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          13                                                            
Network Working Group                                         J.M. Snell
Internet-Draft                                              May 20, 2013
Intended status: Informational
Expires: November 21, 2013


              HTTP/2.0 Discussion: Stored Header Encoding
                      draft-snell-httpbis-bohe-08

Abstract

   This memo describes a proposed alternative encoding for headers that
   combines the best concepts from the proposed Delta and HeaderDiff
   options with the typed value codecs introduced by previous versions
   of this draft.

Status of This Memo

   This Internet-Draft is submitted to IETF 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 November 21, 2013.

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.

Table of Contents

   1.  Stored Header Encoding  . . . . . . . . . . . . . . . . . . .   2



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   2.  State Model . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Header Serialization  . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Header Group Prefix . . . . . . . . . . . . . . . . . . .   6
     3.2.  Index Header Group  . . . . . . . . . . . . . . . . . . .   7
     3.3.  Index Range Header Group  . . . . . . . . . . . . . . . .   7
     3.4.  Cloned Index Header Group . . . . . . . . . . . . . . . .   8
     3.5.  Literal Header Group  . . . . . . . . . . . . . . . . . .   9
   4.  Header Values . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  UTF-8 Text Values . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Numeric Values  . . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Timestamp Values  . . . . . . . . . . . . . . . . . . . .  10
     4.4.  Raw Binary Octet Values . . . . . . . . . . . . . . . . .  10
     4.5.  Unsigned Variable Length Integer Syntax . . . . . . . . .  11
     4.6.  Huffman Coding  . . . . . . . . . . . . . . . . . . . . .  11
   5.  Implementation Considerations . . . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     7.2.  Informational References  . . . . . . . . . . . . . . . .  13
   Appendix A.  Huffman Tables . . . . . . . . . . . . . . . . . . .  13
   Appendix B.  Static Storage Cache . . . . . . . . . . . . . . . .  20
   Appendix C.  Updated Standard Header Definitions  . . . . . . . .  23
   Appendix D.  State Management Alternatives  . . . . . . . . . . .  25
   Appendix E.  Alternative Timestamp encodings  . . . . . . . . . .  26
   Appendix F.  Alternative uvarint encodings  . . . . . . . . . . .  27
     F.1.  Option 1: . . . . . . . . . . . . . . . . . . . . . . . .  28
     F.2.  Option 2: . . . . . . . . . . . . . . . . . . . . . . . .  28
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Stored Header Encoding

   The Stored Header Encoding is an alternative "binary header encoding"
   for HTTP/2.0 that combines the best elements from three other
   proposed encodings, including:

   o  The "Header Delta Compression" scheme proposed by Roberto Peon in
      http://tools.ietf.org/html/draft-rpeon-httpbis-header-
      compression-03

   o  The "Header Diff" encoding proposed by Herve Reullan, Jun
      Fujisawa, Romain Bellessort, and Youenn Fablet in http://
      tools.ietf.org/html/draft-ruellan-headerdiff-00

   o  The "Binary Optimized Header Encoding" proposed by James Snell
      (me) in http://tools.ietf.org/html/draft-snell-httpbis-bohe-03






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   The Stored Header Encoding seeks to find an elegant, efficient and
   simple marriage of the best concepts from each of these separate
   proposals.

2.  State Model

   The compressor and decompressor each maintain a cache of header value
   pairs.  There is a static cache, prepopulated by the specification,
   and a dynamic cache, populated through the compression and
   decompression process.  Each cache contains a maximum of 128
   individual key+value pairs.

   Each item in the index is referenced by an 8-bit identifier.  The
   most significant bit identifies whether an item from the static or
   dynamic cache is being referenced.  Note: the Nil byte (0x00) is a
   valid identifier for the dynamic cache.

   0xxxxxxx  --  Dynamic Cache
   1xxxxxxx  --  Static Cache


   The dynamic cache is managed in a "least recently written" style,
   that is, as the cache fills to capacity in both number of entries and
   maximum stored byte size, the least recently written items are
   dropped and those index positions are reused.

   Index positions from the dynamic cache are assigned in "encounter
   order", beginning from 0x00 and increasing monotonically to 0x7F.
   That is to say, the positions are assigned in precisely the same
   order that they are serialized, and thereby encountered by the
   decompressor upon reading and processing the block.

   Each item in the store consists of a Header Name and a Value.  The
   Name is a lower-case ISO-8859-1 character sequence.  The Value is
   either a UTF-8 string, a number, a Timestamp or an arbitrary sequence
   of binary octets.

   The available size of the stored compression state can be capped by
   the decompressor.  Each stored value contributes to the accumulated
   size of the storage state.  As new key+value pairs are assigned
   positions in the dynamic cache, the least-recently assigned items
   must be removed if necessary to free up the required space.

   The size of string values is measured by the number of UTF-8 bytes
   required for the character sequence.






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   The size of number and timestamp values are measured by the number of
   unsigned variable length integer (uvarint) encoded bytes it takes to
   represent the value (see the section of value types below).

   The size of raw binary values is measured by the number of octets.

   Header names DO NOT contribute to the stored state size of the
   compressor; only the size of the value is considered.  Duplicate
   values MUST be counted individually.

   Header names MUST NOT exceed 255 octets in length.

3.  Header Serialization

   Headers are serialized into four typed header groups, each
   represented by a two-bit identifier.  These groups are serialized
   sequentially.  A serialized header block can contain, at most 256
   header groups.  The first byte of the serialized block is an
   unsigned, 0-based counter indicating the number of groups.

   00 -- Index Header Group
   01 -- Index Range Header Group
   10 -- Cloned Index Header Group
   11 -- Literal Header Group


   The Cloned Index (10) and Literal (11) header group types have an
   additional "ephemeral" property that indicates whether or not the
   group affects the compression state.

   Each header group contains a single 8-bit prefix and up to 32
   distinct header instances.

   If a particular serialization block contains more than 32 intances of
   a given type, then multiple instances of the Header Group Type can be
   included in the serialized block.  For instance, if a given message
   contains 33 index references, the serialized block may contain two
   separate Index Header Groups.

   Wire Format

     header-block               = OCTET *(index-header-group  /
                                    index-range-header-group  /
                                    cloned-index-header-group /
                                    literal-header-group)

     ; Header Group Prefix = 8 bits ...
     ;  First two bits = header-group-type



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     ;  Third bit = ephemeral flag
     ;  Final five bits = instance counter
     ;
     index-header-group-type    = 00
     index-range-group-type     = 01
     cloned-index-group-type    = 10
     literal-group-type         = 11
     count                      = 5bit

     index-header-prefix        = index-header-group-type
                                  unset count ; 000xxxxx
     index-range-header-prefix  = index-header-group-type
                                  unset count ; 010xxxxx
     cloned-index-header-prefix = cloned-index-group-type
                                  bit   count ; 10?xxxxx
     literal-header-prefix      = literal-group-type
                                  bit   count ; 11?xxxxx

     ; Cache Index Identifier = 8 bits ...
     ;  0xxxxxxx = Dynamic Cache Identifier
     ;  1xxxxxxx = Static Cache Identifier
     cache-index                = %x00-FF

     ; Index Header Group
     index-header-group         = index-header-prefix
                                  1*32cache-index

     ; Index-Range Header Group
     ;  Contains a pair of cache-index values, second MUST
     ;  me strictly higher in value than the first...
     index-range-header-group   = index-range-header-prefix
                                  1*32(cache-index cache-index)

     ; Cloned-Index Header Group
     cloned-index-header-group  = cloned-index-header-prefix
                                  1*32(cache-index value)

     ; Literal Header Group
     literal-header-group       = literal-header-prefix
                                  1*32(name value)

     value                      = text-value /
                                  number-value /
                                  timestamp-value /
                                  binary-value

     text-value-type            = 00 ; two bits
     number-value-type          = 01



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     timestamp-value-type       = 10
     binary-value-type          = 11

     text-value-prefix          = text-value-type
                                  unset count    ; 000xxxxx
     number-value-prefix        = number-value-type
                                  unset count    ; 010xxxxx
     timestamp-value-prefix     = timestamp-value-type
                                  unset count    ; 100xxxxx
     binary-value-prefix        = binary-value-type
                                  unset count    ; 110xxxxx

     text-value                 = text-value-prefix *32string
     number-value               = number-value-type *32uvarint
     timestamp-value            = timestamp-value-prefix *32uvarint
     binary-value               = binary-value-prefix uvarint *OCTET

     uvarint                    = *uvarint-continuation uvarint-final
     uvarint-continuation       = %x80-FF
     uvarint-final              = %x00-7F

     name                       = 1*tchar
     tchar                      = "!" / "#" / "$" / "%" / "&" /
                                  "'" / "*" / "+" / "-" / "." /
                                  "^" / "_" / "`" / "|" / "~"
                                  / DIGIT / ALPHA

     string                     = uvarint *(HUFFMAN-ENCODED-CHAR)
                                  HUFFMAN-EOF
                                  padding-to-nearest-byte;
     padding-to-nearest-byte    = *7unset

     bit                        = set / unset
     unset                      = 0
     set                        = 1


3.1.  Header Group Prefix

   The Header Group Prefix is a single octet that provides three
   distinct pieces of information:

     0  1   2   3  4  5  6  7
   +------+---+---------------+
   | TYPE |EPH|   COUNTER     |
   +------+---+---------------+





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   TYPE  The two most significant bits of the header group prefix
      identify the group type.

   EPH  The next bit is the "ephemeral flag" and is used only for Cloned
      and Literal group types.  This bit indicates whether or not the
      group alters the stored compression state.

   COUNTER  The remaining five bits specify the number of header
      instances in the group, with 00000 indicating that the group
      contains 1 instance and 11111 contains 32.  A header group MUST
      contain at least one instance.

   The remaining serialization of the header group depends entirely on
   the group type.

3.2.  Index Header Group

   The serialization of the Index Header Group consists of the Header
   Group Prefix and up to 32 additional octets, each referencing a
   single 8-bit storage index identifier for items in either the Static
   or Dynamic Cache.

   For instance

     00000000 00000000 = References item #0 from
                         the dynamic cache

     00000001 00000000 10000000 = References item #0 from the
                                  dynamic cache and item #0
                                  from the static cache


   Index Header Groups do not affect the stored compression state.  If
   an Index Header Group references a header index that has not yet been
   allocated, the deserialization MUST terminate with an error.  This
   likely means that the compression state has become out of sync and
   needs to be reestablished.

3.3.  Index Range Header Group

   The serialization of the Index Range Header Group consists of the
   Header Group Prefix and up to 32 additional 2-octet (16 bits) pairs
   of 8-bit storage index identifiers.  Each pair specifies a sequential
   range of adjacent ranges.

   For instance:





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   01000000 00000000 00000100 = References items #0-#4 from
                                the dynamic cache.
                                (five distinct items total)


   A range MAY span dynamic and static index values.  Index values are
   treated as unsigned byte values, so indices from the static cache are
   numerically greater than dynamic cache values.. e.g.

   01000000 01111111 10000001 = References item #127 from the
                                dynamic cache, and items #0
                                and #1 from the static cache.


   Index Range Header Groups do not affect the stored compression state.
   If a range references a header index that has not yet been allocated,
   the deserialization MUST terminate with an error.  This likely means
   that the compression state has become out of sync and needs to be
   reestablished.

3.4.  Cloned Index Header Group

   The serialization of the Cloned Index Header Group consists of the
   Header Group Prefix and up to 32 Index+Value pairs.  Each Index+Value
   pair consists of a leading 8-bit storage index of an existing stored
   header followed by a new serialized value.  The serialization of the
   value depends on the value type (see discussion of Value
   serialization below).

   The Cloned Header Group affects the stored compression state if, and
   only if, the "ephemeral" flag in the Header Group Prefix is NOT set.
   If the header group is not marked as being ephemeral, then the
   specified value is stored in the next available storage index using
   the key name from the referenced storage index.

   For instance, assume the dynamic cache currently contains an item at
   index #1 with key name "foo" and value "bar", the following causes a
   new item to be added to the storage with key name "foo" and value
   "baz":

   10000000 00000001 00000000 00000100
   10111000 01001111 10110101 00100000


   An explanation of the value syntax is given a bit later.

   If a Cloned Header Group references a header index that has not yet
   been allocated, the deserialization MUST terminate with an error.



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   This likely means that the compression state has become out of sync
   and needs to be reestablished.

3.5.  Literal Header Group

   The serialization of the Literal Header Group consists of the Header
   Group Prefix and up to 32 Name+Value pairs.  Each Name+Value pair
   consists of a length-prefixed sequence of ASCII bytes specifying the
   Header Name followed by the serialized value.  The serialization of
   the value depends on the value type.  The length prefix is encoded as
   an unsigned variable length integer (uvarint).  The length prefix
   SHOULD NOT be longer than five octets and SHOULD NOT specify a value
   larger than 0xFFFF.

   The Literal Header Group affects the stored compression state if, and
   only if, the "ephemeral" flag in the Header Group Prefix is NOT set.
   If the header group is not marked as being ephemeral, then the
   specified key name and value are stored in the next available storage
   index.

   For instance:

   11000000 00000011 01100110 01101111
   01101111 00000000 00000010 10111000
   01000100 11010010


4.  Header Values

   Header Values can be one of four types, each identified by a two-bit
   identifier.

   o  00 -- UTF-8 Text

   o  01 -- Numeric

   o  10 -- Timetamp

   o  11 -- Raw Binary Octets

   An individual value MAY consist of up to 32 distinct discreet "value
   instances".  A value with multiple instances is considered, for all
   intents and purposes, to be a single value.

   Each serialized value is preceded by an 8-bit Value Prefix.

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



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   | TYPE |RSV|   COUNTER     |
   +------+---+---------------+


   TYPE  The two most significant bits specifies the value type.

   RSV  The next significant bit is a reserved flag.  Future iterations
      of this specification might make use of this bit.

   COUNTER  The remaining bits specify the number of discreet instances
      in the value.  00000 indicates that one instance is included,
      11111 indicates that 32 instances are included.  The value MUST
      contain at least one instance.

   The remaining serialization depends entirely on the type.

4.1.  UTF-8 Text Values

   UTF-8 Text is encoded as a length-prefixed sequence of Huffman-
   encoded UTF-8 octets.  The length prefix is encoded as an unsigned
   variable-length integer specifying the number of octets after
   applying the Huffman-encoding.

4.2.  Numeric Values

   Numeric values are encoded as unsigned variable-length integers
   (uvarint) (Section 4.5) of up to a maximum of 10-octets.  Negative
   values cannot be represented using this syntax.

4.3.  Timestamp Values

   Timestamp values are encoded as unsigned variable-length integers
   (Section 4.5) specifying the number of milliseconds that have passed
   since the standard Epoch (1970-01-01T00:00:00 GMT).  The syntax is
   identical that used for Numeric Values.  Dates prior to the epoch
   cannot be represented using this syntax.

   Representing timestamps in this manner ensures that timestamps will
   always encode using six bytes up and until 2109-05-15T07:35:00 GMT,
   then as seven bytes up to and until 19809-03-05T11:03:41 GMT.

4.4.  Raw Binary Octet Values

   Binary values are encoded as a length prefixed sequence of arbitrary
   octets.  The length prefix is encoded as an unsigned variable length
   integer.





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4.5.  Unsigned Variable Length Integer Syntax

   The uvarint syntax is identical to that used by Google's protobufs.
   They are serialized with the least-significant bytes first in batches
   of 7-bits, with the most significant bit per byte reserved as a
   continuation bit.  Values less than or equal to 127 are serialized
   using at most one byte; values less than or equal to 16383 are
   serialized using at most two bytes; values less than or equal to
   2097151 are serialized using at most three bytes.

   def uvarint(num):
     return [] if num = 0
     ret = []
     while(num != 0):
       m = num >>> 7    ; unsigned shift left 7 bits
       ret.push (byte)((num & ~0x80) | ( m >  0 ? 0x80 : 0x00 ));
       num = m;
     return ret;


   For example, the binary representation of the 32-bit integer 217 is:

   00000000 00000000 00000000 11011001


   The variable length encoding is:

   11011001 00000001


   The binary representation of the 32-bit integer 1386210052 is:

   01010010 10011111 11100011 00000100


   The variable length encoding is:

   10000100 11000110 11111111 10010100 00000101


4.6.  Huffman Coding

   All UTF-8 text values are compressed using a modified static huffman
   code.  "Modified" because the encoded version may contain compact-
   representations of raw, arbitrary UTF-8 bytes that are not covered by
   the static huffman code table.





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   There are two huffman tables in use, one for HTTP Requests and
   another for HTTP Responses, each covers UTF-8 codepoints strictly
   less than 128 as well the fifty possible UTF-8 leading octets.

   The encoded result MUST end with a specific terminal sequence of bits
   called the "HUFFMAN_EOF".  Currently, the HUFFMAN_EOF is the same for
   both the Request and Response tables, but that could change if the
   tables are regenerated.  Currently, the HUFFMAN_EOF sequence is
   101001.

   Codepoints >= 128 are handled by first taking the leading octet of
   the UTF-8 representation and serializing it's associated huffman code
   from the table to the output stream, then, depending on the octets
   value, serializing the six least significant bits from each of the
   remaining trailing octets.

   For instance, the UTF-8 character U+00D4 (LATIN CAPITAL LETTER O WITH
   CIRCUMFLEX), with UTF-8 representation of C394 (hex) is encoded as:

   11000100 01010010 10010000


   The first 8-bits represents the huffman-table prefix, the six most
   significant bytes of the second octet are taken directly from the six
   least significant bits of the second UTF-8 byte (0x94).  Following
   those six bits are the six bits of the HUFFMAN_EOF 101001, followed
   by four unset padding bits.

   The number of raw UTF-8 bits to write depends on the value of the
   leading octet.  If the value is between 0xC2 and 0xDF (inclusive),
   six bits from the second continuation byte is encoded.  If the value
   is between 0xE0 and 0xEF (inclusive), six bits from the second and
   third continuation bytes are encoded.  If the value is between 0xF0
   and 0xF4 (inclusive), six bits from the second, third and fourth
   continuation bytes are encoded.  UTF-8 codepoints that require
   greater than four bytes to encode cannot be represented.

5.  Implementation Considerations

   When implementing the Stored Header Encoding, it is important to note
   that the compression state is managed in encounter order.  This means
   that the compressor must delay storing items in the compression state
   until the order in which frames are to be serialized out to the
   network has been determined.  This is a potential performance
   bottleneck that needs to be fully tested out to determine its impact.

6.  Security Considerations




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   TBD

7.  References

7.1.  Normative References

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

7.2.  Informational References

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

Appendix A.  Huffman Tables

   Request Table

       (  0)  |11111111|11111111|11111111|0 [25]        1fffffe [25]
       (  1)  |11111111|11111111|11111111|1 [25]        1ffffff [25]
       (  2)  |11111111|11111111|11100000 [24]           ffffe0 [24]
       (  3)  |11111111|11111111|11100001 [24]           ffffe1 [24]
       (  4)  |11111111|11111111|11100010 [24]           ffffe2 [24]
       (  5)  |11111111|11111111|11100011 [24]           ffffe3 [24]
       (  6)  |11111111|11111111|11100100 [24]           ffffe4 [24]
       (  7)  |11111111|11111111|11100101 [24]           ffffe5 [24]
       (  8)  |11111111|11111111|11100110 [24]           ffffe6 [24]
       (  9)  |11111111|11111111|11100111 [24]           ffffe7 [24]
       ( 10)  |11111111|11111111|11101000 [24]           ffffe8 [24]
       ( 11)  |11111111|11111111|11101001 [24]           ffffe9 [24]
       ( 12)  |11111111|11111111|11101010 [24]           ffffea [24]
       ( 13)  |11111111|11111111|11101011 [24]           ffffeb [24]
       ( 14)  |11111111|11111111|11101100 [24]           ffffec [24]
       ( 15)  |11111111|11111111|11101101 [24]           ffffed [24]
       ( 16)  |11111111|11111111|11101110 [24]           ffffee [24]
       ( 17)  |11111111|11111111|11101111 [24]           ffffef [24]
       ( 18)  |11111111|11111111|11110000 [24]           fffff0 [24]
       ( 19)  |11111111|11111111|11110001 [24]           fffff1 [24]
       ( 20)  |11111111|11111111|11110010 [24]           fffff2 [24]
       ( 21)  |11111111|11111111|11110011 [24]           fffff3 [24]
       ( 22)  |11111111|11111111|11110100 [24]           fffff4 [24]
       ( 23)  |11111111|11111111|11110101 [24]           fffff5 [24]
       ( 24)  |11111111|11111111|11110110 [24]           fffff6 [24]
       ( 25)  |11111111|11111111|11110111 [24]           fffff7 [24]
       ( 26)  |11111111|11111111|11111000 [24]           fffff8 [24]
       ( 27)  |11111111|11111111|11111001 [24]           fffff9 [24]
       ( 28)  |11111111|11111111|11111010 [24]           fffffa [24]
       ( 29)  |11111111|11111111|11111011 [24]           fffffb [24]



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       ( 30)  |11111111|11111111|11111100 [24]           fffffc [24]
       ( 31)  |11111111|11111111|11111101 [24]           fffffd [24]
   ' ' ( 32)  |11111111|0110 [12]                           ff6 [12]
   '!' ( 33)  |11111111|0111 [12]                           ff7 [12]
   '"' ( 34)  |11111111|111010 [14]                        3ffa [14]
   '#' ( 35)  |11111111|1111100 [15]                       7ffc [15]
   '$' ( 36)  |11111111|1111101 [15]                       7ffd [15]
   '%' ( 37)  |011000 [6]                                    18 [6]
   '&' ( 38)  |1010100 [7]                                   54 [7]
   ''' ( 39)  |11111111|1111110 [15]                       7ffe [15]
   '(' ( 40)  |11111111|1000 [12]                           ff8 [12]
   ')' ( 41)  |11111111|1001 [12]                           ff9 [12]
   '*' ( 42)  |11111111|1010 [12]                           ffa [12]
   '+' ( 43)  |11111111|1011 [12]                           ffb [12]
   ',' ( 44)  |11111011|10 [10]                             3ee [10]
   '-' ( 45)  |011001 [6]                                    19 [6]
   '.' ( 46)  |00010 [5]                                      2 [5]
   '/' ( 47)  |00011 [5]                                      3 [5]
   '0' ( 48)  |011010 [6]                                    1a [6]
   '1' ( 49)  |011011 [6]                                    1b [6]
   '2' ( 50)  |011100 [6]                                    1c [6]
   '3' ( 51)  |011101 [6]                                    1d [6]
   '4' ( 52)  |1010101 [7]                                   55 [7]
   '5' ( 53)  |1010110 [7]                                   56 [7]
   '6' ( 54)  |1010111 [7]                                   57 [7]
   '7' ( 55)  |1011000 [7]                                   58 [7]
   '8' ( 56)  |1011001 [7]                                   59 [7]
   '9' ( 57)  |1011010 [7]                                   5a [7]
   ':' ( 58)  |011110 [6]                                    1e [6]
   ';' ( 59)  |11111011|11 [10]                             3ef [10]
   '<' ( 60)  |11111111|11111111|10 [18]                  3fffe [18]
   '=' ( 61)  |011111 [6]                                    1f [6]
   '>' ( 62)  |11111111|11111110|0 [17]                   1fffc [17]
   '?' ( 63)  |11110110|0 [9]                               1ec [9]
   '@' ( 64)  |11111111|11100 [13]                         1ffc [13]
   'A' ( 65)  |10111010 [8]                                  ba [8]
   'B' ( 66)  |11110110|1 [9]                               1ed [9]
   'C' ( 67)  |10111011 [8]                                  bb [8]
   'D' ( 68)  |10111100 [8]                                  bc [8]
   'E' ( 69)  |11110111|0 [9]                               1ee [9]
   'F' ( 70)  |10111101 [8]                                  bd [8]
   'G' ( 71)  |11111100|00 [10]                             3f0 [10]
   'H' ( 72)  |11111100|01 [10]                             3f1 [10]
   'I' ( 73)  |11110111|1 [9]                               1ef [9]
   'J' ( 74)  |11111100|10 [10]                             3f2 [10]
   'K' ( 75)  |11111111|010 [11]                            7fa [11]
   'L' ( 76)  |11111100|11 [10]                             3f3 [10]
   'M' ( 77)  |11111000|0 [9]                               1f0 [9]



Snell                  Expires November 21, 2013               [Page 14]


Internet-Draft           Stored Header Encoding                 May 2013


   'N' ( 78)  |11111101|00 [10]                             3f4 [10]
   'O' ( 79)  |11111101|01 [10]                             3f5 [10]
   'P' ( 80)  |11111000|1 [9]                               1f1 [9]
   'Q' ( 81)  |11111101|10 [10]                             3f6 [10]
   'R' ( 82)  |11111001|0 [9]                               1f2 [9]
   'S' ( 83)  |11111001|1 [9]                               1f3 [9]
   'T' ( 84)  |11111010|0 [9]                               1f4 [9]
   'U' ( 85)  |11111101|11 [10]                             3f7 [10]
   'V' ( 86)  |11111110|00 [10]                             3f8 [10]
   'W' ( 87)  |11111110|01 [10]                             3f9 [10]
   'X' ( 88)  |11111110|10 [10]                             3fa [10]
   'Y' ( 89)  |11111110|11 [10]                             3fb [10]
   'Z' ( 90)  |11111111|00 [10]                             3fc [10]
   '[' ( 91)  |11111111|111011 [14]                        3ffb [14]
   '\' ( 92)  |11111111|11111111|11111110 [24]           fffffe [24]
   ']' ( 93)  |11111111|111100 [14]                        3ffc [14]
   '^' ( 94)  |11111111|111101 [14]                        3ffd [14]
   '_' ( 95)  |1011011 [7]                                   5b [7]
   '`' ( 96)  |11111111|11111111|110 [19]                 7fffe [19]
   'a' ( 97)  |00100 [5]                                      4 [5]
   'b' ( 98)  |1011100 [7]                                   5c [7]
   'c' ( 99)  |00101 [5]                                      5 [5]
   'd' (100)  |100000 [6]                                    20 [6]
   'e' (101)  |0000 [4]                                       0 [4]
   'f' (102)  |100001 [6]                                    21 [6]
   'g' (103)  |100010 [6]                                    22 [6]
   'h' (104)  |100011 [6]                                    23 [6]
   'i' (105)  |00110 [5]                                      6 [5]
   'j' (106)  |10111110 [8]                                  be [8]
   'k' (107)  |10111111 [8]                                  bf [8]
   'l' (108)  |100100 [6]                                    24 [6]
   'm' (109)  |100101 [6]                                    25 [6]
   'n' (110)  |100110 [6]                                    26 [6]
   'o' (111)  |00111 [5]                                      7 [5]
   'p' (112)  |01000 [5]                                      8 [5]
   'q' (113)  |11111010|1 [9]                               1f5 [9]
   'r' (114)  |01001 [5]                                      9 [5]
   's' (115)  |01010 [5]                                      a [5]
   't' (116)  |01011 [5]                                      b [5]
   'u' (117)  |100111 [6]                                    27 [6]
   'v' (118)  |11000000 [8]                                  c0 [8]
   'w' (119)  |101000 [6]                                    28 [6]
   'x' (120)  |11000001 [8]                                  c1 [8]
   'y' (121)  |11000010 [8]                                  c2 [8]
   'z' (122)  |11111011|0 [9]                               1f6 [9]
   '{' (123)  |11111111|11111110|1 [17]                   1fffd [17]
   '|' (124)  |11111111|1100 [12]                           ffc [12]
   '}' (125)  |11111111|11111111|0 [17]                   1fffe [17]



Snell                  Expires November 21, 2013               [Page 15]


Internet-Draft           Stored Header Encoding                 May 2013


   '~' (126)  |11111111|1101 [12]                           ffd [12]
       (127)  |101001 [6]                                    29 [6]
       (0xC2) |11000011 [8]                                  c3 [8]
       (0xC3) |11000100 [8]                                  c4 [8]
       (0xC4) |11000101 [8]                                  c5 [8]
       (0xC5) |11000110 [8]                                  c6 [8]
       (0xC6) |11000111 [8]                                  c7 [8]
       (0xC7) |11001000 [8]                                  c8 [8]
       (0xC8) |11001001 [8]                                  c9 [8]
       (0xC9) |11001010 [8]                                  ca [8]
       (0xCA) |11001011 [8]                                  cb [8]
       (0xCB) |11001100 [8]                                  cc [8]
       (0xCC) |11001101 [8]                                  cd [8]
       (0xCD) |11001110 [8]                                  ce [8]
       (0xCE) |11001111 [8]                                  cf [8]
       (0xCF) |11010000 [8]                                  d0 [8]
       (0xD0) |11010001 [8]                                  d1 [8]
       (0xD1) |11010010 [8]                                  d2 [8]
       (0xD2) |11010011 [8]                                  d3 [8]
       (0xD3) |11010100 [8]                                  d4 [8]
       (0xD4) |11010101 [8]                                  d5 [8]
       (0xD5) |11010110 [8]                                  d6 [8]
       (0xD6) |11010111 [8]                                  d7 [8]
       (0xD7) |11011000 [8]                                  d8 [8]
       (0xD8) |11011001 [8]                                  d9 [8]
       (0xD9) |11011010 [8]                                  da [8]
       (0xDA) |11011011 [8]                                  db [8]
       (0xDB) |11011100 [8]                                  dc [8]
       (0xDC) |11011101 [8]                                  dd [8]
       (0xDD) |11011110 [8]                                  de [8]
       (0xDE) |11011111 [8]                                  df [8]
       (0xDF) |11100000 [8]                                  e0 [8]
       (0xE0) |11100001 [8]                                  e1 [8]
       (0xE1) |11100010 [8]                                  e2 [8]
       (0xE2) |11100011 [8]                                  e3 [8]
       (0xE3) |11100100 [8]                                  e4 [8]
       (0xE4) |11100101 [8]                                  e5 [8]
       (0xE5) |11100110 [8]                                  e6 [8]
       (0xE6) |11100111 [8]                                  e7 [8]
       (0xE7) |11101000 [8]                                  e8 [8]
       (0xE8) |11101001 [8]                                  e9 [8]
       (0xE9) |11101010 [8]                                  ea [8]
       (0xEA) |11101011 [8]                                  eb [8]
       (0xEB) |11101100 [8]                                  ec [8]
       (0xEC) |11101101 [8]                                  ed [8]
       (0xED) |11101110 [8]                                  ee [8]
       (0xEE) |11101111 [8]                                  ef [8]
       (0xEF) |11110000 [8]                                  f0 [8]



Snell                  Expires November 21, 2013               [Page 16]


Internet-Draft           Stored Header Encoding                 May 2013


       (0xF0) |11110001 [8]                                  f1 [8]
       (0xF1) |11110010 [8]                                  f2 [8]
       (0xF2) |11110011 [8]                                  f3 [8]
       (0xF3) |11110100 [8]                                  f4 [8]
       (0xF4) |11110101 [8]                                  f5 [8]


   Response Table:

       (  0)  |11111111|11111111|11111111|0 [25]        1fffffe [25]
       (  1)  |11111111|11111111|11111111|1 [25]        1ffffff [25]
       (  2)  |11111111|11111111|11100000 [24]           ffffe0 [24]
       (  3)  |11111111|11111111|11100001 [24]           ffffe1 [24]
       (  4)  |11111111|11111111|11100010 [24]           ffffe2 [24]
       (  5)  |11111111|11111111|11100011 [24]           ffffe3 [24]
       (  6)  |11111111|11111111|11100100 [24]           ffffe4 [24]
       (  7)  |11111111|11111111|11100101 [24]           ffffe5 [24]
       (  8)  |11111111|11111111|11100110 [24]           ffffe6 [24]
       (  9)  |11111111|11111111|11100111 [24]           ffffe7 [24]
       ( 10)  |11111111|11111111|11101000 [24]           ffffe8 [24]
       ( 11)  |11111111|11111111|11101001 [24]           ffffe9 [24]
       ( 12)  |11111111|11111111|11101010 [24]           ffffea [24]
       ( 13)  |11111111|11111111|11101011 [24]           ffffeb [24]
       ( 14)  |11111111|11111111|11101100 [24]           ffffec [24]
       ( 15)  |11111111|11111111|11101101 [24]           ffffed [24]
       ( 16)  |11111111|11111111|11101110 [24]           ffffee [24]
       ( 17)  |11111111|11111111|11101111 [24]           ffffef [24]
       ( 18)  |11111111|11111111|11110000 [24]           fffff0 [24]
       ( 19)  |11111111|11111111|11110001 [24]           fffff1 [24]
       ( 20)  |11111111|11111111|11110010 [24]           fffff2 [24]
       ( 21)  |11111111|11111111|11110011 [24]           fffff3 [24]
       ( 22)  |11111111|11111111|11110100 [24]           fffff4 [24]
       ( 23)  |11111111|11111111|11110101 [24]           fffff5 [24]
       ( 24)  |11111111|11111111|11110110 [24]           fffff6 [24]
       ( 25)  |11111111|11111111|11110111 [24]           fffff7 [24]
       ( 26)  |11111111|11111111|11111000 [24]           fffff8 [24]
       ( 27)  |11111111|11111111|11111001 [24]           fffff9 [24]
       ( 28)  |11111111|11111111|11111010 [24]           fffffa [24]
       ( 29)  |11111111|11111111|11111011 [24]           fffffb [24]
       ( 30)  |11111111|11111111|11111100 [24]           fffffc [24]
       ( 31)  |11111111|11111111|11111101 [24]           fffffd [24]
   ' ' ( 32)  |11111111|0110 [12]                           ff6 [12]
   '!' ( 33)  |11111111|0111 [12]                           ff7 [12]
   '"' ( 34)  |11111111|111010 [14]                        3ffa [14]
   '#' ( 35)  |11111111|1111100 [15]                       7ffc [15]
   '$' ( 36)  |11111111|1111101 [15]                       7ffd [15]
   '%' ( 37)  |011000 [6]                                    18 [6]
   '&' ( 38)  |1010100 [7]                                   54 [7]



Snell                  Expires November 21, 2013               [Page 17]


Internet-Draft           Stored Header Encoding                 May 2013


   ''' ( 39)  |11111111|1111110 [15]                       7ffe [15]
   '(' ( 40)  |11111111|1000 [12]                           ff8 [12]
   ')' ( 41)  |11111111|1001 [12]                           ff9 [12]
   '*' ( 42)  |11111111|1010 [12]                           ffa [12]
   '+' ( 43)  |11111111|1011 [12]                           ffb [12]
   ',' ( 44)  |11111011|10 [10]                             3ee [10]
   '-' ( 45)  |011001 [6]                                    19 [6]
   '.' ( 46)  |00010 [5]                                      2 [5]
   '/' ( 47)  |00011 [5]                                      3 [5]
   '0' ( 48)  |011010 [6]                                    1a [6]
   '1' ( 49)  |011011 [6]                                    1b [6]
   '2' ( 50)  |011100 [6]                                    1c [6]
   '3' ( 51)  |011101 [6]                                    1d [6]
   '4' ( 52)  |1010101 [7]                                   55 [7]
   '5' ( 53)  |1010110 [7]                                   56 [7]
   '6' ( 54)  |1010111 [7]                                   57 [7]
   '7' ( 55)  |1011000 [7]                                   58 [7]
   '8' ( 56)  |1011001 [7]                                   59 [7]
   '9' ( 57)  |1011010 [7]                                   5a [7]
   ':' ( 58)  |011110 [6]                                    1e [6]
   ';' ( 59)  |11111011|11 [10]                             3ef [10]
   '<' ( 60)  |11111111|11111111|10 [18]                  3fffe [18]
   '=' ( 61)  |011111 [6]                                    1f [6]
   '>' ( 62)  |11111111|11111110|0 [17]                   1fffc [17]
   '?' ( 63)  |11110110|0 [9]                               1ec [9]
   '@' ( 64)  |11111111|11100 [13]                         1ffc [13]
   'A' ( 65)  |10111010 [8]                                  ba [8]
   'B' ( 66)  |11110110|1 [9]                               1ed [9]
   'C' ( 67)  |10111011 [8]                                  bb [8]
   'D' ( 68)  |10111100 [8]                                  bc [8]
   'E' ( 69)  |11110111|0 [9]                               1ee [9]
   'F' ( 70)  |10111101 [8]                                  bd [8]
   'G' ( 71)  |11111100|00 [10]                             3f0 [10]
   'H' ( 72)  |11111100|01 [10]                             3f1 [10]
   'I' ( 73)  |11110111|1 [9]                               1ef [9]
   'J' ( 74)  |11111100|10 [10]                             3f2 [10]
   'K' ( 75)  |11111111|010 [11]                            7fa [11]
   'L' ( 76)  |11111100|11 [10]                             3f3 [10]
   'M' ( 77)  |11111000|0 [9]                               1f0 [9]
   'N' ( 78)  |11111101|00 [10]                             3f4 [10]
   'O' ( 79)  |11111101|01 [10]                             3f5 [10]
   'P' ( 80)  |11111000|1 [9]                               1f1 [9]
   'Q' ( 81)  |11111101|10 [10]                             3f6 [10]
   'R' ( 82)  |11111001|0 [9]                               1f2 [9]
   'S' ( 83)  |11111001|1 [9]                               1f3 [9]
   'T' ( 84)  |11111010|0 [9]                               1f4 [9]
   'U' ( 85)  |11111101|11 [10]                             3f7 [10]
   'V' ( 86)  |11111110|00 [10]                             3f8 [10]



Snell                  Expires November 21, 2013               [Page 18]


Internet-Draft           Stored Header Encoding                 May 2013


   'W' ( 87)  |11111110|01 [10]                             3f9 [10]
   'X' ( 88)  |11111110|10 [10]                             3fa [10]
   'Y' ( 89)  |11111110|11 [10]                             3fb [10]
   'Z' ( 90)  |11111111|00 [10]                             3fc [10]
   '[' ( 91)  |11111111|111011 [14]                        3ffb [14]
   '\' ( 92)  |11111111|11111111|11111110 [24]           fffffe [24]
   ']' ( 93)  |11111111|111100 [14]                        3ffc [14]
   '^' ( 94)  |11111111|111101 [14]                        3ffd [14]
   '_' ( 95)  |1011011 [7]                                   5b [7]
   '`' ( 96)  |11111111|11111111|110 [19]                 7fffe [19]
   'a' ( 97)  |00100 [5]                                      4 [5]
   'b' ( 98)  |1011100 [7]                                   5c [7]
   'c' ( 99)  |00101 [5]                                      5 [5]
   'd' (100)  |100000 [6]                                    20 [6]
   'e' (101)  |0000 [4]                                       0 [4]
   'f' (102)  |100001 [6]                                    21 [6]
   'g' (103)  |100010 [6]                                    22 [6]
   'h' (104)  |100011 [6]                                    23 [6]
   'i' (105)  |00110 [5]                                      6 [5]
   'j' (106)  |10111110 [8]                                  be [8]
   'k' (107)  |10111111 [8]                                  bf [8]
   'l' (108)  |100100 [6]                                    24 [6]
   'm' (109)  |100101 [6]                                    25 [6]
   'n' (110)  |100110 [6]                                    26 [6]
   'o' (111)  |00111 [5]                                      7 [5]
   'p' (112)  |01000 [5]                                      8 [5]
   'q' (113)  |11111010|1 [9]                               1f5 [9]
   'r' (114)  |01001 [5]                                      9 [5]
   's' (115)  |01010 [5]                                      a [5]
   't' (116)  |01011 [5]                                      b [5]
   'u' (117)  |100111 [6]                                    27 [6]
   'v' (118)  |11000000 [8]                                  c0 [8]
   'w' (119)  |101000 [6]                                    28 [6]
   'x' (120)  |11000001 [8]                                  c1 [8]
   'y' (121)  |11000010 [8]                                  c2 [8]
   'z' (122)  |11111011|0 [9]                               1f6 [9]
   '{' (123)  |11111111|11111110|1 [17]                   1fffd [17]
   '|' (124)  |11111111|1100 [12]                           ffc [12]
   '}' (125)  |11111111|11111111|0 [17]                   1fffe [17]
   '~' (126)  |11111111|1101 [12]                           ffd [12]
       (127)  |101001 [6]                                    29 [6]
       (0xC2) |11000011 [8]                                  c3 [8]
       (0xC3) |11000100 [8]                                  c4 [8]
       (0xC4) |11000101 [8]                                  c5 [8]
       (0xC5) |11000110 [8]                                  c6 [8]
       (0xC6) |11000111 [8]                                  c7 [8]
       (0xC7) |11001000 [8]                                  c8 [8]
       (0xC8) |11001001 [8]                                  c9 [8]



Snell                  Expires November 21, 2013               [Page 19]


Internet-Draft           Stored Header Encoding                 May 2013


       (0xC9) |11001010 [8]                                  ca [8]
       (0xCA) |11001011 [8]                                  cb [8]
       (0xCB) |11001100 [8]                                  cc [8]
       (0xCC) |11001101 [8]                                  cd [8]
       (0xCD) |11001110 [8]                                  ce [8]
       (0xCE) |11001111 [8]                                  cf [8]
       (0xCF) |11010000 [8]                                  d0 [8]
       (0xD0) |11010001 [8]                                  d1 [8]
       (0xD1) |11010010 [8]                                  d2 [8]
       (0xD2) |11010011 [8]                                  d3 [8]
       (0xD3) |11010100 [8]                                  d4 [8]
       (0xD4) |11010101 [8]                                  d5 [8]
       (0xD5) |11010110 [8]                                  d6 [8]
       (0xD6) |11010111 [8]                                  d7 [8]
       (0xD7) |11011000 [8]                                  d8 [8]
       (0xD8) |11011001 [8]                                  d9 [8]
       (0xD9) |11011010 [8]                                  da [8]
       (0xDA) |11011011 [8]                                  db [8]
       (0xDB) |11011100 [8]                                  dc [8]
       (0xDC) |11011101 [8]                                  dd [8]
       (0xDD) |11011110 [8]                                  de [8]
       (0xDE) |11011111 [8]                                  df [8]
       (0xDF) |11100000 [8]                                  e0 [8]
       (0xE0) |11100001 [8]                                  e1 [8]
       (0xE1) |11100010 [8]                                  e2 [8]
       (0xE2) |11100011 [8]                                  e3 [8]
       (0xE3) |11100100 [8]                                  e4 [8]
       (0xE4) |11100101 [8]                                  e5 [8]
       (0xE5) |11100110 [8]                                  e6 [8]
       (0xE6) |11100111 [8]                                  e7 [8]
       (0xE7) |11101000 [8]                                  e8 [8]
       (0xE8) |11101001 [8]                                  e9 [8]
       (0xE9) |11101010 [8]                                  ea [8]
       (0xEA) |11101011 [8]                                  eb [8]
       (0xEB) |11101100 [8]                                  ec [8]
       (0xEC) |11101101 [8]                                  ed [8]
       (0xED) |11101110 [8]                                  ee [8]
       (0xEE) |11101111 [8]                                  ef [8]
       (0xEF) |11110000 [8]                                  f0 [8]
       (0xF0) |11110001 [8]                                  f1 [8]
       (0xF1) |11110010 [8]                                  f2 [8]
       (0xF2) |11110011 [8]                                  f3 [8]
       (0xF3) |11110100 [8]                                  f4 [8]
       (0xF4) |11110101 [8]                                  f5 [8]


Appendix B.  Static Storage Cache




Snell                  Expires November 21, 2013               [Page 20]


Internet-Draft           Stored Header Encoding                 May 2013


   0x80 "date"                        = NIL
   0x81 ":scheme"                     = "https"
   0x82 ":scheme"                     = "http"
   0x83 ":scheme"                     = "ftp"
   0x84 ":method"                     = "get"
   0x85 ":method"                     = "post"
   0x86 ":method"                     = "put"
   0x87 ":method"                     = "delete"
   0x88 ":method"                     = "options"
   0x89 ":method"                     = "patch"
   0x8A ":method"                     = "connect"
   0x8B ":path"                       = "/"
   0x8C ":host"                       = NIL
   0x8D "cookie"                      = NIL
   0x8E ":status"                     = 100
   0x8F ":status"                     = 101
   0x90 ":status"                     = 102
   0x91 ":status"                     = 200
   0x92 ":status"                     = 201
   0x93 ":status"                     = 202
   0x94 ":status"                     = 203
   0x95 ":status"                     = 204
   0x96 ":status"                     = 205
   0x97 ":status"                     = 206
   0x98 ":status"                     = 207
   0x99 ":status"                     = 208
   0x9A ":status"                     = 300
   0x9B ":status"                     = 301
   0x9C ":status"                     = 302
   0x9D ":status"                     = 303
   0x9E ":status"                     = 304
   0x9F ":status"                     = 305
   0xA0 ":status"                     = 307
   0xA1 ":status"                     = 308
   0xA2 ":status"                     = 400
   0xA3 ":status"                     = 401
   0xA4 ":status"                     = 402
   0xA5 ":status"                     = 403
   0xA6 ":status"                     = 404
   0xA7 ":status"                     = 405
   0xA8 ":status"                     = 406
   0xA9 ":status"                     = 407
   0xAA ":status"                     = 408
   0xAB ":status"                     = 409
   0xAC ":status"                     = 410
   0xAD ":status"                     = 411
   0xAE ":status"                     = 412
   0xAF ":status"                     = 413



Snell                  Expires November 21, 2013               [Page 21]


Internet-Draft           Stored Header Encoding                 May 2013


   0xB0 ":status"                     = 414
   0xB1 ":status"                     = 415
   0xB2 ":status"                     = 416
   0xB3 ":status"                     = 417
   0xB4 ":status"                     = 500
   0xB5 ":status"                     = 501
   0xB6 ":status"                     = 502
   0xB7 ":status"                     = 503
   0xB8 ":status"                     = 504
   0xB9 ":status"                     = 505
   0xBA ":status-text"                = "OK"
   0xBB ":version"                    = "1.1"
   0xBC "accept"                      = NIL
   0xBD "accept-charset"              = NIL
   0xBE "accept-encoding"             = NIL
   0xBF "accept-language"             = NIL
   0xC0 "accept-ranges"               = NIL
   0xC1 "allow"                       = NIL
   0xC2 "authorization"               = NIL
   0xC3 "cache-control"               = NIL
   0xC4 "content-base"                = NIL
   0xC5 "content-encoding"            = NIL
   0xC6 "content-length"              = NIL
   0xC7 "content-location"            = NIL
   0xC8 "content-md5"                 = NIL
   0xC9 "content-range"               = NIL
   0xCA "content-type"                = NIL
   0xCB "content-disposition"         = NIL
   0xCC "content-language"            = NIL
   0xCD "etag"                        = NIL
   0xCE "expect"                      = NIL
   0xCF "expires"                     = NIL
   0xD0 "from"                        = NIL
   0xD1 "if-match"                    = NIL
   0xD2 "if-modified-since"           = NIL
   0xD3 "if-none-match"               = NIL
   0xD4 "if-range"                    = NIL
   0xD5 "if-unmodified-since"         = NIL
   0xD6 "last-modified"               = NIL
   0xD7 "location"                    = NIL
   0xD8 "max-forwards"                = NIL
   0xD9 "origin"                      = NIL
   0xDA "pragma"                      = NIL
   0xDB "proxy-authenticate"          = NIL
   0xDC "proxy-authorization"         = NIL
   0xDD "range"                       = NIL
   0xDE "referer"                     = NIL
   0xDF "retry-after"                 = NIL



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   0xE0 "server"                      = NIL
   0xE1 "set-cookie"                  = NIL
   0xE2 "status"                      = NIL
   0xE3 "te"                          = NIL
   0xE4 "trailer"                     = NIL
   0xE5 "transfer-encoding"           = NIL
   0xE6 "upgrade"                     = NIL
   0xE7 "user-agent"                  = NIL
   0xE8 "vary"                        = NIL
   0xE9 "via"                         = NIL
   0xEA "warning"                     = NIL
   0xEB "www-authenticate"            = NIL
   0xEC "access-control-allow-origin" = NIL
   0xED "get-dictionary"              = NIL
   0xEE "p3p"                         = NIL
   0xEF "link"                        = NIL
   0xF0 "prefer"                      = NIL
   0xF1 "preference-applied"          = NIL
   0xF2 "accept-patch"                = NIL
   0xF3 NIL
   0xF4 NIL
   0xF5 NIL
   0xF6 NIL
   0xF7 NIL
   0xF8 NIL
   0xF9 NIL
   0xFA NIL
   0xFB NIL
   0xFC NIL
   0xFD NIL
   0xFE NIL
   0xFF NIL


Appendix C.  Updated Standard Header Definitions

   In order 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, then, will continue to
      be represented as ISO-8859-1 Text unless their standard
      definitions are updated.  The HTTP/2 specification would update
      the definition of specific known header fields (e.g.  content-
      length, date, if-modified-since, etc).





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   o  Extension header fields that use the typed codecs will have
      specific normative transformations to ISO-8859-1 defined.

      *  UTF-8 Text 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.

      *  Binary fields will be Base64-encoded.

   o  There will be no normative transformation from ISO-8859-1 values
      into the typed codecs.  Implementations are free to apply
      transformation where those impls determine it is appropriate, but
      it will be perfectly legal for an implementation to pass a text
      value through even if it is known that a given header type has a
      typed codec equivalent (for instance, Content-Length may come
      through as a number or a text value, either will be valid).  This
      means that when translating from HTTP/1 -> HTTP/2, receiving
      implementations need to be prepared to handle either value form.

   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).               |



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   | 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).               |
   | 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).               |
   +-----------------------+--------------+----------------------------+


Appendix D.  State Management Alternatives

   In the current design, dynamic cache storage slots are assigned by
   the decompressor in "encounter order".  While this is effective, it
   requires that all compressor and decompressor implementations utilize
   the exact same algorithm for assigning storage slots, precluding any
   implementation from experimenting with more efficient algorithms.  An
   alternative approach is to allow the compressor to assign the slots
   and communicate the assigned slots with each header.  This would
   require a single additional byte per header instance in the Literal
   and Cloned Header Groups.




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   The serialization syntax for Cloned and Literal Header Groups would
   change to:

     ; Cloned-Index Header Group
     cloned-index-header-group  = cloned-index-header-prefix
                                  1*32(cache-index cache-index value)

     ; Literal Header Group
     literal-header-group       = literal-header-prefix
                                  1*32(cache-index name value)


   The decompressor would used the specified storage index locations to
   store header values rather than relying on the encounter order,
   increasing the general efficiency of the algorithm with a minimal
   impact on transmission size.

   On the downside, this puts the compressor in greater control over the
   decompressor-maintained state because the decompressor would not know
   how long it needs to hold on to stored items.  This opens the
   decompressor up to possible abuse from malicious compressors.  The
   current least-recently-written cache approach allows the decompressor
   to reliably drop items as space fills without fear of falling out of
   sync with the compressor.

   A second alternative is to utilize separate stored "header groups"
   the way the current Delta Compression proposal does.  Each of these
   represents a separate storage bucket where headers are stored
   persistently by the compressor and decompressor.  The advantage of
   such buckets is that it simplifies and optimizes serialization of
   frames by the compressor.  However, it incurs the cost of a
   potentially significant increase in managed state on the decompressor
   (rather than a single bucket of stored items with max capacity N,
   there are multiple buckets of stored items with max capacity N/number
   of buckets).

Appendix E.  Alternative Timestamp encodings

   This specification currently uses the number of milliseconds from the
   UNIX Epoch to represent timestamps.  This 64-bit number is encoded
   using the same uvarint encoding as Numeric fields.  This means that
   the timestamp is encoded using a variable width that, right now (for
   about the next 100 years or so), will encode in six bytes, then seven
   bytes for the reasonable future.

   One possible alternative approach we can take is similar to NTP's
   handling of Era's.  We can take the current timestamp and generate an
   Era value, with a maximum of 255 (0xFF).  This is used as a



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   multiplier for the timestamp value.  The two values are calculated
   using the following formula:

   m   = 4294967296000
   now = milliseconds since UNIX Epoch
   era = now / m
   ts  = now % m


   The allowable values for "era" would be capped at 255.  This value is
   encoded as a single byte, followed by a uvarint encoding of ts.  This
   ensures that the timestamp will never be encoded using more than
   7-bytes total, though it may be encoded in as few as two bytes on
   extremely rare occasions (specifically, immediately following each
   era rollover).

   Wire Syntax:

   era       = %x00-FF
   ts        = uvarint
   date-time = era ts


   To convert back to the Epoch, the formula is equally simple:

   now = era * m + ts


   The largest date we can encode using this format is
   "36812-02-20T00:36:15.999Z".  Dates prior to the epoch cannot be
   represented.

   The significant drawback with this approach is that current dates
   would encode in 7-bytes until the next era rollover, which will occur
   at approximately 2106-02-07T06:28:15.999Z (give or take a few leap
   seconds thrown in here and there).

   Note: The byte lengths assume we want millisecond precision.  If we
   opted to keep the second precision currently in HTTP/1, then this
   alternative encoding ensures that our timestamps never exceed six-
   bytes in length.

Appendix F.  Alternative uvarint encodings

   The uvarint encoding currently specified by this specification is
   certainly not the only possible option we can use.  I chose it simply
   because it is drop dead simple to implement.  There are quite a few
   other approaches we can take, each of which can be used as drop-in



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   replacements for the current approach.  Below are just a couple
   alternatives.  There are plenty others.  We just need to pick the one
   that we feel will work the best.

   It ought to be noted that while each of these schemes vary in details
   such as endianess, specific wire-format, etc, each will typically
   encode the same numbers using the same number of bytes with
   variations of only a single byte only in the edge cases.  Processing
   time for each is also equivalent when dealing with any number less or
   equal to 64-bits in length.  This means that the choice is largely a
   matter of style than substance.

F.1.  Option 1:

   With this option, leading bits are used to indicate the total number
   of bytes used to encode the number value, with no fixed upper limit.
   Values strictly less than 128 are encoded using a single byte.

   0 xxxxxxx
   10 xxxxxx OCTET
   110 xxxxx 2OCTET
   1110 xxxx 3OCTET
   11110 xxx 4OCTET
   111110 xx 5OCTET
   1111110 x 6OCTET
   11111110  7OCTET
   11111111  0xxxxxxx 7OCTET
   11111111  10xxxxxx 8OCTET
   ...


   The number of leading 1 bits specify the number of additional bytes
   used to serialize the value.  A single 0 bit is used to mark the end
   of this prefix, the remaining bits are used to encode the minimum
   bits necessary to encode the value (with appropriate leading 0 bits
   to ensure proper byte-alignment).

   For instance, the integer value 500, which is represented in binary
   as 00000001 11110100, can be encoded using two bytes, 10000001
   11110100

   The integer value 9770098, which is represented in binary as 10010101
   00010100 01110010, can be encoded using four bytes: 11100000 10010101
   00010100 01110010

F.2.  Option 2:





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   This option is generally identical to the previous with the exception
   of being capped at a maximum of nine encoded octets total.  Rather
   than growing indefinitely, the encoded value must never require more
   then eight continuation bytes to encode.  Because of this
   restriction, there is no need for a trailing 0-bit spilling over past
   the first leading byte.

   0 xxxxxxx
   10 xxxxxx OCTET
   110 xxxxx 2OCTET
   1110 xxxx 3OCTET
   11110 xxx 4OCTET
   111110 xx 5OCTET
   1111110 x 6OCTET
   11111110  7OCTET
   11111111  8OCTET
   ...


   The number of leading 1 bits specify the number of additional bytes
   used to serialize the value.  If the number of bytes required is less
   than 8, a single 0 bit is used to mark the end of this prefix, the
   remaining bits are used to encode the minimum bits necessary to
   encode the value (with appropriate leading 0 bits to ensure proper
   byte-alignment).

   For instance, the integer value 500, which is represented in binary
   as 00000001 11110100, can be encoded using two bytes, 10000001
   11110100

   The integer value 9770098, which is represented in binary as 10010101
   00010100 01110010, can be encoded using four bytes: 11100000 10010101
   00010100 01110010

   This format is not capable of encoding any number requiring more than
   64-bits.

Author's Address

   James M Snell

   Email: jasnell@gmail.com








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