Extensible Prioritization Scheme for HTTP

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.¶

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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.¶

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 favoring instead bespoke server-driven schemes based on heuristics and other hints, like the content type of resources and the 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 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.¶

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 5.1 and Section 5.2) 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 Headers Dictionary ([STRUCTURED-HEADERS]).¶

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 6.¶

Unknown parameters, parameters with out-of-range values or values of unexpected types MUST be ignored.¶

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 6).¶

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 of the cached response, by using header fields that control the caching behavior (e.g., Cache-Control, Vary).¶

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 frame with the priority field value set to u=0.¶

In HTTP/2 and HTTP/3, after a request message is sent on a stream, the stream transitions to a state that prevents the client from sending additional frames on the stream. Therefore, a client cannot reprioritize a response by using the Priority header field. Modifying this behavior would require a semantic change to the protocol, but this is avoided by restricting the stream on which a PRIORITY_UPDATE frame can be sent. In HTTP/2 the frame is on stream zero and in HTTP/3 it is sent on the control stream ([I-D.ietf-quic-http], Section 6.2.1).¶

This document specifies a new PRIORITY_UPDATE frame type for HTTP/2 ([RFC7540]) and HTTP/3 ([I-D.ietf-quic-http]) which enables reprioritization. It carries updated priority parameters and references the target of the reprioritization based on a version-specific identifier; in HTTP/2 this is the Stream ID, in HTTP/3 this is either the Stream ID or Push ID.¶

Unlike the header field, the reprioritization frame is a hop-by-hop signal.¶

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 ¶

and the origin responds with¶

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.¶

A client MAY use priority values to make local scheduling choices about the requests it initiates.¶

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).¶

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.¶

[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. 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?¶

Header field name:

Priority

Applicable protocol:

http

Status:

standard

Author/change controller:

IETF

Specification document(s):

This document

Related information:

n/a

Name:

SETTINGS_DEPRECATE_HTTP2_PRIORITIES

Code:

0x9

Initial value:

0

Specification:

This document

Frame Type:

PRIORITY_UPDATE

Code:

0xF

Specification:

This document

Frame Type:

PRIORITY_UPDATE

Code:

0xF

Specification:

This document

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. In https://github.com/pmeenan/http3-prioritization-proposal, 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.¶