wdiff draft-kazuho-httpbis-priority-02.txt draft-kazuho-httpbis-priority-03.txt

HTTP K. Oku
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July 23,
November 05, 2019

The Priority

Extensible Prioritization Scheme for HTTP Header Field
draft-kazuho-httpbis-priority-02
draft-kazuho-httpbis-priority-03

Abstract

This document describes the Priority a scheme for prioritizing HTTP header field. responses.
This header
field can be used by endpoints to specify scheme expresses the absolute precedence priority of
an 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 way. 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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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. Motivation for Replacing HTTP/2 Priorities . . . . . . . . . 4
3. Negotiating Priorities . . . . . . . . . . . . . . . . . . . 5
3.1. The SETTINGS_PRIORITIES SETTINGS Parameter . . . . . . . 5
3.2. Defined Prioritization Scheme Values . . . . . . . . . . 6
3.2.1. H2_TREE . . . . . . . . . . . . . . . . . . . . . . . 6
3.2.2. URGENCY . . . . . . . . . . . . . . . . . . . . . . . 7
4. The Priority HTTP Header Field . . . . . . . . . . . . . . . 4
2.1. 7
4.1. urgency . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1. 7
4.1.1. prerequisite . . . . . . . . . . . . . . . . . . . . 5
2.1.2. 8
4.1.2. default . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.3. 8
4.1.3. supplementary . . . . . . . . . . . . . . . . . . . . 5
2.1.4. 8
4.1.4. background . . . . . . . . . . . . . . . . . . . . . 6
2.2. 9
4.2. progressive . . . . . . . . . . . . . . . . . . . . . . . 6
3. Merging Client- and Server-Driven Parameters 9
5. Reprioritization . . . . . . . . 7
4. Coexistence with . . . . . . . . . . . . . . 10
5.1. HTTP/2 Priorities PRIORITY_UPDATE Frame . . . . . . . . . . . . . 7
4.1. The SETTINGS_HEADER_BASED_PRIORITY SETTINGS Parameter . 11
5.2. HTTP/3 PRIORITY_UPDATE Frame . 8
5. Considerations . . . . . . . . . . . . . 11
6. Merging Client- and Server-Driven Parameters . . . . . . . . 12
7. Security Considerations . . 8
5.1. Why use an End-to-End Header Field? . . . . . . . . . . . 8
5.2. Why do Urgencies Have Meanings? . . . . . . 13
7.1. Fairness and Coalescing Intermediaries . . . . . . . 9
5.3. Reprioritization . . 13
8. Considerations . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . 14
8.1. Why use an End-to-End Header Field? . . . . . . . . . . . 14
8.2. Why do Urgencies Have Meanings? . . . 10
7. . . . . . . . . . . 14
8.3. Can an Intermediary Send its own Signal? . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. 15
9.1. HTTP Prioritization Scheme Registry . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. 17
10.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. 17
10.2. Informative References . . . . . . . . . . . . . . . . . 11 18
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 12 18
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 12 19
B.1. Since draft-kazuho-httpbis-priority-01 draft-kazuho-httpbis-priority-02 . . . . . . . . . 12 19
B.2. Since draft-kazuho-httpbis-priority-01 . . . . . . . . . 19
B.3. Since draft-kazuho-httpbis-priority-00 . . . . . . . . . 12 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 19

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 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 shortcomings:

o Its complexity has led been
observed to varying levels of support 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
clients 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 servers.

o It 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 hard 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 coordinate with 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 prioritization. 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. But
with the HTTP/2 prioritization scheme, Since client
trees vary, it is impossible for the server to determine how such
images should be prioritized against other responses that use the client-driven prioritization tree,
because every client builds the responses.

The HTTP/2 prioritization scheme allows intermediaries to coalesce multiple client
trees into a single tree in that is used for a
different way.

o It 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 design 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 ported cleanly to used in HTTP/3
([I-D.ietf-quic-http]). One of
[I-D.ietf-quic-http] without changing the primary goals signal and its processing.

Considering the problems with deployment and adaptability to HTTP/3,
retaining the HTTP/2 priority scheme increases the complexity of HTTP/3 is 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
minimize head-of-line blocking. Transmitting the evolving
representation more complex
HTTP/2 setups seen in practice, at least for the web use case.

The problems and insights laid out above are motivation for the
alternative and more straightforward prioritization scheme presented
in this document. In order to support deployment of new schemes, a "prioritization tree" from
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 client 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 requires head-of-line blocking.

Based on these observations, this 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 Priority HTTP
header field that can 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 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
specify know what information from the precedence client is of HTTP responses the most value.

Once the negotiation is complete, endpoints MAY stop sending hop-by-
hop prioritization signals that were not negotiated in a standardized,
extensible, protocol-version- independent, end-to-end format. 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
header-based document defines two prioritization scheme can act as a substitute values for use with
the
HTTP/2 frame-based prioritization SETTINGS_PRIORITIES setting.

3.2.1. H2_TREE

This document defines the priority scheme (see identifier H2_TREE (8-bit
value of 1) that indicates support for HTTP/2-style priorities
([RFC7540], Section 4).

1.1. Notational Conventions 5.3).

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" H2_TREE priority scheme identifier MUST NOT be be sent in an
HTTP/3 settings because there is no defined mapping of this
document are to be interpreted scheme.
Endpoints MUST treat receipt of H2_TREE as described 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 in [RFC2119].

The terms sh-token the present document.

An intermediary connecting to a backend server SHOULD declare support
for the extensible priority scheme when and sh-boolean only when all the
requests that are imported to be sent on that backend connection originates
from
[I-D.ietf-httpbis-header-structure].

Example HTTP requests and responses one client-side connection that has negotiated the use of the HTTP/2-style formatting
from [RFC7540].

2.
extensible priority scheme (see Section 7.1).

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 3). 6).

The value of the Priority header field is a Structured Headers
[I-D.ietf-httpbis-header-structure] Dictionary.
Dictionary ([STRUCTURED-HEADERS]). Each dictionary member represents
a parameter of the Priority header field. This document defines the
"urgency" and "progressive" parameters. Values of these parameters
MUST always be present. 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 MUST be ignored.

2.1.

4.1. urgency

The "urgency" parameter takes an integer between -1 and 6 as shown
below:

+-----------------+-------------------------------+
| Urgency | Definition |
+-----------------+-------------------------------+
| -1 | prerequisite (Section 2.1.1) 4.1.1) |
| 0 | default (Section 2.1.2) 4.1.2) |
| between 1 and 5 | supplementary (Section 2.1.3) 4.1.3) |
| 6 | background (Section 2.1.4) 4.1.4) |
+-----------------+-------------------------------+

Table 1: Urgencies

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

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":

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

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

2.1.1.

4.1.1. prerequisite

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

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.

2.1.2.

4.1.2. default

The default urgency (value 0) 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.

2.1.3.

4.1.3. supplementary

The supplementary urgency indicates 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 and 5 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 3 6 explains how these values might be used.

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

Clients MAY use values 2 to indicate that a request is given
relatively high priority, or 4 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 instead of 3, 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, 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.

2.1.4.

4.1.4. background

The background urgency (value 6) 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.

2.2.

4.2. progressive

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

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

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

A server SHOULD transmit non-progressive 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" and the progressive parameter set to
"1".

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

5. Reprioritization

Once a client sends a request, circumstances might change and mean
that it is 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" (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" (prerequisite).

However, a client cannot reprioritize a response by using the
Priority header field. This is because an HTTP header field can only
be sent as part of an HTTP message. Therefore, to support
reprioritization, it is necessary to define a HTTP-version-dependent
mechanism for transmitting the priority parameters.

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
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 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

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

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.

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

and the origin responds with

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

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

4. Coexistence with HTTP/2 Priorities

Standard HTTP/2 ([RFC7540]) endpoints use frame-based prioritization,
whereby a client sends priority information in dedicated fields
present in HEADERS

7. Security Considerations

7.1. Fairness and PRIORITY frames. A 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 instead
choose have higher
precedence than those coming from others.

It is sometimes beneficial for the server running behind an
intermediary to use header-based prioritization as specified in this
document.

4.1. The SETTINGS_HEADER_BASED_PRIORITY SETTINGS Parameter

To improve communication obey to the value of the client's intended prioritization
scheme, this document specifies Priority header field. As
an example, a new HTTP/2 SETTINGS parameter with resource-constrained server might defer the name "SETTINGS_HEADER_BASED_PRIORITY". The value
transmission of software update files that would have the
parameter MUST be 0 or 1; the initial value is 0. Frame-based
prioritization is respected when background
urgency being associated. However, in the value is 0, or when worst case, the server
does not recognize asymmetry
between the setting.

An HTTP/2 precedence declared by multiple clients might cause
responses going to one end client that uses header-based priority SHOULD send a
"SETTINGS_HEADER_BASED_PRIORITY" parameter with a value of 1 to be delayed totally after those
going to another.

In order to mitigate this fairness problem, when
connecting a server responds to
a server.

An intermediary request that is known to have come through an intermediary, the
server SHOULD send a "SETTINGS_HEADER_BASED_PRIORITY"
parameter with a prioritize the response as if it was assigned the
priority of "urgency=0, progressive=?1" (i.e. round-robin) regardless
of the value of 1 for a connection it establishes when, and
only when, all the requests to be sent over Priority header field being transmitted, unless
the server has the knowledge that connection originate no intermediaries are coalescing
requests from a client that utilizes this header-based prioritization scheme.
Otherwise this settings parameter SHOULD multiple clients. That can be set to 0. determined by the
settings when the intermediaries support this specification (see
Section 3.2.2), or else through configuration.

A client 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 MUST NOT send in a
"SETTINGS_HEADER_BASED_PRIORITY" parameter with round-
robin manner works well when the value of 0 after
previously sending a value 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 1. those
buffered responses based on the prioritization scheme it implements.
A sophisticated server MUST NOT send a "SETTINGS_HEADER_BASED_PRIORITY" parameter.
Upon receipt, MAY use a client weighted round-robin reflecting the
urgencies expressed in the requests, so that supports header-based prioritization MUST
close less urgent responses
would receive less bandwidth in case the connection with a protocol error. Non-supporting clients
will ignore this extension element (see [RFC7540], Section 5.5).

5. bottleneck exists between
the server and the intermediary.

8. Considerations

5.1.

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.

5.2.

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" 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.

5.3. Reprioritization

Once

8.3. Can an Intermediary Send its own Signal?

There might be a client sends benefit in recommending a request, coalescing intermediary to
embed its own prioritization hints into the HTTP request that it cannot reprioritize
forwards to the
corresponding response by using backend server, as otherwise the Priority header field. This is
because an HTTP header
field can only would not be sent as part of an HTTP
message.

Therefore, to support reprioritization, it is necessary helpful to define a
HTTP-version-dependent mechanism for transmitting the priority
parameters. backend (see Section 7.1).

One approach that we can use in HTTP/2 ([RFC7540]) is to use a frame
that carries way of achieving that, without dropping the priority parameters.

Figure 1: Reprioritization frame payload

The Reprioritization frame original signal,
would be sent on stream 0. This frame
carries the stream ID of the response that is being reprioritized,
and to let the updated priority in ASCII text, intermediary express its own signal using the
Priority header field, at the same
represententation as that of time transplanting the Priority original
value to a different header field value. field.

As an example, when a web browser might issue a prefetch request for client sends an
HTML on stream 31, with the urgency parameter of the Priority HTTP request
header field set to "background". Then, when carrying a
priority of "urgency=-1" and the user navigates intermediary wants to instead
associate "urgency=0; progressive=?1", the HTML while prefetch is in action, it intermediary would send a
reprioritization frame with
HTTP request that contains to the stream ID set following two header fields to 31, and the
backend server:

priority
field value set to "urgency=0".

6. Security Considerations

TBD

7. = urgency=0; progressive=?1
original-priority = urgency=-1

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_HEADER_BASED_PRIORITY: SETTINGS_PRIORITIES

Code: 0xTBD 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

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

8.1.

10.1. Normative References

[I-D.ietf-httpbis-header-structure]
Nottingham,

[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 P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-11 Secure Transport", draft-ietf-quic-transport-23 (work
in progress),
July 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>.

8.2.

[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.ietf-quic-http]
Bishop, M., "Hypertext Transfer Protocol Version 3
(HTTP/3)", draft-ietf-quic-http-22

[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>.

[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>.

8.3.

[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.

Many

The negotiation scheme described in this document is based on
[I-D.lassey-priority-setting], authored by Brad Lassey and Lucas
Pardue.

The motivation for defining an alternative to HTTP/2 priorities is
drawn from discussion within the broad HTTP community. Special
thanks to Robin Marx, Patrick Meenan Roberto Peon, Martin Thomson and Ian Swett Netflix for their
feedback. 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-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. Since draft-kazuho-httpbis-priority-01

o Explain how reprioritization might be supported.

B.2.

B.3. 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