QUIC                                                           C. Krasic
Internet-Draft                                                   Netflix
Intended status: Standards Track                               M. Bishop
Expires: May 7, 2020                                 Akamai Technologies
                                                        A. Frindell, Ed.
                                                                Facebook
                                                       November 04, 2019


                  QPACK: Header Compression for HTTP/3
                        draft-ietf-quic-qpack-11

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 May 7, 2020.






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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  . . . . . . . . . . . . . . . . .   4
   2.  Compression Process Overview  . . . . . . . . . . . . . . . .   5
     2.1.  Encoder . . . . . . . . . . . . . . . . . . . . . . . . .   5
       2.1.1.  Reference Tracking  . . . . . . . . . . . . . . . . .   5
       2.1.2.  Blocked Dynamic Table Insertions  . . . . . . . . . .   6
       2.1.3.  Blocked Streams . . . . . . . . . . . . . . . . . . .   7
       2.1.4.  Known Received Count  . . . . . . . . . . . . . . . .   8
     2.2.  Decoder . . . . . . . . . . . . . . . . . . . . . . . . .   8
       2.2.1.  Blocked Decoding  . . . . . . . . . . . . . . . . . .   8
       2.2.2.  State Synchronization . . . . . . . . . . . . . . . .   9
       2.2.3.  Invalid References  . . . . . . . . . . . . . . . . .  10
   3.  Header Tables . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.1.  Static Table  . . . . . . . . . . . . . . . . . . . . . .  10
     3.2.  Dynamic Table . . . . . . . . . . . . . . . . . . . . . .  11
       3.2.1.  Dynamic Table Size  . . . . . . . . . . . . . . . . .  11
       3.2.2.  Dynamic Table Capacity and Eviction . . . . . . . . .  11
       3.2.3.  Maximum Dynamic Table Capacity  . . . . . . . . . . .  12
       3.2.4.  Absolute Indexing . . . . . . . . . . . . . . . . . .  12
       3.2.5.  Relative Indexing . . . . . . . . . . . . . . . . . .  12
       3.2.6.  Post-Base Indexing  . . . . . . . . . . . . . . . . .  13
   4.  Wire Format . . . . . . . . . . . . . . . . . . . . . . . . .  14
     4.1.  Primitives  . . . . . . . . . . . . . . . . . . . . . . .  14
       4.1.1.  Prefixed Integers . . . . . . . . . . . . . . . . . .  14
       4.1.2.  String Literals . . . . . . . . . . . . . . . . . . .  14
     4.2.  Encoder and Decoder Streams . . . . . . . . . . . . . . .  15
     4.3.  Encoder Instructions  . . . . . . . . . . . . . . . . . .  15
       4.3.1.  Set Dynamic Table Capacity  . . . . . . . . . . . . .  16
       4.3.2.  Insert With Name Reference  . . . . . . . . . . . . .  16
       4.3.3.  Insert Without Name Reference . . . . . . . . . . . .  17



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       4.3.4.  Duplicate . . . . . . . . . . . . . . . . . . . . . .  17
     4.4.  Decoder Instructions  . . . . . . . . . . . . . . . . . .  18
       4.4.1.  Header Acknowledgement  . . . . . . . . . . . . . . .  18
       4.4.2.  Stream Cancellation . . . . . . . . . . . . . . . . .  18
       4.4.3.  Insert Count Increment  . . . . . . . . . . . . . . .  19
     4.5.  Header Block Representations  . . . . . . . . . . . . . .  19
       4.5.1.  Header Block Prefix . . . . . . . . . . . . . . . . .  19
       4.5.2.  Indexed Header Field  . . . . . . . . . . . . . . . .  22
       4.5.3.  Indexed Header Field With Post-Base Index . . . . . .  22
       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 . . . . .  24
   5.  Configuration . . . . . . . . . . . . . . . . . . . . . . . .  25
   6.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  25
   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-10  . . . . . . . . . . . . .  35
     C.2.  Since draft-ietf-quic-qpack-09  . . . . . . . . . . . . .  35
     C.3.  Since draft-ietf-quic-qpack-08  . . . . . . . . . . . . .  35
     C.4.  Since draft-ietf-quic-qpack-06  . . . . . . . . . . . . .  35
     C.5.  Since draft-ietf-quic-qpack-05  . . . . . . . . . . . . .  35
     C.6.  Since draft-ietf-quic-qpack-04  . . . . . . . . . . . . .  35
     C.7.  Since draft-ietf-quic-qpack-03  . . . . . . . . . . . . .  35
     C.8.  Since draft-ietf-quic-qpack-02  . . . . . . . . . . . . .  36
     C.9.  Since draft-ietf-quic-qpack-01  . . . . . . . . . . . . .  36
     C.10. Since draft-ietf-quic-qpack-00  . . . . . . . . . . . . .  36
     C.11. Since draft-ietf-quic-qcram-00  . . . . . . . . . . . . .  36
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  37
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

1.  Introduction

   The QUIC transport protocol [QUIC-TRANSPORT] is designed to support
   HTTP semantics, and its design subsumes many of the features of
   HTTP/2 [RFC7540].  HTTP/2 uses HPACK [RFC7541] for header
   compression.  If HPACK were used for HTTP/3 [HTTP3], 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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   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 and post-base indices.
      References to dynamic table entries in header blocks are relative
      to a Base.

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

   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:




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   x (A)  Indicates that x is A bits long

   x (A+)  Indicates that x uses the prefixed integer encoding defined
      in Section 4.1.1, 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 defines unidirectional streams for sending instructions from
   encoder to decoder and vice versa.

2.1.  Encoder

   An encoder converts a header list into a header block by emitting
   either an indexed or a literal representation for each header field
   in the list (see Section 4.5).  Indexed representations achieve high
   compression by replacing the literal name and possibly the value with
   an index to either the static or dynamic table.  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.

2.1.1.  Reference Tracking

   An encoder MUST ensure that a header block which references a dynamic
   table entry is not processed by the decoder after the referenced
   entry has been evicted.  Hence the encoder needs to retain



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   information about each compressed header block that references the
   dynamic table until that header block is acknowledged by the decoder
   (see Section 4.4.1).

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.

   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 order to avoid this, an encoder that uses the dynamic
   table has to keep track of blocking entries.

   Note:  A blocking entry is unrelated to a blocked stream, see
      Section 2.1.3.

2.1.2.1.  Avoiding Blocked Insertions

   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 4.3.4), 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 (see Section 3.2.4) 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.












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

                 Figure 1: Draining Dynamic Table Entries

2.1.3.  Blocked Streams

   Because QUIC does not guarantee order between data on different
   streams, a decoder might encounter a header block that references a
   dynamic table entry that it has not yet received.

   Each header block contains a Required Insert Count (see
   Section 4.5.1), 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.

   When the decoder receives a header block with a Required Insert Count
   greater than its own Insert Count, the stream cannot be processed
   immediately, and is considered "blocked" (see Section 2.2.1).

   The decoder specifies an upper bound on the number of streams which
   can be blocked using the SETTINGS_QPACK_BLOCKED_STREAMS setting (see
   Section 5).  An encoder MUST limit the number of streams which could
   become blocked to the value of SETTINGS_QPACK_BLOCKED_STREAMS at all
   times.  If a decoder encounters more blocked streams than it promised
   to support, it MUST treat this as a connection error of type
   HTTP_QPACK_DECOMPRESSION_FAILED.

   Note that the decoder might not become blocked on every stream which
   risks becoming blocked.

   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
   can avoid 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



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   result in the encoder becoming blocked on congestion or flow control
   limits.

2.1.4.  Known Received Count

   The Known Received Count is the total number of dynamic table
   insertions and duplications acknowledged by the decoder.  The encoder
   tracks the Known Received Count in order to identify which dynamic
   table entries can be referenced without potentially blocking a
   stream.  The decoder tracks the Known Received Count in order to be
   able to send Insert Count Increment instructions (see Section 4.4.3).

   If a header block was potentially blocking, the encoder infers from
   receiving a Header Acknowledgement instruction (Section 4.4.1) that
   the decoder has received all dynamic table state necessary to process
   that header block.  If the Required Insert Count of the acknowledged
   header block is greater than the current Known Received Count, the
   encoder updates the Known Received Count to the value of the Required
   Insert Count of the acknowledged header block.

   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 4.4.3).

2.2.  Decoder

   As in HPACK, the decoder processes header blocks and emits the
   corresponding header lists.  It also processes instructions received
   on the encoder stream that modify the dynamic table.  Note that
   header blocks and encoder stream instructions arrive on separate
   streams.  This is unlike HPACK, where header blocks can contain
   instructions that modify the dynamic table, and there is no dedicated
   stream of HPACK instructions.

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

2.2.1.  Blocked Decoding

   Upon receipt of a header block, the decoder examines the Required
   Insert Count.  When the Required Insert Count is less than or equal
   to the decoder's Insert Count, the header block can be processed
   immediately.  Otherwise, the stream on which the header block was
   received becomes blocked.

   While blocked, header block data SHOULD remain in the blocked
   stream's flow control window.  A stream becomes unblocked when the



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   Insert Count becomes greater than or equal to the Required Insert
   Count for all header blocks the decoder has started reading from the
   stream.

   When processing header blocks, the decoder expects the Required
   Insert Count to exactly match the value defined in Section 2.1.3.  If
   it encounters a smaller value than expected, it MUST treat this as a
   connection error of type HTTP_QPACK_DECOMPRESSION_FAILED (see
   Section 2.2.3).  If it encounters a larger value than expected, it
   MAY treat this as a connection error of type
   HTTP_QPACK_DECOMPRESSION_FAILED.

2.2.2.  State Synchronization

   The decoder signals the following events by emitting decoder
   instructions (Section 4.4) on the decoder stream.

2.2.2.1.  Completed Processing of a Header Block

   After the decoder finishes decoding a header block containing dynamic
   table references, it MUST emit a Header Acknowledgement instruction
   (Section 4.4.1).  A stream may carry multiple header blocks in the
   case of intermediate responses, trailers, and pushed requests.  The
   encoder interprets each Header Acknowledgement instruction as
   acknowledging the earliest unacknowledged header block containing
   dynamic table references sent on the given stream.

2.2.2.2.  Abandonment of a Stream

   When an endpoint receives a stream reset before the end of a stream
   or before all header blocks are processed on that stream, or when it
   abandons reading of a stream, it generates a Stream Cancellation
   instruction (see Section 4.4.2).  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 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.

   The Header Acknowledgement and Stream Cancellation instructions
   permit the encoder to remove references to entries in the dynamic
   table.  When an entry with absolute index lower than the Known
   Received Count has zero references, then it is no longer considered
   blocking (see Section 2.1.2).






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2.2.2.3.  New Table Entries

   After receiving new table entries on the encoder stream, the decoder
   chooses when to emit Insert Count Increment instructions (see
   Section 4.4.3).  Emitting this instruction after adding each new
   dynamic table entry will provide the timeliest 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 4.4.1).  However, delaying too long may lead to compression
   inefficiencies if the encoder waits for an entry to be acknowledged
   before using it.

2.2.3.  Invalid References

   If the decoder encounters a reference in a header block
   representation 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".

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

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.

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

   Note that 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 representation it MUST treat this as a connection error of type
   "HTTP_QPACK_DECOMPRESSION_FAILED".  If this index is received on the



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   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.

   Dynamic table entries can have empty values.

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, 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.  The encoder sends a Set Dynamic Table Capacity
   instruction (Section 4.3.1) with a non-zero capacity to begin using
   the dynamic table.

   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).  The
   encoder MUST NOT cause a blocking dynamic table entry to be evicted
   (see Section 2.1.2).  The new entry is then added to the table.  It
   is an error if the encoder attempts to add an entry that is larger
   than the 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.




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   Whenever the dynamic table capacity is reduced by the encoder (see
   Section 4.3.1), 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 4.3.1).

   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 zero, 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 zero, the encoder MUST NOT insert
   entries into the dynamic table, and MUST NOT send any encoder
   instructions on the encoder stream.

3.2.4.  Absolute Indexing

   Each entry possesses an absolute index which is fixed for the
   lifetime of that entry.  The first entry inserted has an absolute
   index of "0"; indices increase by one with each insertion.

3.2.5.  Relative Indexing

   Relative indices begin at zero and increase in the opposite direction
   from the absolute index.  Determining which entry has a relative
   index of "0" depends on the context of the reference.




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   In encoder instructions (see Section 4.3), a relative index of "0"
   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 - Encoder Stream

   Unlike in encoder instructions, relative indices in header block
   representations 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 header blocks and dynamic table updates are processed
   out of order.

   In a header block a relative index of "0" refers to the entry with
   absolute index equal to Base - 1.

                  Base
                   |
                   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
   In this example, Base = n - 2

      Example Dynamic Table Indexing - Relative Index in Header Block

3.2.6.  Post-Base Indexing

   Post-Base indices are used in header block instructions for entries
   with absolute indices greater than or equal to Base, starting at 0




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   for the entry with absolute index equal to Base, and increasing in
   the same direction as the absolute index.

   Post-Base indices allow an encoder to process a header block in a
   single pass and include references to entries added while processing
   this (or other) header blocks.

                  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
   In this example, Base = n - 2

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

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.  Note, however, that QPACK uses some prefix sizes not
   actually used in HPACK.

   QPACK implementations MUST be able to decode integers up to and
   including 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 bit flag, denoted as 'H' in this document
   (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.




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   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 prefix size, N, can have
   a value between 2 and 8 inclusive.  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.  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_STREAM_CREATION_ERROR.
   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 an encoder stream if it's not going to
   be used (for example if its encoder 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 a decoder stream if its decoder sets
   the maximum capacity of the dynamic table to zero.

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

4.3.  Encoder Instructions

   An encoder sends encoder instructions on the encoder stream to set
   the capacity of the dynamic table and add dynamic table entries.
   Instructions adding table entries can use existing entries to avoid
   transmitting redundant information.  The name can be transmitted as a



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   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.

4.3.1.  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.  This is followed by the new dynamic table capacity
   represented as an integer with a 5-bit prefix (see Section 4.1.1).

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

                   Figure 2: 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.

4.3.2.  Insert With Name Reference

   An encoder adds an entry to the dynamic table where the header field
   name matches the header field name of an entry stored in the static
   or the dynamic table using an instruction that starts with the '1'
   one-bit pattern.  The second ('T') bit indicates whether the
   reference is to the static or dynamic table.  The 6-bit prefix
   integer (see Section 4.1.1) that follows is used to locate the table
   entry for the header name.  When T=1, the number represents the
   static table index; when T=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 4.1.2).



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

                    Insert Header Field -- Indexed Name

4.3.3.  Insert Without Name Reference

   An encoder adds an entry to the dynamic table where both the header
   field name and the header field value are represented as string
   literals using an instruction that starts with the '01' two-bit
   pattern.

   This is followed by the name represented as a 6-bit prefix string
   literal, and the value represented as an 8-bit prefix string literal
   (see Section 4.1.2).

        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

4.3.4.  Duplicate

   An encoder duplicates an existing entry in the dynamic table using an
   instruction that begins with the '000' three-bit pattern.  This is
   followed by the relative index of the existing entry represented as
   an integer with a 5-bit prefix (see Section 4.1.1.

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

                            Figure 3: Duplicate



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   The existing entry is re-inserted into the dynamic table without
   resending either the name or the value.  This is useful to avoid
   adding a reference to an older entry, which might block inserting new
   entries.

4.4.  Decoder Instructions

   A decoder sends decoder instructions on the decoder stream to inform
   the encoder about the processing of header blocks and table updates
   to ensure consistency of the dynamic table.

   This section specifies the following decoder instructions.

4.4.1.  Header Acknowledgement

   After processing a header block whose declared Required Insert Count
   is not zero, the decoder emits a Header Acknowledgement instruction.
   The instruction begins with the '1' one-bit pattern which is followed
   by the header block's associated stream ID encoded as a 7-bit prefix
   integer (see Section 4.1.1).

   This instruction is used as described in Section 2.1.4 and in
   Section 2.2.2.

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

                     Figure 4: Header Acknowledgement

   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".

4.4.2.  Stream Cancellation

   When a stream is reset or reading is abandoned, the decoder emits a
   Stream Cancellation instruction.  The instruction begins with the
   '01' two-bit pattern, which is followed by the stream ID of the
   affected stream encoded as a 6-bit prefix integer.

   This instruction is used as described in Section 2.2.2.







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

                       Figure 5: Stream Cancellation

4.4.3.  Insert Count Increment

   The Insert Count Increment instruction begins with the '00' two-bit
   pattern, followed by the Increment encoded as a 6-bit prefix integer.
   The value of the Increment is the total number of dynamic table
   insertions and duplications processed by the decoder since the last
   time it sent a Header Acknowledgement instruction that increased the
   Known Received Count (see Section 2.1.4) or an Insert Count Increment
   instruction.  The encoder uses this value to update the Known
   Received Count, as described in Section 2.2.2.

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

                     Figure 6: 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".

4.5.  Header Block Representations

   A header block consists of a prefix and a possibly empty sequence of
   representations defined in this section.  Each representation
   corresponds to a single header field.  These representations
   reference the static table or the dynamic table in a particular
   state, but do not modify that state.

   Header blocks are carried in frames on streams defined by the
   enclosing protocol.

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 a
   sign bit ('S') and a Delta Base value with a 7-bit prefix (see
   Section 4.5.1.2).



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

                          Figure 7: Header Block

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:

      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.





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      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

         # Value of 0 must be encoded as 0.
         if ReqInsertCount == 0:
            Error

   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
   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 Required Insert
   Count using a one-bit sign ('S') and the "Delta Base" value.  A sign
   bit of 0 indicates that the Base is greater than or equal to the
   value of the Required Insert Count; the decoder adds the value of
   Delta Base to the Required Insert Count to determine the value of the
   Base.  A sign bit of 1 indicates that the Base is less than the
   Required Insert Count; the decoder subtracts the value of Delta Base
   from the Required Insert Count and also subtracts one to determine
   the value of the Base.  That is:

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



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   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 Base to the value of Required Insert Count.  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 one of the most
   efficient encodings.

   For example, with a Required Insert Count of 9, a decoder receives an
   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
   relative 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 the
   static table, or an entry in the dynamic table with an absolute index
   less than the Base.

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

                           Indexed Header Field

   This representation starts with the '1' 1-bit pattern, followed by
   the 'T' bit indicating whether the reference is into the static or
   dynamic table.  The 6-bit prefix integer (see Section 4.1.1) that
   follows is used to locate the table entry for the header field.  When
   T=1, the number represents the static table index; when T=0, the
   number is the relative index of the entry in the dynamic table.

4.5.3.  Indexed Header Field With Post-Base Index

   An indexed header field with post-base index representation
   identifies an entry in the dynamic table with an absolute index
   greater than or equal to the Base.



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

                 Indexed Header Field with Post-Base Index

   This representation starts with the '0001' 4-bit pattern.  This is
   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 4.1.1).

4.5.4.  Literal Header Field With Name Reference

   A literal header field with name reference representation encodes a
   header field where the header field name matches the header field
   name of an entry in the static table, or the header field name of an
   entry in the dynamic table with an absolute index less than the Base.

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

                 Literal Header Field With Name Reference

   This representation starts with the '01' two-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 for more details).

   The fourth ('T') bit indicates whether the reference is to the static
   or dynamic table.  The 4-bit prefix integer (see Section 4.1.1) that
   follows is used to locate the table entry for the header name.  When
   T=1, the number represents the static table index; when T=0, the
   number is the relative index of the entry in the dynamic table.





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   Only the header field name is taken from the dynamic table entry; the
   header field value is encoded as an 8-bit prefix string literal (see
   Section 4.1.2).

4.5.5.  Literal Header Field With Post-Base Name Reference

   A literal header field with post-base name reference representation
   encodes a header field where the header field name matches the header
   field name of a dynamic table entry with an absolute index greater
   than or equal to the Base.

        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

   This representation starts with the '0000' four-bit pattern.  The
   fifth bit is the 'N' bit as described in Section 4.5.4.  This is
   followed by a post-base index of the dynamic table entry (see
   Section 3.2.6) encoded as an integer with a 3-bit prefix (see
   Section 4.1.1).

   Only the header field name is taken from the dynamic table entry; the
   header field value is encoded as an 8-bit prefix string literal (see
   Section 4.1.2).

4.5.6.  Literal Header Field Without Name Reference

   The literal header field without name reference representation
   encodes a header field name and a header field value as string
   literals.














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        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

   This representation begins with the '001' three-bit pattern.  The
   fourth bit is the 'N' bit as described in Section 4.5.4.  The name
   follows, represented as a 4-bit prefix string literal, then the
   value, represented as an 8-bit prefix string literal (see
   Section 4.1.2).

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.

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 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.






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7.  Security Considerations

   TBD.  Also see Section 7.1 of [RFC7541].

   While the negotiated limit on the dynamic table size accounts for
   much of the memory that can be consumed by a QPACK implementation,
   data which cannot be immediately sent due to flow control is not
   affected by this limit.  Implementations should limit the size of
   unsent data, especially on the decoder stream where flexibility to
   choose what to send is limited.  Possible responses to an excess of
   unsent data might include limiting the ability of the peer to open
   new streams, reading only from the encoder stream, or closing the
   connection.

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 | Default |
       +--------------------------+------+---------------+---------+
       | QPACK_MAX_TABLE_CAPACITY | 0x1  | Section 5     | 0       |
       |                          |      |               |         |
       | QPACK_BLOCKED_STREAMS    | 0x7  | Section 5     | 0       |
       +--------------------------+------+---------------+---------+

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   | Both   |
         |                      |      |               |        |
         | QPACK Decoder Stream | 0x03 | Section 4.2   | 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-24 (work in progress),
              November 2019.

   [QUIC-TRANSPORT]
              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", draft-ietf-quic-
              transport-24 (work in progress), November 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-10

   Editorial changes

C.2.  Since draft-ietf-quic-qpack-09

   o  Decoders MUST emit Header Acknowledgements (#2939)

   o  Updated error code for multiple encoder or decoder streams (#2970)

   o  Added explicit defaults for new SETTINGS (#2974)

C.3.  Since draft-ietf-quic-qpack-08

   o  Endpoints are permitted to create encoder and decoder streams even
      if they can't use them (#2100, #2529)

   o  Maximum values for settings removed (#2766, #2767)

C.4.  Since draft-ietf-quic-qpack-06

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

C.5.  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.6.  Since draft-ietf-quic-qpack-04

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

C.7.  Since draft-ietf-quic-qpack-03

   o  Change HTTP settings defaults (#2038)

   o  Substantial editorial reorganization




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C.8.  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.9.  Since draft-ietf-quic-qpack-01

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

C.10.  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.11.  Since draft-ietf-quic-qcram-00

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




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   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

   Charles 'Buck' Krasic
   Netflix

   Email: ckrasic@netflix.com


   Mike Bishop
   Akamai Technologies

   Email: mbishop@evequefou.be




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   Alan Frindell (editor)
   Facebook

   Email: afrind@fb.com















































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