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Versions: 00 01                                                         
Network Working Group                                           S. Hurst
Internet-Draft                                BBC Research & Development
Intended status: Experimental                             13 August 2021
Expires: 14 February 2022

               An Offset Extension Frame For HTTP/3 Data


   This document specifies an optional extension frame type for HTTP/3
   that extends the functionality of the "DATA" frame type to include an
   offset for the HTTP message payload.  This is useful in situations
   where the HTTP/3 exchange is taking place over an unreliable
   transport mechanism.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 14 February 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  DATA_WITH_OFFSET Extension Frame  . . . . . . . . . . . . . .   3
   4.  Realising HTTP Multipart Range Responses With HTTP/3 Binary
           Framing . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Response Headers  . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Usage of DATA_WITH_OFFSET frame with HTTP Range
           Responses . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Negotiating Support For The DATA_WITH_OFFSET Frame  . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   8
   Appendix B.  Changelog  . . . . . . . . . . . . . . . . . . . . .   8
     B.1.  Changes since
           draft-hurst-quic-http-data-offset-frame-00  . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

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

   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.

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2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

     Type (i) = 0xd00,
     Length (i),
     Offset (i),
     Data (..),

                      Figure 1: DATA_WITH_OFFSET Frame

   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.

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   [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"

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"

4.1.  Response Headers

   [HTTP-SEMANTICS] specifies how a server may respond to an HTTP
   multipart range request using the 206 (Partial Content) status code.
   The response message carries a "Content-Type" response header
   indicating the "multipart/byteranges" media type with its required
   boundary parameter.  This boundary parameter allows each body part to
   carry its own header area containing a "Content-Range" header to
   describe what range of the selected representation this body part
   conveys, as well as a "Content-Type" header (if applicable) which
   describes the actual media type of the selected representation.

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   (Note that section 14.2 of [HTTP-SEMANTICS] describes several reasons
   why a server may choose to deliver a different selection of parts
   than what the client originally requested.)

   Because a selected representation may only contain a single "Content-
   Type" header field with a single value, repeating this header field
   in every body part is highly inefficient.  Moreover, the unbounded
   length of the boundary parameter further reduces transmission

   This specification modifies the syntax of the "Content-Range" header
   and explicitly defines it as a list-based field as per section 5.6.1
   of [HTTP-SEMANTICS] that is carried in the first "HEADERS" block sent
   as part of an HTTP/3 response.  In addition, when used on the same
   QUIC stream as "DATA_WITH_OFFSET" frames, this specification permits
   the "Content-Range" and "Content-Type" HTTP headers to appear in the
   "HEADERS" frame of a 206 (Partial Content) response, enabling it to
   indicate the MIME media type of the whole representation without
   needing to duplicate it for each body part.

   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

                 Figure 2: ABNF for extended Content-Range

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

                 Figure 3: Range-Response header example

   Implementations advertising support for the "DATA_WITH_OFFSET" frame
   as described in Section 5 MUST be able to consume this overloaded
   form of the "Content-Range" HTTP response header.

   A server MAY continue to use the method described in [HTTP-SEMANTICS]
   even if a client has expressed support for the "DATA_WITH_OFFSET"

      *Author's Note:* Is it possibly worth splitting this out into its
      own HTTP setting value?

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4.2.  Usage of DATA_WITH_OFFSET frame with HTTP Range Responses

   The "DATA_WITH_OFFSET" frame may be used in conjunction with HTTP
   Range Requests, as described in [HTTP-SEMANTICS].  When carrying data
   for a byte range response, the Offset field in the frame header MUST
   reflect the starting byte position of the frame's payload in the HTTP
   representation and not the offset within the HTTP/3 exchange.  For
   example, for an HTTP/3 range request made with a request header of
   "range: bytes=1000-1999", the first "DATA_WITH_OFFSET" frame in the
   response MUST carry the value 1000 in the Offset field of the frame

   For HTTP response messages carrying a set of byte ranges, a
   "DATA_WITH_OFFSET" frame MUST NOT carry data for more than one
   contiguous byte range within that set.  An individual byte range MAY
   be carried over multiple instances of the "DATA_WITH_OFFSET" frame.

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].

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7.  IANA Considerations

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

               | Frame Type       | Value | Specification |
               | DATA_WITH_OFFSET | 0xd00 | Section 3     |

                  Table 1: Registered HTTP/3 Frame Type

      *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]).

   |Setting                                |Value|Specification|Default|
   |SETTINGS_ENABLE_DATA_WITH_OFFSET_FRAME |0xd00|Section 5    |0      |

                    Table 2: Registered HTTP/3 Settings

8.  References

8.1.  Normative References

              Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", Work in Progress, Internet-Draft,

              Bishop, M., Ed., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-34, <https://datatracker.ietf.org/doc/html/

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,

8.2.  Informative References

              Crasic, C., Ed., Bishop, M., Ed., and A. Frindell, Ed.,
              "QPACK: Header Compression for HTTP/3", Work in Progress,
              Internet-Draft, draft-ietf-quic-qpack-21,

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.

B.1.  Changes since draft-hurst-quic-http-data-offset-frame-00

   *  Update reference to QUIC transport I-D to [RFC9000].

   *  Update reference to [HTTP-SEMANTICS] I-D.

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Author's Address

   Sam Hurst
   BBC Research & Development

   Email: sam.hurst@bbc.co.uk

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