wdiff draft-kazuho-httpbis-priority-03.txt draft-kazuho-httpbis-priority-04.txt

HTTP K. Oku
Internet-Draft Fastly
Intended status: Standards Track L. Pardue
Expires: May 8, 23, 2020 Cloudflare
November 05, 20, 2019

Extensible Prioritization Scheme for HTTP
draft-kazuho-httpbis-priority-03
draft-kazuho-httpbis-priority-04

Abstract

This document describes a scheme for prioritizing HTTP responses.
This scheme expresses the priority of each HTTP response using
absolute values, rather than as a relative relationship between a
group of HTTP responses.

This document defines the Priority header field for communicating the
initial priority in an HTTP version-independent manner, as well as
HTTP/2 and HTTP/3 frames for reprioritizing the responses. These
share a common format structure that is designed to provide future
extensibility.

Status of This Memo

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provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on May 8, 23, 2020.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. Motivation for Replacing HTTP/2 Priorities . . . . . . . . . 4
3. Negotiating
2.1. Disabling HTTP/2 Priorities . . . . . . . . . . . . . . . 5
3. Priority Parameters . . . . 5
3.1. The SETTINGS_PRIORITIES SETTINGS Parameter . . . . . . . 5
3.2. Defined Prioritization Scheme Values . . . . . . . . . . 6
3.2.1. H2_TREE
3.1. urgency . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2.2. URGENCY
3.1.1. prerequisite . . . . . . . . . . . . . . . . . . . . 7
3.1.2. default . . . 7
4. The Priority HTTP Header Field . . . . . . . . . . . . . . . 7
4.1. urgency . . . . . 7
3.1.3. supplementary . . . . . . . . . . . . . . . . . . . . 7
4.1.1. prerequisite
3.1.4. background . . . . . . . . . . . . . . . . . . . . . 8
4.1.2. default
3.2. incremental . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.3. supplementary
3.3. Defining New Parameters . . . . . . . . . . . . . . . . . 9
4. The Priority HTTP Header Field . . . 8
4.1.4. background . . . . . . . . . . . . 9
5. Reprioritization . . . . . . . . . 9
4.2. progressive . . . . . . . . . . . . . 9
5.1. HTTP/2 PRIORITY_UPDATE Frame . . . . . . . . . . 9
5. Reprioritization . . . . 10
5.2. HTTP/3 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 11
6. Merging Client- and Server-Driven Parameters . . . . 10
5.1. HTTP/2 PRIORITY_UPDATE Frame . . . . 12
7. Security Considerations . . . . . . . . . . 11
5.2. HTTP/3 PRIORITY_UPDATE Frame . . . . . . . . . 13
7.1. Fairness . . . . . . 11
6. Merging Client- and Server-Driven Parameters . . . . . . . . 12
7. Security Considerations . . . . . . . . . . 13
7.1.1. Coalescing Intermediaries . . . . . . . . . . . . . . 13
7.1. Fairness and Coalescing Intermediaries
7.1.2. HTTP/1.x Back Ends . . . . . . . . . 13 . . . . . . . . 14
7.1.3. Intentional Introduction of Unfairness . . . . . . . 15
8. Considerations . . . . . . . . . . . . . . . . . . . . . . . 14 15
8.1. Why use an End-to-End Header Field? . . . . . . . . . . . 14 15
8.2. Why do Urgencies Have Meanings? . . . . . . . . . . . . . 14 15
8.3. Can an Intermediary Send its own Signal? . . . . . . . . 15 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9.1. HTTP Prioritization Scheme Registry . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Normative References . . . . . . . . . . . . . . . . . . 17
10.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 18 19
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 19
B.1. Since draft-kazuho-httpbis-priority-02 draft-kazuho-httpbis-priority-03 . . . . . . . . . 19 20
B.2. Since draft-kazuho-httpbis-priority-01 draft-kazuho-httpbis-priority-02 . . . . . . . . . 19 20
B.3. Since draft-kazuho-httpbis-priority-01 . . . . . . . . . 20
B.4. Since draft-kazuho-httpbis-priority-00 . . . . . . . . . 19 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 20

1. Introduction

It is common for an HTTP ([RFC7230]) resource representation to have
relationships to one or more other resources. Clients will often
discover these relationships while processing a retrieved
representation, leading to further retrieval requests. Meanwhile,
the nature of the relationship determines whether the client is
blocked from continuing to process locally available resources. For
example, visual rendering of an HTML document could be blocked by the
retrieval of a CSS file that the document refers to. In contrast,
inline images do not block rendering and get drawn progressively incrementally as
the chunks of the images arrive.

To provide meaningful representation of a document at the earliest
moment, it is important for an HTTP server to prioritize the HTTP
responses, or the chunks of those HTTP responses, that it sends.

HTTP/2 ([RFC7540]) provides such a prioritization scheme. A client
sends a series of PRIORITY frames to communicate to the server a
"priority tree"; this represents the client's preferred ordering and
weighted distribution of the bandwidth among the HTTP responses.
However, the design and implementation of this scheme has been
observed to have shortcomings, explained in Section 2.

This document defines the Priority HTTP header field that can be used
by both client and server to specify the precedence of HTTP responses
in a standardized, extensible, protocol-version-independent, end-to-
end format. Along with the protocol-version-specific frame for
reprioritization, this prioritization scheme acts as a substitute for
the original prioritization scheme of HTTP/2.

1.1. Notational Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

The terms sh-token and sh-boolean are imported from
[STRUCTURED-HEADERS].

Example HTTP requests and responses use the HTTP/2-style formatting
from [RFC7540].

This document uses the variable-length integer encoding from
[I-D.ietf-quic-transport].

2. Motivation for Replacing HTTP/2 Priorities

An important feature of any implementation of a protocol that
provides multiplexing is the ability to prioritize the sending of
information. This was an important realization in the design of
HTTP/2. Prioritization is a difficult problem, so it will always be
suboptimal, particularly if one endpoint operates in ignorance of the
needs of its peer.

HTTP/2 introduced a complex prioritization signaling scheme that used
a combination of dependencies and weights, formed into an unbalanced
tree. This scheme has suffered from poor deployment and
interoperability.

The rich flexibility of client-driven HTTP/2 prioritization tree
building is rarely exercised; experience shows that clients either
choose a single model optimized for a web use case (and don't vary
it) or do nothing at all. But every client builds their
prioritization tree in a different 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
priority scheme, some favoring instead bespoke server-driven schemes
based on heuristics and other hints, like the content type of
resources and the order in which requests arrive. For example, a
server, with knowledge of the document structure, might want to
prioritize the delivery of images that are critical to user
experience above other images, but below the CSS files. Since client
trees vary, it is impossible for the server to determine how such
images should be prioritized against other responses.

The HTTP/2 scheme allows intermediaries to coalesce multiple client
trees into a single tree that is used for a single upstream HTTP/2
connection. However, most intermediaries do not support this. The
scheme does not define a method that can be used by a server to
express the priority of a response. Without such a method,
intermediaries cannot coordinate client-driven and server-driven
priorities.

HTTP/2 describes denial-of-service considerations for
implementations. On 2019-08-13 Netflix issued an advisory notice
about the discovery of several resource exhaustion vectors affecting
multiple HTTP/2 implementations. One attack, CVE-2019-9513 aka
"Resource Loop", is based on manipulation of the priority tree.

The HTTP/2 scheme depends on in-order delivery of signals, leading to
challenges in porting the scheme to protocols that do not provide
global ordering. For example, the scheme cannot be used in HTTP/3
[I-D.ietf-quic-http] without changing the signal and its processing.

Considering the problems with deployment and adaptability to HTTP/3,
retaining the HTTP/2 priority scheme increases the complexity of the
entire system without any evidence that the value it provides offsets
that complexity. In fact, multiple experiments from independent
research have shown that simpler schemes can reach at least
equivalent performance characteristics compared to the more complex
HTTP/2 setups seen in practice, at least for the web use case.

2.1. Disabling HTTP/2 Priorities

The problems and insights laid set out above are motivation for allowing
endpoints to opt out of using the HTTP/2 priority scheme, in favor of
using an alternative and more straightforward prioritization such as the scheme presented defined in this document. In order to support deployment of new schemes, a
general-purpose negotiation mechanism is specified in Section 3.

3. Negotiating Priorities
specification. The document specifies a negotiation mechanism that allows each peer SETTINGS_DEPRECATE_HTTP2_PRIORITIES setting
described below enables endpoints 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 understand their peer's SETTINGS may arrive first,
so any negotiation must be unilateral and not rely upon receiving
intention. The value of the
peer's SETTINGS value.

Servers are likely to only use one prioritization scheme at once per
each connection, and may parameter MUST be unable to change the scheme once
established, so the setting 0 or 1. Any value
other than 0 or 1 MUST be sent prior to the first request
if it is ever sent. In HTTP/3, treated as a connection error (see
[RFC7540]; Section 5.4.1) of type PROTOCOL_ERROR.

Endpoints MUST send this SETTINGS might arrive after parameter as part of the first
request even if they are sent first. Therefore, future
specifications that define alternative prioritization schemes for
HTTP/3 SHOULD define how
SETTINGS frame. When the server would act when it peer receives a
stream-level priority signal prior to receiving the first 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 frame, it is zero or non-zero.

If
learns the setting sender has a value of zero deprecated the HTTP/2 priority scheme if it indicates no support for
priorities. If either side sends
receives the SETTINGS_DEPRECATE_HTTP2_PRIORITIES 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. 1.

A sender MUST comply with the following restrictions when
constructing a preference list: duplicate 8-bit values (excluding NOT change the SETTINGS_DEPRECATE_HTTP2_PRIORITIES
parameter 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 after the first SETTINGS frame. Detection of these requirements a
change by a receiver MUST treat it be treated 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,
PROTOCOL_ERROR.

Until the setting SHOULD appear in client receives the first SETTINGS frame and
peers MUST NOT process from the setting if it is received multiple times server, the
client SHOULD send both the priority signal defined in order to avoid changing the agreed upon HTTP/2
priority scheme and also that of this prioritization scheme.

If there is a prioritization scheme supported by both Once
the client and
server, then the server's preference order prevails and both peers
SHOULD only use learns that the agreed upon HTTP/2 priority scheme for the remainder of
the session. The server chooses because it is in deprecated, it
SHOULD stop sending the best position
to know what information from HTTP/2 priority signals. If the client is of the most value.

Once
learns that the negotiation HTTP/2 priority scheme is complete, endpoints MAY stop sending hop-by-
hop prioritization signals that were not negotiated in order to
conserve bandwidth. However, endpoints deprecated, it SHOULD
stop sending PRIORITY_UPDATE frames, but MAY continue sending end-
to-end signals (e.g., the
Priority header field), field, as it is an end-to-end signal that might have
meaningful effect be
useful 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 SETTINGS_PRIORITIES setting.

3.2.1. H2_TREE

This document defines behind the priority scheme identifier H2_TREE (8-bit
value of 1) server that indicates support for HTTP/2-style priorities
([RFC7540], Section 5.3). the client is directly
connected to.

The H2_TREE SETTINGS frame precedes any priority scheme identifier MUST NOT be be signal sent in an
HTTP/3 settings because there is no defined mapping of this scheme.
Endpoints MUST treat receipt of H2_TREE as from a connection error of type
H3_SETTINGS_ERROR.

3.2.2. URGENCY

This document defines the priority scheme identifier URGENCY (8-bit
value of 2) that indicates support for the extensible priority scheme
defined client in the present document.

An intermediary connecting to
HTTP/2, so a backend server SHOULD declare support
for can determine if it should respect the extensible priority HTTP/2
scheme when and only when all the
requests that are to be sent on that backend connection originates
from one client-side connection that has negotiated the use before building state.

3. Priority Parameters

The priority information is a sequence of the
extensible key-value pairs, providing
room for future extensions. Each key-value pair represents a
priority scheme (see Section 7.1).

4. The Priority HTTP Header Field parameter.

The Priority HTTP header field can appear in requests and responses.
A client uses it is an end-to-end way to specify the priority transmit this
set of the response. A server
uses it parameters when a request or a response is issued. In order
to inform the client that the priority was overwritten. An
intermediary can use the Priority information from client requests
and server responses reprioritize a request, HTTP-version-specific frames are used by
clients to correct or amend transmit the precedence same information on a single hop. If
intermediaries want to suit specify prioritizaton on a multiplexed HTTP
connection, it
(see Section 6).

The value of SHOULD 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
Structured Headers Dictionary ([STRUCTURED-HEADERS]). Each dictionary member represents
a parameter of the Priority header field.

This document defines the
"urgency" urgency("u") and "progressive" incremental("i")
parameters. Values of When used, these parameters MUST always be present. accompanied by
values. When any of the defined parameters are omitted, or if the
Priority header field is not used, their default values SHOULD be
applied.

Unknown parameters, parameters with out-of-range values or values of
unexpected types MUST be ignored.

4.1.

3.1. urgency

The "urgency" urgency("u") parameter takes an integer between -1 0 and 6 7, in
descending order of priority, as shown below:

+-----------------+-------------------------------+
| Urgency | Definition |
+-----------------+-------------------------------+
| -1 0 | prerequisite (Section 4.1.1) 3.1.1) |
| 0 1 | default (Section 4.1.2) 3.1.2) |
| between 1 2 and 5 6 | supplementary (Section 4.1.3) 3.1.3) |
| 6 7 | background (Section 4.1.4) 3.1.4) |
+-----------------+-------------------------------+

Table 1: Urgencies

The value is encoded as an sh-integer. The default value is zero. 1.

A server SHOULD transmit HTTP responses in the order of their urgency
values. The lower the value, the higher the precedence.

The following example shows a request for a CSS file with the urgency
set to "-1": "0":

:method = GET
:scheme = https
:authority = example.net
:path = /style.css
priority = urgency=-1 u=0

The definition of the urgencies and their expected use-case are
described below. Endpoints SHOULD respect the definition of the
values when assigning urgencies.

4.1.1.

3.1.1. prerequisite

The prerequisite urgency (value -1) 0) indicates that the response
prevents other responses with an urgency of prerequisite or default
from being used. used until it is fully transmitted.

For example, use of an external stylesheet can block a web browser
from rendering the HTML. In such case, the stylesheet is given the
prerequisite urgency.

4.1.2.

3.1.2. default

The default urgency (value 0) 1) indicates a response that is to be used
as it is delivered to the client, but one that does not block other
responses from being used.

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.
When that HTML document uses a custom font, the request for that
custom font SHOULD also be given the default urgency. This is
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
rendered by that font.

4.1.3.

3.1.3. supplementary

The supplementary urgency indicates urgencies (values 2 to 6) indicate a response that
is helpful to the client using a composition of responses, even
though the response itself is not mandatory for using those
responses.

For example, inline images (i.e., images being fetched and displayed
as part of the document) are visually important elements of an HTML
document. As such, users will typically not be prevented from using
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
an improvement for visual clients rather than a prerequisite for all
user agents. Therefore, such images will be given the supplementary
urgency.

Values between 1 2 and 5 6 are used to represent this urgency, to provide
flexibility to the endpoints for giving some responses more or less
precedence than others that belong to the supplementary group.
Section 6 explains how these values might be used.

Clients SHOULD NOT use values 1 2 and 5. 6. Servers MAY use these values
to prioritize a response above or below other supplementary
responses.

Clients MAY use values 2 3 to indicate that a request is given
relatively high priority, or 4 5 to indicate relatively low priority,
within the supplementary urgency group.

For example, an image certain to be visible at the top of the page,
might be assigned a value of 2 3 instead of 3, 4, as it will have a high
visual impact for the user. Conversely, an asynchronously loaded
JavaScript file might be assigned an urgency value of 4, 5, as it is
less likely to have a visual impact.

When none of the considerations above is applicable, the value of 3
SHOULD be used.

4.1.4.

3.1.4. background

The background urgency (value 6) 7) is used for responses of which the
delivery can be postponed without having an impact on using other
responses.

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
loads on the same connection.

4.2. progressive

3.2. incremental

The "progressive" incremental("i") parameter takes an sh-boolean as the value that
indicates if a response can be processed progressively, incrementally, i.e. provide
some meaningful output as chunks of the response arrive.

The default value of the "progressive" incremental parameter is "0".

A server SHOULD distribute the bandwidth of a connection between
progressive
incremental responses that share the same urgency.

A server SHOULD transmit non-progressive non-incremental responses one by one,
preferably in the order the requests were generated. Doing so
maximizes the chance of the client making progress in using the
composition of the HTTP responses at the earliest moment.

The following example shows a request for a JPEG file with the
urgency parameter set to "3" "4" and the progressive incremental parameter set to
"1".

:method = GET
:scheme = https
:authority = example.net
:path = /image.jpg
priority = urgency=3, progressive=?1 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

Once

After a client sends a request, circumstances might change and mean
that it is may be beneficial to change the
priority of the response. As an example, a web browser might issue a
prefetch request for a JavaScript file with the urgency parameter of
the Priority request header field set to "urgency=6" "u=7" (background). Then,
when the user navigates to a page which references the new JavaScript
file, while the prefetch is in progress, the browser would send a
reprioritization frame with the priority field value set to
"urgency=-1" "u=0"
(prerequisite).

However,

In HTTP/2 and HTTP/3, after a request message is sent on a stream,
the stream transitions to a state that prevents the client from
sending additional frames on the stream. Therefore, a client cannot
reprioritize a response by using the Priority header field. This
Modifying this behavior would require a semantic change to the
protocol, but this is because an HTTP header field avoided by restricting the stream on which a
PRIORITY_UPDATE frame can only be sent as part of an HTTP message. Therefore, to support
reprioritization, sent. In HTTP/2 the frame is on stream
zero and in HTTP/3 it is necessary to define a HTTP-version-dependent
mechanism for transmitting sent on the priority parameters. control stream
([I-D.ietf-quic-http], Section 6.2.1).

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 which enables
reprioritization. It carries updated priority parameters and
references the target of the reprioritization based on a version-
specific identifier; in HTTP/2 this is the Stream ID, in HTTP/3 this
is either the Stream ID or Push ID.

In HTTP/2 and HTTP/3 a request message sent on a stream transitions
it into a state that prevents the client from sending additional
frames on the stream. Modifying this behavior requires a semantic
change to the protocol, this is avoided by restricting

Unlike the stream on
which a PRIORITY_UPDATE frame can be sent. In HTTP/2 header field, the reprioritization 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). a hop-by-hop
signal.

5.1. HTTP/2 PRIORITY_UPDATE Frame

The HTTP/2 PRIORITY_UPDATE frame (type=0xF) carries the stream ID of
the response that is being reprioritized, and the updated priority in
ASCII text, using the same representation as that of the Priority
header field value.

The Stream Identifier field ([RFC7540], Section 4.1) in the
PRIORITY_UPDATE frame header MUST be zero (0x0).

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
+---------------------------------------------------------------+
|R| Stream ID (31) |
+---------------------------------------------------------------+
| Priority Field Value (*) ...
+---------------------------------------------------------------+

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
PRIORITY_UPDATE on wrong stream, a PRIORITY_UPDATE with an invalid
ID, etc.

5.2. HTTP/3 PRIORITY_UPDATE Frame

The HTTP/3 PRIORITY_UPDATE frame (type=0xF) carries the identifier of
the element that is being reprioritized, and the updated priority in
ASCII text, using the same representation as that of the Priority
header field value.

The PRIORITY_UPDATE frame MUST be sent on the control stream
([I-D.ietf-quic-http], Section 6.2.1).

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T| Empty | Prioritized Element ID (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Priority Field Value (*) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: HTTP/3 PRIORITY_UPDATE Frame Payload

The PRIORITY_UPDATE frame payload has the following fields:

T (Prioritized Element Type): A one-bit field indicating the type of
element being prioritized. A value of 0 indicates a
reprioritization for a Request Stream, so the Prioritized Element
ID is interpreted as a Stream ID. A value of 1 indicates a
reprioritization for a Push stream, so the Prioritized Element ID
is interpreted as a Push ID.

Empty: A seven-bit field that has no semantic value.

Prioritized Element ID: The stream ID or push ID that is the target
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
PRIORITY_UPDATE on wrong stream, a PRIORITY_UPDATE with an invalid
ID, etc.

6. Merging Client- and Server-Driven Parameters

It is not always the case that the client has the best understanding
of how the HTTP responses deserve to be prioritized. For example,
use of an HTML document might depend heavily on one of the inline
images. Existence of such dependencies is typically best known to
the server.

By using the "Priority" response header, a server can override the
prioritization hints provided by the client. When used, the
parameters found in the response header field overrides those
specified by the client.

For example, when the client sends an HTTP request with
:method = GET
:scheme = https
:authority = example.net
:path = /menu.png
priority = urgency=3, progressive=?1 u=4, i=?1

and the origin responds with

:status = 200
content-type = image/png
priority = urgency=1 u=2

the intermediary's understanding of the urgency is promoted from "3" "4"
to "1", "2", because the server-provided value overrides the value
provided by the client. The progressiveness incremental value continues to be "1",
the value specified by the client, as the server did not specify the
"progressive"
incremental("i") parameter.

7. Security Considerations

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
clients into one HTTP/2 or HTTP/3 connection going to the backend
server, requests that originate from one client might have higher
precedence than those coming from others.

It is sometimes beneficial for the server running behind an
intermediary to obey to the value of the Priority header field. As
an example, a resource-constrained server might defer the
transmission of software update files that would have the background
urgency being associated. However, in the worst case, the asymmetry
between the precedence declared by multiple clients might cause
responses going to one end client to be delayed totally after those
going to another.

In order to mitigate this fairness problem, when a server responds to
a request that is known to have come through an intermediary, the
server SHOULD prioritize the response as if it was assigned the
priority of "urgency=0, progressive=?1" "u=1, i=?1" (i.e. round-robin) regardless of the value of
the Priority header field being transmitted, unless the server has knows
the knowledge that no intermediaries are intermediary is not coalescing 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
configuration, or by consulting if that request contains one of the
following header fields:

o CDN-Loop ([RFC8586])

o Forwarded, X-Forwarded-For ([RFC7239])

o Via ([RFC7230], Section 5.7.1)

Responding to requests coming through an intermediary in a round-
robin manner works well when the network bottleneck exists between
the intermediary and the end client, as the intermediary would be
buffering the responses and then be forwarding the chunks of those
buffered responses based on the prioritization scheme it implements.
A sophisticated server MAY use a weighted round-robin reflecting the
urgencies expressed in the requests, so that less urgent responses
would receive less bandwidth in case the bottleneck exists between
the server and the intermediary.

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

8.1. Why use an End-to-End Header Field?

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.

The rationale is that the Priority header field transmits how each
response affects the client's processing of those responses, rather
than how relatively urgent each response is to others. The way a
client processes a response is a property associated to that client
generating that request. Not that of an intermediary. Therefore, it
is an end-to-end property. How these end-to-end properties carried
by the Priority header field affect the prioritization between the
responses that share a connection is a hop-by-hop issue.

Having the Priority header field defined as end-to-end is important
for caching intermediaries. Such intermediaries can cache the value
of the Priority header field along with the response, and utilize the
value of the cached header field when serving the cached response,
only because the header field is defined as end-to-end rather than
hop-by-hop.

It should also be noted that the use of a header field carrying a
textual value makes the prioritization scheme extensible; see the
discussion below.

8.2. Why do Urgencies Have Meanings?

One of the aims of this specification is to define a mechanism for
merging client- and server-provided hints for prioritizing the
responses. For that to work, each urgency level needs to have a
well-defined meaning. As an example, a server can assign the highest
precedence among the supplementary responses to an HTTP response
carrying an icon, because the meaning of "urgency=1" "u=2" is shared among the
endpoints.

This specification restricts itself to defining a minimum set of
urgency levels in order to provide sufficient granularity for
prioritizing responses for ordinary web browsing, at minimal
complexity.

However, that does not mean that the prioritization scheme would
forever be stuck to the eight levels. The design provides
extensibility. If deemed necessary, it would be possible to
subdivide any of the eight urgency levels that are currently defined.
Or, a graphical user-agent could send a "visible" parameter to
indicate if the resource being requested is within the viewport.

A server can combine the hints provided in the Priority header field
with other information in order to improve the prioritization of
responses. For example, a server that receives requests for a font
[RFC8081] and images with the same urgency might give higher
precedence to the font, so that a visual client can render textual
information at an early moment.

8.3. Can an Intermediary Send its own Signal?

There might be a benefit in recommending a coalescing intermediary to
embed its own prioritization hints into the HTTP request that it
forwards to the backend server, as otherwise the Priority header
field would not be as helpful to the backend (see Section 7.1).

One way of achieving that, without dropping the original signal,
would be to let the intermediary express its own signal using the
Priority header field, at the same time transplanting the original
value to a different header field.

As an example, when a client sends an HTTP request carrying a
priority of "urgency=-1" "u=0" and the intermediary wants to instead associate "urgency=0; progressive=?1",
"u=1; i=?1", the intermediary would send a HTTP request that contains to
the following two header fields to the backend server:

priority = urgency=0; progressive=?1 u=1; i=?1
original-priority = urgency=-1 u=0

9. IANA Considerations

This specification registers the following entry in the Permanent
Message Header Field Names registry established by [RFC3864]:

Header field name: Priority

Applicable protocol: http

Status: standard

Author/change controller: IETF

Specification document(s): This document

Related information: n/a

This specification registers the following entry in the HTTP/2
Settings registry established by [RFC7540]:

Name: SETTINGS_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 SETTINGS_DEPRECATE_HTTP2_PRIORITIES

Code: 0x9

Initial value: 0

Specification: This document

This specification registers the following entry in the HTTP/2 Frame
Type registry established by [RFC7540]:

Frame Type: PRIORITY_UPDATE

Code: 0xF

Specification: This document

This specification registers the following entries in the HTTP/3
Frame Type registry established by [I-D.ietf-quic-http]:

Frame Type: PRIORITY_UPDATE

Code: 0xF

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

10.1. Normative References

[I-D.ietf-quic-http]
Bishop, M., "Hypertext Transfer Protocol Version 3
(HTTP/3)", draft-ietf-quic-http-23 (work in progress),
September 2019.

[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-23 (work
in progress), September 2019.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<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
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/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>.

[STRUCTURED-HEADERS]
Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-14 (work in progress),
October 2019.

10.2. Informative References

[I-D.lassey-priority-setting]
Lassey, B. and L. Pardue, "Declaring Support for HTTP/2
Priorities", draft-lassey-priority-setting-00 (work in
progress), July 2019.

[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<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",
RFC 7239, DOI 10.17487/RFC7239, June 2014,
<https://www.rfc-editor.org/info/rfc7239>.

[RFC8081] Lilley, C., "The "font" Top-Level Media Type", RFC 8081,
DOI 10.17487/RFC8081, February 2017,
<https://www.rfc-editor.org/info/rfc8081>.

[RFC8586] Ludin, S., Nottingham, M., and N. Sullivan, "Loop
Detection in Content Delivery Networks (CDNs)", RFC 8586,
DOI 10.17487/RFC8586, April 2019,
<https://www.rfc-editor.org/info/rfc8586>.

10.3. URIs

[1] http://tools.ietf.org/agenda/83/slides/slides-83-httpbis-5.pdf

[2] https://github.com/pmeenan/http3-prioritization-proposal

Appendix A. Acknowledgements

Roy Fielding presented the idea of using a header field for
representing priorities in http://tools.ietf.org/agenda/83/slides/
slides-83-httpbis-5.pdf [1]. In https://github.com/pmeenan/http3-
prioritization-proposal [2], Patrick Meenan advocates for
representing the priorities using a tuple of urgency and concurrency.
The negotiation scheme described in this document ability to deprecate HTTP/2 priortization is based on
[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
drawn from discussion within the broad HTTP community. Special
thanks to Roberto Peon, Martin Thomson and Netflix for text that was
incorporated explicitly in this document.

In addition to the people above, this document owes a lot to the
extensive discussion in the HTTP priority design team, consisting of
Alan Frindell, Andrew Galloni, Craig Taylor, Ian Swett, Kazuho Oku,
Lucas Pardue, Matthew Cox, Mike Bishop, Roberto Peon, Robin Marx, Roy
Fielding.

Appendix B. Change Log
B.1. Since draft-kazuho-httpbis-priority-03

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 Define SETTINGS_PRIORITIES for negotiation (#58, #69)

o Define PRIORITY_UPDATE frame for HTTP/2 and HTTP/3 (#51)

o Explain fairness issue and mitigations (#56)

B.2.

B.3. Since draft-kazuho-httpbis-priority-01

o Explain how reprioritization might be supported.

B.3.

B.4. Since draft-kazuho-httpbis-priority-00

o Expand urgency levels from 3 to 8.

Authors' Addresses

Kazuho Oku
Fastly

Email: kazuhooku@gmail.com

Lucas Pardue
Cloudflare

Email: lucaspardue.24.7@gmail.com