Extensible Prioritization Scheme for HTTP

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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.

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.

The priority scheme defined by this document is primarily focused on the prioritization of HTTP response messages (see Section 3.4 of [HTTP]). It defines new priority parameters (Section 4) and a means of conveying those parameters (Sections 5 and 7), which is intended to communicate the priority of responses to a server that is responsible for prioritizing them. Section 10 provides considerations for servers about acting on those signals in combination with other inputs and factors.

The CONNECT method (see Section 9.3.6 of [HTTP]) can be used to establish tunnels. Signaling applies similarly to tunnels; additional considerations for server prioritization are given in Section 11.

Section 9 describes how clients can optionally apply elements of this scheme locally to the request messages that they generate.

Some forms of HTTP extensions might change HTTP/2 or HTTP/3 stream behavior or define new data carriage mechanisms. Such extensions can themselves define how this priority scheme is to be applied.

The priority information is a sequence of key-value pairs, providing room for future extensions. Each key-value pair represents a priority parameter.

The Priority HTTP header field (Section 5) is an end-to-end way to transmit this set of priority parameters when a request or a response is issued. After sending a request, a client can change their view of response priority (Section 6) by sending HTTP-version-specific PRIORITY_UPDATE frames as defined in Sections 7.1 and 7.2. Frames transmit priority parameters on a single hop only.

Intermediaries can consume and produce priority signals in a PRIORITY_UPDATE frame or Priority header field. An intermediary that passes only the Priority request header field to the next hop preserves the original end-to-end signal from the client; see Section 14. An intermediary could pass the Priority header field and additionally send a PRIORITY_UPDATE frame. This would have the effect of preserving the original client end-to-end signal, while instructing the next hop to use a different priority, per the guidance in Section 7. An intermediary that replaces or adds a Priority request header field overrides the original client end-to-end signal, which can affect prioritization for all subsequent recipients of the request.

For both the Priority header field and the PRIORITY_UPDATE frame, the set of priority parameters is encoded as a Dictionary (see Section 3.2 of [STRUCTURED-FIELDS]).

This document defines the urgency (u) and incremental (i) priority parameters. When receiving an HTTP request that does not carry these priority parameters, a server SHOULD act as if their default values were specified.

An intermediary can combine signals from requests and responses that it forwards. Note that omission of priority parameters in responses is handled differently from omission in requests; see Section 8.

Receivers parse the Dictionary as described in Section 4.2 of [STRUCTURED-FIELDS]. Where the Dictionary is successfully parsed, this document places the additional requirement that unknown priority parameters, priority parameters with out-of-range values, or values of unexpected types MUST be ignored.

The Priority HTTP header field is a Dictionary that carries priority parameters (see Section 4). It can appear in requests and responses. It is an end-to-end signal that indicates the endpoint's view of how HTTP responses should be prioritized. Section 8 describes how intermediaries can combine the priority information sent from clients and servers. Clients cannot interpret the appearance or omission of a Priority response header field as acknowledgement that any prioritization has occurred. Guidance for how endpoints can act on Priority header values is given in Sections 9 and 10.

An HTTP request with a Priority header field might be cached and reused for subsequent requests; see [CACHING]. When an origin server generates the Priority response header field based on properties of an HTTP request it receives, the server is expected to control the cacheability or the applicability of the cached response by using header fields that control the caching behavior (e.g., Cache-Control, Vary).

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.

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.

It is not always the case that the client has the best understanding of how the HTTP responses deserve to be prioritized. The server might have additional information that can be combined with the client's indicated priority in order to improve the prioritization of the response. For example, use of an HTML document might depend heavily on one of the inline images; the existence of such dependencies is typically best known to the server. Or, a server that receives requests for a font [RFC8081] and images with the same urgency might give higher precedence to the font, so that a visual client can render textual information at an early moment.

An origin can use the Priority response header field to indicate its view on how an HTTP response should be prioritized. An intermediary that forwards an HTTP response can use the priority parameters found in the Priority response header field, in combination with the client Priority request header field, as input to its prioritization process. No guidance is provided for merging priorities; this is left as an implementation decision.

The absence of a priority parameter in an HTTP response indicates the server's disinterest in changing the client-provided value. This is different from the request header field, in which omission of a priority parameter implies the use of its default value (see Section 4).

As a non-normative example, when the client sends an HTTP request with the urgency parameter set to 5 and the incremental parameter set to true

:method = GET
:scheme = https
:authority = example.net
:path = /menu.png
priority = u=5, i

and the origin responds with

:status = 200
content-type = image/png
priority = u=1

the intermediary might alter its understanding of the urgency from 5 to 1, because it prefers the server-provided value over the client's. The incremental value continues to be true, i.e., the value specified by the client, as the server did not specify the incremental (i) parameter.

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

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.

1. At the same urgency level, a non-incremental request for a large resource followed by an incremental request for a small resource.
2. At the same urgency level, an incremental request of indeterminate length followed by a non-incremental large resource.

It is RECOMMENDED that servers avoid such starvation where possible. The method for doing so is an implementation decision. For example, a server might preemptively send responses of a particular incremental type based on other information such as content size.

Optimal scheduling of server push is difficult, especially when pushed resources contend with active concurrent requests. Servers can consider many factors when scheduling, such as the type or size of resource being pushed, the priority of the request that triggered the push, the count of active concurrent responses, the priority of other active concurrent responses, etc. There is no general guidance on the best way to apply these. A server that is too simple could easily push at too high a priority and block client requests, or push at too low a priority and delay the response, negating intended goals of server push.

Priority signals are a factor for server push scheduling. The concept of parameter value defaults applies slightly differently because there is no explicit client-signaled initial priority. A server can apply priority signals provided in an origin response; see the merging guidance given in Section 8. In the absence of origin signals, applying default parameter values could be suboptimal. By whatever means a server decides to schedule a pushed response, it can signal the intended priority to the client by including the Priority field in a PUSH_PROMISE or HEADERS frame.

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.

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.

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.

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.

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.

Field Name:

Priority

Status:

permanent

Reference:

This document

Code:

0x9

Name:

SETTINGS_NO_RFC7540_PRIORITIES

Initial Value:

0

Reference:

This document

Code:

0x10

Frame Type:

PRIORITY_UPDATE

Reference:

This document

Value:

0xF0700-0xF0701

Frame Type:

PRIORITY_UPDATE

Status:

permanent

Reference:

This document

Change Controller:

IETF

Contact:

ietf-http-wg@w3.org

IANA has created the "Hypertext Transfer Protocol (HTTP) Priority" registry at <https://www.iana.org/assignments/http-priority> and has populated it with the entries in Table 1; see Section 4.3.1 for its associated procedures.

Roy Fielding presented the idea of using a header field for representing priorities in <https://www.ietf.org/proceedings/83/slides/slides-83-httpbis-5.pdf>. In <https://github.com/pmeenan/http3-prioritization-proposal>, Patrick Meenan advocated for representing the priorities using a tuple of urgency and concurrency. The ability to disable HTTP/2 prioritization is inspired by [PRIORITY-SETTING], authored by Brad Lassey and Lucas Pardue, with modifications based on feedback that was not incorporated into an update to that document.

The motivation for defining an alternative to HTTP/2 priorities is drawn from discussion within the broad HTTP community. Special thanks to Roberto Peon, Martin Thomson, and Netflix for text that was incorporated explicitly in this document.

In addition to the people above, this document owes a lot to the extensive discussion in the HTTP priority design team, consisting of Alan Frindell, Andrew Galloni, Craig Taylor, Ian Swett, Matthew Cox, Mike Bishop, Roberto Peon, Robin Marx, Roy Fielding, and the authors of this document.

Yang Chi contributed the section on retransmission scheduling.