draft-ietf-quic-qpack.txt

QUIC                                                           C. Krasic
Internet-Draft                                                   Netflix
Intended status: Standards Track                               M. Bishop
Expires: December 23, 2019                           Akamai Technologies
                                                        A. Frindell, Ed.
                                                                Facebook
                                                           June 21, 2019


                  QPACK: Header Compression for HTTP/3
                      draft-ietf-quic-qpack-latest

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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 December 23, 2019.






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Copyright Notice

   Copyright (c) 2019 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  . . . . . . . . . . . . . . . . .   5
   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.  Known Received Count  . . . . . . . . . . . . . . . .   8
     2.2.  Decoder . . . . . . . . . . . . . . . . . . . . . . . . .   8
       2.2.1.  State Synchronization . . . . . . . . . . . . . . . .   9
       2.2.2.  Blocked Decoding  . . . . . . . . . . . . . . . . . .   9
   3.  Header Tables . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Static Table  . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Dynamic Table . . . . . . . . . . . . . . . . . . . . . .  10
       3.2.1.  Dynamic Table Size  . . . . . . . . . . . . . . . . .  10
       3.2.2.  Dynamic Table Capacity and Eviction . . . . . . . . .  10
       3.2.3.  Maximum Dynamic Table Capacity  . . . . . . . . . . .  11
       3.2.4.  Absolute Indexing . . . . . . . . . . . . . . . . . .  12
       3.2.5.  Relative Indexing . . . . . . . . . . . . . . . . . .  12
       3.2.6.  Post-Base Indexing  . . . . . . . . . . . . . . . . .  13
       3.2.7.  Invalid References  . . . . . . . . . . . . . . . . .  13
   4.  Wire Format . . . . . . . . . . . . . . . . . . . . . . . . .  14
     4.1.  Primitives  . . . . . . . . . . . . . . . . . . . . . . .  14
       4.1.1.  Prefixed Integers . . . . . . . . . . . . . . . . . .  14
       4.1.2.  String Literals . . . . . . . . . . . . . . . . . . .  14
     4.2.  Instructions  . . . . . . . . . . . . . . . . . . . . . .  14
       4.2.1.  Encoder and Decoder Streams . . . . . . . . . . . . .  15
     4.3.  Encoder Instructions  . . . . . . . . . . . . . . . . . .  15
       4.3.1.  Insert With Name Reference  . . . . . . . . . . . . .  16
       4.3.2.  Insert Without Name Reference . . . . . . . . . . . .  16



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       4.3.3.  Duplicate . . . . . . . . . . . . . . . . . . . . . .  17
       4.3.4.  Set Dynamic Table Capacity  . . . . . . . . . . . . .  17
     4.4.  Decoder Instructions  . . . . . . . . . . . . . . . . . .  18
       4.4.1.  Insert Count Increment  . . . . . . . . . . . . . . .  18
       4.4.2.  Header Acknowledgement  . . . . . . . . . . . . . . .  18
       4.4.3.  Stream Cancellation . . . . . . . . . . . . . . . . .  19
     4.5.  Header Block Instructions . . . . . . . . . . . . . . . .  20
       4.5.1.  Header Block Prefix . . . . . . . . . . . . . . . . .  20
       4.5.2.  Indexed Header Field  . . . . . . . . . . . . . . . .  22
       4.5.3.  Indexed Header Field With Post-Base Index . . . . . .  23
       4.5.4.  Literal Header Field With Name Reference  . . . . . .  23
       4.5.5.  Literal Header Field With Post-Base Name Reference  .  24
       4.5.6.  Literal Header Field Without Name Reference . . . . .  25
   5.  Configuration . . . . . . . . . . . . . . . . . . . . . . . .  25
   6.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  26
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
     8.1.  Settings Registration . . . . . . . . . . . . . . . . . .  26
     8.2.  Stream Type Registration  . . . . . . . . . . . . . . . .  26
     8.3.  Error Code Registration . . . . . . . . . . . . . . . . .  27
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  27
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  28
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  28
   Appendix A.  Static Table . . . . . . . . . . . . . . . . . . . .  28
   Appendix B.  Sample One Pass Encoding Algorithm . . . . . . . . .  33
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  35
     C.1.  Since draft-ietf-quic-qpack-06  . . . . . . . . . . . . .  35
     C.2.  Since draft-ietf-quic-qpack-05  . . . . . . . . . . . . .  35
     C.3.  Since draft-ietf-quic-qpack-04  . . . . . . . . . . . . .  35
     C.4.  Since draft-ietf-quic-qpack-03  . . . . . . . . . . . . .  35
     C.5.  Since draft-ietf-quic-qpack-02  . . . . . . . . . . . . .  35
     C.6.  Since draft-ietf-quic-qpack-01  . . . . . . . . . . . . .  36
     C.7.  Since draft-ietf-quic-qpack-00  . . . . . . . . . . . . .  36
     C.8.  Since draft-ietf-quic-qcram-00  . . . . . . . . . . . . .  36
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  36
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

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.



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   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].

   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:  A reference point for relative indices.  Dynamic references
      are made relative to a Base in header blocks.

   Insert Count:  The total number of entries inserted in the dynamic
      table.

   QPACK is a name, not an acronym.






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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

   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.




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   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.

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

   A dynamic table entry is considered blocking and cannot be evicted
   until its insertion has been acknowledged and there are no
   outstanding unacknowledged references to the entry.  In particular, a
   dynamic table entry that has never been referenced can still be
   blocking.

   Note:  A blocking entry is unrelated to a blocked stream, which is a
      stream that a decoder cannot decode as a result of references to
      entries that are not yet available.  Any encoder that uses the
      dynamic table has to keep track of blocked entries, whereas
      blocked streams are optional.

   An encoder MUST NOT insert an entry into the dynamic table (or
   duplicate an existing entry) if doing so would evict a blocking
   entry.  In this case, the encoder can send literal representations of
   header fields.

   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 non-blocking 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



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   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               Insertion Point
     Point

                 Figure 1: Draining Dynamic Table Entries

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 Required Insert Count, the lowest
   possible value for the Insert Count with which the header block can
   be decoded.  For a header block with references to the dynamic table,
   the Required Insert Count is one larger than the largest Absolute
   Index of all referenced dynamic table entries.  For a header block
   with no references to the dynamic table, the Required Insert Count is
   zero.

   If the decoder encounters a header block with a Required Insert Count
   value larger than defined above, it MAY treat this as a connection
   error of type HTTP_QPACK_DECOMPRESSION_FAILED.  If the decoder
   encounters a header block with a Required Insert Count value smaller
   than defined above, it MUST treat this as a connection error of type
   HTTP_QPACK_DECOMPRESSION_FAILED as prescribed in Section 3.2.7.

   When the Required Insert Count is zero, the frame contains no
   references to the dynamic table and can always be processed
   immediately.

   If the Required Insert Count is greater than the number of dynamic
   table entries received, the stream is considered "blocked."  While
   blocked, header field data SHOULD remain in the blocked stream's flow
   control window.  A stream becomes unblocked when the Insert Count
   becomes greater than or equal to the Required Insert Count for all
   header blocks the decoder has started reading from the stream.





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   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
   connection 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.  Known Received Count

   In order to identify which dynamic table entries can be safely used
   without a stream becoming blocked, the encoder tracks the number of
   entries received by the decoder.  The Known Received Count tracks the
   total number of acknowledged insertions.

   When blocking references are permitted, the encoder uses header block
   acknowledgement to maintain the Known Received Count, 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 an Insert Count
   Increment 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 encoder instructions received on the encoder
   stream.

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







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2.2.1.  State Synchronization

   The decoder instructions (Section 4.4) signal 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 (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 Insert Count Increment 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 an Insert Count Increment instruction, the decoder might be
   able to coalesce multiple Insert Count Increment 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.

2.2.2.  Blocked Decoding

   To track blocked streams, the Required Insert Count 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.




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   All entries in the static table have a name and a value.  However,
   values can be empty (that is, have a length of 0).

   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 in a header
   block instruction it MUST treat this as a connection 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.  Each HTTP/3 endpoint holds a dynamic
   table that is initially empty.  Entries are added by encoder
   instructions received on the encoder stream (see Section 4.3).

   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.  Dynamic 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.

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

3.2.2.  Dynamic Table Capacity and Eviction

   The encoder sets the capacity of the dynamic table, which serves as
   the upper limit on its size.  The initial capacity of the dynamic
   table is zero.

   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
   is less than or equal to (table capacity - size of new entry) or
   until the table is empty.  The encoder MUST NOT evict a blocking
   dynamic table entry (see Section 2.1.2).

   If the size of the new entry is less than or equal to the dynamic
   table capacity, then that entry is added to the table.  It is an
   error if the encoder attempts to add an entry that is larger than the



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   dynamic table capacity; the decoder MUST treat this 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
   or value if the referenced entry is evicted from the dynamic table
   prior to inserting the new entry.

   Whenever the dynamic table capacity is reduced by the encoder,
   entries are evicted from the end of the dynamic table until the size
   of the dynamic table is less than or equal to the new table capacity.
   This mechanism can be used to completely clear entries from the
   dynamic table by setting a capacity of 0, which can subsequently be
   restored.

3.2.3.  Maximum Dynamic Table Capacity

   To bound the memory requirements of the decoder, the decoder limits
   the maximum value the encoder is permitted to set for the dynamic
   table capacity.  In HTTP/3, this limit is determined by the value of
   SETTINGS_QPACK_MAX_TABLE_CAPACITY sent by the decoder (see
   Section 5).  The encoder MUST not set a dynamic table capacity that
   exceeds this maximum, but it can choose to use a lower dynamic table
   capacity (see Section 4.3.4).

   For clients using 0-RTT data in HTTP/3, the server's maximum table
   capacity is the remembered value of the setting, or zero if the value
   was not previously sent.  When the client's 0-RTT value of the
   SETTING is 0, the server MAY set it to a non-zero value in its
   SETTINGS frame.  If the remembered value is non-zero, the server MUST
   send the same non-zero value in its SETTINGS frame.  If it specifies
   any other value, or omits SETTINGS_QPACK_MAX_TABLE_CAPACITY from
   SETTINGS, the encoder must treat this as a connection error of type
   "HTTP_QPACK_DECODER_STREAM_ERROR".

   For HTTP/3 servers and HTTP/3 clients when 0-RTT is not attempted or
   is rejected, the maximum table capacity is 0 until the encoder
   processes a SETTINGS frame with a non-zero value of
   SETTINGS_QPACK_MAX_TABLE_CAPACITY.

   When the maximum table capacity is 0, the encoder MUST NOT insert
   entries into the dynamic table, and MUST NOT send any encoder
   instructions on the encoder stream.







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3.2.4.  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 "0"; indices increase by one with each insertion.

3.2.5.  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.

   In encoder instructions, 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-1 |      ...      |   d   |  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

   Unlike encoder instructions, relative indices in header block
   instructions are relative to the Base at the beginning of the header
   block (see Section 4.5.1).  This ensures that references are stable
   even if the dynamic table is updated while decoding a header block.

   The Base is encoded as a value relative to the Required Insert Count.
   The Base identifies which dynamic table entries can be referenced
   using relative indexing, starting with 0 at the last entry added.

   Post-Base references are used for entries inserted after base,
   starting at 0 for the first entry added after the Base; see
   Section 3.2.6.







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

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

      Example Dynamic Table Indexing - Relative Index in Header Block

3.2.6.  Post-Base Indexing

   A header block can reference entries added after the entry identified
   by the Base.  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, with the zero
   value being the first entry inserted after the Base.

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

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

     Example Dynamic Table Indexing - Post-Base Index in Header Block

3.2.7.  Invalid References

   If the decoder encounters a reference in a header block instruction
   to a dynamic table entry which has already been evicted or which has
   an absolute index greater than or equal to the declared Required
   Insert Count (see Section 4.5.1), it MUST treat this as a connection
   error of type "HTTP_QPACK_DECOMPRESSION_FAILED".




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   If the decoder encounters a reference in an encoder instruction 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.  Instructions

   There are three separate QPACK instruction spaces.  Encoder
   instructions (Section 4.3) carry table updates, decoder instructions
   (Section 4.4) carry acknowledgments of table modifications and header
   processing, and header block instructions (Section 4.5) convey an
   encoded representation of a header list by referring to the QPACK
   table state.

   Encoder and decoder instructions appear on the unidirectional stream
   types described in this section.  Header block instructions are



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   contained in HEADERS and PUSH_PROMISE frames, which are conveyed on
   request or push streams as described in [HTTP3].

4.2.1.  Encoder and Decoder Streams

   QPACK defines two unidirectional stream types:

   o  An encoder stream is a unidirectional stream of type "0x02".  It
      carries an unframed sequence of encoder instructions from encoder
      to decoder.

   o  A decoder stream is a unidirectional stream of type "0x03".  It
      carries an unframed sequence of decoder instructions from decoder
      to encoder.

   HTTP/3 endpoints contain a QPACK encoder and decoder.  Each endpoint
   MUST initiate at most one encoder stream and at most one decoder
   stream.  Receipt of a second instance of either stream type MUST be
   treated as a connection error of type HTTP_WRONG_STREAM_COUNT.  These
   streams MUST NOT be closed.  Closure of either unidirectional stream
   type MUST be treated as a connection error of type
   HTTP_CLOSED_CRITICAL_STREAM.

   An endpoint MAY avoid creating its own encoder stream if it's not
   going to be used (for example if the endpoint doesn't wish to use the
   dynamic table, or if the maximum size of the dynamic table permitted
   by the peer is zero).

   An endpoint MAY avoid creating its own decoder stream if the maximum
   size of its own dynamic table is zero.

   An endpoint MUST allow its peer to create both encoder and decoder
   streams even if the connection's settings prevent their use.

4.3.  Encoder Instructions

   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.

   This section specifies the following encoder instructions.







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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]).

        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





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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
   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.  Set Dynamic Table Capacity

   An encoder informs the decoder of a change to the dynamic table
   capacity using an instruction which begins with the '001' three-bit
   pattern.  The new dynamic table capacity 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 |   Capacity (5+)   |
   +---+---+---+-------------------+

                   Figure 3: Set Dynamic Table Capacity

   The new capacity MUST be lower than or equal to the limit described
   in Section 3.2.3.  In HTTP/3, this limit is the value of the
   SETTINGS_QPACK_MAX_TABLE_CAPACITY parameter (see Section 5) received
   from the decoder.  The decoder MUST treat a new dynamic table
   capacity value that exceeds this limit as a connection error of type
   "HTTP_QPACK_ENCODER_STREAM_ERROR".

   Reducing the dynamic table capacity can cause entries to be evicted
   (see Section 3.2.2).  This MUST NOT cause the eviction of blocking
   entries (see Section 2.1.2).  Changing the capacity of the dynamic
   table is not acknowledged as this instruction does not insert an
   entry.






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4.4.  Decoder Instructions

   Decoder instructions provide information used to ensure consistency
   of the dynamic table.  They are sent from the decoder to the encoder
   on a decoder stream; 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.

   This section specifies the following decoder instructions.

4.4.1.  Insert Count Increment

   The Insert Count Increment instruction begins with the '00' two-bit
   pattern.  The instruction specifies the total number of dynamic table
   inserts and duplications since the last Insert Count Increment or
   Header Acknowledgement that increased the Known Received Count for
   the dynamic table (see Section 2.1.4).  The Increment field 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 |     Increment (6+)    |
   +---+---+-----------------------+

                     Figure 4: Insert Count Increment

   An encoder that receives an Increment field equal to zero or one that
   increases the Known Received Count 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 Required Insert Count
   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 header block's associated 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 the Known
   Received Count.








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     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 HEADERS
   and PUSH_PROMISE 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 Required
   Insert Count has already been acknowledged, that MUST be treated as a
   connection error of type "HTTP_QPACK_DECODER_STREAM_ERROR".

   When blocking references are permitted, the encoder uses
   acknowledgement of header blocks to update the Known Received Count.
   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 Required Insert Count
   of an acknowledged header block was greater than the encoder's
   current Known Received Count, the block's Required Insert Count
   becomes the new Known Received Count.

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



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   outstanding.  A decoder with a maximum dynamic table capacity equal
   to zero (see Section 3.2.3) 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.  Header Block Instructions

   HTTP/3 endpoints convert header lists to headers blocks and exchange
   them inside HEADERS and PUSH_PROMISE frames.  A decoder interprets
   header block instructions in order to construct a header list.  These
   instructions reference the static table, or dynamic table in a
   particular state without modifying it.

   This section specifies the following header block instructions.

4.5.1.  Header Block Prefix

   Each header block is prefixed with two integers.  The Required Insert
   Count is encoded as an integer with an 8-bit prefix after the
   encoding described in Section 4.5.1.1).  The Base is encoded as sign-
   and-modulus integer, using a single sign bit and a value with a 7-bit
   prefix (see Section 4.5.1.2).

   These two values are followed by instructions for compressed headers.
   The entire block is expected to be framed by the using protocol.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   |   Required Insert Count (8+)  |
   +---+---------------------------+
   | S |      Delta Base (7+)      |
   +---+---------------------------+
   |      Compressed Headers     ...
   +-------------------------------+

                          Figure 7: Frame Payload

4.5.1.1.  Required Insert Count

   Required Insert Count identifies the state of the dynamic table
   needed to process the header block.  Blocking decoders use the
   Required Insert Count to determine when it is safe to process the
   rest of the block.

   The encoder transforms the Required Insert Count as follows before
   encoding:



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      if ReqInsertCount == 0:
         EncInsertCount = 0
      else:
         EncInsertCount = (ReqInsertCount mod (2 * MaxEntries)) + 1

   Here "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.  Hence "MaxEntries" is calculated as

      MaxEntries = floor( MaxTableCapacity / 32 )

   "MaxTableCapacity" is the maximum capacity of the dynamic table as
   specified by the decoder (see Section 3.2.3).

   This encoding limits the length of the prefix on long-lived
   connections.

   The decoder can reconstruct the Required Insert Count using an
   algorithm such as the following.  If the decoder encounters a value
   of EncodedInsertCount that could not have been produced by a
   conformant encoder, it MUST treat this as a connection error of type
   "HTTP_QPACK_DECOMPRESSION_FAILED".

   TotalNumberOfInserts is the total number of inserts into the
   decoder's dynamic table.

      FullRange = 2 * MaxEntries
      if EncodedInsertCount == 0:
         ReqInsertCount = 0
      else:
         if EncodedInsertCount > FullRange:
            Error
         MaxValue = TotalNumberOfInserts + MaxEntries

         # MaxWrapped is the largest possible value of
         # ReqInsertCount that is 0 mod 2*MaxEntries
         MaxWrapped = floor(MaxValue / FullRange) * FullRange
         ReqInsertCount = MaxWrapped + EncodedInsertCount - 1

         # If ReqInsertCount exceeds MaxValue, the Encoder's value
         # must have wrapped one fewer time
         if ReqInsertCount > MaxValue:
            if ReqInsertCount < FullRange:
               Error
            ReqInsertCount -= FullRange

   For example, if the dynamic table is 100 bytes, then the Required
   Insert Count will be encoded modulo 6.  If a decoder has received 10



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   inserts, then an encoded value of 3 indicates that the Required
   Insert Count is 9 for the header block.

4.5.1.2.  Base

   The "Base" is used to resolve references in the dynamic table as
   described in Section 3.2.5.

   To save space, the Base is encoded relative to the Insert Count using
   a one-bit sign and the "Delta Base" value.  A sign bit of 0 indicates
   that the Base is greater than or equal to the value of the Insert
   Count; the value of Delta Base is added to the Insert Count to
   determine the value of the Base.  A sign bit of 1 indicates that the
   Base is less than the Insert Count.  That is:

      if S == 0:
         Base = ReqInsertCount + DeltaBase
      else:
         Base = ReqInsertCount - DeltaBase - 1

   A single-pass encoder determines the Base before encoding a header
   block.  If the encoder inserted entries in the dynamic table while
   encoding the header block, Required Insert Count will be greater than
   the Base, 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, the Base will
   be greater than the Required Insert Count, 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 Required Insert Count and the Base to the same value.  In
   such case, both the sign bit and the Delta Base will be set to zero.

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

   For example, with an Required Insert Count of 9, a decoder receives a
   S bit of 1 and a Delta Base of 2.  This sets the Base to 6 and
   enables post-base indexing for three entries.  In this example, a
   regular index of 1 refers to the 5th entry that was added to the
   table; a post-base index of 1 refers to the 8th entry.

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




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   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 the Base, 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) 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 or equal to the Base, the representation starts with the '0001'
   4-bit pattern, followed by the post-base index (see Section 3.2.6) 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 the Base, this representation starts with
   the '01' two-bit pattern.  If the entry is in the dynamic table with
   an absolute index greater than or equal to the Base, the
   representation starts with the '0000' four-bit pattern.

   Only the header field name stored in the static or dynamic table is
   used.  Any header field value MUST be ignored.





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   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).

        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 the Base, 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
   or equal to the Base, the header field name is represented using the
   post-base index of that entry (see Section 3.2.6) 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






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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
   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_QPACK_MAX_TABLE_CAPACITY (0x1):  The default value is zero.
      See Section 3.2 for usage.  This is the equivalent of the
      SETTINGS_HEADER_TABLE_SIZE from HTTP/2.

   SETTINGS_QPACK_BLOCKED_STREAMS (0x7):  The default value is zero.
      See Section 2.1.3.








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6.  Error Handling

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

   HTTP_QPACK_DECOMPRESSION_FAILED (0x200):  The decoder failed to
      interpret a header block instruction and is not able to continue
      decoding that header block.

   HTTP_QPACK_ENCODER_STREAM_ERROR (0x201):  The decoder failed to
      interpret an encoder instruction received on the encoder stream.

   HTTP_QPACK_DECODER_STREAM_ERROR (0x202):  The encoder failed to
      interpret a decoder instruction received on the decoder stream.

7.  Security Considerations

   TBD.

8.  IANA Considerations

8.1.  Settings Registration

   This document specifies two settings.  The entries in the following
   table are registered in the "HTTP/3 Settings" registry established in
   [HTTP3].

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

8.2.  Stream Type Registration

   This document specifies two stream types.  The entries in the
   following table are registered in the "HTTP/3 Stream Type" registry
   established in [HTTP3].

         +----------------------+------+---------------+--------+
         | Stream Type          | Code | Specification | Sender |
         +----------------------+------+---------------+--------+
         | QPACK Encoder Stream | 0x02 | Section 4.2.1 | Both   |
         |                      |      |               |        |
         | QPACK Decoder Stream | 0x03 | Section 4.2.1 | Both   |
         +----------------------+------+---------------+--------+



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8.3.  Error Code Registration

   This document specifies three error codes.  The entries in the
   following table are registered in the "HTTP/3 Error Code" registry
   established in [HTTP3].

   +-----------------------------+-------+--------------+--------------+
   | Name                        | Code  | Description  | Specificatio |
   |                             |       |              | n            |
   +-----------------------------+-------+--------------+--------------+
   | HTTP_QPACK_DECOMPRESSION_FA | 0x200 | Decompressio | Section 6    |
   | ILED                        |       | n of a       |              |
   |                             |       | header block |              |
   |                             |       | failed       |              |
   |                             |       |              |              |
   | HTTP_QPACK_ENCODER_STREAM_E | 0x201 | Error on the | Section 6    |
   | RROR                        |       | encoder      |              |
   |                             |       | stream       |              |
   |                             |       |              |              |
   | HTTP_QPACK_DECODER_STREAM_E | 0x202 | Error on the | Section 6    |
   | RROR                        |       | decoder      |              |
   |                             |       | stream       |              |
   +-----------------------------+-------+--------------+--------------+

9.  References

9.1.  Normative References

   [HTTP3]    Bishop, M., Ed., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", draft-ietf-quic-http (work in progress), June
              2019.

   [QUIC-TRANSPORT]
              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", draft-ietf-quic-
              transport (work in progress), June 2019.

   [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>.






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   [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>.

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               |                              |



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   |      |                             |                              |
   | 10   | last-modified               |                              |
   |      |                             |                              |
   | 11   | link                        |                              |
   |      |                             |                              |
   | 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                |



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   |      | headers                     |                              |
   |      |                             |                              |
   | 34   | access-control-allow-       | content-type                 |
   |      | headers                     |                              |
   |      |                             |                              |
   | 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-                     |
   |      |                             |                              |



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   | 56   | strict-transport-security   | max-age=31536000             |
   |      |                             |                              |
   | 57   | strict-transport-security   | max-age=31536000;            |
   |      |                             | includesubdomains            |
   |      |                             |                              |
   | 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                     |                              |
   |      |                             |                              |



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   | 77   | access-control-allow-       | get, post, options           |
   |      | methods                     |                              |
   |      |                             |                              |
   | 78   | access-control-allow-       | options                      |
   |      | 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                         |



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   |      |                             |                              |
   | 98   | x-frame-options             | sameorigin                   |
   +------+-----------------------------+------------------------------+

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-06

   o  Clarify initial dynamic table capacity maximums (#2276, #2330,
      #2330)

C.2.  Since draft-ietf-quic-qpack-05

   o  Introduced the terms dynamic table capacity and maximum dynamic
      table capacity.

   o  Renamed SETTINGS_HEADER_TABLE_SIZE to
      SETTINGS_QPACK_MAX_TABLE_CAPACITY.

C.3.  Since draft-ietf-quic-qpack-04

   o  Changed calculation of Delta Base Index to avoid an illegal value
      (#2002, #2005)

C.4.  Since draft-ietf-quic-qpack-03

   o  Change HTTP settings defaults (#2038)

   o  Substantial editorial reorganization

C.5.  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)




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

C.6.  Since draft-ietf-quic-qpack-01

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

C.7.  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)

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

C.8.  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




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   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

   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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