draft-kazuho-httpbis-priority-03.txt   draft-kazuho-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: May 8, 2020 Cloudflare Expires: May 23, 2020 Cloudflare
November 05, 2019 November 20, 2019
Extensible Prioritization Scheme for HTTP Extensible Prioritization Scheme for HTTP
draft-kazuho-httpbis-priority-03 draft-kazuho-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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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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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 May 8, 2020. This Internet-Draft will expire on May 23, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Provisions Relating to IETF Documents Provisions Relating to IETF Documents
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
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
3. Negotiating Priorities . . . . . . . . . . . . . . . . . . . 5 2.1. Disabling HTTP/2 Priorities . . . . . . . . . . . . . . . 5
3.1. The SETTINGS_PRIORITIES SETTINGS Parameter . . . . . . . 5 3. Priority Parameters . . . . . . . . . . . . . . . . . . . . . 6
3.2. Defined Prioritization Scheme Values . . . . . . . . . . 6 3.1. urgency . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2.1. H2_TREE . . . . . . . . . . . . . . . . . . . . . . . 6 3.1.1. prerequisite . . . . . . . . . . . . . . . . . . . . 7
3.2.2. URGENCY . . . . . . . . . . . . . . . . . . . . . . . 7 3.1.2. default . . . . . . . . . . . . . . . . . . . . . . . 7
4. The Priority HTTP Header Field . . . . . . . . . . . . . . . 7 3.1.3. supplementary . . . . . . . . . . . . . . . . . . . . 7
4.1. urgency . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.1.4. background . . . . . . . . . . . . . . . . . . . . . 8
4.1.1. prerequisite . . . . . . . . . . . . . . . . . . . . 8 3.2. incremental . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.2. default . . . . . . . . . . . . . . . . . . . . . . . 8 3.3. Defining New Parameters . . . . . . . . . . . . . . . . . 9
4.1.3. supplementary . . . . . . . . . . . . . . . . . . . . 8 4. The Priority HTTP Header Field . . . . . . . . . . . . . . . 9
4.1.4. background . . . . . . . . . . . . . . . . . . . . . 9 5. Reprioritization . . . . . . . . . . . . . . . . . . . . . . 9
4.2. progressive . . . . . . . . . . . . . . . . . . . . . . . 9 5.1. HTTP/2 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 10
5. Reprioritization . . . . . . . . . . . . . . . . . . . . . . 10
5.1. HTTP/2 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 11
5.2. HTTP/3 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 11 5.2. HTTP/3 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 11
6. Merging Client- and Server-Driven Parameters . . . . . . . . 12 6. Merging Client- and Server-Driven Parameters . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13 7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7.1. Fairness and Coalescing Intermediaries . . . . . . . . . 13 7.1. Fairness . . . . . . . . . . . . . . . . . . . . . . . . 13
8. Considerations . . . . . . . . . . . . . . . . . . . . . . . 14 7.1.1. Coalescing Intermediaries . . . . . . . . . . . . . . 13
8.1. Why use an End-to-End Header Field? . . . . . . . . . . . 14 7.1.2. HTTP/1.x Back Ends . . . . . . . . . . . . . . . . . 14
8.2. Why do Urgencies Have Meanings? . . . . . . . . . . . . . 14 7.1.3. Intentional Introduction of Unfairness . . . . . . . 15
8.3. Can an Intermediary Send its own Signal? . . . . . . . . 15 8. Considerations . . . . . . . . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 8.1. Why use an End-to-End Header Field? . . . . . . . . . . . 15
9.1. HTTP Prioritization Scheme Registry . . . . . . . . . . . 16 8.2. Why do Urgencies Have Meanings? . . . . . . . . . . . . . 15
8.3. Can an Intermediary Send its own Signal? . . . . . . . . 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Normative References . . . . . . . . . . . . . . . . . . 17 10.1. Normative References . . . . . . . . . . . . . . . . . . 17
10.2. Informative References . . . . . . . . . . . . . . . . . 18 10.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 18 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 19
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 19 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 19
B.1. Since draft-kazuho-httpbis-priority-02 . . . . . . . . . 19 B.1. Since draft-kazuho-httpbis-priority-03 . . . . . . . . . 20
B.2. Since draft-kazuho-httpbis-priority-01 . . . . . . . . . 19 B.2. Since draft-kazuho-httpbis-priority-02 . . . . . . . . . 20
B.3. Since draft-kazuho-httpbis-priority-00 . . . . . . . . . 19 B.3. Since draft-kazuho-httpbis-priority-01 . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 B.4. Since draft-kazuho-httpbis-priority-00 . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
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 progressively 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 representation of a document at the earliest To provide meaningful representation 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 ([RFC7540]) 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.
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[I-D.ietf-quic-http] without changing the signal and its processing. [I-D.ietf-quic-http] 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.
The problems and insights laid out above are motivation for the 2.1. Disabling HTTP/2 Priorities
alternative and more straightforward prioritization scheme presented
in this document. In order to support deployment of new schemes, a
general-purpose negotiation mechanism is specified in Section 3.
3. Negotiating Priorities
The document specifies a negotiation mechanism that allows each peer
to communicate which, if any, priority schemes are supported, as well
as the server's ranked preference.
For both HTTP/2 and HTTP/3, either peer's SETTINGS may arrive first,
so any negotiation must be unilateral and not rely upon receiving the
peer's SETTINGS value.
Servers are likely to only use one prioritization scheme at once per
each connection, and may be unable to change the scheme once
established, so the setting MUST be sent prior to the first request
if it is ever sent. In HTTP/3, SETTINGS might arrive after the first
request even if they are sent first. Therefore, future
specifications that define alternative prioritization schemes for
HTTP/3 SHOULD define how the server would act when it receives a
stream-level priority signal prior to receiving the SETTINGS frame.
3.1. The SETTINGS_PRIORITIES SETTINGS Parameter
This document defines a new SETTINGS_PRIORITIES parameter (0x9) for
HTTP/2 and HTTP/3, which allows both peers to indicate which
prioritization schemes they support. The value of this parameter is
interpreted in two ways depending on if it is zero or non-zero.
If the setting has a value of zero it indicates no support for
priorities. If either side sends the parameter with a value of zero,
clients SHOULD NOT send hop-by-hop priority signals (e.g., HTTP/2
PRIORITY frame) and servers SHOULD NOT make any assumptions based on
the presence or lack thereof of such signals.
If the value is non-zero, then it is interpreted as an ordered
preference list of prioritization schemes represented by 8-bit
values. The least significant 8 bits indicate the sender's most
preferred priority scheme, the second least significant 8 bits
indicate the sender's second choice, and so on. This allows
expressing support for 4 schemes in HTTP/2 and 7 in HTTP/3.
A sender MUST comply with the following restrictions when
constructing a preference list: duplicate 8-bit values (excluding the
value 0) MUST NOT be used, and if any byte is 0 then all more
significant bytes MUST also be 0. An endpoint that receives a
setting in violation of these requirements MUST treat it as a
connection error of type PROTOCOL_ERROR for HTTP/2 [RFC7540], or of
type H3_SETTINGS_ERROR for HTTP/3 [I-D.ietf-quic-http].
In HTTP/2, the setting SHOULD appear in the first SETTINGS frame and
peers MUST NOT process the setting if it is received multiple times
in order to avoid changing the agreed upon prioritization scheme.
If there is a prioritization scheme supported by both the client and
server, then the server's preference order prevails and both peers
SHOULD only use the agreed upon priority scheme for the remainder of
the session. The server chooses because it is in the best position
to know what information from the client is of the most value.
Once the negotiation is complete, endpoints MAY stop sending hop-by-
hop prioritization signals that were not negotiated in order to
conserve bandwidth. However, endpoints SHOULD continue sending end-
to-end signals (e.g., the Priority header field), as that might have
meaningful effect to other nodes that handle the HTTP message.
3.2. Defined Prioritization Scheme Values
This document defines two prioritization scheme values for use with The problems and insights set out above are motivation for allowing
the SETTINGS_PRIORITIES setting. endpoints to opt out of using the HTTP/2 priority scheme, in favor of
using an alternative such as the scheme defined in this
specification. The SETTINGS_DEPRECATE_HTTP2_PRIORITIES setting
described below enables endpoints to understand their peer's
intention. The value of the parameter MUST be 0 or 1. Any value
other than 0 or 1 MUST be treated as a connection error (see
[RFC7540]; Section 5.4.1) of type PROTOCOL_ERROR.
3.2.1. H2_TREE Endpoints MUST send this SETTINGS parameter as part of the first
SETTINGS frame. When the peer receives the first SETTINGS frame, it
learns the sender has deprecated the HTTP/2 priority scheme if it
receives the SETTINGS_DEPRECATE_HTTP2_PRIORITIES parameter with the
value of 1.
This document defines the priority scheme identifier H2_TREE (8-bit A sender MUST NOT change the SETTINGS_DEPRECATE_HTTP2_PRIORITIES
value of 1) that indicates support for HTTP/2-style priorities parameter value after the first SETTINGS frame. Detection of a
([RFC7540], Section 5.3). change by a receiver MUST be treated as a connection error of type
PROTOCOL_ERROR.
The H2_TREE priority scheme identifier MUST NOT be be sent in an Until the client receives the SETTINGS frame from the server, the
HTTP/3 settings because there is no defined mapping of this scheme. client SHOULD send both the priority signal defined in the HTTP/2
Endpoints MUST treat receipt of H2_TREE as a connection error of type priority scheme and also that of this prioritization scheme. Once
H3_SETTINGS_ERROR. the client learns that the HTTP/2 priority scheme is deprecated, it
SHOULD stop sending the HTTP/2 priority signals. If the client
learns that the HTTP/2 priority scheme is not deprecated, it SHOULD
stop sending PRIORITY_UPDATE frames, but MAY continue sending the
Priority header field, as it is an end-to-end signal that might be
useful to nodes behind the server that the client is directly
connected to.
3.2.2. URGENCY The SETTINGS frame precedes any priority signal sent from a client in
HTTP/2, so a server can determine if it should respect the HTTP/2
scheme before building state.
This document defines the priority scheme identifier URGENCY (8-bit 3. Priority Parameters
value of 2) that indicates support for the extensible priority scheme
defined in the present document.
An intermediary connecting to a backend server SHOULD declare support The priority information is a sequence of key-value pairs, providing
for the extensible priority scheme when and only when all the room for future extensions. Each key-value pair represents a
requests that are to be sent on that backend connection originates priority parameter.
from one client-side connection that has negotiated the use of the
extensible priority scheme (see Section 7.1).
4. The Priority HTTP Header Field The Priority HTTP header field is an end-to-end way to transmit this
set of parameters when a request or a response is issued. In order
to reprioritize a request, HTTP-version-specific frames are used by
clients to transmit the same information on a single hop. If
intermediaries want to specify prioritizaton on a multiplexed HTTP
connection, it SHOULD use a PRIORITY_UPDATE frame and SHOULD NOT
change the Priority header field.
The Priority HTTP header field can appear in requests and responses. In both cases, the set of priority parameters is encoded as a
A client uses it to specify the priority of the response. A server Structured Headers Dictionary ([STRUCTURED-HEADERS]).
uses it to inform the client that the priority was overwritten. An
intermediary can use the Priority information from client requests
and server responses to correct or amend the precedence to suit it
(see Section 6).
The value of the Priority header field is a Structured Headers This document defines the urgency("u") and incremental("i")
Dictionary ([STRUCTURED-HEADERS]). Each dictionary member represents parameters. When used, these parameters MUST be accompanied by
a parameter of the Priority header field. This document defines the values. When any of the defined parameters are omitted, or if the
"urgency" and "progressive" parameters. Values of these parameters Priority header field is not used, their default values SHOULD be
MUST always be present. When any of the defined parameters are applied.
omitted, or if the Priority header field is not used, their default
values SHOULD be applied.
Unknown parameters MUST be ignored. Unknown parameters, parameters with out-of-range values or values of
unexpected types MUST be ignored.
4.1. urgency 3.1. urgency
The "urgency" parameter takes an integer between -1 and 6 as shown The urgency("u") parameter takes an integer between 0 and 7, in
below: descending order of priority, as shown below:
+-----------------+-------------------------------+ +-----------------+-------------------------------+
| Urgency | Definition | | Urgency | Definition |
+-----------------+-------------------------------+ +-----------------+-------------------------------+
| -1 | prerequisite (Section 4.1.1) | | 0 | prerequisite (Section 3.1.1) |
| 0 | default (Section 4.1.2) | | 1 | default (Section 3.1.2) |
| between 1 and 5 | supplementary (Section 4.1.3) | | between 2 and 6 | supplementary (Section 3.1.3) |
| 6 | background (Section 4.1.4) | | 7 | background (Section 3.1.4) |
+-----------------+-------------------------------+ +-----------------+-------------------------------+
Table 1: Urgencies Table 1: Urgencies
The value is encoded as an sh-integer. The default value is zero. The value is encoded as an sh-integer. The default value is 1.
A server SHOULD transmit HTTP responses in the order of their urgency A server SHOULD transmit HTTP responses in the order of their urgency
values. The lower the value, the higher the precedence. values. The lower the value, the higher the precedence.
The following example shows a request for a CSS file with the urgency The following example shows a request for a CSS file with the urgency
set to "-1": set to "0":
:method = GET :method = GET
:scheme = https :scheme = https
:authority = example.net :authority = example.net
:path = /style.css :path = /style.css
priority = urgency=-1 priority = u=0
The definition of the urgencies and their expected use-case are The definition of the urgencies and their expected use-case are
described below. Endpoints SHOULD respect the definition of the described below. Endpoints SHOULD respect the definition of the
values when assigning urgencies. values when assigning urgencies.
4.1.1. prerequisite 3.1.1. prerequisite
The prerequisite urgency (value -1) indicates that the response The prerequisite urgency (value 0) indicates that the response
prevents other responses with an urgency of prerequisite or default prevents other responses with an urgency of prerequisite or default
from being used. from being used until it is fully transmitted.
For example, use of an external stylesheet can block a web browser For example, use of an external stylesheet can block a web browser
from rendering the HTML. In such case, the stylesheet is given the from rendering the HTML. In such case, the stylesheet is given the
prerequisite urgency. prerequisite urgency.
4.1.2. default 3.1.2. default
The default urgency (value 0) indicates a response that is to be used The default urgency (value 1) indicates a response that is to be used
as it is delivered to the client, but one that does not block other as it is delivered to the client, but one that does not block other
responses from being used. responses from being used.
For example, when a user using a web browser navigates to a new HTML For example, when a user using a web browser navigates to a new HTML
document, the request for that HTML is given the default urgency. document, the request for that HTML is given the default urgency.
When that HTML document uses a custom font, the request for that When that HTML document uses a custom font, the request for that
custom font SHOULD also be given the default urgency. This is custom font SHOULD also be given the default urgency. This is
because the availability of the custom font is likely a precondition because the availability of the custom font is likely a precondition
for the user to use that portion of the HTML document, which is to be for the user to use that portion of the HTML document, which is to be
rendered by that font. rendered by that font.
4.1.3. supplementary 3.1.3. supplementary
The supplementary urgency indicates a response that is helpful to the The supplementary urgencies (values 2 to 6) indicate a response that
client using a composition of responses, even though the response is helpful to the client using a composition of responses, even
itself is not mandatory for using those responses. though the response itself is not mandatory for using those
responses.
For example, inline images (i.e., images being fetched and displayed For example, inline images (i.e., images being fetched and displayed
as part of the document) are visually important elements of an HTML as part of the document) are visually important elements of an HTML
document. As such, users will typically not be prevented from using document. As such, users will typically not be prevented from using
the document, at least to some degree, before any or all of these the document, at least to some degree, before any or all of these
images are loaded. Display of those images are thus considered to be images are loaded. Display of those images are thus considered to be
an improvement for visual clients rather than a prerequisite for all an improvement for visual clients rather than a prerequisite for all
user agents. Therefore, such images will be given the supplementary user agents. Therefore, such images will be given the supplementary
urgency. urgency.
Values between 1 and 5 are used to represent this urgency, to provide Values between 2 and 6 are used to represent this urgency, to provide
flexibility to the endpoints for giving some responses more or less flexibility to the endpoints for giving some responses more or less
precedence than others that belong to the supplementary group. precedence than others that belong to the supplementary group.
Section 6 explains how these values might be used. Section 6 explains how these values might be used.
Clients SHOULD NOT use values 1 and 5. Servers MAY use these values Clients SHOULD NOT use values 2 and 6. Servers MAY use these values
to prioritize a response above or below other supplementary to prioritize a response above or below other supplementary
responses. responses.
Clients MAY use values 2 to indicate that a request is given Clients MAY use values 3 to indicate that a request is given
relatively high priority, or 4 to indicate relatively low priority, relatively high priority, or 5 to indicate relatively low priority,
within the supplementary urgency group. within the supplementary urgency group.
For example, an image certain to be visible at the top of the page, For example, an image certain to be visible at the top of the page,
might be assigned a value of 2 instead of 3, as it will have a high might be assigned a value of 3 instead of 4, as it will have a high
visual impact for the user. Conversely, an asynchronously loaded visual impact for the user. Conversely, an asynchronously loaded
JavaScript file might be assigned an urgency value of 4, as it is JavaScript file might be assigned an urgency value of 5, as it is
less likely to have a visual impact. less likely to have a visual impact.
When none of the considerations above is applicable, the value of 3 When none of the considerations above is applicable, the value of 3
SHOULD be used. SHOULD be used.
4.1.4. background 3.1.4. background
The background urgency (value 6) is used for responses of which the The background urgency (value 7) is used for responses of which the
delivery can be postponed without having an impact on using other delivery can be postponed without having an impact on using other
responses. responses.
As an example, the download of a large file in a web browser would be As an example, the download of a large file in a web browser would be
assigned the background urgency so it would not impact further page assigned the background urgency so it would not impact further page
loads on the same connection. loads on the same connection.
4.2. progressive 3.2. incremental
The "progressive" parameter takes an sh-boolean as the value that The incremental("i") parameter takes an sh-boolean as the value that
indicates if a response can be processed progressively, i.e. provide indicates if a response can be processed incrementally, i.e. provide
some meaningful output as chunks of the response arrive. some meaningful output as chunks of the response arrive.
The default value of the "progressive" parameter is "0". The default value of the incremental parameter is "0".
A server SHOULD distribute the bandwidth of a connection between A server SHOULD distribute the bandwidth of a connection between
progressive responses that share the same urgency. incremental responses that share the same urgency.
A server SHOULD transmit non-progressive responses one by one, A server SHOULD transmit non-incremental responses one by one,
preferably in the order the requests were generated. Doing so preferably in the order the requests were generated. Doing so
maximizes the chance of the client making progress in using the maximizes the chance of the client making progress in using the
composition of the HTTP responses at the earliest moment. composition of the HTTP responses at the earliest moment.
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 "3" and the progressive parameter set to urgency parameter set to "4" and the incremental parameter set to
"1". "1".
:method = GET :method = GET
:scheme = https :scheme = https
:authority = example.net :authority = example.net
:path = /image.jpg :path = /image.jpg
priority = urgency=3, progressive=?1 priority = u=4, i=?1
3.3. Defining New Parameters
When attempting to extend priorities, care must be taken to ensure
any use of existing parameters are either unchanged or modified in a
way that is backwards compatible for peers that are unaware of the
extended meaning.
4. The Priority HTTP Header Field
The Priority HTTP header field can appear in requests and responses.
A client uses it to specify the priority of the response. A server
uses it to inform the client that the priority was overwritten. An
intermediary can use the Priority information from client requests
and server responses to correct or amend the precedence to suit it
(see Section 6).
The Priority header field is an end-to-end signal of the request
priority from the client or the response priority from the server.
As is the ordinary case for HTTP caching ([RFC7234]), a response with
a Priority header field might be cached and re-used for subsequent
requests. When an origin server generates the Priority response
header field based on properties of an HTTP request it receives, the
server is expected to control the cacheability or the applicability
of the cached response, by using header fields that control the
caching behavior (e.g., Cache-Control, Vary).
5. Reprioritization 5. Reprioritization
Once a client sends a request, circumstances might change and mean After a client sends a request, it may be beneficial to change the
that it is beneficial to change the priority of the response. As an priority of the response. As an example, a web browser might issue a
example, a web browser might issue a prefetch request for a prefetch request for a JavaScript file with the urgency parameter of
JavaScript file with the urgency parameter of the Priority request the Priority request header field set to "u=7" (background). Then,
header field set to "urgency=6" (background). Then, when the user when the user navigates to a page which references the new JavaScript
navigates to a page which references the new JavaScript file, while file, while the prefetch is in progress, the browser would send a
the prefetch is in progress, the browser would send a reprioritization frame with the priority field value set to "u=0"
reprioritization frame with the priority field value set to (prerequisite).
"urgency=-1" (prerequisite).
However, a client cannot reprioritize a response by using the In HTTP/2 and HTTP/3, after a request message is sent on a stream,
Priority header field. This is because an HTTP header field can only the stream transitions to a state that prevents the client from
be sent as part of an HTTP message. Therefore, to support sending additional frames on the stream. Therefore, a client cannot
reprioritization, it is necessary to define a HTTP-version-dependent reprioritize a response by using the Priority header field.
mechanism for transmitting the priority parameters. 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 type for HTTP/2
([RFC7540]) and HTTP/3 ([I-D.ietf-quic-http]) that is specialized for ([RFC7540]) and HTTP/3 ([I-D.ietf-quic-http]) which enables
reprioritization. It carries updated priority parameters and reprioritization. It carries updated priority parameters and
references the target of the reprioritization based on a version- references the target of the reprioritization based on a version-
specific identifier; in HTTP/2 this is the Stream ID, in HTTP/3 this specific identifier; in HTTP/2 this is the Stream ID, in HTTP/3 this
is either the Stream ID or Push ID. is either the Stream ID or Push ID.
In HTTP/2 and HTTP/3 a request message sent on a stream transitions Unlike the header field, the reprioritization frame is a hop-by-hop
it into a state that prevents the client from sending additional signal.
frames on the stream. Modifying this behavior requires a semantic
change to the protocol, 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).
5.1. HTTP/2 PRIORITY_UPDATE Frame 5.1. HTTP/2 PRIORITY_UPDATE Frame
The HTTP/2 PRIORITY_UPDATE frame (type=0xF) carries the stream ID of The HTTP/2 PRIORITY_UPDATE frame (type=0xF) carries the stream ID of
the response that is being reprioritized, and the updated priority in the response that is being reprioritized, 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.
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).
skipping to change at page 11, line 27 skipping to change at page 10, line 49
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| 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:
R: A reserved 1-bit field. The semantics of this bit are undefined,
and the bit MUST remain unset (0x0) when sending and MUST be
ignored when receiving.
Stream ID: A 31-bit stream identifier for the stream that is the
target of the priority update.
Priority Field Value: The priority update value in ASCII text,
encoded using Structured Headers.
The HTTP/2 PRIORITY_UPDATE frame MUST NOT be sent prior to opening
the stream. If a PRIORITY_UPDATE is received prior to the stream
being opened, it MAY be treated as a connection error of type
PROTOCOL_ERROR.
TODO: add more description of how to handle things like receiving TODO: add more description of how to handle things like receiving
PRIORITY_UPDATE on wrong stream, a PRIORITY_UPDATE with an invalid PRIORITY_UPDATE on wrong stream, a PRIORITY_UPDATE with an invalid
ID, etc. ID, etc.
5.2. HTTP/3 PRIORITY_UPDATE Frame 5.2. HTTP/3 PRIORITY_UPDATE Frame
The HTTP/3 PRIORITY_UPDATE frame (type=0xF) carries the identifier of The HTTP/3 PRIORITY_UPDATE frame (type=0xF) carries the identifier of
the element that is being reprioritized, and the updated priority in the element that is being reprioritized, 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.
skipping to change at page 12, line 14 skipping to change at page 12, line 7
T (Prioritized Element Type): A one-bit field indicating the type of T (Prioritized Element Type): A one-bit field indicating the type of
element being prioritized. A value of 0 indicates a element being prioritized. A value of 0 indicates a
reprioritization for a Request Stream, so the Prioritized Element reprioritization for a Request Stream, so the Prioritized Element
ID is interpreted as a Stream ID. A value of 1 indicates a ID is interpreted as a Stream ID. A value of 1 indicates a
reprioritization for a Push stream, so the Prioritized Element ID reprioritization for a Push stream, so the Prioritized Element ID
is interpreted as a Push ID. is interpreted as a Push ID.
Empty: A seven-bit field that has no semantic value. Empty: A seven-bit field that has no semantic value.
Prioritized Element ID: The stream ID or push ID that is the target
of the priority update.
Priority Field Value: The priority update value in ASCII text,
encoded using Structured Headers.
The HTTP/3 PRIORITY_UPDATE frame MUST NOT be sent with an invalid
identifier, including before the request stream has been opened or
before a promised request has been received. If a server receives a
PRIORITY_UPDATE specifying a push ID that has not been promised, it
SHOULD be treated as a connection error of type H3_ID_ERROR.
Because the HTTP/3 PRIORITY_UPDATE frame is sent on the control
stream and there are no ordering guarantees between streams, a client
that reprioritizes a request before receiving the response data might
cause the server to receive a PRIORITY_UPDATE for an unknown request.
If the request stream ID is within bidirectional stream limits, the
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 TODO: add more description of how to handle things like receiving
PRIORITY_UPDATE on wrong stream, a PRIORITY_UPDATE with an invalid PRIORITY_UPDATE on wrong stream, a PRIORITY_UPDATE with an invalid
ID, etc. ID, etc.
6. Merging Client- and Server-Driven Parameters 6. 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. For example, of how the HTTP responses deserve to be prioritized. For example,
use of an HTML document might depend heavily on one of the inline use of an HTML document might depend heavily on one of the inline
images. Existence of such dependencies is typically best known to images. Existence of such dependencies is typically best known to
skipping to change at page 12, line 32 skipping to change at page 13, line 4
use of an HTML document might depend heavily on one of the inline use of an HTML document might depend heavily on one of the inline
images. Existence of such dependencies is typically best known to images. Existence of such dependencies is typically best known to
the server. the server.
By using the "Priority" response header, a server can override the By using the "Priority" response header, a server can override the
prioritization hints provided by the client. When used, the prioritization hints provided by the client. When used, the
parameters found in the response header field overrides those parameters found in the response header field overrides those
specified by the client. specified by the client.
For example, when the client sends an HTTP request with For example, when the client sends an HTTP request with
:method = GET :method = GET
:scheme = https :scheme = https
:authority = example.net :authority = example.net
:path = /menu.png :path = /menu.png
priority = urgency=3, progressive=?1 priority = u=4, i=?1
and the origin responds with and the origin responds with
:status = 200 :status = 200
content-type = image/png content-type = image/png
priority = urgency=1 priority = u=2
the intermediary's understanding of the urgency is promoted from "3" the intermediary's understanding of the urgency is promoted from "4"
to "1", because the server-provided value overrides the value to "2", because the server-provided value overrides the value
provided by the client. The progressiveness continues to be "1", the provided by the client. The incremental value continues to be "1",
value specified by the client, as the server did not specify the the value specified by the client, as the server did not specify the
"progressive" parameter. incremental("i") parameter.
7. Security Considerations 7. Security Considerations
7.1. Fairness and Coalescing Intermediaries 7.1. Fairness
As a general guideline, a server SHOULD NOT use priority information
for making schedule decisions across multiple connections, unless it
knows that those connections originate from the same client. Due to
this, priority information conveyed over a non-coalesced HTTP
connection (e.g., HTTP/1.1) might go unused.
The remainder of this section discusses scenarios where unfairness is
problematic and presents possible mitigations, or where unfairness is
desirable.
TODO: Discuss if we should add a signal that mitigates this issue.
For example, we might add a SETTINGS parameter that indicates the
next hop that the connection is NOT coalesced (see
https://github.com/kazuho/draft-kazuho-httpbis-priority/issues/99).
7.1.1. 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 end client 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, when a server responds to
a request that is known to have come through an intermediary, the a request that is known to have come through an intermediary, the
server SHOULD prioritize the response as if it was assigned the server SHOULD prioritize the response as if it was assigned the
priority of "urgency=0, progressive=?1" (i.e. round-robin) regardless priority of "u=1, i=?1" (i.e. round-robin) regardless of the value of
of the value of the Priority header field being transmitted, unless the Priority header field being transmitted, unless the server knows
the server has the knowledge that no intermediaries are coalescing the intermediary is not coalescing requests from multiple clients.
requests from multiple clients. That can be determined by the
settings when the intermediaries support this specification (see
Section 3.2.2), or else through configuration.
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 CDN-Loop ([RFC8586]) o CDN-Loop ([RFC8586])
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)
skipping to change at page 14, line 5 skipping to change at page 14, line 36
Responding to requests coming through an intermediary in a round- Responding to requests coming through an intermediary in a round-
robin manner works well when the network bottleneck exists between robin manner works well when the network bottleneck exists between
the intermediary and the end client, as the intermediary would be the intermediary and the end client, as the intermediary would be
buffering the responses and then be forwarding the chunks of those buffering the responses and then be forwarding the chunks of those
buffered responses based on the prioritization scheme it implements. buffered responses based on the prioritization scheme it implements.
A sophisticated server MAY use a weighted round-robin reflecting the A sophisticated server MAY use a weighted round-robin reflecting the
urgencies expressed in the requests, so that less urgent responses urgencies expressed in the requests, so that less urgent responses
would receive less bandwidth in case the bottleneck exists between would receive less bandwidth in case the bottleneck exists between
the server and the intermediary. the server and the intermediary.
7.1.2. HTTP/1.x Back Ends
It is common for CDN infrastructure to support different HTTP
versions on the front end and back end. For instance, the client-
facing edge might support HTTP/2 and HTTP/3 while communication to
back end servers is done using HTTP/1.1. Unlike with connection
coalescing, the CDN will "de-mux" requests into discrete connections
to the back end. As HTTP/1.1 and older do not provide a way to
concurrently transmit multiple responses, there is no immediate
fairness issue in protocol. However, back end servers MAY still use
client headers for request scheduling. Back end servers SHOULD only
schedule based on client priority information where that information
can be scoped to individual end clients. Authentication and other
session information might provide this linkability.
7.1.3. Intentional Introduction of Unfairness
It is sometimes beneficial to deprioritize the transmission of one
connection over others, knowing that doing so introduces a certain
amount of unfairness between the connections and therefore between
the requests served on those connections.
For example, a server might use a scavenging congestion controller on
connections that only convey background priority responses such as
software update images. Doing so improves responsiveness of other
connections at the cost of delaying the delivery of updates.
Also, a client MAY use the priority values for making local
scheduling choices for the requests it initiates.
8. Considerations 8. Considerations
8.1. Why use an End-to-End Header Field? 8.1. 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
skipping to change at page 14, line 39 skipping to change at page 16, line 5
textual value makes the prioritization scheme extensible; see the textual value makes the prioritization scheme extensible; see the
discussion below. discussion below.
8.2. Why do Urgencies Have Meanings? 8.2. Why do Urgencies Have Meanings?
One of the aims of this specification is to define a mechanism for One of the aims of this specification is to define a mechanism for
merging client- and server-provided hints for prioritizing the merging client- and server-provided hints for prioritizing the
responses. For that to work, each urgency level needs to have a responses. For that to work, each urgency level needs to have a
well-defined meaning. As an example, a server can assign the highest well-defined meaning. As an example, a server can assign the highest
precedence among the supplementary responses to an HTTP response precedence among the supplementary responses to an HTTP response
carrying an icon, because the meaning of "urgency=1" is shared among carrying an icon, because the meaning of "u=2" is shared among the
the endpoints. endpoints.
This specification restricts itself to defining a minimum set of This specification restricts itself to defining a minimum set of
urgency levels in order to provide sufficient granularity for urgency levels in order to provide sufficient granularity for
prioritizing responses for ordinary web browsing, at minimal prioritizing responses for ordinary web browsing, at minimal
complexity. complexity.
However, that does not mean that the prioritization scheme would However, that does not mean that the prioritization scheme would
forever be stuck to the eight levels. The design provides forever be stuck to the eight levels. The design provides
extensibility. If deemed necessary, it would be possible to extensibility. If deemed necessary, it would be possible to
subdivide any of the eight urgency levels that are currently defined. subdivide any of the eight urgency levels that are currently defined.
skipping to change at page 15, line 25 skipping to change at page 16, line 40
embed its own prioritization hints into the HTTP request that it embed its own prioritization hints into the HTTP request that it
forwards to the backend server, as otherwise the Priority header forwards to the backend server, as otherwise the Priority header
field would not be as helpful to the backend (see Section 7.1). field would not be as helpful to the backend (see Section 7.1).
One way of achieving that, without dropping the original signal, One way of achieving that, without dropping the original signal,
would be to let the intermediary express its own signal using the would be to let the intermediary express its own signal using the
Priority header field, at the same time transplanting the original Priority header field, at the same time transplanting the original
value to a different header field. value to a different header field.
As an example, when a client sends an HTTP request carrying a As an example, when a client sends an HTTP request carrying a
priority of "urgency=-1" and the intermediary wants to instead priority of "u=0" and the intermediary wants to instead associate
associate "urgency=0; progressive=?1", the intermediary would send a "u=1; i=?1", the intermediary would send a HTTP request that contains
HTTP request that contains to the following two header fields to the the following two header fields to the backend server:
backend server:
priority = urgency=0; progressive=?1 priority = u=1; i=?1
original-priority = urgency=-1 original-priority = u=0
9. IANA Considerations 9. 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
skipping to change at page 16, line 5 skipping to change at page 17, line 20
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_PRIORITIES Name: SETTINGS_DEPRECATE_HTTP2_PRIORITIES
Code: 0x9
Initial value: 0
Specification: This document
This specification registers the following entry in the HTTP/2
Settings registry established by [I-D.ietf-quic-http]:
Name: SETTINGS_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]:
skipping to change at page 16, line 42 skipping to change at page 17, line 46
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: 0xF
Specification: This document Specification: This document
9.1. HTTP Prioritization Scheme Registry
This document establishes a registry for HTTP prioritization scheme
codes to be used in conjunction with the SETTINGS_PRIORITIES
parameter. The "HTTP Prioritization Scheme" registry manages an
8-bit space. The "HTTP Prioritization Scheme" registry operates
under either of the "IETF Review" or "IESG Approval" policies
[RFC5226] for values between 0x00 and 0xef, with values between 0xf0
and 0xff being reserved for Experimental Use.
New entries in this registry require the following information:
Prioritization Scheme: A name or label for the prioritization
scheme.
Code: The 8-bit code assigned to the prioritization scheme.
Specification: A reference to a specification that includes a
description of the prioritization scheme.
The entries in the following table are registered by this document.
+-----------------------+------+---------------+
| Prioritization Scheme | Code | Specification |
+-----------------------+------+---------------+
| H2_TREE | 1 | Section 3.2.1 |
| URGENCY | 2 | Section 3.2.2 |
+-----------------------+------+---------------+
10. References 10. References
10.1. Normative References 10.1. 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-23 (work in progress), (HTTP/3)", draft-ietf-quic-http-23 (work in progress),
September 2019. September 2019.
[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-23 (work and Secure Transport", draft-ietf-quic-transport-23 (work
in progress), September 2019. in progress), September 2019.
[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>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<https://www.rfc-editor.org/info/rfc5226>.
[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>.
skipping to change at page 18, line 27 skipping to change at page 18, line 47
[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.
[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/info/rfc3864>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<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>.
[RFC8586] Ludin, S., Nottingham, M., and N. Sullivan, "Loop [RFC8586] Ludin, S., Nottingham, M., and N. Sullivan, "Loop
Detection in Content Delivery Networks (CDNs)", RFC 8586, Detection in Content Delivery Networks (CDNs)", RFC 8586,
skipping to change at page 19, line 4 skipping to change at page 19, line 31
[2] https://github.com/pmeenan/http3-prioritization-proposal [2] https://github.com/pmeenan/http3-prioritization-proposal
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 [1]. In https://github.com/pmeenan/http3- slides-83-httpbis-5.pdf [1]. In https://github.com/pmeenan/http3-
prioritization-proposal [2], Patrick Meenan advocates for prioritization-proposal [2], Patrick Meenan advocates for
representing the priorities using a tuple of urgency and concurrency. representing the priorities using a tuple of urgency and concurrency.
The ability to deprecate HTTP/2 priortization is based on
The negotiation scheme described in this document is based on
[I-D.lassey-priority-setting], authored by Brad Lassey and Lucas [I-D.lassey-priority-setting], authored by Brad Lassey and Lucas
Pardue. Pardue, with modifications based on feedback that was not
incorporated into an update to that document.
The motivation for defining an alternative to HTTP/2 priorities is The motivation for defining an alternative to HTTP/2 priorities is
drawn from discussion within the broad HTTP community. Special drawn from discussion within the broad HTTP community. Special
thanks to Roberto Peon, Martin Thomson and Netflix for text that was thanks to Roberto Peon, Martin Thomson and Netflix for text that was
incorporated explicitly in this document. incorporated explicitly in this document.
In addition to the people above, this document owes a lot to the In addition to the people above, this document owes a lot to the
extensive discussion in the HTTP priority design team, consisting of extensive discussion in the HTTP priority design team, consisting of
Alan Frindell, Andrew Galloni, Craig Taylor, Ian Swett, Kazuho Oku, Alan Frindell, Andrew Galloni, Craig Taylor, Ian Swett, 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-kazuho-httpbis-priority-03
B.1. Since draft-kazuho-httpbis-priority-02 o Changed numbering from [-1,6] to [0,7] (#78)
o Replaced priority scheme negotiation with HTTP/2 priority
deprecation (#100)
o Shorten parameter names (#108)
o Expand on considerations (#105, #107, #109, #110, #111, #113)
B.2. 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.2. Since draft-kazuho-httpbis-priority-01 B.3. Since draft-kazuho-httpbis-priority-01
o Explain how reprioritization might be supported. o Explain how reprioritization might be supported.
B.3. Since draft-kazuho-httpbis-priority-00 B.4. 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
 End of changes. 73 change blocks. 
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