An Offset Extension Frame For HTTP/3 Data

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Table of Contents

1. Introduction

HTTP/3 [QUIC-HTTP] supports the transfer of HTTP semantics over the QUIC transport protocol [RFC9000]. In a conventional HTTP/3 message exchange, messages consist of a header field section sent as a single HEADERS frame, an optional HTTP message payload sent as a series of DATA frames, followed optionally by a trailer field section sent as a single HEADERS frame. Each DATA frame does not describe its position within the HTTP message payload; rather this is calculated from the position within the QUIC stream minus the overhead from HTTP/3 frame headers and the contents of the header field section.¶

In the case where the message exchange is taking place across a partially reliable or unreliable profile of [RFC9000], packet loss could result in a lack of synchronisation in the receiver between the perceived HTTP/3 DATA frame offset and the QUIC STREAM frame offset, potentially resulting in a corrupt HTTP representation at the receiver.¶

In addition, there are other use cases, such as HTTP multipart range requests, where the HTTP/3 payload offset has no direct mapping to the value calculated by the method described above.¶

This document introduces an extension frame type DATA_WITH_OFFSET which can be used to explicitly signal the offset in the original representation of the data being conveyed within the frame.¶

2. Conventions and Terminology

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 variable-length integer encoding from [RFC9000]. The packet and frame diagrams in this document use the bespoke format specified in [RFC9000].¶

3. DATA_WITH_OFFSET Extension Frame

Based on the DATA frame defined in [QUIC-HTTP], the DATA_WITH_OFFSET frame conveys arbitrary, variable-length sequences of bytes at a defined offset of an HTTP representation. By carrying an explicit payload offset in the HTTP/3 frame header, the HTTP message payload offset is decoupled from the QUIC STREAM frame header offset value. The additional payload offset field takes the form of a variable-length integer, as shown in Figure 1 below.¶

DATA_WITH_OFFSET Frame {
  Type (i) = 0xd00,
  Length (i),
  Offset (i),
  Data (..),
}

If its peer has indicated support for the DATA_WITH_OFFSET extension frame type (as described in Section 5 below) a sender MAY choose to use either DATA frames or DATA_WITH_OFFSET frames to transmit an HTTP representation. Senders MUST NOT mix the use of DATA and DATA_WITH_OFFSET frames on the same QUIC stream (i.e. in the same HTTP message).¶

• Author's Note: The author welcomes comments about relaxation of the requirement to not mix the usage of DATA and DATA_WITH_OFFSET frames in the same HTTP message.¶

[QUIC-HTTP] defines three stream types: control stream, request stream and push stream. The DATA_WITH_OFFSET frame is only permitted to appear on request streams and push streams. A DATA_WITH_OFFSET frame MUST NOT appear on control streams. If a DATA_WITH_OFFSET frame is received on a control stream, the recipient MUST respond with a connection error of type H3_FRAME_UNEXPECTED, as described in [QUIC-HTTP].¶

The purpose of the DATA_WITH_OFFSET frame is only to assist in locating a particular slice of data carried as part of an HTTP message payload, and not as a means to send data out of order. Senders MUST send data in order, i.e. with increasing values in the Offset field. In cases where the underlying transport does not guarantee in-order delivery of HTTP/3 frames, the receiver MUST be prepared to deal with out-of-order reception of DATA_WITH_OFFSET frames.¶

4. Realising HTTP Multipart Range Responses With HTTP/3 Binary Framing

HTTP Range Requests, described in [HTTP-SEMANTICS], is an optional feature of HTTP that allows a client to request transfer of one or more subranges of a given representation. Despite the move to binary framing for HTTP in [RFC7540] and subsequently [QUIC-HTTP], multiple part HTTP Range Requests still rely on textual encoding - including boundary strings - which is inefficient. These boundary strings then preface additional HTTP headers for each body part, which always carry a Content-Range field, and may additionally carry a Content-Type field which is likely to be repeated across every body part. It is not possible to compress these headers using [QUIC-QPACK].¶

By using the DATA_WITH_OFFSET frame described in Section 3, the network efficiency of multiple part range requests is improved by no longer needing to encode a boundary string into the representation response. Instead, the offset of each part of a representation is simply encoded in the header of a fresh HTTP DATA_WITH_OFFSET frame.¶

Content-Range       = 1#range-item
range-item          = range-unit SP
                      ( range-resp / unsatisfied-range )

range-resp          = incl-range "/" ( complete-length / "*" )
incl-range          = first-pos "-" last-pos
unsatisfied-range   = "*/" complete-length

complete-length     = 1*DIGIT

:status = 206
content-type = video/mp4
content-range = bytes 10000-17999/18879543, bytes 24000-41999/18879543

5. Negotiating Support For The DATA_WITH_OFFSET Frame

The DATA_WITH_OFFSET frame described in Section 3 is an optional extension to the regular HTTP/3 protocol specification and, as such, usage of the frame type must be negotiated as described in section 9 of [QUIC-HTTP]. For a conventional HTTP/3 connection, this is done using HTTP/3 SETTINGS frames carried on the control streams.¶

This specification defines the following setting:¶

SETTINGS_ENABLE_DATA_WITH_OFFSET_FRAME (0xd00):

A boolean value with a default value of 0 (false). Any non-zero value is true.¶

• Author's Note: It is intended that the value of the H3 SETTINGS frame should mirror the value of the frame to indicate which version of the DATA_WITH_OFFSET frame it understands, should subsequent revisions of this draft change the frame type.¶

An endpoint that implements this specification SHOULD send a SETTINGS_ENABLE_DATA_WITH_OFFSET_FRAME setting at the beginning of the connection to indicate that it is able to process DATA_WITH_OFFSET frames from its peer.¶

An endpoint MUST NOT send a DATA_WITH_OFFSET frame unless it has received a positive (i.e. non-zero) SETTINGS_ENABLE_DATA_WITH_OFFSET_FRAME setting from its peer.¶

6. Security Considerations

This document introduces no new security considerations beyond those discussed in [QUIC-HTTP].¶

7. IANA Considerations

This specification registers a new frame type in the "HTTP/3 Frame Type" registry ([QUIC-HTTP]).¶

• Author's Note: The final, intended value of the frame type is 0xd0f, but in order to allow for this extension to naturally evolve and allow for the frame format to change, it starts at 0xd00 and subsequent revisions of this extension can take incrementally higher frame type values between 0xd00 and 0xd0e.¶

This specification registers a new setting in the "HTTP/3 Settings" registry ([QUIC-HTTP]).¶

Appendix A. Acknowledgements

The author would like to thank the following for their contributions to the design described in the present document: Lucas Pardue, Richard Bradbury and David Waring.¶

I am also grateful for Chris Poole's helpful review comments.¶

Appendix B. Changelog

• RFC Editor's Note: Please remove this section prior to publication of a final version of this document.¶

Author's Address