draft-ietf-quic-qpack.txt

QUIC                                                           C. Krasic
Internet-Draft                                                   Netflix
Intended status: Standards Track                               M. Bishop
Expires: 10 July 2021                                Akamai Technologies
                                                        A. Frindell, Ed.
                                                                Facebook
                                                          6 January 2021


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

Abstract

   This specification defines QPACK, a compression format for
   efficiently representing HTTP fields, to be used in HTTP/3.  This is
   a variation of HPACK 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 (mailto:quic@ietf.org)), which is
   archived at https://mailarchive.ietf.org/arch/
   search/?email_list=quic.

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

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
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   This Internet-Draft will expire on 10 July 2021.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction
     1.1.  Conventions and Definitions
     1.2.  Notational Conventions
   2.  Compression Process Overview
     2.1.  Encoder
       2.1.1.  Limits on Dynamic Table Insertions
       2.1.2.  Blocked Streams
       2.1.3.  Avoiding Flow Control Deadlocks
       2.1.4.  Known Received Count
     2.2.  Decoder
       2.2.1.  Blocked Decoding
       2.2.2.  State Synchronization
       2.2.3.  Invalid References
   3.  Reference Tables
     3.1.  Static Table
     3.2.  Dynamic Table
       3.2.1.  Dynamic Table Size
       3.2.2.  Dynamic Table Capacity and Eviction
       3.2.3.  Maximum Dynamic Table Capacity
       3.2.4.  Absolute Indexing
       3.2.5.  Relative Indexing
       3.2.6.  Post-Base Indexing
   4.  Wire Format
     4.1.  Primitives
       4.1.1.  Prefixed Integers
       4.1.2.  String Literals
     4.2.  Encoder and Decoder Streams
     4.3.  Encoder Instructions
       4.3.1.  Set Dynamic Table Capacity
       4.3.2.  Insert With Name Reference
       4.3.3.  Insert With Literal Name
       4.3.4.  Duplicate
     4.4.  Decoder Instructions
       4.4.1.  Section Acknowledgment
       4.4.2.  Stream Cancellation
       4.4.3.  Insert Count Increment
     4.5.  Field Line Representations
       4.5.1.  Encoded Field Section Prefix
       4.5.2.  Indexed Field Line
       4.5.3.  Indexed Field Line With Post-Base Index
       4.5.4.  Literal Field Line With Name Reference
       4.5.5.  Literal Field Line With Post-Base Name Reference
       4.5.6.  Literal Field Line With Literal Name
   5.  Configuration
   6.  Error Handling
   7.  Security Considerations
     7.1.  Probing Dynamic Table State
       7.1.1.  Applicability to QPACK and HTTP
       7.1.2.  Mitigation
       7.1.3.  Never-Indexed Literals
     7.2.  Static Huffman Encoding
     7.3.  Memory Consumption
     7.4.  Implementation Limits
   8.  IANA Considerations
     8.1.  Settings Registration
     8.2.  Stream Type Registration
     8.3.  Error Code Registration
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  Static Table
   Appendix B.  Encoding and Decoding Examples
     B.1.  Literal Field Line With Name Reference
     B.2.  Dynamic Table
     B.3.  Speculative Insert
     B.4.  Duplicate Instruction, Stream Cancellation
     B.5.  Dynamic Table Insert, Eviction
   Appendix C.  Sample One Pass Encoding Algorithm
   Appendix D.  Change Log
     D.1.  Since draft-ietf-quic-qpack-19
     D.2.  Since draft-ietf-quic-qpack-18
     D.3.  Since draft-ietf-quic-qpack-17
     D.4.  Since draft-ietf-quic-qpack-16
     D.5.  Since draft-ietf-quic-qpack-15
     D.6.  Since draft-ietf-quic-qpack-14
     D.7.  Since draft-ietf-quic-qpack-13
     D.8.  Since draft-ietf-quic-qpack-12
     D.9.  Since draft-ietf-quic-qpack-11
     D.10. Since draft-ietf-quic-qpack-10
     D.11. Since draft-ietf-quic-qpack-09
     D.12. Since draft-ietf-quic-qpack-08
     D.13. Since draft-ietf-quic-qpack-06
     D.14. Since draft-ietf-quic-qpack-05
     D.15. Since draft-ietf-quic-qpack-04
     D.16. Since draft-ietf-quic-qpack-03
     D.17. Since draft-ietf-quic-qpack-02
     D.18. Since draft-ietf-quic-qpack-01
     D.19. Since draft-ietf-quic-qpack-00
     D.20. Since draft-ietf-quic-qcram-00
   Acknowledgments
   Authors' Addresses

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 compression of
   the header and trailer sections.  If HPACK were used for HTTP/3
   ([HTTP3]), it would induce head-of-line blocking for field sections
   due to built-in assumptions of a total ordering across frames on all
   streams.

   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:

   HTTP fields:  Metadata sent as part of an HTTP message.  The term
      encompasses both header and trailer fields.  Colloquially, the
      term "headers" has often been used to refer to HTTP header fields
      and trailer fields; this document uses "fields" for generality.

   HTTP field line:  A name-value pair sent as part of an HTTP field
      section.  See Sections 6.3 and Section 6.5 of [SEMANTICS].

   HTTP field value:  Data associated with a field name, composed from
      all field line values with that field name in that section,
      concatenated together and separated with commas.

   Field section:  An ordered collection of HTTP field lines associated
      with an HTTP message.  A field section can contain multiple field
      lines with the same name.  It can also contain duplicate field
      lines.  An HTTP message can include both header field and trailer
      field sections.

   Representation:  An instruction that represents a field line,
      possibly by reference to the dynamic and static tables.

   Encoder:  An implementation that encodes field sections.

   Decoder:  An implementation that decodes encoded field sections.

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

   Base:  A reference point for relative and post-base indices.
      Representations that reference dynamic table entries 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:

   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 field lines
   ("headers") to indices.  The static table (Section 3.1) is predefined
   and contains common header field lines (some of them with an empty
   value).  The dynamic table (Section 3.2) is built up over the course
   of the connection and can be used by the encoder to index both header
   and trailer field lines in the encoded field sections.

   QPACK defines unidirectional streams for sending instructions from
   encoder to decoder and vice versa.

2.1.  Encoder

   An encoder converts a header or trailer field section into a series
   of representations by emitting either an indexed or a literal
   representation for each field line 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 acknowledgment 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 field lines it is compressing.

   QPACK preserves the ordering of field lines within each field
   section.  An encoder MUST emit field representations in the order
   they appear in the input field section.

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

2.1.1.  Limits on Dynamic Table Insertions

   Inserting entries into the dynamic table might not be possible if the
   table contains entries that cannot be evicted.

   A dynamic table entry cannot be evicted immediately after insertion,
   even if it has never been referenced.  Once the insertion of a
   dynamic table entry has been acknowledged and there are no
   outstanding references to the entry in unacknowledged
   representations, the entry becomes evictable.  Note that references
   on the encoder stream never preclude the eviction of an entry,
   because those references are guaranteed to be processed before the
   instruction evicting the entry.

   If the dynamic table does not contain enough room for a new entry
   without evicting other entries, and the entries that would be evicted
   are not evictable, the encoder MUST NOT insert that entry into the
   dynamic table (including duplicates of existing entries).  In order
   to avoid this, an encoder that uses the dynamic table has to keep
   track of each dynamic table entry referenced by each field section
   until those representations are acknowledged by the decoder; see
   Section 4.4.1.

2.1.1.1.  Avoiding Prohibited 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 (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 (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.

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

                  Figure 1: Draining Dynamic Table Entries

2.1.2.  Blocked Streams

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

   Each encoded field section contains a Required Insert Count
   (Section 4.5.1), the lowest possible value for the Insert Count with
   which the field section can be decoded.  For a field section encoded
   using 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 field section encoded with no references to the
   dynamic table, the Required Insert Count is zero.

   When the decoder receives an encoded field section 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 that
   can be blocked using the SETTINGS_QPACK_BLOCKED_STREAMS setting; see
   Section 5.  An encoder MUST limit the number of streams that 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
   QPACK_DECOMPRESSION_FAILED.

   Note that the decoder might not become blocked on every stream that
   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 that have been acknowledged, but this could mean using
   literals.  Since literals make the encoded field section larger, this
   can result in the encoder becoming blocked on congestion or flow
   control limits.

2.1.3.  Avoiding Flow Control Deadlocks

   Writing instructions on streams that are limited by flow control can
   produce deadlocks.

   A decoder might stop issuing flow control credit on the stream that
   carries an encoded field section until the necessary updates are
   received on the encoder stream.  If the granting of flow control
   credit on the encoder stream (or the connection as a whole) depends
   on the consumption and release of data on the stream carrying the
   encoded field section, a deadlock might result.

   More generally, a stream containing a large instruction can become
   deadlocked if the decoder withholds flow control credit until the
   instruction is completely received.

   To avoid these deadlocks, an encoder SHOULD avoid writing an
   instruction unless sufficient stream and connection flow control
   credit is available for the entire instruction.

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.

   A Section Acknowledgment instruction (Section 4.4.1) implies that the
   decoder has received all dynamic table state necessary to decode the
   field section.  If the Required Insert Count of the acknowledged
   field section is greater than the current Known Received Count, Known
   Received Count is updated to the value of the Required Insert Count.

   An Insert Count Increment instruction (Section 4.4.3) increases the
   Known Received Count by its Increment parameter.  See Section 2.2.2.3
   for guidance.

2.2.  Decoder

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

   The decoder MUST emit field lines in the order their representations
   appear in the encoded field section.

2.2.1.  Blocked Decoding

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

   While blocked, encoded field section 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 encoded field sections the decoder has started
   reading from the stream.

   When processing encoded field sections, the decoder expects the
   Required Insert Count to equal the lowest possible value for the
   Insert Count with which the field section can be decoded, as
   prescribed in Section 2.1.2.  If it encounters a Required Insert
   Count smaller than expected, it MUST treat this as a connection error
   of type QPACK_DECOMPRESSION_FAILED; see Section 2.2.3.  If it
   encounters a Required Insert Count larger than expected, it MAY treat
   this as a connection error of type 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 Field Section

   After the decoder finishes decoding a field section encoded using
   representations containing dynamic table references, it MUST emit a
   Section Acknowledgment instruction (Section 4.4.1).  A stream may
   carry multiple field sections in the case of intermediate responses,
   trailers, and pushed requests.  The encoder interprets each
   Section Acknowledgment instruction as acknowledging the earliest
   unacknowledged field section 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 encoded field sections 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 (Section 3.2.3) equal to zero MAY omit sending Stream
   Cancellations, because the encoder cannot have any dynamic table
   references.  An encoder cannot infer from this instruction that any
   updates to the dynamic table have been received.

   The Section Acknowledgment 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 considered evictable;
   see Section 2.1.1.

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 Section Acknowledgments; 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 field line representation
   to a dynamic table entry that has already been evicted or that has an
   absolute index greater than or equal to the declared Required Insert
   Count (Section 4.5.1), it MUST treat this as a connection error of
   type QPACK_DECOMPRESSION_FAILED.

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

3.  Reference 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 list of field lines, 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 field
   line representation it MUST treat this as a connection error of type
   QPACK_DECOMPRESSION_FAILED.  If this index is received on the encoder
   stream, this MUST be treated as a connection error of type
   QPACK_ENCODER_STREAM_ERROR.

3.2.  Dynamic Table

   The dynamic table consists of a list of field lines 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 dynamic table entry to be evicted unless
   that entry is evictable; see Section 2.1.1.  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
   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
   (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
   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 that 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.

   In encoder instructions (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

         Figure 2: Example Dynamic Table Indexing - Encoder Stream

   Unlike in encoder instructions, relative indices in field line
   representations are relative to the Base at the beginning of the
   encoded field section; see Section 4.5.1.  This ensures that
   references are stable even if encoded field sections and dynamic
   table updates are processed out of order.

   In a field line representation, 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

        Figure 3: Example Dynamic Table Indexing - Relative Index in
                               Representation

3.2.6.  Post-Base Indexing

   Post-Base indices are used in field line representations for entries
   with absolute indices greater than or equal to Base, starting at 0
   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 field section in a
   single pass and include references to entries added while processing
   this (or other) field sections.

                  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

       Figure 4: Example Dynamic Table Indexing - Post-Base Index in
                               Representation

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 and Length
   indicates the size of the string after encoding.

   This document expands the definition of string literals by permitting
   them to begin other than on a byte boundary.  An "N-bit prefix string
   literal" begins mid-byte, with the first (8-N) bits allocated to a
   previous field.  The string uses one bit for the 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:

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

   *  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 H3_STREAM_CREATION_ERROR.

   These streams MUST NOT be closed.  Closure of either unidirectional
   stream type MUST be treated as a connection error of type
   H3_CLOSED_CRITICAL_STREAM.

   An endpoint MAY avoid creating an encoder stream if it will not be
   used (for example if its encoder does not 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
   reference to an existing entry in the static or the dynamic table or
   as a string literal.  For entries that already exist in the dynamic
   table, the full entry can also be used by reference, creating a
   duplicate entry.

4.3.1.  Set Dynamic Table Capacity

   An encoder informs the decoder of a change to the dynamic table
   capacity using an instruction that starts with the '001' 3-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 5: 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 (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
   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 entries that
   are not evictable; see Section 2.1.1.  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 field name
   matches the field name of an entry stored in the static or the
   dynamic table using an instruction that starts with the '1' 1-bit
   pattern.  The second ('T') bit indicates whether the reference is to
   the static or dynamic table.  The 6-bit prefix integer
   (Section 4.1.1) that follows is used to locate the table entry for
   the field 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 field name reference is followed by the field value represented
   as a string literal; see Section 4.1.2.

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

                Figure 6: Insert Field Line -- Indexed Name

4.3.3.  Insert With Literal Name

   An encoder adds an entry to the dynamic table where both the field
   name and the field value are represented as string literals using an
   instruction that starts with the '01' 2-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)  |
      +-------------------------------+

                  Figure 7: Insert Field Line -- New Name

4.3.4.  Duplicate

   An encoder duplicates an existing entry in the dynamic table using an
   instruction that starts with the '000' 3-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 8: Duplicate

   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 field sections and table updates
   to ensure consistency of the dynamic table.

4.4.1.  Section Acknowledgment

   After processing an encoded field section whose declared Required
   Insert Count is not zero, the decoder emits a Section Acknowledgment
   instruction.  The instruction starts with the '1' 1-bit pattern,
   followed by the field section'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 9: Section Acknowledgment

   If an encoder receives a Section Acknowledgment instruction referring
   to a stream on which every encoded field section with a non-zero
   Required Insert Count has already been acknowledged, this MUST be
   treated as a connection error of type QPACK_DECODER_STREAM_ERROR.

   The Section Acknowledgment instruction might increase the Known
   Received Count; see Section 2.1.4.

4.4.2.  Stream Cancellation

   When a stream is reset or reading is abandoned, the decoder emits a
   Stream Cancellation instruction.  The instruction starts with the
   '01' 2-bit pattern, 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.

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

                       Figure 10: Stream Cancellation

4.4.3.  Insert Count Increment

   The Insert Count Increment instruction starts with the '00' 2-bit
   pattern, followed by the Increment encoded as a 6-bit prefix integer.
   This instruction increases the Known Received Count (Section 2.1.4)
   by the value of the Increment parameter.  The decoder should send an
   Increment value that increases the Known Received Count to the total
   number of dynamic table insertions and duplications processed so far.

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

                     Figure 11: 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
   QPACK_DECODER_STREAM_ERROR.

4.5.  Field Line Representations

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

   Encoded field sections are carried in frames on streams defined by
   the enclosing protocol.

4.5.1.  Encoded Field Section Prefix

   Each encoded field section is prefixed with two integers.  The
   Required Insert Count is encoded as an integer with an 8-bit prefix
   using 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.

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

                      Figure 12: Encoded Field Section

4.5.1.1.  Required Insert Count

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

   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
   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

         # 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 4 indicates that the Required
   Insert Count is 9 for the field section.

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

   A single-pass encoder determines the Base before encoding a field
   section.  If the encoder inserted entries in the dynamic table while
   encoding the field section and is referencing them, 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 field section was not
   encoded using representations that 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 field
   section 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 field section that was encoded without references to 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 Field Line

   An indexed field line representation identifies an entry in the
   static table, or an entry in the dynamic table with an absolute index
   less than the value of the Base.

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

                       Figure 13: Indexed Field Line

   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 (Section 4.1.1) that follows
   is used to locate the table entry for the field line.  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 Field Line With Post-Base Index

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

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

             Figure 14: Indexed Field Line with Post-Base Index

   This representation starts with the '0001' 4-bit pattern.  This is
   followed by the post-base index (Section 3.2.6) of the matching field
   line, represented as an integer with a 4-bit prefix; see
   Section 4.1.1.

4.5.4.  Literal Field Line With Name Reference

   A literal field line with name reference representation encodes a
   field line where the field name matches the field name of an entry in
   the static table, or the field name of an entry in the dynamic table
   with an absolute index less than the value of 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)  |
      +-------------------------------+

             Figure 15: Literal Field Line With Name Reference

   This representation starts with the '01' 2-bit pattern.  The
   following bit, 'N', indicates whether an intermediary is permitted to
   add this field line to the dynamic table on subsequent hops.  When
   the 'N' bit is set, the encoded field line MUST always be encoded
   with a literal representation.  In particular, when a peer sends a
   field line that it received represented as a literal field line with
   the 'N' bit set, it MUST use a literal representation to forward this
   field line.  This bit is intended for protecting 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 (Section 4.1.1) that
   follows is used to locate the table entry for the field 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.

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

4.5.5.  Literal Field Line With Post-Base Name Reference

   A literal field line with post-base name reference representation
   encodes a field line where the field name matches the field name of a
   dynamic table entry with an absolute index greater than or equal to
   the value of 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)  |
      +-------------------------------+

        Figure 16: Literal Field Line With Post-Base Name Reference

   This representation starts with the '0000' 4-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 (Section 3.2.6)
   encoded as an integer with a 3-bit prefix; see Section 4.1.1.

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

4.5.6.  Literal Field Line With Literal Name

   The literal field line with literal name representation encodes a
   field name and a field value as string literals.

        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)  |
      +-------------------------------+

              Figure 17: Literal Field Line With Literal Name

   This representation starts with the '001' 3-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 for 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 2.1.2.

6.  Error Handling

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

   QPACK_DECOMPRESSION_FAILED (0x200):  The decoder failed to interpret
      an encoded field section and is not able to continue decoding that
      field section.

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

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

7.  Security Considerations

   This section describes potential areas of security concern with
   QPACK:

   *  Use of compression as a length-based oracle for verifying guesses
      about secrets that are compressed into a shared compression
      context.

   *  Denial of service resulting from exhausting processing or memory
      capacity at a decoder.

7.1.  Probing Dynamic Table State

   QPACK reduces the encoded size of field sections by exploiting the
   redundancy inherent in protocols like HTTP.  The ultimate goal of
   this is to reduce the amount of data that is required to send HTTP
   requests or responses.

   The compression context used to encode header and trailer fields can
   be probed by an attacker who can both define fields to be encoded and
   transmitted and observe the length of those fields once they are
   encoded.  When an attacker can do both, they can adaptively modify
   requests in order to confirm guesses about the dynamic table state.
   If a guess is compressed into a shorter length, the attacker can
   observe the encoded length and infer that the guess was correct.

   This is possible even over the Transport Layer Security Protocol
   (TLS, see [TLS]), because while TLS provides confidentiality
   protection for content, it only provides a limited amount of
   protection for the length of that content.

   Note:  Padding schemes only provide limited protection against an
      attacker with these capabilities, potentially only forcing an
      increased number of guesses to learn the length associated with a
      given guess.  Padding schemes also work directly against
      compression by increasing the number of bits that are transmitted.

   Attacks like CRIME ([CRIME]) demonstrated the existence of these
   general attacker capabilities.  The specific attack exploited the
   fact that DEFLATE ([RFC1951]) removes redundancy based on prefix
   matching.  This permitted the attacker to confirm guesses a character
   at a time, reducing an exponential-time attack into a linear-time
   attack.

7.1.1.  Applicability to QPACK and HTTP

   QPACK mitigates but does not completely prevent attacks modeled on
   CRIME ([CRIME]) by forcing a guess to match an entire field line,
   rather than individual characters.  An attacker can only learn
   whether a guess is correct or not, so is reduced to a brute force
   guess for the field values associated with a given field name.

   The viability of recovering specific field values therefore depends
   on the entropy of values.  As a result, values with high entropy are
   unlikely to be recovered successfully.  However, values with low
   entropy remain vulnerable.

   Attacks of this nature are possible any time that two mutually
   distrustful entities control requests or responses that are placed
   onto a single HTTP/3 connection.  If the shared QPACK compressor
   permits one entity to add entries to the dynamic table, and the other
   to access those entries to encode chosen field lines, then the
   attacker can learn the state of the table by observing the length of
   the encoded output.

   Having requests or responses from mutually distrustful entities
   occurs when an intermediary either:

   *  sends requests from multiple clients on a single connection toward
      an origin server, or

   *  takes responses from multiple origin servers and places them on a
      shared connection toward a client.

   Web browsers also need to assume that requests made on the same
   connection by different web origins ([RFC6454]) are made by mutually
   distrustful entities.

7.1.2.  Mitigation

   Users of HTTP that require confidentiality for header or trailer
   fields can use values with entropy sufficient to make guessing
   infeasible.  However, this is impractical as a general solution
   because it forces all users of HTTP to take steps to mitigate
   attacks.  It would impose new constraints on how HTTP is used.

   Rather than impose constraints on users of HTTP, an implementation of
   QPACK can instead constrain how compression is applied in order to
   limit the potential for dynamic table probing.

   An ideal solution segregates access to the dynamic table based on the
   entity that is constructing the message.  Field values that are added
   to the table are attributed to an entity, and only the entity that
   created a particular value can extract that value.

   To improve compression performance of this option, certain entries
   might be tagged as being public.  For example, a web browser might
   make the values of the Accept-Encoding header field available in all
   requests.

   An encoder without good knowledge of the provenance of field values
   might instead introduce a penalty for many field lines with the same
   field name and different values.  This penalty could cause a large
   number of attempts to guess a field value to result in the field not
   being compared to the dynamic table entries in future messages,
   effectively preventing further guesses.

   Note:  Simply removing entries corresponding to the field from the
      dynamic table can be ineffectual if the attacker has a reliable
      way of causing values to be reinstalled.  For example, a request
      to load an image in a web browser typically includes the Cookie
      header field (a potentially highly valued target for this sort of
      attack), and web sites can easily force an image to be loaded,
      thereby refreshing the entry in the dynamic table.

   This response might be made inversely proportional to the length of
   the field value.  Disabling access to the dynamic table for a given
   field name might occur for shorter values more quickly or with higher
   probability than for longer values.

7.1.3.  Never-Indexed Literals

   Implementations can also choose to protect sensitive fields by not
   compressing them and instead encoding their value as literals.

   Refusing to insert a field line into the dynamic table is only
   effective if doing so is avoided on all hops.  The never-indexed
   literal bit (see Section 4.5.4) can be used to signal to
   intermediaries that a particular value was intentionally sent as a
   literal.

   An intermediary MUST NOT re-encode a value that uses a literal
   representation with the 'N' bit set with another representation that
   would index it.  If QPACK is used for re-encoding, a literal
   representation with the 'N' bit set MUST be used.  If HPACK is used
   for re-encoding, the never-indexed literal representation (see
   Section 6.2.3 of [RFC7541]) MUST be used.

   The choice to mark that a field value should never be indexed depends
   on several factors.  Since QPACK does not protect against guessing an
   entire field value, short or low-entropy values are more readily
   recovered by an adversary.  Therefore, an encoder might choose not to
   index values with low entropy.

   An encoder might also choose not to index values for fields that are
   considered to be highly valuable or sensitive to recovery, such as
   the Cookie or Authorization header fields.

   On the contrary, an encoder might prefer indexing values for fields
   that have little or no value if they were exposed.  For instance, a
   User-Agent header field does not commonly vary between requests and
   is sent to any server.  In that case, confirmation that a particular
   User-Agent value has been used provides little value.

   Note that these criteria for deciding to use a never-indexed literal
   representation will evolve over time as new attacks are discovered.

7.2.  Static Huffman Encoding

   There is no currently known attack against a static Huffman encoding.
   A study has shown that using a static Huffman encoding table created
   an information leakage, however this same study concluded that an
   attacker could not take advantage of this information leakage to
   recover any meaningful amount of information (see [PETAL]).

7.3.  Memory Consumption

   An attacker can try to cause an endpoint to exhaust its memory.
   QPACK is designed to limit both the peak and stable amounts of memory
   allocated by an endpoint.

   The amount of memory used by the encoder is limited by the protocol
   using QPACK through the definition of the maximum size of the dynamic
   table, and the maximum number of blocking streams.  In HTTP/3, these
   values are controlled by the decoder through the settings parameters
   SETTINGS_QPACK_MAX_TABLE_CAPACITY and SETTINGS_QPACK_BLOCKED_STREAMS,
   respectively (see Section 3.2.3 and Section 2.1.2).  The limit on the
   size of the dynamic table takes into account the size of the data
   stored in the dynamic table, plus a small allowance for overhead.
   The limit on the number of blocked streams is only a proxy for the
   maximum amount of memory required by the decoder.  The actual maximum
   amount of memory will depend on how much memory the decoder uses to
   track each blocked stream.

   A decoder can limit the amount of state memory used for the dynamic
   table by setting an appropriate value for the maximum size of the
   dynamic table.  In HTTP/3, this is realized by setting an appropriate
   value for the SETTINGS_QPACK_MAX_TABLE_CAPACITY parameter.  An
   encoder can limit the amount of state memory it uses by signaling a
   lower dynamic table size than the decoder allows (see Section 3.2.2).

   A decoder can limit the amount of state memory used for blocked
   streams by setting an appropriate value for the maximum number of
   blocked streams.  In HTTP/3, this is realized by setting an
   appropriate value for the QPACK_BLOCKED_STREAMS parameter.  Streams
   which risk becoming blocked consume no additional state memory on the
   encoder.

   An encoder allocates memory to track all dynamic table references in
   unacknowledged field sections.  An implementation can directly limit
   the amount of state memory by only using as many references to the
   dynamic table as it wishes to track; no signaling to the decoder is
   required.  However, limiting references to the dynamic table will
   reduce compression effectiveness.

   The amount of temporary memory consumed by an encoder or decoder can
   be limited by processing field lines sequentially.  A decoder
   implementation does not need to retain a complete list of field lines
   while decoding a field section.  An encoder implementation does not
   need to retain a complete list of field lines while encoding a field
   section if it is using a single-pass algorithm.  Note that it might
   be necessary for an application to retain a complete list of field
   lines for other reasons; even if QPACK does not force this to occur,
   application constraints might make this necessary.

   While the negotiated limit on the dynamic table size accounts for
   much of the memory that can be consumed by a QPACK implementation,
   data that 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.

7.4.  Implementation Limits

   An implementation of QPACK needs to ensure that large values for
   integers, long encoding for integers, or long string literals do not
   create security weaknesses.

   An implementation has to set a limit for the values it accepts for
   integers, as well as for the encoded length; see Section 4.1.1.  In
   the same way, it has to set a limit to the length it accepts for
   string literals; see Section 4.1.2.

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       |
       +--------------------------+------+---------------+---------+

                                  Table 1

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   |
         +----------------------+------+---------------+--------+

                                 Table 2

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 | Specification |
   +============================+=======+=============+===============+
   | QPACK_DECOMPRESSION_FAILED | 0x200 | Decoding of | Section 6     |
   |                            |       | a field     |               |
   |                            |       | section     |               |
   |                            |       | failed      |               |
   +----------------------------+-------+-------------+---------------+
   | QPACK_ENCODER_STREAM_ERROR | 0x201 | Error on    | Section 6     |
   |                            |       | the encoder |               |
   |                            |       | stream      |               |
   +----------------------------+-------+-------------+---------------+
   | QPACK_DECODER_STREAM_ERROR | 0x202 | Error on    | Section 6     |
   |                            |       | the decoder |               |
   |                            |       | stream      |               |
   +----------------------------+-------+-------------+---------------+

                                 Table 3

9.  References

9.1.  Normative References

   [HTTP3]    Bishop, M., Ed., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http, 6 January 2021,
              <https://tools.ietf.org/html/draft-ietf-quic-http>.

   [QUIC-TRANSPORT]
              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", Work in Progress,
              Internet-Draft, draft-ietf-quic-transport, 6 January 2021,
              <https://tools.ietf.org/html/draft-ietf-quic-transport>.

   [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>.

   [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>.

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

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

   [SEMANTICS]
              Fielding, R., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-13, 14 December 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-httpbis-
              semantics-13.txt>.

9.2.  Informative References

   [CRIME]    Wikipedia, "CRIME", May 2015, <http://en.wikipedia.org/w/
              index.php?title=CRIME&oldid=660948120>.

   [PETAL]    Tan, J. and J. Nahata, "PETAL: Preset Encoding
              Table Information Leakage", April 2013,
              <http://www.pdl.cmu.edu/PDL-FTP/associated/CMU-PDL-
              13-106.pdf>.

   [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification
              version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996,
              <https://www.rfc-editor.org/info/rfc1951>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <https://www.rfc-editor.org/info/rfc6454>.

   [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>.

   [TLS]      Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

Appendix A.  Static Table

   This table was generated by analyzing actual Internet traffic in 2018
   and including the most common header fields, after filtering out some
   unsupported and non-standard values.  Due to this methodology, some
   of the entries may be inconsistent or appear multiple times with
   similar but not identical values.  The order of the entries is
   optimized to encode the most common header fields with the smallest
   number of bytes.

   +=======+==================================+=======================+
   | Index | Name                             | Value                 |
   +=======+==================================+=======================+
   | 0     | :authority                       |                       |
   +-------+----------------------------------+-----------------------+
   | 1     | :path                            | /                     |
   +-------+----------------------------------+-----------------------+
   | 2     | age                              | 0                     |
   +-------+----------------------------------+-----------------------+
   | 3     | content-disposition              |                       |
   +-------+----------------------------------+-----------------------+
   | 4     | content-length                   | 0                     |
   +-------+----------------------------------+-----------------------+
   | 5     | cookie                           |                       |
   +-------+----------------------------------+-----------------------+
   | 6     | date                             |                       |
   +-------+----------------------------------+-----------------------+
   | 7     | etag                             |                       |
   +-------+----------------------------------+-----------------------+
   | 8     | if-modified-since                |                       |
   +-------+----------------------------------+-----------------------+
   | 9     | if-none-match                    |                       |
   +-------+----------------------------------+-----------------------+
   | 10    | last-modified                    |                       |
   +-------+----------------------------------+-----------------------+
   | 11    | link                             |                       |
   +-------+----------------------------------+-----------------------+
   | 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-headers     | cache-control         |
   +-------+----------------------------------+-----------------------+
   | 34    | access-control-allow-headers     | content-type          |
   +-------+----------------------------------+-----------------------+
   | 35    | access-control-allow-origin      | *                     |
   +-------+----------------------------------+-----------------------+
   | 36    | cache-control                    | max-age=0             |
   +-------+----------------------------------+-----------------------+
   | 37    | cache-control                    | max-age=2592000       |
   +-------+----------------------------------+-----------------------+
   | 38    | cache-control                    | max-age=604800        |
   +-------+----------------------------------+-----------------------+
   | 39    | cache-control                    | no-cache              |
   +-------+----------------------------------+-----------------------+
   | 40    | cache-control                    | no-store              |
   +-------+----------------------------------+-----------------------+
   | 41    | cache-control                    | public, max-          |
   |       |                                  | age=31536000          |
   +-------+----------------------------------+-----------------------+
   | 42    | content-encoding                 | br                    |
   +-------+----------------------------------+-----------------------+
   | 43    | content-encoding                 | gzip                  |
   +-------+----------------------------------+-----------------------+
   | 44    | content-type                     | application/dns-      |
   |       |                                  | message               |
   +-------+----------------------------------+-----------------------+
   | 45    | content-type                     | application/          |
   |       |                                  | javascript            |
   +-------+----------------------------------+-----------------------+
   | 46    | content-type                     | application/json      |
   +-------+----------------------------------+-----------------------+
   | 47    | content-type                     | application/x-www-    |
   |       |                                  | form-urlencoded       |
   +-------+----------------------------------+-----------------------+
   | 48    | content-type                     | image/gif             |
   +-------+----------------------------------+-----------------------+
   | 49    | content-type                     | image/jpeg            |
   +-------+----------------------------------+-----------------------+
   | 50    | content-type                     | image/png             |
   +-------+----------------------------------+-----------------------+
   | 51    | content-type                     | text/css              |
   +-------+----------------------------------+-----------------------+
   | 52    | content-type                     | text/html;            |
   |       |                                  | charset=utf-8         |
   +-------+----------------------------------+-----------------------+
   | 53    | content-type                     | text/plain            |
   +-------+----------------------------------+-----------------------+
   | 54    | content-type                     | text/                 |
   |       |                                  | plain;charset=utf-8   |
   +-------+----------------------------------+-----------------------+
   | 55    | range                            | bytes=0-              |
   +-------+----------------------------------+-----------------------+
   | 56    | strict-transport-security        | max-age=31536000      |
   +-------+----------------------------------+-----------------------+
   | 57    | strict-transport-security        | max-age=31536000;     |
   |       |                                  | includesubdomains     |
   +-------+----------------------------------+-----------------------+
   | 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-credentials | FALSE                 |
   +-------+----------------------------------+-----------------------+
   | 74    | access-control-allow-credentials | TRUE                  |
   +-------+----------------------------------+-----------------------+
   | 75    | access-control-allow-headers     | *                     |
   +-------+----------------------------------+-----------------------+
   | 76    | access-control-allow-methods     | get                   |
   +-------+----------------------------------+-----------------------+
   | 77    | access-control-allow-methods     | get, post, options    |
   +-------+----------------------------------+-----------------------+
   | 78    | access-control-allow-methods     | options               |
   +-------+----------------------------------+-----------------------+
   | 79    | access-control-expose-headers    | content-length        |
   +-------+----------------------------------+-----------------------+
   | 80    | access-control-request-headers   | content-type          |
   +-------+----------------------------------+-----------------------+
   | 81    | access-control-request-method    | get                   |
   +-------+----------------------------------+-----------------------+
   | 82    | access-control-request-method    | post                  |
   +-------+----------------------------------+-----------------------+
   | 83    | alt-svc                          | clear                 |
   +-------+----------------------------------+-----------------------+
   | 84    | authorization                    |                       |
   +-------+----------------------------------+-----------------------+
   | 85    | content-security-policy          | script-src 'none';    |
   |       |                                  | object-src 'none';    |
   |       |                                  | base-uri 'none'       |
   +-------+----------------------------------+-----------------------+
   | 86    | early-data                       | 1                     |
   +-------+----------------------------------+-----------------------+
   | 87    | expect-ct                        |                       |
   +-------+----------------------------------+-----------------------+
   | 88    | forwarded                        |                       |
   +-------+----------------------------------+-----------------------+
   | 89    | if-range                         |                       |
   +-------+----------------------------------+-----------------------+
   | 90    | origin                           |                       |
   +-------+----------------------------------+-----------------------+
   | 91    | purpose                          | prefetch              |
   +-------+----------------------------------+-----------------------+
   | 92    | server                           |                       |
   +-------+----------------------------------+-----------------------+
   | 93    | timing-allow-origin              | *                     |
   +-------+----------------------------------+-----------------------+
   | 94    | upgrade-insecure-requests        | 1                     |
   +-------+----------------------------------+-----------------------+
   | 95    | user-agent                       |                       |
   +-------+----------------------------------+-----------------------+
   | 96    | x-forwarded-for                  |                       |
   +-------+----------------------------------+-----------------------+
   | 97    | x-frame-options                  | deny                  |
   +-------+----------------------------------+-----------------------+
   | 98    | x-frame-options                  | sameorigin            |
   +-------+----------------------------------+-----------------------+

                                 Table 4

Appendix B.  Encoding and Decoding Examples

   The following examples represent a series of exchanges between an
   encoder and a decoder.  The exchanges are designed to exercise most
   QPACK instructions, and highlight potentially common patterns and
   their impact on dynamic table state.  The encoder sends three encoded
   field sections containing one field line each, as wells as two
   speculative inserts that are not referenced.

   The state of the encoder's dynamic table is shown, along with its
   current size.  Each entry is shown with the Absolute Index of the
   entry (Abs), the current number of outstanding encoded field sections
   with references to that entry (Ref), along with the name and value.
   Entries above the 'acknowledged' line have been acknowledged by the
   decoder.

B.1.  Literal Field Line With Name Reference

   The encoder sends an encoded field section containing a literal
   representation of a field with a static name reference.

   Data                | Interpretation
                                | Encoder's Dynamic Table

   Stream: 0
   0000                | Required Insert Count = 0, Base = 0
   510b 2f69 6e64 6578 | Literal Field Line with Name Reference
   2e68 746d 6c        |  Static Table, Index=1
                       |  (:path=/index.html)

                                 Abs Ref Name        Value
                                 ^-- acknowledged --^
                                 Size=0

B.2.  Dynamic Table

   The encoder sets the dynamic table capacity, inserts a header with a
   dynamic name reference, then sends a potentially blocking, encoded
   field section referencing this new entry.  The decoder acknowledges
   processing the encoded field section, which implicitly acknowledges
   all dynamic table insertions up to the Required Insert Count.

   Stream: Encoder
   3fbd01              | Set Dynamic Table Capacity=220
   c00f 7777 772e 6578 | Insert With Name Reference
   616d 706c 652e 636f | Static Table, Index=0
   6d                  |  (:authority=www.example.com)
   c10c 2f73 616d 706c | Insert With Name Reference
   652f 7061 7468      |  Static Table, Index=1
                       |  (:path=/sample/path)

                                 Abs Ref Name        Value
                                 ^-- acknowledged --^
                                  1   0  :authority  www.example.com
                                  2   0  :path       /sample/path
                                 Size=106

   Stream: 4
   0381                | Required Insert Count = 2, Base = 0
   10                  | Indexed Field Line With Post-Base Index
                       |  Absolute Index = Base(0) + Index(0) + 1 = 1
                       |  (:authority=www.example.com)
   11                  | Indexed Field Line With Post-Base Index
                       |  Absolute Index = Base(0) + Index(1) + 1 = 2
                       |  (:path=/sample/path)

                                 Abs Ref Name        Value
                                 ^-- acknowledged --^
                                  1   1  :authority  www.example.com
                                  2   1  :path       /sample/path
                                 Size=106

   Stream: Decoder
   84                  | Section Acknowledgment (stream=4)

                                 Abs Ref Name        Value
                                  1   0  :authority  www.example.com
                                  2   0  :path       /sample/path
                                 ^-- acknowledged --^
                                 Size=106

B.3.  Speculative Insert

   The encoder inserts a header into the dynamic table with a literal
   name.  The decoder acknowledges receipt of the entry.  The encoder
   does not send any encoded field sections.

   Stream: Encoder
   4a63 7573 746f 6d2d | Insert With Literal Name
   6b65 790c 6375 7374 |  (custom-key=custom-value)
   6f6d 2d76 616c 7565 |

                                 Abs Ref Name        Value
                                  1   0  :authority  www.example.com
                                  2   0  :path       /sample/path
                                 ^-- acknowledged --^
                                  3   0  custom-key  custom-value
                                 Size=160

   Stream: Decoder
   01                  | Insert Count Increment (1)

                                 Abs Ref Name        Value
                                  1   0  :authority  www.example.com
                                  2   0  :path       /sample/path
                                  3   0  custom-key  custom-value
                                 ^-- acknowledged --^
                                 Size=160

B.4.  Duplicate Instruction, Stream Cancellation

   The encoder duplicates an existing entry in the dynamic table, then
   sends an encoded field section referencing the dynamic table entries
   including the duplicated entry.  The packet containing the encoder
   stream data is delayed.  Before the packet arrives, the decoder
   cancels the stream and notifies the encoder that the encoded field
   section was not processed.

   Stream: Encoder
   02                  | Duplicate (Relative Index=2)

                                 Abs Ref Name        Value
                                  1   0  :authority  www.example.com
                                  2   0  :path       /sample/path
                                  3   0  custom-key  custom-value
                                 ^-- acknowledged --^
                                  4   0  :authority  www.example.com
                                 Size=217

   Stream: 8
   0500                | Required Insert Count = 4, Base = 4
   80                  | Indexed Field Line, Dynamic Table
                       |  Absolute Index = Base(4) - Index(0) = 4
                       |  (:authority=www.example.com)
   c1                  | Indexed Field Line, Static Table Index = 1
                       |  (:path=/)
   81                  | Indexed Field Line, Dynamic Table
                       |  Absolute Index = Base(4) - Index(1) = 3
                       |  (custom-key=custom-value)

                                 Abs Ref Name        Value
                                  1   0  :authority  www.example.com
                                  2   0  :path       /sample/path
                                  3   1  custom-key  custom-value
                                 ^-- acknowledged --^
                                  4   1  :authority  www.example.com
                                 Size=217

   Stream: Decoder
   48                  | Stream Cancellation (Stream=8)

                                 Abs Ref Name        Value
                                  1   0  :authority  www.example.com
                                  2   0  :path       /sample/path
                                  3   0  custom-key  custom-value
                                 ^-- acknowledged --^
                                  4   0  :authority  www.example.com
                                 Size=215

B.5.  Dynamic Table Insert, Eviction

   The encoder inserts another header into the dynamic table, which
   evicts the oldest entry.  The encoder does not send any encoded field
   sections.

   Stream: Encoder
   810d 6375 7374 6f6d | Insert With Name Reference
   2d76 616c 7565 32   |  Dynamic Table, Absolute Index=2
                       |  (custom-key=custom-value2)

                                 Abs Ref Name        Value
                                  2   0  :path       /sample/path
                                  3   0  custom-key  custom-value
                                 ^-- acknowledged --^
                                  4   0  :authority  www.example.com
                                  5   0  custom-key  custom-value2
                                 Size=215

Appendix C.  Sample One Pass Encoding Algorithm

   Pseudo-code for single pass encoding, excluding handling of
   duplicates, non-blocking mode, available encoder stream flow control
   and reference tracking.

   base = dynamicTable.getInsertCount()
   requiredInsertCount = 0
   for line in field_lines:
     staticIndex = staticTable.findIndex(line)
     if staticIndex is not None:
       encodeIndexReference(streamBuffer, staticIndex)
       continue

     dynamicIndex = dynamicTable.findIndex(line)
     if dynamicIndex is None:
       # No matching entry.  Either insert+index or encode literal
       staticNameIndex = staticTable.findName(line.name)
       if staticNameIndex is None:
          dynamicNameIndex = dynamicTable.findName(line.name)

       if shouldIndex(line) and dynamicTable.canIndex(line):
         encodeInsert(encoderBuffer, staticNameIndex,
                      dynamicNameIndex, line)
         dynamicIndex = dynamicTable.add(line)

     if dynamicIndex is None:
       # Could not index it, literal
       if dynamicNameIndex is not None:
         # Encode literal with dynamic name, possibly above base
         encodeDynamicLiteral(streamBuffer, dynamicNameIndex,
                              base, line)
         requiredInsertCount = max(requiredInsertCount,
                                   dynamicNameIndex)
       else:
         # Encodes a literal with a static name or literal name
         encodeLiteral(streamBuffer, staticNameIndex, line)
     else:
       # Dynamic index reference
       assert(dynamicIndex is not None)
       requiredInsertCount = max(requiredInsertCount, dynamicIndex)
       # Encode dynamicIndex, possibly above base
       encodeDynamicIndexReference(streamBuffer, dynamicIndex, base)

   # encode the prefix
   if requiredInsertCount == 0:
     encodeIndexReference(prefixBuffer, 0, 0, 8)
     encodeIndexReference(prefixBuffer, 0, 0, 7)
   else:
     wireRIC = (
       requiredInsertCount
       % (2 * getMaxEntries(maxTableCapacity))
     ) + 1;
     encodeInteger(prefixBuffer, 0x00, wireRIC, 8)
     if base >= requiredInsertCount:
       encodeInteger(prefixBuffer, 0, base - requiredInsertCount, 7)
     else:
       encodeInteger(prefixBuffer, 0x80,
                     requiredInsertCount  - base - 1, 7)

   return encoderBuffer, prefixBuffer + streamBuffer

Appendix D.  Change Log

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

D.1.  Since draft-ietf-quic-qpack-19

   Editorial changes only

D.2.  Since draft-ietf-quic-qpack-18

   Editorial changes only

D.3.  Since draft-ietf-quic-qpack-17

   Editorial changes only

D.4.  Since draft-ietf-quic-qpack-16

   Editorial changes only

D.5.  Since draft-ietf-quic-qpack-15

   No changes

D.6.  Since draft-ietf-quic-qpack-14

   Added security considerations

D.7.  Since draft-ietf-quic-qpack-13

   No changes

D.8.  Since draft-ietf-quic-qpack-12

   Editorial changes only

D.9.  Since draft-ietf-quic-qpack-11

   Editorial changes only

D.10.  Since draft-ietf-quic-qpack-10

   Editorial changes only

D.11.  Since draft-ietf-quic-qpack-09

   *  Decoders MUST emit Header Acknowledgments (#2939)

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

   *  Added explicit defaults for new SETTINGS (#2974)

D.12.  Since draft-ietf-quic-qpack-08

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

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

D.13.  Since draft-ietf-quic-qpack-06

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

D.14.  Since draft-ietf-quic-qpack-05

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

   *  Renamed SETTINGS_HEADER_TABLE_SIZE to
      SETTINGS_QPACK_MAX_TABLE_CAPACITY.

D.15.  Since draft-ietf-quic-qpack-04

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

D.16.  Since draft-ietf-quic-qpack-03

   *  Change HTTP settings defaults (#2038)

   *  Substantial editorial reorganization

D.17.  Since draft-ietf-quic-qpack-02

   *  Largest Reference encoded modulo MaxEntries (#1763)

   *  New Static Table (#1355)

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

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

   *  Implementations must handle 62 bit integers (#1760)

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

   *  Preserve header field order (#1725)

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

D.18.  Since draft-ietf-quic-qpack-01

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

D.19.  Since draft-ietf-quic-qpack-00

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

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

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

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

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

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

D.20.  Since draft-ietf-quic-qcram-00

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

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

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

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

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

Acknowledgments

   The IETF QUIC Working Group received an enormous amount of support
   from many people.

   The compression design team did substantial work exploring the
   problem space and influencing the initial draft.  The contributions
   of design team members Roberto Peon, Martin Thomson, and Dmitri
   Tikhonov are gratefully acknowledged.

   The following people also provided substantial contributions to this
   document:

   *  Bence Beky

   *  Alessandro Ghedini

   *  Ryan Hamilton

   *  Robin Marx

   *  Patrick McManus

   *  奥 一穂 (Kazuho Oku)

   *  Lucas Pardue

   *  Biren Roy

   *  Ian Swett

   This draft draws heavily on the text of [RFC7541].  The indirect
   input of those authors is also gratefully acknowledged.

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

   A 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