QPACK: Header Compression for HTTP over QUIC
draft-ietf-quic-qpack-00

QUIC                                                           C. Krasic
Internet-Draft                                               Google, Inc
Intended status: Standards Track                               M. Bishop
Expires: November 24, 2018                           Akamai Technologies
                                                        A. Frindell, Ed.
                                                                Facebook
                                                            May 23, 2018

              QPACK: Header Compression for HTTP over QUIC
                        draft-ietf-quic-qpack-00

Abstract

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

Note to Readers

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

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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on November 24, 2018.

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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Head-of-Line Blocking in HPACK  . . . . . . . . . . . . .   3
     1.2.  Avoiding Head-of-Line Blocking in HTTP/QUIC . . . . . . .   4
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   5
     2.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   5
   3.  Wire Format . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Primitives  . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Indexing  . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  QPACK Encoder Stream  . . . . . . . . . . . . . . . . . .   8
       3.3.1.  Insert With Name Reference  . . . . . . . . . . . . .   8
       3.3.2.  Insert Without Name Reference . . . . . . . . . . . .   9
       3.3.3.  Duplicate . . . . . . . . . . . . . . . . . . . . . .   9
       3.3.4.  Dynamic Table Size Update . . . . . . . . . . . . . .  10
     3.4.  QPACK Decoder Stream  . . . . . . . . . . . . . . . . . .  10
       3.4.1.  Table State Synchronize . . . . . . . . . . . . . . .  11
       3.4.2.  Header Acknowledgement  . . . . . . . . . . . . . . .  11
     3.5.  Request and Push Streams  . . . . . . . . . . . . . . . .  11
       3.5.1.  Header Data Prefix  . . . . . . . . . . . . . . . . .  12
       3.5.2.  Instructions  . . . . . . . . . . . . . . . . . . . .  12
   4.  Encoding Strategies . . . . . . . . . . . . . . . . . . . . .  15
     4.1.  Single pass encoding  . . . . . . . . . . . . . . . . . .  15
     4.2.  Preventing Eviction Races . . . . . . . . . . . . . . . .  15
     4.3.  Reference Tracking  . . . . . . . . . . . . . . . . . . .  15
       4.3.1.  Blocked Eviction  . . . . . . . . . . . . . . . . . .  16
       4.3.2.  Blocked Decoding  . . . . . . . . . . . . . . . . . .  16
     4.4.  Speculative table updates . . . . . . . . . . . . . . . .  16
     4.5.  Sample One Pass Encoding Algorithm  . . . . . . . . . . .  17
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  18

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     7.2.  Informative References  . . . . . . . . . . . . . . . . .  18
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Change Log  . . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     B.1.  Since draft-ietf-quic-qcram-00  . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   The QUIC transport protocol was designed from the outset to support
   HTTP semantics, and its design subsumes many of the features of
   HTTP/2.  QUIC's stream multiplexing comes into some conflict with
   header compression.  A key goal of the design of QUIC is to improve
   stream multiplexing relative to HTTP/2 by eliminating HoL (head of
   line) blocking, which can occur in HTTP/2.  HoL blocking can happen
   because all HTTP/2 streams are multiplexed onto a single TCP
   connection with its in-order semantics.  QUIC can maintain
   independence between streams because it implements core transport
   functionality in a fully stream-aware manner.  However, the HTTP/QUIC
   mapping is still subject to HoL blocking if HPACK is used directly.
   HPACK exploits multiplexing for greater compression, shrinking the
   representation of headers that have appeared earlier on the same
   connection.  In the context of QUIC, this imposes a vulnerability to
   HoL blocking (see Section 1.1).

   QUIC is described in [QUIC-TRANSPORT].  The HTTP/QUIC mapping is
   described in [QUIC-HTTP].  For a full description of HTTP/2, see
   [RFC7540].  The description of HPACK is [RFC7541], with important
   terminology in Section 1.3.

   QPACK modifies HPACK to allow correctness in the presence of out-of-
   order delivery, with flexibility for implementations to balance
   between resilience against HoL 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.

   QPACK is intended to be a relatively non-intrusive extension to
   HPACK; an implementation should be easily shared within stacks
   supporting both HTTP/2 over (TLS+)TCP and HTTP/QUIC.

1.1.  Head-of-Line Blocking in HPACK

   HPACK enables several types of header representations, one of which
   also adds the header to a dynamic table of header values.  These
   values are then available for reuse in subsequent header blocks
   simply by referencing the entry number in the table.

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   If the packet containing a header is lost, that stream cannot
   complete header processing until the packet is retransmitted.  This
   is unavoidable.  However, other streams which rely on the state
   created by that packet _also_ cannot make progress.  This is the
   problem which QUIC solves in general, but which is reintroduced by
   HPACK when the loss includes a HEADERS frame.

1.2.  Avoiding Head-of-Line Blocking in HTTP/QUIC

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

   Each header block contains a Largest Reference (see Section 3.5.1)
   which identifies the table state necessary for decoding.  If the
   greatest absolute index in the dynamic table is less than the value
   of the Largest Reference, the stream is considered "blocked."  While
   blocked, header field data should remain in the blocked stream's flow
   control window.  When the Largest Reference is zero, the frame
   contains no references to the dynamic table and can always be
   processed immediately.  A stream becomes unblocked when the greatest
   absolute index in the dynamic table becomes greater than or equal to
   the Largest Reference for all header blocks the decoder has started
   reading from the stream.

   A decoder can permit the possibility of blocked streams by setting
   SETTINGS_QPACK_BLOCKED_STREAMS to a non-zero value.  This setting
   specifies an upper bound on the number of streams which can be
   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 be improved by referencing dynamic table
   entries that are still in transit, but if there is loss or reordering
   the stream can become blocked at the decoder.  An encoder avoids the
   risk of blocking by only referencing dynamic table entries which have
   been acknowledged, but this means using literals.  Since literals
   make the header block larger, this can result in the encoder becoming
   blocked on congestion or flow control limits.

   An encoder MUST limit the number of streams which could become
   blocked to the value of SETTINGS_QPACK_BLOCKED_STREAMS at all times.
   Note that the decoder might not actually become blocked on every
   stream which risks becoming blocked.  If the decoder encounters more
   blocked streams than it promised to support, it SHOULD treat this as
   a stream error of type HTTP_QPACK_DECOMPRESSION_FAILED.

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2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Definitions of terms that are used in this document:

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

   Header set:  The full collection of headers associated with an HTTP
      message.

   Header block:  The compressed representation of a header set.

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

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

   QPACK is a name, not an acronym.

2.1.  Notational Conventions

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

   x (A)  Indicates that x is A bits long

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

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

3.  Wire Format

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

   o  Table updates are carried by a unidirectional stream from encoder
      to decoder.  Instructions on this stream modify the dynamic table
      state without generating output to any particular request.

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   o  Acknowledgements of table modifications and header processing are
      carried by a unidirectional stream from decoder to encoder.

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

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

   All table updates occur on the control stream.  Request streams only
   carry header blocks that do not modify the state of the table.

3.1.  Primitives

   The prefixed integer from Section 5.1 of [RFC7541] is used heavily
   throughout this document.  The string literal, defined by Section 5.2
   of [RFC7541], is used with the following modification.

   HPACK defines string literals to begin on a byte boundary.  They
   begin with a single flag (indicating whether the string is Huffman-
   coded), followed by the Length encoded as a 7-bit prefix integer, and
   finally Length octets of data.

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

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

3.2.  Indexing

   Entries in the QPACK static and dynamic tables are addressed
   separately.

   Entries in the static table have the same indices at all times.  The
   static table is defined in Appendix A of [RFC7541].  Note that
   because HPACK did not use zero-based references, there is no value at
   index zero of the static table.

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

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   On the control stream, a relative index of "0" always refers to the
   most recently inserted value in the dynamic table.  Note that this
   means the entry referenced by a given relative index can change while
   interpreting a HEADERS frame as new entries are inserted.

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

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

              Example Dynamic Table Indexing - Control Stream

   Because frames from request streams can be delivered out of order
   with instructions on the control stream, relative indices are
   relative to the Base Index at the beginning of the header block (see
   Section 3.5.1).  The Base Index is the absolute index of the entry
   which has the relative index of zero when interpreting the frame.
   The relative indices of entries do not change while interpreting
   headers on a request or push stream.

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

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

              Example Dynamic Table Indexing - Request Stream

   Entries with an absolute index greater than a frame's Base Index can
   be referenced using specific Post-Base instructions.  The relative
   indices of Post-Base references count up from Base Index.

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

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

               Dynamic Table Indexing - Post-Base References

   If the decoder encounters a reference to an entry which has already
   been dropped from the table or which is greater than the declared
   Largest Reference, this MUST be treated as a stream error of type
   "HTTP_QPACK_DECOMPRESSION_FAILED" error code.  If this reference
   occurs on the control stream, this MUST be treated as a session
   error.

3.3.  QPACK Encoder Stream

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

   Each set of encoder instructions is prefaced by its length, encoded
   as a variable length integer with an 8-bit prefix.  Instructions MUST
   NOT span more than one block.

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      |       Block Length (8+)       |
      +-------------------------------+
      |     Instruction Block (*)   ...
      +-------------------------------+

                         Encoder instruction block

3.3.1.  Insert With Name Reference

   An addition to the header table where the header field name matches
   the header field name of an entry stored in the static table or the
   dynamic table starts with the '1' one-bit pattern.  The "S" bit

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   indicates whether the reference is to the static (S=1) or dynamic
   (S=0) table.  The header field name is represented using the relative
   index of that entry, which is represented as an integer with a 6-bit
   prefix (see Section 5.1 of [RFC7541]).

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

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

                    Insert Header Field -- Indexed Name

3.3.2.  Insert Without Name Reference

   An addition to the header table where both the header field name and
   the header field value are represented as string literals (see
   Section 3.1) starts with the '01' two-bit pattern.

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

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

                      Insert Header Field -- New Name

3.3.3.  Duplicate

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

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

                            Figure 1: Duplicate

   The existing entry is re-inserted into the dynamic table without
   resending either the name or the value.  This is useful to mitigate
   the eviction of older entries which are frequently referenced, both
   to avoid the need to resend the header and to avoid the entry in the
   table blocking the ability to insert new headers.

3.3.4.  Dynamic Table Size Update

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

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

                Figure 2: Maximum Dynamic Table Size Change

   The new maximum size MUST be lower than or equal to the limit
   determined by the protocol using QPACK.  A value that exceeds this
   limit MUST be treated as a decoding error.  In HTTP/QUIC, this limit
   is the value of the SETTINGS_HEADER_TABLE_SIZE parameter (see
   [QUIC-HTTP]) received from the decoder.

   Reducing the maximum size of the dynamic table can cause entries to
   be evicted (see Section 4.3 of [RFC7541]).  This MUST NOT cause the
   eviction of entries with outstanding references (see Section 4.3).

3.4.  QPACK Decoder Stream

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

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3.4.1.  Table State Synchronize

   After processing a set of instructions on the encoder stream, the
   decoder will emit a Table State Synchronize instruction on the
   decoder stream.  The instruction begins with the '1' one-bit pattern.
   The instruction specifies the total number of dynamic table inserts
   and duplications since the last Table State Synchronize, encoded as a
   7-bit prefix integer.  The encoder uses this value to determine which
   table entries are vulnerable to head-of-line blocking.  A decoder MAY
   coalesce multiple synchronization updates into a single update.

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

                     Figure 3: Table Size Synchronize

3.4.2.  Header Acknowledgement

   After processing a header block on a request or push stream, the
   decoder emits a Header Acknowledgement instruction on the decoder
   stream.  The instruction begins with the '0' one-bit pattern and
   includes the request stream's stream ID, encoded as a 7-bit prefix
   integer.  It is used by the peer's QPACK encoder to know when it is
   safe to evict an entry.

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

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

                     Figure 4: Header Acknowledgement

3.5.  Request and Push Streams

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

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3.5.1.  Header Data Prefix

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

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

                          Figure 5: Frame Payload

   "Largest Reference" identifies the largest absolute dynamic index
   referenced in the block.  Blocking decoders use the Largest Reference
   to determine when it is safe to process the rest of the block.

   "Base Index" is used to resolve references in the dynamic table as
   described in Section 3.2.  To save space, Base Index is encoded
   relative to Largest Reference using a one-bit sign flag.

   baseIndex = largestReference + deltaBaseIndex

   If the encoder inserted entries to the table while the encoding the
   block, Largest Reference will be greater than Base Index, so
   deltaBaseIndex will be negative and encoded with S=1.  If the block
   did not reference the most recent entry in the table and did not
   insert any new entries, Largest Reference will be less than Base
   Index, so deltaBaseIndex will be positive and encoded with S=0.  When
   Largest Reference and Base Index are equal, deltaBaseIndex is 0 and
   encoded with S=0.

3.5.2.  Instructions

3.5.2.1.  Indexed Header Field

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

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

                           Indexed Header Field

   If the entry is in the static table, or in the dynamic table with an
   absolute index less than or equal to Base Index, this representation
   starts with the '1' 1-bit pattern, followed by the "S" bit indicating
   whether the reference is into the static (S=1) or dynamic (S=0)
   table.  Finally, the relative index of the matching header field is
   represented as an integer with a 6-bit prefix (see Section 5.1 of
   [RFC7541]).

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

                           Indexed Header Field

   If the entry is in the dynamic table with an absolute index greater
   than Base Index, the representation starts with the '0100' 4-bit
   pattern, followed by the post-base index (see Section 3.2) of the
   matching header field, represented as an integer with a 4-bit prefix
   (see Section 5.1 of [RFC7541]).

3.5.2.2.  Literal Header Field With Name Reference

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

   If the entry is in the static table, or in the dynamic table with an
   absolute index less than or equal to Base Index, this representation
   starts with the '00' two-bit pattern.  If the entry is in the dynamic
   table with an absolute index greater than Base Index, the
   representation starts with the '0101' four-bit pattern.

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

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

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

                 Literal Header Field With Name Reference

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

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

            Literal Header Field With Post-Base Name Reference

   For entries in the dynamic table with an absolute index greater than
   Base Index, the header field name is represented using the post-base
   index of that entry (see Section 3.2) encoded as an integer with a
   3-bit prefix.

3.5.2.3.  Literal Header Field Without Name Reference

   An addition to the header table where both the header field name and
   the header field value are represented as string literals (see
   Section 3.1) starts with the '011' three-bit pattern.

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

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   with the 'N' bit set, it MUST use a literal representation to forward
   this header field.  This bit is intended for protecting header field
   values that are not to be put at risk by compressing them (see
   Section 7.1 of [RFC7541] for more details).

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

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

                Literal Header Field Without Name Reference

4.  Encoding Strategies

4.1.  Single pass encoding

   An encoder making a single pass over a list of headers must choose
   Base Index before knowing Largest Reference.  When trying to
   reference a header inserted to the table after encoding has begun,
   the entry is encoded with different instructions that tell the
   decoder to use an absolute index greater than the Base Index.

4.2.  Preventing Eviction Races

   Due to out-of-order arrival, QPACK's eviction algorithm requires
   changes (relative to HPACK) to avoid the possibility that an indexed
   representation is decoded after the referenced entry has already been
   evicted.  QPACK employs a two-phase eviction algorithm, in which the
   encoder will not evict entries that have outstanding (unacknowledged)
   references.

4.3.  Reference Tracking

   An encoder MUST ensure that a header block which references a dynamic
   table entry is not received by the decoder after the referenced entry
   has already been evicted.  An encoder also respects the limit set by
   the decoder on the number of streams that are allowed to become
   blocked.  Even if the decoder is willing to tolerate blocked streams,
   the encoder might choose to avoid them in certain cases.

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   In order to enable this, the encoder will need to track outstanding
   (unacknowledged) header blocks and table updates using feedback
   received from the decoder.

4.3.1.  Blocked Eviction

   The encoder MUST NOT permit an entry to be evicted while a reference
   to that entry remains unacknowledged.  If a new header to be inserted
   into the dynamic table would cause the eviction of such an entry, the
   encoder MUST NOT emit the insert instruction until the reference has
   been processed by the decoder and acknowledged.

   The encoder can emit a literal representation for the new header in
   order to avoid encoding delays, and MAY insert the header into the
   table later if desired.

   To ensure that the blocked eviction case is rare, references to the
   oldest entries in the dynamic table SHOULD be avoided.  When one of
   the oldest entries in the table is still actively used for
   references, the encoder SHOULD emit an Duplicate representation
   instead (see Section 3.3.3).

4.3.2.  Blocked Decoding

   For header blocks encoded in non-blocking mode, the encoder needs to
   forego indexed representations that refer to table updates which have
   not yet been acknowledged with Section 3.4.  Since all table updates
   are processed in sequence on the control stream, an index into the
   dynamic table is sufficient to track which entries have been
   acknowledged.

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

4.4.  Speculative table updates

   Implementations can _speculatively_ send header frames on the HTTP
   Control Streams which are not needed for any current HTTP request or
   response.  Such headers could be used strategically to improve
   performance.  For instance, the encoder might decide to _refresh_ by
   sending Duplicate representations for popular header fields
   (Section 3.3.3), ensuring they have small indices and hence minimal
   size on the wire.

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4.5.  Sample One Pass Encoding Algorithm

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

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

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

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

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

   return controlBuffer, prefixBuffer + streamBuffer

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5.  Security Considerations

   TBD.

6.  IANA Considerations

   None.

7.  References

7.1.  Normative References

   [QUIC-HTTP]
              Bishop, M., "Hypertext Transfer Protocol (HTTP) over
              QUIC", draft-ietf-quic-http-12 (work in progress), April
              2018.

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

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

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

7.2.  Informative References

   [QUIC-TRANSPORT]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-11 (work
              in progress), April 2018.

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

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

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7.3.  URIs

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

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

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

Acknowledgments

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

   o  Ryan Hamilton

   o  Patrick McManus

   o  Kazuho Oku

   o  Biren Roy

   o  Ian Swett

   o  Dmitri Tikhonov

Change Log

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

B.1.  Since draft-ietf-quic-qcram-00

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

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

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

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

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

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Authors' Addresses

   Charles 'Buck' Krasic
   Google, Inc

   Email: ckrasic@google.com

   Mike Bishop
   Akamai Technologies

   Email: mbishop@evequefou.be

   Alan Frindell (editor)
   Facebook

   Email: afrind@fb.com

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