Extensible Prioritization Scheme for HTTP
draft-ietf-httpbis-priority-07
The information below is for an old version of the document.
| Document | Type |
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| Authors | Kazuho Oku , Lucas Pardue | ||
| Last updated | 2021-10-25 | ||
| Replaces | draft-kazuho-httpbis-priority | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-httpbis-priority-07
HTTP K. Oku
Internet-Draft Fastly
Intended status: Standards Track L. Pardue
Expires: 28 April 2022 Cloudflare
25 October 2021
Extensible Prioritization Scheme for HTTP
draft-ietf-httpbis-priority-07
Abstract
This document describes a scheme that allows an HTTP client to
communicate its preferences for how the upstream server prioritizes
responses to its requests, and also allows a server to hint to a
downstream intermediary how its responses should be prioritized when
they are forwarded. 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
responses. These share a common format structure that is designed to
provide future extensibility.
Note to Readers
_RFC EDITOR: please remove this section before publication_
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/
(https://lists.w3.org/Archives/Public/ietf-http-wg/).
Working Group information can be found at https://httpwg.org/
(https://httpwg.org/); source code and issues list for this draft can
be found at https://github.com/httpwg/http-extensions/labels/
priorities (https://github.com/httpwg/http-extensions/labels/
priorities).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on 28 April 2022.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4
2. Motivation for Replacing RFC 7540 Priorities . . . . . . . . 5
2.1. Disabling RFC 7540 Priorities . . . . . . . . . . . . . . 6
2.1.1. Advice when Using Extensible Priorities as the
Alternative . . . . . . . . . . . . . . . . . . . . . 7
3. Applicability of the Extensible Priority Scheme . . . . . . . 7
4. Priority Parameters . . . . . . . . . . . . . . . . . . . . . 8
4.1. Urgency . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Incremental . . . . . . . . . . . . . . . . . . . . . . . 9
4.3. Defining New Parameters . . . . . . . . . . . . . . . . . 10
4.3.1. Registration . . . . . . . . . . . . . . . . . . . . 10
5. The Priority HTTP Header Field . . . . . . . . . . . . . . . 11
6. Reprioritization . . . . . . . . . . . . . . . . . . . . . . 12
7. The PRIORITY_UPDATE Frame . . . . . . . . . . . . . . . . . . 12
7.1. HTTP/2 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 13
7.2. HTTP/3 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 14
8. Merging Client- and Server-Driven Parameters . . . . . . . . 15
9. Client Scheduling . . . . . . . . . . . . . . . . . . . . . . 16
10. Server Scheduling . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Intermediaries with Multiple Backend Connections . . . . 18
11. Scheduling and the CONNECT Method . . . . . . . . . . . . . . 19
12. Retransmission Scheduling . . . . . . . . . . . . . . . . . . 19
13. Fairness . . . . . . . . . . . . . . . . . . . . . . . . . . 20
13.1. Coalescing Intermediaries . . . . . . . . . . . . . . . 20
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13.2. HTTP/1.x Back Ends . . . . . . . . . . . . . . . . . . . 21
13.3. Intentional Introduction of Unfairness . . . . . . . . . 21
14. Why use an End-to-End Header Field? . . . . . . . . . . . . . 21
15. Security Considerations . . . . . . . . . . . . . . . . . . . 22
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
17.1. Normative References . . . . . . . . . . . . . . . . . . 23
17.2. Informative References . . . . . . . . . . . . . . . . . 24
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 25
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 25
B.1. Since draft-ietf-httpbis-priority-06 . . . . . . . . . . 25
B.2. Since draft-ietf-httpbis-priority-05 . . . . . . . . . . 26
B.3. Since draft-ietf-httpbis-priority-04 . . . . . . . . . . 26
B.4. Since draft-ietf-httpbis-priority-03 . . . . . . . . . . 26
B.5. Since draft-ietf-httpbis-priority-02 . . . . . . . . . . 26
B.6. Since draft-ietf-httpbis-priority-01 . . . . . . . . . . 26
B.7. Since draft-ietf-httpbis-priority-00 . . . . . . . . . . 27
B.8. Since draft-kazuho-httpbis-priority-04 . . . . . . . . . 27
B.9. Since draft-kazuho-httpbis-priority-03 . . . . . . . . . 27
B.10. Since draft-kazuho-httpbis-priority-02 . . . . . . . . . 27
B.11. Since draft-kazuho-httpbis-priority-01 . . . . . . . . . 27
B.12. Since draft-kazuho-httpbis-priority-00 . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction
It is common for representations of an HTTP [HTTP] resource to have
relationships to one or more other resources. Clients will often
discover these relationships while processing a retrieved
representation, which may lead to further retrieval requests.
Meanwhile, the nature of the relationship determines whether the
client is blocked from continuing to process locally available
resources. An example of this is visual rendering of an HTML
document, which 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 incrementally as the chunks of the images
arrive.
HTTP/2 [HTTP2] and HTTP/3 [HTTP3] support multiplexing of requests
and responses in a single connection. An important feature of any
implementation of a protocol that provides multiplexing is the
ability to prioritize the sending of information. For example, to
provide meaningful presentation of an HTML 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 to a
client.
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A server that operates in ignorance of how clients issue requests and
consume responses can cause suboptimal client application
performance. Priority signals allow clients to communicate their
view of request priority. Servers have their own needs that are
independent from client needs, so they often combine priority signals
with other available information in order to inform scheduling of
response data.
RFC 7540 [RFC7540] stream priority allowed a client to send a series
of priority signals that communicate to the server a "priority tree";
the structure of this tree represents the client's preferred relative
ordering and weighted distribution of the bandwidth among HTTP
responses. Servers could use these priority signals as input into
prioritization decision making.
The design and implementation of RFC 7540 stream priority was
observed to have shortcomings, explained in Section 2. HTTP/2
[HTTP2] has consequently deprecated the use of these stream priority
signals.
This document describes an extensible scheme for prioritizing HTTP
responses that uses absolute values. Section 4 defines priority
parameters, which are a standardized and extensible format of
priority information. Section 5 defines the Priority HTTP header
field, a protocol-version-independent and end-to-end priority signal.
Clients can use this header to signal priority to servers in order to
specify the precedence of HTTP responses. Similarly, servers behind
an intermediary can use it to signal priority to the intermediary.
Section 7.1 and Section 7.2 define version-specific frames that carry
parameters, which clients can use for reprioritization.
Header field and frame priority signals are input to a server's
response prioritization process. They are only a suggestion and do
not guarantee any particular processing or transmission order for one
response relative to any other response. Section 10 and Section 12
provide consideration and guidance about how servers might act upon
signals.
The prioritization scheme and priority signals defined herein can act
as a substitute for RFC 7540 stream priority.
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].
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The terms Dictionary, sf-boolean, sf-dictionary, and sf-integer are
imported from [STRUCTURED-FIELDS].
Example HTTP requests and responses use the HTTP/2-style formatting
from [HTTP2].
This document uses the variable-length integer encoding from [QUIC].
The term control stream is used to describe the HTTP/2 stream with
identifier 0x0, and HTTP/3 control stream; see Section 6.2.1 of
[HTTP3].
The term HTTP/2 priority signal is used to describe the priority
information sent from clients to servers in HTTP/2 frames; see
Section 5.3.2 of [HTTP2].
2. Motivation for Replacing RFC 7540 Priorities
RFC 7540 stream priority (see Section 5.3 of [RFC7540]) is a complex
system where clients signal stream dependencies and weights to
describe an unbalanced tree. It suffered from limited deployment and
interoperability and was deprecated in a revision of HTTP/2 [HTTP2].
However, in order to maintain wire compatibility, HTTP/2 priority
signals are still mandatory to handle (see Section 5.3.2 of [HTTP2]).
Clients can build RFC 7540 trees with rich flexibility but experience
has shown this is rarely exercised. Instead they tend to choose a
single model optimized for a single use case and experiment within
the model constraints, or do nothing at all. Furthermore, many
clients build their prioritization tree in a unique way, which makes
it difficult for servers to understand their intent and act or
intervene accordingly.
Many RFC 7540 server implementations do not act on HTTP/2 priority
signals. Some instead favor custom server-driven schemes based on
heuristics or other hints, such as resource content type or request
generation order. For example, a server, with knowledge of an HTML
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.
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RFC 7540 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. Additionally, RFC
7540 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.
RFC 7540 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 RFC 7540 implementations. One attack, [CVE-2019-9513] aka
"Resource Loop", is based on using priority signals to manipulate the
server's stored prioritization state.
HTTP/2 priority associated with an HTTP request is signalled as a
value relative to those of other requests sharing the same HTTP/2
connection. Therefore, in order to prioritize requests, endpoints
are compelled to have the knowledge of the underlying HTTP version
and how the requests are coalesced. This has been a burden to HTTP
endpoints that generate or forward requests in a version-agnostic
manner.
HTTP/2 priority signals are required to be delivered and processed in
the order they are sent so that the receiver handling is
deterministic. Porting HTTP/2 priority signals to protocols that do
not provide ordering guarantees presents challenges. For example,
HTTP/3 [HTTP3] lacks global ordering across streams that would carry
priority signals. Early attempts to port HTTP/2 priority signals to
HTTP/3 required adding additional information to the signals, leading
to more complicated processing. Problems found with this approach
could not be resolved and definition of a HTTP/3 priority signalling
feature was removed before publication.
Considering the deployment problems and the design restrictions of
RFC 7540 stream priority, as well as the difficulties in adapting it
to HTTP/3, continuing to base prioritization on this mechanism risks
increasing the complexity of systems. Multiple experiments from
independent research have shown that simpler schemes can reach at
least equivalent performance characteristics compared to the more
complex RFC 7540 setups seen in practice, at least for the web use
case.
2.1. Disabling RFC 7540 Priorities
The problems and insights set out above provided the motivation for
deprecating RFC 7540 stream priority (see Section 5.3 of [RFC7540]).
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The SETTINGS_NO_RFC7540_PRIORITIES HTTP/2 setting is defined by this
document in order to allow endpoints to explicitly opt out of using
HTTP/2 priority signals (see Section 5.3.2 of [HTTP2]). Endpoints
are encouraged to use alternative priority signals (for example,
Section 5 or Section 7.1) but there is no requirement to use a
specific signal type.
The value of SETTINGS_NO_RFC7540_PRIORITIES MUST be 0 or 1. Any
value other than 0 or 1 MUST be treated as a connection error (see
Section 5.4.1 of [HTTP2]) of type PROTOCOL_ERROR. The initial value
is 0.
Endpoints MUST send this SETTINGS parameter as part of the first
SETTINGS frame. A sender MUST NOT change the
SETTINGS_NO_RFC7540_PRIORITIES parameter value after the first
SETTINGS frame. Detection of a change by a receiver MUST be treated
as a connection error of type PROTOCOL_ERROR.
The SETTINGS frame precedes any HTTP/2 priority signal sent from a
client, so a server can determine if it needs to allocate any
resource to signal handling before they arrive. A server that
receives SETTINGS_NO_RFC7540_PRIORITIES with value of 1 MUST ignore
HTTP/2 priority signals.
2.1.1. Advice when Using Extensible Priorities as the Alternative
Until the client receives the SETTINGS frame from the server, the
client SHOULD send both the HTTP/2 priority signals and the signals
of this prioritization scheme (see Section 5 and Section 7.1). When
the client receives the first SETTINGS frame that contains the
SETTINGS_NO_RFC7540_PRIORITIES parameter with value of 1, it SHOULD
stop sending the HTTP/2 priority signals. If the value was 0 or if
the settings parameter was absent, it SHOULD stop sending
PRIORITY_UPDATE frames (Section 7.1), but MAY continue sending the
Priority header field (Section 5), 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. Applicability of the Extensible Priority Scheme
The priority scheme defined by this document considers only the
prioritization of HTTP messages and tunnels, see Section 9,
Section 10, and Section 11.
Where HTTP extensions change stream behavior or define new data
carriage mechanisms, they can also define how this priority scheme
can be applied.
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4. Priority Parameters
The priority information is a sequence of key-value pairs, providing
room for future extensions. Each key-value pair represents a
priority parameter.
The Priority HTTP header field (Section 5) is an end-to-end way to
transmit this set of parameters when a request or a response is
issued. In order to reprioritize a request, HTTP-version-specific
PRIORITY_UPDATE frames (Section 7.1 and Section 7.2) are used by
clients to transmit the same information on a single hop. If
intermediaries want to specify prioritization on a multiplexed HTTP
connection, they SHOULD use a PRIORITY_UPDATE frame and SHOULD NOT
change the Priority header field.
In both cases, the set of priority parameters is encoded as a
Structured Fields Dictionary (see Section 3.2 of
[STRUCTURED-FIELDS]).
This document defines the urgency(u) and incremental(i) parameters.
When receiving an HTTP request that does not carry these priority
parameters, a server SHOULD act as if their default values were
specified. Note that handling of omitted parameters is different
when processing an HTTP response; see Section 8.
Receivers parse the Dictionary as defined in Section 4.2 of
[STRUCTURED-FIELDS]. Where the Dictionary is successfully parsed,
this document places the additional requirement that unknown priority
parameters, parameters with out-of-range values, or values of
unexpected types MUST be ignored.
4.1. Urgency
The urgency parameter (u) takes an integer between 0 and 7, in
descending order of priority.
The value is encoded as an sf-integer. The default value is 3.
Endpoints use this parameter to communicate their view of the
precedence of HTTP responses. The chosen value of urgency can be
based on the expectation that servers might use this information to
transmit HTTP responses in the order of their urgency. The smaller
the value, the higher the precedence.
The following example shows a request for a CSS file with the urgency
set to 0:
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:method = GET
:scheme = https
:authority = example.net
:path = /style.css
priority = u=0
A client that fetches a document that likely consists of multiple
HTTP resources (e.g., HTML) SHOULD assign the default urgency level
to the main resource. This convention allows servers to refine the
urgency using knowledge specific to the web-site (see Section 8).
The lowest urgency level (7) is reserved for background tasks such as
delivery of software updates. This urgency level SHOULD NOT be used
for fetching responses that have impact on user interaction.
4.2. Incremental
The incremental parameter (i) takes an sf-boolean as the value that
indicates if an HTTP response can be processed incrementally, i.e.,
provide some meaningful output as chunks of the response arrive.
The default value of the incremental parameter is false (0).
If a client makes concurrent requests with the incremental parameter
set to false, there is no benefit serving responses with the same
urgency concurrently because the client is not going to process those
responses incrementally. Serving non-incremental responses with the
same urgency one by one, in the order in which those requests were
generated is considered to be the best strategy.
If a client makes concurrent requests with the incremental parameter
set to true, serving requests with the same urgency concurrently
might be beneficial. Doing this distributes the connection
bandwidth, meaning that responses take longer to complete.
Incremental delivery is most useful where multiple partial responses
might provide some value to clients ahead of a complete response
being available.
The following example shows a request for a JPEG file with the
urgency parameter set to 5 and the incremental parameter set to true.
:method = GET
:scheme = https
:authority = example.net
:path = /image.jpg
priority = u=5, i
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4.3. Defining New Parameters
When attempting to define new parameters, care must be taken so that
they do not adversely interfere with prioritization performed by
existing endpoints or intermediaries that do not understand the newly
defined parameter. Since unknown parameters are ignored, new
parameters should not change the interpretation of, or modify, the
urgency (see Section 4.1) or incremental (see Section 4.2) parameters
in a way that is not backwards compatible or fallback safe.
For example, if there is a need to provide more granularity than
eight urgency levels, it would be possible to subdivide the range
using an additional parameter. Implementations that do not recognize
the parameter can safely continue to use the less granular eight
levels.
Alternatively, the urgency can be augmented. For example, a
graphical user agent could send a visible parameter to indicate if
the resource being requested is within the viewport.
Generic parameters are preferred over vendor-specific, application-
specific or deployment-specific values. If a generic value cannot be
agreed upon in the community, the parameter's name should be
correspondingly specific (e.g., with a prefix that identifies the
vendor, application or deployment).
4.3.1. Registration
New Priority parameters can be defined by registering them in the
HTTP Priority Parameters Registry. The registry governs the keys
(short textual strings) used in Structured Fields Dictionary (see
Section 3.2 of [STRUCTURED-FIELDS]). Since each HTTP request can
have associated priority signals, there is value in having short key
lengths, especially single-character strings. In order to encourage
extension while avoiding unintended conflict among attractive key
values, the HTTP Priority Parameters Registry operates two
registration policies depending on key length.
* Registration requests for parameters with a key length of one use
the Specification Required policy, as per Section 4.6 of
[RFC8126].
* Registration requests for parameters with a key length greater
than one use the Expert Review policy, as per Section 4.5 of
[RFC8126]. A specification document is appreciated, but not
required.
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When reviewing registration requests, the designated expert(s) can
consider the additional guidance provided in Section 4.3 but cannot
use it as a basis for rejection.
Registration requests should use the following template:
Name: [a name for the Priority Parameter that matches key]
Description: [a description of the parameter semantics and value]
Reference: [to a specification defining this parameter]
See the registry at https://iana.org/assignments/http-priority
(https://iana.org/assignments/http-priority) for details on where to
send registration requests.
5. The Priority HTTP Header Field
The Priority HTTP header field carries priority parameters Section 4.
It can appear in requests and responses. It is an end-to-end signal
of the request priority from the client or the response priority from
the server. Section 8 describes how intermediaries can combine the
priority information from client requests and server responses to
correct or amend the precedence. Clients cannot interpret the
appearance or omission of a Priority response header as
acknowledgement that any prioritization has occurred. Guidance for
how endpoints can act on Priority header values is given in
Section 10 and Section 9.
Priority is a Dictionary (Section 3.2 of [STRUCTURED-FIELDS]):
Priority = sf-dictionary
As is the ordinary case for HTTP caching [CACHING], 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).
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6. Reprioritization
After a client sends a request, it may be beneficial to change the
priority of the response. As an example, a web browser might issue a
prefetch request for a JavaScript file with the urgency parameter of
the Priority request header field set to 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 signal with the priority field value set to u=0.
The PRIORITY_UPDATE frame (Section 7) can be used for such
reprioritization.
7. The PRIORITY_UPDATE Frame
This document specifies a new PRIORITY_UPDATE frame for HTTP/2
[HTTP2] and HTTP/3 [HTTP3]. It carries priority parameters and
references the target of the prioritization based on a version-
specific identifier. In HTTP/2, this identifier is the Stream ID; in
HTTP/3, the identifier is either the Stream ID or Push ID. Unlike
the Priority header field, the PRIORITY_UPDATE frame is a hop-by-hop
signal.
PRIORITY_UPDATE frames are sent by clients on the control stream,
allowing them to be sent independent from the stream that carries the
response. This means they can be used to reprioritize a response or
a push stream; or signal the initial priority of a response instead
of the Priority header field.
A PRIORITY_UPDATE frame communicates a complete set of all parameters
in the Priority Field Value field. Omitting a parameter is a signal
to use the parameter's default value. Failure to parse the Priority
Field Value MUST be treated as a connection error. In HTTP/2 the
error is of type PROTOCOL_ERROR; in HTTP/3 the error is of type
H3_FRAME_ERROR.
A client MAY send a PRIORITY_UPDATE frame before the stream that it
references is open (except for HTTP/2 push streams; see Section 7.1).
Furthermore, HTTP/3 offers no guaranteed ordering across streams,
which could cause the frame to be received earlier than intended.
Either case leads to a race condition where a server receives a
PRIORITY_UPDATE frame that references a request stream that is yet to
be opened. To solve this condition, for the purposes of scheduling,
the most recently received PRIORITY_UPDATE frame can be considered as
the most up-to-date information that overrides any other signal.
Servers SHOULD buffer the most recently received PRIORITY_UPDATE
frame and apply it once the referenced stream is opened. Holding
PRIORITY_UPDATE frames for each stream requires server resources,
which can can be bound by local implementation policy. Although
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there is no limit to the number of PRIORITY_UPDATES that can be sent,
storing only the most recently received frame limits resource
commitment.
7.1. HTTP/2 PRIORITY_UPDATE Frame
The HTTP/2 PRIORITY_UPDATE frame (type=0x10) is used by clients to
signal the initial priority of a response, or to reprioritize a
response or push stream. It carries the stream ID of the response
and the priority in ASCII text, using the same representation as the
Priority header field value.
The Stream Identifier field (see Section 5.1.1 of [HTTP2]) in the
PRIORITY_UPDATE frame header MUST be zero (0x0). Receiving a
PRIORITY_UPDATE frame with a field of any other value MUST be treated
as a connection error of type PROTOCOL_ERROR.
HTTP/2 PRIORITY_UPDATE Frame {
Length (24),
Type (i) = 10,
Unused Flags (8).
Reserved (1),
Stream Identifier (31),
Reserved (1),
Prioritized Stream ID (31),
Priority Field Value (..),
}
Figure 1: HTTP/2 PRIORITY_UPDATE Frame Payload
The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
fields are described in Section 4 of [HTTP2]. The frame payload of
PRIORITY_UPDATE frame payload contains the following additional
fields:
Reserved: 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.
Prioritized 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 Fields. This is the same representation
as the Priority header field value.
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When the PRIORITY_UPDATE frame applies to a request stream, clients
SHOULD provide a Prioritized Stream ID that refers to a stream in the
"open", "half-closed (local)", or "idle" state. Servers can discard
frames where the Prioritized Stream ID refers to a stream in the
"half-closed (local)" or "closed" state. The number of streams which
have been prioritized but remain in the "idle" state plus the number
of active streams (those in the "open" or either "half-closed" state;
see Section 5.1.2 of [HTTP2]) MUST NOT exceed the value of the
SETTINGS_MAX_CONCURRENT_STREAMS parameter. Servers that receive such
a PRIORITY_UPDATE MUST respond with a connection error of type
PROTOCOL_ERROR.
When the PRIORITY_UPDATE frame applies to a push stream, clients
SHOULD provide a Prioritized Stream ID that refers to a stream in the
"reserved (remote)" or "half-closed (local)" state. Servers can
discard frames where the Prioritized Stream ID refers to a stream in
the "closed" state. Clients MUST NOT provide a Prioritized Stream ID
that refers to a push stream in the "idle" state. Servers that
receive a PRIORITY_UPDATE for a push stream in the "idle" state MUST
respond with a connection error of type PROTOCOL_ERROR.
If a PRIORITY_UPDATE frame is received with a Prioritized Stream ID
of 0x0, the recipient MUST respond with a connection error of type
PROTOCOL_ERROR.
If a client receives a PRIORITY_UPDATE frame, it MUST respond with a
connection error of type PROTOCOL_ERROR.
7.2. HTTP/3 PRIORITY_UPDATE Frame
The HTTP/3 PRIORITY_UPDATE frame (type=0xF0700 or 0xF0701) is used by
clients to signal the initial priority of a response, or to
reprioritize a response or push stream. It carries the identifier of
the element that is being prioritized, and the updated priority in
ASCII text, using the same representation as that of the Priority
header field value. PRIORITY_UPDATE with a frame type of 0xF0700 is
used for request streams, while PRIORITY_UPDATE with a frame type of
0xF0701 is used for push streams.
The PRIORITY_UPDATE frame MUST be sent on the client control stream
(see Section 6.2.1 of [HTTP3]). Receiving a PRIORITY_UPDATE frame on
a stream other than the client control stream MUST be treated as a
connection error of type H3_FRAME_UNEXPECTED.
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HTTP/3 PRIORITY_UPDATE Frame {
Type (i) = 0xF0700..0xF0701,
Length (i),
Prioritized Element ID (i),
Priority Field Value (..),
}
Figure 2: HTTP/3 PRIORITY_UPDATE Frame
The PRIORITY_UPDATE frame payload has the following fields:
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 Fields. This is the same representation
as the Priority header field value.
The request-stream variant of PRIORITY_UPDATE (type=0xF0700) MUST
reference a request stream. If a server receives a PRIORITY_UPDATE
(type=0xF0700) for a Stream ID that is not a request stream, this
MUST be treated as a connection error of type H3_ID_ERROR. The
Stream ID MUST be within the client-initiated bidirectional stream
limit. If a server receives a PRIORITY_UPDATE (type=0xF0700) with a
Stream ID that is beyond the stream limits, this SHOULD be treated as
a connection error of type H3_ID_ERROR. Generating an error is not
mandatory because HTTP/3 implementations might have practical
barriers to determining the active stream concurrency limit that is
applied by the QUIC layer.
The push-stream variant PRIORITY_UPDATE (type=0xF0701) MUST reference
a promised push stream. If a server receives a PRIORITY_UPDATE
(type=0xF0701) with a Push ID that is greater than the maximum Push
ID or which has not yet been promised, this MUST be treated as a
connection error of type H3_ID_ERROR.
PRIORITY_UPDATE frames of either type are only sent by clients. If a
client receives a PRIORITY_UPDATE frame, this MUST be treated as a
connection error of type H3_FRAME_UNEXPECTED.
8. 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. The server
might have additional information that can be combined with the
client's indicated priority in order to improve the prioritization of
the response. For example, use of an HTML document might depend
heavily on one of the inline images; existence of such dependencies
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is typically best known to the server. Or, 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.
An origin can use the Priority response header field to indicate its
view on how an HTTP response should be prioritized. An intermediary
that forwards an HTTP response can use the parameters found in the
Priority response header field, in combination with the client
Priority request header field, as input to its prioritization
process. No guidance is provided for merging priorities, this is
left as an implementation decision.
Absence of a priority parameter in an HTTP response indicates the
server's disinterest in changing the client-provided value. This is
different from the logic being defined for the request header field,
in which omission of a priority parameter implies the use of their
default values (see Section 4).
As a non-normative example, when the client sends an HTTP request
with the urgency parameter set to 5 and the incremental parameter set
to true
:method = GET
:scheme = https
:authority = example.net
:path = /menu.png
priority = u=5, i
and the origin responds with
:status = 200
content-type = image/png
priority = u=1
the intermediary might alter its understanding of the urgency from 5
to 1, because it prefers the server-provided value over the client's.
The incremental value continues to be true, the value specified by
the client, as the server did not specify the incremental(i)
parameter.
9. Client Scheduling
A client MAY use priority values to make local processing or
scheduling choices about the requests it initiates.
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10. Server Scheduling
Priority signals are input to a prioritization process. They do not
guarantee any particular processing or transmission order for one
response relative to any other response. An endpoint cannot force a
peer to process concurrent request in a particular order using
priority. Expressing priority is therefore only a suggestion.
A server can use priority signals along with other inputs to make
scheduling decisions. No guidance is provided about how this can or
should be done. Factors such as implementation choices or deployment
environment also play a role. Any given connection is likely to have
many dynamic permutations. For these reasons, there is no unilateral
perfect scheduler and this document only provides some basic
recommendations for implementations.
Clients cannot depend on particular treatment based on priority
signals. Servers can use other information to prioritize responses.
It is RECOMMENDED that, when possible, servers respect the urgency
parameter (Section 4.1), sending higher urgency responses before
lower urgency responses.
The incremental parameter indicates how a client processes response
bytes as they arrive. It is RECOMMENDED that, when possible, servers
respect the incremental parameter (Section 4.2). Non-incremental
resources can only be used when all of the response payload has been
received. Therefore, non-incremental responses of the same urgency
SHOULD be served in their entirety, one-by-one, based on the stream
ID, which corresponds to the order in which clients make requests.
Doing so ensures that clients can use request ordering to influence
response order.
Incremental responses of the same urgency SHOULD be served by sharing
bandwidth amongst them. Incremental resources are used as parts, or
chunks, of the response payload are received. A client might benefit
more from receiving a portion of all these resources rather than the
entirety of a single resource. How large a portion of the resource
is needed to be useful in improving performance varies. Some
resource types place critical elements early, others can use
information progressively. This scheme provides no explicit mandate
about how a server should use size, type or any other input to decide
how to prioritize.
There can be scenarios where a server will need to schedule multiple
incremental and non-incremental responses at the same urgency level.
Strictly abiding the scheduling guidance based on urgency and request
generation order might lead to sub-optimal results at the client, as
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early non-incremental responses might prevent serving of incremental
responses issued later. The following are examples of such
challenges.
1. At the same urgency level, a non-incremental request for a large
resource followed by an incremental request for a small resource.
2. At the same urgency level, an incremental request of
indeterminate length followed by a non-incremental large
resource.
It is RECOMMENDED that servers avoid such starvation where possible.
The method to do so is an implementation decision. For example, a
server might pre-emptively send responses of a particular incremental
type based on other information such as content size.
Optimal scheduling of server push is difficult, especially when
pushed resources contend with active concurrent requests. Servers
can consider many factors when scheduling, such as the type or size
of resource being pushed, the priority of the request that triggered
the push, the count of active concurrent responses, the priority of
other active concurrent responses, etc. There is no general guidance
on the best way to apply these. A server that is too simple could
easily push at too high a priority and block client requests, or push
at too low a priority and delay the response, negating intended goals
of server push.
Priority signals are a factor for server push scheduling. The
concept of parameter value defaults applies slightly differently
because there is no explicit client-signalled initial priority. A
server can apply priority signals provided in an origin response; see
the merging guidance given in Section 8. In the absence of origin
signals, applying default parameter values could be suboptimal. By
whatever means a server decides to schedule a pushed response, it can
signal the intended priority to the client by including the Priority
field in a PUSH_PROMISE or HEADERS frame.
10.1. Intermediaries with Multiple Backend Connections
An intermediary serving an HTTP connection might split requests over
multiple backend connections. When it applies prioritization rules
strictly, low priority requests cannot make progress while requests
with higher priorities are inflight. This blocking can propagate to
backend connections, which the peer might interpret as a connection
stall. Endpoints often implement protections against stalls, such as
abruptly closing connections after a certain time period. To reduce
the possibility of this occurring, intermediaries can avoid strictly
following prioritization and instead allocate small amounts of
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bandwidth for all the requests that they are forwarding, so that
every request can make some progress over time.
Similarly, servers SHOULD allocate some amount of bandwidths to
streams acting as tunnels.
11. Scheduling and the CONNECT Method
When a request stream carries the CONNECT method, the scheduling
guidance in this document applies to the frames on the stream. A
client that issues multiple CONNECT requests can set the incremental
parameter to true, servers that implement the recommendation in
Section 10 will schedule these fairly.
12. Retransmission Scheduling
Transport protocols such as TCP and QUIC provide reliability by
detecting packet losses and retransmitting lost information. While
this document specifies HTTP-layer prioritization, its effectiveness
can be further enhanced if the transport layer factors priority into
scheduling both new data and retransmission data. The remainder of
this section discusses considerations when using QUIC.
Section 13.3 of [QUIC] states "Endpoints SHOULD prioritize
retransmission of data over sending new data, unless priorities
specified by the application indicate otherwise". When an HTTP/3
application uses the priority scheme defined in this document and the
QUIC transport implementation supports application indicated stream
priority, a transport that considers the relative priority of streams
when scheduling both new data and retransmission data might better
match the expectations of the application. However, there are no
requirements on how a transport chooses to schedule based on this
information because the decision depends on several factors and
trade-offs. It could prioritize new data for a higher urgency stream
over retransmission data for a lower priority stream, or it could
prioritize retransmission data over new data irrespective of
urgencies.
Section 6.2.4 of [QUIC-RECOVERY], also highlights consideration of
application priorities when sending probe packets after Probe Timeout
timer expiration. A QUIC implementation supporting application-
indicated priorities might use the relative priority of streams when
choosing probe data.
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13. Fairness
As a general guideline, a server SHOULD NOT use priority information
for making scheduling 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.
13.1. 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 user agent to be delayed totally after those
going to another.
In order to mitigate this fairness problem, a server could use
knowledge about the intermediary as another signal in its
prioritization decisions. For instance, if a server knows the
intermediary is coalescing requests, then it could avoid serving the
responses in their entirety and instead distribute bandwidth (for
example, in a round-robin manner). This can work if the constrained
resource is network capacity between the intermediary and the user
agent, as the intermediary buffers responses and forwards the chunks
based on the prioritization scheme it implements.
A server can determine if a request came from an intermediary through
configuration, or by consulting if that request contains one of the
following header fields:
* Forwarded [FORWARDED], X-Forwarded-For
* Via (see Section 7.6.3 of [HTTP])
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13.2. 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. HTTP/1.1 and older do not support response
multiplexing in a single connection, so there is not a fairness
problem. 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.
13.3. 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.
14. 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.
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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.
15. Security Considerations
[RFC7540] stream prioritization relies on dependencies.
Considerations are presented to implementations, describing how
limiting state or work commitments can avoid some types of problems.
In addition, [CVE-2019-9513] aka "Resource Loop", is an example of a
DoS attack that abuses stream dependencies. Extensible priorities
does not use dependencies, which avoids these issues.
Section 7 describes considerations for server buffering of
PRIORITY_UPDATE frames.
Section 10 presents examples where servers that prioritize responses
in a certain way might be starved of the ability to transmit payload.
The security considerations from [STRUCTURED-FIELDS] apply to
processing of priority parameters defined in Section 4.
16. IANA Considerations
This specification registers the following entry in the the Hypertext
Transfer Protocol (HTTP) Field Name Registry established by [HTTP]:
Field name: Priority
Status: permanent
Specification document(s): This document
This specification registers the following entry in the HTTP/2
Settings registry established by [RFC7540]:
Name: SETTINGS_NO_RFC7540_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
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Code: 0x10
Specification: This document
This specification registers the following entries in the HTTP/3
Frame Type registry established by [HTTP3]:
Frame Type: PRIORITY_UPDATE
Code: 0xF0700 and 0xF0701
Specification: This document
Upon publication, please create the HTTP Priority Parameters registry
at https://iana.org/assignments/http-priority
(https://iana.org/assignments/http-priority) and populate it with the
types defined in Section 4; see Section 4.3.1 for its associated
procedures.
17. References
17.1. Normative References
[HTTP] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Semantics", Work in Progress, Internet-Draft, draft-ietf-
httpbis-semantics-19, 12 September 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
semantics-19>.
[HTTP2] Thomson, M. and C. Benfield, "Hypertext Transfer Protocol
Version 2 (HTTP/2)", Work in Progress, Internet-Draft,
draft-ietf-httpbis-http2bis-05, 26 September 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
http2bis-05>.
[HTTP3] Bishop, M., "Hypertext Transfer Protocol Version 3
(HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
quic-http-34, 2 February 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
http-34>.
[QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>.
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[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/rfc/rfc2119>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/rfc8126>.
[STRUCTURED-FIELDS]
Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/rfc/rfc8941>.
17.2. Informative References
[CACHING] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Caching", Work in Progress, Internet-Draft, draft-ietf-
httpbis-cache-19, 12 September 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
cache-19>.
[CVE-2019-9513]
Common Vulnerabilities and Exposures, "CVE-2019-9513", 1
March 2019, <https://cve.mitre.org/cgi-bin/
cvename.cgi?name=CVE-2019-9513>.
[FORWARDED]
Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
RFC 7239, DOI 10.17487/RFC7239, June 2014,
<https://www.rfc-editor.org/rfc/rfc7239>.
[I-D.lassey-priority-setting]
Lassey, B. and L. Pardue, "Declaring Support for HTTP/2
Priorities", Work in Progress, Internet-Draft, draft-
lassey-priority-setting-00, 25 July 2019,
<https://datatracker.ietf.org/doc/html/draft-lassey-
priority-setting-00>.
[QUIC-RECOVERY]
Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
May 2021, <https://www.rfc-editor.org/rfc/rfc9002>.
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[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/rfc/rfc7540>.
[RFC8081] Lilley, C., "The "font" Top-Level Media Type", RFC 8081,
DOI 10.17487/RFC8081, February 2017,
<https://www.rfc-editor.org/rfc/rfc8081>.
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 (http://tools.ietf.org/agenda/83/slides/
slides-83-httpbis-5.pdf). In https://github.com/pmeenan/http3-
prioritization-proposal (https://github.com/pmeenan/http3-
prioritization-proposal), Patrick Meenan advocated for representing
the priorities using a tuple of urgency and concurrency. The ability
to disable HTTP/2 prioritization is inspired by
[I-D.lassey-priority-setting], authored by Brad Lassey and Lucas
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.
Yang Chi contributed the section on retransmission scheduling.
Appendix B. Change Log
B.1. Since draft-ietf-httpbis-priority-06
* Focus on editorial changes
* Clarify rules about Sf-Dictionary handling in headers
* Split policy for parameter IANA registry into two sections based
on key length
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B.2. Since draft-ietf-httpbis-priority-05
* Renamed SETTINGS_DEPRECATE_RFC7540_PRIORITIES to
SETTINGS_NO_RFC7540_PRIORITIES
* Clarify that senders of the HTTP/2 setting can use any alternative
(#1679, #1705)
B.3. Since draft-ietf-httpbis-priority-04
* Renamed SETTINGS_DEPRECATE_HTTP2_PRIORITIES to
SETTINGS_DEPRECATE_RFC7540_PRIORITIES (#1601)
* Reoriented text towards RFC7540bis (#1561, #1601)
* Clarify intermediary behavior (#1562)
B.4. Since draft-ietf-httpbis-priority-03
* Add statement about what this scheme applies to. Clarify
extensions can use it but must define how themselves (#1550,
#1559)
* Describe scheduling considerations for the CONNECT method (#1495,
#1544)
* Describe scheduling considerations for retransmitted data (#1429,
#1504)
* Suggest intermediaries might avoid strict prioritization (#1562)
B.5. Since draft-ietf-httpbis-priority-02
* Describe considerations for server push prioritization (#1056,
#1345)
* Define HTTP/2 PRIORITY_UPDATE ID limits in HTTP/2 terms (#1261,
#1344)
* Add a Parameters registry (#1371)
B.6. Since draft-ietf-httpbis-priority-01
* PRIORITY_UPDATE frame changes (#1096, #1079, #1167, #1262, #1267,
#1271)
* Add section to describe server scheduling considerations (#1215,
#1232, #1266)
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* Remove specific instructions related to intermediary fairness
(#1022, #1264)
B.7. Since draft-ietf-httpbis-priority-00
* Move text around (#1217, #1218)
* Editorial change to the default urgency. The value is 3, which
was always the intent of previous changes.
B.8. Since draft-kazuho-httpbis-priority-04
* Minimize semantics of Urgency levels (#1023, #1026)
* Reduce guidance about how intermediary implements merging priority
signals (#1026)
* Remove mention of CDN-Loop (#1062)
* Editorial changes
* Make changes due to WG adoption
* Removed outdated Consideration (#118)
B.9. Since draft-kazuho-httpbis-priority-03
* Changed numbering from [-1,6] to [0,7] (#78)
* Replaced priority scheme negotiation with HTTP/2 priority
deprecation (#100)
* Shorten parameter names (#108)
* Expand on considerations (#105, #107, #109, #110, #111, #113)
B.10. Since draft-kazuho-httpbis-priority-02
* Consolidation of the problem statement (#61, #73)
* Define SETTINGS_PRIORITIES for negotiation (#58, #69)
* Define PRIORITY_UPDATE frame for HTTP/2 and HTTP/3 (#51)
* Explain fairness issue and mitigations (#56)
B.11. Since draft-kazuho-httpbis-priority-01
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* Explain how reprioritization might be supported.
B.12. Since draft-kazuho-httpbis-priority-00
* 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
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