HTTP Header Compression
draft-ietf-httpbis-header-compression-00

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HTTPbis Working Group                                            R. Peon
Internet-Draft                                               Google, Inc
Intended status: Informational                                H. Ruellan
Expires: December 27, 2013                                     Canon CRF
                                                           June 25, 2013

                        HTTP Header Compression
                draft-ietf-httpbis-header-compression-00

Abstract

   This document describes a format adapted to efficiently represent
   HTTP headers in the context of HTTP/2.0.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 27, 2013.

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   Copyright (c) 2013 IETF Trust and the persons identified as the
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   described in the Simplified BSD License.

Table of Contents

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   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   2
     2.1.  Outline . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Header Encoding . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Encoding Components . . . . . . . . . . . . . . . . . . .   3
     3.2.  Header Table  . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Header Representation . . . . . . . . . . . . . . . . . .   5
       3.3.1.  Literal Representation  . . . . . . . . . . . . . . .   5
       3.3.2.  Indexed Representation  . . . . . . . . . . . . . . .   5
     3.4.  Differential Coding . . . . . . . . . . . . . . . . . . .   5
   4.  Detailed Format . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Header Blocks . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Low-level representations . . . . . . . . . . . . . . . .   7
       4.2.1.  Integer representation  . . . . . . . . . . . . . . .   7
       4.2.2.  String literal representation . . . . . . . . . . . .   9
     4.3.  Indexed Header Representation . . . . . . . . . . . . . .   9
     4.4.  Literal Header Representation . . . . . . . . . . . . . .   9
       4.4.1.  Literal Header without Indexing . . . . . . . . . . .   9
       4.4.2.  Literal Header with Incremental Indexing  . . . . . .  10
       4.4.3.  Literal Header with Substitution Indexing . . . . . .  10
   5.  Parameter Negotiation . . . . . . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  11
   Appendix A.  Initial header names . . . . . . . . . . . . . . . .  12
     A.1.  Requests  . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.2.  Responses . . . . . . . . . . . . . . . . . . . . . . . .  13
   Appendix B.  Example  . . . . . . . . . . . . . . . . . . . . . .  14
     B.1.  First header set  . . . . . . . . . . . . . . . . . . . .  14
     B.2.  Second header set . . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   This document describes a format adapted to efficiently represent
   HTTP headers in the context of HTTP/2.0.

2.  Overview

   In HTTP/1.X, HTTP headers, which are necessary for the functioning of
   the protocol, are transmitted with no transformations.
   Unfortunately, the amount of redundancy in both the keys and the
   values of these headers is astonishingly high, and is the cause of
   increased latency on lower bandwidth links.  This indicates that an
   alternate encoding for headers would be beneficial to latency, and
   that is what is proposed here.  As shown by SPDY [SPDY], Deflate
   compresses HTTP very effectively.  However, the use of a compression

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   scheme which allows for arbitrary matches against the previously
   encoded data (such as Deflate) exposes users to security issues.  In
   particular, the compression of sensitive data, together with other
   data controlled by an attacker, may lead to leakage of that sensitive
   data, even when the resultant bytes are transmitted over an encrypted
   channel.  Another consideration is that processing and memory costs
   of a compressor such as Deflate may also be too high for some classes
   of devices, for example when doing forward or reverse proxying.

2.1.  Outline

   The HTTP header representation described in this document is based on
   indexing tables that store (name, value) pairs, called header tables
   in the remainder of this document.  This scheme is believed to be
   safe for all known attacks against the compression context today.
   Header tables are incrementally updated during the whole HTTP/2.0
   session.  Two independent header tables are used during a HTTP/2.0
   session, one for HTTP request headers and one for HTTP response
   headers.

   The encoder is responsible for deciding which headers to insert as
   (name, value) pairs in the header table.  The decoder then does
   exactly what the encoder prescribes, ending in a state that exactly
   matches the encoder's state.  This enables decoders to remain simple
   and understand a wide variety of encoders.

   A header may be represented as a literal or as an index.  If
   represented as a literal, the representation specifies whether this
   header is used to update the indexing table.  The different
   representations are described in Section 3.3.

   A set of headers is coded as a difference from the previous set of
   headers.

   An example illustrating the use these different mechanisms to
   represent headers is available in Appendix B.

3.  Header Encoding

3.1.  Encoding Components

   The encoding and decoding of headers relies on a few components.
   First, a header table (see Section 3.2) is used to associate headers
   to index values.  Second, a set of headers is encoded as a difference
   from the previous reference set of headers (see Section 3.4).

   As messages are exchanged in two directions, from client to server
   and from server to client, there are two sets of components: one for

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   each direction.  All the headers sent in messages from the client to
   the server are encoded (and decoded) using one set of components.
   All the headers sent in messages from the server to the client
   (including headers contained in PUSH_PROMISE frame) are encoded using
   the other set of compotents.

3.2.  Header Table

   A header table consists of an ordered list of (name, value) pairs.  A
   pair is either inserted at the end of the table or replaces an
   existing pair depending on the chosen representation.  A pair can be
   represented as an index which is its position in the table, starting
   with 0 for the first entry.

   Header names are always represented as lower-case strings.  An input
   header name matches the header name of a (name, value) pair stored in
   the Header Table if they are equal using a character-based, _case
   insensitive_ comparison.  An input header value matches the header
   value of a (name, value) pair stored in the Header Table if they are
   equal using a character-based, _case sensitive_ comparison.  An input
   header (name, value) pair matches a pair in the Header Table if both
   the name and value are matching as per above.

   The header table is progressively updated based on headers
   represented as literal (as defined in Section 3.3.1).  Two update
   mechanisms are defined:

   o  Incremental indexing: the represented header is inserted at the
      end of the header table as a (name, value) pair.  The inserted
      pair index is set to the next free index in the table: it is equal
      to the number of headers in the table before its insertion.

   o  Substitution indexing: the represented header contains an index to
      an existing (name, value) pair.  The existing pair value is
      replaced by the pair representing the new header.

   Incremental and substitution indexing are optional.  If none of them
   is selected in a header representation, the header table is not
   updated.  In particular, no update happens on the header table when
   processing an indexed representation.

   The header table size can be bounded so as to limit the memory
   requirements (see the SETTINGS_MAX_BUFFER_SIZE in Section 5).  The
   header table size is defined as the sum of the size of each entry of
   the table.  The size of an entry is the sum of the length in bytes
   (as defined in Section 4.2.2) of its name, of value's length in bytes
   and of 32 bytes (for accounting for the entry structure overhead).

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   When an entry is added to the header table, if the header table size
   is greater than the limit, the table size is reduced by dropping the
   entries at the beginning of the table until the header table size
   becomes lower than or equal to the limit.  Dropping entries from the
   beginning of the table causes a renumbering of the remaining entries.
   [[Feedback is needed on this automatic eviction strategy.  ]]

   To optimize the representation of the headers exchanged at the
   beginning of an HTTP/2.0 session, the header table is initialized
   with common headers.  Two lists of initial headers are provided in
   Appendix A.  One is for messages sent from a client to a server, the
   other is for messages sent from a server to a client.

3.3.  Header Representation

3.3.1.  Literal Representation

   The literal representation defines a new header.  A literal header is
   represented as:

   o  A header name, with two possible representations:

      *  A literal string, as described in Section 4.2.2.

      *  A index in the header table referencing the name of the
         corresponding header.  The index is represented as an integer,
         as described in Section 4.2.1.

   o  The header value, represented as a literal string, as described in
      Section 4.2.2.

3.3.2.  Indexed Representation

   The indexed representation defines a header as a match to a (name,
   value) pair in the header table.  An indexed header is represented
   as:

   o  An integer representing the index of the matching (name, value)
      pair, as described in Section 4.2.1.

3.4.  Differential Coding

   A set of headers is encoded as a difference from the previous
   reference set of headers.  The initial reference set of headers is
   the empty set.

   An indexed representation toggles the presence of the header in the
   current set of headers.  If the header corresponding to the indexed

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   representation was not in the set, it is added to the set.  If the
   header index was in the set, it is removed from it.

   A literal representation adds a header to the current set of headers
   if the header is added to the header table (either by incremental
   indexing or by substitution indexing).

   To ensure a correct decoding of a set of headers, the following steps
   or equivalent ones MUST be executed by the decoder.

   First, upon starting the decoding of a new set of headers, the
   reference set of headers is interpreted into the working set of
   headers: for each header in the reference set, an entry is added to
   the working set, containing the header name, its value, and its
   current index in the header table.

   Then, the header representations are processed in their order of
   occurrence in the frame.

   For an indexed representation, the decoder checks whether the index
   is present in the working set.  If true, the corresponding entry is
   removed from the working set.  If several entries correspond to this
   encoded index, all these entries are removed from the working set.
   If the index is not present in the working set, it is used to
   retrieve the corresponding header from the header table, and a new
   entry is added to the working set representing this header.

   For a literal representation, a new entry is added to the working set
   representing this header.  If the literal representation specifies
   that the header is to be indexed, the header is added accordingly to
   the header table, and its index is included in the entry in the
   working set.  Otherwise, the entry in the working set contains an
   undefined index.

   When all the header representations have been processed, the working
   set contains all the headers of the set of headers.

   The new reference set of headers is computed by removing from the
   working set all the headers that are not present in the header table.

   It should be noted that during the decoding of the header
   representations, the same index may be associated to different
   headers in the working set and in the header table.

4.  Detailed Format

4.1.  Header Blocks

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   A header block consists of a set of header fields, which are name-
   value pairs.  Each header field is encoded using one of the header
   representation.

4.2.  Low-level representations

4.2.1.  Integer representation

   Integers are used to represent name indexes, pair indexes or string
   lengths.  The integer representation keeps byte-alignment as much as
   possible as this allows various processing optimizations as well as
   efficient use of DEFLATE.  For that purpose, an integer
   representation always finishes at the end of a byte.

   An integer is represented in two parts: a prefix that fills the
   current byte and an optional list of bytes that are used if the
   integer value does not fit in 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 byte.  If the value is
   small enough (strictly less than 2^N-1), it is encoded within the
   N-bit prefix.  Otherwise all the bits of the prefix are set to 1 and
   the value is encoded using an unsigned variable length integer [1]
   representation.

   The algorithm to represent an integer I is as follows:

   1.  If I < 2^N - 1, encode I on N bits

   2.  Else, encode 2^N - 1 on N bits and do the following steps:

   3.

       1.  Set I to (I - (2^N - 1)) and Q to 1

       2.  While Q > 0

       3.

           1.  Compute Q and R, quotient and remainder of I divided by
               2^7

           2.  If Q is strictly greater than 0, write one 1 bit;
               otherwise, write one 0 bit

           3.  Encode R on the next 7 bits

           4.  I = Q

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

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

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

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

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

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

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

   o

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

   o  The value to represent on next bytes is I = 1337 - (2^5 - 1) =
      1306.

   o

      *  1306 = 128*10 + 26, i.e.  Q=10 and R=26.

      *  Q is greater than 1, bit 8 is set to 1.

      *  The remainder R=26 is encoded on next 7 bits.

      *  I is replaced by the quotient Q=10.

   o  The value to represent on next bytes is I = 10.

   o

      *  10 = 128*0 + 10, i.e.  Q=0 and R=10.

      *  Q is equal to 0, bit 16 is set to 0.

      *  The remainder R=10 is encoded on next 7 bits.

      *  I is replaced by the quotient Q=0.

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   o  The process ends.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | X | X | X | 1 | 1 | 1 | 1 | 1 |   Prefix = 31
   | 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 |   Q>=1, R=26
   | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 |   Q=0 , R=10
   +---+---+---+---+---+---+---+---+

4.2.2.  String literal representation

   Literal strings can represent header names or header values.  They
   are encoded in two parts:

   1.  The string length, defined as the number of bytes needed to store
       its UTF-8 representation, is represented as an integer with a
       zero bits prefix.  If the string length is strictly less than
       128, it is represented as one byte.

   2.  The string value represented as a list of UTF-8 characters.

4.3.  Indexed Header Representation

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

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

4.4.  Literal Header Representation

4.4.1.  Literal Header without Indexing

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

   This representation, which does not involve updating the header
   table, starts with the '011' 3-bit pattern.

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   If the header name matches the header name of a (name, value) pair
   stored in the Header Table, the index of the pair increased by one
   (index + 1) is represented as an integer with a 5-bit prefix.  Note
   that if the index is strictly below 31, one byte is used.

   If the header name does not match a header name entry, the value 0 is
   represented on 5 bits followed by the header name, represented as a
   literal string.

   Header name representation is followed by the header value
   represented as a literal string as described in Section 4.2.2.

4.4.2.  Literal Header with Incremental Indexing

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

   This representation starts with the '010' 3-bit pattern.

   If the header name matches the header name of a (name, value) pair
   stored in the Header Table, the index of the pair increased by one
   (index + 1) is represented as an integer with a 5-bit prefix.  Note
   that if the index is strictly below 31, one byte is used.

   If the header name does not match a header name entry, the value 0 is
   represented on 5 bits followed by the header name, represented as a
   literal string.

   Header name representation is followed by the header value
   represented as a literal string as described in Section 4.2.2.

4.4.3.  Literal Header with Substitution Indexing

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

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

   If the header name matches the header name of a (name, value) pair
   stored in the Header Table, the index of the pair increased by one
   (index + 1) is represented as an integer with a 6-bit prefix.  Note
   that if the index is strictly below 62, one byte is used.

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   If the header name does not match a header name entry, the value 0 is
   represented on 6 bits followed by the header name, represented as a
   literal string.

   The index of the substituted (name, value) pair is inserted after the
   header name representation as a 0-bit prefix integer.

   This index is followed by the header value represented as a literal
   string as described in Section 4.2.2.

5.  Parameter Negotiation

   A few parameters can be used to accomodate client and server
   processing and memory requirements.

   SETTINGS_MAX_BUFFER_SIZE:  Allows the sender to inform the remote
      endpoint of the maximum size it accepts for the header table.
      The default value is 4096 bytes.
      [[Is this default value OK?  Do we need a maximum size?  Do we
      want to allow infinite buffer?]]
      When the remote endpoint receives a SETTINGS frame containing a
      SETTINGS_MAX_BUFFER_SIZE setting with a value smaller than the one
      currently in use, it MUST send as soon as possible a HEADER frame
      with a stream identifier of 0x0 containing a value smaller than or
      equal to the received setting value.
      [[This changes slightly the behaviour of the HEADERS frame, which
      should be updated as follows: ]]
      A HEADER frame with a stream identifier of 0x0 indicates that the
      sender has reduced the maximum size of the header table.  The new
      maximum size of the header table is encoded on 32-bit.  The
      decoder MUST reduce its own header table by dropping entries from
      it until the size of the header table is lower than or equal to
      the transmitted maximum size.

6.  Security Considerations

   TODO?

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Informative References

   [SPDY]     Belshe, M. and R. Peon, "SPDY Protocol", February 2012,
              <http://tools.ietf.org/html/draft-mbelshe-httpbis-spdy>.

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Appendix A.  Initial header names

   [[The tables in this section should be updated based on statistical
   analysis of header names frequency and specific HTTP 2.0 header rules
   (like removal of some headers).  ]]
   [[These tables are not adapted for headers contained in PUSH_PROMISE
   frames.  Either the tables can be merged, or the table for responses
   can be updated.  ]]

A.1.  Requests

   The following table lists the pre-defined headers that make-up the
   initial header table user to represent requests sent from a client to
   a server.

              +-------+---------------------+--------------+
              | Index | Header Name         | Header Value |
              +-------+---------------------+--------------+
              | 0     | :scheme             | http         |
              | 1     | :scheme             | https        |
              | 2     | :host               |              |
              | 3     | :path               | /            |
              | 4     | :method             | get          |
              | 5     | accept              |              |
              | 6     | accept-charset      |              |
              | 7     | accept-encoding     |              |
              | 8     | accept-language     |              |
              | 9     | cookie              |              |
              | 10    | if-modified-since   |              |
              | 11    | keep-alive          |              |
              | 12    | user-agent          |              |
              | 13    | proxy-connection    |              |
              | 14    | referer             |              |
              | 15    | accept-datetime     |              |
              | 16    | authorization       |              |
              | 17    | allow               |              |
              | 18    | cache-control       |              |
              | 19    | connection          |              |
              | 20    | content-length      |              |
              | 21    | content-md5         |              |
              | 22    | content-type        |              |
              | 23    | date                |              |
              | 24    | expect              |              |
              | 25    | from                |              |
              | 26    | if-match            |              |
              | 27    | if-none-match       |              |
              | 28    | if-range            |              |
              | 29    | if-unmodified-since |              |

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              | 30    | max-forwards        |              |
              | 31    | pragma              |              |
              | 32    | proxy-authorization |              |
              | 33    | range               |              |
              | 34    | te                  |              |
              | 35    | upgrade             |              |
              | 36    | via                 |              |
              | 37    | warning             |              |
              +-------+---------------------+--------------+

                                  Table 1

A.2.  Responses

   The following table lists the pre-defined headers that make-up the
   initial header table used to represent responses sent from a server
   to a client.  The same header table is also used to represent request
   headers sent from a server to a client in a PUSH_PROMISE frame.

          +-------+-----------------------------+--------------+
          | Index | Header Name                 | Header Value |
          +-------+-----------------------------+--------------+
          | 0     | :status                     | 200          |
          | 1     | age                         |              |
          | 2     | cache-control               |              |
          | 3     | content-length              |              |
          | 4     | content-type                |              |
          | 5     | date                        |              |
          | 6     | etag                        |              |
          | 7     | expires                     |              |
          | 8     | last-modified               |              |
          | 9     | server                      |              |
          | 10    | set-cookie                  |              |
          | 11    | vary                        |              |
          | 12    | via                         |              |
          | 13    | access-control-allow-origin |              |
          | 14    | accept-ranges               |              |
          | 15    | allow                       |              |
          | 16    | connection                  |              |
          | 17    | content-disposition         |              |
          | 18    | content-encoding            |              |
          | 19    | content-language            |              |
          | 20    | content-location            |              |
          | 21    | content-md5                 |              |
          | 22    | content-range               |              |
          | 23    | link                        |              |
          | 24    | location                    |              |
          | 25    | p3p                         |              |

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          | 26    | pragma                      |              |
          | 27    | proxy-authenticate          |              |
          | 28    | refresh                     |              |
          | 29    | retry-after                 |              |
          | 30    | strict-transport-security   |              |
          | 31    | trailer                     |              |
          | 32    | transfer-encoding           |              |
          | 33    | warning                     |              |
          | 34    | www-authenticate            |              |
          +-------+-----------------------------+--------------+

                                  Table 2

Appendix B.  Example

   Here is an example that illustrates different representations and how
   tables are updated.  [[This section needs to be updated to integrate
   differential coding.]]

B.1.  First header set

   The first header set to represent is the following:

   :path: /my-example/index.html
   user-agent: my-user-agent
   x-my-header: first

   The header table is empty, all headers are represented as literal
   headers with indexing.  The 'x-my-header' header name is not in the
   header name table and is encoded literally.  This gives the following
   representation:

   0x44      (literal header with incremental indexing, name index = 3)
   0x16      (header value string length = 22)
   /my-example/index.html
   0x4D      (literal header with incremental indexing, name index = 12)
   0x0D      (header value string length = 13)
   my-user-agent
   0x40      (literal header with incremental indexing, new name)
   0x0B      (header name string length = 11)
   x-my-header
   0x05      (header value string length = 5)
   first

   The header table is as follows after the processing of these headers:

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   Header table
   +---------+----------------+---------------------------+
   |  Index  | Header Name    | Header Value              |
   +---------+----------------+---------------------------+
   |    0    | :scheme        | http                      |
   +---------+----------------+---------------------------+
   |    1    | :scheme        | https                     |
   +---------+----------------+---------------------------+
   |   ...   | ...            | ...                       |
   +---------+----------------+---------------------------+
   |   37    | warning        |                           |
   +---------+----------------+---------------------------+
   |   38    | :path          | /my-example/index.html    | added header
   +---------+----------------+---------------------------+
   |   39    | user-agent     | my-user-agent             | added header
   +---------+----------------+---------------------------+
   |   40    | x-my-header    | first                     | added header
   +---------+----------------+---------------------------+

   As all the headers in the first header set are indexed in the header
   table, all are kept in the reference set of headers, which is:

   Reference Set:
   :path, /my-example/index.html
   user-agent, my-user-agent
   x-my-header, first

B.2.  Second header set

   The second header set to represent is the following:

   :path: /my-example/resources/script.js
   user-agent: my-user-agent
   x-my-header: second

   Comparing this second header set to the reference set, the first and
   third headers are from the reference set are not present in this
   second header set and must be removed.  In addition, in this new set,
   the first and third headers have to be encoded.  The path header is
   represented as a literal header with substitution indexing.  The x
   -my-header will be represented as a literal header with incremental
   indexing.

   0xa6       (indexed header, index = 38: removal from reference set)
   0xa8       (indexed header, index = 40: removal from reference set)

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   0x04       (literal header, substitution indexing, name index = 3)
   0x26       (replaced entry index = 38)
   0x1f       (header value string length = 31)
   /my-example/resources/script.js
   0x5f 0x0a  (literal header, incremental indexing, name index = 40)
   0x06       (header value string length = 6)
   second

   The header table is updated as follow:

   Header table
   +---------+----------------+---------------------------+
   |  Index  | Header Name    | Header Value              |
   +---------+----------------+---------------------------+
   |    0    | :scheme        | http                      |
   +---------+----------------+---------------------------+
   |    1    | :scheme        | https                     |
   +---------+----------------+---------------------------+
   |   ...   | ...            | ...                       |
   +---------+----------------+---------------------------+
   |   37    | warning        |                           |
   +---------+----------------+---------------------------+
   |   38    | :path          | /my-example/resources/    | replaced
   |         |                |     script.js             | header
   +---------+----------------+---------------------------+
   |   39    | user-agent     | my-user-agent             |
   +---------+----------------+---------------------------+
   |   40    | x-my-header    | first                     |
   +---------+----------------+---------------------------+
   |   41    | x-my-header    | second                    | added header
   +---------+----------------+---------------------------+

   All the headers in this second header set are indexed in the header
   table, therefore, all are kept in the reference set of headers, which
   becomes:

   Reference Set:
   :path, /my-example/resources/script.js
   user-agent, my-user-agent
   x-my-header, second

Authors' Addresses

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   Roberto Peon
   Google, Inc

   EMail: fenix@google.com

   Herve Ruellan
   Canon CRF

   EMail: herve.ruellan@crf.canon.fr

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