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Versions: (draft-ietf-quic-qcram) 00 01 02 03 04

QUIC                                                           C. Krasic
Internet-Draft                                                   Netflix
Intended status: Standards Track                               M. Bishop
Expires: June 7, 2019                                Akamai Technologies
                                                        A. Frindell, Ed.
                                                                Facebook
                                                       December 04, 2018


              QPACK: Header Compression for HTTP over QUIC
                        draft-ietf-quic-qpack-04

Abstract

   This specification defines QPACK, a compression format for
   efficiently representing HTTP header fields, to be used in HTTP/3.
   This is a variation of HPACK header compression that seeks to reduce
   head-of-line blocking.

Note to Readers

   Discussion of this draft takes place on the QUIC working group
   mailing list (quic@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/search/?email_list=quic [1].

   Working Group information can be found at https://github.com/quicwg
   [2]; source code and issues list for this draft can be found at
   https://github.com/quicwg/base-drafts/labels/-qpack [3].

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 June 7, 2019.






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Copyright Notice

   Copyright (c) 2018 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 and restrictions with respect
   to this document.  Code Components 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   4
     1.2.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  Compression Process Overview  . . . . . . . . . . . . . . . .   5
     2.1.  Encoder . . . . . . . . . . . . . . . . . . . . . . . . .   5
       2.1.1.  Reference Tracking  . . . . . . . . . . . . . . . . .   6
       2.1.2.  Blocked Dynamic Table Insertions  . . . . . . . . . .   6
       2.1.3.  Avoiding Head-of-Line Blocking  . . . . . . . . . . .   7
       2.1.4.  Largest Known Received  . . . . . . . . . . . . . . .   7
     2.2.  Decoder . . . . . . . . . . . . . . . . . . . . . . . . .   8
       2.2.1.  State Synchronization . . . . . . . . . . . . . . . .   8
       2.2.2.  Blocked Decoding  . . . . . . . . . . . . . . . . . .   9
   3.  Header Tables . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Static Table  . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Dynamic Table . . . . . . . . . . . . . . . . . . . . . .   9
       3.2.1.  Maximum Table Size  . . . . . . . . . . . . . . . . .  10
       3.2.2.  Calculating Table Size  . . . . . . . . . . . . . . .  10
       3.2.3.  Absolute Indexing . . . . . . . . . . . . . . . . . .  11
       3.2.4.  Relative Indexing . . . . . . . . . . . . . . . . . .  11
       3.2.5.  Post-Base Indexing  . . . . . . . . . . . . . . . . .  12
       3.2.6.  Invalid References  . . . . . . . . . . . . . . . . .  12
   4.  Wire Format . . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  Primitives  . . . . . . . . . . . . . . . . . . . . . . .  13
       4.1.1.  Prefixed Integers . . . . . . . . . . . . . . . . . .  13
       4.1.2.  String Literals . . . . . . . . . . . . . . . . . . .  13
     4.2.  Stream Types  . . . . . . . . . . . . . . . . . . . . . .  13
     4.3.  Encoder Stream  . . . . . . . . . . . . . . . . . . . . .  14
       4.3.1.  Insert With Name Reference  . . . . . . . . . . . . .  14
       4.3.2.  Insert Without Name Reference . . . . . . . . . . . .  15
       4.3.3.  Duplicate . . . . . . . . . . . . . . . . . . . . . .  15
       4.3.4.  Dynamic Table Size Update . . . . . . . . . . . . . .  16



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     4.4.  Decoder Stream  . . . . . . . . . . . . . . . . . . . . .  16
       4.4.1.  Table State Synchronize . . . . . . . . . . . . . . .  16
       4.4.2.  Header Acknowledgement  . . . . . . . . . . . . . . .  17
       4.4.3.  Stream Cancellation . . . . . . . . . . . . . . . . .  18
     4.5.  Request and Push Streams  . . . . . . . . . . . . . . . .  18
       4.5.1.  Header Data Prefix  . . . . . . . . . . . . . . . . .  18
       4.5.2.  Indexed Header Field  . . . . . . . . . . . . . . . .  20
       4.5.3.  Indexed Header Field With Post-Base Index . . . . . .  21
       4.5.4.  Literal Header Field With Name Reference  . . . . . .  21
       4.5.5.  Literal Header Field With Post-Base Name Reference  .  22
       4.5.6.  Literal Header Field Without Name Reference . . . . .  22
   5.  Configuration . . . . . . . . . . . . . . . . . . . . . . . .  23
   6.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  23
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  24
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
     8.1.  Settings Registration . . . . . . . . . . . . . . . . . .  24
     8.2.  Stream Type Registration  . . . . . . . . . . . . . . . .  24
     8.3.  Error Code Registration . . . . . . . . . . . . . . . . .  24
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  25
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  26
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  26
   Appendix A.  Static Table . . . . . . . . . . . . . . . . . . . .  26
   Appendix B.  Sample One Pass Encoding Algorithm . . . . . . . . .  31
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  33
     C.1.  Since draft-ietf-quic-qpack-03  . . . . . . . . . . . . .  33
     C.2.  Since draft-ietf-quic-qpack-02  . . . . . . . . . . . . .  33
     C.3.  Since draft-ietf-quic-qpack-01  . . . . . . . . . . . . .  33
     C.4.  Since draft-ietf-quic-qpack-00  . . . . . . . . . . . . .  33
     C.5.  Since draft-ietf-quic-qcram-00  . . . . . . . . . . . . .  34
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

1.  Introduction

   The QUIC transport protocol was designed from the outset to support
   HTTP semantics, and its design subsumes many of the features of
   HTTP/2.  HTTP/2 uses HPACK ([RFC7541]) for header compression, but
   QUIC's stream multiplexing comes into some conflict with HPACK.  A
   key goal of the design of QUIC is to improve stream multiplexing
   relative to HTTP/2 by reducing head-of-line blocking.  If HPACK were
   used for HTTP/3, it would induce head-of-line blocking due to built-
   in assumptions of a total ordering across frames on all streams.

   QUIC is described in [QUIC-TRANSPORT].  The HTTP/3 mapping is
   described in [HTTP3].  For a full description of HTTP/2, see
   [RFC7540].  The description of HPACK is [RFC7541].




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   QPACK reuses core concepts from HPACK, but is redesigned to allow
   correctness in the presence of out-of-order delivery, with
   flexibility for implementations to balance between resilience against
   head-of-line blocking and optimal compression ratio.  The design
   goals are to closely approach the compression ratio of HPACK with
   substantially less head-of-line blocking under the same loss
   conditions.

1.1.  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.

   Definitions of terms that are used in this document:

   Header field:  A name-value pair sent as part of an HTTP message.

   Header list:  An ordered collection of header fields associated with
      an HTTP message.  A header list can contain multiple header fields
      with the same name.  It can also contain duplicate header fields.

   Header block:  The compressed representation of a header list.

   Encoder:  An implementation which transforms a header list into a
      header block.

   Decoder:  An implementation which transforms a header block into a
      header list.

   Absolute Index:  A unique index for each entry in the dynamic table.

   Base Index:  An absolute index in a header block from which relative
      indices are made.

   Largest Reference:  The largest absolute index of an entry referenced
      in a header block.

   QPACK is a name, not an acronym.

1.2.  Notational Conventions

   Diagrams use the format described in Section 3.1 of [RFC2360], with
   the following additional conventions:

   x (A)  Indicates that x is A bits long



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   x (A+)  Indicates that x uses the prefixed integer encoding defined
      in Section 5.1 of [RFC7541], beginning with an A-bit prefix.

   x ...  Indicates that x is variable-length and extends to the end of
      the region.

2.  Compression Process Overview

   Like HPACK, QPACK uses two tables for associating header fields to
   indices.  The static table (see Section 3.1) is predefined and
   contains common header fields (some of them with an empty value).
   The dynamic table (see Section 3.2) is built up over the course of
   the connection and can be used by the encoder to index header fields
   in the encoded header lists.

   QPACK instructions appear in three different types of streams:

   o  The encoder uses a unidirectional stream to modify the state of
      the dynamic table without emitting header fields associated with
      any particular request.

   o  HEADERS and PUSH_PROMISE frames on request and push streams
      reference the table state without modifying it.

   o  The decoder sends feedback to the encoder on a unidirectional
      stream.  This feedback enables the encoder to manage dynamic table
      state.

2.1.  Encoder

   An encoder compresses a header list by emitting either an indexed or
   a literal representation for each header field in the list.
   References to the static table and literal representations do not
   require any dynamic state and never risk head-of-line blocking.
   References to the dynamic table risk head-of-line blocking if the
   encoder has not received an acknowledgement indicating the entry is
   available at the decoder.

   An encoder MAY insert any entry in the dynamic table it chooses; it
   is not limited to header fields it is compressing.

   QPACK preserves the ordering of header fields within each header
   list.  An encoder MUST emit header field representations in the order
   they appear in the input header list.

   QPACK is designed to contain the more complex state tracking to the
   encoder, while the decoder is relatively simple.




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2.1.1.  Reference Tracking

   An encoder MUST ensure that a header block which references a dynamic
   table entry is not received by the decoder after the referenced entry
   has been evicted.  Hence the encoder needs to track information about
   each compressed header block that references the dynamic table until
   that header block is acknowledged by the decoder.

2.1.2.  Blocked Dynamic Table Insertions

   An encoder MUST NOT insert an entry into the dynamic table (or
   duplicate an existing entry) if doing so would evict an entry with
   unacknowledged references.  For header blocks that might rely on the
   newly added entry, the encoder can use a literal representation and
   maybe insert the entry later.

   To ensure that the encoder is not prevented from adding new entries,
   the encoder can avoid referencing entries that are close to eviction.
   Rather than reference such an entry, the encoder can emit a Duplicate
   instruction (see Section 4.3.3), and reference the duplicate instead.

   Determining which entries are too close to eviction to reference is
   an encoder preference.  One heuristic is to target a fixed amount of
   available space in the dynamic table: either unused space or space
   that can be reclaimed by evicting unreferenced entries.  To achieve
   this, the encoder can maintain a draining index, which is the
   smallest absolute index in the dynamic table that it will emit a
   reference for.  As new entries are inserted, the encoder increases
   the draining index to maintain the section of the table that it will
   not reference.  If the encoder does not create new references to
   entries with an absolute index lower than the draining index, the
   number of unacknowledged references to those entries will eventually
   become zero, allowing them to be evicted.

      +----------+---------------------------------+--------+
      | Draining |          Referenceable          | Unused |
      | Entries  |             Entries             | Space  |
      +----------+---------------------------------+--------+
      ^          ^                                 ^
      |          |                                 |
    Dropping    Draining Index               Base Index /
     Point                                   Insertion Point

                 Figure 1: Draining Dynamic Table Entries







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2.1.3.  Avoiding Head-of-Line Blocking

   Because QUIC does not guarantee order between data on different
   streams, a header block might reference an entry in the dynamic table
   that has not yet been received.

   Each header block contains a Largest Reference (Section 4.5.1) which
   identifies the table state necessary for decoding.  If the greatest
   absolute index in the dynamic table is less than the value of the
   Largest Reference, the stream is considered "blocked."  While
   blocked, header field data SHOULD remain in the blocked stream's flow
   control window.  When the Largest Reference is zero, the frame
   contains no references to the dynamic table and can always be
   processed immediately.  A stream becomes unblocked when the greatest
   absolute index in the dynamic table becomes greater than or equal to
   the Largest Reference for all header blocks the decoder has started
   reading from the stream.  If the decoder encounters a header block
   where the actual largest reference is not equal to the Largest
   Reference declared in the prefix, it MAY treat this as a stream error
   of type HTTP_QPACK_DECOMPRESSION_FAILED.

   The SETTINGS_QPACK_BLOCKED_STREAMS setting (see Section 5) specifies
   an upper bound on the number of streams which can be blocked.  An
   encoder MUST limit the number of streams which could become blocked
   to the value of SETTINGS_QPACK_BLOCKED_STREAMS at all times.  Note
   that the decoder might not actually become blocked on every stream
   which risks becoming blocked.  If the decoder encounters more blocked
   streams than it promised to support, it MUST treat this as a stream
   error of type HTTP_QPACK_DECOMPRESSION_FAILED.

   An encoder can decide whether to risk having a stream become blocked.
   If permitted by the value of SETTINGS_QPACK_BLOCKED_STREAMS,
   compression efficiency can often be improved by referencing dynamic
   table entries that are still in transit, but if there is loss or
   reordering the stream can become blocked at the decoder.  An encoder
   avoids the risk of blocking by only referencing dynamic table entries
   which have been acknowledged, but this could mean using literals.
   Since literals make the header block larger, this can result in the
   encoder becoming blocked on congestion or flow control limits.

2.1.4.  Largest Known Received

   In order to identify which dynamic table entries can be safely used
   without a stream becoming blocked, the encoder tracks the absolute
   index of the decoder's Largest Known Received entry.






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   When blocking references are permitted, the encoder uses header block
   acknowledgement to identify the Largest Known Received index, as
   described in Section 4.4.2.

   To acknowledge dynamic table entries which are not referenced by
   header blocks, for example because the encoder or the decoder have
   chosen not to risk blocked streams, the decoder sends a Table State
   Synchronize instruction (see Section 4.4.1).

2.2.  Decoder

   As in HPACK, the decoder processes header blocks and emits the
   corresponding header lists.  It also processes dynamic table
   modifications from instructions on the encoder stream.

   The decoder MUST emit header fields in the order their
   representations appear in the input header block.

2.2.1.  State Synchronization

   The decoder stream (Section 4.4) signals key events at the decoder
   that permit the encoder to track the decoder's state.  These events
   are:

   o  Complete processing of a header block

   o  Abandonment of a stream which might have remaining header blocks

   o  Receipt of new dynamic table entries

   Knowledge that a header block with references to the dynamic table
   has been processed permits the encoder to evict entries to which no
   unacknowledged references remain, regardless of whether those
   references were potentially blocking (see Section 2.1.2).  When a
   stream is reset or abandoned, the indication that these header blocks
   will never be processed serves a similar function; see Section 4.4.3.

   The decoder chooses when to emit Table State Synchronize instructions
   (see Section 4.4.1).  Emitting an instruction after adding each new
   dynamic table entry will provide the most timely feedback to the
   encoder, but could be redundant with other decoder feedback.  By
   delaying a Table State Synchronize instruction, the decoder might be
   able to coalesce multiple Table State Synchronize instructions, or
   replace them entirely with Header Acknowledgements (see
   Section 4.4.2).  However, delaying too long may lead to compression
   inefficiencies if the encoder waits for an entry to be acknowledged
   before using it.




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2.2.2.  Blocked Decoding

   To track blocked streams, the necessary Largest Reference value for
   each stream can be used.  Whenever the decoder processes a table
   update, it can begin decoding any blocked streams that now have their
   dependencies satisfied.

3.  Header Tables

   Unlike in HPACK, entries in the QPACK static and dynamic tables are
   addressed separately.  The following sections describe how entries in
   each table are addressed.

3.1.  Static Table

   The static table consists of a predefined static list of header
   fields, each of which has a fixed index over time.  Its entries are
   defined in Appendix A.

   Note the QPACK static table is indexed from 0, whereas the HPACK
   static table is indexed from 1.

   When the decoder encounters an invalid static table index on a
   request stream or push stream it MUST treat this as a stream error of
   type "HTTP_QPACK_DECOMPRESSION_FAILED".  If this index is received on
   the encoder stream, this MUST be treated as a connection error of
   type "HTTP_QPACK_ENCODER_STREAM_ERROR".

3.2.  Dynamic Table

   The dynamic table consists of a list of header fields maintained in
   first-in, first-out order.  The dynamic table is initially empty.
   Entries are added by instructions on the encoder stream (see
   Section 4.3).

   The maximum size of the dynamic table can be modified by the encoder,
   subject to a decoder-controlled limit (see Section 5 and
   Section 4.3.4).  The initial maximum size is determined by the
   corresponding setting when HTTP requests or responses are first
   permitted to be sent.  For clients using 0-RTT data in HTTP/3, the
   table size is the remembered value of the setting, even if the server
   later specifies a larger maximum in its SETTINGS frame.  For HTTP/3
   servers and HTTP/3 clients when 0-RTT is not attempted or is
   rejected, the initial maximum table size is the value of the setting
   in the peer's SETTINGS frame.

   Before a new entry is added to the dynamic table, entries are evicted
   from the end of the dynamic table until the size of the dynamic table



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   is less than or equal to (maximum size - new entry size) or until the
   table is empty.  The encoder MUST NOT evict a dynamic table entry
   unless it has first been acknowledged by the decoder.

   If the size of the new entry is less than or equal to the maximum
   size, that entry is added to the table.  It is an error to attempt to
   add an entry that is larger than the maximum size; this MUST be
   treated as a connection error of type
   "HTTP_QPACK_ENCODER_STREAM_ERROR".

   A new entry can reference an entry in the dynamic table that will be
   evicted when adding this new entry into the dynamic table.
   Implementations are cautioned to avoid deleting the referenced name
   if the referenced entry is evicted from the dynamic table prior to
   inserting the new entry.

   The dynamic table can contain duplicate entries (i.e., entries with
   the same name and same value).  Therefore, duplicate entries MUST NOT
   be treated as an error by the decoder.

3.2.1.  Maximum Table Size

   The encoder decides how to update the dynamic table and as such can
   control how much memory is used by the dynamic table.  To limit the
   memory requirements of the decoder, the dynamic table size is
   strictly bounded.

   The decoder determines the maximum size that the encoder is permitted
   to use for the dynamic table.  In HTTP/3, this value is determined by
   the SETTINGS_HEADER_TABLE_SIZE setting (see Section 5).

   An encoder can choose to use less capacity than this maximum size
   (see Section 4.3.4), but the chosen size MUST stay lower than or
   equal to the maximum set by the decoder.  Whenever the maximum size
   for the dynamic table is reduced, entries are evicted from the end of
   the dynamic table until the size of the dynamic table is less than or
   equal to the maximum size.

   This mechanism can be used to completely clear entries from the
   dynamic table by setting a maximum size of 0, which can subsequently
   be restored.

3.2.2.  Calculating Table Size

   The size of the dynamic table is the sum of the size of its entries.

   The size of an entry is the sum of its name's length in bytes (as
   defined in Section 4.1.2), its value's length in bytes, and 32.



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   The size of an entry is calculated using the length of its name and
   value without any Huffman encoding applied.

   "MaxEntries" is the maximum number of entries that the dynamic table
   can have.  The smallest entry has empty name and value strings and
   has the size of 32.  The MaxEntries is calculated as

      MaxEntries = floor( MaxTableSize / 32 )

   MaxTableSize is the maximum size of the dynamic table as specified by
   the decoder (see Section 3.2.1).

3.2.3.  Absolute Indexing

   Each entry possesses both an absolute index which is fixed for the
   lifetime of that entry and a relative index which changes based on
   the context of the reference.  The first entry inserted has an
   absolute index of "1"; indices increase sequentially with each
   insertion.

3.2.4.  Relative Indexing

   The relative index begins at zero and increases in the opposite
   direction from the absolute index.  Determining which entry has a
   relative index of "0" depends on the context of the reference.

   On the encoder stream, a relative index of "0" always refers to the
   most recently inserted value in the dynamic table.  Note that this
   means the entry referenced by a given relative index will change
   while interpreting instructions on the encoder stream.

       +---+---------------+-----------+
       | n |      ...      |   d + 1   |  Absolute Index
       + - +---------------+ - - - - - +
       | 0 |      ...      | n - d - 1 |  Relative Index
       +---+---------------+-----------+
         ^                       |
         |                       V
   Insertion Point         Dropping Point

   n = count of entries inserted
   d = count of entries dropped

              Example Dynamic Table Indexing - Control Stream

   Because frames from request streams can be delivered out of order
   with instructions on the encoder stream, relative indices are
   relative to the Base Index at the beginning of the header block (see



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   Section 4.5.1).  The Base Index is an absolute index.  When
   interpreting the rest of the frame, the entry identified by Base
   Index has a relative index of zero.  The relative indices of entries
   do not change while interpreting headers on a request or push stream.

                Base Index
                    |
                    V
       +---+-----+-----+-----+-------+
       | n | n-1 | n-2 | ... |  d+1  |  Absolute Index
       +---+-----+  -  +-----+   -   +
                 |  0  | ... | n-d-3 |  Relative Index
                 +-----+-----+-------+

   n = count of entries inserted
   d = count of entries dropped

     Example Dynamic Table Indexing - Relative Index on Request Stream

3.2.5.  Post-Base Indexing

   A header block on the request stream can reference entries added
   after the entry identified by the Base Index.  This allows an encoder
   to process a header block in a single pass and include references to
   entries added while processing this (or other) header blocks.  Newly
   added entries are referenced using Post-Base instructions.  Indices
   for Post-Base instructions increase in the same direction as absolute
   indices, but the zero value is one higher than the Base Index.

                Base Index
                    |
                    V
       +---+-----+-----+-----+-----+
       | n | n-1 | n-2 | ... | d+1 |  Absolute Index
       +---+-----+-----+-----+-----+
       | 1 |  0  |                    Post-Base Index
       +---+-----+

   n = count of entries inserted
   d = count of entries dropped

    Example Dynamic Table Indexing - Post-Base Index on Request Stream

3.2.6.  Invalid References

   If the decoder encounters a reference on a request or push stream to
   a dynamic table entry which has already been evicted or which has an
   absolute index greater than the declared Largest Reference (see



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   Section 4.5.1), it MUST treat this as a stream error of type
   "HTTP_QPACK_DECOMPRESSION_FAILED".

   If the decoder encounters a reference on the encoder stream to a
   dynamic table entry which has already been dropped, it MUST treat
   this as a connection error of type "HTTP_QPACK_ENCODER_STREAM_ERROR".

4.  Wire Format

4.1.  Primitives

4.1.1.  Prefixed Integers

   The prefixed integer from Section 5.1 of [RFC7541] is used heavily
   throughout this document.  The format from [RFC7541] is used
   unmodified.  QPACK implementations MUST be able to decode integers up
   to 62 bits long.

4.1.2.  String Literals

   The string literal defined by Section 5.2 of [RFC7541] is also used
   throughout.  This string format includes optional Huffman encoding.

   HPACK defines string literals to begin on a byte boundary.  They
   begin with a single flag (indicating whether the string is Huffman-
   coded), followed by the Length encoded as a 7-bit prefix integer, and
   finally Length bytes of data.  When Huffman encoding is enabled, the
   Huffman table from Appendix B of [RFC7541] is used without
   modification.

   This document expands the definition of string literals and permits
   them to begin other than on a byte boundary.  An "N-bit prefix string
   literal" begins with the same Huffman flag, followed by the length
   encoded as an (N-1)-bit prefix integer.  The remainder of the string
   literal is unmodified.

   A string literal without a prefix length noted is an 8-bit prefix
   string literal and follows the definitions in [RFC7541] without
   modification.

4.2.  Stream Types

   QPACK instructions occur in three locations, each of which uses a
   separate instruction space:

   o  The encoder stream is a unidirectional stream of type "0x48"
      (ASCII 'H') which carries table updates from encoder to decoder.




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   o  The decoder stream is a unidirectional stream of type "0x68"
      (ASCII 'h') which carries acknowledgements of table modifications
      and header processing from decoder to encoder.

   o  Finally, the contents of HEADERS and PUSH_PROMISE frames on
      request streams and push streams reference the QPACK table state.

   There MUST be exactly one of each unidirectional stream type in each
   direction.  Receipt of a second instance of either stream type MUST
   be treated as a connection error of HTTP_WRONG_STREAM_COUNT.  Closure
   of either unidirectional stream MUST be treated as a connection error
   of type HTTP_CLOSED_CRITICAL_STREAM.

   This section describes the instructions which are possible on each
   stream type.

4.3.  Encoder Stream

   Table updates can add a table entry, possibly using existing entries
   to avoid transmitting redundant information.  The name can be
   transmitted as a reference to an existing entry in the static or the
   dynamic table or as a string literal.  For entries which already
   exist in the dynamic table, the full entry can also be used by
   reference, creating a duplicate entry.

   The contents of the encoder stream are an unframed sequence of the
   following instructions.

4.3.1.  Insert With Name Reference

   An addition to the header table where the header field name matches
   the header field name of an entry stored in the static table or the
   dynamic table starts with the '1' one-bit pattern.  The "S" bit
   indicates whether the reference is to the static (S=1) or dynamic
   (S=0) table.  The 6-bit prefix integer (see Section 5.1 of [RFC7541])
   that follows is used to locate the table entry for the header name.
   When S=1, the number represents the static table index; when S=0, the
   number is the relative index of the entry in the dynamic table.

   The header name reference is followed by the header field value
   represented as a string literal (see Section 5.2 of [RFC7541]).










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        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      | 1 | S |    Name Index (6+)    |
      +---+---+-----------------------+
      | H |     Value Length (7+)     |
      +---+---------------------------+
      |  Value String (Length bytes)  |
      +-------------------------------+

                    Insert Header Field -- Indexed Name

4.3.2.  Insert Without Name Reference

   An addition to the header table where both the header field name and
   the header field value are represented as string literals (see
   Section 4.1) starts with the '01' two-bit pattern.

   The name is represented as a 6-bit prefix string literal, while the
   value is represented as an 8-bit prefix string literal.

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      | 0 | 1 | H | Name Length (5+)  |
      +---+---+---+-------------------+
      |  Name String (Length bytes)   |
      +---+---------------------------+
      | H |     Value Length (7+)     |
      +---+---------------------------+
      |  Value String (Length bytes)  |
      +-------------------------------+

                      Insert Header Field -- New Name

4.3.3.  Duplicate

   Duplication of an existing entry in the dynamic table starts with the
   '000' three-bit pattern.  The relative index of the existing entry is
   represented as an integer with a 5-bit prefix.

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

                            Figure 2: Duplicate

   The existing entry is re-inserted into the dynamic table without
   resending either the name or the value.  This is useful to mitigate



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   the eviction of older entries which are frequently referenced, both
   to avoid the need to resend the header and to avoid the entry in the
   table blocking the ability to insert new headers.

4.3.4.  Dynamic Table Size Update

   An encoder informs the decoder of a change to the size of the dynamic
   table using an instruction which begins with the '001' three-bit
   pattern.  The new maximum table size is represented as an integer
   with a 5-bit prefix (see Section 5.1 of [RFC7541]).

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 0 | 1 |   Max size (5+)   |
   +---+---+---+-------------------+

                Figure 3: Maximum Dynamic Table Size Change

   The new maximum size MUST be lower than or equal to the limit
   determined by the protocol using QPACK.  A value that exceeds this
   limit MUST be treated as a connection error of type
   "HTTP_QPACK_ENCODER_STREAM_ERROR".  In HTTP/3, this limit is the
   value of the SETTINGS_HEADER_TABLE_SIZE parameter (see Section 5)
   received from the decoder.

   Reducing the maximum size of the dynamic table can cause entries to
   be evicted (see Section 4.3 of [RFC7541]).  This MUST NOT cause the
   eviction of entries with outstanding references (see Section 2.1.1).
   Changing the size of the dynamic table is not acknowledged as this
   instruction does not insert an entry.

4.4.  Decoder Stream

   The decoder stream carries information used to ensure consistency of
   the dynamic table.  Information is sent from the decoder to the
   encoder; that is, the server informs the client about the processing
   of the client's header blocks and table updates, and the client
   informs the server about the processing of the server's header blocks
   and table updates.

   The contents of the decoder stream are an unframed sequence of the
   following instructions.

4.4.1.  Table State Synchronize

   The Table State Synchronize instruction begins with the '00' two-bit
   pattern.  The instruction specifies the total number of dynamic table
   inserts and duplications since the last Table State Synchronize or



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   Header Acknowledgement that increased the Largest Known Received
   dynamic table entry (see Section 2.1.4).  This is encoded as a 6-bit
   prefix integer.  The encoder uses this value to determine which table
   entries might cause a stream to become blocked, as described in
   Section 2.2.1.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 0 |   Insert Count (6+)   |
   +---+---+-----------------------+

                     Figure 4: Table State Synchronize

   An encoder that receives an Insert Count equal to zero or one that
   increases Largest Known Received beyond what the encoder has sent
   MUST treat this as a connection error of type
   "HTTP_QPACK_DECODER_STREAM_ERROR".

4.4.2.  Header Acknowledgement

   After processing a header block whose declared Largest Reference is
   not zero, the decoder emits a Header Acknowledgement instruction on
   the decoder stream.  The instruction begins with the '1' one-bit
   pattern and includes the request stream's stream ID, encoded as a
   7-bit prefix integer.  It is used by the peer's encoder to know when
   it is safe to evict an entry, and possibly update Largest Known
   Received.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1 |      Stream ID (7+)       |
   +---+---------------------------+

                     Figure 5: Header Acknowledgement

   The same Stream ID can be identified multiple times, as multiple
   header blocks can be sent on a single stream in the case of
   intermediate responses, trailers, and pushed requests.  Since header
   frames on each stream are received and processed in order, this gives
   the encoder precise feedback on which header blocks within a stream
   have been fully processed.

   If an encoder receives a Header Acknowledgement instruction referring
   to a stream on which every header block with a non-zero Largest
   Reference has already been acknowledged, that MUST be treated as a
   connection error of type "HTTP_QPACK_DECODER_STREAM_ERROR".





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   When blocking references are permitted, the encoder uses
   acknowledgement of header blocks to update the Largest Known Received
   index.  If a header block was potentially blocking, the
   acknowledgement implies that the decoder has received all dynamic
   table state necessary to process the header block.  If the Largest
   Reference of an acknowledged header block was greater than the
   encoder's current Largest Known Received index, the block's Largest
   Reference becomes the new Largest Known Received.

4.4.3.  Stream Cancellation

   The instruction begins with the '01' two-bit pattern.  The
   instruction includes the stream ID of the affected stream - a request
   or push stream - encoded as a 6-bit prefix integer.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 1 |     Stream ID (6+)    |
   +---+---+-----------------------+

                       Figure 6: Stream Cancellation

   A stream that is reset might have multiple outstanding header blocks
   with dynamic table references.  When an endpoint receives a stream
   reset before the end of a stream, it generates a Stream Cancellation
   instruction on the decoder stream.  Similarly, when an endpoint
   abandons reading of a stream it needs to signal this using the Stream
   Cancellation instruction.  This signals to the encoder that all
   references to the dynamic table on that stream are no longer
   outstanding.  A decoder with a maximum dynamic table size equal to
   zero MAY omit sending Stream Cancellations, because the encoder
   cannot have any dynamic table references.

   An encoder cannot infer from this instruction that any updates to the
   dynamic table have been received.

4.5.  Request and Push Streams

   HEADERS and PUSH_PROMISE frames on request and push streams reference
   the dynamic table in a particular state without modifying it.  Frames
   on these streams emit the headers for an HTTP request or response.

4.5.1.  Header Data Prefix

   Header data is prefixed with two integers, "Largest Reference" and
   "Base Index".





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     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   |     Largest Reference (8+)    |
   +---+---------------------------+
   | S |   Delta Base Index (7+)   |
   +---+---------------------------+
   |      Compressed Headers     ...
   +-------------------------------+

                          Figure 7: Frame Payload

4.5.1.1.  Largest Reference

   "Largest Reference" identifies the largest absolute dynamic index
   referenced in the block.  Blocking decoders use the Largest Reference
   to determine when it is safe to process the rest of the block.  If
   Largest Reference is greater than zero, the encoder transforms it as
   follows before encoding:

      LargestReference = (LargestReference mod (2 * MaxEntries)) + 1

   The decoder reconstructs the Largest Reference using the following
   algorithm:

      if LargestReference > 0:
         LargestReference -= 1
         CurrentWrapped = TotalNumberOfInserts mod (2 * MaxEntries)

         if CurrentWrapped >= LargestReference + MaxEntries:
            # Largest Reference wrapped around 1 extra time
            LargestReference += 2 * MaxEntries
         else if CurrentWrapped + MaxEntries < LargestReference
            # Decoder wrapped around 1 extra time
            CurrentWrapped += 2 * MaxEntries

         LargestReference += TotalNumberOfInserts - CurrentWrapped

   TotalNumberOfInserts is the total number of inserts into the
   decoder's dynamic table.  This encoding limits the length of the
   prefix on long-lived connections.

4.5.1.2.  Base Index

   "Base Index" is used to resolve references in the dynamic table as
   described in Section 3.2.4.

   To save space, Base Index is encoded relative to Largest Reference
   using a one-bit sign and the "Delta Base Index" value.  A sign bit of



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   0 indicates that the Base Index has an absolute index that is greater
   than or equal to the Largest Reference; the value of Delta Base Index
   is added to the Largest Reference to determine the absolute value of
   the Base Index.  A sign bit of 1 indicates that the Base Index is
   less than the Largest Reference.  That is:

      if sign == 0:
         baseIndex = largestReference + deltaBaseIndex
      else:
         baseIndex = largestReference - deltaBaseIndex

   A single-pass encoder determines the absolute value of Base Index
   before encoding a header block.  If the encoder inserted entries in
   the dynamic table while encoding the header block, Largest Reference
   will be greater than Base Index, so the encoded difference is
   negative and the sign bit is set to 1.  If the header block did not
   reference the most recent entry in the table and did not insert any
   new entries, Base Index will be greater than the Largest Reference,
   so the delta will be positive and the sign bit is set to 0.

   An encoder that produces table updates before encoding a header block
   might set Largest Reference and Base Index to the same value.  When
   Largest Reference and Base Index are equal, the Delta Base Index is
   encoded with a zero sign bit.  A sign bit set to 1 when the Delta
   Base Index is 0 MUST be treated as a decoder error.

   A header block that does not reference the dynamic table can use any
   value for Base Index; setting both Largest Reference and Base Index
   to zero is the most efficient encoding.

4.5.2.  Indexed Header Field

   An indexed header field representation identifies an entry in either
   the static table or the dynamic table and causes that header field to
   be added to the decoded header list, as described in Section 3.2 of
   [RFC7541].

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1 | S |      Index (6+)       |
   +---+---+-----------------------+

                           Indexed Header Field

   If the entry is in the static table, or in the dynamic table with an
   absolute index less than or equal to Base Index, this representation
   starts with the '1' 1-bit pattern, followed by the "S" bit indicating
   whether the reference is into the static (S=1) or dynamic (S=0)



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   table.  Finally, the relative index of the matching header field is
   represented as an integer with a 6-bit prefix (see Section 5.1 of
   [RFC7541]).

4.5.3.  Indexed Header Field With Post-Base Index

   If the entry is in the dynamic table with an absolute index greater
   than Base Index, the representation starts with the '0001' 4-bit
   pattern, followed by the post-base index (see Section 3.2.5) of the
   matching header field, represented as an integer with a 4-bit prefix
   (see Section 5.1 of [RFC7541]).

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

                 Indexed Header Field with Post-Base Index

4.5.4.  Literal Header Field With Name Reference

   A literal header field with a name reference represents a header
   where the header field name matches the header field name of an entry
   stored in the static table or the dynamic table.

   If the entry is in the static table, or in the dynamic table with an
   absolute index less than or equal to Base Index, this representation
   starts with the '01' two-bit pattern.  If the entry is in the dynamic
   table with an absolute index greater than Base Index, the
   representation starts with the '0000' four-bit pattern.

   The following bit, 'N', indicates whether an intermediary is
   permitted to add this header to the dynamic header table on
   subsequent hops.  When the 'N' bit is set, the encoded header MUST
   always be encoded with a literal representation.  In particular, when
   a peer sends a header field that it received represented as a literal
   header field with the 'N' bit set, it MUST use a literal
   representation to forward this header field.  This bit is intended
   for protecting header field values that are not to be put at risk by
   compressing them (see Section 7.1 of [RFC7541] for more details).











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        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      | 0 | 1 | N | S |Name Index (4+)|
      +---+---+---+---+---------------+
      | H |     Value Length (7+)     |
      +---+---------------------------+
      |  Value String (Length bytes)  |
      +-------------------------------+

                 Literal Header Field With Name Reference

   For entries in the static table or in the dynamic table with an
   absolute index less than or equal to Base Index, the header field
   name is represented using the relative index of that entry, which is
   represented as an integer with a 4-bit prefix (see Section 5.1 of
   [RFC7541]).  The "S" bit indicates whether the reference is to the
   static (S=1) or dynamic (S=0) table.

4.5.5.  Literal Header Field With Post-Base Name Reference

   For entries in the dynamic table with an absolute index greater than
   Base Index, the header field name is represented using the post-base
   index of that entry (see Section 3.2.5) encoded as an integer with a
   3-bit prefix.

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      | 0 | 0 | 0 | 0 | N |NameIdx(3+)|
      +---+---+---+---+---+-----------+
      | H |     Value Length (7+)     |
      +---+---------------------------+
      |  Value String (Length bytes)  |
      +-------------------------------+

            Literal Header Field With Post-Base Name Reference

4.5.6.  Literal Header Field Without Name Reference

   An addition to the header table where both the header field name and
   the header field value are represented as string literals (see
   Section 4.1) starts with the '001' three-bit pattern.

   The fourth bit, 'N', indicates whether an intermediary is permitted
   to add this header to the dynamic header table on subsequent hops.
   When the 'N' bit is set, the encoded header MUST always be encoded
   with a literal representation.  In particular, when a peer sends a
   header field that it received represented as a literal header field
   with the 'N' bit set, it MUST use a literal representation to forward



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   this header field.  This bit is intended for protecting header field
   values that are not to be put at risk by compressing them (see
   Section 7.1 of [RFC7541] for more details).

   The name is represented as a 4-bit prefix string literal, while the
   value is represented as an 8-bit prefix string literal.

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      | 0 | 0 | 1 | N | H |NameLen(3+)|
      +---+---+---+---+---+-----------+
      |  Name String (Length bytes)   |
      +---+---------------------------+
      | H |     Value Length (7+)     |
      +---+---------------------------+
      |  Value String (Length bytes)  |
      +-------------------------------+

                Literal Header Field Without Name Reference

5.  Configuration

   QPACK defines two settings which are included in the HTTP/3 SETTINGS
   frame.

   SETTINGS_HEADER_TABLE_SIZE (0x1):  An integer with a maximum value of
      2^30 - 1.  The default value is zero bytes.  See Section 3.2 for
      usage.

   SETTINGS_QPACK_BLOCKED_STREAMS (0x7):  An integer with a maximum
      value of 2^16 - 1.  The default value is zero.  See Section 2.1.3.

6.  Error Handling

   The following error codes are defined for HTTP/3 to indicate failures
   of QPACK which prevent the stream or connection from continuing:

   HTTP_QPACK_DECOMPRESSION_FAILED (TBD):  The decoder failed to
      interpret an instruction on a request or push stream and is not
      able to continue decoding that header block.

   HTTP_QPACK_ENCODER_STREAM_ERROR (TBD):  The decoder failed to
      interpret an instruction on the encoder stream.

   HTTP_QPACK_DECODER_STREAM_ERROR (TBD):  The encoder failed to
      interpret an instruction on the decoder stream.





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   Upon encountering an error, an implementation MAY elect to treat it
   as a connection error even if this document prescribes that it MUST
   be treated as a stream error.

7.  Security Considerations

   TBD.

8.  IANA Considerations

8.1.  Settings Registration

   This document creates two new settings in the "HTTP/3 Settings"
   registry established in [HTTP3].

   The entries in the following table are registered by this document.

             +-----------------------+------+---------------+
             | Setting Name          | Code | Specification |
             +-----------------------+------+---------------+
             | HEADER_TABLE_SIZE     | 0x1  | Section 5     |
             |                       |      |               |
             | QPACK_BLOCKED_STREAMS | 0x7  | Section 5     |
             +-----------------------+------+---------------+

8.2.  Stream Type Registration

   This document creates two new settings in the "HTTP/3 Stream Type"
   registry established in [HTTP3].

   The entries in the following table are registered by this document.

         +----------------------+------+---------------+--------+
         | Stream Type          | Code | Specification | Sender |
         +----------------------+------+---------------+--------+
         | QPACK Encoder Stream | 0x48 | Section 4     | Both   |
         |                      |      |               |        |
         | QPACK Decoder Stream | 0x68 | Section 4     | Both   |
         +----------------------+------+---------------+--------+

8.3.  Error Code Registration

   This document establishes the following new error codes in the
   "HTTP/3 Error Code" registry established in [HTTP3].







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   +------------------------------+------+--------------+--------------+
   | Name                         | Code | Description  | Specificatio |
   |                              |      |              | n            |
   +------------------------------+------+--------------+--------------+
   | HTTP_QPACK_DECOMPRESSION_FAI | TBD  | Decompressio | Section 6    |
   | LED                          |      | n of a       |              |
   |                              |      | header block |              |
   |                              |      | failed       |              |
   |                              |      |              |              |
   | HTTP_QPACK_ENCODER_STREAM_ER | TBD  | Error on the | Section 6    |
   | ROR                          |      | encoder      |              |
   |                              |      | stream       |              |
   |                              |      |              |              |
   | HTTP_QPACK_DECODER_STREAM_ER | TBD  | Error on the | Section 6    |
   | ROR                          |      | decoder      |              |
   |                              |      | stream       |              |
   +------------------------------+------+--------------+--------------+

9.  References

9.1.  Normative References

   [HTTP3]    Bishop, M., Ed., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", draft-ietf-quic-http-16 (work in progress),
              December 2018.

   [QUIC-TRANSPORT]
              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", draft-ietf-quic-
              transport-16 (work in progress), December 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7541]  Peon, R. and H. Ruellan, "HPACK: Header Compression for
              HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
              <https://www.rfc-editor.org/info/rfc7541>.

   [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>.








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9.2.  Informative References

   [RFC2360]  Scott, G., "Guide for Internet Standards Writers", BCP 22,
              RFC 2360, DOI 10.17487/RFC2360, June 1998,
              <https://www.rfc-editor.org/info/rfc2360>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

9.3.  URIs

   [1] https://mailarchive.ietf.org/arch/search/?email_list=quic

   [2] https://github.com/quicwg

   [3] https://github.com/quicwg/base-drafts/labels/-qpack

Appendix A.  Static Table

   +------+-----------------------------+------------------------------+
   | Inde | Name                        | Value                        |
   | x    |                             |                              |
   +------+-----------------------------+------------------------------+
   | 0    | :authority                  |                              |
   |      |                             |                              |
   | 1    | :path                       | /                            |
   |      |                             |                              |
   | 2    | age                         | 0                            |
   |      |                             |                              |
   | 3    | content-disposition         |                              |
   |      |                             |                              |
   | 4    | content-length              | 0                            |
   |      |                             |                              |
   | 5    | cookie                      |                              |
   |      |                             |                              |
   | 6    | date                        |                              |
   |      |                             |                              |
   | 7    | etag                        |                              |
   |      |                             |                              |
   | 8    | if-modified-since           |                              |
   |      |                             |                              |
   | 9    | if-none-match               |                              |
   |      |                             |                              |
   | 10   | last-modified               |                              |
   |      |                             |                              |
   | 11   | link                        |                              |



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   |      |                             |                              |
   | 12   | location                    |                              |
   |      |                             |                              |
   | 13   | referer                     |                              |
   |      |                             |                              |
   | 14   | set-cookie                  |                              |
   |      |                             |                              |
   | 15   | :method                     | CONNECT                      |
   |      |                             |                              |
   | 16   | :method                     | DELETE                       |
   |      |                             |                              |
   | 17   | :method                     | GET                          |
   |      |                             |                              |
   | 18   | :method                     | HEAD                         |
   |      |                             |                              |
   | 19   | :method                     | OPTIONS                      |
   |      |                             |                              |
   | 20   | :method                     | POST                         |
   |      |                             |                              |
   | 21   | :method                     | PUT                          |
   |      |                             |                              |
   | 22   | :scheme                     | http                         |
   |      |                             |                              |
   | 23   | :scheme                     | https                        |
   |      |                             |                              |
   | 24   | :status                     | 103                          |
   |      |                             |                              |
   | 25   | :status                     | 200                          |
   |      |                             |                              |
   | 26   | :status                     | 304                          |
   |      |                             |                              |
   | 27   | :status                     | 404                          |
   |      |                             |                              |
   | 28   | :status                     | 503                          |
   |      |                             |                              |
   | 29   | accept                      | */*                          |
   |      |                             |                              |
   | 30   | accept                      | application/dns-message      |
   |      |                             |                              |
   | 31   | accept-encoding             | gzip, deflate, br            |
   |      |                             |                              |
   | 32   | accept-ranges               | bytes                        |
   |      |                             |                              |
   | 33   | access-control-allow-       | cache-control                |
   |      | headers                     |                              |
   |      |                             |                              |
   | 34   | access-control-allow-       | content-type                 |
   |      | headers                     |                              |



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   |      |                             |                              |
   | 35   | access-control-allow-origin | *                            |
   |      |                             |                              |
   | 36   | cache-control               | max-age=0                    |
   |      |                             |                              |
   | 37   | cache-control               | max-age=2592000              |
   |      |                             |                              |
   | 38   | cache-control               | max-age=604800               |
   |      |                             |                              |
   | 39   | cache-control               | no-cache                     |
   |      |                             |                              |
   | 40   | cache-control               | no-store                     |
   |      |                             |                              |
   | 41   | cache-control               | public, max-age=31536000     |
   |      |                             |                              |
   | 42   | content-encoding            | br                           |
   |      |                             |                              |
   | 43   | content-encoding            | gzip                         |
   |      |                             |                              |
   | 44   | content-type                | application/dns-message      |
   |      |                             |                              |
   | 45   | content-type                | application/javascript       |
   |      |                             |                              |
   | 46   | content-type                | application/json             |
   |      |                             |                              |
   | 47   | content-type                | application/x-www-form-      |
   |      |                             | urlencoded                   |
   |      |                             |                              |
   | 48   | content-type                | image/gif                    |
   |      |                             |                              |
   | 49   | content-type                | image/jpeg                   |
   |      |                             |                              |
   | 50   | content-type                | image/png                    |
   |      |                             |                              |
   | 51   | content-type                | text/css                     |
   |      |                             |                              |
   | 52   | content-type                | text/html; charset=utf-8     |
   |      |                             |                              |
   | 53   | content-type                | text/plain                   |
   |      |                             |                              |
   | 54   | content-type                | text/plain;charset=utf-8     |
   |      |                             |                              |
   | 55   | range                       | bytes=0-                     |
   |      |                             |                              |
   | 56   | strict-transport-security   | max-age=31536000             |
   |      |                             |                              |
   | 57   | strict-transport-security   | max-age=31536000;            |
   |      |                             | includesubdomains            |



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   |      |                             |                              |
   | 58   | strict-transport-security   | max-age=31536000;            |
   |      |                             | includesubdomains; preload   |
   |      |                             |                              |
   | 59   | vary                        | accept-encoding              |
   |      |                             |                              |
   | 60   | vary                        | origin                       |
   |      |                             |                              |
   | 61   | x-content-type-options      | nosniff                      |
   |      |                             |                              |
   | 62   | x-xss-protection            | 1; mode=block                |
   |      |                             |                              |
   | 63   | :status                     | 100                          |
   |      |                             |                              |
   | 64   | :status                     | 204                          |
   |      |                             |                              |
   | 65   | :status                     | 206                          |
   |      |                             |                              |
   | 66   | :status                     | 302                          |
   |      |                             |                              |
   | 67   | :status                     | 400                          |
   |      |                             |                              |
   | 68   | :status                     | 403                          |
   |      |                             |                              |
   | 69   | :status                     | 421                          |
   |      |                             |                              |
   | 70   | :status                     | 425                          |
   |      |                             |                              |
   | 71   | :status                     | 500                          |
   |      |                             |                              |
   | 72   | accept-language             |                              |
   |      |                             |                              |
   | 73   | access-control-allow-       | FALSE                        |
   |      | credentials                 |                              |
   |      |                             |                              |
   | 74   | access-control-allow-       | TRUE                         |
   |      | credentials                 |                              |
   |      |                             |                              |
   | 75   | access-control-allow-       | *                            |
   |      | headers                     |                              |
   |      |                             |                              |
   | 76   | access-control-allow-       | get                          |
   |      | methods                     |                              |
   |      |                             |                              |
   | 77   | access-control-allow-       | get, post, options           |
   |      | methods                     |                              |
   |      |                             |                              |
   | 78   | access-control-allow-       | options                      |



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   |      | methods                     |                              |
   |      |                             |                              |
   | 79   | access-control-expose-      | content-length               |
   |      | headers                     |                              |
   |      |                             |                              |
   | 80   | access-control-request-     | content-type                 |
   |      | headers                     |                              |
   |      |                             |                              |
   | 81   | access-control-request-     | get                          |
   |      | method                      |                              |
   |      |                             |                              |
   | 82   | access-control-request-     | post                         |
   |      | method                      |                              |
   |      |                             |                              |
   | 83   | alt-svc                     | clear                        |
   |      |                             |                              |
   | 84   | authorization               |                              |
   |      |                             |                              |
   | 85   | content-security-policy     | script-src 'none'; object-   |
   |      |                             | src 'none'; base-uri 'none'  |
   |      |                             |                              |
   | 86   | early-data                  | 1                            |
   |      |                             |                              |
   | 87   | expect-ct                   |                              |
   |      |                             |                              |
   | 88   | forwarded                   |                              |
   |      |                             |                              |
   | 89   | if-range                    |                              |
   |      |                             |                              |
   | 90   | origin                      |                              |
   |      |                             |                              |
   | 91   | purpose                     | prefetch                     |
   |      |                             |                              |
   | 92   | server                      |                              |
   |      |                             |                              |
   | 93   | timing-allow-origin         | *                            |
   |      |                             |                              |
   | 94   | upgrade-insecure-requests   | 1                            |
   |      |                             |                              |
   | 95   | user-agent                  |                              |
   |      |                             |                              |
   | 96   | x-forwarded-for             |                              |
   |      |                             |                              |
   | 97   | x-frame-options             | deny                         |
   |      |                             |                              |
   | 98   | x-frame-options             | sameorigin                   |
   +------+-----------------------------+------------------------------+




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Appendix B.  Sample One Pass Encoding Algorithm

   Pseudo-code for single pass encoding, excluding handling of
   duplicates, non-blocking mode, and reference tracking.















































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   baseIndex = dynamicTable.baseIndex
   largestReference = 0
   for header in headers:
     staticIdx = staticTable.getIndex(header)
     if staticIdx:
       encodeIndexReference(streamBuffer, staticIdx)
       continue

     dynamicIdx = dynamicTable.getIndex(header)
     if !dynamicIdx:
       # No matching entry.  Either insert+index or encode literal
       nameIdx = getNameIndex(header)
       if shouldIndex(header) and dynamicTable.canIndex(header):
         encodeLiteralWithIncrementalIndex(controlBuffer, nameIdx,
                                           header)
         dynamicTable.add(header)
         dynamicIdx = dynamicTable.baseIndex

     if !dynamicIdx:
       # Couldn't index it, literal
       if nameIdx <= staticTable.size:
         encodeLiteral(streamBuffer, nameIndex, header)
       else:
         # encode literal, possibly with nameIdx above baseIndex
         encodeDynamicLiteral(streamBuffer, nameIndex, baseIndex,
                              header)
         largestReference = max(largestReference,
                                dynamicTable.toAbsolute(nameIdx))
     else:
       # Dynamic index reference
       assert(dynamicIdx)
       largestReference = max(largestReference, dynamicIdx)
       # Encode dynamicIdx, possibly with dynamicIdx above baseIndex
       encodeDynamicIndexReference(streamBuffer, dynamicIdx,
                                   baseIndex)

   # encode the prefix
   encodeInteger(prefixBuffer, 0x00, largestReference, 8)
   if baseIndex >= largestReference:
     encodeInteger(prefixBuffer, 0, baseIndex - largestReference, 7)
   else:
     encodeInteger(prefixBuffer, 0x80,
                   largestReference  - baseIndex, 7)

   return controlBuffer, prefixBuffer + streamBuffer






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Appendix C.  Change Log

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

C.1.  Since draft-ietf-quic-qpack-03

   Substantial editorial reorganization; no technical changes.

C.2.  Since draft-ietf-quic-qpack-02

   o  Largest Reference encoded modulo MaxEntries (#1763)

   o  New Static Table (#1355)

   o  Table Size Update with Insert Count=0 is a connection error
      (#1762)

   o  Stream Cancellations are optional when
      SETTINGS_HEADER_TABLE_SIZE=0 (#1761)

   o  Implementations must handle 62 bit integers (#1760)

   o  Different error types for each QPACK stream, other changes to
      error handling (#1726)

   o  Preserve header field order (#1725)

   o  Initial table size is the maximum permitted when table is first
      usable (#1642)

C.3.  Since draft-ietf-quic-qpack-01

   o  Only header blocks that reference the dynamic table are
      acknowledged (#1603, #1605)

C.4.  Since draft-ietf-quic-qpack-00

   o  Renumbered instructions for consistency (#1471, #1472)

   o  Decoder is allowed to validate largest reference (#1404, #1469)

   o  Header block acknowledgments also acknowledge the associated
      largest reference (#1370, #1400)

   o  Added an acknowledgment for unread streams (#1371, #1400)

   o  Removed framing from encoder stream (#1361,#1467)



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   o  Control streams use typed unidirectional streams rather than fixed
      stream IDs (#910,#1359)

C.5.  Since draft-ietf-quic-qcram-00

   o  Separate instruction sets for table updates and header blocks
      (#1235, #1142, #1141)

   o  Reworked indexing scheme (#1176, #1145, #1136, #1130, #1125,
      #1314)

   o  Added mechanisms that support one-pass encoding (#1138, #1320)

   o  Added a setting to control the number of blocked decoders (#238,
      #1140, #1143)

   o  Moved table updates and acknowledgments to dedicated streams
      (#1121, #1122, #1238)

Acknowledgments

   This draft draws heavily on the text of [RFC7541].  The indirect
   input of those authors is gratefully acknowledged, as well as ideas
   from:

   o  Ryan Hamilton

   o  Patrick McManus

   o  Kazuho Oku

   o  Biren Roy

   o  Ian Swett

   o  Dmitri Tikhonov

   Buck's contribution was supported by Google during his employment
   there.

   A substantial portion of Mike's contribution was supported by
   Microsoft during his employment there.

Authors' Addresses







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   Charles 'Buck' Krasic
   Netflix

   Email: ckrasic@netflix.com


   Mike Bishop
   Akamai Technologies

   Email: mbishop@evequefou.be


   Alan Frindell (editor)
   Facebook

   Email: afrind@fb.com



































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