HTTP Datagram Prioritization

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Table of Contents

1. Introduction

Application protocols using the QUIC transport protocol [QUIC] rely on streams, and optionally the DATAGRAM extension [QUIC-DATAGRAM], to carry application data. Streams and datagrams can be multiplexed but QUIC provides no interoperable prioritization scheme or signaling mechanism itself. The HTTP Extensible Prioritization scheme [I-D.ietf-httpbis-priority] describes how to prioritize streams in HTTP/2 and HTTP/3. This document adopts the scheme to support HTTP datagrams [HTTP-DATAGRAM].¶

The Extensible Priorities scheme for HTTP describes how clients can send priority signals related to requests in order to suggest how a server allocates resources to serving responses. When the protocol is HTTP/2, responses are carried on streams. When the protocol is HTTP/3, responses are carries on QUIC streams.¶

While QUIC streams support multiplexing natively via use of a stream identifier, the QUIC DATAGRAM extension does not provide any such identifier. HTTP datagrams [HTTP-DATAGRAM] supports multiplexing using a set of application-level identifiers that can be controlled and accessed by HTTP/3. One identifer relates to a request stream, the second, optional, identifer relates to an abstract context. [HTTP-DATAGRAM] does not, however, define any means for multiplexed datagram prioritization.¶

When the application protocol is HTTP/3, HTTP Datagrams can map directly to QUIC datagrams or they can be carried on streams using a DATAGRAM Capsule; see Section 4.4 of [HTTP-DATAGRAM].¶

This document describes how the Extensible Priorities scheme applies to HTTP datagrams. Priority signals sent by clients, related to requests, can also be considered input to server scheduling decisions for HTTP datagrams mapped to QUIC datagrams.¶

2. Signalling Datagram Priority

The Extensible Prioritization scheme [I-D.ietf-httpbis-priority] provides a framework for communicating and acting upon priority parameters, using [STRUCTURED-FIELDS] formats. It defines the urgency and incremental parameters and provides guidance to implementers about how to act on these parameters, in combination with other inputs, to make resource allocation and scheduling choices. Urgency communicates the client-view of request importance, and incremental communicates how the client intends to process response data as it arrives. Parameters are communicated in HTTP headers or version-specific frames. A client omitting the urgency or incremental parameters can be interpreted by the server as a signal to apply default priorities. The core scheme is extensible, new parameters can be defined to augment the base ones.¶

This specification defines the datagram-urgency (du) extension parameter that operates in addition to the base urgency. There is no extension to the base incremental behavior; individual datragrams, even if belonging to the same identifier, are messages that are expected to be processed individually as they arrive.¶

:method = GET
:scheme = https
:authority = example.net
:path = /style.css
priority = u=0, du=2

3. Client Scheduling

Clients MAY use datagram-urgency to make local processing or scheduling choices about HTTP datagrams related to the requests it initiates.¶

4. Server Scheduling

Priority signals are input to a prioritization process. Expressing priority is only a suggestion. The datagram-urgency parameter introduces new scheduling considerations on top of those presented in Section 10 of [I-D.ietf-httpbis-priority].¶

It is RECOMMENDED that, when possible, servers send higher urgency HTTP datagrams before lower urgency datagrams.¶

Where streams and datagrams have equal urgency and datagram-urgency, it is RECOMMENDED that servers alternate emitting HTTP datagrams and stream bytes. Where servers implement the recommendations in Section 10 of [I-D.ietf-httpbis-priority], alternating between datagram and stream data will result in fair scheduling. This recommendation holds whether stream are incremental or not.¶

It is RECOMMENDED that servers schedule DATAGRAM capsules the same as response data.¶

5. Retransmission Scheduling

Section 12 of [I-D.ietf-httpbis-priority] provides guidance about scheduling of retransmission data vs. new data. Since QUIC datagrams are not retransmitted, endpoints that prioritize QUIC stream retransmission data could delay datagrams. Furthermore, since DATAGRAM capsules are sent as stream data, they are subject to retransmission and could also delay native QUIC datagrams.¶

6. Security Considerations

There are believed to be no additional considerations to those presented in [I-D.ietf-httpbis-priority].¶

7. IANA Considerations

This specification registers the following entry in the HTTP Priority Parameters Registry¶

Name:

datagram-urgency¶

Description:

Priority of HTTP datagrams¶

Reference:

This document¶

Appendix A. Acknowledgements

This document is inspired by discussion by many people across HTTP, QUIC and MASQUE WGs.¶

Author's Address