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Extensible Prioritization Scheme for HTTP
draft-ietf-httpbis-priority-02
Abstract
This document describes a scheme for prioritizing HTTP responses.
This scheme expresses the priority of each HTTP response using
absolute values, rather than as a relative relationship between a
group of HTTP responses.
This document defines the Priority header field for communicating the
initial priority in an HTTP version-independent manner, as well as
HTTP/2 and HTTP/3 frames for reprioritizing the responses. These
share a common format structure that is designed to provide future
extensibility.
Note to Readers
_RFC EDITOR: please remove this section before publication_
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/ [1].
Working Group information can be found at https://httpwg.org/ [2];
source code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions/labels/priorities [3].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 4, 2021.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. Motivation for Replacing HTTP/2 Priorities . . . . . . . . . 4
2.1. Disabling HTTP/2 Priorities . . . . . . . . . . . . . . . 5
3. Priority Parameters . . . . . . . . . . . . . . . . . . . . . 6
3.1. Urgency . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Incremental . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Defining New Parameters . . . . . . . . . . . . . . . . . 8
4. The Priority HTTP Header Field . . . . . . . . . . . . . . . 8
5. Reprioritization . . . . . . . . . . . . . . . . . . . . . . 9
6. The PRIORITY_UPDATE Frame . . . . . . . . . . . . . . . . . . 9
6.1. HTTP/2 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 10
6.2. HTTP/3 PRIORITY_UPDATE Frame . . . . . . . . . . . . . . 11
7. Merging Client- and Server-Driven Parameters . . . . . . . . 12
8. Client Scheduling . . . . . . . . . . . . . . . . . . . . . . 13
9. Server Scheduling . . . . . . . . . . . . . . . . . . . . . . 13
10. Fairness . . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1. Coalescing Intermediaries . . . . . . . . . . . . . . . 15
10.2. HTTP/1.x Back Ends . . . . . . . . . . . . . . . . . . . 15
10.3. Intentional Introduction of Unfairness . . . . . . . . . 16
11. Why use an End-to-End Header Field? . . . . . . . . . . . . . 16
12. Security Considerations . . . . . . . . . . . . . . . . . . . 16
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
14.1. Normative References . . . . . . . . . . . . . . . . . . 18
14.2. Informative References . . . . . . . . . . . . . . . . . 19
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14.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 20
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 20
B.1. Since draft-ietf-httpbis-priority-01 . . . . . . . . . . 20
B.2. Since draft-ietf-httpbis-priority-00 . . . . . . . . . . 20
B.3. Since draft-kazuho-httpbis-priority-04 . . . . . . . . . 21
B.4. Since draft-kazuho-httpbis-priority-03 . . . . . . . . . 21
B.5. Since draft-kazuho-httpbis-priority-02 . . . . . . . . . 21
B.6. Since draft-kazuho-httpbis-priority-01 . . . . . . . . . 21
B.7. Since draft-kazuho-httpbis-priority-00 . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
It is common for an HTTP ([RFC7230]) resource representation to have
relationships to one or more other resources. Clients will often
discover these relationships while processing a retrieved
representation, leading to further retrieval requests. Meanwhile,
the nature of the relationship determines whether the client is
blocked from continuing to process locally available resources. For
example, visual rendering of an HTML document could be blocked by the
retrieval of a CSS file that the document refers to. In contrast,
inline images do not block rendering and get drawn incrementally as
the chunks of the images arrive.
To provide meaningful presentation of a document at the earliest
moment, it is important for an HTTP server to prioritize the HTTP
responses, or the chunks of those HTTP responses, that it sends.
HTTP/2 ([RFC7540]) provides such a prioritization scheme. A client
sends a series of PRIORITY frames to communicate to the server a
"priority tree"; this represents the client's preferred ordering and
weighted distribution of the bandwidth among the HTTP responses.
However, the design and implementation of this scheme has been
observed to have shortcomings, explained in Section 2.
This document defines the Priority HTTP header field that can be used
by both client and server to specify the precedence of HTTP responses
in a standardized, extensible, protocol-version-independent, end-to-
end format. Along with the protocol-version-specific frame for
reprioritization, this prioritization scheme acts as a substitute for
the original prioritization scheme of HTTP/2.
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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The terms sf-token and sf-boolean are imported from
[STRUCTURED-FIELDS].
Example HTTP requests and responses use the HTTP/2-style formatting
from [RFC7540].
This document uses the variable-length integer encoding from
[I-D.ietf-quic-transport].
The term control stream is used to describe the HTTP/2 stream with
identifier 0x0, and HTTP/3 control stream; see [I-D.ietf-quic-http],
Section 6.2.1.
2. Motivation for Replacing HTTP/2 Priorities
An important feature of any implementation of a protocol that
provides multiplexing is the ability to prioritize the sending of
information. This was an important realization in the design of
HTTP/2. Prioritization is a difficult problem, so it will always be
suboptimal, particularly if one endpoint operates in ignorance of the
needs of its peer.
HTTP/2 introduced a complex prioritization signaling scheme that used
a combination of dependencies and weights, formed into an unbalanced
tree. This scheme has suffered from poor deployment and
interoperability.
The rich flexibility of client-driven HTTP/2 prioritization tree
building is rarely exercised. Experience has shown that clients tend
to choose a single model optimized for a web use case and experiment
within the model constraints, or do nothing at all. Furthermore,
many clients build their prioritization tree in a unique way, which
makes it difficult for servers to understand their intent and act or
intervene accordingly.
Many HTTP/2 server implementations do not include support for the
priority scheme. Some instead favor custom server-driven schemes
based on heuristics or other hints, such as resource content type or
request generation order. For example, a server, with knowledge of
the document structure, might want to prioritize the delivery of
images that are critical to user experience above other images, but
below the CSS files. Since client trees vary, it is impossible for
the server to determine how such images should be prioritized against
other responses.
The HTTP/2 scheme allows intermediaries to coalesce multiple client
trees into a single tree that is used for a single upstream HTTP/2
connection. However, most intermediaries do not support this. The
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scheme does not define a method that can be used by a server to
express the priority of a response. Without such a method,
intermediaries cannot coordinate client-driven and server-driven
priorities.
HTTP/2 describes denial-of-service considerations for
implementations. On 2019-08-13 Netflix issued an advisory notice
about the discovery of several resource exhaustion vectors affecting
multiple HTTP/2 implementations. One attack, [CVE-2019-9513] aka
"Resource Loop", is based on manipulation of the priority tree.
The HTTP/2 scheme depends on in-order delivery of signals, leading to
challenges in porting the scheme to protocols that do not provide
global ordering. For example, the scheme cannot be used in HTTP/3
[I-D.ietf-quic-http] without changing the signal and its processing.
Considering the problems with deployment and adaptability to HTTP/3,
retaining the HTTP/2 priority scheme increases the complexity of the
entire system without any evidence that the value it provides offsets
that complexity. In fact, multiple experiments from independent
research have shown that simpler schemes can reach at least
equivalent performance characteristics compared to the more complex
HTTP/2 setups seen in practice, at least for the web use case.
2.1. Disabling HTTP/2 Priorities
The problems and insights set out above are motivation for allowing
endpoints to opt out of using the HTTP/2 priority scheme, in favor of
using an alternative such as the scheme defined in this
specification. The SETTINGS_DEPRECATE_HTTP2_PRIORITIES setting
described below enables endpoints to understand their peer's
intention. The value of the parameter MUST be 0 or 1. Any value
other than 0 or 1 MUST be treated as a connection error (see
[RFC7540], Section 5.4.1) of type PROTOCOL_ERROR.
Endpoints MUST send this SETTINGS parameter as part of the first
SETTINGS frame. When the peer receives the first SETTINGS frame, it
learns the sender has deprecated the HTTP/2 priority scheme if it
receives the SETTINGS_DEPRECATE_HTTP2_PRIORITIES parameter with the
value of 1.
A sender MUST NOT change the SETTINGS_DEPRECATE_HTTP2_PRIORITIES
parameter value after the first SETTINGS frame. Detection of a
change by a receiver MUST be treated as a connection error of type
PROTOCOL_ERROR.
Until the client receives the SETTINGS frame from the server, the
client SHOULD send both the priority signal defined in the HTTP/2
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priority scheme and also that of this prioritization scheme. Once
the client learns that the HTTP/2 priority scheme is deprecated, it
SHOULD stop sending the HTTP/2 priority signals. If the client
learns that the HTTP/2 priority scheme is not deprecated, it SHOULD
stop sending PRIORITY_UPDATE frames (Section 6.1), but MAY continue
sending the Priority header field (Section 4), as it is an end-to-end
signal that might be useful to nodes behind the server that the
client is directly connected to.
The SETTINGS frame precedes any priority signal sent from a client in
HTTP/2, so a server can determine if it should respect the HTTP/2
scheme before building state. A server that receives
SETTINGS_DEPRECATE_HTTP2_PRIORITIES MUST ignore HTTP/2 priority
signals.
Where both endpoints disable HTTP/2 priorities, the client is
expected to send this scheme's priority signal. Handling of omitted
signals is described in Section 3.
3. 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 4) is an end-to-end way to
transmit this set of parameters when a request or a response is
issued. In order to reprioritize a request, HTTP-version-specific
frames (Section 6.1 and Section 6.2) are used by clients to transmit
the same information on a single hop. If intermediaries want to
specify prioritization on a multiplexed HTTP connection, they SHOULD
use a PRIORITY_UPDATE frame and SHOULD NOT change the Priority header
field.
In both cases, the set of priority parameters is encoded as a
Structured Fields Dictionary ([STRUCTURED-FIELDS]).
This document defines the urgency("u") and incremental("i")
parameters. When receiving an HTTP request that does not carry these
priority parameters, a server SHOULD act as if their default values
were specified. Note that handling of omitted parameters is
different when processing an HTTP response; see Section 7.
Unknown parameters, parameters with out-of-range values or values of
unexpected types MUST be ignored.
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3.1. Urgency
The urgency parameter ("u") takes an integer between 0 and 7, in
descending order of priority. This range provides sufficient
granularity for prioritizing responses for ordinary web browsing, at
minimal complexity.
The value is encoded as an sf-integer. The default value is 3.
This parameter indicates the sender's recommendation, based on the
expectation that the server would transmit HTTP responses in the
order of their urgency values if possible. 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 web-site (see Section 7).
The lowest urgency level (7) is reserved for background tasks such as
delivery of software updates. This urgency level SHOULD NOT be used
for fetching responses that have impact on user interaction.
3.2. Incremental
The incremental parameter ("i") takes an sf-boolean as the value that
indicates if an HTTP response can be processed incrementally, i.e.
provide some meaningful output as chunks of the response arrive.
The default value of the incremental parameter is false ("0").
A server might distribute the bandwidth of a connection between
incremental responses that share the same urgency, hoping that
providing those responses in parallel would be more helpful to the
client than delivering the responses one by one.
If a client makes concurrent requests with the incremental parameter
set to false, there is no benefit serving responses in parallel
because the client is not going to process those responses
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incrementally. Serving non-incremental responses one by one, in the
order in which those requests were generated is considered to be the
best strategy.
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
3.3. Defining New Parameters
When attempting to extend priorities, care must be taken to ensure
any use of existing parameters leaves them either unchanged or
modified in a way that is backwards compatible for peers that are
unaware of the extended meaning.
For example, if there is a need to provide more granularity than
eight urgency levels, it would be possible to subdivide the range
using an additional parameter. Implementations that do not recognize
the parameter can safely continue to use the less granular eight
levels.
Alternatively, the urgency can be augmented. For example, a
graphical user agent could send a "visible" parameter to indicate if
the resource being requested is within the viewport.
4. The Priority HTTP Header Field
The Priority HTTP header field can appear in requests and responses.
A client uses it to specify the priority of the response. A server
uses it to inform the client that the priority was overwritten. An
intermediary can use the Priority information from client requests
and server responses to correct or amend the precedence to suit it
(see Section 7).
The Priority header field is an end-to-end signal of the request
priority from the client or the response priority from the server.
As is the ordinary case for HTTP caching ([RFC7234]), a response with
a Priority header field might be cached and re-used for subsequent
requests. When an origin server generates the Priority response
header field based on properties of an HTTP request it receives, the
server is expected to control the cacheability or the applicability
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of the cached response, by using header fields that control the
caching behavior (e.g., Cache-Control, Vary).
An endpoint that fails to parse the Priority header field SHOULD use
default parameter values.
5. Reprioritization
After a client sends a request, it may be beneficial to change the
priority of the response. As an example, a web browser might issue a
prefetch request for a JavaScript file with the urgency parameter of
the Priority request header field set to "u=7" (background). Then,
when the user navigates to a page which references the new JavaScript
file, while the prefetch is in progress, the browser would send a
reprioritization signal with the priority field value set to "u=0".
The PRIORITY_UPDATE frame (Section 6) can be used for such
reprioritization.
6. The PRIORITY_UPDATE Frame
This document specifies a new PRIORITY_UPDATE frame for HTTP/2
([RFC7540]) and HTTP/3 ([I-D.ietf-quic-http]). It carries priority
parameters and references the target of the prioritization based on a
version-specific identifier. In HTTP/2, this identifier is the
Stream ID; in HTTP/3, the identifier is either the Stream ID or Push
ID. Unlike the Priority header field, the PRIORITY_UPDATE frame is a
hop-by-hop signal.
PRIORITY_UPDATE frames are sent by clients on the control stream,
allowing them to be sent independent from the stream that carries the
response. This means they can be used to reprioritize a response or
a push stream; or signal the initial priority of a response instead
of the Priority header field.
A PRIORITY_UPDATE frame communicates a complete set of all parameters
in the Priority Field Value field. Omitting a parameter is a signal
to use the parameter's default value. Failure to parse the Priority
Field Value MUST be treated as a connection error. In HTTP/2 the
error is of type PROTOCOL_ERROR; in HTTP/3 the error is of type
H3_FRAME_ERROR.
A client MAY send a PRIORITY_UPDATE frame before the stream that it
references is open. 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
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frame can be considered as the most up-to-date information that
overrides any other signal. Servers SHOULD buffer the most recently
received PRIORITY_UPDATE frame and apply it once the referenced
stream is opened. Holding PRIORITY_UPDATE frames for each stream
requires server resources, which can can be bound by local
implementation policy. (TODO: consider resolving #1261, and adding
more text about bounds). Although there is no limit to the number
PRIORITY_UPDATES that can be sent, storing only the most recently
received frame limits resource commitment.
6.1. 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 ([RFC7540], Section 4.1) 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.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
|R| Prioritized Stream ID (31) |
+---------------------------------------------------------------+
| Priority Field Value (*) ...
+---------------------------------------------------------------+
Figure 1: HTTP/2 PRIORITY_UPDATE Frame Payload
The PRIORITY_UPDATE frame payload has the following fields:
R: A reserved 1-bit field. The semantics of this bit are undefined,
and the bit MUST remain unset (0x0) when sending and MUST be
ignored when receiving.
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.
The Prioritized Stream ID MUST be within the stream limit. If a
server receives a PRIORITY_UPDATE with a Prioritized Stream ID that
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is beyond the stream limits, this SHOULD be treated as a connection
error of type PROTOCOL_ERROR.
If a PRIORITY_UPDATE frame is received with a Prioritized Stream ID
of 0x0, the recipient MUST respond with a connection error of type
PROTOCOL_ERROR.
If a client receives a PRIORITY_UPDATE frame, it MUST respond with a
connection error of type PROTOCOL_ERROR.
6.2. HTTP/3 PRIORITY_UPDATE Frame
The HTTP/3 PRIORITY_UPDATE frame (type=0xF0700 or 0xF0701) is used by
clients to signal the initial priority of a response, or to
reprioritize a response or push stream. It carries the identifier of
the element that is being prioritized, and the updated priority in
ASCII text, using the same representation as that of the Priority
header field value. PRIORITY_UPDATE with a frame type of 0xF0700 is
used for request streams, while PRIORITY_UPDATE with a frame type of
0xF0701 is used for push streams.
The PRIORITY_UPDATE frame MUST be sent on the client control stream
([I-D.ietf-quic-http], Section 6.2.1). 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.
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
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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.
The push-stream variant PRIORITY_UPDATE (type=0xF0701) MUST reference
a promised push stream. If a server receives a PRIORITY_UPDATE
(type=0xF0701) with a Push ID that is greater than the maximum Push
ID or which has not yet been promised, this MUST be treated as a
connection error of type H3_ID_ERROR.
PRIORITY_UPDATE frames of either type are only sent by clients. If a
client receives a PRIORITY_UPDATE frame, this MUST be treated as a
connection error of type H3_FRAME_UNEXPECTED.
7. Merging Client- and Server-Driven Parameters
It is not always the case that the client has the best understanding
of how the HTTP responses deserve to be prioritized. The server
might have additional information that can be combined with the
client's indicated priority in order to improve the prioritization of
the response. For example, use of an HTML document might depend
heavily on one of the inline images; existence of such dependencies
is typically best known to the server. Or, a server that receives
requests for a font [RFC8081] and images with the same urgency might
give higher precedence to the font, so that a visual client can
render textual information at an early moment.
An origin can use the Priority response header field to indicate its
view on how an HTTP response should be prioritized. An intermediary
that forwards an HTTP response can use the parameters found in the
Priority response header field, in combination with the client
Priority request header field, as input to its prioritization
process. No guidance is provided for merging priorities, this is
left as an implementation decision.
Absence of a priority parameter in an HTTP response indicates the
server's disinterest in changing the client-provided value. This is
different from the logic being defined for the request header field,
in which omission of a priority parameter implies the use of their
default values (see Section 3).
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"
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:method = GET
:scheme = https
:authority = example.net
:path = /menu.png
priority = u=5, i
and the origin responds with
:status = 200
content-type = image/png
priority = u=1
the intermediary might alter its understanding of the urgency from
"5" to "1", because it prefers the server-provided value over the
client's. The incremental value continues to be "true", the value
specified by the client, as the server did not specify the
incremental("i") parameter.
8. Client Scheduling
A client MAY use priority values to make local processing or
scheduling choices about the requests it initiates.
9. Server Scheduling
Priority signals are input to a prioritization process. They do not
guarantee any particular processing or transmission order for one
response relative to any other response. An endpoint cannot force a
peer to process concurrent request in a particular order using
priority. Expressing priority is therefore only a suggestion.
A server can use priority signals along with other inputs to make
scheduling decisions. No guidance is provided about how this can or
should be done. Factors such as implementation choices or deployment
environment also play a role. Any given connection is likely to have
many dynamic permutations. For these reasons, there is no unilateral
perfect scheduler and this document only provides some basic
recommendations for implementations.
Clients cannot depend on particular treatment based on priority
signals. Servers can use other information to prioritize responses.
It is RECOMMENDED that, when possible, servers respect the urgency
parameter (Section 3.1), sending higher urgency responses before
lower urgency responses.
It is RECOMMENDED that, when possible, servers respect the
incremental parameter (Section 3.2). Non-incremental responses of
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the same urgency SHOULD be served one-by-one based on the Stream ID,
which corresponds to the order in which clients make requests. Doing
so ensures that clients can use request ordering to influence
response order. Incremental responses of the same urgency SHOULD be
served in round-robin manner.
The incremental parameter indicates how a client processes response
bytes as they arrive. Non-incremental resources are only used when
all of the response payload has been received. Incremental resources
are used as parts, or chunks, of the response payload are received.
Therefore, the timing of response data reception at the client, such
as the time to early bytes or the time to receive the entire payload,
plays an important role in perceived performance. Timings depend on
resource size but this scheme provides no explicit guidance about how
a server should use size as an input to prioritization. Instead, the
following examples demonstrate how a server that strictly abides the
scheduling guidance based on urgency and request generation order
could find that early requests prevent serving of later requests.
1. At the same urgency level, a non-incremental request for a large
resource followed by an incremental request for a small resource.
2. At the same urgency level, an incremental request of
indeterminate length followed by a non-incremental large
resource.
It is RECOMMENDED that servers avoid such starvation where possible.
The method to do so is an implementation decision. For example, a
server might pre-emptively send responses of a particular incremental
type based on other information such as content size.
10. Fairness
As a general guideline, a server SHOULD NOT use priority information
for making schedule decisions across multiple connections, unless it
knows that those connections originate from the same client. Due to
this, priority information conveyed over a non-coalesced HTTP
connection (e.g., HTTP/1.1) might go unused.
The remainder of this section discusses scenarios where unfairness is
problematic and presents possible mitigations, or where unfairness is
desirable.
TODO: Discuss if we should add a signal that mitigates this issue.
For example, we might add a SETTINGS parameter that indicates the
next hop that the connection is NOT coalesced (see
https://github.com/kazuho/draft-kazuho-httpbis-priority/issues/99).
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10.1. Coalescing Intermediaries
When an intermediary coalesces HTTP requests coming from multiple
clients into one HTTP/2 or HTTP/3 connection going to the backend
server, requests that originate from one client might have higher
precedence than those coming from others.
It is sometimes beneficial for the server running behind an
intermediary to obey to the value of the Priority header field. As
an example, a resource-constrained server might defer the
transmission of software update files that would have the background
urgency being associated. However, in the worst case, the asymmetry
between the precedence declared by multiple clients might cause
responses going to one user agent to be delayed totally after those
going to another.
In order to mitigate this fairness problem, a server could use
knowledge about the intermediary as another signal in its
prioritization decisions. For instance, if a server knows the
intermediary is coalescing requests, then it could serve the
responses in round-robin manner. This can work if the constrained
resource is network capacity between the intermediary and the user
agent, as the intermediary buffers responses and forwards the chunks
based on the prioritization scheme it implements.
A server can determine if a request came from an intermediary through
configuration, or by consulting if that request contains one of the
following header fields:
o Forwarded, X-Forwarded-For ([RFC7239])
o Via ([RFC7230], Section 5.7.1)
10.2. HTTP/1.x Back Ends
It is common for CDN infrastructure to support different HTTP
versions on the front end and back end. For instance, the client-
facing edge might support HTTP/2 and HTTP/3 while communication to
back end servers is done using HTTP/1.1. Unlike with connection
coalescing, the CDN will "de-mux" requests into discrete connections
to the back end. As HTTP/1.1 and older do not provide a way to
concurrently transmit multiple responses, there is no immediate
fairness issue in protocol. However, back end servers MAY still use
client headers for request scheduling. Back end servers SHOULD only
schedule based on client priority information where that information
can be scoped to individual end clients. Authentication and other
session information might provide this linkability.
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10.3. 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.
11. Why use an End-to-End Header Field?
Contrary to the prioritization scheme of HTTP/2 that uses a hop-by-
hop frame, the Priority header field is defined as end-to-end.
The rationale is that the Priority header field transmits how each
response affects the client's processing of those responses, rather
than how relatively urgent each response is to others. The way a
client processes a response is a property associated to that client
generating that request. Not that of an intermediary. Therefore, it
is an end-to-end property. How these end-to-end properties carried
by the Priority header field affect the prioritization between the
responses that share a connection is a hop-by-hop issue.
Having the Priority header field defined as end-to-end is important
for caching intermediaries. Such intermediaries can cache the value
of the Priority header field along with the response, and utilize the
value of the cached header field when serving the cached response,
only because the header field is defined as end-to-end rather than
hop-by-hop.
It should also be noted that the use of a header field carrying a
textual value makes the prioritization scheme extensible; see the
discussion below.
12. Security Considerations
[CVE-2019-9513] aka "Resource Loop", is a DoS attack based on
manipulation of the HTTP/2 priority tree. Extensible priorities does
not use stream dependencies, which mitigates this vulnerability.
TBD: depending on the outcome of reprioritization discussions,
following paragraphs may change or be removed.
[RFC7540], Section 5.3.4 describes a scenario where closure of
streams in the priority tree could cause suboptimal prioritization.
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To avoid this, [RFC7540] states that "an endpoint SHOULD retain
stream prioritization state for a period after streams become
closed". Retaining state for streams no longer counted towards
stream concurrency consumes server resources. Furthermore, [RFC7540]
identifies that reprioritization of a closed stream could affect
dependents; it recommends updating the priority tree if sufficient
state is stored, which will also consume server resources. To limit
this commitment, it is stated that "The amount of prioritization
state that is retained MAY be limited" and "If a limit is applied,
endpoints SHOULD maintain state for at least as many streams as
allowed by their setting for SETTINGS_MAX_CONCURRENT_STREAMS.".
Extensible priorities does not use stream dependencies, which
minimizes most of the resource concerns related to this scenario.
[RFC7540], Section 5.3.4 also presents considerations about the state
required to store priority information about streams in an "idle"
state. This state can be limited by adopting the guidance about
concurrency limits described above. Extensible priorities is subject
to a similar consideration because PRIORITY_UPDATE frames may arrive
before the request that they reference. A server is required to
store the information in order to apply the most up-to-date signal to
the request. However, HTTP/3 implementations might have practical
barriers to determining reasonable stream concurrency limits
depending on the information that is available to them from the QUIC
transport layer. TODO: so what can we suggest?
13. IANA Considerations
This specification registers the following entry in the Permanent
Message Header Field Names registry established by [RFC3864]:
Header field name: Priority
Applicable protocol: http
Status: standard
Author/change controller: IETF
Specification document(s): This document
Related information: n/a
This specification registers the following entry in the HTTP/2
Settings registry established by [RFC7540]:
Name: SETTINGS_DEPRECATE_HTTP2_PRIORITIES
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Code: 0x9
Initial value: 0
Specification: This document
This specification registers the following entry in the HTTP/2 Frame
Type registry established by [RFC7540]:
Frame Type: PRIORITY_UPDATE
Code: 0x10
Specification: This document
This specification registers the following entries in the HTTP/3
Frame Type registry established by [I-D.ietf-quic-http]:
Frame Type: PRIORITY_UPDATE
Code: 0xF0700 and 0xF0701
Specification: This document
14. References
14.1. Normative References
[I-D.ietf-quic-http]
Bishop, M., "Hypertext Transfer Protocol Version 3
(HTTP/3)", draft-ietf-quic-http-31 (work in progress),
September 2020.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-31 (work
in progress), September 2020.
[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>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
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[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[STRUCTURED-FIELDS]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", draft-ietf-httpbis-header-structure-19 (work in
progress), June 2020.
14.2. Informative References
[CVE-2019-9513]
Common Vulnerabilities and Exposures, "CVE-2019-9513",
March 2019, <https://cve.mitre.org/cgi-bin/
cvename.cgi?name=CVE-2019-9513>.
[I-D.lassey-priority-setting]
Lassey, B. and L. Pardue, "Declaring Support for HTTP/2
Priorities", draft-lassey-priority-setting-00 (work in
progress), July 2019.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
[RFC7239] Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
RFC 7239, DOI 10.17487/RFC7239, June 2014,
<https://www.rfc-editor.org/info/rfc7239>.
[RFC8081] Lilley, C., "The "font" Top-Level Media Type", RFC 8081,
DOI 10.17487/RFC8081, February 2017,
<https://www.rfc-editor.org/info/rfc8081>.
14.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] https://httpwg.org/
[3] https://github.com/httpwg/http-extensions/labels/priorities
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[4] http://tools.ietf.org/agenda/83/slides/slides-83-httpbis-5.pdf
[5] https://github.com/pmeenan/http3-prioritization-proposal
Appendix A. Acknowledgements
Roy Fielding presented the idea of using a header field for
representing priorities in http://tools.ietf.org/agenda/83/slides/
slides-83-httpbis-5.pdf [4]. In https://github.com/pmeenan/http3-
prioritization-proposal [5], Patrick Meenan advocates for
representing the priorities using a tuple of urgency and concurrency.
The ability to deprecate HTTP/2 prioritization is based on
[I-D.lassey-priority-setting], authored by Brad Lassey and Lucas
Pardue, with modifications based on feedback that was not
incorporated into an update to that document.
The motivation for defining an alternative to HTTP/2 priorities is
drawn from discussion within the broad HTTP community. Special
thanks to Roberto Peon, Martin Thomson and Netflix for text that was
incorporated explicitly in this document.
In addition to the people above, this document owes a lot to the
extensive discussion in the HTTP priority design team, consisting of
Alan Frindell, Andrew Galloni, Craig Taylor, Ian Swett, Kazuho Oku,
Lucas Pardue, Matthew Cox, Mike Bishop, Roberto Peon, Robin Marx, Roy
Fielding.
Appendix B. Change Log
B.1. Since draft-ietf-httpbis-priority-01
o PRIORITY_UPDATE frame changes (#1096, #1079, #1167, #1262, #1267,
#1271)
o Add section to describe server scheduling considerations (#1215,
#1232, #1266)
o Remove specific instructions related to intermediary fairness
(#1022, #1264)
B.2. Since draft-ietf-httpbis-priority-00
o Move text around (#1217, #1218)
o Editorial change to the default urgency. The value is 3, which
was always the intent of previous changes.
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B.3. Since draft-kazuho-httpbis-priority-04
o Minimize semantics of Urgency levels (#1023, #1026)
o Reduce guidance about how intermediary implements merging priority
signals (#1026)
o Remove mention of CDN-Loop (#1062)
o Editorial changes
o Make changes due to WG adoption
o Removed outdated Consideration (#118)
B.4. Since draft-kazuho-httpbis-priority-03
o Changed numbering from "[-1,6]" to "[0,7]" (#78)
o Replaced priority scheme negotiation with HTTP/2 priority
deprecation (#100)
o Shorten parameter names (#108)
o Expand on considerations (#105, #107, #109, #110, #111, #113)
B.5. Since draft-kazuho-httpbis-priority-02
o Consolidation of the problem statement (#61, #73)
o Define SETTINGS_PRIORITIES for negotiation (#58, #69)
o Define PRIORITY_UPDATE frame for HTTP/2 and HTTP/3 (#51)
o Explain fairness issue and mitigations (#56)
B.6. Since draft-kazuho-httpbis-priority-01
o Explain how reprioritization might be supported.
B.7. Since draft-kazuho-httpbis-priority-00
o Expand urgency levels from 3 to 8.
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Authors' Addresses
Kazuho Oku
Fastly
Email: kazuhooku@gmail.com
Lucas Pardue
Cloudflare
Email: lucaspardue.24.7@gmail.com
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