QUIC Working Group                                             M. Bishop
Internet-Draft                                                 Microsoft
Intended status: Standards Track                       November 15, 2016
Expires: May 19, 2017

            HTTP over QUIC - Mapping and Header Compression


   HTTP/2 [RFC7540] uses HPACK [RFC7541] for header compression.
   However, HPACK relies on the in-order message-based semantics of the
   HTTP/2 framing layer in order to function.  Messages can only be
   successfully decoded if processed by the receiver in the same order
   as generated by the sender.  This draft refines HPACK to loosen the
   ordering requirements for use over QUIC
   [I-D.hamilton-quic-transport-protocol] and describes changes to
   [I-D.shade-quic-http2-mapping] to leverage the new compression.

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 http://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 May 19, 2017.

Copyright Notice

   Copyright (c) 2016 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
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must

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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  QPACK . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Changes to Static and Dynamic Tables  . . . . . . . . . .   3
       2.1.1.  Dynamic table state synchronization . . . . . . . . .   4
     2.2.  Changes to Binary Format  . . . . . . . . . . . . . . . .   5
       2.2.1.  Literal Header Field Representation . . . . . . . . .   5
       2.2.2.  Deletion  . . . . . . . . . . . . . . . . . . . . . .   6
       2.2.3.  The QPACK-ACK frame . . . . . . . . . . . . . . . . .   7
   3.  HTTP over QUIC Mapping  . . . . . . . . . . . . . . . . . . .   7
     3.1.  Stream usage  . . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  On-Stream Framing Definition  . . . . . . . . . . . . . .   8
       3.2.1.  DATA  . . . . . . . . . . . . . . . . . . . . . . . .   9
       3.2.2.  HEADERS . . . . . . . . . . . . . . . . . . . . . . .   9
       3.2.3.  PUSH_PROMISE  . . . . . . . . . . . . . . . . . . . .  10
       3.2.4.  PRIORITY  . . . . . . . . . . . . . . . . . . . . . .  10
       3.2.5.  SETTINGS  . . . . . . . . . . . . . . . . . . . . . .  11
       3.2.6.  Other frames not mentioned  . . . . . . . . . . . . .  11
     3.3.  HTTP Message Exchanges  . . . . . . . . . . . . . . . . .  11
   4.  Performance Considerations  . . . . . . . . . . . . . . . . .  13
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   HPACK has a number of features that were intended to provide
   performance advantages to HTTP/2, but which don't live well in an
   out-of-order environment such as that provided by QUIC.

   The largest challenge is the fact that elements are referenced by a
   very fluid index.  Not only is the index implicit when an item is
   added to the header table, the index will change without notice as
   other items are added to the header table.  Static entries occupy the
   first 61 values, followed by dynamic entries.  A newly-added dynamic
   entry would cause older dynamic entries to be evicted, and the
   retained items are then renumbered beginning with 62.  This means
   that, without processing all preceding header sets, no index into the

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   dynamic table can be interpreted, and the index of a given entry
   cannot be predicted.

   Any solution to the above will almost certainly fall afoul of the
   memory constraints the decompressor imposes.  The automatic eviction
   of entries is done based on the compressor's declared dynamic table
   size, which MUST be less than the maximum permitted by the
   decompressor (and relayed using an HTTP/2 SETTINGS value).

   In the following sections, this document proposes a new version of
   HPACK which makes different trade-offs, enabling out-of-order
   interpretation and bounded memory consumption with minimal head-of-
   line blocking.  None of the proposed improvements to HPACK (strongly-
   typed fields, binary compression of common header syntax) are
   currently included, but certainly could be.

1.1.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
   [RFC2119] and indicate requirement levels for compliant STuPiD


2.1.  Changes to Static and Dynamic Tables

   QPACK uses two tables for associating header fields to indexes.  The
   static table is unchanged from [RFC7541].

   The dynamic table is a map from index to header field.  Indices are
   arbitrary numbers greater than the last index of the static table.
   Each insert instruction will specify the index being modified.  While
   any index MAY be chosen for a new entry, smaller numbers will yield
   better compression performance.  Once an index has been assigned, its
   value is immutable for the lifetime of that dynamic table.

   In order to improve resiliency to reordering, an encoder MAY send
   multiple insert instructions for the same value to the same index.
   However, any attempt to insert a different value to an occupied index
   is a fatal error.

   The dynamic table is still constrained to the size specified by the
   receiver.  An attempt to add a header to the dynamic table which
   causes it to exceed the maximum size MUST be treated as an error by a

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   Because it is possible for QPACK frames to arrive which reference
   indices which have not yet been defined, such frames MUST wait until
   another frame has arrived and defined the index.  In order to guard
   against malicious senders, implementations SHOULD impose a time limit
   and treat expiration of the timer as a decoding error.  However, if
   the implementation chooses not to abort the connection, the remainder
   of the header block MUST be decoded and the output discarded.

2.1.1.  Dynamic table state synchronization

   No entries are evicted from the dynamic table.  Size management is
   purely the responsibility of the sender, which MUST NOT exceed the
   declared memory size of the receiver.

   Both sender and receiver will maintain a count of references to the
   indexed entry.  This count includes:

   o  Insertions to the field, both the initial and any redundant
      indexed literal emissions.

   o  Literal values which use the indexed entry to provide the header

   o  Explicit emissions of the indexed value

   The sender MUST consider memory as committed beginning with the first
   time the indexed entry is assigned.  An encoder MAY repeat the
   insertion instruction in other frames rather than leveraging the
   index while it waits for the frame to arrive.

   When the sender wishes to delete an inserted value, it flows through
   the following set of states:

   1.  *Delete requested.* The sender emits a delete instruction
       including the terminal value of the reference counter.  The
       sender MUST NOT reference the entry in any subsequent frame until
       this state machine has completed and MUST continue to include the
       entry in its calculation of consumed memory.

   2.  *Delete pending.* The receiver receives the delete instruction
       and compares the sender's counter with its own.  If the
       receiver's counter is less than the sender's, it stores the
       sender's counter and waits for other QPACK frames to arrive.

   3.  *Delete acknowledged.* The receiver has received all QPACK frames
       which reference the deleted value, and can safely delete the
       entry.  The receiver SHOULD promptly emit a QPACK-ACK frame, but
       MAY delay briefly waiting for other pending deletes as well.

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   4.  *Delete completed.* When the sender receives a QPACK-ACK frame
       acknowledging the delete, it no longer counts the size of the
       deleted entry against the table size and MAY emit insert
       instructions for the field with a new value.

   The decoder can receive a delete instruction for a vacant table
   entry.  A decoder MUST NOT consider this to be an error, but MUST
   handle the delete as usual even while waiting for the definition to

2.2.  Changes to Binary Format

2.2.1.  Literal Header Field Representation

   (This section replaces [RFC7541], Section 6.2.1.)

   A literal header field with indexing representation results in
   inserting a header field to the decoded header list and inserting it
   as a new entry into the dynamic table.

        0   1   2   3   4   5   6   7
      | 0 | 1 |    New Index (6+)     |
      |          Name Index (8+)      |
      | H |     Value Length (7+)     |
      | Value String (Length octets)  |

            Literal Header Field with Indexing -- Indexed Name

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        0   1   2   3   4   5   6   7
      | 0 | 1 |    New Index (6+)     |
      |               0               |
      | H |     Name Length (7+)      |
      |  Name String (Length octets)  |
      | H |     Value Length (7+)     |
      | Value String (Length octets)  |

              Literal Header Field with Indexing -- New Name

   A literal header field with incremental indexing representation
   starts with the '01' 2-bit pattern, followed by the new index of the
   header represented as an integer with a 6-bit prefix.  This value is
   always greater than the number of entries in the static table.

   If the header field name matches the header field name of an entry
   stored in the static table or the dynamic table, the header field
   name can be represented using the index of that entry.  In this case,
   the index of the entry is represented as an integer with an 8-bit
   prefix (see Section 5.1 of [RFC7231]).  This value is always non-

   Otherwise, the header field name is represented as a string literal
   (see Section 5.2 of [RFC7231]).  A value 0 is used in place of the
   8-bit index, followed by the header field name.

   Either form of header field name representation is followed by the
   header field value represented as a string literal (see Section 5.2).

   An encoder MUST NOT attempt to place a value at an index not known to
   be vacant.  An encoder MAY insert the same value to the same vacant
   slot multiple times in different frames, to reduce the risk of
   blocking from out-of-order frame interpretation.  However, a decoder
   MUST treat the attempt to insert a different header field into an
   occupied slot as a fatal error.

2.2.2.  Deletion

   (This section replaces [RFC7541], Section 6.2.3.)

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   *DISCUSS:* I stole the never-indexed instruction code to avoid
   renumbering _all_ the instructions to fit a new one.  If we think we
   still need this in QUIC, we'll have to do the renumbering later.

 0   1   2   3   4   5   6   7    +---+---+---+---+---+---+---+---+    | 0 | 0 | 0 | 1 | RefCount (4+) |    +---+---+---+---+---------------+    |          Index (8+)           |    +-------------------------------+ ~~~~~~~~~~

                           Header Field Deletion

   The sender may delete an entry in the dynamic header table at any
   time in order to stay below the receiver's declared memory boundary.
   This instruction tells the receiver that they should prepare to
   delete the specified entry after all preceding frames referencing it
   have been received.  The delete instruction includes the count of
   such frames to facilitate the receiver's garbage collection process.

2.2.3.  The QPACK-ACK frame

   Each peer MUST periodically emit a QPACK-ACK frame (0xTBD) on QUIC
   stream 3 to reflect the current state of its header table.  A peer
   MAY omit sending a new QPACK-ACK frame if the dynamic table has not
   changed since the last frame.

   The QPACK-ACK frame defines no flags and consists of a bitmap.  The
   first bit in the bitmap reflects the first index after the static
   table (currently 62), and each successive bit indicates the next
   integer value.  Each bit MUST be set if the indexed entry has had a
   delete complete since the preceding QUIC frame and MUST be unset
   otherwise.  Indices beyond the end of the QPACK-ACK frame are assumed
   to be unset.

   Upon receipt, an encoder uses the table to confirm which items have
   been deleted.  At this point, the space can be recovered by the
   encoder and the encoder can safely reuse the index for future

3.  HTTP over QUIC Mapping

   [I-D.shade-quic-http2-mapping] refers to QUIC Stream 3 as carrying
   "headers," but more accurately, it carries a nearly-complete HTTP/2
   session, complete with framing and multiplexing.  The mapping deletes
   certain elements of HTTP/2's framing layer which can be delegated to
   the QUIC transport layer.

   This was done in large part for expediency, reusing HTTP/2 code in
   place anywhere no QUIC-specific approach had yet been added.  A
   primary driver of this is the need for in-order reliable delivery of

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   While the ability to reuse HTTP/2 framing is useful, the double-mux
   layer is unwieldy and has proved unpopular in the working group.
   This section presents an alternate mapping preserving some HTTP/2
   code, but delegating all multiplexing to the QUIC layer.

   QPACK would permit header data to be on-stream with the request/
   response bodies, but some framing is still required.  It would be
   possible (and perhaps desirable) to introduce a simplified version of
   HTTP/2's framing on each QUIC stream.

3.1.  Stream usage

   In both QUIC and HTTP/2, odd-numbered streams are client-initiated,
   while even-numbered streams are server-initiated.  A single HTTP
   transaction spans two streams, differentiated by the next stream bit.
   This means that the client's first request occurs on QUIC streams 5
   and 7, the second on stream 9 and 11, and so on.  This amounts to the
   second least-significant bit differentiating the two streams in a

   The payload of each frame type is unmodified from HTTP/2 unless
   otherwise noted.  Frames which would be sent on stream zero in HTTP/2
   are sent on QUIC stream 3.

   Because stream creation does not depend on particular frames, the
   requirement that a stream begin only with HEADERS is omitted.

   The second stream is used to carry any message payload, eliminating
   the DATA frame.  The first stream is the request control stream and
   is used to carry all other frames which would have been on-stream in

3.2.  On-Stream Framing Definition

   Many framing concepts from HTTP/2 can be elided away on QUIC, because
   the transport deals with them.  Because these frames would already be
   on a stream, they can omit the stream number.  Because the frames do
   not block multiplexing (QUIC's multiplexing occurs below this layer),
   the support for variable-maximum-length packets can be removed.
   Because stream termination is handled by QUIC, an END_STREAM flag is
   not required.

   On QUIC streams other than Stream 1, the general frame format is as

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        0   1   2   3   4   5   6   7
      |          Length (16)          |
      |                               |
      |            Type (8)           |
      |            Flags (8)          |
      |        Frame Payload        ...

                          HTTP/QUIC frame format

   The fields are defined as in [RFC7540].  The frames currently defined
   are described in this section:

3.2.1.  DATA

   DATA frames (type=0x0) do not exist.

3.2.2.  HEADERS

   The HEADERS frame (type=0x1) is used to carry part of a header set,
   compressed using QPACK ({QPACK}).  The PRIORITY-equivalent regions
   have been removed, since a stream MAY begin with a PRIORITY frame and
   the size of the QUIC stream format requires changes to how these
   fields are handled.

   Padding MUST NOT be used.  The flags defined are:

   o  End Header Block (0x4): This frame concludes a header block.

   The next frame on the same stream after a HEADERS frame without the
   EHB flag set MUST be another HEADERS frame.  A receiver MUST treat
   the receipt of any other type of frame as a stream error.  (Note that
   QUIC can intersperse data from other streams between frames, or even
   during transmission of frames, so multiplexing is not blocked by this

   A full header block is contained in a sequence of zero or more
   HEADERS frames without EHB set, followed by a HEADERS frame with EHB

   HEADERS frames from various streams may be processed by the QPACK
   decoder in any order, completely or partially.  It is not necessary
   to withhold decoding results until the end of the header block has
   arrived.  However, depending on the contents, the processing of one

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   frame might depend on other QPACK frames.  The results of decoding
   MUST be emitted in the same order as the HEADERS frames were placed
   on the stream.


   The PUSH_PROMISE frame (type=0x02) is used to carry a request header
   set from server to client, as in HTTP/2.  It contains the same flags
   as HEADERS.

   The payload contains a QPACK headers block encoding the request whose
   response is promised, preceded by a 32-bit Stream ID indicating the
   QUIC stream on which the response headers will be sent.  (The
   response body stream is implied by the headers stream, as defined in
   Section 3.1.)

   *TODO:* QUIC stream space may be enlarged; would need to redefine
   Promised Stream field in this case.

3.2.4.  PRIORITY

   The PRIORITY frame (type=0x2) specifies the sender-advised priority
   of a stream and is sent on Stream 3.  It can refer to a stream in any
   state, including idle or closed streams.

       |                  Prioritized Stream (32)                      |
       |                   Dependent Stream (32)                       |
       |   Weight (8)  |

                          PRIORITY Frame Payload

   The payload of a PRIORITY frame contains the following fields:

   Prioritized Stream:  The 32-bit stream identifier for the stream
      whose priority is being modified.

   Stream Dependency:
   :The 32-bit stream identifier for the stream that this stream depends

   Weight: :An unsigned 8-bit integer representing a priority weight for
   the stream (see Section 5.3).  Add one to the value to obtain a
   weight between 1 and 256.

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   The PRIORITY frame defines one flag:

   EXCLUSIVE (0x01):
   :Indicates that the stream dependency is exclusive (see [RFC7540]
   Section 5.3).

   If a PRIORITY frame is received which attempts to modify a stream
   which is not a request control scheme, the recipient MUST respond
   with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

   The PRIORITY frame can affect a stream in any state.  Note that this
   frame could arrive after processing or frame sending has completed,
   which would cause it to have no effect on the identified stream.  For
   a stream that is in the "half-closed (remote)" or "closed" state,
   this frame can only affect processing of the identified stream and
   its dependent streams; it does not affect frame transmission on that

   The PRIORITY frame can create a dependency on a stream in the "idle"
   or "closed" state.  This allows for the reprioritization of a group
   of dependent streams by altering the priority of an unused or closed
   parent stream.

   A PRIORITY frame with a length other than 9 octets MUST be treated as
   a connection error of type FRAME_SIZE_ERROR.

3.2.5.  SETTINGS

   The EXTENDED_SETTINGS frame as defined in
   [I-D.bishop-httpbis-extended-settings] will be renamed SETTINGS and
   will replace the HTTP/2 SETTINGS frame.

   *TODO:* SETTINGS_ACK and stream state.  Do we need to emit the ACK on
   every active stream?  What about idle streams and in-flight data?

3.2.6.  Other frames not mentioned

   QUIC stream 3 is equivalent to HTTP/2's stream 0, and the same
   framing is used as for other streams.  SETTINGS frames remain on
   stream 3, as do any other HTTP/2 stream-zero frames.  This enables
   HTTP/2 extension frames which do not have a hard cross-stream
   ordering requirement to continue to function.

3.3.  HTTP Message Exchanges

   A client sends an HTTP request on a new pair of QUIC stream.  A
   server sends an HTTP response on the same streams as the request.

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   An HTTP message (request or response) consists of:

   1.  for a response only, zero or more header blocks (a sequence of
       HEADERS frames with End Header Block set on the last) on the
       control stream containing the message headers of informational
       (1xx) HTTP responses (see [RFC7230], Section 3.2 and [RFC7231],
       Section 6.2),

   2.  one header block on the control stream containing the message
       headers (see [RFC7230], Section 3.2),

   3.  the payload body (see [RFC7230], Section 3.3), sent on the data

   4.  optionally, one header block on the control stream containing the
       trailer-part, if present (see [RFC7230], Section 4.1.2).

   If the message does not contain a body, the corresponding data stream
   MUST still be half-closed without transferring any data.  The
   "chunked" transfer encoding defined in Section 4.1 of [RFC7230] MUST
   NOT be used.

   Trailing header fields are carried in a header block following the
   body.  Such a header block is a sequence of HEADERS frames with End
   Header Block set on the last frame.  Header blocks after the first
   but before the end of the stream are invalid.  These MUST be decoded
   to maintain QPACK decoder state, but the resulting output MUST be

   An HTTP request/response exchange fully consumes a pair of streams.
   After sending a request, a client closes the streams for sending;
   after sending a response, the server closes its streams for sending
   and the QUIC streams are fully closed.

   A server can send a complete response prior to the client sending an
   entire request if the response does not depend on any portion of the
   request that has not been sent and received.  When this is true, a
   server MAY request that the client abort transmission of a request
   without error by sending a RST_STREAM with an error code of NO_ERROR
   after sending a complete response and closing its stream.  Clients
   MUST NOT discard responses as a result of receiving such a
   RST_STREAM, though clients can always discard responses at their
   discretion for other reasons.

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4.  Performance Considerations

   While QPACK is designed to minimize head-of-line blocking between
   streams on header decoding, there are some situations in which lost
   or delayed packets can still impact the performance of header

   References to indexed entries will block if the frame containing the
   entry definition is lost or delayed.  Encoders MAY choose to trade
   off compression efficiency and avoid blocking by repeating the
   literal-with-indexing instruction rather than referencing the dynamic
   table until the insertion is known to be complete.

   Delayed frames which prevent deletes from completing can prevent the
   encoder from adding any new entries due to the maximum table size.
   This does not block the encoder from continuing to make requests, but
   could sharply limit compression performance.  Encoders would be well-
   served to delete entries in advance of encountering the table
   maximum.  Decoders SHOULD be prompt about emitting QPACK-ACK frames
   to enable the sender to recover the table space.

5.  Security Considerations

   The security considerations for QPACK are believed to be the same as
   for HPACK.

6.  IANA Considerations

   This document currently makes no request of IANA, but probably

7.  Acknowledgements

   This draft draws heavily on the text of [RFC7540] and [RFC7541], as
   well as the ideas of [I-D.shade-quic-http2-mapping].  The indirect
   input of those authors is gratefully acknowledged, as well as ideas
   gleefully stolen from:

   o  Jana Iyengar

   o  Patrick McManus

   o  Martin Thomson

   o  Charles 'Buck' Krasic

   o  Kyle Rose

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8.  References

8.1.  Normative References

              Bishop, M., "HTTP/2 Extended SETTINGS Extension", draft-
              bishop-httpbis-extended-settings-00 (work in progress),
              June 2016.

              Hamilton, R., Iyengar, J., Swett, I., and A. Wilk, "QUIC:
              A UDP-Based Multiplexed and Secure Transport", draft-
              hamilton-quic-transport-protocol-01 (work in progress),
              October 2016.

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

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,

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

   [RFC7541]  Peon, R. and H. Ruellan, "HPACK: Header Compression for
              HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,

8.2.  Informative References

              Shade, R. and M. Warres, "HTTP/2 Semantics Using The QUIC
              Transport Protocol", draft-shade-quic-http2-mapping-00
              (work in progress), July 2016.

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Author's Address

   Mike Bishop

   Email: michael.bishop@microsoft.com

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