Internet Engineering Task Force (IETF) 奥 一穂 (K. Oku)
Request for Comments: 9218 Fastly
Category: Standards Track L. Pardue
ISSN: 2070-1721 Cloudflare
June 2022
Extensible Prioritization Scheme for HTTP
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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9218.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
1.1. Notational Conventions
2. Motivation for Replacing RFC 7540 Stream Priorities
2.1. Disabling RFC 7540 Stream Priorities
2.1.1. Advice when Using Extensible Priorities as the
Alternative
3. Applicability of the Extensible Priority Scheme
4. Priority Parameters
4.1. Urgency
4.2. Incremental
4.3. Defining New Priority Parameters
4.3.1. Registration
5. The Priority HTTP Header Field
6. Reprioritization
7. The PRIORITY_UPDATE Frame
7.1. HTTP/2 PRIORITY_UPDATE Frame
7.2. HTTP/3 PRIORITY_UPDATE Frame
8. Merging Client- and Server-Driven Priority Parameters
9. Client Scheduling
10. Server Scheduling
10.1. Intermediaries with Multiple Backend Connections
11. Scheduling and the CONNECT Method
12. Retransmission Scheduling
13. Fairness
13.1. Coalescing Intermediaries
13.2. HTTP/1.x Back Ends
13.3. Intentional Introduction of Unfairness
14. Why Use an End-to-End Header Field?
15. Security Considerations
16. IANA Considerations
17. References
17.1. Normative References
17.2. Informative References
Acknowledgements
Authors' Addresses
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 relationships determines whether a
client is blocked from continuing to process locally available
resources. An example of this is the visual rendering of an HTML
document, which could be blocked by the retrieval of a Cascading
Style Sheets (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 [HTTP/2] and HTTP/3 [HTTP/3] 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.
HTTP/2 and HTTP/3 servers can schedule transmission of concurrent
response data by any means they choose. Servers can ignore client
priority signals and still successfully serve HTTP responses.
However, servers that operate 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 of 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 decisions.
The design and implementation of RFC 7540 stream priority were
observed to have shortcomings, as explained in Section 2. HTTP/2
[HTTP/2] has consequently deprecated the use of these stream priority
signals. The prioritization scheme and priority signals defined
herein can act as a substitute for RFC 7540 stream priority.
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, which is an end-to-end priority signal that is independent of
protocol version. Clients can send this header field to signal their
view of how responses should be prioritized. Similarly, servers
behind an intermediary can use it to signal priority to the
intermediary. After sending a request, a client can change their
view of response priority (see Section 6) by sending HTTP-version-
specific frames as defined in Sections 7.1 and 7.2.
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. Sections 10 and 12 provide
considerations and guidance about how servers might act upon signals.
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document uses the following terminology from Section 3 of
[STRUCTURED-FIELDS] to specify syntax and parsing: "Boolean",
"Dictionary", and "Integer".
Example HTTP requests and responses use the HTTP/2-style formatting
from [HTTP/2].
This document uses the variable-length integer encoding from [QUIC].
The term "control stream" is used to describe both the HTTP/2 stream
with identifier 0x0 and the HTTP/3 control stream; see Section 6.2.1
of [HTTP/3].
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 [HTTP/2].
2. Motivation for Replacing RFC 7540 Stream 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 has been deprecated in a revision of HTTP/2
[HTTP/2]. HTTP/2 retains these protocol elements in order to
maintain wire compatibility (see Section 5.3.2 of [HTTP/2]), which
means that they might still be used even in the presence of
alternative signaling, such as the scheme this document describes.
Many RFC 7540 server implementations do not act on HTTP/2 priority
signals.
Prioritization can use information that servers have about resources
or the order in which requests are generated. 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. With RFC 7540, it is difficult for
servers to interpret signals from clients for prioritization, as the
same conditions could result in very different signaling from
different clients. This document describes signaling that is simpler
and more constrained, requiring less interpretation and allowing less
variation.
RFC 7540 does not define a method that can be used by a server to
provide a priority signal for intermediaries.
RFC 7540 stream priority is expressed relative to other requests
sharing the same connection at the same time. It is difficult to
incorporate such a design into applications that generate requests
without knowledge of how other requests might share a connection, or
into protocols that do not have strong ordering guarantees across
streams, like HTTP/3 [HTTP/3].
Experiments from independent research [MARX] 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 Stream Priorities
The problems and insights set out above provided the motivation for
an alternative to RFC 7540 stream priority (see Section 5.3 of
[HTTP/2]).
The SETTINGS_NO_RFC7540_PRIORITIES HTTP/2 setting is defined by this
document in order to allow endpoints to omit or ignore HTTP/2
priority signals (see Section 5.3.2 of [HTTP/2]), as described below.
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 [HTTP/2]) of type PROTOCOL_ERROR. The initial value
is 0.
If endpoints use SETTINGS_NO_RFC7540_PRIORITIES, they MUST send it in
the first SETTINGS frame. Senders MUST NOT change the
SETTINGS_NO_RFC7540_PRIORITIES value after the first SETTINGS frame.
Receivers that detect a change MAY treat it as a connection error of
type PROTOCOL_ERROR.
Clients can send SETTINGS_NO_RFC7540_PRIORITIES with a value of 1 to
indicate that they are not using HTTP/2 priority signals. The
SETTINGS frame precedes any HTTP/2 priority signal sent from clients,
so servers can determine whether they need to allocate any resources
to signal handling before signals arrive. A server that receives
SETTINGS_NO_RFC7540_PRIORITIES with a value of 1 MUST ignore HTTP/2
priority signals.
Servers can send SETTINGS_NO_RFC7540_PRIORITIES with a value of 1 to
indicate that they will ignore HTTP/2 priority signals sent by
clients.
Endpoints that send SETTINGS_NO_RFC7540_PRIORITIES are encouraged to
use alternative priority signals (for example, see Section 5 or
Section 7.1), but there is no requirement to use a specific signal
type.
2.1.1. Advice when Using Extensible Priorities as the Alternative
Before receiving a SETTINGS frame from a server, a client does not
know if the server is ignoring HTTP/2 priority signals. Therefore,
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 Sections 5 and 7.1).
Once the client receives the first SETTINGS frame that contains the
SETTINGS_NO_RFC7540_PRIORITIES parameter with a value of 1, it SHOULD
stop sending the HTTP/2 priority signals. This avoids sending
redundant signals that are known to be ignored.
Similarly, if the client receives SETTINGS_NO_RFC7540_PRIORITIES with
a value of 0 or if the settings parameter was absent, it SHOULD stop
sending PRIORITY_UPDATE frames (Section 7.1), since those frames are
likely to be ignored. However, the client 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 is primarily focused on
the prioritization of HTTP response messages (see Section 3.4 of
[HTTP]). It defines new priority parameters (Section 4) and a means
of conveying those parameters (Sections 5 and 7), which is intended
to communicate the priority of responses to a server that is
responsible for prioritizing them. Section 10 provides
considerations for servers about acting on those signals in
combination with other inputs and factors.
The CONNECT method (see Section 9.3.6 of [HTTP]) can be used to
establish tunnels. Signaling applies similarly to tunnels;
additional considerations for server prioritization are given in
Section 11.
Section 9 describes how clients can optionally apply elements of this
scheme locally to the request messages that they generate.
Some forms of HTTP extensions might change HTTP/2 or HTTP/3 stream
behavior or define new data carriage mechanisms. Such extensions can
themselves define how this priority scheme is to be applied.
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 priority parameters when a request or a response
is issued. After sending a request, a client can change their view
of response priority (Section 6) by sending HTTP-version-specific
PRIORITY_UPDATE frames as defined in Sections 7.1 and 7.2. Frames
transmit priority parameters on a single hop only.
Intermediaries can consume and produce priority signals in a
PRIORITY_UPDATE frame or Priority header field. An intermediary that
passes only the Priority request header field to the next hop
preserves the original end-to-end signal from the client; see
Section 14. An intermediary could pass the Priority header field and
additionally send a PRIORITY_UPDATE frame. This would have the
effect of preserving the original client end-to-end signal, while
instructing the next hop to use a different priority, per the
guidance in Section 7. An intermediary that replaces or adds a
Priority request header field overrides the original client end-to-
end signal, which can affect prioritization for all subsequent
recipients of the request.
For both the Priority header field and the PRIORITY_UPDATE frame, the
set of priority parameters is encoded as a Dictionary (see
Section 3.2 of [STRUCTURED-FIELDS]).
This document defines the urgency (u) and incremental (i) priority
parameters. When receiving an HTTP request that does not carry these
priority parameters, a server SHOULD act as if their default values
were specified.
An intermediary can combine signals from requests and responses that
it forwards. Note that omission of priority parameters in responses
is handled differently from omission in requests; see Section 8.
Receivers parse the Dictionary as described in Section 4.2 of
[STRUCTURED-FIELDS]. Where the Dictionary is successfully parsed,
this document places the additional requirement that unknown priority
parameters, priority parameters with out-of-range values, or values
of unexpected types MUST be ignored.
4.1. Urgency
The urgency (u) parameter value is Integer (see Section 3.3.1 of
[STRUCTURED-FIELDS]), between 0 and 7 inclusive, in descending order
of priority. The default 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:
: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 website (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 any impact on user interaction.
4.2. Incremental
The incremental (i) parameter value is Boolean (see Section 3.3.6 of
[STRUCTURED-FIELDS]). It 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 in 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
4.3. Defining New Priority Parameters
When attempting to define new priority 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 priority parameters. Since unknown priority parameters
are ignored, new priority parameters should not change the
interpretation of, or modify, the urgency (see Section 4.1) or
incremental (see Section 4.2) priority 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 priority 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 priority parameter to
indicate if the resource being requested is within the viewport.
Generic priority 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" registry. This registry governs the keys (short
textual strings) used in the 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
extensions while avoiding unintended conflict among attractive key
values, the "HTTP Priority" registry operates two registration
policies, depending on key length.
* Registration requests for priority parameters with a key length of
one use the Specification Required policy, per Section 4.6 of
[RFC8126].
* Registration requests for priority parameters with a key length
greater than one use the Expert Review policy, per Section 4.5 of
[RFC8126]. A specification document is appreciated but not
required.
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 the parameter
key]
Description: [a description of the priority parameter semantics and
value]
Reference: [to a specification defining this priority parameter]
See the registry at <https://www.iana.org/assignments/http-priority>
for details on where to send registration requests.
5. The Priority HTTP Header Field
The Priority HTTP header field is a Dictionary that carries priority
parameters (see Section 4). It can appear in requests and responses.
It is an end-to-end signal that indicates the endpoint's view of how
HTTP responses should be prioritized. Section 8 describes how
intermediaries can combine the priority information sent from clients
and servers. Clients cannot interpret the appearance or omission of
a Priority response header field as acknowledgement that any
prioritization has occurred. Guidance for how endpoints can act on
Priority header values is given in Sections 9 and 10.
An HTTP request with a Priority header field might be cached and
reused for subsequent requests; see [CACHING]. 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).
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 that 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
[HTTP/2] and HTTP/3 [HTTP/3]. 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 independently of the stream that carries the
response. This means they can be used to reprioritize a response or
a push stream, or to signal the initial priority of a response
instead of the Priority header field.
A PRIORITY_UPDATE frame communicates a complete set of all priority
parameters in the Priority Field Value field. Omitting a priority
parameter is a signal to use its default value. Failure to parse the
Priority Field Value MAY 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_GENERAL_PROTOCOL_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 be bounded by local implementation policy. Although there
is no limit to the number of PRIORITY_UPDATE frames 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 [HTTP/2]) 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 (8) = 0x10,
Unused Flags (8),
Reserved (1),
Stream Identifier (31),
Reserved (1),
Prioritized Stream ID (31),
Priority Field Value (..),
}
Figure 1: HTTP/2 PRIORITY_UPDATE Frame Format
The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
fields are described in Section 4 of [HTTP/2]. The PRIORITY_UPDATE
frame payload contains the following additional fields:
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.
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 (i.e., streams where
data might still be received). Servers can discard frames where the
prioritized stream ID refers to a stream in the "half-closed (local)"
or "closed" state (i.e., streams where no further data will be sent).
The number of streams that have been prioritized but remain in the
"idle" state plus the number of active streams (those in the "open"
state or in either of the "half-closed" states; see Section 5.1.2 of
[HTTP/2]) 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.
Servers MUST NOT send PRIORITY_UPDATE frames. 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 that uses 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 [HTTP/3]). 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.
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 of 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 that has not yet been promised, this MUST be
treated as a connection error of type H3_ID_ERROR.
Servers MUST NOT send PRIORITY_UPDATE frames of either type. 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 Priority 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; the existence of such
dependencies 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 priority 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.
The 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 request header field, in which omission of a
priority parameter implies the use of its default value (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, i.e., 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.
10. Server Scheduling
It is generally beneficial for an HTTP server to send all responses
as early as possible. However, when serving multiple requests on a
single connection, there could be competition between the requests
for resources such as connection bandwidth. This section describes
considerations regarding how servers can schedule the order in which
the competing responses will be sent when such competition exists.
Server scheduling is a prioritization process based on many inputs,
with priority signals being only one form of input. 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, it is not possible to describe a universal
scheduling algorithm. This document provides some basic, non-
exhaustive recommendations for how servers might act on priority
parameters. It does not describe in detail how servers might combine
priority signals with other factors. Endpoints cannot depend on
particular treatment based on priority signals. Expressing priority
is only a suggestion.
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 responses of the same urgency SHOULD be served by
prioritizing bandwidth allocation in ascending order of 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 among them. The message content of incremental responses
is used as parts, or chunks, 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 by the scheduling guidance based on urgency and
request generation order might lead to suboptimal results at the
client, as early non-incremental responses might prevent the 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 for doing so is an implementation decision. For example,
a server might preemptively 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-signaled 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 in flight. This blocking can propagate to
backend connections, which the peer might interpret as a connection
stall. Endpoints often implement protections against stalls, such as
abruptly closing connections after a certain time period. To reduce
the possibility of this occurring, intermediaries can avoid strictly
following prioritization and instead allocate small amounts of
bandwidth for all the requests that they are forwarding, so that
every request can make some progress over time.
Similarly, servers SHOULD allocate some amount of bandwidths to
streams acting as tunnels.
11. Scheduling and the CONNECT Method
When a stream carries a CONNECT request, 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 recommendations for handling of the
incremental parameter (Section 10) are likely to schedule these
fairly, preventing one CONNECT stream from blocking others.
12. Retransmission Scheduling
Transport protocols such as TCP and QUIC provide reliability by
detecting packet losses and retransmitting lost information. In
addition to the considerations in Section 10, scheduling of
retransmission data could compete with new data. The remainder of
this section discusses considerations when using QUIC.
Section 13.3 of [QUIC] states the following: "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 considerations
regarding 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.
13. Fairness
Typically, HTTP implementations depend on the underlying transport to
maintain fairness between connections competing for bandwidth. When
an intermediary receives HTTP requests on client connections, it
forwards them to backend connections. Depending on how the
intermediary coalesces or splits requests across different backend
connections, different clients might experience dissimilar
performance. This dissimilarity might expand if the intermediary
also uses priority signals when forwarding requests. Sections 13.1
and 13.2 discuss mitigations of this expansion of unfairness.
Conversely, Section 13.3 discusses how servers might intentionally
allocate unequal bandwidth to some connections, depending on the
priority signals.
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 carry signals
indicating higher priority than those coming from others.
It is sometimes beneficial for the server running behind an
intermediary to obey Priority header field values. As an example, a
resource-constrained server might defer the transmission of software
update files that have the background urgency level (7). However, in
the worst case, the asymmetry between the priority declared by
multiple clients might cause all responses going to one user agent to
be delayed until all responses going to another user agent have been
sent.
In order to mitigate this fairness problem, a server could use
knowledge about the intermediary as another input 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 can check to see if the request contains one of the
following header fields:
* Forwarded [FORWARDED], X-Forwarded-For
* Via (see Section 7.6.3 of [HTTP])
13.2. HTTP/1.x Back Ends
It is common for Content Delivery Network (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 backend servers is done using HTTP/1.1.
Unlike connection coalescing, the CDN will "demux" requests into
discrete connections to the back end. Response multiplexing in a
single connection is not supported by HTTP/1.1 (or older), so there
is not a fairness problem. However, backend servers MAY still use
client headers for request scheduling. Backend 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?
In contrast to the prioritization scheme of HTTP/2, which uses a hop-
by-hop frame, the Priority header field is defined as "end-to-end".
The way that a client processes a response is a property associated
with the 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.
15. Security Considerations
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
responses.
The security considerations from [STRUCTURED-FIELDS] apply to the
processing of priority parameters defined in Section 4.
16. IANA Considerations
This specification registers the following entry in the "Hypertext
Transfer Protocol (HTTP) Field Name Registry" defined in [HTTP/2]:
Field Name: Priority
Status: permanent
Reference: This document
This specification registers the following entry in the "HTTP/2
Settings" registry defined in [HTTP/2]:
Code: 0x9
Name: SETTINGS_NO_RFC7540_PRIORITIES
Initial Value: 0
Reference: This document
This specification registers the following entry in the "HTTP/2 Frame
Type" registry defined in [HTTP/2]:
Code: 0x10
Frame Type: PRIORITY_UPDATE
Reference: This document
This specification registers the following entry in the "HTTP/3 Frame
Types" registry established by [HTTP/3]:
Value: 0xF0700-0xF0701
Frame Type: PRIORITY_UPDATE
Status: permanent
Reference: This document
Change Controller: IETF
Contact: ietf-http-wg@w3.org
IANA has created the "Hypertext Transfer Protocol (HTTP) Priority"
registry at <https://www.iana.org/assignments/http-priority> and has
populated it with the entries in Table 1; see Section 4.3.1 for its
associated procedures.
+======+==================================+=============+
| Name | Description | Reference |
+======+==================================+=============+
| u | The urgency of an HTTP response. | Section 4.1 |
+------+----------------------------------+-------------+
| i | Whether an HTTP response can be | Section 4.2 |
| | processed incrementally. | |
+------+----------------------------------+-------------+
Table 1: Initial Priority Parameters
17. References
17.1. Normative References
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[HTTP/2] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
DOI 10.17487/RFC9113, June 2022,
<https://www.rfc-editor.org/info/rfc9113>.
[HTTP/3] Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
June 2022, <https://www.rfc-editor.org/info/rfc9114>.
[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/info/rfc9000>.
[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>.
[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/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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/info/rfc8941>.
17.2. Informative References
[CACHING] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Caching", STD 98, RFC 9111,
DOI 10.17487/RFC9111, June 2022,
<https://www.rfc-editor.org/info/rfc9111>.
[FORWARDED]
Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
RFC 7239, DOI 10.17487/RFC7239, June 2014,
<https://www.rfc-editor.org/info/rfc7239>.
[MARX] Marx, R., De Decker, T., Quax, P., and W. Lamotte, "Of the
Utmost Importance: Resource Prioritization in HTTP/3 over
QUIC", SCITEPRESS Proceedings of the 15th International
Conference on Web Information Systems and Technologies
(pages 130-143), DOI 10.5220/0008191701300143, September
2019, <https://www.doi.org/10.5220/0008191701300143>.
[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/info/rfc9002>.
[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>.
[RFC8081] Lilley, C., "The "font" Top-Level Media Type", RFC 8081,
DOI 10.17487/RFC8081, February 2017,
<https://www.rfc-editor.org/info/rfc8081>.
Acknowledgements
Roy Fielding presented the idea of using a header field for
representing priorities in
<https://www.ietf.org/proceedings/83/slides/slides-83-httpbis-5.pdf>.
In <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 [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, Matthew Cox,
Mike Bishop, Roberto Peon, Robin Marx, Roy Fielding, and the authors
of this document.
Yang Chi contributed the section on retransmission scheduling.
Authors' Addresses
Kazuho Oku
Fastly
Email: kazuhooku@gmail.com
Additional contact information:
奥 一穂
Fastly