HTTPbis Working Group                                            R. Peon
Internet-Draft                                               Google, Inc
Intended status: Standards Track                              H. Ruellan
Expires: August 17, October 05, 2014                                      Canon CRF
                                                       February 13,
                                                          April 03, 2014

                 HPACK - Header Compression for HTTP/2
                draft-ietf-httpbis-header-compression-06
                draft-ietf-httpbis-header-compression-07

Abstract

   This specification defines HPACK, a compression format for
   efficiently representing HTTP header fields in the context of 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 [1]. <http://
   lists.w3.org/Archives/Public/ietf-http-wg/>.

   Working Group information and related documents can be found at [2]
   (Wiki) and [3] (source code and issues tracker). <http://tools.ietf.org/wg/
   httpbis/>; that specific to HTTP/2 are at <http://http2.github.io/>.

   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
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3   4
     2.1.  Outline . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Header Field Encoding . . . . . . . . . . . . . . . . . . . .   4   5
     3.1.  Encoding Concepts . . . . . . . . . . . . . . . . . . . .   4   5
       3.1.1.  Encoding Context  . . . . . . . . . . . . . . . . . .   5   6
       3.1.2.  Header Table  . . . . . . . . . . . . . . . . . . . .   5   6
       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  . . . . . . . . . . . . . . .   9  10
       3.2.3.  Header Set Completion . . . . . . . . . . . . . . . .   9  10
     3.3.  Header Table Management . . . . . . . . . . . . . . . . .   9  10
       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  . . . . . . .  10  11
   4.  Detailed Format . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Low-level representations . . . . . . . . . . . . . . . .  11
       4.1.1.  Integer representation  . . . . . . . . . . . . . . .  11
       4.1.2.  String Literal Representation . . . . . . . . . . . .  13  12
     4.2.  Indexed Header Field Representation . . . . . . . . . . .  15  13
     4.3.  Literal Header Field Representation . . . . . . . . . . .  15  14
       4.3.1.  Literal Header Field with Incremental Indexing  . . .  14
       4.3.2.  Literal Header Field without Indexing . . . . . . . .  15
       4.3.2.
       4.3.3.  Literal Header Field with Incremental Indexing never Indexed  . . . . . . . . .  16
     4.4.  Encoding Context Update . . . . . . . . . . . . . . . . .  17
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
     5.1.  Compression-based Attacks . . . . . . . . . . . . . . . .  18
     5.2.  Memory Consumption  . . . . . . . . . . . . . . . . . . .  19
     5.3.  Implementation Limits . . . . . . . . . . . . . . . . . .  19
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19  20
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  19  20
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  19  20
   Appendix A.  Change Log (to be removed by RFC Editor before
                publication  . . . . . . . . . . . . . . . . . . . .  20  21
     A.1.  Since draft-ietf-httpbis-header-compression-05 draft-ietf-httpbis-header-compression-06  . . . . .  20  21
     A.2.  Since draft-ietf-httpbis-header-compression-04 draft-ietf-httpbis-header-compression-05  . . . . .  20  21
     A.3.  Since draft-ietf-httpbis-header-compression-03 draft-ietf-httpbis-header-compression-04  . . . . .  21
     A.4.  Since draft-ietf-httpbis-header-compression-02 draft-ietf-httpbis-header-compression-03  . . . . .  21
     A.5.  Since draft-ietf-httpbis-header-compression-01 draft-ietf-httpbis-header-compression-02  . . . . .  21  22
     A.6.  Since draft-ietf-httpbis-header-compression-01  . . . . .  22
     A.7.  Since draft-ietf-httpbis-header-compression-00  . . . . .  21  22
   Appendix B.  Static Table . . . . . . . . . . . . . . . . . . . .  22  23
   Appendix C.  Huffman Codes  . . . . . . . . . . . . . . . . . . .  24  25
   Appendix D.  Examples . . . . . . . . . . . . . . . . . . . . . .  29  31
     D.1.  Integer Representation Examples . . . . . . . . . . . . .  31
       D.1.1.  Example 1: Encoding 10 using a 5-bit prefix . . . . .  31
       D.1.2.  Example 2: Encoding 1337 using a 5-bit prefix . . . .  31
       D.1.3.  Example 3: Encoding 42 starting at an
               octet-boundary  . . . . . . . . . . . . . . . . . . .  32
     D.2.  Header Field Representation Examples  . . . . . . . . . .  30
       D.1.1.  32
       D.2.1.  Literal Header Field with Indexing  . . . . . . . . .  30
       D.1.2.  32
       D.2.2.  Literal Header Field without Indexing . . . . . . . .  30
       D.1.3.  33
       D.2.3.  Indexed Header Field  . . . . . . . . . . . . . . . .  31
       D.1.4.  34
       D.2.4.  Indexed Header Field from Static Table  . . . . . . .  32
     D.2.  35
     D.3.  Request Examples without Huffman  . . . . . . . . . . . .  33
       D.2.1.  35
       D.3.1.  First request . . . . . . . . . . . . . . . . . . . .  33
       D.2.2.  35
       D.3.2.  Second request  . . . . . . . . . . . . . . . . . . .  34
       D.2.3.  37
       D.3.3.  Third request . . . . . . . . . . . . . . . . . . . .  35
     D.3.  38
     D.4.  Request Examples with Huffman . . . . . . . . . . . . . .  37
       D.3.1.  40
       D.4.1.  First request . . . . . . . . . . . . . . . . . . . .  37
       D.3.2.  40
       D.4.2.  Second request  . . . . . . . . . . . . . . . . . . .  38
       D.3.3.  41
       D.4.3.  Third request . . . . . . . . . . . . . . . . . . . .  40
     D.4.  42
     D.5.  Response Examples without Huffman . . . . . . . . . . . .  42
       D.4.1.  44
       D.5.1.  First response  . . . . . . . . . . . . . . . . . . .  42
       D.4.2.  44
       D.5.2.  Second response . . . . . . . . . . . . . . . . . . .  44
       D.4.3.  46
       D.5.3.  Third response  . . . . . . . . . . . . . . . . . . .  45
     D.5.  47
     D.6.  Response Examples with Huffman  . . . . . . . . . . . . .  47
       D.5.1.  49
       D.6.1.  First response  . . . . . . . . . . . . . . . . . . .  47
       D.5.2.  49
       D.6.2.  Second response . . . . . . . . . . . . . . . . . . .  50
       D.5.3.  52
       D.6.3.  Third response  . . . . . . . . . . . . . . . . . . .  51  53

1.  Introduction
   This specification defines HPACK, a compression format for
   efficiently representing HTTP header fields in the context of HTTP/2
   (see [HTTP2]).

2.  Overview

   In HTTP/1.1 (see [HTTP-p1]), header fields are 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 measurably increase latency and unnecessarily consume bandwidth
   (see [PERF1] and [PERF2]).

   SPDY [SPDY] initially addressed this redundancy by compressing header
   fields using the DEFLATE format [DEFLATE], which proved very
   effective at efficiently representing the redundant header fields.
   However, that approach exposed a security risk as demonstrated by the
   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 requirement for
   use in constrained environments.

2.1.  Outline

   The HTTP header field encoding defined in this document is based on a
   header table that maps name-value pairs to index values.  The header
   table is incrementally updated during the HTTP/2 connection.

   A set of header fields is treated as an unordered collection of name-
   value pairs.  Names and values are considered to be opaque sequences
   of 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 is coded as a difference from the previous set.  The
   goal is to only encode the changes (header fields present in one of
   the sets that are absent from the other) between the two sets of
   header fields.

   A header field 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 a set of 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 encoded as changes to
   the set of references.

   The 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 in the process.
   This enables decoders to remain simple and understand a wide variety
   of encoders.

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

   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 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 are added to the current
      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.

   HTTP messages are exchanged between a client and a server in both
   directions.  The encoding of header fields in each direction is
   independent from the other direction.  There is a single encoding
   context for each direction used to encode all header fields sent in
   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 of the 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 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 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 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

      Three different literal representations are provided:

      *  A literal representation that does not add the header field to
         the header table (see Section 4.3.1). 4.3.2).

      *  A literal representation that does not add the header field to
         the header table and require that this header field always use
         a literal representation, in particular when re-encoded by an
         intermediary (see Section 4.3.3).

      *  A literal representation that adds the header field as a new
         entry at the beginning of the header table (see Section 4.3.2). 4.3.1).

   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 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 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 table) + 1 refers to the first
      entry in the static table.

         Index 0 signals a modification of the encoding context: either
         the reference set 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. a decoding error.

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

                            Index Address Space

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, 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 set, except if this new entry didn't fit in the
         header table). 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 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 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 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 (maximum size - new entry
   size), or until the table is empty.

   If the size of the new entry is less than or equal to 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 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 [4]
   representation.
   representation (see <http://en.wikipedia.org/wiki/Variable-
   length_quantity>).  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 I < 2^N - 1, encode I on N bits
   else
       encode (2^N - 1) on N bits
       I = I - (2^N - 1)
       while I >= 128
            encode (I % 128 + 128) on 8 bits
            I = I / 128
       encode 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

   Examples illustrating the encoding of integers are available in
   Appendix D.1.

   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

4.1.2.  String Literal Representation

   Header field names and header field values can be encoded with a 5-bit prefix.

   o  10 is less than 31 (= 2^5 - 1) and is represented using as
   literal string.  A literal string is encoded as a sequence of octets,
   either by directly encoding the 5-bit
      prefix. literal string's octets, or by using
   a canonical [CANON] Huffman encoding [HUFF].

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | X | X | X | 0 | 1 H | 0    String Length (7+)     | 1
   +---+---------------------------+
   | 0  String Data (Length octets)  |   10 stored on 5 bits
   +---+---+---+---+---+---+---+---+

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

                       String Literal Representation

   A literal string representation contains the following fields:

   H: A one bit flag, H, indicating whether or not the octets of the
      string are Huffman encoded.

   String Length:  The value I=1337 is number of octets used to be encode the string
      literal, encoded as an integer with a 5-bit prefix.

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

         The 5-bit 7-bit prefix is filled with its max value (31).

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

         I (1306) is greater than or equal to 128, (see
      Section 4.1.1).

   String Data:  The encoded data of the while loop body
         executes:

            I % 128 == 26

            26 + 128 == 154
            154 string literal.  If H is '0',
      then the encoded in 8 bits as: 10011010

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

            I the raw octets of the string literal.  If
      H is no longer greater than or equal to 128, '1', then the while loop
            terminates.

         I, now 10, encoded data is the Huffman encoding of the
      string literal.

   String literals which use Huffman encoding are 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&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 with 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
   Huffman codes defined in
   three parts:

   1.  One bit, H, indicating whether or not the octets are Appendix C (see examples inRequest Examples
   with Huffman
       encoded.

   2.  The number of octets required to hold the result of the next
       step, represented as an integer Appendix D.4 and in Response Examples with a 7-bit prefix (see
       Section 4.1.1), immediately following the first bit.

   3. Huffman
   Appendix D.6).  The encoded data of the string:

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

       *  If H is '0', then the encoded string is the octets of literal.

   As the
          field value without modification.

   Padding is necessary when doing Huffman encoding to ensure that the
   remaining bits between the actual end of the encoded data and doesn't always end at an octet boundary,
   some padding is inserted after it up to the next octet
   boundary are not boundary.  To
   prevent this padding to be misinterpreted as part of the input data.

   When padding for Huffman encoding, string
   literal, the most significant bits from of the EOS (end-of-
   string) (end-of-string) entry
   in the Huffman table are used, starting with used.

   Upon decoding, an incomplete Huffman code at the MSB
   (most significant bit).  This entry end of the encoded
   data is guaranteed to be at least 8 considered as padding and discarded.  A padding
   strictly longer than 7 bits long.

   String literals which use MUST be treated as a decoding error.  A
   padding not corresponding to the most significant bits of the EOS
   entry MUST be treated as a decoding error.  A Huffman encoding are encoded with string
   literal containing the
   Huffman Codes Appendix C (see examples in Request Examples with
   Huffman Appendix D.3 and in Response Examples with Huffman
   Appendix D.5).

   The EOS symbol is represented with value 256, and is used solely to
   signal the end of entry MUST be treated as a decoding error.

4.2.  Indexed Header Field Representation

   An indexed header field representation either identifies an entry in
   the Huffman-encoded key data header table or the end of the
   Huffman-encoded value data.  Given that only between 0-7 bits static table.  The processing of the
   EOS symbol an indexed
   header field representation is included described 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. Section 3.2.1.

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

                   String
   +---+---------------------------+

                           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 not used.  It MUST be treated as a decoding
   error if found in an indexed header field representation.

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 with Huffman Encoding 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 |  Value Length Prefix (7) 1 |
   +---+---+---+---+---+---+---+---+      Index (6+)       |
   +---+---+-----------------------+
   | H |     Value Length (0-N octets)   |
   +---+---+---+---+---+---+---+---+
   ...
   +---+---+---+---+---+---+---+---+ (7+)     |  Field Bytes without Encoding
   +---+---------------------------+
   |
   +---+---+---+---+---+---+---+---+ Value String Literal 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. (Length octets)  |
   +-------------------------------+

       Literal Header Field with Incremental Indexing - Indexed Name

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

                           Indexed
   |  Name String (Length octets)  |
   +---+---------------------------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+

         Literal Header Field with Incremental Indexing - New Name

   This representation starts with the '1' 1-bit pattern, followed by '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 matching pair, index of the entry, index (which is strictly greater than
   0), is represented as an integer with a
   7-bit prefix. 6-bit prefix (see
   Section 4.1.1).

   Otherwise, the header field name is represented as a literal.  The index
   value of 0 is reserved for signalling changes in represented on 6 bits followed by the
   encoding context header field name
   (see Section 4.4).

4.3.  Literal Header Field Representation

   Literal 4.1.2).

   The header field representations contain a literal name representation is followed by the header field
   value.  Header field names are either provided
   value represented as a literal or by
   reference to an existing header table or static table entry.

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

4.3.1. Section 4.1.2.

4.3.2.  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 0 | 0 | 0 |  Index (6+) (4+)   |
   +---+---+---+-------------------+
   +---+---+-----------------------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+

           Literal Header Field without Indexing - Indexed Name

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

             Literal Header Field without Indexing - New Name

   This

   The literal header field without indexing representation starts with
   the '01' 2-bit '0000' 4-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 4 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.

4.3.3.  Literal Header Field with Incremental Indexing never Indexed

   A literal header field with incremental indexing adds never indexed causes the emission of a new entry to header
   field without altering the header table.

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

             Literal Header Field with Incremental Indexing Field never Indexed - Indexed Name

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

               Literal Header Field with Incremental Indexing never Indexed - New Name

   This

   The literal header field never indexed representation starts with the '00' 2-bit
   '0001' 4-bit pattern.

   If the header field name matches the header field name of

   When 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 a literal header field never
   indexed, it MUST always be encoded with this same representation.  In
   particular, when a peer sends a 6-bit prefix (see
   Section 4.1.1).

   Otherwise, the header field name is that it received
   represented as a literal.  The
   value 0 is represented on 6 bits followed by literal header field never indexed, it MUST use the
   same representation to forward this header field.

   This representation is intended for protecting header field name values
   that are not to be put at risk by compressing them (see Section 4.1.2). 5.1
   for more details).

   The header field name encoding of the representation is followed by the header field
   value represented same as a for the literal string as described in
   header field without indexing representation (see Section 4.1.2. 4.3.2).

4.4.  Encoding Context Update

   An indexed value encoding context update causes the immediate application of 0 is reserved for signalling changes in a
   change to the encoding context.  The

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

                              Context Update

   An encoding context update starts with the '001' 3-bit pattern.

   It is followed by a flag specifying the type of the change is encoded on change, and by
   any data necessary to describe the following
   octet(s).  Any change in the encoding context is applied immediately. itself.

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

                          Reference Set Emptying

   An octet with its high
   +---+---------------------------+

                          Reference Set Emptying

   The flag bit being 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 0 | 1 | 0 | Max size (7+) (4+) |
   +---+---------------------------+

                     Maximum Header Table Size Change

   An octet with its high

   The flag bit being set to '0' signals that a change to 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 4-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

5.1.  Compression-based Attacks

   Compression can be
   efficiently probed create a weak point allowing an attacker to reveal secrets.  A conformant implementation recover
   secret data.  For example, the CRIME attack (see [CRIME]) took
   advantage of
   this specification should be fairly safe against that kind the DEFLATE mechanism (see [DEFLATE]) of attack,
   as SPDY (see
   [SPDY]) to efficiently probe the reaping compression context.  The full-text
   compression mechanism of any DEFLATE allowed the attacker to learn some
   information from each failed attempt at guessing the secret.

   For this reason, HPACK provides only limited compression context
   requires more work than guessing and verifying mechanisms
   in the plain text data
   directly with form of an indexing table and of a static Huffman encoding.

   The indexing table can still provide information to an attacker that
   would be able to probe the server.  As with any secret, however, compression context.  However, this
   information is limited to the longer knowledge of whether the length attacker's
   guess is correct or not.

   Still, an attacker could take advantage of this limited information
   for breaking low-entropy secrets using a brute-force attack.  A
   server usually has some protections against such brute-force attack.
   Here, the secret, attack would target the client, where it would be harder to
   detect.  The attack would be even more difficult dangerous if the secret attacker is
   able to guess.
   It is inadvisable prevent the traffic generated by its brute-force attack from
   reaching the server.

   To offer some protection against such type of attacks, HPACK enables
   an endpoint to have short cookies indicate that are relied upon to
   remain secret for a header field must never be compressed,
   across any duration hop up to the other endpoint (see Section 4.3.3).  An
   endpoint MUST use this feature to prevent the compression of time.

   A proper security-conscious implementation will also need any
   header field whose value contains a secret which could be put at risk
   by a brute-force attack.

   For optimal processing, a sensitive value (for example a cookie)
   needs to have an entropy high enough to prevent
   timing attacks not be endangered by ensuring that the amount of time it takes a brute-
   force attack, in order to do
   string comparisons take advantage of HPACK indexing.

   There is always a function currently no known threat taking advantage of the total length use of the
   strings, and not a function of the number of matched characters.
   fixed Huffman encoding.  A decoder needs to ensure study has shown that larger values or encodings of integers
   do using a fixed Huffman
   encoding table created an information leakage, however this same
   study concluded that an attacker could not permit exploitation.  Decoders MUST limit the size take advantage of
   integers, both in value and encoded length, that it accepts this
   information leakage to recover any meaningful amount of information
   (see
   Section 4.1.1).

   Another common security problem is when the remote endpoint
   successfully causes the local [PETAL]).

5.2.  Memory Consumption

   An attacker can try to cause an endpoint to exhaust its memory.  This
   compressor attempts
   HPACK is designed to deal with the most obvious ways that this
   could occur by limiting limit both the peak and the steady-state state amounts of memory
   allocated by an endpoint.

   The amount of memory consumed in used by the compressor state, state is limited by providing ways for the
   application to consume/flush
   value of the emitted header fields in small
   chunks, and by considering overhead in setting SETTINGS_HEADER_TABLE_SIZE.  This limitation
   takes into account both the state size calculation.
   Implementors must still be careful of the data stored in the creation of APIs to an
   implementation of this compressor by ensuring that header field keys
   table, and values are either emitted as a stream, or that the compression
   implementation have a limit on overhead required by the maximum size of a key or value.
   Failure to implement these kinds of safeguards may still result in a
   scenario where table structure itself.

   For the local decoding side, an endpoint exhausts its memory.

   A particular care should be can limit the amount of state
   memory used by setting an appropriate value for
   SETTINGS_HEADER_TABLE_SIZE.  For the maximum size of encoding side, the header
   table.  While an endpoint can fully control
   limit the amount of state memory it uses by defining a header table
   maximum size lower than the value of SETTINGS_HEADER_TABLE_SIZE
   defined by its peer (see Section 4.4).

   The amount of temporary memory consumed is linked to the maximum size set of its
   header table for the decoding size, fields emitted or received.  However, this amount of temporary
   memory can be limited by using
   SETTINGS_HEADER_TABLE_SIZE, the maximum size processing these header fields in a
   streaming manner.

5.3.  Implementation Limits

   An implementation of the HPACK needs to ensure that large values for
   integers, long encoding size is
   controlled by for integers, or long string literal do not
   create security weaknesses.

   An implementation has to set a limit for the remote peer.  The endpoint should check values it accepts for
   integers, as well as for the
   SETTINGS_HEADER_TABLE_SIZE defined by encoded length (see Section 4.1.1).  In
   the remote peer, and decrease same way, it has to set a limit to the maximum size length it accepts for the encoding size if needed.
   string literals (see Section 4.1.2).

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", draft-ietf-httpbis-http2-10
              (work in progress), 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 Attack", September
              2012, <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", March 2012, <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>.

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

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

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

A.1.  Since draft-ietf-httpbis-header-compression-06

   o  Updated format to include literal headers that must never be
      compressed.

   o  Updated security considerations.

   o  Moved integer encoding examples to the appendix.

   o  Updated Huffman table.

   o  Updated static header table (adding and removing status values).

   o  Updated examples.

A.2.  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.

A.3.  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.3.

A.4.  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.4.

A.5.  Since draft-ietf-httpbis-header-compression-02

   o  Corrected error in integer encoding pseudocode.

A.5.

A.6.  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.6.

A.7.  Since 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, table) + 1, and the last entry in the table is
   represented by the index len(header table)+len(static table) + len(static table).

   [[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
   header field rules (like removal of some header fields).  This set of
   header fields is currently very likely incomplete, and should be made
   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                     | 500 204          |
          | 10    | :status                     | 404 206          |
          | 11    | :status                     | 403 304          |
          | 12    | :status                     | 400          |
          | 13    | :status                     | 401 404          |
          | 14    | accept-charset :status                     | 500          |
          | 15    | accept-encoding accept-charset              |              |
          | 16    | accept-language accept-encoding             |              |
          | 17    | accept-ranges accept-language             |              |
          | 18    | accept accept-ranges               |              |
          | 19    | access-control-allow-origin accept                      |              |
          | 20    | age access-control-allow-origin |              |
          | 21    | allow age                         |              |
          | 22    | authorization allow                       |              |
          | 23    | cache-control authorization               |              |
          | 24    | content-disposition cache-control               |              |
          | 25    | content-disposition         |              |
          | 26    | content-encoding            |              |
          | 26 27    | content-language            |              |
          | 27 28    | content-length              |              |
          | 28 29    | content-location            |              |
          | 29 30    | content-range               |              |
          | 30 31    | content-type                |              |
          | 31 32    | cookie                      |              |
          | 32 33    | date                        |              |
          | 33 34    | etag                        |              |
          | 34 35    | expect                      |              |
          | 35 36    | expires                     |              |
          | 36 37    | from                        |              |
          | 37 38    | host                        |              |
          | 38 39    | if-match                    |              |
          | 39 40    | if-modified-since           |              |
          | 40 41    | if-none-match               |              |
          | 41 42    | if-range                    |              |
          | 42 43    | if-unmodified-since         |              |
          | 43 44    | last-modified               |              |
          | 44 45    | link                        |              |
          | 45 46    | location                    |              |
          | 46 47    | max-forwards                |              |
          | 47 48    | proxy-authenticate          |              |
          | 48 49    | proxy-authorization         |              |
          | 49 50    | range                       |              |
          | 50 51    | referer                     |              |
          | 51 52    | refresh                     |              |
          | 52 53    | retry-after                 |              |
          | 53 54    | server                      |              |
          | 54 55    | set-cookie                  |              |
          | 55 56    | strict-transport-security   |              |
          | 56 57    | transfer-encoding           |              |
          | 57 58    | user-agent                  |              |
          | 58 59    | vary                        |              |
          | 59 60    | via                         |              |
          | 60 61    | 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

   The following Huffman codes are used when encoding string literals.

   [[This literals with an
   Huffman coding (see Section 4.1.2).

   Each row in the table will specifies one Huffman code:

   sym:  The symbol to be regenerated.  ]]

              aligned                                  aligned represented.  It is the decimal value of an
      octet, possibly prepended with its ASCII representation.  A
      specific symbol, "EOS", is used to                            len indicate the end of a string
      literal.

   code as bits:  The Huffman code for the symbol represented as a
      base-2 integer.

   code as hex:  The Huffman code for the symbol, represented as a
      hexadecimal integer, aligned on the least significant bit.

   len:  The number of bits for the Huffman code of the symbol.

   As an example, the Huffman code for the symbol 48 (corresponding to     len
               MSB                            in        LSB
   the ASCII character "0") consists in
        sym    as the 5 bits "0", "0", "1", "0",
   "1".  This corresponds to the value 5 encoded on 5 bits.

                                               code
                       code as bits           as hex   len
        sym           aligned to MSB          aligned   in
                                              to LSB   bits
       (  0) |11111111|11111111|11110111|010 [27]      7ffffba [27]  |11111111|11111111|11101110|10  3ffffba  [26]
       (  1) |11111111|11111111|11110111|011 [27]      7ffffbb [27]  |11111111|11111111|11101110|11  3ffffbb  [26]
       (  2) |11111111|11111111|11110111|100 [27]      7ffffbc [27]  |11111111|11111111|11101111|00  3ffffbc  [26]
       (  3) |11111111|11111111|11110111|101 [27]      7ffffbd [27]  |11111111|11111111|11101111|01  3ffffbd  [26]
       (  4) |11111111|11111111|11110111|110 [27]      7ffffbe [27]  |11111111|11111111|11101111|10  3ffffbe  [26]
       (  5) |11111111|11111111|11110111|111 [27]      7ffffbf [27]  |11111111|11111111|11101111|11  3ffffbf  [26]
       (  6) |11111111|11111111|11111000|000 [27]      7ffffc0 [27]  |11111111|11111111|11110000|00  3ffffc0  [26]
       (  7) |11111111|11111111|11111000|001 [27]      7ffffc1 [27]  |11111111|11111111|11110000|01  3ffffc1  [26]
       (  8) |11111111|11111111|11111000|010 [27]      7ffffc2 [27]  |11111111|11111111|11110000|10  3ffffc2  [26]
       (  9) |11111111|11111111|11111000|011 [27]      7ffffc3 [27]  |11111111|11111111|11110000|11  3ffffc3  [26]
       ( 10) |11111111|11111111|11111000|100 [27]      7ffffc4 [27]  |11111111|11111111|11110001|00  3ffffc4  [26]
       ( 11) |11111111|11111111|11111000|101 [27]      7ffffc5 [27]  |11111111|11111111|11110001|01  3ffffc5  [26]
       ( 12) |11111111|11111111|11111000|110 [27]      7ffffc6 [27]  |11111111|11111111|11110001|10  3ffffc6  [26]
       ( 13) |11111111|11111111|11111000|111 [27]      7ffffc7 [27]  |11111111|11111111|11110001|11  3ffffc7  [26]
       ( 14) |11111111|11111111|11111001|000 [27]      7ffffc8 [27]  |11111111|11111111|11110010|00  3ffffc8  [26]
       ( 15) |11111111|11111111|11111001|001 [27]      7ffffc9 [27]  |11111111|11111111|11110010|01  3ffffc9  [26]
       ( 16) |11111111|11111111|11111001|010 [27]      7ffffca [27]  |11111111|11111111|11110010|10  3ffffca  [26]
       ( 17) |11111111|11111111|11111001|011 [27]      7ffffcb [27]  |11111111|11111111|11110010|11  3ffffcb  [26]
       ( 18) |11111111|11111111|11111001|100 [27]      7ffffcc [27]  |11111111|11111111|11110011|00  3ffffcc  [26]
       ( 19) |11111111|11111111|11111001|101 [27]      7ffffcd [27]  |11111111|11111111|11110011|01  3ffffcd  [26]
       ( 20) |11111111|11111111|11111001|110 [27]      7ffffce [27]  |11111111|11111111|11110011|10  3ffffce  [26]
       ( 21) |11111111|11111111|11111001|111 [27]      7ffffcf [27]  |11111111|11111111|11110011|11  3ffffcf  [26]
       ( 22) |11111111|11111111|11111010|000 [27]      7ffffd0 [27]  |11111111|11111111|11110100|00  3ffffd0  [26]
       ( 23) |11111111|11111111|11111010|001 [27]      7ffffd1 [27]  |11111111|11111111|11110100|01  3ffffd1  [26]
       ( 24) |11111111|11111111|11111010|010 [27]      7ffffd2 [27]  |11111111|11111111|11110100|10  3ffffd2  [26]
       ( 25) |11111111|11111111|11111010|011 [27]      7ffffd3 [27]  |11111111|11111111|11110100|11  3ffffd3  [26]
       ( 26) |11111111|11111111|11111010|100 [27]      7ffffd4 [27]  |11111111|11111111|11110101|00  3ffffd4  [26]
       ( 27) |11111111|11111111|11111010|101 [27]      7ffffd5 [27]  |11111111|11111111|11110101|01  3ffffd5  [26]
       ( 28) |11111111|11111111|11111010|110 [27]      7ffffd6 [27]  |11111111|11111111|11110101|10  3ffffd6  [26]
       ( 29) |11111111|11111111|11111010|111 [27]      7ffffd7 [27]  |11111111|11111111|11110101|11  3ffffd7  [26]
       ( 30) |11111111|11111111|11111011|000 [27]      7ffffd8 [27]  |11111111|11111111|11110110|00  3ffffd8  [26]
       ( 31) |11111111|11111111|11111011|001 [27]      7ffffd9 [27]  |11111111|11111111|11110110|01  3ffffd9  [26]
   ' ' ( 32) |11101000| [8]                                  e8 [8]  |00110                                6  [ 5]
   '!' ( 33) |11111111|1100 [12]                           ffc [12]  |11111111|11100                    1ffc  [13]
   '"' ( 34) |11111111|111010 [14]                        3ffa [14]  |11111000|0                         1f0  [ 9]
   '#' ( 35) |11111111|1111100 [15]                       7ffc [15]  |11111111|111100                   3ffc  [14]
   '$' ( 36) |11111111|1111101 [15]                       7ffd  |11111111|1111100                  7ffc  [15]
   '%' ( 37) |100100 [6]                                     24 [6]  |011110                              1e  [ 6]
   '&' ( 38) |1101110 [7]                                    6e [7]  |1100100                             64  [ 7]
   ''' ( 39) |11111111|1111110 [15]                       7ffe [15]  |11111111|11101                    1ffd  [13]
   '(' ( 40) |11111111|010 [11]                            7fa [11]  |11111110|10                        3fa  [10]
   ')' ( 41) |11111111|011 [11]                            7fb [11]  |11111000|1                         1f1  [ 9]
   '*' ( 42) |11111110|10 [10]                             3fa  |11111110|11                        3fb  [10]
   '+' ( 43) |11111111|100 [11]                            7fc [11]  |11111111|00                        3fc  [10]
   ',' ( 44) |11101001| [8]                                  e9 [8]  |1100101                             65  [ 7]
   '-' ( 45) |100101 [6]                                     25 [6]  |1100110                             66  [ 7]
   '.' ( 46) |00100 [5]                                       4 [5]  |011111                              1f  [ 6]
   '/' ( 47) |0000 [4]                                        0 [4]  |00111                                7  [ 5]
   '0' ( 48) |00101 [5]                                       5 [5]  |0000                                 0  [ 4]
   '1' ( 49) |00110 [5]                                       6 [5]  |0001                                 1  [ 4]
   '2' ( 50) |00111 [5]                                       7 [5]  |0010                                 2  [ 4]
   '3' ( 51) |100110 [6]                                     26 [6]  |01000                                8  [ 5]
   '4' ( 52) |100111 [6]                                     27 [6]  |100000                              20  [ 6]
   '5' ( 53) |101000 [6]                                     28 [6]  |100001                              21  [ 6]
   '6' ( 54) |101001 [6]                                     29 [6]  |100010                              22  [ 6]
   '7' ( 55) |101010 [6]                                     2a [6]  |100011                              23  [ 6]
   '8' ( 56) |101011 [6]                                     2b [6]  |100100                              24  [ 6]
   '9' ( 57) |101100 [6]                                     2c [6]  |100101                              25  [ 6]
   ':' ( 58) |11110110|0 [9]                                1ec [9]  |100110                              26  [ 6]
   ';' ( 59) |11101010| [8]                                  ea [8]  |11101100|                           ec  [ 8]
   '<' ( 60) |11111111|11111111|10 [18]                  3fffe [18]  |11111111|11111110|0              1fffc  [17]
   '=' ( 61) |101101 [6]                                     2d [6]  |100111                              27  [ 6]
   '>' ( 62) |11111111|11111110|0 [17]                   1fffc [17]  |11111111|1111101                  7ffd  [15]
   '?' ( 63) |11110110|1 [9]                                1ed [9]  |11111111|01                        3fd  [10]
   '@' ( 64) |11111111|111011 [14]                        3ffb [14]  |11111111|1111110                  7ffe  [15]
   'A' ( 65) |1101111 [7]                                    6f [7]  |1100111                             67  [ 7]
   'B' ( 66) |11101011| [8]                                  eb [8]  |11101101|                           ed  [ 8]
   'C' ( 67) |11101100| [8]                                  ec [8]  |11101110|                           ee  [ 8]
   'D' ( 68) |11101101| [8]                                  ed [8]  |1101000                             68  [ 7]
   'E' ( 69) |11101110| [8]                                  ee [8]  |11101111|                           ef  [ 8]
   'F' ( 70) |1110000 [7]                                    70 [7]  |1101001                             69  [ 7]
   'G' ( 71) |11110111|0 [9]                                1ee [9]  |1101010                             6a  [ 7]
   'H' ( 72) |11110111|1 [9]                                1ef [9]  |11111001|0                         1f2  [ 9]
   'I' ( 73) |11111000|0 [9]                                1f0 [9]  |11110000|                           f0  [ 8]
   'J' ( 74) |11111000|1 [9]                                1f1 [9]  |11111001|1                         1f3  [ 9]
   'K' ( 75) |11111110|11 [10]                             3fb [10]  |11111010|0                         1f4  [ 9]
   'L' ( 76) |11111001|0 [9]                                1f2 [9]  |11111010|1                         1f5  [ 9]
   'M' ( 77) |11101111| [8]                                  ef [8]  |1101011                             6b  [ 7]
   'N' ( 78) |11111001|1 [9]                                1f3 [9]  |1101100                             6c  [ 7]
   'O' ( 79) |11111010|0 [9]                                1f4 [9]  |11110001|                           f1  [ 8]
   'P' ( 80) |11111010|1 [9]                                1f5 [9]  |11110010|                           f2  [ 8]
   'Q' ( 81)  |11111011|0 [9]                         1f6 [9]  [ 9]
   'R' ( 82)  |11111011|1 [9]                         1f7 [9]  [ 9]
   'S' ( 83) |11110000| [8]                                  f0 [8]  |1101101                             6d  [ 7]
   'T' ( 84) |11110001| [8]                                  f1 [8]  |101000                              28  [ 6]
   'U' ( 85) |11111100|0 [9]                                1f8 [9]  |11110011|                           f3  [ 8]
   'V' ( 86) |11111100|1 [9]                                1f9 [9]  |11111100|0                         1f8  [ 9]
   'W' ( 87) |11111101|0 [9]                                1fa [9]  |11111100|1                         1f9  [ 9]
   'X' ( 88) |11111101|1 [9]                                1fb [9]  |11110100|                           f4  [ 8]
   'Y' ( 89) |11111110|0 [9]                                1fc [9]  |11111101|0                         1fa  [ 9]
   'Z' ( 90) |11111111|00 [10]                             3fc [10]  |11111101|1                         1fb  [ 9]
   '[' ( 91) |11111111|111100 [14]                        3ffc [14]  |11111111|100                       7fc  [11]
   '\' ( 92) |11111111|11111111|11111011|010 [27]      7ffffda [27]  |11111111|11111111|11110110|10  3ffffda  [26]
   ']' ( 93) |11111111|11100 [13]                         1ffc [13]  |11111111|101                       7fd  [11]
   '^' ( 94)  |11111111|111101 [14]                   3ffd  [14]
   '_' ( 95) |101110 [6]                                     2e [6]  |1101110                             6e  [ 7]
   '`' ( 96) |11111111|11111111|110 [19]                 7fffe [19]  |11111111|11111111|10             3fffe  [18]
   'a' ( 97) |01000 [5]                                       8 [5]  |01001                                9  [ 5]
   'b' ( 98) |101111 [6]                                     2f [6]  |1101111                             6f  [ 7]
   'c' ( 99) |01001 [5]                                       9 [5]  |01010                                a  [ 5]
   'd' (100) |110000 [6]                                     30 [6]  |101001                              29  [ 6]
   'e' (101) |0001 [4]                                        1 [4]  |01011                                b  [ 5]
   'f' (102) |110001 [6]                                     31 [6]  |1110000                             70  [ 7]
   'g' (103) |110010 [6]                                     32 [6]  |101010                              2a  [ 6]
   'h' (104) |110011 [6]                                     33 [6]  |101011                              2b  [ 6]
   'i' (105) |01010 [5]                                       a [5]  |01100                                c  [ 5]
   'j' (106) |1110001 [7]                                    71 [7]  |11110101|                           f5  [ 8]
   'k' (107) |1110010 [7]                                    72 [7]  |11110110|                           f6  [ 8]
   'l' (108) |01011 [5]                                       b [5]  |101100                              2c  [ 6]
   'm' (109) |110100 [6]                                     34 [6]  |101101                              2d  [ 6]
   'n' (110) |01100 [5]                                       c [5]  |101110                              2e  [ 6]
   'o' (111)  |01101 [5]                                d [5]  [ 5]
   'p' (112) |01110 [5]                                       e [5]  |101111                              2f  [ 6]
   'q' (113) |11110010| [8]                                  f2 [8]  |11111110|0                         1fc  [ 9]
   'r' (114) |01111 [5]                                       f [5]  |110000                              30  [ 6]
   's' (115) |10000 [5]                                      10 [5]  |110001                              31  [ 6]
   't' (116) |10001 [5]                                      11 [5]  |01110                                e  [ 5]
   'u' (117) |110101 [6]                                     35 [6]  |1110001                             71  [ 7]
   'v' (118) |1110011 [7]                                    73 [7]  |1110010                             72  [ 7]
   'w' (119) |110110 [6]                                     36 [6]  |1110011                             73  [ 7]
   'x' (120) |11110011| [8]                                  f3 [8]  |1110100                             74  [ 7]
   'y' (121) |11110100| [8]                                  f4 [8]  |1110101                             75  [ 7]
   'z' (122) |11110101| [8]                                  f5 [8]  |11110111|                           f7  [ 8]
   '{' (123)  |11111111|11111110|1 [17]              1fffd  [17]
   '|' (124) |11111111|101 [11]                            7fd [11]  |11111111|1100                      ffc  [12]
   '}' (125)  |11111111|11111111|0 [17]              1fffe  [17]
   '~' (126)  |11111111|1101 [12]                      ffd  [12]
       (127) |11111111|11111111|11111011|011 [27]      7ffffdb [27]  |11111111|11111111|11110110|11  3ffffdb  [26]
       (128) |11111111|11111111|11111011|100 [27]      7ffffdc [27]  |11111111|11111111|11110111|00  3ffffdc  [26]
       (129) |11111111|11111111|11111011|101 [27]      7ffffdd [27]  |11111111|11111111|11110111|01  3ffffdd  [26]
       (130) |11111111|11111111|11111011|110 [27]      7ffffde [27]  |11111111|11111111|11110111|10  3ffffde  [26]
       (131) |11111111|11111111|11111011|111 [27]      7ffffdf [27]  |11111111|11111111|11110111|11  3ffffdf  [26]
       (132) |11111111|11111111|11111100|000 [27]      7ffffe0 [27]  |11111111|11111111|11111000|00  3ffffe0  [26]
       (133) |11111111|11111111|11111100|001 [27]      7ffffe1 [27]  |11111111|11111111|11111000|01  3ffffe1  [26]
       (134) |11111111|11111111|11111100|010 [27]      7ffffe2 [27]  |11111111|11111111|11111000|10  3ffffe2  [26]
       (135) |11111111|11111111|11111100|011 [27]      7ffffe3 [27]  |11111111|11111111|11111000|11  3ffffe3  [26]
       (136) |11111111|11111111|11111100|100 [27]      7ffffe4 [27]  |11111111|11111111|11111001|00  3ffffe4  [26]
       (137) |11111111|11111111|11111100|101 [27]      7ffffe5 [27]  |11111111|11111111|11111001|01  3ffffe5  [26]
       (138) |11111111|11111111|11111100|110 [27]      7ffffe6 [27]  |11111111|11111111|11111001|10  3ffffe6  [26]
       (139) |11111111|11111111|11111100|111 [27]      7ffffe7 [27]  |11111111|11111111|11111001|11  3ffffe7  [26]
       (140) |11111111|11111111|11111101|000 [27]      7ffffe8 [27]  |11111111|11111111|11111010|00  3ffffe8  [26]
       (141) |11111111|11111111|11111101|001 [27]      7ffffe9 [27]  |11111111|11111111|11111010|01  3ffffe9  [26]
       (142) |11111111|11111111|11111101|010 [27]      7ffffea [27]  |11111111|11111111|11111010|10  3ffffea  [26]
       (143) |11111111|11111111|11111101|011 [27]      7ffffeb [27]  |11111111|11111111|11111010|11  3ffffeb  [26]
       (144) |11111111|11111111|11111101|100 [27]      7ffffec [27]  |11111111|11111111|11111011|00  3ffffec  [26]
       (145) |11111111|11111111|11111101|101 [27]      7ffffed [27]  |11111111|11111111|11111011|01  3ffffed  [26]
       (146) |11111111|11111111|11111101|110 [27]      7ffffee [27]  |11111111|11111111|11111011|10  3ffffee  [26]
       (147) |11111111|11111111|11111101|111 [27]      7ffffef [27]  |11111111|11111111|11111011|11  3ffffef  [26]
       (148) |11111111|11111111|11111110|000 [27]      7fffff0 [27]  |11111111|11111111|11111100|00  3fffff0  [26]
       (149) |11111111|11111111|11111110|001 [27]      7fffff1 [27]  |11111111|11111111|11111100|01  3fffff1  [26]
       (150) |11111111|11111111|11111110|010 [27]      7fffff2 [27]  |11111111|11111111|11111100|10  3fffff2  [26]
       (151) |11111111|11111111|11111110|011 [27]      7fffff3 [27]  |11111111|11111111|11111100|11  3fffff3  [26]
       (152) |11111111|11111111|11111110|100 [27]      7fffff4 [27]  |11111111|11111111|11111101|00  3fffff4  [26]
       (153) |11111111|11111111|11111110|101 [27]      7fffff5 [27]  |11111111|11111111|11111101|01  3fffff5  [26]
       (154) |11111111|11111111|11111110|110 [27]      7fffff6 [27]  |11111111|11111111|11111101|10  3fffff6  [26]
       (155) |11111111|11111111|11111110|111 [27]      7fffff7 [27]  |11111111|11111111|11111101|11  3fffff7  [26]
       (156) |11111111|11111111|11111111|000 [27]      7fffff8 [27]  |11111111|11111111|11111110|00  3fffff8  [26]
       (157) |11111111|11111111|11111111|001 [27]      7fffff9 [27]  |11111111|11111111|11111110|01  3fffff9  [26]
       (158) |11111111|11111111|11111111|010 [27]      7fffffa [27]  |11111111|11111111|11111110|10  3fffffa  [26]
       (159) |11111111|11111111|11111111|011 [27]      7fffffb [27]  |11111111|11111111|11111110|11  3fffffb  [26]
       (160) |11111111|11111111|11111111|100 [27]      7fffffc [27]  |11111111|11111111|11111111|00  3fffffc  [26]
       (161) |11111111|11111111|11111111|101 [27]      7fffffd [27]  |11111111|11111111|11111111|01  3fffffd  [26]
       (162) |11111111|11111111|11111111|110 [27]      7fffffe [27]
       (163) |11111111|11111111|11111111|111 [27]      7ffffff [27]
       (164) |11111111|11111111|11100000|00  |11111111|11111111|11111111|10  3fffffe  [26]       3ffff80
       (163)  |11111111|11111111|11111111|11  3ffffff  [26]
       (164)  |11111111|11111111|11000000|0   1ffff80  [25]
       (165) |11111111|11111111|11100000|01 [26]       3ffff81 [26]  |11111111|11111111|11000000|1   1ffff81  [25]
       (166) |11111111|11111111|11100000|10 [26]       3ffff82 [26]  |11111111|11111111|11000001|0   1ffff82  [25]
       (167) |11111111|11111111|11100000|11 [26]       3ffff83 [26]  |11111111|11111111|11000001|1   1ffff83  [25]
       (168) |11111111|11111111|11100001|00 [26]       3ffff84 [26]  |11111111|11111111|11000010|0   1ffff84  [25]
       (169) |11111111|11111111|11100001|01 [26]       3ffff85 [26]  |11111111|11111111|11000010|1   1ffff85  [25]
       (170) |11111111|11111111|11100001|10 [26]       3ffff86 [26]  |11111111|11111111|11000011|0   1ffff86  [25]
       (171) |11111111|11111111|11100001|11 [26]       3ffff87 [26]  |11111111|11111111|11000011|1   1ffff87  [25]
       (172) |11111111|11111111|11100010|00 [26]       3ffff88 [26]  |11111111|11111111|11000100|0   1ffff88  [25]
       (173) |11111111|11111111|11100010|01 [26]       3ffff89 [26]  |11111111|11111111|11000100|1   1ffff89  [25]
       (174) |11111111|11111111|11100010|10 [26]       3ffff8a [26]  |11111111|11111111|11000101|0   1ffff8a  [25]
       (175) |11111111|11111111|11100010|11 [26]       3ffff8b [26]  |11111111|11111111|11000101|1   1ffff8b  [25]
       (176) |11111111|11111111|11100011|00 [26]       3ffff8c [26]  |11111111|11111111|11000110|0   1ffff8c  [25]
       (177) |11111111|11111111|11100011|01 [26]       3ffff8d [26]  |11111111|11111111|11000110|1   1ffff8d  [25]
       (178) |11111111|11111111|11100011|10 [26]       3ffff8e [26]  |11111111|11111111|11000111|0   1ffff8e  [25]
       (179) |11111111|11111111|11100011|11 [26]       3ffff8f [26]  |11111111|11111111|11000111|1   1ffff8f  [25]
       (180) |11111111|11111111|11100100|00 [26]       3ffff90 [26]  |11111111|11111111|11001000|0   1ffff90  [25]
       (181) |11111111|11111111|11100100|01 [26]       3ffff91 [26]  |11111111|11111111|11001000|1   1ffff91  [25]
       (182) |11111111|11111111|11100100|10 [26]       3ffff92 [26]  |11111111|11111111|11001001|0   1ffff92  [25]
       (183) |11111111|11111111|11100100|11 [26]       3ffff93 [26]  |11111111|11111111|11001001|1   1ffff93  [25]
       (184) |11111111|11111111|11100101|00 [26]       3ffff94 [26]  |11111111|11111111|11001010|0   1ffff94  [25]
       (185) |11111111|11111111|11100101|01 [26]       3ffff95 [26]  |11111111|11111111|11001010|1   1ffff95  [25]
       (186) |11111111|11111111|11100101|10 [26]       3ffff96 [26]  |11111111|11111111|11001011|0   1ffff96  [25]
       (187) |11111111|11111111|11100101|11 [26]       3ffff97 [26]  |11111111|11111111|11001011|1   1ffff97  [25]
       (188) |11111111|11111111|11100110|00 [26]       3ffff98 [26]  |11111111|11111111|11001100|0   1ffff98  [25]
       (189) |11111111|11111111|11100110|01 [26]       3ffff99 [26]  |11111111|11111111|11001100|1   1ffff99  [25]
       (190) |11111111|11111111|11100110|10 [26]       3ffff9a [26]  |11111111|11111111|11001101|0   1ffff9a  [25]
       (191) |11111111|11111111|11100110|11 [26]       3ffff9b [26]  |11111111|11111111|11001101|1   1ffff9b  [25]
       (192) |11111111|11111111|11100111|00 [26]       3ffff9c [26]  |11111111|11111111|11001110|0   1ffff9c  [25]
       (193) |11111111|11111111|11100111|01 [26]       3ffff9d [26]  |11111111|11111111|11001110|1   1ffff9d  [25]
       (194) |11111111|11111111|11100111|10 [26]       3ffff9e [26]  |11111111|11111111|11001111|0   1ffff9e  [25]
       (195) |11111111|11111111|11100111|11 [26]       3ffff9f [26]  |11111111|11111111|11001111|1   1ffff9f  [25]
       (196) |11111111|11111111|11101000|00 [26]       3ffffa0 [26]  |11111111|11111111|11010000|0   1ffffa0  [25]
       (197) |11111111|11111111|11101000|01 [26]       3ffffa1 [26]  |11111111|11111111|11010000|1   1ffffa1  [25]
       (198) |11111111|11111111|11101000|10 [26]       3ffffa2 [26]  |11111111|11111111|11010001|0   1ffffa2  [25]
       (199) |11111111|11111111|11101000|11 [26]       3ffffa3 [26]  |11111111|11111111|11010001|1   1ffffa3  [25]
       (200) |11111111|11111111|11101001|00 [26]       3ffffa4 [26]  |11111111|11111111|11010010|0   1ffffa4  [25]
       (201) |11111111|11111111|11101001|01 [26]       3ffffa5 [26]  |11111111|11111111|11010010|1   1ffffa5  [25]
       (202) |11111111|11111111|11101001|10 [26]       3ffffa6 [26]  |11111111|11111111|11010011|0   1ffffa6  [25]
       (203) |11111111|11111111|11101001|11 [26]       3ffffa7 [26]  |11111111|11111111|11010011|1   1ffffa7  [25]
       (204) |11111111|11111111|11101010|00 [26]       3ffffa8 [26]  |11111111|11111111|11010100|0   1ffffa8  [25]
       (205) |11111111|11111111|11101010|01 [26]       3ffffa9 [26]  |11111111|11111111|11010100|1   1ffffa9  [25]
       (206) |11111111|11111111|11101010|10 [26]       3ffffaa [26]  |11111111|11111111|11010101|0   1ffffaa  [25]
       (207) |11111111|11111111|11101010|11 [26]       3ffffab [26]  |11111111|11111111|11010101|1   1ffffab  [25]
       (208) |11111111|11111111|11101011|00 [26]       3ffffac [26]  |11111111|11111111|11010110|0   1ffffac  [25]
       (209) |11111111|11111111|11101011|01 [26]       3ffffad [26]  |11111111|11111111|11010110|1   1ffffad  [25]
       (210) |11111111|11111111|11101011|10 [26]       3ffffae [26]  |11111111|11111111|11010111|0   1ffffae  [25]
       (211) |11111111|11111111|11101011|11 [26]       3ffffaf [26]  |11111111|11111111|11010111|1   1ffffaf  [25]
       (212) |11111111|11111111|11101100|00 [26]       3ffffb0 [26]  |11111111|11111111|11011000|0   1ffffb0  [25]
       (213) |11111111|11111111|11101100|01 [26]       3ffffb1 [26]  |11111111|11111111|11011000|1   1ffffb1  [25]
       (214) |11111111|11111111|11101100|10 [26]       3ffffb2 [26]  |11111111|11111111|11011001|0   1ffffb2  [25]
       (215) |11111111|11111111|11101100|11 [26]       3ffffb3 [26]  |11111111|11111111|11011001|1   1ffffb3  [25]
       (216) |11111111|11111111|11101101|00 [26]       3ffffb4 [26]  |11111111|11111111|11011010|0   1ffffb4  [25]
       (217) |11111111|11111111|11101101|01 [26]       3ffffb5 [26]  |11111111|11111111|11011010|1   1ffffb5  [25]
       (218) |11111111|11111111|11101101|10 [26]       3ffffb6 [26]  |11111111|11111111|11011011|0   1ffffb6  [25]
       (219) |11111111|11111111|11101101|11 [26]       3ffffb7 [26]  |11111111|11111111|11011011|1   1ffffb7  [25]
       (220) |11111111|11111111|11101110|00 [26]       3ffffb8 [26]  |11111111|11111111|11011100|0   1ffffb8  [25]
       (221) |11111111|11111111|11101110|01 [26]       3ffffb9 [26]  |11111111|11111111|11011100|1   1ffffb9  [25]
       (222) |11111111|11111111|11101110|10 [26]       3ffffba [26]  |11111111|11111111|11011101|0   1ffffba  [25]
       (223) |11111111|11111111|11101110|11 [26]       3ffffbb [26]  |11111111|11111111|11011101|1   1ffffbb  [25]
       (224) |11111111|11111111|11101111|00 [26]       3ffffbc [26]  |11111111|11111111|11011110|0   1ffffbc  [25]
       (225) |11111111|11111111|11101111|01 [26]       3ffffbd [26]  |11111111|11111111|11011110|1   1ffffbd  [25]
       (226) |11111111|11111111|11101111|10 [26]       3ffffbe [26]  |11111111|11111111|11011111|0   1ffffbe  [25]
       (227) |11111111|11111111|11101111|11 [26]       3ffffbf [26]  |11111111|11111111|11011111|1   1ffffbf  [25]
       (228) |11111111|11111111|11110000|00 [26]       3ffffc0 [26]  |11111111|11111111|11100000|0   1ffffc0  [25]
       (229) |11111111|11111111|11110000|01 [26]       3ffffc1 [26]  |11111111|11111111|11100000|1   1ffffc1  [25]
       (230) |11111111|11111111|11110000|10 [26]       3ffffc2 [26]  |11111111|11111111|11100001|0   1ffffc2  [25]
       (231) |11111111|11111111|11110000|11 [26]       3ffffc3 [26]  |11111111|11111111|11100001|1   1ffffc3  [25]
       (232) |11111111|11111111|11110001|00 [26]       3ffffc4 [26]  |11111111|11111111|11100010|0   1ffffc4  [25]
       (233) |11111111|11111111|11110001|01 [26]       3ffffc5 [26]  |11111111|11111111|11100010|1   1ffffc5  [25]
       (234) |11111111|11111111|11110001|10 [26]       3ffffc6 [26]  |11111111|11111111|11100011|0   1ffffc6  [25]
       (235) |11111111|11111111|11110001|11 [26]       3ffffc7 [26]  |11111111|11111111|11100011|1   1ffffc7  [25]
       (236) |11111111|11111111|11110010|00 [26]       3ffffc8 [26]  |11111111|11111111|11100100|0   1ffffc8  [25]
       (237) |11111111|11111111|11110010|01 [26]       3ffffc9 [26]  |11111111|11111111|11100100|1   1ffffc9  [25]
       (238) |11111111|11111111|11110010|10 [26]       3ffffca [26]  |11111111|11111111|11100101|0   1ffffca  [25]
       (239) |11111111|11111111|11110010|11 [26]       3ffffcb [26]  |11111111|11111111|11100101|1   1ffffcb  [25]
       (240) |11111111|11111111|11110011|00 [26]       3ffffcc [26]  |11111111|11111111|11100110|0   1ffffcc  [25]
       (241) |11111111|11111111|11110011|01 [26]       3ffffcd [26]  |11111111|11111111|11100110|1   1ffffcd  [25]
       (242) |11111111|11111111|11110011|10 [26]       3ffffce [26]  |11111111|11111111|11100111|0   1ffffce  [25]
       (243) |11111111|11111111|11110011|11 [26]       3ffffcf [26]  |11111111|11111111|11100111|1   1ffffcf  [25]
       (244) |11111111|11111111|11110100|00 [26]       3ffffd0 [26]  |11111111|11111111|11101000|0   1ffffd0  [25]
       (245) |11111111|11111111|11110100|01 [26]       3ffffd1 [26]  |11111111|11111111|11101000|1   1ffffd1  [25]
       (246) |11111111|11111111|11110100|10 [26]       3ffffd2 [26]  |11111111|11111111|11101001|0   1ffffd2  [25]
       (247) |11111111|11111111|11110100|11 [26]       3ffffd3 [26]  |11111111|11111111|11101001|1   1ffffd3  [25]
       (248) |11111111|11111111|11110101|00 [26]       3ffffd4 [26]  |11111111|11111111|11101010|0   1ffffd4  [25]
       (249) |11111111|11111111|11110101|01 [26]       3ffffd5 [26]  |11111111|11111111|11101010|1   1ffffd5  [25]
       (250) |11111111|11111111|11110101|10 [26]       3ffffd6 [26]
       (251) |11111111|11111111|11110101|11 [26]       3ffffd7 [26]  |11111111|11111111|11101011|0   1ffffd6  [25]
       (251)  |11111111|11111111|11101011|1   1ffffd7  [25]
       (252) |11111111|11111111|11110110|00 [26]       3ffffd8 [26]  |11111111|11111111|11101100|0   1ffffd8  [25]
       (253) |11111111|11111111|11110110|01 [26]       3ffffd9 [26]  |11111111|11111111|11101100|1   1ffffd9  [25]
       (254) |11111111|11111111|11110110|10 [26]       3ffffda [26]  |11111111|11111111|11101101|0   1ffffda  [25]
       (255) |11111111|11111111|11110110|11 [26]       3ffffdb [26]  |11111111|11111111|11101101|1   1ffffdb  [25]
   EOS (256) |11111111|11111111|11110111|00 [26]       3ffffdc [26]  |11111111|11111111|11101110|0   1ffffdc  [25]

Appendix D.  Examples

   A number of examples are worked through here, covering integer
   encoding, header field representation, and the encoding of whole sets
   of header fields, for both requests and responses, and with and without Huffman coding.

D.1.
   without Huffman coding.

D.1.  Integer Representation Examples

   This section shows the representation of integer values in details
   (see Section 4.1.1).

D.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
   +---+---+---+---+---+---+---+---+

D.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, encode(10), done
   +---+---+---+---+---+---+---+---+

D.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
   +---+---+---+---+---+---+---+---+

D.2.  Header Field Representation Examples

   This section show shows several independent representation examples.

D.1.1.

D.2.1.  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

   400a 6375 7374 6f6d 2d6b 6579 0d63 7573 | ..custom-key.cus @.custom-key.cus
   746f 6d2d 6865 6164 6572                | tom-header

   Decoding process:

   00

   40                                      | == 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

D.1.2.

D.2.2.  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

   040c 2f73 616d 706c 652f 7061 7468      | D./sample/path ../sample/path

   Decoding process:

   44

   04                                      | == 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

D.1.3.

D.2.3.  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

D.1.4.

D.2.4.  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

D.2.

D.3.  Request Examples without Huffman

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

D.2.1.

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 8644 0f77 7777 2e65 7861 6d70 6c65 | .....www.example ...D.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
   44                                      | == 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

D.2.2.

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:

   1b08

   5c08 6e6f 2d63 6163 6865                | ..no-cache \.no-cache

   Decoding process:

   1b

   5c                                      | == Literal indexed ==
                                           |   Indexed name (idx = 27) 28)
                                           |     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

D.2.3.

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:

   8080 858c 8b84 000a 6375 7374 6f6d 2d6b | ........custom-k
   6579 0c63

   3085 8c8b 8440 0a63 7573 746f 6d2d 7661 6c75 65 6b65 | 0....@.custom-ke
   790c 6375 7374 6f6d 2d76 616c 7565      | ey.custom-value y.custom-value
   Decoding process:

   80 80

   30                                      | == 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
   40                                      | == 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

D.3.

D.4.  Request Examples with Huffman

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

D.3.1.

D.4.1.  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 8644 8ce7 cf9b ebe8 9b6f b16f a9b6 | ...D.......o.o..
   ff                                      | ......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
   44                                      | == Literal indexed ==
                                           |   Indexed name (idx = 4)
                                           |     :authority
   8b
   8c                                      |   Literal value (len = 15)
                                           |     Huffman encoded:
   db6d 883e 68d1 cb12 25ba 7f
   e7cf 9beb e89b 6fb1 6fa9 b6ff           | .m..h...%.. ......o.o...
                                           |     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

D.3.2.

D.4.2.  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

   5c86 b9b9 9495 56bf                     | ..ceJ... \.....V.

   Decoding process:

   1b

   5c                                      | == Literal indexed ==
                                           |   Indexed name (idx = 27) 28)
                                           |     cache-control
   86                                      |   Literal value (len = 8)
                                           |     Huffman encoded:
   6365 4a13 98ff
   b9b9 9495 56bf                          | ceJ... ....V.
                                           |     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

D.3.3.

D.4.3.  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:

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

   3085 8c8b 8440 8857 1c5c db73 7b2f af89 | 0....@.W.\.s{/..
   571c 5cdb 7372 4d9c 57                  | .N..t..... W.\.srM.W

   Decoding process:

   80 80

   30                                      | == 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
   40                                      | == Literal indexed ==
   88                                      |   Literal name (len = 10)
                                           |     Huffman encoded:
   4eb0 8b74 9790 fa7f
   571c 5cdb 737b 2faf                     | N..t.... W.\.s{/.
                                           |     Decoded:
                                           | custom-key
   89                                      |   Literal value (len = 12)
                                           |     Huffman encoded:
   4eb0 8b74 979a 17a8 ff
   571c 5cdb 7372 4d9c 57                  | N..t..... W.\.srM.W
                                           |     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

D.4.

D.5.  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.

D.4.1.

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:

   0803

   4803 3330 3218 3259 0770 7269 7661 7465 221d 631d | ..302..private". H.302Y.privatec.
   4d6f 6e2c 2032 3120 4f63 7420 3230 3133 | Mon, 21 Oct 2013
   2032 303a 3133 3a32 3120 474d 5430 5471 1768 |  20:13:21 GMT0.h GMTq.h
   7474 7073 3a2f 2f77 7777 2e65 7861 6d70 | ttps://www.examp
   6c65 2e63 6f6d                          | le.com
   Decoding process:

   08

   48                                      | == Literal indexed ==
                                           |   Indexed name (idx = 8)
                                           |     :status
   03                                      |   Literal value (len = 3)
   3330 32                                 | 302
                                           | -> :status: 302
   18
   59                                      | == Literal indexed ==
                                           |   Indexed name (idx = 24) 25)
                                           |     cache-control
   07                                      |   Literal value (len = 7)
   7072 6976 6174 65                       | private
                                           | -> cache-control: private
   22
   63                                      | == Literal indexed ==
                                           |   Indexed name (idx = 34) 35)
                                           |     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
   71                                      | == Literal indexed ==
                                           |   Indexed name (idx = 48) 49)
                                           |     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

D.4.2.

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,
   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:

   8c                                      | .

   Decoding process:

   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

D.4.3.

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:

   8484 031d 431d 4d6f 6e2c 2032 3120 4f63 7420 | ....Mon, ..C.Mon, 21 Oct
   3230 3133 2032 303a 3133 3a32 3220 474d | 2013 20:13:22 GM
   541d
   545e 0467 7a69 7084 8483 833a 837b 3866 6f6f | T..gzip....:8foo T^.gzip....{8foo
   3d41 5344 4a4b 4851 4b42 5a58 4f51 5745 | =ASDJKHQKBZXOQWE
   4f50 4955 4158 5157 454f 4955 3b20 6d61 | OPIUAXQWEOIU; ma
   782d 6167 653d 3336 3030 3b20 7665 7273 | x-age=3600; vers
   696f 6e3d 31                            | ion=1

   Decoding process:

   84                                      | == Indexed - Remove ==
                                           |   idx = 4
                                           | -> cache-control: private
   84                                      | == Indexed - Add ==
                                           |   idx = 4
                                           | -> cache-control: private
   03
   43                                      | == 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
   5e                                      | == Literal indexed ==
                                           |   Indexed name (idx = 29) 30)
                                           |     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
   7b                                      | == Literal indexed ==
                                           |   Indexed name (idx = 58) 59)
                                           |     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

D.5.

D.6.  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.

D.5.1.

D.6.1.  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 98a7 1885 73d5 cd11 1f22 98ef 6b3a

   4882 4017 5985 bf06 724b 9763 93d6 dbb2 | H.@.Y...rK.c....
   9884 de2a 7188 0506 2098 5131 09b5 6ba3 | ...*q... .Q1..k.
   7191 adce bf19 8e7e 7cf9 bebe 89b6 fb16 | ......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 q.......|.......
   fa9b 6f                                 | ..9bD.O ..o

   Decoding process:

   08

   48                                      | == Literal indexed ==
                                           |   Indexed name (idx = 8)
                                           |     :status
   82                                      |   Literal value (len = 3)
                                           |     Huffman encoded:
   98a7
   4017                                    | .. @.
                                           |     Decoded:
                                           | 302
                                           | -> :status: 302
   18
   59                                      | == Literal indexed ==
                                           |   Indexed name (idx = 24) 25)
                                           |     cache-control
   85                                      |   Literal value (len = 7)
                                           |     Huffman encoded:
   73d5 cd11 1f
   bf06 724b 97                            | s.... ..rK.
                                           |     Decoded:
                                           | private
                                           | -> cache-control: private
   22
   63                                      | == Literal indexed ==
                                           |   Indexed name (idx = 34) 35)
                                           |     date
   98
   93                                      |   Literal value (len = 29)
                                           |     Huffman encoded:
   ef6b 3a7a 0e6e 8fa2 63d0 729a 6e83 97d8 | .k:z.n..c.r.n...
   69bd 8737 47bb bfc7
   d6db b298 84de 2a71 8805 0620 9851 3109 | ......*q... .Q1.
   b56b a3                                 | i..7G... .k.
                                           |     Decoded:
                                           | Mon, 21 Oct 2013 20:13:21 \
                                           | GMT
                                           | -> date: Mon, 21 Oct 2013 \
                                           |   20:13:21 GMT
   30
   71                                      | == Literal indexed ==
                                           |   Indexed name (idx = 48) 49)
                                           |     location
   90
   91                                      |   Literal value (len = 23)
                                           |     Huffman encoded:
   ce31 743d 801b 6db1 07cd 1a39 6244 b74f
   adce bf19 8e7e 7cf9 bebe 89b6 fb16 fa9b | ......|.........
   6f                                      | .1t=..m....9bD.O 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

D.5.2.

D.6.2.  Second response

   The (":status", "302") header field is evicted from the header table
   to free space to allow adding the (":status", "200") header field,
   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:

   8c                                      | .

   Decoding process:

   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

D.5.3.

D.6.3.  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:

   8484 0398 ef6b 3a7a 0e6e 8fa2 63d0 729a | .....k:z.n..c.r.
   6e83 97d8 69bd 873f 47bb bfc7 1d83 cbd5 | n...i..?G.......
   4e84 4393 d6db b298 84de 2a71 8805 0620 | ..C.......*q...
   9851 3111 b56b a35e 84ab dd97 ff84 8483 833a 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 | IK..._.5.. .Q1..k.^........
   837b b1e0 d6cf 9f6e 8f9f d3e5 f6fa 76fe | .{.....n......v.
   fd3c 7edf 9eff 1f2f 0f3c fe9f 6fcf 7f8f | ......./....o...
   879f 61ad 4f4c c9a9 73a2 200e c372 5e18 | ..a.OL..s. ..r^.
   b1b7 4e3f                               | ..N?

   Decoding process:

   84                                      | == Indexed - Remove ==
                                           |   idx = 4
                                           | -> cache-control: private
   84                                      | == Indexed - Add ==
                                           |   idx = 4
                                           | -> cache-control: private
   03
   43                                      | == Literal indexed ==
                                           |   Indexed name (idx = 3)
                                           |     date
   98
   93                                      |   Literal value (len = 29)
                                           |     Huffman encoded:
   ef6b 3a7a 0e6e 8fa2 63d0 729a 6e83 97d8 | .k:z.n..c.r.n...
   69bd 873f 47bb bfc7
   d6db b298 84de 2a71 8805 0620 9851 3111 | ......*q... .Q1.
   b56b a3                                 | i..?G... .k.
                                           |     Decoded:
                                           | Mon, 21 Oct 2013 20:13:22 \
                                           | GMT
                                           | - evict: cache-control: pr\
                                           |   ivate
                                           | -> date: Mon, 21 Oct 2013 \
                                           |   20:13:22 GMT
   1d
   5e                                      | == Literal indexed ==
                                           |   Indexed name (idx = 29) 30)
                                           |     content-encoding
   83
   84                                      |   Literal value (len = 4)
                                           |     Huffman encoded:
   cbd5 4e
   abdd 97ff                               | ..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
   7b                                      | == Literal indexed ==
                                           |   Indexed name (idx = 58) 59)
                                           |     set-cookie
   b3
   b1                                      |   Literal value (len = 56)
                                           |     Huffman encoded:
   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
   e0d6 cf9f 6e8f 9fd3 e5f6 fa76 fefd 3c7e | ....n......v....
   df9e ff1f 2f0f 3cfe 9f6f cf7f 8f87 9f61 | ..../....o.....a
   ad4f 4cc9 a973 a220 0ec3 725e 18b1 b74e | .OL..s. ..r^...N
   3f                                      | 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