HTTPbis Working Group                                            R. Peon
Internet-Draft                                               Google, Inc
Intended status: Informational Standards Track                              H. Ruellan
Expires: June 7, August 17, 2014                                       Canon CRF
                                                        December 4, 2013
                                                       February 13, 2014

                 HPACK - Header Compression for HTTP/2.0
                draft-ietf-httpbis-header-compression-05 HTTP/2
                draft-ietf-httpbis-header-compression-06

Abstract

   This document describes specification defines HPACK, a compression format adapted to for
   efficiently
   represent representing HTTP header fields in the context of HTTP/2.0. HTTP/2.

Editorial Note (To be removed by RFC Editor)

   Discussion of this draft takes place on the HTTPBIS working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   <http://lists.w3.org/Archives/Public/ietf-http-wg/>. [1].

   Working Group information and related documents can be found at
   <http://tools.ietf.org/wg/httpbis/> [2]
   (Wiki) and
   <https://github.com/http2/http2-spec> [3] (source code and issues tracker).

   The changes in this draft are summarized in Appendix A.1.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on June 7, August 17, 2014.

Copyright Notice

   Copyright (c) 2013 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  4   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . . .  4   3
     2.1.  Outline . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Header Field Encoding . . . . . . . . . . . . . . . . . . . .  5   4
     3.1.  Encoding Concepts . . . . . . . . . . . . . . . . . . . .  5   4
       3.1.1.  Encoding Context  . . . . . . . . . . . . . . . . . . .   5
       3.1.2.  Header Table  . . . . . . . . . . . . . . . . . . . . .  6   5
       3.1.3.  Reference Set . . . . . . . . . . . . . . . . . . . .   6
       3.1.4.  Header Field Representation . . . . . . . . . . . . .   7
       3.1.5.  Header Field Emission . . . . . . . . . . . . . . . .   8
     3.2.  Header Block Decoding . . . . . . . . . . . . . . . . . .   8
       3.2.1.  Header Field Representation Processing  . . . . . . . .   8
       3.2.2.  Reference Set Emission  . . . . . . . . . . . . . . . . 10   9
       3.2.3.  Header Set Completion . . . . . . . . . . . . . . . . 10   9
     3.3.  Header Table Management . . . . . . . . . . . . . . . . . 10   9
       3.3.1.  Maximum Table Size  . . . . . . . . . . . . . . . . . .  10
       3.3.2.  Entry Eviction When Header Table Size Changes . . . .  10
       3.3.3.  Entry Eviction when Adding New Entries  . . . . . . . . 11  10
   4.  Detailed Format . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Low-level representations . . . . . . . . . . . . . . . .  11
       4.1.1.  Integer representation  . . . . . . . . . . . . . . . .  11
       4.1.2.  String Literal Representation . . . . . . . . . . . .  13
     4.2.  Indexed Header Field Representation . . . . . . . . . . .  15
     4.3.  Literal Header Field Representation . . . . . . . . . . .  15
       4.3.1.  Literal Header Field without Indexing . . . . . . . .  15
       4.3.2.  Literal Header Field with Incremental Indexing  . . . .  16
     4.4.  Encoding Context Update . . . . . . . . . . . . . . . . .  17
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   6.  References  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   7.  References  . . . . . 18
     6.1. . . . . . . . . . . . . . . . . . . . .  19
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 18
     6.2.  19
     7.2.  Informative References  . . . . . . . . . . . . . . . . . .  19
   Appendix A.  Change Log (to be removed by RFC Editor before
                publication  . . . . . . . . . . . . . . . . . . . . . 19  20
     A.1.  Since draft-ietf-httpbis-header-compression-04 . draft-ietf-httpbis-header-compression-05  . . . . . 19  20
     A.2.  Since draft-ietf-httpbis-header-compression-03 . draft-ietf-httpbis-header-compression-04  . . . . .  20
     A.3.  Since draft-ietf-httpbis-header-compression-02 . draft-ietf-httpbis-header-compression-03  . . . . . 20  21
     A.4.  Since draft-ietf-httpbis-header-compression-01 . draft-ietf-httpbis-header-compression-02  . . . . . 20  21
     A.5.  Since draft-ietf-httpbis-header-compression-01  . . . . .  21
     A.6.  Since draft-ietf-httpbis-header-compression-00  . . . . .  21
   Appendix B.  Static Table . . . . . . . . . . . . . . . . . . . . 21  22
   Appendix C.  Huffman Codes For Requests  . . . . . .  . . . . . . . 23
   Appendix D.  Huffman Codes for Responses . . . . . . . . . . . . . 29  24
   Appendix E. D.  Examples . . . . . . . . . . . . . . . . . . . . . . 34
     E.1.  29
     D.1.  Header Field Representation Examples  . . . . . . . . . . . 34
       E.1.1.  30
       D.1.1.  Literal Header Field with Indexing  . . . . . . . . . . 34
       E.1.2.  30
       D.1.2.  Literal Header Field without Indexing . . . . . . . . 35
       E.1.3.  30
       D.1.3.  Indexed Header Field  . . . . . . . . . . . . . . . . . 36
       E.1.4.  31
       D.1.4.  Indexed Header Field from Static Table  . . . . . . . . 37
     E.2.  32
     D.2.  Request Examples without Huffman  . . . . . . . . . . . . . 37
       E.2.1.  33
       D.2.1.  First request . . . . . . . . . . . . . . . . . . . . 37
       E.2.2.  33
       D.2.2.  Second request  . . . . . . . . . . . . . . . . . . . . 38
       E.2.3.  34
       D.2.3.  Third request . . . . . . . . . . . . . . . . . . . . 40
     E.3.  35
     D.3.  Request Examples with Huffman . . . . . . . . . . . . . . 42
       E.3.1.  37
       D.3.1.  First request . . . . . . . . . . . . . . . . . . . . 42
       E.3.2.  37
       D.3.2.  Second request  . . . . . . . . . . . . . . . . . . . . 43
       E.3.3.  38
       D.3.3.  Third request . . . . . . . . . . . . . . . . . . . . 44
     E.4.  40
     D.4.  Response Examples without Huffman . . . . . . . . . . . . 46
       E.4.1.  42
       D.4.1.  First response  . . . . . . . . . . . . . . . . . . . . 46
       E.4.2.  42
       D.4.2.  Second response . . . . . . . . . . . . . . . . . . . 48
       E.4.3.  44
       D.4.3.  Third response  . . . . . . . . . . . . . . . . . . . . 49
     E.5.  45
     D.5.  Response Examples with Huffman  . . . . . . . . . . . . . . 51
       E.5.1.  47
       D.5.1.  First response  . . . . . . . . . . . . . . . . . . . . 51
       E.5.2.  47
       D.5.2.  Second response . . . . . . . . . . . . . . . . . . . 53
       E.5.3.  50
       D.5.3.  Third response  . . . . . . . . . . . . . . . . . . . . 54  51

1.  Introduction

   This document describes specification defines HPACK, a compression format adapted to for
   efficiently
   represent representing HTTP header fields in the context of HTTP/2.0 HTTP/2
   (see [HTTP2]).

2.  Overview

   In HTTP HTTP/1.1 (see [HTTP-p1]), header fields are sent encoded without any
   form of compression.  As web pages have grown to include dozens to
   hundreds of requests, the redundant header fields in these requests
   now pose a
   problem of measurable measurably increase latency and unnecessary unnecessarily consume bandwidth
   (see [PERF1] and [PERF2]).

   SPDY [SPDY] initially addressed this redundancy by compressing header
   fields with Deflate, using the DEFLATE format [DEFLATE], which proved very
   effective at eliminating efficiently representing the redundant header fields.
   However, that approach exposed a security risk as demonstrated by the
   CRIME [CRIME]. attack (see [CRIME]).

   This document describes HPACK, a new compressor for header fields
   which eliminates redundant header fields, is not vulnerable to known
   security attacks, and which also has a bounded memory cost requirement for
   use in constrained environments.

2.1.  Outline

   The HTTP header field encoding described defined in this document is based on a
   header table that map maps name-value pairs to index values.  Header
   tables are  The header
   table is incrementally updated during the HTTP/2.0 session.

   The encoder is responsible for deciding which HTTP/2 connection.

   A set of header fields to insert is treated as new entries in the header table.  The decoder then does exactly
   what the encoder prescribes, ending in a state that exactly matches
   the encoder's state.  This enables decoders to remain simple an unordered collection of name-
   value pairs.  Names and
   understand a wide variety values are considered to be opaque sequences
   of encoders. octets.  The order of header fields is not guaranteed to be
   preserved after being compressed and decompressed.

   As two consecutive sets of header fields often have header fields in
   common, each set of header fields is coded as a difference from the previous set of header fields. set.  The
   goal is to only encode the changes (header fields present in one of
   the set and not in sets that are absent from the other) between the two sets of
   header fields.

   HTTP

   A header field compression treats is represented either literally or as a reference to a
   name-value pair in the header table.  A set of header fields is
   stored as an
   unordered collection a set of name-value pairs.  Names and values references to entries in the header table
   (possibly keeping only a subset of it, as some header fields may be
   missing a corresponding entry in the header table).  Differences
   between consecutive sets of header fields are
   opaque sequences encoded as changes to
   the set of octets. references.

   The order encoder is responsible for deciding which header fields to insert
   as new entries in the header table.  The decoder executes the
   modifications to the header table and reference set prescribed by the
   encoder, reconstructing the set of header fields is not
   guaranteed in the process.
   This enables decoders to be preserved after being compression remain simple and decompression. understand a wide variety
   of encoders.

   Examples illustrating the use of these different mechanisms to
   represent header fields are available in Appendix E. D.

3.  Header Field Encoding

3.1.  Encoding Concepts

   The encoding and decoding of header fields relies on some components
   and concepts:

   Header Field:  A name-value pair.  Both the name and value are
      treated as opaque sequences of octets.

   Header Table:  The header table (see Section 3.1.2) is a component
      used to associate stored header fields to index values.  The data
      stored in this table is in first-in, first-out order.

   Static Table:  The static table (see Appendix B) is a component used
      to associate static header fields to index values.  This data is
      ordered, read-only, always accessible, and may be shared amongst
      all encoding contexts.

   Reference Set:  The reference set (see Section 3.1.3) is a component
      containing an unordered set of references to entries in the header
      table.  This is used for the differential encoding of a new header
      set.

   Header Set:  A header set is a potentially ordered an unordered group of header fields that
      are encoded jointly.  A complete set of key-value pairs contained
      in a HTTP request or response is a header set.

   Header Field Representation:  A header field can be represented in
      encoded form either as a literal or as an index (see
      Section 3.1.4).

   Header Block:  The entire set of encoded header field representations
      which, when decoded, yield a complete header set.

   Header Field Emission:  When decoding a set of header field
      representations, some operations emit a header field (see
      Section 3.1.5).  Emitted header fields can be safely passed are added to the
      upper processing layers as part of the current Header Set.
      header set and cannot be removed.

3.1.1.  Encoding Context

   The set of mutable structures used within an encoding context include
   a header table and a reference set.  Everything else is either
   immutable or conceptual.

   Using HTTP,

   HTTP messages are exchanged between a client and a server in both direction.  To keep the
   directions.  The encoding of header fields in each direction is
   independent from the other direction, there direction.  There is one a single encoding
   context for each direction.

   The direction used to encode all header fields contained sent in a PUSH_PROMISE frame sent by a server
   to a client are encoded within the same context as the header fields
   contained in the HEADERS frame corresponding to a response sent from
   the server to the client.

3.1.2.  Header Table

   A header table consists of
   that direction.

3.1.2.  Header Table

   A header table consists of a list of header fields maintained in
   first-in, first-out order.  The first and newest entry in a header
   table is always at index 1, and the oldest entry of a header table is
   at the index len(header table).

   The header table is initially empty.

   There is typically no need for the header table to contain duplicate
   entries.  However, duplicate entries MUST NOT be treated as an error
   by a decoder.

   The encoder decides how to update the header table and as such can
   control how much memory is used by the header table.  To limit the
   memory requirements on of the decoder side, decoder, the header table size is strictly
   bounded (see Section 3.3.1).

   The header table is updated during the processing of a set of header
   field representations (see header field representation processing
   (Section 3.2.1)). Section 3.2.1).

3.1.3.  Reference Set

   A reference set is an unordered set of references to entries of the
   header table.

   The reference set is initially empty.

   The reference set is updated during the processing of a set of header
   field representations (see header field representation processing
   (Section 3.2.1)). Section 3.2.1).

   The reference set enables differential encoding, whereby only
   differences between the previous header set and the current header
   set need to be encoded.  The use of differential encoding is optional
   for any header set.

   When an entry is evicted from the header table, if it was referenced
   from the reference set, its reference is removed from the reference
   set.

   To limit the memory requirements on the decoder side for handling the
   reference set, only entries within the header table can be contained
   in the reference set.  To still allow entries from the static table
   to take advantage of the differential encoding, when a header field
   is represented as a reference to an entry of the static table, this
   entry is inserted into the header table ((see (see Section 3.2.1).

3.1.4.  Header Field Representation

   An encoded header field can be represented either as a literal or as
   an index.

   Literal Representation:  A literal representation defines a new
      header field.  The header field name is represented either
      literally or as a reference to an entry of the header table.  The
      header field value is represented literally.

      Two different literal representations are provided:

      *  A literal representation that does not add the header field to
         the header table (see Section 4.3.1).

      *  A literal representation that adds the header field as a new
         entry at the beginning of the header table (see Section 4.3.2).

   Indexed Representation:  The indexed representation defines a header
      field as a reference to an entry in either the header table or the
      static table(see table (see Section 4.2).

          <----------  Index Address Space ---------->
          <-- Header  Table -->  <-- Static  Table -->
          +---+-----------+---+  +---+-----------+---+
          | 1 |    ...    | k |  |k+1|    ...    | n |
          +---+-----------+---+  +---+-----------+---+
          ^                   |
          |                   V
   Insertion Point       Drop Point

                            Index Address Space

         Indices between 1 and len(header table), inclusive, refer to
         elements in the header table, with index 1 referring to the
         beginning of the table.

         Indices between len(header table)+1 and len(header table)+
         len(static
         table)+len(static table), inclusive, refer to elements in the
         static table, where the index len(header table)+1 refers to the
         first entry in the static table.

         Index 0 signals that a modification of the encoding context: either
         the reference set MUST be emptied. is emptied, or the maximum size of the header
         table is updated (see Section 4.4).

         Any other indices MUST be treated as erroneous, and the
         compression context considered corrupt and unusable.

3.1.5.  Header Field Emission

   The emission of a header field is the process of marking a header
   field as belonging to the current header set.  Once a header has been
   emitted, it cannot be removed from the current header set.

   On the decoding side, an emitted header field can be safely passed to
   the upper processing layer as part of the current header set.  The
   decoder MAY pass the emitted header fields to the upper processing
   layer in any order.

   By emitting header fields instead of emitting header sets, the
   decoder can be implemented in a streaming way, and as such has only
   to keep in memory the header table and the reference set.  This
   bounds the amount of memory used by the decoder, even in presence of
   a very large set of header fields.  The management of memory for
   handling very large sets of header fields can therefore be deferred
   to the upper processing layers.

3.2.  Header Block Decoding

   The processing of a header block to obtain a header set is defined in
   this section.  To ensure that the decoding will successfully produce
   a header set, a decoder MUST obey the following rules.

3.2.1.  Header Field Representation Processing

   All the header field representations contained in a header block are
   processed in the order in which they are presented, as specified
   below.

   An _indexed representation_ with an index value of 0 entails one of
   the following actions: actions, depending on what is encoded next:

   o  The reference set is emptied.

   o  The maximum size of the header table is updated.

   An _indexed representation_ corresponding to an entry _present_ in
   the reference set entails the following actions:

   o  The entry is removed from the reference set.

   An _indexed representation_ corresponding to an entry _not present_
   in the reference set entails the following actions:

   o  If referencing an element of the static table:

      *  The header field corresponding to the referenced entry is
         emitted.

      *  The referenced static entry is inserted at the beginning of the
         header table.

      *  A reference to this new header table entry is added to the
         reference set (except if this new entry didn't fit in the
         header table).

   o  If referencing an element of the header table:

      *  The header field corresponding to the referenced entry is
         emitted.

      *  The referenced header table entry is added to the reference
         set.

   A _literal representation_ that is _not added_ to the header table
   entails the following action:

   o  The header field is emitted.

   A _literal representation_ that is _added_ to the header table
   entails the following actions:

   o  The header field is emitted.

   o  The header field is inserted at the beginning of the header table.

   o  A reference to the new entry is added to the reference set (except
      if this new entry didn't fit in the header table).

3.2.2.  Reference Set Emission

   Once all the representations contained in a header block have been
   processed, the header fields referenced in the reference set which
   have not previously been emitted during this processing are emitted.

3.2.3.  Header Set Completion

   Once all of the header field representations have been processed, and
   the remaining items in the reference set have been emitted, the
   header set is complete.

3.3.  Header Table Management
3.3.1.  Maximum Table Size

   To limit the memory requirements on the decoder side, the size of the
   header table is bounded.  The size of the header table MUST stay
   lower than or equal to its maximum size.

   By default, the maximum size of the header table is equal to the
   value of the HTTP/2.0 HTTP/2 setting SETTINGS_HEADER_TABLE_SIZE defined by the
   decoder (see [HTTP2]).  The encoder can change this maximum size (see
   Section 4.4), but it must stay lower than or equal to the value of
   SETTINGS_HEADER_TABLE_SIZE.

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

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

   The lengths are measured on the non-encoded entry name and entry
   value (for the case when a Huffman encoding is used to transmit
   string values).

   The 32 octets are an accounting for the entry structure overhead.
   For example, an entry structure using two 64-bits pointers to
   reference the name and the value and the entry, and two 64-bits
   integer for counting the number of references to these name and value
   would use 32 octets.

3.3.2.  Entry Eviction When Header Table Size Changes

   Whenever an entry is evicted from the header table, any reference to
   that entry contained by the reference set is removed.

   Whenever SETTINGS_HEADER_TABLE_SIZE the maximum size for the header table is made smaller,
   entries are evicted from the end of the header table until the size
   of the header table is less than or equal to SETTINGS_HEADER_TABLE_SIZE. the maximum size.

   The eviction of an entry from the header table causes the index of
   the entries in the static table to be reduced by one.

3.3.3.  Entry Eviction when Adding New Entries

   Whenever a new entry is to be added to the table, any name referenced
   by the representation of this new entry is cached, and then entries
   are evicted from the end of the header table until the size of the
   header table is less than or equal to SETTINGS_HEADER_TABLE_SIZE (maximum size - new entry size,
   size), or until the table is empty.

   If the size of the new entry is less than or equal to
   SETTINGS_HEADER_TABLE_SIZE, the maximum
   size, that entry is added to the table.  It is not an error to
   attempt to add an entry that is larger than
   SETTINGS_HEADER_TABLE_SIZE. the maximum size.

4.  Detailed Format

4.1.  Low-level representations

4.1.1.  Integer representation

   Integers are used to represent name indexes, pair indexes or string
   lengths.  To allow for optimized processing, an integer
   representation always finishes at the end of an octet.

   An integer is represented in two parts: a prefix that fills the
   current octet and an optional list of octets that are used if the
   integer value does not fit within the prefix.  The number of bits of
   the prefix (called N) is a parameter of the integer representation.

   The N-bit prefix allows filling the current octet.  If the value is
   small enough (strictly less than 2^N-1), it is encoded within the
   N-bit prefix.  Otherwise all the bits of the prefix are set to 1 and
   the value is encoded using an unsigned variable length integer [1] [4]
   representation.  N is always between 1 and 8 bits.  An integer
   starting at an octet-boundary will have an 8-bit prefix.

   The algorithm to represent an integer I is as follows:

   If

   if I < 2^N - 1, encode I on N bits
   Else
   else
       encode 2^N (2^N - 1 1) on N bits
       I = I - (2^N - 1)
       While
       while I >= 128
            Encode
            encode (I % 128 + 128) on 8 bits
            I = I / 128
       encode (I) I on 8 bits

   For informational purpose, the algorithm to decode an integer I is as
   follows:

   decode I from the next N bits
   if I < 2^N - 1, return I
   else
       M = 0
       repeat
           B = next octet
           I = I + (B & 127) * 2^M
           M = M + 7
       while B & 128 == 128
       return I

   This integer representation allows for values of indefinite size.  It
   is also possible for an encoder to send a large number of zero
   values, which can waste octets and could be used to overflow integer
   values.  Excessively large integer encodings - in value or octet
   length - MUST be treated as a decoding error.  Different limits can
   be set for each of the different uses of integers, based on
   implementation constraints.

4.1.1.1.  Example 1: Encoding 10 using a 5-bit prefix

   The value 10 is to be encoded with a 5-bit prefix.

   o  10 is less than 31 (= 2^5 - 1) and is represented using the 5-bit
      prefix.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | X | X | X | 0 | 1 | 0 | 1 | 0 |   10 stored on 5 bits
   +---+---+---+---+---+---+---+---+

4.1.1.2.  Example 2: Encoding 1337 using a 5-bit prefix

   The value I=1337 is to be encoded with a 5-bit prefix.

      1337 is greater than 31 (= 2^5 - 1).

         The 5-bit prefix is filled with its max value (31).

      I = 1337 - (2^5 - 1) = 1306.

         I (1306) is greater than or equal to 128, the while loop body
         executes:

            I % 128 == 26

            26 + 128 == 154
            154 is encoded in 8 bits as: 10011010

            I is set to 10 (1306 / 128 == 10)

            I is no longer greater than or equal to 128, the while loop
            terminates.

         I, now 10, is encoded on 8 bits as: 00001010

      The process ends.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | X | X | X | 1 | 1 | 1 | 1 | 1 |  Prefix = 31, I = 1306
   | 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 |  1306>=128, encode(154), I=1306/128
   | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 |  10<128,  10&lt;128, encode(10), done
   +---+---+---+---+---+---+---+---+

4.1.1.3.  Example 3: Encoding 42 starting at an octet-boundary

   The value 42 is to be encoded starting at an octet-boundary.  This
   implies that a 8-bit prefix is used.

   o  42 is less than 255 (= 2^8 - 1) and is represented using the 8-bit
      prefix.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 0 | 1 | 0 | 1 | 0 | 1 | 0 |   42 stored on 8 bits
   +---+---+---+---+---+---+---+---+

4.1.2.  String Literal Representation

   Header field names and header field values are encoded as sequences
   of octets.  A header field name or a header field value is encoded in
   three parts:

   1.  One bit, H, indicating whether or not the octets are Huffman
       encoded.

   2.  The number of octets required to hold the result of the next
       step, represented as a variable-length-quantity (Section 4.1.1),
       starting an integer with a 7-bit prefix (see
       Section 4.1.1), immediately following the first bit.

   3.  The encoded data of the string:

       1.

       *  If H is true, '1', then the encoded string data is the bitwise
          concatenation of the canonical [CANON]Huffman [CANON] Huffman code [HUFF]
          corresponding to each octet of the data, followed by between
          0-7 bits of padding.

       2.

       *  If H is false, '0', then the encoded string is the octets of the
          field value without modification.

   Padding is necessary when doing Huffman encoding to ensure that the
   remaining bits between the actual end of the data and the next octet
   boundary are not misinterpreted as part of the input data.

   When padding for Huffman encoding, use the bits from the EOS (end-of-
   string) entry in the Huffman table, table are used, starting with the MSB
   (most significant bit).  This entry is guaranteed to be at least 8
   bits long.

   String literals sent in the client to server direction which use Huffman encoding are encoded with the codes within the request
   Huffman code table (Appendix C) Codes Appendix C (see examples in Request Examples With with
   Huffman
   (Appendix E.3)).

   String literals sent Appendix D.3 and in the server to client direction which use
   Huffman encoding are encoded with the codes within the response
   Huffman code table (Appendix D) (see Response Examples With with Huffman
   (Appendix E.5)).
   Appendix D.5).

   The EOS symbol is represented with value 256, and is used solely to
   signal the end of the Huffman-encoded key data or the end of the
   Huffman-encoded value data.  Given that only between 0-7 bits of the
   EOS symbol is included in any Huffman-encoded string, and given that
   the EOS symbol is at least 8 bits long, it is expected that it should
   never be successfully decoded.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1 |  Value Length Prefix (7)  |
   +---+---+---+---+---+---+---+---+
   |   Value Length (0-N octets)   |
   +---+---+---+---+---+---+---+---+
   ...
   +---+---+---+---+---+---+---+---+
   | Huffman Encoded Data  |Padding|
   +---+---+---+---+---+---+---+---+

                   String Literal With with Huffman Encoding

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 |  Value Length Prefix (7)  |
   +---+---+---+---+---+---+---+---+
   |   Value Length (0-N octets)   |
   +---+---+---+---+---+---+---+---+
   ...
   +---+---+---+---+---+---+---+---+
   |  Field Bytes Without without Encoding |
   +---+---+---+---+---+---+---+---+

                  String Literal Without without Huffman Encoding

4.2.  Indexed Header Field Representation

   An indexed header field representation either identifies an entry in
   the header table or static table.  The processing of an indexed
   header field representation is described in Section 3.2.1.

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

                           Indexed Header Field

   This representation starts with the '1' 1-bit pattern, followed by
   the index of the matching pair, represented as an integer with a
   7-bit prefix.

   The index value of 0 is reserved for signalling that changes in the reference
   set is emptied.
   encoding context (see Section 4.4).

4.3.  Literal Header Field Representation

   Literal header field representations contain a literal header field
   value.  Header field names are either provided as a literal or by
   reference to an existing header table or static table entry.

   Literal representations all result in the emission of a header field
   when decoded.

4.3.1.  Literal Header Field without Indexing

   A literal header field without indexing causes the emission of a
   header field without altering the header table.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 1 |      Index (6+)       |
   +---+---+---+-------------------+
   | H |     Value Length (8+) (7+)     |
   +-------------------------------+
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+

           Literal Header Field without Indexing - Indexed Name

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 1 |           0           |
   +---+---+---+-------------------+
   | H |     Name Length (8+) (7+)      |
   +-------------------------------+
   +---+---------------------------+
   |  Name String (Length octets)  |
   +-------------------------------+
   +---+---------------------------+
   | H |     Value Length (8+) (7+)     |
   +-------------------------------+
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+

             Literal Header Field without Indexing - New Name

   This representation starts with the '01' 2-bit pattern.

   If the header field name matches the header field name of a (name,
   value) pair stored in the Header Table or Static Table, the header
   field name can be represented using the index of that entry.  In this
   case, the index of the entry, index (which is strictly greater than
   0), is represented as an integer with a 6-bit prefix (see
   Section 4.1.1).

   Otherwise, the header field name is represented as a literal.  The
   value 0 is represented on 6 bits followed by the header field name
   (see Section 4.1.2).

   The header field name representation is followed by the header field
   value represented as a literal string as described in Section 4.1.2.

4.3.2.  Literal Header Field with Incremental Indexing

   A literal header field with incremental indexing adds a new entry to
   the header table.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 0 |      Index (6+)       |
   +---+---+---+-------------------+
   | H |     Value Length (8+) (7+)     |
   +-------------------------------+
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+
       Literal Header Field with Incremental Indexing - Indexed Name

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 0 |           0           |
   +---+---+---+-------------------+
   | H |     Name Length (8+) (7+)      |
   +-------------------------------+
   +---+---------------------------+
   |  Name String (Length octets)  |
   +-------------------------------+
   +---+---------------------------+
   | H |     Value Length (8+) (7+)     |
   +-------------------------------+
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+

         Literal Header Field with Incremental Indexing - New Name

   This representation starts with the '00' 2-bit pattern.

   If the header field name matches the header field name of a (name,
   value) pair stored in the Header Table or Static Table, the header
   field name can be represented using the index of that entry.  In this
   case, the index of the entry, index (which is strictly greater than
   0), is represented as an integer with a 6-bit prefix (see
   Section 4.1.1).

   Otherwise, the header field name is represented as a literal.  The
   value 0 is represented on 6 bits followed by the header field name
   (see Section 4.1.2).

   The header field name representation is followed by the header field
   value represented as a literal string as described in Section 4.1.2.

4.4.  Encoding Context Update

   An indexed value of 0 is reserved for signalling changes in the
   encoding context.  The type of the change is encoded on the following
   octet(s).  Any change in the encoding context is applied immediately.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1 |             0             |
   +---+---------------------------+

                          Reference Set Emptying

   An octet with its high bit set to '1' signals that the reference set
   is emptied.  The remaining bits are set to '0'.

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

                     Maximum Header Table Size Change

   An octet with its high bit set to '0' signals the new maximum size of
   the header table.  This new maximum size MUST be lower than or equal
   to the value of the setting SETTINGS_HEADER_TABLE_SIZE (see [HTTP2]).

   The new maximum size is encoded as an integer with a 7-bit prefix.

   Change in the maximum size of the header table can trigger entry
   evictions (see Section 3.3.2).

5.  Security Considerations

   This compressor exists to solve security issues present in stream
   compressors such as DEFLATE whereby the compression context can be
   efficiently probed to reveal secrets.  A conformant implementation of
   this specification should be fairly safe against that kind of attack,
   as the reaping of any information from the compression context
   requires more work than guessing and verifying the plain text data
   directly with the server.  As with any secret, however, the longer
   the length of the secret, the more difficult the secret is to guess.
   It is inadvisable to have short cookies that are relied upon to
   remain secret for any duration of time.

   A proper security-conscious implementation will also need to prevent
   timing attacks by ensuring that the amount of time it takes to do
   string comparisons is always a function of the total length of the
   strings, and not a function of the number of matched characters.

   A decoder needs to ensure that larger values or encodings of integers
   do not permit exploitation.  Decoders MUST limit the size of
   integers, both in value and encoded length, that it accepts (see
   Section 4.1.1).

   Another common security problem is when the remote endpoint
   successfully causes the local endpoint to exhaust its memory.  This
   compressor attempts to deal with the most obvious ways that this
   could occur by limiting both the peak and the steady-state amount of
   memory consumed in the compressor state, by providing ways for the
   application to consume/flush the emitted header fields in small
   chunks, and by considering overhead in the state size calculation.
   Implementors must still be careful in the creation of APIs to an
   implementation of this compressor by ensuring that header field keys
   and values are either emitted as a stream, or that the compression
   implementation have a limit on the maximum size of a key or value.
   Failure to implement these kinds of safeguards may still result in a
   scenario where the local endpoint exhausts its memory.

6.  References

6.1.  Normative References

   [HTTP-p1]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              draft-ietf-httpbis-p1-messaging-25 (work in progress),
              November 2013.

   [HTTP2]    Belshe, M., Peon, R., Thomson, M., Ed., and A. Melnikov,
              Ed.,

   A particular care should be used for the maximum size of the header
   table.  While an endpoint can fully control the maximum size of its
   header table for the decoding size, by using
   SETTINGS_HEADER_TABLE_SIZE, the maximum size of the encoding size is
   controlled by the remote peer.  The endpoint should check the
   SETTINGS_HEADER_TABLE_SIZE defined by the remote peer, and decrease
   the maximum size for the encoding size if needed.

6.  Acknowledgements

   This document includes substantial editorial contributions from the
   following individuals: Mike Bishop, Jeff Pinner, Julian Reschke,
   Martin Thomson.

7.  References

7.1.  Normative References

   [HTTP-p1]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing", draft-
              ietf-httpbis-p1-messaging-26 (work in progress), February
              2014.

   [HTTP2]    Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol version 2.0",
              draft-ietf-httpbis-http2-08 2", draft-ietf-httpbis-http2-10
              (work in progress),
              November 2013.

6.2. February 2014.

7.2.  Informative References

   [CANON]    Schwartz, E. and B. Kallick, "Generating a canonical
              prefix encoding", Communications of the ACM Volume 7 Issue
              3, pp. 166-169, March 1964,
              <http://dl.acm.org/citation.cfm?id=363991>.

   [CRIME]    Rizzo, J. and T. Duong, "The Crime CRIME Attack", September
              2012, <https://docs.google.com/a/twist.com/
              presentation/d/ <https://docs.google.com/a/twist.com/presentation/d/
              11eBmGiHbYcHR9gL5nDyZChu_-lCa2GizeuOfaLU2HOU/
              edit#slide=id.g1eb6c1b5_3_6>.

   [DEFLATE]  Deutsch, P., "DEFLATE Compressed Data Format Specification
              version 1.3", RFC 1951, May 1996.

   [HUFF]     Huffman, D., "A Method for the Construction of Minimum
              Redundancy Codes", Proceedings of the Institute of Radio
              Engineers Volume 40, Number 9, pp. 1098-1101, September
              1952, <http://ieeexplore.ieee.org/xpl/
              articleDetails.jsp?arnumber=4051119>.

   [PERF1]    Belshe, M., "IETF83: SPDY and What to Consider for
              HTTP/2.0", HTTP/
              2.0", March 2012, <http://www.ietf.org/proceedings/
              83/slides/slides-83-httpbis-3>. <http://www.ietf.org/proceedings/83/
              slides/slides-83-httpbis-3>.

   [PERF2]    McManus, P., "SPDY: What I Like About You", September
              2011, <http://bitsup.blogspot.com/2011/09/
              spdy-what-i-like-about-you.html>. <http://bitsup.blogspot.com/2011/09/spdy-what-i
              -like-about-you.html>.

   [SPDY]     Belshe, M. and R. Peon, "SPDY Protocol",
              draft-mbelshe-httpbis-spdy-00 draft-mbelshe-
              httpbis-spdy-00 (work in progress), February 2012.

URIs

   [1]  <http://en.wikipedia.org/wiki/Variable-length_quantity>

Appendix A.  Change Log (to be removed by RFC Editor before publication

A.1.  Since draft-ietf-httpbis-header-compression-05

   o  Regenerated examples.

   o  Only one Huffman table for requests and responses.

   o  Added maximum size for header table, independent of
      SETTINGS_HEADER_TABLE_SIZE.

   o  Added pseudo-code for integer decoding.

   o  Improved examples (removing unnecessary removals).

A.2.  Since draft-ietf-httpbis-header-compression-04

   o  Updated examples: take into account changes in the spec, and show
      more features.

   o  Use 'octet' everywhere instead of having both 'byte' and 'octet'.

   o  Added reference set emptying.

   o  Editorial changes and clarifications.

   o  Added "host" header to the static table.

   o  Ordering for list of values (either NULL- or comma-separated).

A.2.

A.3.  Since draft-ietf-httpbis-header-compression-03

   o  A large number of editorial changes; changed the description of
      evicting/adding new entries.

   o  Removed substitution indexing

   o  Changed 'initial headers' to 'static headers', as per issue #258

   o  Merged 'request' and 'response' static headers, as per issue #259

   o  Changed text to indicate that new headers are added at index 0 and
      expire from the largest index, as per issue #233

A.3.

A.4.  Since draft-ietf-httpbis-header-compression-02

   o  Corrected error in integer encoding pseudocode.

A.4.

A.5.  Since draft-ietf-httpbis-header-compression-01

   o  Refactored of Header Encoding Section: split definitions and
      processing rule.

   o  Backward incompatible change: Updated reference set management as
      per issue #214.  This changes how the interaction between the
      reference set and eviction works.  This also changes the working
      of the reference set in some specific cases.

   o  Backward incompatible change: modified initial header list, as per
      issue #188.

   o  Added example of 32 octets entry structure (issue #191).

   o  Added Header Set Completion section.  Reflowed some text.
      Clarified some writing which was akward.  Added text about
      duplicate header entry encoding.  Clarified some language w.r.t
      Header Set.  Changed x-my-header to mynewheader.  Added text in
      the HeaderEmission section indicating that the application may
      also be able to free up memory more quickly.  Added information in
      Security Considerations section.

A.5.

A.6.  Since draft-ietf-httpbis-header-compression-01 draft-ietf-httpbis-header-compression-00

      Fixed bug/omission in integer representation algorithm.

      Changed the document title.

      Header matching text rewritten.

      Changed the definition of header emission.

      Changed the name of the setting which dictates how much memory the
      compression context should use.

      Removed "specific use cases" section

      Corrected erroneous statement about what index can be contained in
      one octet

      Added descriptions of opcodes

      Removed security claims from introduction.

Appendix B.  Static Table

   The static table consists of an unchangeable ordered list of (name,
   value) pairs.  The first entry in the table is always represented by
   the index len(header table)+1, and the last entry in the table is
   represented by the index len(header table)+len(static table).

   [[anchor9: The

   [[The ordering of these tables is currently arbitrary.  The tables in
   this section should be updated and ordered such that the table
   entries with the smallest indices are those which, based on a
   statistical analysis of the frequency of use weighted by size,
   achieve the largest decrease in octets transmitted subject to HTTP
   2.0 2
   header field rules (like removal of some header fields).  This set of
   header fields is currently very likely incomplete, and should be made complete.]]
   complete.  ]]

   The following table lists the pre-defined header fields that make-up
   the static header table.

          +-------+-----------------------------+--------------+
          | Index | Header Name                 | Header Value |
          +-------+-----------------------------+--------------+
          | 1     | :authority                  |              |
          | 2     | :method                     | GET          |
          | 3     | :method                     | POST         |
          | 4     | :path                       | /            |
          | 5     | :path                       | /index.html  |
          | 6     | :scheme                     | http         |
          | 7     | :scheme                     | https        |
          | 8     | :status                     | 200          |
          | 9     | :status                     | 500          |
          | 10    | :status                     | 404          |
          | 11    | :status                     | 403          |
          | 12    | :status                     | 400          |
          | 13    | :status                     | 401          |
          | 14    | accept-charset              |              |
          | 15    | accept-encoding             |              |
          | 16    | accept-language             |              |
          | 17    | accept-ranges               |              |
          | 18    | accept                      |              |
          | 19    | access-control-allow-origin |              |
          | 20    | age                         |              |
          | 21    | allow                       |              |
          | 22    | authorization               |              |
          | 23    | cache-control               |              |
          | 24    | content-disposition         |              |
          | 25    | content-encoding            |              |
          | 26    | content-language            |              |
          | 27    | content-length              |              |
          | 28    | content-location            |              |
          | 29    | content-range               |              |
          | 30    | content-type                |              |
          | 31    | cookie                      |              |
          | 32    | date                        |              |
          | 33    | etag                        |              |
          | 34    | expect                      |              |
          | 35    | expires                     |              |
          | 36    | from                        |              |
          | 37    | host                        |              |
          | 38    | if-match                    |              |
          | 39    | if-modified-since           |              |
          | 40    | if-none-match               |              |
          | 41    | if-range                    |              |
          | 42    | if-unmodified-since         |              |
          | 43    | last-modified               |              |
          | 44    | link                        |              |
          | 45    | location                    |              |
          | 46    | max-forwards                |              |
          | 47    | proxy-authenticate          |              |
          | 48    | proxy-authorization         |              |
          | 49    | range                       |              |
          | 50    | referer                     |              |
          | 51    | refresh                     |              |
          | 52    | retry-after                 |              |
          | 53    | server                      |              |
          | 54    | set-cookie                  |              |
          | 55    | strict-transport-security   |              |
          | 56    | transfer-encoding           |              |
          | 57    | user-agent                  |              |
          | 58    | vary                        |              |
          | 59    | via                         |              |
          | 60    | www-authenticate            |              |
          +-------+-----------------------------+--------------+

                       Table 1: Static Table Entries

   The table give the index of each entry in the static table.  The full
   index of each entry, to be used for encoding a reference to this
   entry, is computed by adding the number of entries in the header
   table to this index.

Appendix C.  Huffman Codes For Requests

   The following Huffman codes are used when encoding string literals in
   the client to server direction.

   [[anchor10: This literals.

   [[This table may need to will be regenerated.]] regenerated.  ]]

              aligned                                  aligned
               to                            len        to     len
               MSB                            in        LSB     in
        sym    as bits                       bits      as hex  bits
       (  0) |11111111|11111111|11110111|010 [27]      7ffffba [27]
       (  1) |11111111|11111111|11110111|011 [27]      7ffffbb [27]
       (  2) |11111111|11111111|11110111|100 [27]      7ffffbc [27]
       (  3) |11111111|11111111|11110111|101 [27]      7ffffbd [27]
       (  4) |11111111|11111111|11110111|110 [27]      7ffffbe [27]
       (  5) |11111111|11111111|11110111|111 [27]      7ffffbf [27]
       (  6) |11111111|11111111|11111000|000 [27]      7ffffc0 [27]
       (  7) |11111111|11111111|11111000|001 [27]      7ffffc1 [27]
       (  8) |11111111|11111111|11111000|010 [27]      7ffffc2 [27]
       (  9) |11111111|11111111|11111000|011 [27]      7ffffc3 [27]
       ( 10) |11111111|11111111|11111000|100 [27]      7ffffc4 [27]
       ( 11) |11111111|11111111|11111000|101 [27]      7ffffc5 [27]
       ( 12) |11111111|11111111|11111000|110 [27]      7ffffc6 [27]
       ( 13) |11111111|11111111|11111000|111 [27]      7ffffc7 [27]
       ( 14) |11111111|11111111|11111001|000 [27]      7ffffc8 [27]
       ( 15) |11111111|11111111|11111001|001 [27]      7ffffc9 [27]
       ( 16) |11111111|11111111|11111001|010 [27]      7ffffca [27]
       ( 17) |11111111|11111111|11111001|011 [27]      7ffffcb [27]
       ( 18) |11111111|11111111|11111001|100 [27]      7ffffcc [27]
       ( 19) |11111111|11111111|11111001|101 [27]      7ffffcd [27]
       ( 20) |11111111|11111111|11111001|110 [27]      7ffffce [27]
       ( 21) |11111111|11111111|11111001|111 [27]      7ffffcf [27]
       ( 22) |11111111|11111111|11111010|000 [27]      7ffffd0 [27]
       ( 23) |11111111|11111111|11111010|001 [27]      7ffffd1 [27]
       ( 24) |11111111|11111111|11111010|010 [27]      7ffffd2 [27]
       ( 25) |11111111|11111111|11111010|011 [27]      7ffffd3 [27]
       ( 26) |11111111|11111111|11111010|100 [27]      7ffffd4 [27]
       ( 27) |11111111|11111111|11111010|101 [27]      7ffffd5 [27]
       ( 28) |11111111|11111111|11111010|110 [27]      7ffffd6 [27]
       ( 29) |11111111|11111111|11111010|111 [27]      7ffffd7 [27]
       ( 30) |11111111|11111111|11111011|000 [27]      7ffffd8 [27]
       ( 31) |11111111|11111111|11111011|001 [27]      7ffffd9 [27]
   ' ' ( 32) |11101000| [8]                                  e8 [8]
   '!' ( 33) |11111111|1100 [12]                           ffc [12]
   '"' ( 34) |11111111|111010 [14]                        3ffa [14]
   '#' ( 35) |11111111|1111100 [15]                       7ffc [15]
   '$' ( 36) |11111111|1111101 [15]                       7ffd [15]
   '%' ( 37) |100100 [6]                                     24 [6]
   '&' ( 38) |1101110 [7]                                    6e [7]
   ''' ( 39) |11111111|1111110 [15]                       7ffe [15]
   '(' ( 40) |11111111|010 [11]                            7fa [11]
   ')' ( 41) |11111111|011 [11]                            7fb [11]
   '*' ( 42) |11111110|10 [10]                             3fa [10]
   '+' ( 43) |11111111|100 [11]                            7fc [11]
   ',' ( 44) |11101001| [8]                                  e9 [8]
   '-' ( 45) |100101 [6]                                     25 [6]
   '.' ( 46) |00100 [5]                                       4 [5]
   '/' ( 47) |0000 [4]                                        0 [4]
   '0' ( 48) |00101 [5]                                       5 [5]
   '1' ( 49) |00110 [5]                                       6 [5]
   '2' ( 50) |00111 [5]                                       7 [5]
   '3' ( 51) |100110 [6]                                     26 [6]
   '4' ( 52) |100111 [6]                                     27 [6]
   '5' ( 53) |101000 [6]                                     28 [6]
   '6' ( 54) |101001 [6]                                     29 [6]
   '7' ( 55) |101010 [6]                                     2a [6]
   '8' ( 56) |101011 [6]                                     2b [6]
   '9' ( 57) |101100 [6]                                     2c [6]
   ':' ( 58) |11110110|0 [9]                                1ec [9]
   ';' ( 59) |11101010| [8]                                  ea [8]
   '<' ( 60) |11111111|11111111|10 [18]                  3fffe [18]
   '=' ( 61) |101101 [6]                                     2d [6]
   '>' ( 62) |11111111|11111110|0 [17]                   1fffc [17]
   '?' ( 63) |11110110|1 [9]                                1ed [9]
   '@' ( 64) |11111111|111011 [14]                        3ffb [14]
   'A' ( 65) |1101111 [7]                                    6f [7]
   'B' ( 66) |11101011| [8]                                  eb [8]
   'C' ( 67) |11101100| [8]                                  ec [8]
   'D' ( 68) |11101101| [8]                                  ed [8]
   'E' ( 69) |11101110| [8]                                  ee [8]
   'F' ( 70) |1110000 [7]                                    70 [7]
   'G' ( 71) |11110111|0 [9]                                1ee [9]
   'H' ( 72) |11110111|1 [9]                                1ef [9]
   'I' ( 73) |11111000|0 [9]                                1f0 [9]
   'J' ( 74) |11111000|1 [9]                                1f1 [9]
   'K' ( 75) |11111110|11 [10]                             3fb [10]
   'L' ( 76) |11111001|0 [9]                                1f2 [9]
   'M' ( 77) |11101111| [8]                                  ef [8]
   'N' ( 78) |11111001|1 [9]                                1f3 [9]
   'O' ( 79) |11111010|0 [9]                                1f4 [9]
   'P' ( 80) |11111010|1 [9]                                1f5 [9]
   'Q' ( 81) |11111011|0 [9]                                1f6 [9]
   'R' ( 82) |11111011|1 [9]                                1f7 [9]
   'S' ( 83) |11110000| [8]                                  f0 [8]
   'T' ( 84) |11110001| [8]                                  f1 [8]
   'U' ( 85) |11111100|0 [9]                                1f8 [9]
   'V' ( 86) |11111100|1 [9]                                1f9 [9]
   'W' ( 87) |11111101|0 [9]                                1fa [9]
   'X' ( 88) |11111101|1 [9]                                1fb [9]
   'Y' ( 89) |11111110|0 [9]                                1fc [9]
   'Z' ( 90) |11111111|00 [10]                             3fc [10]
   '[' ( 91) |11111111|111100 [14]                        3ffc [14]
   '\' ( 92) |11111111|11111111|11111011|010 [27]      7ffffda [27]
   ']' ( 93) |11111111|11100 [13]                         1ffc [13]
   '^' ( 94) |11111111|111101 [14]                        3ffd [14]
   '_' ( 95) |101110 [6]                                     2e [6]
   '`' ( 96) |11111111|11111111|110 [19]                 7fffe [19]
   'a' ( 97) |01000 [5]                                       8 [5]
   'b' ( 98) |101111 [6]                                     2f [6]
   'c' ( 99) |01001 [5]                                       9 [5]
   'd' (100) |110000 [6]                                     30 [6]
   'e' (101) |0001 [4]                                        1 [4]
   'f' (102) |110001 [6]                                     31 [6]
   'g' (103) |110010 [6]                                     32 [6]
   'h' (104) |110011 [6]                                     33 [6]
   'i' (105) |01010 [5]                                       a [5]
   'j' (106) |1110001 [7]                                    71 [7]
   'k' (107) |1110010 [7]                                    72 [7]
   'l' (108) |01011 [5]                                       b [5]
   'm' (109) |110100 [6]                                     34 [6]
   'n' (110) |01100 [5]                                       c [5]
   'o' (111) |01101 [5]                                       d [5]
   'p' (112) |01110 [5]                                       e [5]
   'q' (113) |11110010| [8]                                  f2 [8]
   'r' (114) |01111 [5]                                       f [5]
   's' (115) |10000 [5]                                      10 [5]
   't' (116) |10001 [5]                                      11 [5]
   'u' (117) |110101 [6]                                     35 [6]
   'v' (118) |1110011 [7]                                    73 [7]
   'w' (119) |110110 [6]                                     36 [6]
   'x' (120) |11110011| [8]                                  f3 [8]
   'y' (121) |11110100| [8]                                  f4 [8]
   'z' (122) |11110101| [8]                                  f5 [8]
   '{' (123) |11111111|11111110|1 [17]                   1fffd [17]
   '|' (124) |11111111|101 [11]                            7fd [11]
   '}' (125) |11111111|11111111|0 [17]                   1fffe [17]
   '~' (126) |11111111|1101 [12]                           ffd [12]
       (127) |11111111|11111111|11111011|011 [27]      7ffffdb [27]
       (128) |11111111|11111111|11111011|100 [27]      7ffffdc [27]
       (129) |11111111|11111111|11111011|101 [27]      7ffffdd [27]
       (130) |11111111|11111111|11111011|110 [27]      7ffffde [27]
       (131) |11111111|11111111|11111011|111 [27]      7ffffdf [27]
       (132) |11111111|11111111|11111100|000 [27]      7ffffe0 [27]
       (133) |11111111|11111111|11111100|001 [27]      7ffffe1 [27]
       (134) |11111111|11111111|11111100|010 [27]      7ffffe2 [27]
       (135) |11111111|11111111|11111100|011 [27]      7ffffe3 [27]
       (136) |11111111|11111111|11111100|100 [27]      7ffffe4 [27]
       (137) |11111111|11111111|11111100|101 [27]      7ffffe5 [27]
       (138) |11111111|11111111|11111100|110 [27]      7ffffe6 [27]
       (139) |11111111|11111111|11111100|111 [27]      7ffffe7 [27]
       (140) |11111111|11111111|11111101|000 [27]      7ffffe8 [27]
       (141) |11111111|11111111|11111101|001 [27]      7ffffe9 [27]
       (142) |11111111|11111111|11111101|010 [27]      7ffffea [27]
       (143) |11111111|11111111|11111101|011 [27]      7ffffeb [27]
       (144) |11111111|11111111|11111101|100 [27]      7ffffec [27]
       (145) |11111111|11111111|11111101|101 [27]      7ffffed [27]
       (146) |11111111|11111111|11111101|110 [27]      7ffffee [27]
       (147) |11111111|11111111|11111101|111 [27]      7ffffef [27]
       (148) |11111111|11111111|11111110|000 [27]      7fffff0 [27]
       (149) |11111111|11111111|11111110|001 [27]      7fffff1 [27]
       (150) |11111111|11111111|11111110|010 [27]      7fffff2 [27]
       (151) |11111111|11111111|11111110|011 [27]      7fffff3 [27]
       (152) |11111111|11111111|11111110|100 [27]      7fffff4 [27]
       (153) |11111111|11111111|11111110|101 [27]      7fffff5 [27]
       (154) |11111111|11111111|11111110|110 [27]      7fffff6 [27]
       (155) |11111111|11111111|11111110|111 [27]      7fffff7 [27]
       (156) |11111111|11111111|11111111|000 [27]      7fffff8 [27]
       (157) |11111111|11111111|11111111|001 [27]      7fffff9 [27]
       (158) |11111111|11111111|11111111|010 [27]      7fffffa [27]
       (159) |11111111|11111111|11111111|011 [27]      7fffffb [27]
       (160) |11111111|11111111|11111111|100 [27]      7fffffc [27]
       (161) |11111111|11111111|11111111|101 [27]      7fffffd [27]
       (162) |11111111|11111111|11111111|110 [27]      7fffffe [27]
       (163) |11111111|11111111|11111111|111 [27]      7ffffff [27]
       (164) |11111111|11111111|11100000|00 [26]       3ffff80 [26]
       (165) |11111111|11111111|11100000|01 [26]       3ffff81 [26]
       (166) |11111111|11111111|11100000|10 [26]       3ffff82 [26]
       (167) |11111111|11111111|11100000|11 [26]       3ffff83 [26]
       (168) |11111111|11111111|11100001|00 [26]       3ffff84 [26]
       (169) |11111111|11111111|11100001|01 [26]       3ffff85 [26]
       (170) |11111111|11111111|11100001|10 [26]       3ffff86 [26]
       (171) |11111111|11111111|11100001|11 [26]       3ffff87 [26]
       (172) |11111111|11111111|11100010|00 [26]       3ffff88 [26]
       (173) |11111111|11111111|11100010|01 [26]       3ffff89 [26]
       (174) |11111111|11111111|11100010|10 [26]       3ffff8a [26]
       (175) |11111111|11111111|11100010|11 [26]       3ffff8b [26]
       (176) |11111111|11111111|11100011|00 [26]       3ffff8c [26]
       (177) |11111111|11111111|11100011|01 [26]       3ffff8d [26]
       (178) |11111111|11111111|11100011|10 [26]       3ffff8e [26]
       (179) |11111111|11111111|11100011|11 [26]       3ffff8f [26]
       (180) |11111111|11111111|11100100|00 [26]       3ffff90 [26]
       (181) |11111111|11111111|11100100|01 [26]       3ffff91 [26]
       (182) |11111111|11111111|11100100|10 [26]       3ffff92 [26]
       (183) |11111111|11111111|11100100|11 [26]       3ffff93 [26]
       (184) |11111111|11111111|11100101|00 [26]       3ffff94 [26]
       (185) |11111111|11111111|11100101|01 [26]       3ffff95 [26]
       (186) |11111111|11111111|11100101|10 [26]       3ffff96 [26]
       (187) |11111111|11111111|11100101|11 [26]       3ffff97 [26]
       (188) |11111111|11111111|11100110|00 [26]       3ffff98 [26]
       (189) |11111111|11111111|11100110|01 [26]       3ffff99 [26]
       (190) |11111111|11111111|11100110|10 [26]       3ffff9a [26]
       (191) |11111111|11111111|11100110|11 [26]       3ffff9b [26]
       (192) |11111111|11111111|11100111|00 [26]       3ffff9c [26]
       (193) |11111111|11111111|11100111|01 [26]       3ffff9d [26]
       (194) |11111111|11111111|11100111|10 [26]       3ffff9e [26]
       (195) |11111111|11111111|11100111|11 [26]       3ffff9f [26]
       (196) |11111111|11111111|11101000|00 [26]       3ffffa0 [26]
       (197) |11111111|11111111|11101000|01 [26]       3ffffa1 [26]
       (198) |11111111|11111111|11101000|10 [26]       3ffffa2 [26]
       (199) |11111111|11111111|11101000|11 [26]       3ffffa3 [26]
       (200) |11111111|11111111|11101001|00 [26]       3ffffa4 [26]
       (201) |11111111|11111111|11101001|01 [26]       3ffffa5 [26]
       (202) |11111111|11111111|11101001|10 [26]       3ffffa6 [26]
       (203) |11111111|11111111|11101001|11 [26]       3ffffa7 [26]
       (204) |11111111|11111111|11101010|00 [26]       3ffffa8 [26]
       (205) |11111111|11111111|11101010|01 [26]       3ffffa9 [26]
       (206) |11111111|11111111|11101010|10 [26]       3ffffaa [26]
       (207) |11111111|11111111|11101010|11 [26]       3ffffab [26]
       (208) |11111111|11111111|11101011|00 [26]       3ffffac [26]
       (209) |11111111|11111111|11101011|01 [26]       3ffffad [26]
       (210) |11111111|11111111|11101011|10 [26]       3ffffae [26]
       (211) |11111111|11111111|11101011|11 [26]       3ffffaf [26]
       (212) |11111111|11111111|11101100|00 [26]       3ffffb0 [26]
       (213) |11111111|11111111|11101100|01 [26]       3ffffb1 [26]
       (214) |11111111|11111111|11101100|10 [26]       3ffffb2 [26]
       (215) |11111111|11111111|11101100|11 [26]       3ffffb3 [26]
       (216) |11111111|11111111|11101101|00 [26]       3ffffb4 [26]
       (217) |11111111|11111111|11101101|01 [26]       3ffffb5 [26]
       (218) |11111111|11111111|11101101|10 [26]       3ffffb6 [26]
       (219) |11111111|11111111|11101101|11 [26]       3ffffb7 [26]
       (220) |11111111|11111111|11101110|00 [26]       3ffffb8 [26]
       (221) |11111111|11111111|11101110|01 [26]       3ffffb9 [26]
       (222) |11111111|11111111|11101110|10 [26]       3ffffba [26]
       (223) |11111111|11111111|11101110|11 [26]       3ffffbb [26]
       (224) |11111111|11111111|11101111|00 [26]       3ffffbc [26]
       (225) |11111111|11111111|11101111|01 [26]       3ffffbd [26]
       (226) |11111111|11111111|11101111|10 [26]       3ffffbe [26]
       (227) |11111111|11111111|11101111|11 [26]       3ffffbf [26]
       (228) |11111111|11111111|11110000|00 [26]       3ffffc0 [26]
       (229) |11111111|11111111|11110000|01 [26]       3ffffc1 [26]
       (230) |11111111|11111111|11110000|10 [26]       3ffffc2 [26]
       (231) |11111111|11111111|11110000|11 [26]       3ffffc3 [26]
       (232) |11111111|11111111|11110001|00 [26]       3ffffc4 [26]
       (233) |11111111|11111111|11110001|01 [26]       3ffffc5 [26]
       (234) |11111111|11111111|11110001|10 [26]       3ffffc6 [26]
       (235) |11111111|11111111|11110001|11 [26]       3ffffc7 [26]
       (236) |11111111|11111111|11110010|00 [26]       3ffffc8 [26]
       (237) |11111111|11111111|11110010|01 [26]       3ffffc9 [26]
       (238) |11111111|11111111|11110010|10 [26]       3ffffca [26]
       (239) |11111111|11111111|11110010|11 [26]       3ffffcb [26]
       (240) |11111111|11111111|11110011|00 [26]       3ffffcc [26]
       (241) |11111111|11111111|11110011|01 [26]       3ffffcd [26]
       (242) |11111111|11111111|11110011|10 [26]       3ffffce [26]
       (243) |11111111|11111111|11110011|11 [26]       3ffffcf [26]
       (244) |11111111|11111111|11110100|00 [26]       3ffffd0 [26]
       (245) |11111111|11111111|11110100|01 [26]       3ffffd1 [26]
       (246) |11111111|11111111|11110100|10 [26]       3ffffd2 [26]
       (247) |11111111|11111111|11110100|11 [26]       3ffffd3 [26]
       (248) |11111111|11111111|11110101|00 [26]       3ffffd4 [26]
       (249) |11111111|11111111|11110101|01 [26]       3ffffd5 [26]
       (250) |11111111|11111111|11110101|10 [26]       3ffffd6 [26]
       (251) |11111111|11111111|11110101|11 [26]       3ffffd7 [26]
       (252) |11111111|11111111|11110110|00 [26]       3ffffd8 [26]
         (253) |11111111|11111111|11110110|01 [26]       3ffffd9 [26]
         (254) |11111111|11111111|11110110|10 [26]       3ffffda [26]
         (255) |11111111|11111111|11110110|11 [26]       3ffffdb [26]
     EOS (256) |11111111|11111111|11110111|00 [26]       3ffffdc [26]

Appendix D.  Huffman Codes for Responses

   The following Huffman codes are used when encoding string literals in
   the server to client direction.  These codes apply for both responses
   to client requests and for push-promises.

   [[anchor11: This table may need to be regenerated.]]

                aligned                                  aligned
                 to                           len         to     len
                 MSB                           in         LSB     in
          sym    as bits                      bits       as hex  bits
         (  0) |11111111|11111111|11011110|0 [25]        1ffffbc [25]
         (  1) |11111111|11111111|11011110|1 [25]        1ffffbd [25]
         (  2) |11111111|11111111|11011111|0 [25]        1ffffbe [25]
         (  3) |11111111|11111111|11011111|1 [25]        1ffffbf [25]
         (  4) |11111111|11111111|11100000|0 [25]        1ffffc0 [25]
         (  5) |11111111|11111111|11100000|1 [25]        1ffffc1 [25]
         (  6) |11111111|11111111|11100001|0 [25]        1ffffc2 [25]
         (  7) |11111111|11111111|11100001|1 [25]        1ffffc3 [25]
         (  8) |11111111|11111111|11100010|0 [25]        1ffffc4 [25]
         (  9) |11111111|11111111|11100010|1 [25]        1ffffc5 [25]
         ( 10) |11111111|11111111|11100011|0 [25]        1ffffc6 [25]
         ( 11) |11111111|11111111|11100011|1 [25]        1ffffc7 [25]
         ( 12) |11111111|11111111|11100100|0 [25]        1ffffc8 [25]
         ( 13) |11111111|11111111|11100100|1 [25]        1ffffc9 [25]
         ( 14) |11111111|11111111|11100101|0 [25]        1ffffca [25]
         ( 15) |11111111|11111111|11100101|1 [25]        1ffffcb [25]
         ( 16) |11111111|11111111|11100110|0 [25]        1ffffcc [25]
         ( 17) |11111111|11111111|11100110|1 [25]        1ffffcd [25]
         ( 18) |11111111|11111111|11100111|0 [25]        1ffffce [25]
         ( 19) |11111111|11111111|11100111|1 [25]        1ffffcf [25]
         ( 20) |11111111|11111111|11101000|0 [25]        1ffffd0 [25]
         ( 21) |11111111|11111111|11101000|1 [25]        1ffffd1 [25]
         ( 22) |11111111|11111111|11101001|0 [25]        1ffffd2 [25]
         ( 23) |11111111|11111111|11101001|1 [25]        1ffffd3 [25]
         ( 24) |11111111|11111111|11101010|0 [25]        1ffffd4 [25]
         ( 25) |11111111|11111111|11101010|1 [25]        1ffffd5 [25]
         ( 26) |11111111|11111111|11101011|0 [25]        1ffffd6 [25]
         ( 27) |11111111|11111111|11101011|1 [25]        1ffffd7 [25]
         ( 28) |11111111|11111111|11101100|0 [25]        1ffffd8 [25]
         ( 29) |11111111|11111111|11101100|1 [25]        1ffffd9 [25]
         ( 30) |11111111|11111111|11101101|0 [25]        1ffffda [25]
         ( 31) |11111111|11111111|11101101|1 [25]        1ffffdb [25]
     ' ' ( 32) |0000 [4]                                        0 [4]
     '!' ( 33) |11111111|1010 [12]                           ffa [12]
     '"' ( 34) |1101010 [7]                                    6a [7]
     '#' ( 35) |11111111|11010 [13]                         1ffa [13]
     '$' ( 36) |11111111|111100 [14]                        3ffc [14]
     '%' ( 37) |11110110|0 [9]                                1ec [9]
     '&' ( 38) |11111110|00 [10]                             3f8 [10]
     ''' ( 39) |11111111|11011 [13]                         1ffb [13]
     '(' ( 40) |11110110|1 [9]                                1ed [9]
     ')' ( 41) |11110111|0 [9]                                1ee [9]
     '*' ( 42) |11111111|1011 [12]                           ffb [12]
     '+' ( 43) |11111111|010 [11]                            7fa [11]
     ',' ( 44) |100010 [6]                                     22 [6]
     '-' ( 45) |100011 [6]                                     23 [6]
     '.' ( 46) |100100 [6]                                     24 [6]
     '/' ( 47) |1101011 [7]                                    6b [7]
     '0' ( 48) |0001 [4]                                        1 [4]
     '1' ( 49) |0010 [4]                                        2 [4]
     '2' ( 50) |0011 [4]                                        3 [4]
     '3' ( 51) |01000 [5]                                       8 [5]
     '4' ( 52) |01001 [5]                                       9 [5]
     '5' ( 53) |01010 [5]                                       a [5]
     '6' ( 54) |100101 [6]                                     25 [6]
     '7' ( 55) |100110 [6]                                     26 [6]
     '8' ( 56) |01011 [5]                                       b [5]
     '9' ( 57) |01100 [5]                                       c [5]
     ':' ( 58) |01101 [5]                                       d [5]
     ';' ( 59) |11110111|1 [9]                                1ef [9]
     '<' ( 60) |11111111|11111010| [16]                     fffa [16]
     '=' ( 61) |1101100 [7]                                    6c [7]
     '>' ( 62) |11111111|11100 [13]                         1ffc [13]
     '?' ( 63) |11111111|1100 [12]                           ffc [12]
     '@' ( 64) |11111111|11111011| [16]                     fffb [16]
     'A' ( 65) |1101101 [7]                                    6d [7]
     'B' ( 66) |11101010| [8]                                  ea [8]
     'C' ( 67) |11101011| [8]                                  eb [8]
     'D' ( 68) |11101100| [8]                                  ec [8]
     'E' ( 69) |11101101| [8]                                  ed [8]
     'F' ( 70) |11101110| [8]                                  ee [8]
     'G' ( 71) |100111 [6]                                     27 [6]
     'H' ( 72) |11111000|0 [9]                                1f0 [9]
     'I' ( 73) |11101111| [8]                                  ef [8]
     'J' ( 74) |11110000| [8]                                  f0 [8]
     'K' ( 75) |11111110|01 [10]                             3f9 [10]
     'L' ( 76) |11111000|1 [9]                                1f1 [9]
     'M' ( 77) |101000 [6]                                     28 [6]
     'N' ( 78) |11110001| [8]                                  f1 [8]
     'O' ( 79) |11110010| [8]                                  f2 [8]
     'P' ( 80) |11111001|0 [9]                                1f2 [9]
     'Q' ( 81) |11111110|10 [10]                             3fa [10]
     'R' ( 82) |11111001|1 [9]                                1f3 [9]
     'S' ( 83) |101001 [6]                                     29 [6]
     'T' ( 84) |01110 [5]                                       e [5]
     'U' ( 85) |11111010|0 [9]                                1f4 [9]
     'V' ( 86) |11111010|1 [9]                                1f5 [9]
     'W' ( 87) |11110011| [8]                                  f3 [8]
     'X' ( 88) |11111110|11 [10]                             3fb [10]
     'Y' ( 89) |11111011|0 [9]                                1f6 [9]
     'Z' ( 90) |11111111|00 [10]                             3fc [10]
     '[' ( 91) |11111111|011 [11]                            7fb [11]
     '\' ( 92) |11111111|11101 [13]                         1ffd [13]
     ']' ( 93) |11111111|100 [11]                            7fc [11]
     '^' ( 94) |11111111|1111100 [15]                       7ffc [15]
     '_' ( 95) |11111011|1 [9]                                1f7 [9]
     '`' ( 96) |11111111|11111111|0 [17]                   1fffe [17]
     'a' ( 97) |01111 [5]                                       f [5]
     'b' ( 98) |1101110 [7]                                    6e [7]
     'c' ( 99) |101010 [6]                                     2a [6]
     'd' (100) |101011 [6]                                     2b [6]
     'e' (101) |10000 [5]                                      10 [5]
     'f' (102) |1101111 [7]                                    6f [7]
     'g' (103) |1110000 [7]                                    70 [7]
     'h' (104) |1110001 [7]                                    71 [7]
     'i' (105) |101100 [6]                                     2c [6]
     'j' (106) |11111100|0 [9]                                1f8 [9]
     'k' (107) |11111100|1 [9]                                1f9 [9]
     'l' (108) |1110010 [7]                                    72 [7]
     'm' (109) |101101 [6]                                     2d [6]
     'n' (110) |101110 [6]                                     2e [6]
     'o' (111) |101111 [6]                                     2f [6]
     'p' (112) |110000 [6]                                     30 [6]
     'q' (113) |11111101|0 [9]                                1fa [9]
     'r' (114) |110001 [6]                                     31 [6]
     's' (115) |110010 [6]                                     32 [6]
     't' (116) |110011 [6]                                     33 [6]
     'u' (117) |110100 [6]                                     34 [6]
     'v' (118) |1110011 [7]                                    73 [7]
     'w' (119) |11110100| [8]                                  f4 [8]
     'x' (120) |1110100 [7]                                    74 [7]
     'y' (121) |11110101| [8]                                  f5 [8]
     'z' (122) |11111101|1 [9]                                1fb [9]
     '{' (123) |11111111|11111100| [16]                     fffc [16]
     '|' (124) |11111111|111101 [14]                        3ffd [14]
     '}' (125) |11111111|11111101| [16]                     fffd [16]
     '~' (126) |11111111|11111110| [16]                     fffe [16]
         (127) |11111111|11111111|11101110|0 [25]        1ffffdc [25]
         (128) |11111111|11111111|11101110|1 [25]        1ffffdd [25]
         (129) |11111111|11111111|11101111|0 [25]        1ffffde [25]
         (130) |11111111|11111111|11101111|1 [25]        1ffffdf [25]
         (131) |11111111|11111111|11110000|0 [25]        1ffffe0 [25]
         (132) |11111111|11111111|11110000|1 [25]        1ffffe1 [25]
         (133) |11111111|11111111|11110001|0 [25]        1ffffe2 [25]
         (134) |11111111|11111111|11110001|1 [25]        1ffffe3 [25]
         (135) |11111111|11111111|11110010|0 [25]        1ffffe4 [25]
         (136) |11111111|11111111|11110010|1 [25]        1ffffe5 [25]
         (137) |11111111|11111111|11110011|0 [25]        1ffffe6 [25]
         (138) |11111111|11111111|11110011|1 [25]        1ffffe7 [25]
         (139) |11111111|11111111|11110100|0 [25]        1ffffe8 [25]
         (140) |11111111|11111111|11110100|1 [25]        1ffffe9 [25]
         (141) |11111111|11111111|11110101|0 [25]        1ffffea [25]
         (142) |11111111|11111111|11110101|1 [25]        1ffffeb [25]
         (143) |11111111|11111111|11110110|0 [25]        1ffffec [25]
         (144) |11111111|11111111|11110110|1 [25]        1ffffed [25]
         (145) |11111111|11111111|11110111|0 [25]        1ffffee [25]
         (146) |11111111|11111111|11110111|1 [25]        1ffffef [25]
         (147) |11111111|11111111|11111000|0 [25]        1fffff0 [25]
         (148) |11111111|11111111|11111000|1 [25]        1fffff1 [25]
         (149) |11111111|11111111|11111001|0 [25]        1fffff2 [25]
         (150) |11111111|11111111|11111001|1 [25]        1fffff3 [25]
         (151) |11111111|11111111|11111010|0 [25]        1fffff4 [25]
         (152) |11111111|11111111|11111010|1 [25]        1fffff5 [25]
         (153) |11111111|11111111|11111011|0 [25]        1fffff6 [25]
         (154) |11111111|11111111|11111011|1 [25]        1fffff7 [25]
         (155) |11111111|11111111|11111100|0 [25]        1fffff8 [25]
         (156) |11111111|11111111|11111100|1 [25]        1fffff9 [25]
         (157) |11111111|11111111|11111101|0 [25]        1fffffa [25]
         (158) |11111111|11111111|11111101|1 [25]        1fffffb [25]
         (159) |11111111|11111111|11111110|0 [25]        1fffffc [25]
         (160) |11111111|11111111|11111110|1 [25]        1fffffd [25]
         (161) |11111111|11111111|11111111|0 [25]        1fffffe [25]
         (162) |11111111|11111111|11111111|1 [25]        1ffffff [25]
         (163) |11111111|11111111|10000000| [24]          ffff80 [24]
         (164) |11111111|11111111|10000001| [24]          ffff81 [24]
         (165) |11111111|11111111|10000010| [24]          ffff82 [24]
         (166) |11111111|11111111|10000011| [24]          ffff83 [24]
         (167) |11111111|11111111|10000100| [24]          ffff84 [24]
         (168) |11111111|11111111|10000101| [24]          ffff85 [24]
         (169) |11111111|11111111|10000110| [24]          ffff86 [24]
         (170) |11111111|11111111|10000111| [24]          ffff87 [24]
         (171) |11111111|11111111|10001000| [24]          ffff88 [24]
         (172) |11111111|11111111|10001001| [24]          ffff89 [24]
         (173) |11111111|11111111|10001010| [24]          ffff8a [24]
         (174) |11111111|11111111|10001011| [24]          ffff8b [24]
         (175) |11111111|11111111|10001100| [24]          ffff8c [24]
         (176) |11111111|11111111|10001101| [24]          ffff8d [24]
         (177) |11111111|11111111|10001110| [24]          ffff8e [24]
         (178) |11111111|11111111|10001111| [24]          ffff8f [24]
         (179) |11111111|11111111|10010000| [24]          ffff90 [24]
         (180) |11111111|11111111|10010001| [24]          ffff91 [24]
         (181) |11111111|11111111|10010010| [24]          ffff92 [24]
         (182) |11111111|11111111|10010011| [24]          ffff93 [24]
         (183) |11111111|11111111|10010100| [24]          ffff94 [24]
         (184) |11111111|11111111|10010101| [24]          ffff95 [24]
         (185) |11111111|11111111|10010110| [24]          ffff96 [24]
         (186) |11111111|11111111|10010111| [24]          ffff97 [24]
         (187) |11111111|11111111|10011000| [24]          ffff98 [24]
         (188) |11111111|11111111|10011001| [24]          ffff99 [24]
         (189) |11111111|11111111|10011010| [24]          ffff9a [24]
         (190) |11111111|11111111|10011011| [24]          ffff9b [24]
         (191) |11111111|11111111|10011100| [24]          ffff9c [24]
         (192) |11111111|11111111|10011101| [24]          ffff9d [24]
         (193) |11111111|11111111|10011110| [24]          ffff9e [24]
         (194) |11111111|11111111|10011111| [24]          ffff9f [24]
         (195) |11111111|11111111|10100000| [24]          ffffa0 [24]
         (196) |11111111|11111111|10100001| [24]          ffffa1 [24]
         (197) |11111111|11111111|10100010| [24]          ffffa2 [24]
         (198) |11111111|11111111|10100011| [24]          ffffa3 [24]
         (199) |11111111|11111111|10100100| [24]          ffffa4 [24]
         (200) |11111111|11111111|10100101| [24]          ffffa5 [24]
         (201) |11111111|11111111|10100110| [24]          ffffa6 [24]
         (202) |11111111|11111111|10100111| [24]          ffffa7 [24]
         (203) |11111111|11111111|10101000| [24]          ffffa8 [24]
         (204) |11111111|11111111|10101001| [24]          ffffa9 [24]
         (205) |11111111|11111111|10101010| [24]          ffffaa [24]
         (206) |11111111|11111111|10101011| [24]          ffffab [24]
         (207) |11111111|11111111|10101100| [24]          ffffac [24]
         (208) |11111111|11111111|10101101| [24]          ffffad [24]
         (209) |11111111|11111111|10101110| [24]          ffffae [24]
         (210) |11111111|11111111|10101111| [24]          ffffaf [24]
         (211) |11111111|11111111|10110000| [24]          ffffb0 [24]
         (212) |11111111|11111111|10110001| [24]          ffffb1 [24]
         (213) |11111111|11111111|10110010| [24]          ffffb2 [24]
         (214) |11111111|11111111|10110011| [24]          ffffb3 [24]
         (215) |11111111|11111111|10110100| [24]          ffffb4 [24]
         (216) |11111111|11111111|10110101| [24]          ffffb5 [24]
         (217) |11111111|11111111|10110110| [24]          ffffb6 [24]
         (218) |11111111|11111111|10110111| [24]          ffffb7 [24]
         (219) |11111111|11111111|10111000| [24]          ffffb8 [24]
         (220) |11111111|11111111|10111001| [24]          ffffb9 [24]
         (221) |11111111|11111111|10111010| [24]          ffffba [24]
         (222) |11111111|11111111|10111011| [24]          ffffbb [24]
         (223) |11111111|11111111|10111100| [24]          ffffbc [24]
         (224) |11111111|11111111|10111101| [24]          ffffbd [24]
         (225) |11111111|11111111|10111110| [24]          ffffbe [24]
         (226) |11111111|11111111|10111111| [24]          ffffbf [24]
         (227) |11111111|11111111|11000000| [24]          ffffc0 [24]
         (228) |11111111|11111111|11000001| [24]          ffffc1 [24]
         (229) |11111111|11111111|11000010| [24]          ffffc2 [24]
         (230) |11111111|11111111|11000011| [24]          ffffc3 [24]
         (231) |11111111|11111111|11000100| [24]          ffffc4 [24]
         (232) |11111111|11111111|11000101| [24]          ffffc5 [24]
         (233) |11111111|11111111|11000110| [24]          ffffc6 [24]
         (234) |11111111|11111111|11000111| [24]          ffffc7 [24]
         (235) |11111111|11111111|11001000| [24]          ffffc8 [24]
         (236) |11111111|11111111|11001001| [24]          ffffc9 [24]
         (237) |11111111|11111111|11001010| [24]          ffffca [24]
         (238) |11111111|11111111|11001011| [24]          ffffcb [24]
         (239) |11111111|11111111|11001100| [24]          ffffcc [24]
         (240) |11111111|11111111|11001101| [24]          ffffcd [24]
         (241) |11111111|11111111|11001110| [24]          ffffce [24]
         (242) |11111111|11111111|11001111| [24]          ffffcf [24]
         (243) |11111111|11111111|11010000| [24]          ffffd0 [24]
         (244) |11111111|11111111|11010001| [24]          ffffd1 [24]
         (245) |11111111|11111111|11010010| [24]          ffffd2 [24]
         (246) |11111111|11111111|11010011| [24]          ffffd3 [24]
         (247) |11111111|11111111|11010100| [24]          ffffd4 [24]
         (248) |11111111|11111111|11010101| [24]          ffffd5 [24]
         (249) |11111111|11111111|11010110| [24]          ffffd6 [24]
         (250) |11111111|11111111|11010111| [24]          ffffd7 [24]
         (251) |11111111|11111111|11011000| [24]          ffffd8 [24]
         (252) |11111111|11111111|11011001| [24]          ffffd9 [24] [26]
       (253) |11111111|11111111|11011010| [24]          ffffda [24] |11111111|11111111|11110110|01 [26]       3ffffd9 [26]
       (254) |11111111|11111111|11011011| [24]          ffffdb [24] |11111111|11111111|11110110|10 [26]       3ffffda [26]
       (255) |11111111|11111111|11011100| [24]          ffffdc [24] |11111111|11111111|11110110|11 [26]       3ffffdb [26]
   EOS (256) |11111111|11111111|11011101| [24]          ffffdd [24] |11111111|11111111|11110111|00 [26]       3ffffdc [26]

Appendix E. D.  Examples

   A number of examples are worked through here, for both requests and
   responses, and with and without Huffman coding.

E.1.

D.1.  Header Field Representation Examples

   This section show several independent representation examples.

E.1.1.

D.1.1.  Literal Header Field with Indexing

   The header field representation uses a literal name and a literal
   value.

   Header set to encode:

   custom-key: custom-header

   Reference set: empty.

   Hex dump of encoded data:

   000a 6375 7374 6f6d 2d6b 6579 0d63 7573 | ..custom-key.cus
   746f 6d2d 6865 6164 6572                | tom-header

   Decoding process:

   00                                      | == Literal indexed ==
   0a                                      |   Literal name (len = 10)
   6375 7374 6f6d 2d6b 6579                | custom-key
   0d                                      |   Literal value (len = 13)
   6375 7374 6f6d 2d68 6561 6465 72        | custom-header
                                           | -> custom-key: custom-head\
                                           |   er

   Header Table (after decoding):

   [  1] (s =  55) custom-key: custom-header
         Table size:  55

   Decoded header set:

   custom-key: custom-header

E.1.2.

D.1.2.  Literal Header Field without Indexing

   The header field representation uses an indexed name and a literal
   value.

   Header set to encode:

   :path: /sample/path

   Reference set: empty.

   Hex dump of encoded data:

   440c 2f73 616d 706c 652f 7061 7468      | D./sample/path

   Decoding process:

   44                                      | == Literal not indexed ==
                                           |   Indexed name (idx = 4)
                                           |     :path
   0c                                      |   Literal value (len = 12)
   2f73 616d 706c 652f 7061 7468           | /sample/path
                                           | -> :path: /sample/path

   Header table (after decoding): empty.

   Decoded header set:

   :path: /sample/path

E.1.3.

D.1.3.  Indexed Header Field

   The header field representation uses an indexed header field, from
   the static table.  Upon using it, the static table entry is copied
   into the header table.

   Header set to encode:

   :method: GET

   Reference set: empty.

   Hex dump of encoded data:

   82                                      | .

   Decoding process:

   82                                      | == Indexed - Add ==
                                           |   idx = 2
                                           | -> :method: GET

   Header Table (after decoding):

   [  1] (s =  42) :method: GET
         Table size:  42

   Decoded header set:

   :method: GET

E.1.4.

D.1.4.  Indexed Header Field from Static Table

   The header field representation uses an indexed header field, from
   the static table.  In this example, the SETTINGS_HEADER_TABLE_SIZE is
   set to 0, therefore, the entry is not copied into the header table.

   Header set to encode:

   :method: GET

   Reference set: empty.

   Hex dump of encoded data:

   82                                      | .

   Decoding process:

   82                                      | == Indexed - Add ==
                                           |   idx = 2
                                           | -> :method: GET

   Header table (after decoding): empty.

   Decoded header set:

   :method: GET

E.2.

D.2.  Request Examples without Huffman

   This section shows several consecutive header sets, corresponding to
   HTTP requests, on the same connection.

E.2.1.

D.2.1.  First request

   Header set to encode:

   :method: GET
   :scheme: http
   :path: /
   :authority: www.example.com

   Reference set: empty.

   Hex dump of encoded data:

   8287 8604 0f77 7777 2e65 7861 6d70 6c65 | .....www.example
   2e63 6f6d                               | .com

   Decoding process:

   82                                      | == Indexed - Add ==
                                           |   idx = 2
                                           | -> :method: GET
   87                                      | == Indexed - Add ==
                                           |   idx = 7
                                           | -> :scheme: http
   86                                      | == Indexed - Add ==
                                           |   idx = 6
                                           | -> :path: /
   04                                      | == Literal indexed ==
                                           |   Indexed name (idx = 4)
                                           |     :authority
   0f                                      |   Literal value (len = 15)
   7777 772e 6578 616d 706c 652e 636f 6d   | www.example.com
                                           | -> :authority: www.example\
                                           |   .com

   Header Table (after decoding):

   [  1] (s =  57) :authority: www.example.com
   [  2] (s =  38) :path: /
   [  3] (s =  43) :scheme: http
   [  4] (s =  42) :method: GET
         Table size: 180

   Decoded header set:

   :method: GET
   :scheme: http
   :path: /
   :authority: www.example.com

E.2.2.

D.2.2.  Second request

   This request takes advantage of the differential encoding of header
   sets.

   Header set to encode:

   :method: GET
   :scheme: http
   :path: /
   :authority: www.example.com
   cache-control: no-cache

   Reference set:

   [  1] :authority: www.example.com
   [  2] :path: /
   [  3] :scheme: http
   [  4] :method: GET

   Hex dump of encoded data:

   1b08 6e6f 2d63 6163 6865                | ..no-cache

   Decoding process:

   1b                                      | == Literal indexed ==
                                           |   Indexed name (idx = 27)
                                           |     cache-control
   08                                      |   Literal value (len = 8)
   6e6f 2d63 6163 6865                     | no-cache
                                           | -> cache-control: no-cache

   Header Table (after decoding):

   [  1] (s =  53) cache-control: no-cache
   [  2] (s =  57) :authority: www.example.com
   [  3] (s =  38) :path: /
   [  4] (s =  43) :scheme: http
   [  5] (s =  42) :method: GET
         Table size: 233

   Decoded header set:

   cache-control: no-cache
   :authority: www.example.com
   :path: /
   :scheme: http
   :method: GET

E.2.3.

D.2.3.  Third request

   This request has not enough headers in common with the previous
   request to take advantage of the differential encoding.  Therefore,
   the reference set is emptied before encoding the header fields.

   Header set to encode:

   :method: GET
   :scheme: https
   :path: /index.html
   :authority: www.example.com
   custom-key: custom-value

   Reference set:

   [  1] cache-control: no-cache
   [  2] :authority: www.example.com
   [  3] :path: /
   [  4] :scheme: http
   [  5] :method: GET

   Hex dump of encoded data:

   8085 8c8b 8400 0a63 7573 746f 6d2d 6b65 | .......custom-ke
   790c

   8080 858c 8b84 000a 6375 7374 6f6d 2d76 616c 7565 2d6b | ........custom-k
   6579 0c63 7573 746f 6d2d 7661 6c75 65   | y.custom-value ey.custom-value

   Decoding process:

   80 80                                   | == Empty reference set ==
                                           |   idx = 0
                                           |   flag = 1
   85                                      | == Indexed - Add ==
                                           |   idx = 5
                                           | -> :method: GET
   8c                                      | == Indexed - Add ==
                                           |   idx = 12
                                           | -> :scheme: https
   8b                                      | == Indexed - Add ==
                                           |   idx = 11
                                           | -> :path: /index.html
   84                                      | == Indexed - Add ==
                                           |   idx = 4
                                           | -> :authority: www.example\
                                           |   .com
   00                                      | == Literal indexed ==
   0a                                      |   Literal name (len = 10)
   6375 7374 6f6d 2d6b 6579                | custom-key
   0c                                      |   Literal value (len = 12)
   6375 7374 6f6d 2d76 616c 7565           | custom-value
                                           | -> custom-key: custom-valu\
                                           |   e

   Header Table (after decoding):

   [  1] (s =  54) custom-key: custom-value
   [  2] (s =  48) :path: /index.html
   [  3] (s =  44) :scheme: https
   [  4] (s =  53) cache-control: no-cache
   [  5] (s =  57) :authority: www.example.com
   [  6] (s =  38) :path: /
   [  7] (s =  43) :scheme: http
   [  8] (s =  42) :method: GET
         Table size: 379

   Decoded header set:

   :method: GET
   :scheme: https
   :path: /index.html
   :authority: www.example.com
   custom-key: custom-value

E.3.

D.3.  Request Examples with Huffman

   This section shows the same examples as the previous section, but
   using Huffman encoding for the literal values.

E.3.1.

D.3.1.  First request

   Header set to encode:

   :method: GET
   :scheme: http
   :path: /
   :authority: www.example.com

   Reference set: empty.

   Hex dump of encoded data:

   8287 8604 8bdb 6d88 3e68 d1cb 1225 ba7f | ......m..h...%..

   Decoding process:

   82                                      | == Indexed - Add ==
                                           |   idx = 2
                                           | -> :method: GET
   87                                      | == Indexed - Add ==
                                           |   idx = 7
                                           | -> :scheme: http
   86                                      | == Indexed - Add ==
                                           |   idx = 6
                                           | -> :path: /
   04                                      | == Literal indexed ==
                                           |   Indexed name (idx = 4)
                                           |     :authority
   8b                                      |   Literal value (len = 15)
                                           |     Huffman encoded:
   db6d 883e 68d1 cb12 25ba 7f             | .m..h...%..
                                           |     Decoded:
                                           | www.example.com
                                           | -> :authority: www.example\
                                           |   .com

   Header Table (after decoding):

   [  1] (s =  57) :authority: www.example.com
   [  2] (s =  38) :path: /
   [  3] (s =  43) :scheme: http
   [  4] (s =  42) :method: GET
         Table size: 180

   Decoded header set:

   :method: GET
   :scheme: http
   :path: /
   :authority: www.example.com

E.3.2.

D.3.2.  Second request

   This request takes advantage of the differential encoding of header
   sets.

   Header set to encode:

   :method: GET
   :scheme: http
   :path: /
   :authority: www.example.com
   cache-control: no-cache

   Reference set:

   [  1] :authority: www.example.com
   [  2] :path: /
   [  3] :scheme: http
   [  4] :method: GET

   Hex dump of encoded data:

   1b86 6365 4a13 98ff                     | ..ceJ...

   Decoding process:

   1b                                      | == Literal indexed ==
                                           |   Indexed name (idx = 27)
                                           |     cache-control
   86                                      |   Literal value (len = 8)
                                           |     Huffman encoded:
   6365 4a13 98ff                          | ceJ...
                                           |     Decoded:
                                           | no-cache
                                           | -> cache-control: no-cache

   Header Table (after decoding):

   [  1] (s =  53) cache-control: no-cache
   [  2] (s =  57) :authority: www.example.com
   [  3] (s =  38) :path: /
   [  4] (s =  43) :scheme: http
   [  5] (s =  42) :method: GET
         Table size: 233

   Decoded header set:

   cache-control: no-cache
   :authority: www.example.com
   :path: /
   :scheme: http
   :method: GET

E.3.3.

D.3.3.  Third request

   This request has not enough headers in common with the previous
   request to take advantage of the differential encoding.  Therefore,
   the reference set is emptied before encoding the header fields.

   Header set to encode:

   :method: GET
   :scheme: https
   :path: /index.html
   :authority: www.example.com
   custom-key: custom-value

   Reference set:

   [  1] cache-control: no-cache
   [  2] :authority: www.example.com
   [  3] :path: /
   [  4] :scheme: http
   [  5] :method: GET

   Hex dump of encoded data:

   8085 8c8b 8400 884e b08b 7497 90fa 7f89 | .......N..t.....

   8080 858c 8b84 0088 4eb0 8b74 979a 17a8 ff 9790 fa7f | N..t..... ........N..t....
   894e b08b 7497 9a17 a8ff                | .N..t.....

   Decoding process:

   80 80                                   | == Empty reference set ==
                                           |   idx = 0
                                           |   flag = 1
   85                                      | == Indexed - Add ==
                                           |   idx = 5
                                           | -> :method: GET
   8c                                      | == Indexed - Add ==
                                           |   idx = 12
                                           | -> :scheme: https
   8b                                      | == Indexed - Add ==
                                           |   idx = 11
                                           | -> :path: /index.html
   84                                      | == Indexed - Add ==
                                           |   idx = 4
                                           | -> :authority: www.example\
                                           |   .com
   00                                      | == Literal indexed ==
   88                                      |   Literal name (len = 10)
                                           |     Huffman encoded:
   4eb0 8b74 9790 fa7f                     | N..t....
                                           |     Decoded:
                                           | custom-key
   89                                      |   Literal value (len = 12)
                                           |     Huffman encoded:
   4eb0 8b74 979a 17a8 ff                  | N..t.....
                                           |     Decoded:
                                           | custom-value
                                           | -> custom-key: custom-valu\
                                           |   e

   Header Table (after decoding):

   [  1] (s =  54) custom-key: custom-value
   [  2] (s =  48) :path: /index.html
   [  3] (s =  44) :scheme: https
   [  4] (s =  53) cache-control: no-cache
   [  5] (s =  57) :authority: www.example.com
   [  6] (s =  38) :path: /
   [  7] (s =  43) :scheme: http
   [  8] (s =  42) :method: GET
         Table size: 379

   Decoded header set:

   :method: GET
   :scheme: https
   :path: /index.html
   :authority: www.example.com
   custom-key: custom-value

E.4.

D.4.  Response Examples without Huffman

   This section shows several consecutive header sets, corresponding to
   HTTP responses, on the same connection.  SETTINGS_HEADER_TABLE_SIZE
   is set to the value of 256 octets, causing some evictions to occur.

E.4.1.

D.4.1.  First response

   Header set to encode:

   :status: 302
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:21 GMT
   location: https://www.example.com

   Reference set: empty.

   Hex dump of encoded data:

   0803 3330 3218 0770 7269 7661 7465 221d | ..302..private".
   4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
   2032 303a 3133 3a32 3120 474d 5430 1768 |  20:13:21 GMT0.h
   7474 7073 3a2f 2f77 7777 2e65 7861 6d70 | ttps://www.examp
   6c65 2e63 6f6d                          | le.com

   Decoding process:

   08                                      | == Literal indexed ==
                                           |   Indexed name (idx = 8)
                                           |     :status
   03                                      |   Literal value (len = 3)
   3330 32                                 | 302
                                           | -> :status: 302
   18                                      | == Literal indexed ==
                                           |   Indexed name (idx = 24)
                                           |     cache-control
   07                                      |   Literal value (len = 7)
   7072 6976 6174 65                       | private
                                           | -> cache-control: private
   22                                      | == Literal indexed ==
                                           |   Indexed name (idx = 34)
                                           |     date
   1d                                      |   Literal value (len = 29)
   4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
   2032 303a 3133 3a32 3120 474d 54        |  20:13:21 GMT
                                           | -> date: Mon, 21 Oct 2013 \
                                           |   20:13:21 GMT
   30                                      | == Literal indexed ==
                                           |   Indexed name (idx = 48)
                                           |     location
   17                                      |   Literal value (len = 23)
   6874 7470 733a 2f2f 7777 772e 6578 616d | https://www.exam
   706c 652e 636f 6d                       | ple.com
                                           | -> location: https://www.e\
                                           |   xample.com

   Header Table (after decoding):

   [  1] (s =  63) location: https://www.example.com
   [  2] (s =  65) date: Mon, 21 Oct 2013 20:13:21 GMT
   [  3] (s =  52) cache-control: private
   [  4] (s =  42) :status: 302
         Table size: 222

   Decoded header set:

   :status: 302
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:21 GMT
   location: https://www.example.com

E.4.2.

D.4.2.  Second response

   The (":status", "302") header field is evicted from the header table
   to free space to allow adding the (":status", "200") header field to
   be field,
   copied from the static table into the header table.  The (":status",
   "302") header field doesn't need to be removed from the reference set
   as it is evicted from the header table.

   Header set to encode:

   :status: 200
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:21 GMT
   location: https://www.example.com

   Reference set:

   [  1] location: https://www.example.com
   [  2] date: Mon, 21 Oct 2013 20:13:21 GMT
   [  3] cache-control: private
   [  4] :status: 302

   Hex dump of encoded data:

   848c

   8c                                      | .. .

   Decoding process:

   84                                      | == Indexed - Remove ==
                                           |   idx = 4
                                           | -> :status: 302

   8c                                      | == Indexed - Add ==
                                           |   idx = 12
                                           | - evict: :status: 302
                                           | -> :status: 200

   Header Table (after decoding):

   [  1] (s =  42) :status: 200
   [  2] (s =  63) location: https://www.example.com
   [  3] (s =  65) date: Mon, 21 Oct 2013 20:13:21 GMT
   [  4] (s =  52) cache-control: private
         Table size: 222

   Decoded header set:

   :status: 200
   location: https://www.example.com
   date: Mon, 21 Oct 2013 20:13:21 GMT
   cache-control: private

E.4.3.

D.4.3.  Third response

   Several header fields are evicted from the header table during the
   processing of this header set.  Before evicting a header belonging to
   the reference set, it is emitted, by coding it twice as an Indexed
   Representation.  The first representation removes the header field
   from the reference set, the second one adds it again to the reference
   set, also emitting it.

   Header set to encode:

   :status: 200
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:22 GMT
   location: https://www.example.com
   content-encoding: gzip
   set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1

   Reference set:

   [  1] :status: 200
   [  2] location: https://www.example.com
   [  3] date: Mon, 21 Oct 2013 20:13:21 GMT
   [  4] cache-control: private

   Hex dump of encoded data:

   8384 8403 1d4d 6f6e 2c20 3231 204f 6374

   8484 031d 4d6f 6e2c 2032 3120 4f63 7420 | .....Mon, ....Mon, 21 Oct
   2032 3031 3320
   3230 3a31 333a 3232 2047 3133 2032 303a 3133 3a32 3220 474d | 2013 20:13:22 G
   4d54 1d04 677a 6970 8484 8383 3a38 666f | MT..gzip....:8fo
   6f3d 4153 444a 4b48 514b 425a 584f 5157 GM
   541d 0467 7a69 7084 8483 833a 3866 6f6f | o=ASDJKHQKBZXOQW
   454f 5049 5541 5851 T..gzip....:8foo
   3d41 5344 4a4b 4851 4b42 5a58 4f51 5745 4f49 553b 206d | EOPIUAXQWEOIU; m
   6178 2d61 6765 3d33 3630 303b 2076 6572 =ASDJKHQKBZXOQWE
   4f50 4955 4158 5157 454f 4955 3b20 6d61 | OPIUAXQWEOIU; ma
   782d 6167 653d 3336 3030 3b20 7665 7273 | ax-age=3600; ver
   7369 6f6e 3d31 x-age=3600; vers
   696f 6e3d 31                            | sion=1 ion=1

   Decoding process:

   83                                      | == Indexed - Remove ==
                                           |   idx = 3
                                           | -> date: Mon, 21 Oct 2013 \
                                           |   20:13:21 GMT

   84                                      | == Indexed - Remove ==
                                           |   idx = 4
                                           | -> cache-control: private
   84                                      | == Indexed - Add ==
                                           |   idx = 4
                                           | -> cache-control: private
   03                                      | == Literal indexed ==
                                           |   Indexed name (idx = 3)
                                           |     date
   1d                                      |   Literal value (len = 29)
   4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
   2032 303a 3133 3a32 3220 474d 54        |  20:13:22 GMT
                                           | - evict: cache-control: pr\
                                           |   ivate
                                           | -> date: Mon, 21 Oct 2013 \
                                           |   20:13:22 GMT
   1d                                      | == Literal indexed ==
                                           |   Indexed name (idx = 29)
                                           |     content-encoding
   04                                      |   Literal value (len = 4)
   677a 6970                               | gzip
                                           | - evict: date: Mon, 21 Oct\
                                           |    2013 20:13:21 GMT
                                           | -> content-encoding: gzip
   84                                      | == Indexed - Remove ==
                                           |   idx = 4
                                           | -> location: https://www.e\
                                           |   xample.com
   84                                      | == Indexed - Add ==
                                           |   idx = 4
                                           | -> location: https://www.e\
                                           |   xample.com
   83                                      | == Indexed - Remove ==
                                           |   idx = 3
                                           | -> :status: 200
   83                                      | == Indexed - Add ==
                                           |   idx = 3
                                           | -> :status: 200
   3a                                      | == Literal indexed ==
                                           |   Indexed name (idx = 58)
                                           |     set-cookie
   38                                      |   Literal value (len = 56)
   666f 6f3d 4153 444a 4b48 514b 425a 584f | foo=ASDJKHQKBZXO
   5157 454f 5049 5541 5851 5745 4f49 553b | QWEOPIUAXQWEOIU;
   206d 6178 2d61 6765 3d33 3630 303b 2076 |  max-age=3600; v
   6572 7369 6f6e 3d31                     | ersion=1
                                           | - evict: location: https:/\
                                           |   /www.example.com
                                           | - evict: :status: 200
                                           | -> set-cookie: foo=ASDJKHQ\
                                           |   KBZXOQWEOPIUAXQWEOIU; ma\
                                           |   x-age=3600; version=1

   Header Table (after decoding):

   [  1] (s =  98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age\
                    =3600; version=1
   [  2] (s =  52) content-encoding: gzip
   [  3] (s =  65) date: Mon, 21 Oct 2013 20:13:22 GMT
         Table size: 215

   Decoded header set:

   cache-control: private
   date: Mon, 21 Oct 2013 20:13:22 GMT
   content-encoding: gzip
   location: https://www.example.com
   :status: 200
   set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1

E.5.

D.5.  Response Examples with Huffman

   This section shows the same examples as the previous section, but
   using Huffman encoding for the literal values.  The eviction
   mechanism uses the length of the decoded literal values, so the same
   evictions occurs as in the previous section.

E.5.1.

D.5.1.  First response

   Header set to encode:

   :status: 302
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:21 GMT
   location: https://www.example.com

   Reference set: empty.

   Hex dump of encoded data:

   0882 409f 1886 c31b 39bf 387f 2292 a2fb | ..@.....9.8."...
   a203 20f2 ab30 3124 018b 490d 3209 e877 | .. ..01$..I.2..w
   3093 e39e 7864 dd7a fd3d 3d24 8747 db87 | 0...xd.z.==$.G..
   2849 55f6 ff 98a7 1885 73d5 cd11 1f22 98ef 6b3a | ......s...."..k:
   7a0e 6e8f a263 d072 9a6e 8397 d869 bd87 | z.n..c.r.n...i..
   3747 bbbf c730 90ce 3174 3d80 1b6d b107 | 7G...0..1t=..m..
   cd1a 3962 44b7 4f                       | (IU.. ..9bD.O

   Decoding process:

   08                                      | == Literal indexed ==
                                           |   Indexed name (idx = 8)
                                           |     :status
   82                                      |   Literal value (len = 3)
                                           |     Huffman encoded:
   409f
   98a7                                    | @. ..
                                           |     Decoded:
                                           | 302
                                           | -> :status: 302
   18                                      | == Literal indexed ==
                                           |   Indexed name (idx = 24)
                                           |     cache-control
   86
   85                                      |   Literal value (len = 7)
                                           |     Huffman encoded:
   c31b 39bf 387f
   73d5 cd11 1f                            | ..9.8. s....
                                           |     Decoded:
                                           | private
                                           | -> cache-control: private
   22                                      | == Literal indexed ==
                                           |   Indexed name (idx = 34)
                                           |     date
   92
   98                                      |   Literal value (len = 29)
                                           |     Huffman encoded:
   a2fb a203 20f2 ab30 3124 018b 490d 3209 | .... ..01$..I.2.
   e877
   ef6b 3a7a 0e6e 8fa2 63d0 729a 6e83 97d8 | .k:z.n..c.r.n...
   69bd 8737 47bb bfc7                     | .w i..7G...
                                           |     Decoded:
                                           | Mon, 21 Oct 2013 20:13:21 \
                                           | GMT
                                           | -> date: Mon, 21 Oct 2013 \
                                           |   20:13:21 GMT
   30                                      | == Literal indexed ==
                                           |   Indexed name (idx = 48)
                                           |     location
   93
   90                                      |   Literal value (len = 23)
                                           |     Huffman encoded:
   e39e 7864 dd7a fd3d 3d24 8747 db87 2849 | ..xd.z.==$.G..(I
   55f6 ff
   ce31 743d 801b 6db1 07cd 1a39 6244 b74f | U.. .1t=..m....9bD.O
                                           |     Decoded:
                                           | https://www.example.com
                                           | -> location: https://www.e\
                                           |   xample.com

   Header Table (after decoding):

   [  1] (s =  63) location: https://www.example.com
   [  2] (s =  65) date: Mon, 21 Oct 2013 20:13:21 GMT
   [  3] (s =  52) cache-control: private
   [  4] (s =  42) :status: 302
         Table size: 222

   Decoded header set:

   :status: 302
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:21 GMT
   location: https://www.example.com

E.5.2.

D.5.2.  Second response

   The (":status", "302") header field is evicted from the header table
   to free space to allow adding the (":status", "200") header field to
   be field,
   copied from the static table into the header table.  The (":status",
   "302") header field doesn't need to be removed from the reference set
   as it is evicted from the header table.

   Header set to encode:

   :status: 200
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:21 GMT
   location: https://www.example.com

   Reference set:

   [  1] location: https://www.example.com
   [  2] date: Mon, 21 Oct 2013 20:13:21 GMT
   [  3] cache-control: private
   [  4] :status: 302

   Hex dump of encoded data:

   848c

   8c                                      | .. .

   Decoding process:

   84                                      | == Indexed - Remove ==
                                           |   idx = 4
                                           | -> :status: 302

   8c                                      | == Indexed - Add ==
                                           |   idx = 12
                                           | - evict: :status: 302
                                           | -> :status: 200

   Header Table (after decoding):

   [  1] (s =  42) :status: 200
   [  2] (s =  63) location: https://www.example.com
   [  3] (s =  65) date: Mon, 21 Oct 2013 20:13:21 GMT
   [  4] (s =  52) cache-control: private
         Table size: 222

   Decoded header set:

   :status: 200
   location: https://www.example.com
   date: Mon, 21 Oct 2013 20:13:21 GMT
   cache-control: private

E.5.3.

D.5.3.  Third response

   Several header fields are evicted from the header table during the
   processing of this header set.  Before evicting a header belonging to
   the reference set, it is emitted, by coding it twice as an Indexed
   Representation.  The first representation removes the header field
   from the reference set, the second one adds it again to the reference
   set, also emitting it.

   Header set to encode:

   :status: 200
   cache-control: private
   date: Mon, 21 Oct 2013 20:13:22 GMT
   location: https://www.example.com
   content-encoding: gzip
   set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1
   Reference set:

   [  1] :status: 200
   [  2] location: https://www.example.com
   [  3] date: Mon, 21 Oct 2013 20:13:21 GMT
   [  4] cache-control: private

   Hex dump of encoded data:

   8384 8403 92a2 fba2 0320 f2ab 3031 2401 | ......... ..01$.
   8b49 0d33 09e8 771d 84e1 fbb3 0f84 8483

   8484 0398 ef6b 3a7a 0e6e 8fa2 63d0 729a | .....k:z.n..c.r.
   6e83 97d8 69bd 873f 47bb bfc7 1d83 cbd5 | .I.3..w......... n...i..?G.......
   4e84 8483 833a b3df 7dfb 36d3 d9e1 fcfc 3faf e7ab | .:..}.6.....?...
   fcfe fcbf af3e df2f 977f d36f f7fd 79f6 b3c5 adb7 7f87 6fc7 fbf7 | N....:......o...
   fdbf bebf f3f7 f4fb 7ebb be9f 5f87 e37f | ............_...
   efed faee fa7c 3f1d 5d1a 23ce 5464 36cd | .....|?.].#.Td6.
   494b d5d1 cc5f 0535 969b                | ......./...o..y.
   f977 fd3d e16b fa46 fe10 d889 447d e1ce | .w.=.k.F....D}..
   18e5 65f7 6c2f                          | ..e.l/ IK..._.5..

   Decoding process:

   83                                      | == Indexed - Remove ==
                                           |   idx = 3
                                           | -> date: Mon, 21 Oct 2013 \
                                           |   20:13:21 GMT

   84                                      | == Indexed - Remove ==
                                           |   idx = 4
                                           | -> cache-control: private
   84                                      | == Indexed - Add ==
                                           |   idx = 4
                                           | -> cache-control: private
   03                                      | == Literal indexed ==
                                           |   Indexed name (idx = 3)
                                           |     date
   92
   98                                      |   Literal value (len = 29)
                                           |     Huffman encoded:
   a2fb a203 20f2 ab30 3124 018b 490d 3309 | .... ..01$..I.3.
   e877
   ef6b 3a7a 0e6e 8fa2 63d0 729a 6e83 97d8 | .k:z.n..c.r.n...
   69bd 873f 47bb bfc7                     | .w i..?G...
                                           |     Decoded:
                                           | Mon, 21 Oct 2013 20:13:22 \
                                           | GMT
                                           | - evict: cache-control: pr\
                                           |   ivate
                                           | -> date: Mon, 21 Oct 2013 \
                                           |   20:13:22 GMT
   1d                                      | == Literal indexed ==
                                           |   Indexed name (idx = 29)
                                           |     content-encoding
   84
   83                                      |   Literal value (len = 4)
                                           |     Huffman encoded:
   e1fb b30f
   cbd5 4e                                 | .... ..N
                                           |     Decoded:
                                           | gzip
                                           | - evict: date: Mon, 21 Oct\
                                           |    2013 20:13:21 GMT
                                           | -> content-encoding: gzip
   84                                      | == Indexed - Remove ==
                                           |   idx = 4
                                           | -> location: https://www.e\
                                           |   xample.com
   84                                      | == Indexed - Add ==
                                           |   idx = 4
                                           | -> location: https://www.e\
                                           |   xample.com
   83                                      | == Indexed - Remove ==
                                           |   idx = 3
                                           | -> :status: 200
   83                                      | == Indexed - Add ==
                                           |   idx = 3
                                           | -> :status: 200
   3a                                      | == Literal indexed ==
                                           |   Indexed name (idx = 58)
                                           |     set-cookie
   b3                                      |   Literal value (len = 56)
                                           |     Huffman encoded:
   df7d fb36 d3d9 e1fc fc3f afe7 abfc fefc | .}.6.....?......
   bfaf 3edf 2f97 7fd3 6ff7 fd79 f6f9 77fd | ..../...o..y..w.
   3de1 6bfa 46fe 10d8 8944 7de1 ce18 e565 | =.k.F....D}....e
   f76c 2f
   c5ad b77f 876f c7fb f7fd bfbe bff3 f7f4 | .....o..........
   fb7e bbbe 9f5f 87e3 7fef edfa eefa 7c3f | ....._........|?
   1d5d 1a23 ce54 6436 cd49 4bd5 d1cc 5f05 | .].#.Td6.IK..._.
   3596 9b                                 | .l/ 5..
                                           |     Decoded:
                                           | foo=ASDJKHQKBZXOQWEOPIUAXQ\
                                           | WEOIU; max-age=3600; versi\
                                           | on=1
                                           | - evict: location: https:/\
                                           |   /www.example.com
                                           | - evict: :status: 200
                                           | -> set-cookie: foo=ASDJKHQ\
                                           |   KBZXOQWEOPIUAXQWEOIU; ma\
                                           |   x-age=3600; version=1

   Header Table (after decoding):

   [  1] (s =  98) set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age\
                    =3600; version=1
   [  2] (s =  52) content-encoding: gzip
   [  3] (s =  65) date: Mon, 21 Oct 2013 20:13:22 GMT
         Table size: 215

   Decoded header set:

   cache-control: private
   date: Mon, 21 Oct 2013 20:13:22 GMT
   content-encoding: gzip
   location: https://www.example.com
   :status: 200
   set-cookie: foo=ASDJKHQKBZXOQWEOPIUAXQWEOIU; max-age=3600; version=1

Authors' Addresses

   Roberto Peon
   Google, Inc

   EMail: fenix@google.com

   Herve Ruellan
   Canon CRF

   EMail: herve.ruellan@crf.canon.fr