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Network Working Group                                           J. Snell
Internet-Draft
Intended status: Informational                           August 14, 2013
Expires: February 15, 2014


              HTTP/2.0 Discussion: Stored Header Encoding
                      draft-snell-httpbis-bohe-13

Abstract

   This memo describes a proposed alternative optimized encoding for
   ordered sets of header field (name,value) pairs in HTTP/2 HEADERS and
   PUSH_PROMISE frames.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 15, 2014.

Copyright Notice

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

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





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Table of Contents

   1.  Stored Header Encoding  . . . . . . . . . . . . . . . . . . .   2
   2.  Model . . . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Header Encoding and Decoding  . . . . . . . . . . . . . . . .   3
     3.1.  Literal (name,value) Representation . . . . . . . . . . .   5
       3.1.1.  Dealing with invalid name or value encodings  . . . .   7
     3.2.  Indexed Representation  . . . . . . . . . . . . . . . . .   7
     3.3.  Non-Indexed Literal Representation  . . . . . . . . . . .   8
     3.4.  Indexed Literal Representation  . . . . . . . . . . . . .   8
   4.  Unsigned Variable Length Integer Syntax . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     6.2.  Informational References  . . . . . . . . . . . . . . . .   9
   Appendix A.  Initial Cache Entries  . . . . . . . . . . . . . . .   9
   Appendix B.  Updated Standard Header Definitions  . . . . . . . .  11
   Appendix C.  Example  . . . . . . . . . . . . . . . . . . . . . .  13
     C.1.  First Header Set: . . . . . . . . . . . . . . . . . . . .  13
     C.2.  Second Header Set:  . . . . . . . . . . . . . . . . . . .  14
     C.3.  Third Header Set: . . . . . . . . . . . . . . . . . . . .  15
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Stored Header Encoding

   The Stored Header Encoding is a proposed alternative "compressed
   header encoding" for HTTP/2.0 that offers reasonably good compression
   ratios, support for a range of compressor strategies, efficient value
   type codecs, constrained state requirements, routing-friendly header
   value ordering, and easier implementation relative to the current
   header compression proposal.

2.  Model

   A "header" is a (name,value) pair.  The name is a sequence of lower-
   case ASCII characters.  The value is either an HTTP 1.1 defined
   field-value (see [I-D.ietf-httpbis-p1-messaging]), a sequence of
   UTF-8 encoded octets, an integer, a timestamp, or an opaque sequence
   of binary octets.

   The compressor and decompressor each maintain a synchronized cache of
   up to 256 headers.  Every header stored in the cache is referenced by
   an 8-bit identifier ranging from 0x00-FF (inclusive).

   The cache is managed in a "least recently written" manner, that is,
   as the cache fills to capacity in both number of entries and maximum
   stored byte size, the least recently written items are cleared and
   their index positions can be reused.



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   The specific index position to which a header is assigned is
   determined by the encoder using any algorithm the encoder determines
   to be appropriate.  If a specific index position already has an
   assigned header, the existing header is replaced.

   The maximum total size of the cache can be limited by the
   decompressor using the SETTINGS_MAX_BUFFER_SIZE setting.  Each stored
   header contributes a certain number of octets to the total
   accumulated size of the cache.  If adding a new header to the cache
   will cause the total size to grow beyond the set limit, the least-
   recently written items are removed in the order written until enough
   space is available to add the new item.  Clearing existing items does
   not change the index positions of the remaining items in the cache.

   The SETTINGS_MAX_BUFFER_SIZE setting has an initial default value of
   4096 bytes.  The decompressor can establish a new maximum buffer size
   at any time, possibly causing header (name,value) pairs to be evicted
   from the cache if the newly established limit is less than the
   current total size.

   The decompressor can disable use of the storage cache completely by
   setting the SETTINGS_MAX_BUFFER_SIZE setting to 0, forcing the cache
   to empty completely and making it impossible to add new headers.

   The size of a header is calculated as: The number of octets required
   for the name plus the number of octets required for the value plus
   32-octets to account for any internal storage overhead.  The number
   of octets required for the value depends on the value type:

   o  String values are measured by the number of UTF-8 encoded octets
      required to represent the character sequence.

   o  Number and Date-Time values are measured by the number of unsigned
      variable length integer (uvarint) encoded octets required to
      represent the value using a 5-bit prefix.

   o  Legacy (HTTP/1.1) values are measured by the number of octets
      required to represent the value.

   o  Binary values are measured by the number of octets contained by
      the sequence.

3.  Header Encoding and Decoding

   The set of headers is encoded for transmission using the following
   process:





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   1.  For each header, determine if the (name,value) pair already
       exists in the cache.

       *  If an exact match is found in the cache, encode the indexed
          position of the header as an Indexed Reference and advance to
          the next header (name,value) pair.

       *  Otherwise, move to step #2.

   2.  Determine if a header (name,value) pair with the same name
       already exists in the cache.  If a matching name is found, make
       note of the indexed position of the matching name and continue to
       step #3.

   3.  Determine whether the new header (name,value) pair ought to be
       assigned to the cache.

       *  If the header is not to be cached, encode the header as a Non-
          Indexed Literal Representation and continue to the next header
          (name,value) pair.

       *  Otherwise, assign an index position for the header
          (name,value) pair and encode the header as an Indexed Literal
          Representation.

   Following these steps, headers are serialized into one of four
   representation types, each represented by a two-bit prefix code.  The
   types and their codes are:

   o  10 - Indexed

   o  00 - Non-Indexed Literal

   o  01 - Indexed Literal

   Headers can be encoded into groups of up to 64 instances.  Each group
   is prefixed by a single octet.  The two most significant bits of this
   prefix identify the representation type, the six remaining bits
   specify the number of instances, with 000000 indicating a single
   instance and 111111 indicating 64.

   Decoding simply reverses the encoding steps:

   1.  First initialize an empty working set of headers.

   2.  Begin iterating through each representation group:





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       *  If it is an Indexed group, iterate through each index included
          in the group, look up the corresponding (name,value) pair in
          the cache and add that to the working set.  If no matching
          (name,value) is found, terminate and report an error.

       *  If it's a Non-Indexed Literal group, iterate through each
          (name,value) pair included in the group and add that to the
          working set.

       *  If it's an Indexed Literal group, iterate through each
          (name,value) pair, assign it to the specified position in the
          cache and add it to the working set.

   3.  Continue with each representation group until the full block has
       been decoded.

   When a single header name is used multiple times with different
   values, the order in which those values are serialized and processed
   is significant.  The working set created by the decoding process
   above MUST preserve the ordering of those values as received.

3.1.  Literal (name,value) Representation

   The structure of an encoded (name,value) pair consists of:

   o  A 3-bit value type identifier,

   o  The name, encoded either as a literal sequence of ASCII octets or
      as the cache index position of another existing header sharing the
      same name, and

   o  The encoded value.

   The three most-sigificant bits of the first octet identify the value
   type.

   This design allows for a maximum of 7 value types, five of which are
   defined by this specification.  The two remaining types are reserved
   for future use.  The currently defined value types are:

      UTF-8 (000)

      Integer (001)

      Timestamp (010)

      Legacy (100)




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      Opaque Binary (111)

   Of the five types, the Legacy type is reserved for encoding header
   values conforming to the field-value construct defined by
   [I-D.ietf-httpbis-p1-messaging], and is used specifically for
   backwards compatibility with header fields that have not yet been
   updated to use a more specific type value (see Appendix B).

   If the name is encoded using an index reference to another existing
   (name,value) pair in the cache, the remaining five least significant
   bits of the first octet are set to zero and the next octet identifies
   the referenced cache index position.  This octet MUST NOT reference a
   cache index position that is not currently assigned.

   If the name is encoded as a sequence of ASCII octets, the number of
   octets required to represent the name is encoded as a unsigned
   variable length integer with a five-bit prefix, filling the
   5-remaining least significant bits of the first octet, followed by
   the sequence of ASCII octets conforming to the following header-name
   construct:

   Header name ABNF:

    header-name = [":"] 1*header-char

    header-char = "!" / "#" / "$" / "%" / "&" / "'" /
                  "*" / "+" / "-" / "." / "^" / "_" /
                  "`" / "|" / "~" / DIGIT / LOWERALPHA

    LOWERALPHA = %x61-7A


   The encoding of the value depends on the value type.

   UTF-8:
      First, the number of UTF-8 encoded bytes required to represent the
      value is encoded as an unsigned variable length integer with a
      0-bit prefix, followed by the full sequence of UTF-8 octets.

   Integer
      Integer values ranging from 0 to 2^64-1 are encoded as unsigned
      variable length integers with a 0-bit prefix.  Negative or
      fractional numbers cannot be represented.

   Timestamp






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      Timestamps is represented as the number of milliseconds ellapsed
      since the standard Epoch (1970-01-01T00:00:00 GMT), encoded as an
      unsigned variable length integer with a 0-bit prefix.  Timestamps
      that predate the Epoch cannot be represented.

   Legacy
      First, the number of octets required to represent the value is
      encoded as an unsigned variable length integer with a 0-bit
      prefix, followed by the full sequence of octets.

   Opaque
      The number of octets in the sequence is encoded as an unsigned
      variable length integer with a 0-bit prefix, followed by the full
      sequence of octets.

3.1.1.  Dealing with invalid name or value encodings

   Implementations encountering invalid name or value encodings MUST
   signal an error and terminate processing of the header block.
   Examples of such errors include:

   o  Header names that include any octets not explicitly permitted by
      the above header-name ABNF construction;

   o  UTF-8 values that include a byte order mark, over-long or invalid
      octet sequences, or octets representing invalid Unicode
      codepoints;

   o  Integer or Date-Time values that encode numbers strictly larger
      than 2^64-1;

3.2.  Indexed Representation

   The serialization of an Indexed Representation consists of a single
   octet prefix followed by up to 64 single-octet cache index position
   references.

   +--------+--------+     +---------+
   |10xxxxxx| IDX[1] | ... | IDX[64] |
   +--------+---------     +---------+


   For instance:

   0x80 0x00
      References item #0 from the cache.

   0x81 0x00 0x01



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      References items #0 and #1 from the cache.

   Indexed Representations do not cause the cache state to be modified
   in any way.  If an Indexed References specifies an index position
   that has not yet been assigned or whose value has been cleared,
   decoding MUST terminate with an error.

3.3.  Non-Indexed Literal Representation

   The serialization of a group of Non-Indexed Literal representations
   consists of a single-octet prefix followed by up to 64 Literal
   (name,value) Representations.

   +--------+-----------------+     +------------------+
   |00xxxxxx| (name,value)[1] | ... | (name,value)[64] |
   +--------+-----------------+     +------------------+


   For instance:

   0x00 0x01 0x61 0x01 0x62
      Specifies a single header with name "a" and a UTF-8 value of "b"
      is to be handled as a Non-Indexed header (it is not added to the
      cache).

3.4.  Indexed Literal Representation

   The serialization of a group of Indexed Literal representations
   consists of a single-octet prefix followed by up to 64 (index
   position, Literal (name,value) representation) pairs.

   +--------+------+---------------+     +-------+----------------+
   |01xxxxxx|IDX[1]|(name,value)[1]| ... |IDX[64]|(name,value)[64]|
   +--------+------+---------------+     +-------+----------------+


   For instance:

   0x40 0x03 0x01 0x61 0x01 0x62
      Specifies that a single header with name "a" and a UTF-8 String
      value of "b" is to be assigned to the cache at index position #3.

   0x40 0x03 0x21 0x61 0x04
      Specifies that a single header with name "a" and Integer value of
      3 is to be assigned to the cache at index position #4.

4.  Unsigned Variable Length Integer Syntax




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   Unsigned integers are encoded as defined in
   [I-D.ietf-httpbis-header-compression].

5.  Security Considerations

   TBD

6.  References

6.1.  Normative References

   [I-D.ietf-httpbis-header-compression]
              Peon, R. and H. Ruellan, "HTTP/2.0 Header Compression",
              draft-ietf-httpbis-header-compression-01 (work in
              progress), July 2013.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

6.2.  Informational References

   [I-D.ietf-httpbis-p1-messaging]
              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", draft-ietf-
              httpbis-p1-messaging-23 (work in progress), July 2013.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.

Appendix A.  Initial Cache Entries

         +-------+-----------------------------+-------+---------+
         | Index | Name                        | Value | Type    |
         +-------+-----------------------------+-------+---------+
         | 0     | :scheme                     | http  | Text    |
         | 1     | :scheme                     | https | Text    |
         | 2     | :host                       |       |         |
         | 3     | :path                       | /     |         |
         | 4     | :method                     | GET   | Text    |
         | 5     | accept                      |       |         |
         | 6     | accept-charset              |       |         |
         | 7     | accept-encoding             |       |         |
         | 8     | accept-language             |       |         |
         | 9     | cookie                      |       |         |
         | 10    | if-modified-since           |       |         |



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         | 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         |       |         |
         | 30    | max-forwards                |       |         |
         | 31    | pragma                      |       |         |
         | 32    | proxy-authorization         |       |         |
         | 33    | range                       |       |         |
         | 34    | te                          |       |         |
         | 35    | upgrade                     |       |         |
         | 36    | via                         |       |         |
         | 37    | warning                     |       |         |
         | 38    | :status                     | 200   | Integer |
         | 39    | age                         |       |         |
         | 40    | cache-control               |       |         |
         | 41    | content-length              |       |         |
         | 42    | content-type                |       |         |
         | 43    | date                        |       |         |
         | 44    | etag                        |       |         |
         | 45    | expires                     |       |         |
         | 46    | last-modified               |       |         |
         | 47    | server                      |       |         |
         | 48    | set-cookie                  |       |         |
         | 49    | vary                        |       |         |
         | 50    | via                         |       |         |
         | 51    | access-control-allow-origin |       |         |
         | 52    | accept-ranges               |       |         |
         | 53    | allow                       |       |         |
         | 54    | connection                  |       |         |
         | 55    | content-disposition         |       |         |
         | 56    | content-encoding            |       |         |
         | 57    | content-language            |       |         |
         | 58    | content-location            |       |         |



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         | 59    | content-md5                 |       |         |
         | 60    | content-range               |       |         |
         | 61    | link                        |       |         |
         | 62    | location                    |       |         |
         | 63    | p3p                         |       |         |
         | 64    | pragma                      |       |         |
         | 65    | proxy-authenticate          |       |         |
         | 66    | refresh                     |       |         |
         | 67    | retry-after                 |       |         |
         | 68    | strict-transport-security   |       |         |
         | 69    | trailer                     |       |         |
         | 70    | transfer-encoding           |       |         |
         | 71    | warning                     |       |         |
         | 72    | www-authenticate            |       |         |
         | 73    | user-agent                  |       |         |
         +-------+-----------------------------+-------+---------+


Appendix B.  Updated Standard Header Definitions

   To properly deal with the backwards compatibility concerns for HTTP/
   1, there are several important rules for use of Typed Codecs in HTTP
   headers:

   o  All header fields MUST be explicitly defined to use the new header
      types.  All existing HTTP/1 header fields will continue to be
      represented in conformance to the field-value construct defined by
      [I-D.ietf-httpbis-p1-messaging] unless their specific definitions
      are updated.  Such fields MUST specify the Legacy value type when
      serialized.  The HTTP/2 specification would update the definitions
      of specific known header fields (e.g. content-length, date, if-
      modified-since, etc).

   o  For translation to HTTP/1.1, header fields that use the typed
      codecs will have specific normative transformations defined.

      *  UTF-8 will be converted to ISO-8859-1 with extended characters
         pct-encoded

      *  Numbers will be converted to their ASCII equivalent values.

      *  Date Times will be converted to their HTTP-Date equivalent
         values.

      *  Opaque fields will be Base64-encoded.

      *  Legacy fields are passed through untranslated.




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   o  There will be no normative transformation from legacy values into
      the typed codecs.  Implementations are free to apply
      transformation where they determine it to be appropriate, but it
      will be perfectly acceptable for an implementation to pass a text
      value through as a Legacy type even if it is known that a given
      header has a typed codec equivalent.

   A Note of warning: Individual header fields MAY be defined such that
   they can be represented using multiple types.  Numeric fields, for
   instance, can be represented using either the uvarint encoding or
   using the equivalent sequence of ASCII numbers.  Implementers will
   need to be capable of supporting each of the possible variations.
   Designers of header field definitions need to be aware of the
   additional complexity and possible issues that allowing for such
   alternatives can introduce for implementers.

   Based on an initial survey of header fields currently defined by the
   HTTPbis specification documents, the following header field
   definitions can be updated to make use of the new types

   +---------------------+---------------+-----------------------------+
   | Field               | Type          | Description                 |
   +---------------------+---------------+-----------------------------+
   | content-length      | Numeric or    | Can be represented as       |
   |                     | Text          | either an unsigned,         |
   |                     |               | variable-length integer or  |
   |                     |               | a sequence of ASCII         |
   |                     |               | numbers.                    |
   | date                | Timestamp or  | Can be represented as       |
   |                     | Text          | either a uvarint encoded    |
   |                     |               | timestamp or as text (HTTP- |
   |                     |               | date).                      |
   | max-forwards        | Numeric or    | Can be represented as       |
   |                     | Text          | either an unsigned,         |
   |                     |               | variable-length integer or  |
   |                     |               | a sequence of ASCII         |
   |                     |               | numbers.                    |
   | retry-after         | Timestamp,    | Can be represented as       |
   |                     | Numeric or    | either a uvarint encoded    |
   |                     | Text          | timestamp, an unsigned,     |
   |                     |               | variable-length integer, or |
   |                     |               | the text equivalents of     |
   |                     |               | either (HTTP-date or        |
   |                     |               | sequence of ASCII numbers)  |
   | if-modified-since   | Timestamp or  | Can be represented as       |
   |                     | Text          | either a uvarint encoded    |
   |                     |               | timestamp or as text (HTTP- |
   |                     |               | date).                      |



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   | if-unmodified-since | Timestamp or  | Can be represented as       |
   |                     | Text          | either a uvarint encoded    |
   |                     |               | timestamp or as text (HTTP- |
   |                     |               | date).                      |
   | last-modified       | Timestamp or  | Can be represented as       |
   |                     | Text          | either a uvarint encoded    |
   |                     |               | timestamp or as text (HTTP- |
   |                     |               | date).                      |
   | age                 | Numeric or    | Can be represented as       |
   |                     | Text          | either an unsigned,         |
   |                     |               | variable-length integer or  |
   |                     |               | a sequence of ASCII         |
   |                     |               | numbers.                    |
   | expires             | Timestamp or  | Can be represented as       |
   |                     | Text          | either a uvarint encoded    |
   |                     |               | timestamp or as text (HTTP- |
   |                     |               | date).                      |
   | etag                | Binary or     | Can be represented as       |
   |                     | Text          | either an opaque sequence   |
   |                     |               | of binary octets or using   |
   |                     |               | the currently defined text  |
   |                     |               | format. When represented as |
   |                     |               | binary octets, the Entity   |
   |                     |               | Tag MUST be considered to   |
   |                     |               | be a Strong Entity tag.     |
   |                     |               | Weak Entity Tags cannot be  |
   |                     |               | represented using the       |
   |                     |               | binary octet option.        |
   +---------------------+---------------+-----------------------------+


Appendix C.  Example

C.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 cache is prefilled as defined in Appendix A, however, none of the
   values represented in the initial set can be found in the cache.  All
   headers, then, are encoding using the Indexed Literal Representation:

   43 4a 00 03 16 2f 6d 79 2d 65
   78 61 6d 70 6c 65 2f 69 6e 64



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   65 78 2e 68 74 6d 6c 4B 00 49
   6d 79 2d 75 73 65 72 2d 61 67
   65 6e 74 4c 0b 78 2d 6d 79 2d
   68 65 61 64 65 72 05 66 69 72
   73 74


   Three new entries are added to the cache:

             +-------+-------------+------------------------+
             | Index | Name        | Value                  |
             +-------+-------------+------------------------+
             | 74    | :path       | /my-example/index.html |
             | 75    | user-agent  | my-user-agent          |
             | 76    | x-my-header | first                  |
             +-------+-------------+------------------------+


C.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 first, we see that the :path
   and x-my-header headers have new values, while the user-agent value
   remains unchanged.

   80 4b 41 4a 00 4a 1f 2f 6d 79
   2d 65 78 61 6d 70 6c 65 2f 72
   65 73 6f 75 72 63 65 73 2f 73
   63 72 69 70 74 2e 6a 73 4c 00
   4c 06 73 65 63 6f 6e 64


   Items #74 and #76 added by the previous header set are replaced:

         +-------+-------------+---------------------------------+
         | Index | Name        | Value                           |
         +-------+-------------+---------------------------------+
         | 74    | :path       | /my-example/resources/script.js |
         | 75    | user-agent  | my-user-agent                   |
         | 76    | x-my-header | second                          |
         +-------+-------------+---------------------------------+




Snell                   Expires February 15, 2014              [Page 14]


Internet-Draft           Stored Header Encoding              August 2013


C.3.  Third Header Set:

   Let's suppose a third header set that is identical to the second is
   sent:

   82 4b 4c 4d


Author's Address

   James M Snell

   Email: jasnell@gmail.com






































Snell                   Expires February 15, 2014              [Page 15]

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