RFC 9218 - Extensible Prioritization Scheme for HTTP

RFC 9218 - Extensible Prioritization Scheme for HTTP
RFC 9218
HTTP Priorities
June 2022
Oku & Pardue
Standards Track
[Page]
Stream:
Internet Engineering Task Force (IETF)
RFC:
9218
Category:
Standards Track
Published:
June 2022
ISSN:
2070-1721
Authors:
奥 一穂
K. Oku
Fastly
L. Pardue
Cloudflare
RFC 9218
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
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with
respect to this document. Code Components extracted from this
document must include Revised BSD License text as described in
Section 4.e of the Trust Legal Provisions and are provided without
warranty as described in the Revised BSD License.
Table of Contents
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
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
and
), 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 (
) and incremental (
) 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 (
) 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
: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 (
) 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
).
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
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
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
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
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
to
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 (
) 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:
At the same urgency level, a non-incremental request for a large resource
followed by an incremental request for a small resource.
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:
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
and has populated it with the entries in
Table 1
; see
Section 4.3.1
for its associated procedures.
Table 1
Initial Priority Parameters
Name
Description
Reference
The urgency of an HTTP response.
Section 4.1
Whether an HTTP response can be processed incrementally.
Section 4.2
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
[HTTP/2]
Thomson, M., Ed.
and
C. Benfield, Ed.
"HTTP/2"
RFC 9113
DOI 10.17487/RFC9113
June 2022
[HTTP/3]
Bishop, M., Ed.
"HTTP/3"
RFC 9114
DOI 10.17487/RFC9114
June 2022
[QUIC]
Iyengar, J., Ed.
and
M. Thomson, Ed.
"QUIC: A UDP-Based Multiplexed and Secure Transport"
RFC 9000
DOI 10.17487/RFC9000
May 2021
[RFC2119]
Bradner, S.
"Key words for use in RFCs to Indicate Requirement Levels"
BCP 14
RFC 2119
DOI 10.17487/RFC2119
March 1997
[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
[RFC8174]
Leiba, B.
"Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"
BCP 14
RFC 8174
DOI 10.17487/RFC8174
May 2017
[STRUCTURED-FIELDS]
Nottingham, M.
and
P-H. Kamp
"Structured Field Values for HTTP"
RFC 8941
DOI 10.17487/RFC8941
February 2021
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
[FORWARDED]
Petersson, A.
and
M. Nilsson
"Forwarded HTTP Extension"
RFC 7239
DOI 10.17487/RFC7239
June 2014
[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
[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
[QUIC-RECOVERY]
Iyengar, J., Ed.
and
I. Swett, Ed.
"QUIC Loss Detection and Congestion Control"
RFC 9002
DOI 10.17487/RFC9002
May 2021
[RFC7540]
Belshe, M.
Peon, R.
, and
M. Thomson, Ed.
"Hypertext Transfer Protocol Version 2 (HTTP/2)"
RFC 7540
DOI 10.17487/RFC7540
May 2015
[RFC8081]
Lilley, C.
"The "font" Top-Level Media Type"
RFC 8081
DOI 10.17487/RFC8081
February 2017
Acknowledgements
Roy Fielding
presented the idea of using a header field for representing
priorities in
In
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
Lucas Pardue
Cloudflare
Email:
lucaspardue.24.7@gmail.com
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- Proposed Standard
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