HTTP priority order extension

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Table of Contents

1. Introduction

The extensible prioritization scheme for HTTP provides guidance for servers using priority signals to schedule the sending of stream data; Section 10 of [RFC9218]. It recommends that when there are multiple items with the same urgency servers choose how to allocate bandwidth by considering the stream ID and incremental parameter, and possibly other signals.¶

In web use cases, an HTML document can include other resources and user agents tend to issue requests in an ordered sequence that matches the actions of HTML or HTTP receiver processing. Requests are made using ascending stream IDs where lower-numbered streams typically relate to objects that were discovered earlier than higher-numbered streams. Because stream IDs represent an ordering that closely matches the receiver processing needs, the scheduling guidance in [RFC9218] can be effectively used to send data that is optimal for this consumption pattern. As a simple example, a client that issues non-incremental requests of the same urgency on streams 0, 4, 8 and 12 would expect a server , that is following Section 10 of [RFC9218], to serve the response in the order 0, 4, 8 and 12.¶

A strictly singular serving order is unlikely to meet the needs of HTTP-based applications. The urgency and incremental parameters augment stream ID, urgency allows more important resources (from a processing perspective) to be sent ahead of less important resources, while incremental streams share bandwidth among resources that can be effectively loaded in parallel.¶

There are other use cases where the user agent may not be able to influence the order of request emission in the most optimal sequence, or may discover an important resource late on and would like the response to "jump the queue" of other streams within a single urgency level. The send-order parameter defined by this document can be used in these situations. While it might be possible to achieve a similar outcome by careful use of the urgency parameter, such as reserving an urgency level, the limited number of levels can make this approach difficult in practice. Alternatively, rebalancing or reprioritization of concurrent stream urgencies might also achieve these goals but encounters issues related to churn and responsiveness due to the latency of reprioritization signals.¶

2. Conventions and Definitions

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].¶

3. The send-order parameter

The send-order (bikeshed-order-name) parameter value is Integer (see Section 3.3.1 of [STRUCTURED-FIELDS]), between 0 and 2³². The range is ascending, higher values have precedence over lower values.¶

Endpoints use send-order to communicate an ordering precedence that is independent of the request order. There is no default value, omission of the parameter means that there is no additional ordering preference.¶

The send-order parameter satisfies the urgency and incremental parameter compatibility requirements in Section 4.3 of [RFC9218].¶

The following example shows an HTTP/3 client issuing multiple requests on the same connection, without the send-order parameter, and responses sent by a server following the guidance in Section 10 of [RFC9218].¶

Client                              Server

HEADERS (stream 0)
  :method = GET
  :scheme = https
  :authority = example.net
  :path = /image1.jpg
  priority = u=1

HEADERS (stream 4)
  :method = GET
  :scheme = https
  :authority = example.net
  :path = /image2.jpg
  priority = u=1

HEADERS (stream 8)
  :method = GET
  :scheme = https
  :authority = example.net
  :path = /image3.jpg
  priority = u=1

                                  HEADERS (stream 0)
                                    :status = 200
                                    content-type = image/jpeg

                                  DATA (stream 0)
                                    {binary data}

                                  HEADERS (stream 4)
                                    :status = 200
                                    content-type = image/jpeg

                                  DATA (stream 4)
                                    {binary data}

                                  HEADERS (stream 8)
                                    :status = 200
                                    content-type = image/jpeg

                                  DATA (stream 8)
                                    {binary data}

The following example shows an HTTP/3 client issuing multiple requests on the same connection, without the send-order parameter, and responses sent by a server following the guidance in Section 4.¶

Client                              Server

HEADERS (stream 0)
  :method = GET
  :scheme = https
  :authority = example.net
  :path = /image1.jpg
  priority = u=1

HEADERS (stream 4)
  :method = GET
  :scheme = https
  :authority = example.net
  :path = /image2.jpg
  priority = u=1,bikeshed-order-name=25

HEADERS (stream 8)
  :method = GET
  :scheme = https
  :authority = example.net
  :path = /image3.jpg
  priority = u=1,bikeshed-order-name=15

                                  HEADERS (stream 4)
                                    :status = 200
                                    content-type = image/jpeg

                                  DATA (stream 4)
                                    {binary data}

                                  HEADERS (stream 8)
                                    :status = 200
                                    content-type = image/jpeg

                                  DATA (stream 8)
                                    {binary data}

                                  HEADERS (stream 0)
                                    :status = 200
                                    content-type = image/jpeg

                                  DATA (stream 0)
                                    {binary data}

4. Server Scheduling

This document updates the scheduling guidance in Section 10 of [RFC9218].¶

Non-incremental responses of the same urgency SHOULD be served by prioritizing bandwidth allocation in ascending order of send-order. If there are no responses with a send-order, allocation SHOULD use ascending order of stream ID.¶

Incremental responses of the same urgency are not affected by send-order.¶

5. Security Considerations

The considerations in [RFC9218] apply. There are not believed to be any additional considerations.¶

6. IANA Considerations

TODO¶

Acknowledgments

TODO acknowledge.¶

Author's Address