, [RFC7540] states that "an endpoint SHOULD retain this, [HTTP2] states that "an endpoint SHOULD retain stream
stream prioritization state for a period after streams become prioritization state for a period after streams become closed".
closed". Retaining state for streams no longer counted towards Retaining state for streams no longer counted towards stream
stream concurrency consumes server resources. Furthermore, [RFC7540] concurrency consumes server resources. Furthermore, [HTTP2]
identifies that reprioritization of a closed stream could affect identifies that reprioritization of a closed stream could affect
dependents; it recommends updating the priority tree if sufficient dependents; it recommends updating the priority tree if sufficient
state is stored, which will also consume server resources. To limit state is stored, which will also consume server resources. To limit
this commitment, it is stated that "The amount of prioritization this commitment, it is stated that "The amount of prioritization
state that is retained MAY be limited" and "If a limit is applied, state that is retained MAY be limited" and "If a limit is applied,
endpoints SHOULD maintain state for at least as many streams as endpoints SHOULD maintain state for at least as many streams as
allowed by their setting for SETTINGS_MAX_CONCURRENT_STREAMS.". allowed by their setting for SETTINGS_MAX_CONCURRENT_STREAMS.".
Extensible priorities does not use stream dependencies, which Extensible priorities does not use stream dependencies, which
minimizes most of the resource concerns related to this scenario. minimizes most of the resource concerns related to this scenario.
[RFC7540], Section 5.3.4 also presents considerations about the state [HTTP2], Section 5.3.4 also presents considerations about the state
required to store priority information about streams in an "idle" required to store priority information about streams in an "idle"
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?
13. IANA Considerations 16. 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 [HTTP2]:
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 [HTTP2]:
Frame Type: PRIORITY_UPDATE Frame Type: PRIORITY_UPDATE
Code: 0x10 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 [HTTP3]:
Frame Type: PRIORITY_UPDATE Frame Type: PRIORITY_UPDATE
Code: 0xF0700 and 0xF0701 Code: 0xF0700 and 0xF0701
Specification: This document Specification: This document
Upon publication, please create the HTTP Priority Parameters registry Upon publication, please create the HTTP Priority Parameters registry
at https://iana.org/assignments/http-priority at https://iana.org/assignments/http-priority
(https://iana.org/assignments/http-priority) and populate it with the (https://iana.org/assignments/http-priority) and populate it with the
types defined in Section 3; see Section 3.3.1 for its associated types defined in Section 4; see Section 4.3.1 for its associated
procedures. procedures.
14. References 17. References
14.1. Normative References 17.1. Normative References
[I-D.ietf-quic-http] [HTTP2] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Bishop, M., "Hypertext Transfer Protocol Version 3 Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/rfc/rfc7540>.
[HTTP3] Bishop, M., "Hypertext Transfer Protocol Version 3
(HTTP/3)", Work in Progress, Internet-Draft, draft-ietf- (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
quic-http-33, 15 December 2020, <http://www.ietf.org/ quic-http-34, 2 February 2021,
internet-drafts/draft-ietf-quic-http-33.txt>. <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
http-34>.
[I-D.ietf-quic-transport] [QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed Multiplexed and Secure Transport", RFC 9000,
and Secure Transport", Work in Progress, Internet-Draft, DOI 10.17487/RFC9000, May 2021,
draft-ietf-quic-transport-33, 13 December 2020, <https://www.rfc-editor.org/rfc/rfc9000>.
<http://www.ietf.org/internet-drafts/draft-ietf-quic-
transport-33.txt>.
[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/rfc/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/rfc/rfc7230>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[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/info/rfc8126>. <https://www.rfc-editor.org/rfc/rfc8126>.
[STRUCTURED-FIELDS] [STRUCTURED-FIELDS]
Nottingham, M. and P. Kamp, "Structured Field Values for Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", Work in Progress, Internet-Draft, draft-ietf- HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
httpbis-header-structure-19, 3 June 2020, <https://www.rfc-editor.org/rfc/rfc8941>.
<http://www.ietf.org/internet-drafts/draft-ietf-httpbis-
header-structure-19.txt>.
14.2. Informative References 17.2. Informative References
[CVE-2019-9513] [CVE-2019-9513]
Common Vulnerabilities and Exposures, "CVE-2019-9513", 1 Common Vulnerabilities and Exposures, "CVE-2019-9513", 1
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", 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,
<http://www.ietf.org/internet-drafts/draft-lassey- <https://datatracker.ietf.org/doc/html/draft-lassey-
priority-setting-00.txt>. priority-setting-00>.
[QUIC-RECOVERY]
Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
May 2021, <https://www.rfc-editor.org/rfc/rfc9002>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864, Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004, DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>. <https://www.rfc-editor.org/rfc/rfc3864>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014, RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>. <https://www.rfc-editor.org/rfc/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/rfc/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/rfc/rfc8081>.
Appendix A. Acknowledgements Appendix A. 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 http://tools.ietf.org/agenda/83/slides/ representing priorities in http://tools.ietf.org/agenda/83/slides/
slides-83-httpbis-5.pdf (http://tools.ietf.org/agenda/83/slides/ slides-83-httpbis-5.pdf (http://tools.ietf.org/agenda/83/slides/
slides-83-httpbis-5.pdf). In https://github.com/pmeenan/http3- slides-83-httpbis-5.pdf). In https://github.com/pmeenan/http3-
prioritization-proposal (https://github.com/pmeenan/http3- prioritization-proposal (https://github.com/pmeenan/http3-
prioritization-proposal), Patrick Meenan advocates for representing prioritization-proposal), Patrick Meenan advocates for representing
the priorities using a tuple of urgency and concurrency. The ability the priorities using a tuple of urgency and concurrency. The ability
skipping to change at page 22, line 22 skipping to change at page 24, line 18
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-01 B.1. Since draft-ietf-httpbis-priority-03
* Describe considerations for server push prioritisation (#1056, * 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.2. Since draft-ietf-httpbis-priority-02
* Describe considerations for server push prioritization (#1056,
#1345) #1345)
* Define HTTP/2 PRIORITY_UPDATE ID limits in HTTP/2 terms (#1261, * Define HTTP/2 PRIORITY_UPDATE ID limits in HTTP/2 terms (#1261,
#1344) #1344)
* Add a Parameters registry (#1371) * Add a Parameters registry (#1371)
B.2. Since draft-ietf-httpbis-priority-01 B.3. Since draft-ietf-httpbis-priority-01
* PRIORITY_UPDATE frame changes (#1096, #1079, #1167, #1262, #1267, * PRIORITY_UPDATE frame changes (#1096, #1079, #1167, #1262, #1267,
#1271) #1271)
* Add section to describe server scheduling considerations (#1215, * Add section to describe server scheduling considerations (#1215,
#1232, #1266) #1232, #1266)
* Remove specific instructions related to intermediary fairness * Remove specific instructions related to intermediary fairness
(#1022, #1264) (#1022, #1264)
B.3. Since draft-ietf-httpbis-priority-00 B.4. Since draft-ietf-httpbis-priority-00
* Move text around (#1217, #1218) * Move text around (#1217, #1218)
* Editorial change to the default urgency. The value is 3, which * Editorial change to the default urgency. The value is 3, which
was always the intent of previous changes. was always the intent of previous changes.
B.4. Since draft-kazuho-httpbis-priority-04 B.5. Since draft-kazuho-httpbis-priority-04
* Minimize semantics of Urgency levels (#1023, #1026) * Minimize semantics of Urgency levels (#1023, #1026)
* Reduce guidance about how intermediary implements merging priority * Reduce guidance about how intermediary implements merging priority
signals (#1026) signals (#1026)
* Remove mention of CDN-Loop (#1062) * Remove mention of CDN-Loop (#1062)
* Editorial changes * Editorial changes
* Make changes due to WG adoption * Make changes due to WG adoption
* Removed outdated Consideration (#118) * Removed outdated Consideration (#118)
B.5. Since draft-kazuho-httpbis-priority-03 B.6. Since draft-kazuho-httpbis-priority-03
* Changed numbering from "[-1,6]" to "[0,7]" (#78) * Changed numbering from "[-1,6]" to "[0,7]" (#78)
* Replaced priority scheme negotiation with HTTP/2 priority * Replaced priority scheme negotiation with HTTP/2 priority
deprecation (#100) deprecation (#100)
* Shorten parameter names (#108) * Shorten parameter names (#108)
* Expand on considerations (#105, #107, #109, #110, #111, #113) * Expand on considerations (#105, #107, #109, #110, #111, #113)
B.6. Since draft-kazuho-httpbis-priority-02 B.7. Since draft-kazuho-httpbis-priority-02
* Consolidation of the problem statement (#61, #73) * Consolidation of the problem statement (#61, #73)
* Define SETTINGS_PRIORITIES for negotiation (#58, #69) * Define SETTINGS_PRIORITIES for negotiation (#58, #69)
* Define PRIORITY_UPDATE frame for HTTP/2 and HTTP/3 (#51) * Define PRIORITY_UPDATE frame for HTTP/2 and HTTP/3 (#51)
* Explain fairness issue and mitigations (#56) * Explain fairness issue and mitigations (#56)
B.7. Since draft-kazuho-httpbis-priority-01 B.8. Since draft-kazuho-httpbis-priority-01
* Explain how reprioritization might be supported. * Explain how reprioritization might be supported.
B.8. Since draft-kazuho-httpbis-priority-00 B.9. Since draft-kazuho-httpbis-priority-00
* Expand urgency levels from 3 to 8. * Expand urgency levels from 3 to 8.
Authors' Addresses Authors' Addresses
Kazuho Oku Kazuho Oku
Fastly Fastly
Email: kazuhooku@gmail.com Email: kazuhooku@gmail.com
Lucas Pardue Lucas Pardue
Cloudflare Cloudflare
Email: lucaspardue.24.7@gmail.com Email: lucaspardue.24.7@gmail.com
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