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Versions: 00 01 02 03

HTTPbis Working Group                                            R. Peon
Internet-Draft                                               Google, Inc
Intended status: Informational                              Mar 18, 2013
Expires: September 19, 2013


                 Header Delta-Compression for HTTP/2.0
               draft-rpeon-httpbis-header-compression-03

Abstract

   This document describes a mechanism for compressing streams of groups
   of key-value pairs, often known as Headers in an HTTP session.  See
   RFC 2616 [RFC2616] or successors for more information about headers.

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 September 19, 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.  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.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  How it works . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Header pre-processing  . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Mapping the first-line . . . . . . . . . . . . . . . . . .  4
     4.2.  Mapping HTTP key-values  . . . . . . . . . . . . . . . . .  4
   5.  Compressor and Decompressor State  . . . . . . . . . . . . . .  5
   6.  Header Block Wire Format . . . . . . . . . . . . . . . . . . .  6
   7.  String Encoding  . . . . . . . . . . . . . . . . . . . . . . .  8
   8.  Operations . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   9.  Decompressor algorithm . . . . . . . . . . . . . . . . . . . .  9
   10. Compression  . . . . . . . . . . . . . . . . . . . . . . . . . 12
   11. Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     11.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 15
     11.2. Example Serialization  . . . . . . . . . . . . . . . . . . 16
   12. Unfinished components  . . . . . . . . . . . . . . . . . . . . 25
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   14. Requirements Notation  . . . . . . . . . . . . . . . . . . . . 25
   15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
   16. Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . 25
   17. Appendix B . . . . . . . . . . . . . . . . . . . . . . . . . . 27
   18. Appendix C . . . . . . . . . . . . . . . . . . . . . . . . . . 32
   19. Normative References . . . . . . . . . . . . . . . . . . . . . 38
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 38

























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1.  Overview

   There have been several problems pointed out with the use of the gzip
   compressor in SPDY [SPDY].  The biggest of these problems is that it
   is possible for a smart attacker to inject content into the
   compressor, and then to test hypotheses about the prior contents of
   the compressor by examining the output size after each such content
   injection.  The other issue is that gzip often consumes more CPU than
   many would like, especially in situations where one is doing forward
   or reverse proxying.  The compressor proposed here intends to solve
   the first issue and significantly mitigate the second, while
   providing compression that is not too much worse than gzip.


2.  How it works

   The 'delta' compressor works by examining the difference between what
   it is told to compress and the state that it has stored about what it
   knows about the past.  The previous state is encoded in two separate
   pieces: An LRU of key-value pairs which the compressor 'saw' in the
   past (including a static group of key-value pairs which every
   compressor is assumed to have seen), and a set of references into the
   LRU which is called a header-group, which the compressor uses to
   determine what has changed between the current input and the past
   input.
   It then encodes this difference by changing the header-group by
   adding references to stored key-values, and it removes references to
   key-values which should no longer be part of the output.  If a key-
   value exists in the to-be-compressed data, but is not present in the
   LRU, then the LRU is modified by having new data added.  When new
   data is added, a reference to that new data is added to the header-
   group.  The mechanism of adding new data takes two forms: Adding an
   entire new key-value, or by referring to the key part of a stored
   key-value, and providing a new value.
   When the LRU has reached its size limit, The oldest elements are
   popped off the end, and, any reference to that element is removed.
   All keys are assumed to have been lowercased, and if not, will be.


3.  Definitions

   user-agent:  The program or device which a human interacts with
         directly and which typically initiates the transport layer
         connection or session







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   client:  Synonym for user-agent

   server:  The computer or device which typically accepts a connection,
         stores, and serves data

   proxy:  An entity acting as a server for the client, and a client for
         the server

   header:  A complete set of key-value pairs, either request-headers,
         or response-headers as defined in RFC2616 [RFC2616] section 5.3
         or 6.2, respectively


4.  Header pre-processing

4.1.  Mapping the first-line

   Before the data is input into the compressor (which works only on
   key-value pairs), the first line of the HTTP message must be made
   into key-value pairs.

   Requests are mapped as follows:
   "METHOD PATH VERSION" becomes:

                 [
                   (":method",  "METHOD"),
                   (":path",    "PATH"),
                   (":version", "VERSION")
                 ]

   Responses are mapped as follows:
   "VERSION STATUS-CODE PHRASE" becomes:

                 [
                   (":version",     "VERSION"),
                   (":status",      "STATUS-CODE"),
                   (":status-text", "PHRASE")
                 ]

4.2.  Mapping HTTP key-values

   The rest of the HTTP key-values are simply added to the key-values as
   mapped from the first-line, with the keys made to be all lowercase,
   and with cookies split into crumbs by breaking apart the cookie
   string on semicolons and treating each as if it were a separate
   header-line.

   As an example, the following key-value pairs:



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                 Host: www.foo.com
                 User-Agent: Browser/1.x (FooOS; Bar) baz
                 Accept-Language: en-US,en;q=0.5
                 Cookie: foo;bar; baz

   become:

                 [
                   ("host", "www.foo.com")
                   ("user-agent", "Browser/1.x (FooOS; Bar) baz"),
                   ("accept-language", "en-US,en;q=0.5"),
                   ("cookie", "foo"),
                   ("cookie", "bar"),
                   ("cookie", "baz")
                 ]


5.  Compressor and Decompressor State

   The header delta de/compression scheme consists of a state machine
   which executes opcodes, emits output, and modifies internal,
   persistent, state.  The de/compressor state consists of:

   static_entries:
         a number of static (unchanging) entries consisting of key-value
         pairs.  These are listed in appendix A.
         e.g. static_entries=[ ("key1", "val1"), ("key2", "val2), ...]
         an lru-idx references into the static key-value pairs if the
         value of the lru-idx is < len(static-entries)
         static_entries[lru_idx]

   lru:
         a queue of key-value pairs
         e.g. lru = deque([(RefCntString("key1"), "val1"),
         (RefCntstring("key2", "val2")), ...])
         an lru-idx references a value in the lru if the lru-idx is >=
         len(static_entries).  The mapping of an lru-idx to a offset
         from the front of the LRU is as follows:

              if lru.first_idx > lru_idx:
                queue_idx = 2**16 - lru.first_idx + lru_idx - len(static_entries)
              else:
                queue_idx = lru_idx - lru.first_idx

         The oldest elements of the LRU are popped before inserting a
         new value if either:





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               Adding a new entry would exceed the maximum allowable
               length

               Adding a new entry would exceed the maximum allowable
               byte length

   header_groups: (default-size: 1)(max: 255)
         a map of group-id to set of lru-idx.  An lru-idx is a reference
         into either the static key-value pairs or the lru's key-value
         pairs.  The maximum number of header-groups is limited by
         default to 1, unless a higher-level of the protocol changes
         this.  The maximum number of header_groups is 255.  It is not
         currently allowed to assert that there are '0' allowed header
         groups.
         e.g. header_groups = {0: set([1,4,6,15122]), 1: set([6,76,3],
         ...)}

   lru.first_idx:
         an int indicating the lru-idx of the oldest element in the
         queue of key-value pairs

   max_byte_size: (default: 4k) (max:2**32-1)
         an int indicating the maximum allowable amount of storage used
         by the strings of the queue's key-value pairs.  Unless a
         higher-level of the protocol changes this, this is assumed to
         be 4k

   max_lru_entries: (default: 1024) (max:2**16-1)
         an int indicating the maximum allowable number of key-value
         pairs in the queue.  Unless a higher-level of the protocol
         changes this, this is assumed to be 1024

   lru.length:
         an int indicating the total number of key-value pairs currently
         stored in the lru

   lru.stored_byte_size:
         an int indicating the total number of bytes of storage used by
         strings in the queue.  Note that the bytes in a ref-counted
         string are counted only once, regardless of how many times that
         string is referenced.


6.  Header Block Wire Format

   The decompressor is fed a header-block which may span multiple
   HEADERs frames by the HTTP/2 framing layer, the format of which
   follows:



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         All ints are in network byte order.

         header-block: group-id
           ( (ekvsto-opcode ekvsto-count ekvsto-field{ekvsto-count})* |
             (eclone-opcode eclone-count eclone-field{eclone-count})* |
             (etrang-opcode etrang-count etrang-field{etrang-count})* |
             (strang-opcode etrang-count strang-field{etrang-count})* |
             (etoggl-opcode etoggl-count etoggl-field{etoggl-count})* |
             (stoggl-opcode stoggl-count stoggl-field{stoggl-count})* )*
           (clone-opcode clone-count clone-field{clone-count})*
           (kvsto-opcode kvsto-count kvsto-field{kvsto-count})*
           ;

         group-id: UINT8;
         etoggl-count: UINT8;
         stoggl-count: UINT8;
         etrang-count: UINT8;
         strang-count: UINT8;
         eclone-count: UINT8;
         sclone-count: UINT8;
         ekvsto-count: UINT8;
         skvsto-count: UINT8;

         stoggl-field: lru-idx;
         etoggl-field: lru-idx;
         strang-field: lru-idx lru-idx;
         etrang-field: lru-idx lru-idx;
         eclone-field: lru-idx string;
         sclone-field: lru-idx string;
         ekvsto-field: string  string;
         skvsto-field: string  string;

         stoggl-opcode: UINT8(0x00);
         etoggl-opcode: UINT8(0x01);
         strang-opcode: UINT8(0x02);
         etrang-opcode: UINT8(0x03);
         skvsto-opcode: UINT8(0x04);
         ekvsto-opcode: UINT8(0x05);
         sclone-opcode: UINT8(0x06);
         eclone-opcode: UINT8(0x07);

         lru_idx: UINT16;

         string: (HUFFMAN-ENCODED-CHAR)* HUFFMAN-EOF
                 padding-to-nearest-byte-boundary;
         padding-to-nearest-byte-boundary: 0{0-7};





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7.  String Encoding

   Strings are huffman encoded [HUFF] using a canonical huffman coding
   [CANON].  In the future, the opcode byte will be permuted to allow
   alternate encodings, such as raw text, binary, or perhaps other
   options.

   The huffman code is constructed by taking the frequency-tables in
   Appendix B, adding 1 to all entries, then generating a canonical
   huffman coding.  If/while this results in a code with a max-length of
   greater than 32 bits, divide all frequencies by two, capping the
   minimum frequency at '1', and regenerate until the max code-length is
   32 bits or less.  The EOF symbol, when decoded, is represented as
   256, which allows for any 8-bit value to be encoded and decoded.


8.  Operations

   For all operations below, the 's' prefix stands for 'State
   modifying', whereas the 'e' prefix stands for 'Ephemeral', and does
   not modify state.

   The *kvsto family of opcodes encode a new key-value entirely by
   providing a new string for key and a new string for val.

   The *clone family of opcodes encode a backreference to the key part
   of a pre-existing key-value from either the static-entries or the
   lru, and a new string value.

   The *toggl family of opcodes encode a backreference to an entire key-
   value from either the static-entries, or the lru.

   The *trang family of opcodes is the same as the toggle family, except
   that it encodes a range of indices instead of a single index

   With four families of opcodes, and two variations (ephemeral vs
   state-changing) per family, we have eight valid opcodes:

   skvsto: (Stateful Key-Value STOre)
         state-modifying kvsto.  The new key and value are inserted into
         the headers and also inserted into the LRU.

   ekvsto: (Ephemeral Key-Value STOre)
         ephemeral, non-state-modifying kvsto.  The new key and value
         are inserted into the headers but the LRU is untouched.






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   sclone (Stateful key CLONE):
         state-modifying clone.  The key part of the referenced key-
         value is paired with the new value and inserted into the
         headers and also inserted into the LRU

   eclone (Ephemeral key CLONE):
         ephemeral, non-state-modifying clone.  The key part of the
         referenced key-value is paired with the new value and inserted
         into the headers.  No persistent state is modified.

   stoggl (Stateful TOGGLe):
         state-modifying toggle.  If the index exists in the current
         header group, it will be turned off, else it will be turned on.

   etoggl (Ephemeral TOGGLe):
         ephemeral, non-state-modifying toggle.  If the provided index
         does not exist in the current header group after all stoggles
         have modified it, then the key-value as referenced by the
         provided index will be present in the output, else, that index
         of the current header group will be temporarily suppressed and
         will not be included in the headers

   strang (Stateful Toggle RANGe):
         encodes a range of stoggls

   etrang (Ephemeral ToggleRANGe):
         encodes a range of etoggls


9.  Decompressor algorithm

   The pseudo-code below provides a definition of how the header-block
   is executed by the decompressor.

        ParseAndExecuteHeaderBlock(header_block):
          store_later = deque()
          etoggles = set()
          stoggles = set()
          headers = dict()
          # the HTTP/2 framing layer determines when the header_block
          # has finished reading.
          group_id  = header_block.read_uint8()
          current_header_group = header_groups[group_id]

          while data in header_block:
            opcode = header_block.read_uint8()
            num_fields = header_block.read_uint8()
            if opcode == stoggl:



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              repeat num_fields times:
                lru_idx = header_block.read_uint16()
                stoggles = set_symmetric_difference(stoggles, [lru_idx])
            elif opcode == etoggl:
              repeat num_fields times:
                lru_idx = header_block.read_uint16()
                etoggles = set_symmetric_difference(etoggles, [lru_idx])
            elif opcode == strang:
              repeat num_fields times:
                lru_idx_first = header_block.read_uint16()
                lru_idx_last = header_block.read_uint16()
                for lru_idx in (lru_idx_first, lru_idx_last) inclusive:
                  stoggles = set_symmetric_difference(stoggles, [lru_idx])
                  stoggles.add(lru_idx)
            elif opcode == etrang:
              repeat num_fields times:
                lru_idx_first = header_block.read_uint16()
                lru_idx_last = header_block.read_uint16()
                for lru_idx in (lru_idx_first, lru_idx_last) inclusive:
                  etoggles = set_symmetric_difference(etoggles, [lru_idx])
            elif opcode == sclone:
              repeat num_fields times:
                lru_idx = header_block.read_uint16()
                val = header_block.read_huffman_string()
                kv = lookup_idx_from_static_entries_or_lru(lru_idx)
                AddToCurrentHeaders(headers, kv.key, val)
                store_later.append(KV(kv.key, val))
            elif opcode == eclone:
              repeat num_fields times:
                lru_idx = header_block.read_uint16()
                val = header_block.read_huffman_string()
                kv = lookup_idx_from_static_entries_or_lru(lru_idx)
                AddToCurrentHeaders(headers, kv.key, val)
            elif opcode == skvsto:
              repeat num_fields times:
                key = header_block.read_huffman_string()
                val = header_block.read_huffman_string()
                AddToCurrentHeaders(headers, key, val)
                store_later.append(KV(key, val))
            elif opcode == ekvsto:
              repeat num_fields times:
                key = header_block.read_huffman_string()
                val = header_block.read_huffman_string()
                AddToCurrentHeaders(headers, key, val)

          # store the state changes to the header-group.
          current_header_group = \
            set_symmetric_difference(current_header_group, stoggles)



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          kv_references = set_symmetric_difference(current_header_group,
                                                   etoggles)

          for lru_idx in sorted(kv_references):
            kv = lookup_idx_from_static_entries_or_lru(lru_idx)
            AddToCurrentHeaders(headers, kv.key, kv.val)

          if 'cookie' in headers:
            headers['cookie'] = headers['cookie'].replace('\0', '; ')

          older_headers = []
          for lru_idx in sorted(current_header_group):
            # sorting by idx is suboptimal when the idxs wrap 2**16.
            # As a refinement, we probably want to change this in the
            # future to something which sorts based on the order in
            # which the elements were first mentioned, which can be
            # done by a smart implementation without actually sorting.
            kv = lookup_idx_from_static_entries_or_lru(lru_idx)
            older_headers.append(kv)
          store_later = older_headers + store_later

          # make state changes to the LRU. Note that this may remove
          # items from the header-group if elements that the header-group
          # refers to are removed from the LRU
          for kv in store_later:
            new_lru_idx = lru.store(kv.key, kv.val)

          return headers

        lru.clear():
          while length > 0:
            pop_oldest()

        lru.store(key, val):
          reserve_size = val.size + key.size
          if max_lru_entries == 0 or
             max_byte_size < reserve_size):
             lru.clear()
             return -1
          while length + 1 >= max_lru_entries:
            pop_oldest()
          while true:
            reserve_size = val.size
            if key.refcnt == 1:
              reserve_size += key.size
            if stored_byte_size + reserve_size < max_byte_size:
              break
            pop_oldest()



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          push(KV(key, val))
          new_lru_idx = lru.first + length
          if new_lru_idx >= 2**16:
            new_lru_idx -= 2**16
            new_lru_idx += static_entries.size:
          return new_lru_idx

        lru.pop_oldest():
          kv = queue.front()
          length -= 1
          if kv.key.refcnt == 1:
            stored_byte_size -= kv.key.size
          stored_byte_size -= kv.val.size
          for header_group in header_groups:
            if first_idx in header_group:
              header_group.remove(first_idx)
          first_idx = get_next_idx(first_idx)
          queue.pop_front()

        lru.push(kv):
          length += 1
          if kv.key.refcnt == 1:
            stored_byte_size += kv.key.size
          stored_byte_size += kv.val.size
          queue.push_back(kv)

        lru.get_next_idx(idx):
          idx += 1
          if idx >= 2**16 - 1:
            return decompressor.static_entries.size
          return idx

        lookup_idx_from_static_entries_or_lru(lru_idx):
          if lru_idx < static_entries.size:
            return static_entries[lru_idx]:
          if lru.first_idx > lru_idx:
            queue_idx = 2**16 - lru.first_idx + lru_idx - static_entries.size
          else:
            queue_idx = lru_idx - lru.first_idx
          return lru.queue[queue_idx]




10.  Compression

   The compressor generates a sequence of instructions which the
   decompressor executes.  There are various ways by which the



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   compressor can determine how to construct these operations.  Pseudo-
   code follows showing one approach. group_id is defined by the sender,
   but must always be less than the maximum allowed number.  A
   reasonable implementation might assign the same group_id to a set of
   headers which are likely to be similar, for instance those which go
   to the same hostname or the same hostname suffix.

        # assumptions: headers is a dict(), where multiple key:values with
        # the same key are encoded as key:value1\0value2\0value3...
        # group_id is provided by some other implementation-dependent
        # code

        # This compressor does not use all of the opcodes and serves simply
        # as an example of a workable, if suboptimal, implementation
        MakeOperations(self, headers, group_id):
          headers_set = set()
          for (key, val) in headers:
            splittoken = '\0'
            if key == 'cookie':
              splittoken = ';'
            for partial_val in split(val, ';'):
              headers_set.add( (key, partial_val) )
              # Note that this discards duplicates.
              # If we decide we care about that generate an 'ekvsto' or
              # 'eclone' for that (duplicate) key-value here.

          keep_set = set()
          done_set = set()
          for idx in header_groups[group_id]:
            kv = lookup_idx_from_static_entries_or_lru(idx)
            if kv in headers_set:
              # If the KV referenced by the idx in the header-group
              # is also in the to-be-compressed headers, then we
              # keep using that reference (don't remove it from the
              # header-group)
              keep_set.add(idx)  # we want to keep this one
              headers_set.remove(kv)
            else:
              # If we're not finding the KV referenced by the idx in
              # the header-group in the to-be-compressed-header, then
              # this idx needs to be removed from the header-group.
              done_set.add(idx)  # we'll want to remove it

          instructions = dict()
          toggls = set()
          clones = []
          kvstos = []
          erefs = []



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          for (key, val) in headers_set:
            # The following 'if' block is a demonstration of an
            # optimization-- since path and referer are rarely
            # backreferenced, and since they are often large and
            # they would, if included in the LRU, cause other entries
            # to be expired from the LRU, we ensure that these don't
            # get stored in the LRU by emitting an 'ephemeral' operation
            if key in [":path", "referer"]:
              instructions['ekvsto'].append( (key, val) )
              continue
            # FindEntryIdx looks for a matching key-value in the LRU, and then
            # in the static-entries, recording the first matching key it finds
            # while searching for the whole match. If it does find a whole
            # match then v_idx will be valid. If it finds an entry with a key
            # which matches, then k_idx will be valid.
            (k_idx, v_idx) = FindEntryIdx(key, val)
            if both k_idx and v_idx are valid:
              # if we found a index for all of the kv, we'll generate
              # a new toggle which backreferences that entire kv.
              toggls.add(v_idx)
            elif only k_idx is valid:
              # Otherwise, if we did't find all of the kv pre-existing,
              # but there was something that already had that key,
              # generate a clone, which backreferences the key and provides
              # a new value.
              instructions['sclone'].append( (k_idx, val) )
            else:
              # Otherwise, we'll need to store a new key and value, both.
              instructions['skvstos'].append( (key, val) )

          full_toggl_list = union(toggls, done_set)
          # convert runs of toggls into trangs
          (trangs, toggls) = ComputeTrangsFromRawToggles(full_toggl_list)
          instructions['stoggl'] = toggls
          instructions['strang'] = trangs

          header_block = SerializeInstructions(instructions, group_id)

          # Execute the instructions just like you would when decompressing.
          # We're throwing away the computed headers here, because all
          # we care about is the side-effects to the header_groups and the
          # lru from executing the generated instructions.
          ParseAndExecuteHeaderBlock(header_block)
        return header_block


       SerializeInstructions(instructions, group_id):
         outbuf.write_uint8(group_id)



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         for opcode in ['stoggl', 'etoggl',
                        'strang', 'etrang',
                        'eclone', 'ekvsto',
                        'sclone', 'skvsto']:
           if not opcode in instructions:
             continue
           ops_idx = 0
           ops_len = len(instructions[opcode])
           while ops_len > ops_idx:
             ops_to_go = ops_len - ops_idx
             outbuf.write_uint8(OpcodeToOpcodeVal(opcode))
             # a value of '0' in this field means '1'.
             # a value of '255' in this field means '256',
             # thus, subtract one from the actual value when
             # preparing to write to the wire.
             outbuf.write_uint8(min(256, ops_to_go) - 1)
             orig_idx = ops_idx
             for i in xrange(ops_to_go):
               if opcode in ['stoggl', 'etoggl']:
                 outbuf.write_uint16(instructions[ops_idx])
               elif opcode in ['strang', 'etrang']:
                 outbuf.write_uint16(instructions[ops_idx][0])
                 outbuf.write_uint16(instructions[ops_idx][1])
               elif opcide in ['sclone', 'eclone']:
                 outbuf.write_uint16(instructions[ops_idx][0])
                 outbuf.encode_and_write_string(instructions[ops_idx][1])
               elif opcode in ['skvsto', 'ekvsto']:
                 outbuf.encode_and_write_string(instructions[ops_idx][0])
                 outbuf.encode_and_write_string(instructions[ops_idx][1])
               ops_idx += 1
         return outbuf


   If the resulting output buffer is larger than the maximum allowed
   frame size, then the buffer shall be split into maximum-allowed-
   payload-size or smaller sections, and sent in separate HEADERS
   frames, with only the last indicating that the frame is finished by
   asserting the FRAME_FINISHED flag.


11.  Example

11.1.  Background

   Here is a simple example showing an input, the changing part of the
   compressor state, and an ascii-ified version of what would be
   serialized on the wire.




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               GET / HTTP/1.0
               Host: www.foo.com
               User-Agent: bar-ua baz stuff
               Accept-Language: en-US,en;q=0.5

   The first stage of processing is to break the first line into key-
   value pairs.  The first line becomes:

               [
               (":method": "GET"),
               (":path": "/"),
               (":version": "HTTP/1.0")
               ]

   This gets integrated with the rest of the headers, becoming:

                 [
                 (":method", "GET"),
                 (":path", "/"),
                 (":version", "HTTP/1.0"),
                 ("host", "www.foo.com"),
                 ("user-agent", "bar-ua baz stuff"),
                 ("accept-language", "en-US,en;q=0.5"),
                 ]

   The compressor now goes through each key-value, determining if it is
   already present in the header-group, in the LRU or static state, and
   determines what it needs to emit.

11.2.  Example Serialization

   This is sample output from a program which implements the compression
   specification above.  It prints out the stream ID, then the group ID,
   then the instructions that the encoder created, then the serialized
   form of these instructions, followed last by the decompressed output.



* http2-demo: 2 req messages
################################################################################
# delta2 request 1 (of 2) for http2-demo

stream_id: 1234 group_id: 1
{'opcode': 'eclone', 'index':     0, 'val': '/http2_sample.html'}
{'opcode': 'stoggl', 'index':     1}
{'opcode': 'stoggl', 'index':     3}
{'opcode': 'sclone', 'index':    38, 'val': 'no-cache'}
{'opcode': 'sclone', 'index':    10, 'val': 'ISO-8859-1,utf-8;q=0.7,*;q=0.3'}



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{'opcode': 'sclone', 'index':    11, 'val': 'gzip,deflate,sdch'}
{'opcode': 'sclone', 'index':     9, 'val': 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8'}
{'opcode': 'sclone', 'index':    16, 'val': 'no-cache'}
{'opcode': 'sclone', 'index':     4, 'val': 'http2-demo'}
{'opcode': 'sclone', 'index':    12, 'val': 'en-US,en;q=0.8'}
{'opcode': 'sclone', 'index':    51, 'val': 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.28 Safari/537.31'}


00 FE 08 01 80 00 04 D2 01 05 00 00 00 0C C8 9E | ................
9B A2 AD 5F A0 9B 32 D7 49 00 00 01 00 01 00 03 | ..._..2.I.......
04 07 00 26 6B A7 5A 97 98 C8 00 0A F6 FD 3E 33 | ...&k.Z.......>3
F3 E7 4D 73 A3 F8 D8 B1 9F 9F A3 F5 69 CD 57 F1 | ..Ms........i.W.
FF 5E 8F D5 A7 35 12 00 0B CB F6 C7 FF 18 0E 3A | .^...5.........:
28 87 F8 8E 0B CE 40 00 09 21 F2 20 CC B5 D3 F8 | (.....@..!. ....
53 DF A3 16 A2 63 9A 1E 59 96 BA 7F DF 96 BA 7F | S....c..Y.......
0A 7B F4 62 D4 4C 73 43 CB 5D 3D 1F AB 4E 6A FF | .{.b.LsC.]=..Nj.
8F FA 0F FA F4 7E AD 39 B9 C8 00 10 6B A7 5A 97 | .....~.9....k.Z.
98 C8 00 04 CC 89 E9 9F 01 D5 D2 00 0C 16 CF F6 | ................
FA 7F 02 DF 47 EA D3 9B 9C 80 00 33 F7 BB F6 CD | ....G......3....
34 50 52 63 FF B7 FC 7E 50 8F 47 FB 7B 98 DD BC | 4PRc...~P.G.{...
BF DB CB 9F 2B B9 71 FF 9F F6 EC 7B F4 1F C0 DF | ....+.q....{....
FE 98 41 4D 15 1A 84 7F B7 FC 7F AF 67 D7 DF EE | ..AM........g...
FE 3F DB 46 78 0F FB 7A C5 7E 0E FF 9F F6 EE CE | .?.Fx..z.~......
0E D4 41 3C 8C 73 23 8A 0E 64 F3 FF 6F A2 B1 54 | ..A<.s#..d..o..T
18 14 D1 51 A8 44 80                            | ...Q.D.

###### decompressed ######
get /http2_sample.html HTTP/1.1
accept-language: en-US,en;q=0.8
accept-encoding: gzip,deflate,sdch
accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
user-agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.28 Safari/537.31
:scheme: http
accept-charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3
pragma: no-cache
cache-control: no-cache
host: http2-demo

################################################################################
# delta2 request 2 (of 2) for http2-demo

stream_id: 1234 group_id: 1
{'opcode': 'eclone', 'index':     0, 'val': '/s/http2_fractal.jpg'}
{'opcode': 'sclone', 'index':    42, 'val': 'http://http2-demo/http2_sample.html'}
{'opcode': 'sclone', 'index':    70, 'val': '*/*'}
{'opcode': 'strang', 'index':    69, 'index_start':    67}
{'opcode': 'strang', 'index':    74, 'index_start':    71}




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00 3E 08 01 80 00 04 D2 01 05 00 00 00 08 86 64 | .>.............d
4F 4D D8 C1 4B 25 68 6E AF CA 40 04 01 00 2A CC | OM..K%hn..@...*.
89 F6 00 66 44 F4 CF 80 EA E0 CC 89 E9 BA 2A D5 | ...fD.........*.
FA 09 B3 2D 74 90 00 46 FF A0 FF A9 00 02 01 00 | ...-t..F........
45 00 43 00 4A 00 47                            | E.C.J.G

###### decompressed ######
get /s/http2_fractal.jpg HTTP/1.1
accept-language: en-US,en;q=0.8
pragma: no-cache
accept: */*
user-agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.28 Safari/537.31
:scheme: http
accept-charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3
referer: http://http2-demo/http2_sample.html
cache-control: no-cache
host: http2-demo
accept-encoding: gzip,deflate,sdch

                  size  time | ratio min   max   std
  delta2           334  0.00 | 1.00  1.00  1.00  0.00

* http2-demo: 2 res messages
################################################################################
# delta2 response 1 (of 2) for http2-demo

stream_id: 1234 group_id: 1
{'opcode': 'stoggl', 'index':     6}
{'opcode': 'sclone', 'index':    16, 'val': 'public, max-age=3600000000'}
{'opcode': 'sclone', 'index':    27, 'val': 'Fri, 1 Jan 2100 12:00:00 GMT'}
{'opcode': 'sclone', 'index':    18, 'val': 'gzip'}
{'opcode': 'sclone', 'index':    13, 'val': 'bytes'}
{'opcode': 'sclone', 'index':    52, 'val': 'Accept-Encoding'}
{'opcode': 'sclone', 'index':    19, 'val': '797'}
{'opcode': 'sclone', 'index':    34, 'val': 'Thu, 08 Nov 2012 17:24:16 GMT'}
{'opcode': 'sclone', 'index':    24, 'val': 'Tue, 12 Mar 2013 23:12:44 GMT'}
{'opcode': 'sclone', 'index':    44, 'val': 'Apache/2.2.22 (Ubuntu)'}
{'opcode': 'sclone', 'index':    23, 'val': 'text/html'}


00 99 08 01 80 00 04 D2 01 00 00 00 06 04 09 00 | ................
10 C3 3D BC 6D AE 60 E5 0F 39 E1 BE 3A 91 40 88 | ..=.m.`..9..:.@.
88 88 8C 80 00 1B EC C6 D9 80 83 B6 17 01 90 88 | ................
11 B8 45 C2 21 4D 4F 90 00 12 DF FB B7 09 00 00 | ..E.!MO.........
0D DB E9 94 79 C8 00 34 D7 5D 71 C3 29 FA EE AE | ....y..4.]q.)...
FB 2D BB 7C 80 00 13 5B 57 20 00 22 7F 0C E6 01 | .-.|...[W ."....
60 77 5F E2 06 24 60 4B 71 A5 C5 40 53 53 E4 00 | `w_..$`Kq..@SS..
18 7E 71 98 08 C2 A8 62 06 24 80 34 38 8D C9 48 | .~q....b.$.48..H



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53 53 E4 00 2C D7 84 2B E1 1E 83 D2 7A 4C C3 D7 | SS..,..+....zL..
F5 DB 9D D9 67 EC 90 00 17 CA 3E 79 74 70 CB 97 | ....g.....>ytp..
19 00                                           | ..

###### decompressed ######
HTTP/1.1 200 ?
content-length: 797
content-encoding: gzip
accept-ranges: bytes
expires: Fri, 1 Jan 2100 12:00:00 GMT
vary: Accept-Encoding
server: Apache/2.2.22 (Ubuntu)
last-modified: Thu, 08 Nov 2012 17:24:16 GMT
cache-control: public, max-age=3600000000
date: Tue, 12 Mar 2013 23:12:44 GMT
content-type: text/html

################################################################################
# delta2 response 2 (of 2) for http2-demo

stream_id: 1234 group_id: 1
{'opcode': 'stoggl', 'index':    69}
{'opcode': 'sclone', 'index':    75, 'val': 'image/jpeg'}
{'opcode': 'sclone', 'index':    71, 'val': '365'}
{'opcode': 'sclone', 'index':    72, 'val': 'Tue, 23 Oct 2012 02:26:33 GMT'}
{'opcode': 'strang', 'index':    67, 'index_start':    66}
{'opcode': 'strang', 'index':    74, 'index_start':    73}


00 35 08 01 80 00 04 D2 01 00 00 00 45 04 02 00 | .5..........E...
4B B7 94 37 C7 A3 F1 84 77 C8 00 47 45 0A 90 00 | K..7....w..GE...
48 7E 71 98 0D 01 DF 5E 40 62 46 02 6E 3A 1C 84 | H~q....^@bF.n:..
05 35 3E 40 02 01 00 43 00 42 00 4A 00 49       | .5>@...C.B.J.I

###### decompressed ######
HTTP/1.1 200 ?
content-length: 365
accept-ranges: bytes
expires: Fri, 1 Jan 2100 12:00:00 GMT
server: Apache/2.2.22 (Ubuntu)
last-modified: Tue, 23 Oct 2012 02:26:33 GMT
cache-control: public, max-age=3600000000
date: Tue, 12 Mar 2013 23:12:44 GMT
content-type: image/jpeg

                  size  time | ratio min   max   std
  delta2           224  0.00 | 1.00  1.00  1.00  0.00




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* http2-demo1: 5 req messages
################################################################################
# delta2 request 1 (of 5) for http2-demo1

stream_id: 1234 group_id: 2
{'opcode': 'eclone', 'index':     0, 'val': '/s/0.png'}
{'opcode': 'sclone', 'index':    81, 'val': 'http2-demo1'}
{'opcode': 'strang', 'index':    80, 'index_start':    75}
{'opcode': 'strang', 'index':    85, 'index_start':    82}


00 20 08 01 80 00 04 D2 02 05 00 00 00 08 81 CC | . ..............
F6 E5 20 04 00 00 51 CC 89 E9 9F 01 D5 C8 90 02 | .. ...Q.........
01 00 50 00 4B 00 55 00 52                      | ..P.K.U.R

###### decompressed ######
get /s/0.png HTTP/1.1
accept-language: en-US,en;q=0.8
accept: */*
user-agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.28 Safari/537.31
:scheme: http
accept-charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3
referer: http://http2-demo/http2_sample.html
pragma: no-cache
cache-control: no-cache
host: http2-demo1
accept-encoding: gzip,deflate,sdch

################################################################################
# delta2 request 2 (of 5) for http2-demo1

stream_id: 1234 group_id: 2
{'opcode': 'eclone', 'index':     0, 'val': '/s/6.png'}
{'opcode': 'stoggl', 'index':    96}


00 0E 08 01 80 00 04 D2 02 05 00 00 00 08 87 23 | ...............#
3D B9 48 00 00 00 60                            | =.H...`

###### decompressed ######
get /s/6.png HTTP/1.1
accept-language: en-US,en;q=0.8
accept: */*
user-agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.28 Safari/537.31
:scheme: http
accept-charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3
referer: http://http2-demo/http2_sample.html
pragma: no-cache



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cache-control: no-cache
host: http2-demo1
accept-encoding: gzip,deflate,sdch

################################################################################
# delta2 request 3 (of 5) for http2-demo1

stream_id: 1234 group_id: 2
{'opcode': 'eclone', 'index':     0, 'val': '/s/12.png'}


00 0B 08 01 80 00 04 D2 02 05 00 00 00 08 82 12 | ................
67 B7 29 00                                     | g.).

###### decompressed ######
get /s/12.png HTTP/1.1
accept-language: en-US,en;q=0.8
accept: */*
user-agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.28 Safari/537.31
:scheme: http
accept-charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3
referer: http://http2-demo/http2_sample.html
pragma: no-cache
cache-control: no-cache
host: http2-demo1
accept-encoding: gzip,deflate,sdch

################################################################################
# delta2 request 4 (of 5) for http2-demo1

stream_id: 1234 group_id: 2
{'opcode': 'eclone', 'index':     0, 'val': '/s/18.png'}


00 0B 08 01 80 00 04 D2 02 05 00 00 00 08 82 39 | ...............9
99 ED CA 40                                     | ...@

###### decompressed ######
get /s/18.png HTTP/1.1
accept-language: en-US,en;q=0.8
accept: */*
user-agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.28 Safari/537.31
:scheme: http
accept-charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3
referer: http://http2-demo/http2_sample.html
pragma: no-cache
cache-control: no-cache
host: http2-demo1



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accept-encoding: gzip,deflate,sdch

################################################################################
# delta2 request 5 (of 5) for http2-demo1

stream_id: 1234 group_id: 2
{'opcode': 'eclone', 'index':     0, 'val': '/s/24.png'}


00 0B 08 01 80 00 04 D2 02 05 00 00 00 08 82 78 | ...............x
99 ED CA 40                                     | ...@


###### decompressed ######
get /s/24.png HTTP/1.1
accept-language: en-US,en;q=0.8
accept: */*
user-agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.28 Safari/537.31
:scheme: http
accept-charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3
referer: http://http2-demo/http2_sample.html
pragma: no-cache
cache-control: no-cache
host: http2-demo1
accept-encoding: gzip,deflate,sdch


* http2-demo1: 5 res messages
################################################################################
# delta2 response 1 (of 5) for http2-demo1

stream_id: 1234 group_id: 2
{'opcode': 'sclone', 'index':    82, 'val': 'image/png'}
{'opcode': 'sclone', 'index':    84, 'val': 'Sat, 23 Jun 2012 02:03:47 GMT'}
{'opcode': 'sclone', 'index':    83, 'val': '338'}
{'opcode': 'strang', 'index':    81, 'index_start':    76}


00 2C 08 01 80 00 04 D2 02 04 02 00 52 B7 94 37 | .,..........R..7
C7 A3 0B B7 C8 00 54 D9 0C A6 03 40 76 E7 70 18 | ......T....@v.p.
91 80 9B 85 0E 4A C2 9A 9F 20 00 53 42 19 20 02 | .....J... .SB. .
00 00 51 00 4C                                  | ..Q.L

###### decompressed ######
HTTP/1.1 200 ?
content-length: 338
accept-ranges: bytes
expires: Fri, 1 Jan 2100 12:00:00 GMT



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server: Apache/2.2.22 (Ubuntu)
last-modified: Sat, 23 Jun 2012 02:03:47 GMT
cache-control: public, max-age=3600000000
date: Tue, 12 Mar 2013 23:12:44 GMT
content-type: image/png

################################################################################
# delta2 response 2 (of 5) for http2-demo1

stream_id: 1234 group_id: 2
{'opcode': 'strang', 'index':    93, 'index_start':    91}


00 06 08 01 80 00 04 D2 02 02 00 00 5D 00 5B    | ............].[

###### decompressed ######
HTTP/1.1 200 ?
content-length: 338
accept-ranges: bytes
expires: Fri, 1 Jan 2100 12:00:00 GMT
server: Apache/2.2.22 (Ubuntu)
last-modified: Sat, 23 Jun 2012 02:03:47 GMT
cache-control: public, max-age=3600000000
date: Tue, 12 Mar 2013 23:12:44 GMT
content-type: image/png

################################################################################
# delta2 response 3 (of 5) for http2-demo1

stream_id: 1234 group_id: 2
{'opcode': 'stoggl', 'index':    93}
{'opcode': 'sclone', 'index':   102, 'val': '358'}


00 0B 08 01 80 00 04 D2 02 00 00 00 5D 04 00 00 | ............]...
66 42 99 20                                     | fB.

###### decompressed ######
HTTP/1.1 200 ?
content-length: 358
accept-ranges: bytes
expires: Fri, 1 Jan 2100 12:00:00 GMT
server: Apache/2.2.22 (Ubuntu)
last-modified: Sat, 23 Jun 2012 02:03:47 GMT
cache-control: public, max-age=3600000000
date: Tue, 12 Mar 2013 23:12:44 GMT
content-type: image/png




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################################################################################
# delta2 response 4 (of 5) for http2-demo1

stream_id: 1234 group_id: 2
{'opcode': 'sclone', 'index':   111, 'val': '397'}


00 07 08 01 80 00 04 D2 02 04 00 00 6F 43 57 20 | ............oCW

###### decompressed ######
HTTP/1.1 200 ?
content-length: 397
accept-ranges: bytes
expires: Fri, 1 Jan 2100 12:00:00 GMT
server: Apache/2.2.22 (Ubuntu)
last-modified: Sat, 23 Jun 2012 02:03:47 GMT
cache-control: public, max-age=3600000000
date: Tue, 12 Mar 2013 23:12:44 GMT
content-type: image/png

################################################################################
# delta2 response 5 (of 5) for http2-demo1

stream_id: 1234 group_id: 2
{'opcode': 'stoggl', 'index':    92}
{'opcode': 'sclone', 'index':   119, 'val': 'Sat, 23 Jun 2012 02:03:46 GMT'}
{'opcode': 'sclone', 'index':   120, 'val': '345'}


00 20 08 01 80 00 04 D2 02 00 00 00 5C 04 01 00 | . ..........\...
77 D9 0C A6 03 40 76 E7 70 18 91 80 9B 85 0E 4D | w....@v.p......M
01 4D 4F 90 00 78 42 55 20                      | .MO..xBU

###### decompressed ######
HTTP/1.1 200 ?
content-length: 345
accept-ranges: bytes
expires: Fri, 1 Jan 2100 12:00:00 GMT
server: Apache/2.2.22 (Ubuntu)
last-modified: Sat, 23 Jun 2012 02:03:46 GMT
cache-control: public, max-age=3600000000
date: Tue, 12 Mar 2013 23:12:44 GMT
content-type: image/png








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12.  Unfinished components

   These are components that must be added in the future.

      Encoding of raw or ascii bytes

      Describing safe mechanisms for changing the allowable compressor
      state size downwards.

      The frequency table used to generate the huffman encoding should
      be updated with a more comprehensive analysis of header-character
      frequency.


13.  Security Considerations

   The compressor algorithm described here is expected to be immune to
   the current attacks against encrypted stream-based compressors such
   as TLS+gzip, but more scrutiny is warranted.  The reason that it is
   believed that the algorithm(s) expressed here is immune is that any
   backreference to a header key or value always requires a whole-text
   match, and thus any probe of the compression context confirms no
   hypothesis unless the attacker has guessed the entire plaintext key
   and value simultaneously.


14.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


15.  Acknowledgements


16.  Appendix A

           Appendix A (static-entries) :

           # (key, val)
           # Order does matter...
           static_entries = [
               (':path', '/'),
               (':scheme', 'http'),
               (':scheme', 'https'),
               (':method', 'get'),
               (':host', ''),



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               ('cookie', ''),
               (':status', '200'),
               (':status-text', 'OK'),
               (':version', '1.1'),
               ('accept', ''),
               ('accept-charset', ''),
               ('accept-encoding', ''),
               ('accept-language', ''),
               ('accept-ranges', ''),
               ('allow', ''),
               ('authorizations', ''),
               ('cache-control', ''),
               ('content-base', ''),
               ('content-encoding', ''),
               ('content-length', ''),
               ('content-location', ''),
               ('content-md5', ''),
               ('content-range', ''),
               ('content-type', ''),
               ('date', ''),
               ('etag', ''),
               ('expect', ''),
               ('expires', ''),
               ('from', ''),
               ('if-match', ''),
               ('if-modified-since', ''),
               ('if-none-match', ''),
               ('if-range', ''),
               ('if-unmodified-since', ''),
               ('last-modified', ''),
               ('location', ''),
               ('max-forwards', ''),
               ('origin', ''),
               ('pragma', ''),
               ('proxy-authenticate', ''),
               ('proxy-authorization', ''),
               ('range', ''),
               ('referer', ''),
               ('retry-after', ''),
               ('server', ''),
               ('set-cookie', ''),
               ('status', ''),
               ('te', ''),
               ('trailer', ''),
               ('transfer-encoding', ''),
               ('upgrade', ''),
               ('user-agent', ''),
               ('vary', ''),



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               ('via', ''),
               ('warning', ''),
               ('www-authenticate', ''),
               ('access-control-allow-origin', ''),
               ('content-disposition', ''),
               ('get-dictionary', ''),
               ('p3p', ''),
               ('x-content-type-options', ''),
               ('x-frame-options', ''),
               ('x-powered-by', ''),
               ('x-xss-protection', ''),
               ]



17.  Appendix B

   Appendix B huffman code-table for requests

                aligned                                  aligned
                 to                                       to
                 MSB                                      LSB
          sym    as bits                       len       as hex  len
         (  0) |11111111|11111111|11110111|100 [27]      7ffffbc [27]
         (  1) |11111111|11111111|11110111|101 [27]      7ffffbd [27]
         (  2) |11111111|11111111|11110111|110 [27]      7ffffbe [27]
         (  3) |11111111|11111111|11110111|111 [27]      7ffffbf [27]
         (  4) |11111111|11111111|11111000|000 [27]      7ffffc0 [27]
         (  5) |11111111|11111111|11111000|001 [27]      7ffffc1 [27]
         (  6) |11111111|11111111|11111000|010 [27]      7ffffc2 [27]
         (  7) |11111111|11111111|11111000|011 [27]      7ffffc3 [27]
         (  8) |11111111|11111111|11111000|100 [27]      7ffffc4 [27]
         (  9) |11111111|11111111|11111000|101 [27]      7ffffc5 [27]
         ( 10) |11111111|11111111|11111000|110 [27]      7ffffc6 [27]
         ( 11) |11111111|11111111|11111000|111 [27]      7ffffc7 [27]
         ( 12) |11111111|11111111|11111001|000 [27]      7ffffc8 [27]
         ( 13) |11111111|11111111|11111001|001 [27]      7ffffc9 [27]
         ( 14) |11111111|11111111|11111001|010 [27]      7ffffca [27]
         ( 15) |11111111|11111111|11111001|011 [27]      7ffffcb [27]
         ( 16) |11111111|11111111|11111001|100 [27]      7ffffcc [27]
         ( 17) |11111111|11111111|11111001|101 [27]      7ffffcd [27]
         ( 18) |11111111|11111111|11111001|110 [27]      7ffffce [27]
         ( 19) |11111111|11111111|11111001|111 [27]      7ffffcf [27]
         ( 20) |11111111|11111111|11111010|000 [27]      7ffffd0 [27]
         ( 21) |11111111|11111111|11111010|001 [27]      7ffffd1 [27]
         ( 22) |11111111|11111111|11111010|010 [27]      7ffffd2 [27]
         ( 23) |11111111|11111111|11111010|011 [27]      7ffffd3 [27]
         ( 24) |11111111|11111111|11111010|100 [27]      7ffffd4 [27]



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         ( 25) |11111111|11111111|11111010|101 [27]      7ffffd5 [27]
         ( 26) |11111111|11111111|11111010|110 [27]      7ffffd6 [27]
         ( 27) |11111111|11111111|11111010|111 [27]      7ffffd7 [27]
         ( 28) |11111111|11111111|11111011|000 [27]      7ffffd8 [27]
         ( 29) |11111111|11111111|11111011|001 [27]      7ffffd9 [27]
         ( 30) |11111111|11111111|11111011|010 [27]      7ffffda [27]
         ( 31) |11111111|11111111|11111011|011 [27]      7ffffdb [27]
     ' ' ( 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) |100110 [6]                                    26 [6]
     '&' ( 38) |1110000 [7]                                   70 [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) |11111110|00 [10]                             3f8 [10]
     '-' ( 45) |100111 [6]                                    27 [6]
     '.' ( 46) |00110 [5]                                      6 [5]
     '/' ( 47) |0000 [4]                                       0 [4]
     '0' ( 48) |00111 [5]                                      7 [5]
     '1' ( 49) |01000 [5]                                      8 [5]
     '2' ( 50) |01001 [5]                                      9 [5]
     '3' ( 51) |101000 [6]                                    28 [6]
     '4' ( 52) |1110001 [7]                                   71 [7]
     '5' ( 53) |101001 [6]                                    29 [6]
     '6' ( 54) |1110010 [7]                                   72 [7]
     '7' ( 55) |101010 [6]                                    2a [6]
     '8' ( 56) |1110011 [7]                                   73 [7]
     '9' ( 57) |101011 [6]                                    2b [6]
     ':' ( 58) |101100 [6]                                    2c [6]
     ';' ( 59) |11110100|0 [9]                               1e8 [9]
     '<' ( 60) |11111111|11111111|10 [18]                  3fffe [18]
     '=' ( 61) |101101 [6]                                    2d [6]
     '>' ( 62) |11111111|11111110|0 [17]                   1fffc [17]
     '?' ( 63) |11110100|1 [9]                               1e9 [9]
     '@' ( 64) |11111111|11100 [13]                         1ffc [13]
     'A' ( 65) |11101100 [8]                                  ec [8]
     'B' ( 66) |11101101 [8]                                  ed [8]
     'C' ( 67) |11101110 [8]                                  ee [8]
     'D' ( 68) |11101111 [8]                                  ef [8]
     'E' ( 69) |11110101|0 [9]                               1ea [9]
     'F' ( 70) |1110100 [7]                                   74 [7]
     'G' ( 71) |11110101|1 [9]                               1eb [9]
     'H' ( 72) |11110110|0 [9]                               1ec [9]



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     'I' ( 73) |11110110|1 [9]                               1ed [9]
     'J' ( 74) |11111110|01 [10]                             3f9 [10]
     'K' ( 75) |11111111|010 [11]                            7fa [11]
     'L' ( 76) |11110111|0 [9]                               1ee [9]
     'M' ( 77) |11110111|1 [9]                               1ef [9]
     'N' ( 78) |11111000|0 [9]                               1f0 [9]
     'O' ( 79) |11111000|1 [9]                               1f1 [9]
     'P' ( 80) |11111001|0 [9]                               1f2 [9]
     'Q' ( 81) |11111110|10 [10]                             3fa [10]
     'R' ( 82) |11111001|1 [9]                               1f3 [9]
     'S' ( 83) |11111010|0 [9]                               1f4 [9]
     'T' ( 84) |11111010|1 [9]                               1f5 [9]
     'U' ( 85) |11111011|0 [9]                               1f6 [9]
     'V' ( 86) |11111011|1 [9]                               1f7 [9]
     'W' ( 87) |11111100|0 [9]                               1f8 [9]
     'X' ( 88) |11111100|1 [9]                               1f9 [9]
     'Y' ( 89) |11111110|11 [10]                             3fb [10]
     'Z' ( 90) |11111111|00 [10]                             3fc [10]
     '[' ( 91) |11111111|111011 [14]                        3ffb [14]
     '\' ( 92) |11111111|11111111|11111011|100 [27]      7ffffdc [27]
     ']' ( 93) |11111111|111100 [14]                        3ffc [14]
     '^' ( 94) |11111111|111101 [14]                        3ffd [14]
     '_' ( 95) |101110 [6]                                    2e [6]
     '`' ( 96) |11111111|11111111|110 [19]                 7fffe [19]
     'a' ( 97) |01010 [5]                                      a [5]
     'b' ( 98) |101111 [6]                                    2f [6]
     'c' ( 99) |01011 [5]                                      b [5]
     'd' (100) |110000 [6]                                    30 [6]
     'e' (101) |0001 [4]                                       1 [4]
     'f' (102) |110001 [6]                                    31 [6]
     'g' (103) |110010 [6]                                    32 [6]
     'h' (104) |110011 [6]                                    33 [6]
     'i' (105) |01100 [5]                                      c [5]
     'j' (106) |1110101 [7]                                   75 [7]
     'k' (107) |11110000 [8]                                  f0 [8]
     'l' (108) |110100 [6]                                    34 [6]
     'm' (109) |110101 [6]                                    35 [6]
     'n' (110) |01101 [5]                                      d [5]
     'o' (111) |01110 [5]                                      e [5]
     'p' (112) |01111 [5]                                      f [5]
     'q' (113) |11111101|0 [9]                               1fa [9]
     'r' (114) |10000 [5]                                     10 [5]
     's' (115) |10001 [5]                                     11 [5]
     't' (116) |0010 [4]                                       2 [4]
     'u' (117) |110110 [6]                                    36 [6]
     'v' (118) |11110001 [8]                                  f1 [8]
     'w' (119) |110111 [6]                                    37 [6]
     'x' (120) |11110010 [8]                                  f2 [8]



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     'y' (121) |11110011 [8]                                  f3 [8]
     'z' (122) |11111101|1 [9]                               1fb [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) |11111111|11111111|11111011|101 [27]      7ffffdd [27]
         (128) |11111111|11111111|11111011|110 [27]      7ffffde [27]
         (129) |11111111|11111111|11111011|111 [27]      7ffffdf [27]
         (130) |11111111|11111111|11111100|000 [27]      7ffffe0 [27]
         (131) |11111111|11111111|11111100|001 [27]      7ffffe1 [27]
         (132) |11111111|11111111|11111100|010 [27]      7ffffe2 [27]
         (133) |11111111|11111111|11111100|011 [27]      7ffffe3 [27]
         (134) |11111111|11111111|11111100|100 [27]      7ffffe4 [27]
         (135) |11111111|11111111|11111100|101 [27]      7ffffe5 [27]
         (136) |11111111|11111111|11111100|110 [27]      7ffffe6 [27]
         (137) |11111111|11111111|11111100|111 [27]      7ffffe7 [27]
         (138) |11111111|11111111|11111101|000 [27]      7ffffe8 [27]
         (139) |11111111|11111111|11111101|001 [27]      7ffffe9 [27]
         (140) |11111111|11111111|11111101|010 [27]      7ffffea [27]
         (141) |11111111|11111111|11111101|011 [27]      7ffffeb [27]
         (142) |11111111|11111111|11111101|100 [27]      7ffffec [27]
         (143) |11111111|11111111|11111101|101 [27]      7ffffed [27]
         (144) |11111111|11111111|11111101|110 [27]      7ffffee [27]
         (145) |11111111|11111111|11111101|111 [27]      7ffffef [27]
         (146) |11111111|11111111|11111110|000 [27]      7fffff0 [27]
         (147) |11111111|11111111|11111110|001 [27]      7fffff1 [27]
         (148) |11111111|11111111|11111110|010 [27]      7fffff2 [27]
         (149) |11111111|11111111|11111110|011 [27]      7fffff3 [27]
         (150) |11111111|11111111|11111110|100 [27]      7fffff4 [27]
         (151) |11111111|11111111|11111110|101 [27]      7fffff5 [27]
         (152) |11111111|11111111|11111110|110 [27]      7fffff6 [27]
         (153) |11111111|11111111|11111110|111 [27]      7fffff7 [27]
         (154) |11111111|11111111|11111111|000 [27]      7fffff8 [27]
         (155) |11111111|11111111|11111111|001 [27]      7fffff9 [27]
         (156) |11111111|11111111|11111111|010 [27]      7fffffa [27]
         (157) |11111111|11111111|11111111|011 [27]      7fffffb [27]
         (158) |11111111|11111111|11111111|100 [27]      7fffffc [27]
         (159) |11111111|11111111|11111111|101 [27]      7fffffd [27]
         (160) |11111111|11111111|11111111|110 [27]      7fffffe [27]
         (161) |11111111|11111111|11111111|111 [27]      7ffffff [27]
         (162) |11111111|11111111|11100000|00 [26]       3ffff80 [26]
         (163) |11111111|11111111|11100000|01 [26]       3ffff81 [26]
         (164) |11111111|11111111|11100000|10 [26]       3ffff82 [26]
         (165) |11111111|11111111|11100000|11 [26]       3ffff83 [26]
         (166) |11111111|11111111|11100001|00 [26]       3ffff84 [26]
         (167) |11111111|11111111|11100001|01 [26]       3ffff85 [26]
         (168) |11111111|11111111|11100001|10 [26]       3ffff86 [26]



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         (169) |11111111|11111111|11100001|11 [26]       3ffff87 [26]
         (170) |11111111|11111111|11100010|00 [26]       3ffff88 [26]
         (171) |11111111|11111111|11100010|01 [26]       3ffff89 [26]
         (172) |11111111|11111111|11100010|10 [26]       3ffff8a [26]
         (173) |11111111|11111111|11100010|11 [26]       3ffff8b [26]
         (174) |11111111|11111111|11100011|00 [26]       3ffff8c [26]
         (175) |11111111|11111111|11100011|01 [26]       3ffff8d [26]
         (176) |11111111|11111111|11100011|10 [26]       3ffff8e [26]
         (177) |11111111|11111111|11100011|11 [26]       3ffff8f [26]
         (178) |11111111|11111111|11100100|00 [26]       3ffff90 [26]
         (179) |11111111|11111111|11100100|01 [26]       3ffff91 [26]
         (180) |11111111|11111111|11100100|10 [26]       3ffff92 [26]
         (181) |11111111|11111111|11100100|11 [26]       3ffff93 [26]
         (182) |11111111|11111111|11100101|00 [26]       3ffff94 [26]
         (183) |11111111|11111111|11100101|01 [26]       3ffff95 [26]
         (184) |11111111|11111111|11100101|10 [26]       3ffff96 [26]
         (185) |11111111|11111111|11100101|11 [26]       3ffff97 [26]
         (186) |11111111|11111111|11100110|00 [26]       3ffff98 [26]
         (187) |11111111|11111111|11100110|01 [26]       3ffff99 [26]
         (188) |11111111|11111111|11100110|10 [26]       3ffff9a [26]
         (189) |11111111|11111111|11100110|11 [26]       3ffff9b [26]
         (190) |11111111|11111111|11100111|00 [26]       3ffff9c [26]
         (191) |11111111|11111111|11100111|01 [26]       3ffff9d [26]
         (192) |11111111|11111111|11100111|10 [26]       3ffff9e [26]
         (193) |11111111|11111111|11100111|11 [26]       3ffff9f [26]
         (194) |11111111|11111111|11101000|00 [26]       3ffffa0 [26]
         (195) |11111111|11111111|11101000|01 [26]       3ffffa1 [26]
         (196) |11111111|11111111|11101000|10 [26]       3ffffa2 [26]
         (197) |11111111|11111111|11101000|11 [26]       3ffffa3 [26]
         (198) |11111111|11111111|11101001|00 [26]       3ffffa4 [26]
         (199) |11111111|11111111|11101001|01 [26]       3ffffa5 [26]
         (200) |11111111|11111111|11101001|10 [26]       3ffffa6 [26]
         (201) |11111111|11111111|11101001|11 [26]       3ffffa7 [26]
         (202) |11111111|11111111|11101010|00 [26]       3ffffa8 [26]
         (203) |11111111|11111111|11101010|01 [26]       3ffffa9 [26]
         (204) |11111111|11111111|11101010|10 [26]       3ffffaa [26]
         (205) |11111111|11111111|11101010|11 [26]       3ffffab [26]
         (206) |11111111|11111111|11101011|00 [26]       3ffffac [26]
         (207) |11111111|11111111|11101011|01 [26]       3ffffad [26]
         (208) |11111111|11111111|11101011|10 [26]       3ffffae [26]
         (209) |11111111|11111111|11101011|11 [26]       3ffffaf [26]
         (210) |11111111|11111111|11101100|00 [26]       3ffffb0 [26]
         (211) |11111111|11111111|11101100|01 [26]       3ffffb1 [26]
         (212) |11111111|11111111|11101100|10 [26]       3ffffb2 [26]
         (213) |11111111|11111111|11101100|11 [26]       3ffffb3 [26]
         (214) |11111111|11111111|11101101|00 [26]       3ffffb4 [26]
         (215) |11111111|11111111|11101101|01 [26]       3ffffb5 [26]
         (216) |11111111|11111111|11101101|10 [26]       3ffffb6 [26]



Peon                   Expires September 19, 2013              [Page 31]

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         (217) |11111111|11111111|11101101|11 [26]       3ffffb7 [26]
         (218) |11111111|11111111|11101110|00 [26]       3ffffb8 [26]
         (219) |11111111|11111111|11101110|01 [26]       3ffffb9 [26]
         (220) |11111111|11111111|11101110|10 [26]       3ffffba [26]
         (221) |11111111|11111111|11101110|11 [26]       3ffffbb [26]
         (222) |11111111|11111111|11101111|00 [26]       3ffffbc [26]
         (223) |11111111|11111111|11101111|01 [26]       3ffffbd [26]
         (224) |11111111|11111111|11101111|10 [26]       3ffffbe [26]
         (225) |11111111|11111111|11101111|11 [26]       3ffffbf [26]
         (226) |11111111|11111111|11110000|00 [26]       3ffffc0 [26]
         (227) |11111111|11111111|11110000|01 [26]       3ffffc1 [26]
         (228) |11111111|11111111|11110000|10 [26]       3ffffc2 [26]
         (229) |11111111|11111111|11110000|11 [26]       3ffffc3 [26]
         (230) |11111111|11111111|11110001|00 [26]       3ffffc4 [26]
         (231) |11111111|11111111|11110001|01 [26]       3ffffc5 [26]
         (232) |11111111|11111111|11110001|10 [26]       3ffffc6 [26]
         (233) |11111111|11111111|11110001|11 [26]       3ffffc7 [26]
         (234) |11111111|11111111|11110010|00 [26]       3ffffc8 [26]
         (235) |11111111|11111111|11110010|01 [26]       3ffffc9 [26]
         (236) |11111111|11111111|11110010|10 [26]       3ffffca [26]
         (237) |11111111|11111111|11110010|11 [26]       3ffffcb [26]
         (238) |11111111|11111111|11110011|00 [26]       3ffffcc [26]
         (239) |11111111|11111111|11110011|01 [26]       3ffffcd [26]
         (240) |11111111|11111111|11110011|10 [26]       3ffffce [26]
         (241) |11111111|11111111|11110011|11 [26]       3ffffcf [26]
         (242) |11111111|11111111|11110100|00 [26]       3ffffd0 [26]
         (243) |11111111|11111111|11110100|01 [26]       3ffffd1 [26]
         (244) |11111111|11111111|11110100|10 [26]       3ffffd2 [26]
         (245) |11111111|11111111|11110100|11 [26]       3ffffd3 [26]
         (246) |11111111|11111111|11110101|00 [26]       3ffffd4 [26]
         (247) |11111111|11111111|11110101|01 [26]       3ffffd5 [26]
         (248) |11111111|11111111|11110101|10 [26]       3ffffd6 [26]
         (249) |11111111|11111111|11110101|11 [26]       3ffffd7 [26]
         (250) |11111111|11111111|11110110|00 [26]       3ffffd8 [26]
         (251) |11111111|11111111|11110110|01 [26]       3ffffd9 [26]
         (252) |11111111|11111111|11110110|10 [26]       3ffffda [26]
         (253) |11111111|11111111|11110110|11 [26]       3ffffdb [26]
         (254) |11111111|11111111|11110111|00 [26]       3ffffdc [26]
         (255) |11111111|11111111|11110111|01 [26]       3ffffdd [26]
         (256) |10010 [5]                                     12 [5]



18.  Appendix C

   Appendix C huffman code-table for responses

                aligned                                  aligned



Peon                   Expires September 19, 2013              [Page 32]

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                 to                                       to
                 MSB                                      LSB
          sym    as bits                      len        as hex  len
         (  0) |11111111|11111111|11101111|10 [26]       3ffffbe [26]
         (  1) |11111111|11111111|11101111|11 [26]       3ffffbf [26]
         (  2) |11111111|11111111|11110000|00 [26]       3ffffc0 [26]
         (  3) |11111111|11111111|11110000|01 [26]       3ffffc1 [26]
         (  4) |11111111|11111111|11110000|10 [26]       3ffffc2 [26]
         (  5) |11111111|11111111|11110000|11 [26]       3ffffc3 [26]
         (  6) |11111111|11111111|11110001|00 [26]       3ffffc4 [26]
         (  7) |11111111|11111111|11110001|01 [26]       3ffffc5 [26]
         (  8) |11111111|11111111|11110001|10 [26]       3ffffc6 [26]
         (  9) |11111111|11111111|11110001|11 [26]       3ffffc7 [26]
         ( 10) |11111111|11111111|11110010|00 [26]       3ffffc8 [26]
         ( 11) |11111111|11111111|11110010|01 [26]       3ffffc9 [26]
         ( 12) |11111111|11111111|11110010|10 [26]       3ffffca [26]
         ( 13) |11111111|11111111|11110010|11 [26]       3ffffcb [26]
         ( 14) |11111111|11111111|11110011|00 [26]       3ffffcc [26]
         ( 15) |11111111|11111111|11110011|01 [26]       3ffffcd [26]
         ( 16) |11111111|11111111|11110011|10 [26]       3ffffce [26]
         ( 17) |11111111|11111111|11110011|11 [26]       3ffffcf [26]
         ( 18) |11111111|11111111|11110100|00 [26]       3ffffd0 [26]
         ( 19) |11111111|11111111|11110100|01 [26]       3ffffd1 [26]
         ( 20) |11111111|11111111|11110100|10 [26]       3ffffd2 [26]
         ( 21) |11111111|11111111|11110100|11 [26]       3ffffd3 [26]
         ( 22) |11111111|11111111|11110101|00 [26]       3ffffd4 [26]
         ( 23) |11111111|11111111|11110101|01 [26]       3ffffd5 [26]
         ( 24) |11111111|11111111|11110101|10 [26]       3ffffd6 [26]
         ( 25) |11111111|11111111|11110101|11 [26]       3ffffd7 [26]
         ( 26) |11111111|11111111|11110110|00 [26]       3ffffd8 [26]
         ( 27) |11111111|11111111|11110110|01 [26]       3ffffd9 [26]
         ( 28) |11111111|11111111|11110110|10 [26]       3ffffda [26]
         ( 29) |11111111|11111111|11110110|11 [26]       3ffffdb [26]
         ( 30) |11111111|11111111|11110111|00 [26]       3ffffdc [26]
         ( 31) |11111111|11111111|11110111|01 [26]       3ffffdd [26]
     ' ' ( 32) |0000 [4]                                       0 [4]
     '!' ( 33) |11111111|1010 [12]                           ffa [12]
     '"' ( 34) |1101000 [7]                                   68 [7]
     '#' ( 35) |11111111|111010 [14]                        3ffa [14]
     '$' ( 36) |11111111|1111100 [15]                       7ffc [15]
     '%' ( 37) |11110101|0 [9]                               1ea [9]
     '&' ( 38) |11111110|00 [10]                             3f8 [10]
     ''' ( 39) |11111111|11100 [13]                         1ffc [13]
     '(' ( 40) |11110101|1 [9]                               1eb [9]
     ')' ( 41) |11110110|0 [9]                               1ec [9]
     '*' ( 42) |11111111|1011 [12]                           ffb [12]
     '+' ( 43) |11111110|01 [10]                             3f9 [10]
     ',' ( 44) |100110 [6]                                    26 [6]



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     '-' ( 45) |100111 [6]                                    27 [6]
     '.' ( 46) |1101001 [7]                                   69 [7]
     '/' ( 47) |11101000 [8]                                  e8 [8]
     '0' ( 48) |0001 [4]                                       1 [4]
     '1' ( 49) |0010 [4]                                       2 [4]
     '2' ( 50) |0011 [4]                                       3 [4]
     '3' ( 51) |01000 [5]                                      8 [5]
     '4' ( 52) |01001 [5]                                      9 [5]
     '5' ( 53) |01010 [5]                                      a [5]
     '6' ( 54) |101000 [6]                                    28 [6]
     '7' ( 55) |01011 [5]                                      b [5]
     '8' ( 56) |01100 [5]                                      c [5]
     '9' ( 57) |01101 [5]                                      d [5]
     ':' ( 58) |01110 [5]                                      e [5]
     ';' ( 59) |11110110|1 [9]                               1ed [9]
     '<' ( 60) |11111111|11111100 [16]                      fffc [16]
     '=' ( 61) |1101010 [7]                                   6a [7]
     '>' ( 62) |11111111|111011 [14]                        3ffb [14]
     '?' ( 63) |11111111|1100 [12]                           ffc [12]
     '@' ( 64) |11111111|11111110|0 [17]                   1fffc [17]
     'A' ( 65) |1101011 [7]                                   6b [7]
     'B' ( 66) |11110111|0 [9]                               1ee [9]
     'C' ( 67) |11101001 [8]                                  e9 [8]
     'D' ( 68) |11101010 [8]                                  ea [8]
     'E' ( 69) |11101011 [8]                                  eb [8]
     'F' ( 70) |11101100 [8]                                  ec [8]
     'G' ( 71) |101001 [6]                                    29 [6]
     'H' ( 72) |11110111|1 [9]                               1ef [9]
     'I' ( 73) |11111000|0 [9]                               1f0 [9]
     'J' ( 74) |11101101 [8]                                  ed [8]
     'K' ( 75) |11111110|10 [10]                             3fa [10]
     'L' ( 76) |11111000|1 [9]                               1f1 [9]
     'M' ( 77) |101010 [6]                                    2a [6]
     'N' ( 78) |11101110 [8]                                  ee [8]
     'O' ( 79) |11101111 [8]                                  ef [8]
     'P' ( 80) |11111001|0 [9]                               1f2 [9]
     'Q' ( 81) |11111001|1 [9]                               1f3 [9]
     'R' ( 82) |11111010|0 [9]                               1f4 [9]
     'S' ( 83) |1101100 [7]                                   6c [7]
     'T' ( 84) |01111 [5]                                      f [5]
     'U' ( 85) |11111010|1 [9]                               1f5 [9]
     'V' ( 86) |11111011|0 [9]                               1f6 [9]
     'W' ( 87) |11110000 [8]                                  f0 [8]
     'X' ( 88) |11111110|11 [10]                             3fb [10]
     'Y' ( 89) |11111111|00 [10]                             3fc [10]
     'Z' ( 90) |11111111|01 [10]                             3fd [10]
     '[' ( 91) |11111111|1101 [12]                           ffd [12]
     '\' ( 92) |11111111|111100 [14]                        3ffc [14]



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     ']' ( 93) |11111111|100 [11]                            7fc [11]
     '^' ( 94) |11111111|1111101 [15]                       7ffd [15]
     '_' ( 95) |11111011|1 [9]                               1f7 [9]
     '`' ( 96) |11111111|11111111|10 [18]                  3fffe [18]
     'a' ( 97) |10000 [5]                                     10 [5]
     'b' ( 98) |1101101 [7]                                   6d [7]
     'c' ( 99) |101011 [6]                                    2b [6]
     'd' (100) |101100 [6]                                    2c [6]
     'e' (101) |10001 [5]                                     11 [5]
     'f' (102) |1101110 [7]                                   6e [7]
     'g' (103) |1101111 [7]                                   6f [7]
     'h' (104) |1110000 [7]                                   70 [7]
     'i' (105) |101101 [6]                                    2d [6]
     'j' (106) |11111100|0 [9]                               1f8 [9]
     'k' (107) |11111100|1 [9]                               1f9 [9]
     'l' (108) |1110001 [7]                                   71 [7]
     'm' (109) |1110010 [7]                                   72 [7]
     'n' (110) |101110 [6]                                    2e [6]
     'o' (111) |101111 [6]                                    2f [6]
     'p' (112) |110000 [6]                                    30 [6]
     'q' (113) |11111101|0 [9]                               1fa [9]
     'r' (114) |110001 [6]                                    31 [6]
     's' (115) |1110011 [7]                                   73 [7]
     't' (116) |110010 [6]                                    32 [6]
     'u' (117) |110011 [6]                                    33 [6]
     'v' (118) |11110001 [8]                                  f1 [8]
     'w' (119) |11110010 [8]                                  f2 [8]
     'x' (120) |11110011 [8]                                  f3 [8]
     'y' (121) |11110100 [8]                                  f4 [8]
     'z' (122) |11111101|1 [9]                               1fb [9]
     '{' (123) |11111111|11111110|1 [17]                   1fffd [17]
     '|' (124) |11111111|111101 [14]                        3ffd [14]
     '}' (125) |11111111|11111111|0 [17]                   1fffe [17]
     '~' (126) |11111111|11111101 [16]                      fffd [16]
         (127) |11111111|11111111|11110111|10 [26]       3ffffde [26]
         (128) |11111111|11111111|11110111|11 [26]       3ffffdf [26]
         (129) |11111111|11111111|11111000|00 [26]       3ffffe0 [26]
         (130) |11111111|11111111|11111000|01 [26]       3ffffe1 [26]
         (131) |11111111|11111111|11111000|10 [26]       3ffffe2 [26]
         (132) |11111111|11111111|11111000|11 [26]       3ffffe3 [26]
         (133) |11111111|11111111|11111001|00 [26]       3ffffe4 [26]
         (134) |11111111|11111111|11111001|01 [26]       3ffffe5 [26]
         (135) |11111111|11111111|11111001|10 [26]       3ffffe6 [26]
         (136) |11111111|11111111|11111001|11 [26]       3ffffe7 [26]
         (137) |11111111|11111111|11111010|00 [26]       3ffffe8 [26]
         (138) |11111111|11111111|11111010|01 [26]       3ffffe9 [26]
         (139) |11111111|11111111|11111010|10 [26]       3ffffea [26]
         (140) |11111111|11111111|11111010|11 [26]       3ffffeb [26]



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         (141) |11111111|11111111|11111011|00 [26]       3ffffec [26]
         (142) |11111111|11111111|11111011|01 [26]       3ffffed [26]
         (143) |11111111|11111111|11111011|10 [26]       3ffffee [26]
         (144) |11111111|11111111|11111011|11 [26]       3ffffef [26]
         (145) |11111111|11111111|11111100|00 [26]       3fffff0 [26]
         (146) |11111111|11111111|11111100|01 [26]       3fffff1 [26]
         (147) |11111111|11111111|11111100|10 [26]       3fffff2 [26]
         (148) |11111111|11111111|11111100|11 [26]       3fffff3 [26]
         (149) |11111111|11111111|11111101|00 [26]       3fffff4 [26]
         (150) |11111111|11111111|11111101|01 [26]       3fffff5 [26]
         (151) |11111111|11111111|11111101|10 [26]       3fffff6 [26]
         (152) |11111111|11111111|11111101|11 [26]       3fffff7 [26]
         (153) |11111111|11111111|11111110|00 [26]       3fffff8 [26]
         (154) |11111111|11111111|11111110|01 [26]       3fffff9 [26]
         (155) |11111111|11111111|11111110|10 [26]       3fffffa [26]
         (156) |11111111|11111111|11111110|11 [26]       3fffffb [26]
         (157) |11111111|11111111|11111111|00 [26]       3fffffc [26]
         (158) |11111111|11111111|11111111|01 [26]       3fffffd [26]
         (159) |11111111|11111111|11111111|10 [26]       3fffffe [26]
         (160) |11111111|11111111|11111111|11 [26]       3ffffff [26]
         (161) |11111111|11111111|11000000|0 [25]        1ffff80 [25]
         (162) |11111111|11111111|11000000|1 [25]        1ffff81 [25]
         (163) |11111111|11111111|11000001|0 [25]        1ffff82 [25]
         (164) |11111111|11111111|11000001|1 [25]        1ffff83 [25]
         (165) |11111111|11111111|11000010|0 [25]        1ffff84 [25]
         (166) |11111111|11111111|11000010|1 [25]        1ffff85 [25]
         (167) |11111111|11111111|11000011|0 [25]        1ffff86 [25]
         (168) |11111111|11111111|11000011|1 [25]        1ffff87 [25]
         (169) |11111111|11111111|11000100|0 [25]        1ffff88 [25]
         (170) |11111111|11111111|11000100|1 [25]        1ffff89 [25]
         (171) |11111111|11111111|11000101|0 [25]        1ffff8a [25]
         (172) |11111111|11111111|11000101|1 [25]        1ffff8b [25]
         (173) |11111111|11111111|11000110|0 [25]        1ffff8c [25]
         (174) |11111111|11111111|11000110|1 [25]        1ffff8d [25]
         (175) |11111111|11111111|11000111|0 [25]        1ffff8e [25]
         (176) |11111111|11111111|11000111|1 [25]        1ffff8f [25]
         (177) |11111111|11111111|11001000|0 [25]        1ffff90 [25]
         (178) |11111111|11111111|11001000|1 [25]        1ffff91 [25]
         (179) |11111111|11111111|11001001|0 [25]        1ffff92 [25]
         (180) |11111111|11111111|11001001|1 [25]        1ffff93 [25]
         (181) |11111111|11111111|11001010|0 [25]        1ffff94 [25]
         (182) |11111111|11111111|11001010|1 [25]        1ffff95 [25]
         (183) |11111111|11111111|11001011|0 [25]        1ffff96 [25]
         (184) |11111111|11111111|11001011|1 [25]        1ffff97 [25]
         (185) |11111111|11111111|11001100|0 [25]        1ffff98 [25]
         (186) |11111111|11111111|11001100|1 [25]        1ffff99 [25]
         (187) |11111111|11111111|11001101|0 [25]        1ffff9a [25]
         (188) |11111111|11111111|11001101|1 [25]        1ffff9b [25]



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         (189) |11111111|11111111|11001110|0 [25]        1ffff9c [25]
         (190) |11111111|11111111|11001110|1 [25]        1ffff9d [25]
         (191) |11111111|11111111|11001111|0 [25]        1ffff9e [25]
         (192) |11111111|11111111|11001111|1 [25]        1ffff9f [25]
         (193) |11111111|11111111|11010000|0 [25]        1ffffa0 [25]
         (194) |11111111|11111111|11010000|1 [25]        1ffffa1 [25]
         (195) |11111111|11111111|11010001|0 [25]        1ffffa2 [25]
         (196) |11111111|11111111|11010001|1 [25]        1ffffa3 [25]
         (197) |11111111|11111111|11010010|0 [25]        1ffffa4 [25]
         (198) |11111111|11111111|11010010|1 [25]        1ffffa5 [25]
         (199) |11111111|11111111|11010011|0 [25]        1ffffa6 [25]
         (200) |11111111|11111111|11010011|1 [25]        1ffffa7 [25]
         (201) |11111111|11111111|11010100|0 [25]        1ffffa8 [25]
         (202) |11111111|11111111|11010100|1 [25]        1ffffa9 [25]
         (203) |11111111|11111111|11010101|0 [25]        1ffffaa [25]
         (204) |11111111|11111111|11010101|1 [25]        1ffffab [25]
         (205) |11111111|11111111|11010110|0 [25]        1ffffac [25]
         (206) |11111111|11111111|11010110|1 [25]        1ffffad [25]
         (207) |11111111|11111111|11010111|0 [25]        1ffffae [25]
         (208) |11111111|11111111|11010111|1 [25]        1ffffaf [25]
         (209) |11111111|11111111|11011000|0 [25]        1ffffb0 [25]
         (210) |11111111|11111111|11011000|1 [25]        1ffffb1 [25]
         (211) |11111111|11111111|11011001|0 [25]        1ffffb2 [25]
         (212) |11111111|11111111|11011001|1 [25]        1ffffb3 [25]
         (213) |11111111|11111111|11011010|0 [25]        1ffffb4 [25]
         (214) |11111111|11111111|11011010|1 [25]        1ffffb5 [25]
         (215) |11111111|11111111|11011011|0 [25]        1ffffb6 [25]
         (216) |11111111|11111111|11011011|1 [25]        1ffffb7 [25]
         (217) |11111111|11111111|11011100|0 [25]        1ffffb8 [25]
         (218) |11111111|11111111|11011100|1 [25]        1ffffb9 [25]
         (219) |11111111|11111111|11011101|0 [25]        1ffffba [25]
         (220) |11111111|11111111|11011101|1 [25]        1ffffbb [25]
         (221) |11111111|11111111|11011110|0 [25]        1ffffbc [25]
         (222) |11111111|11111111|11011110|1 [25]        1ffffbd [25]
         (223) |11111111|11111111|11011111|0 [25]        1ffffbe [25]
         (224) |11111111|11111111|11011111|1 [25]        1ffffbf [25]
         (225) |11111111|11111111|11100000|0 [25]        1ffffc0 [25]
         (226) |11111111|11111111|11100000|1 [25]        1ffffc1 [25]
         (227) |11111111|11111111|11100001|0 [25]        1ffffc2 [25]
         (228) |11111111|11111111|11100001|1 [25]        1ffffc3 [25]
         (229) |11111111|11111111|11100010|0 [25]        1ffffc4 [25]
         (230) |11111111|11111111|11100010|1 [25]        1ffffc5 [25]
         (231) |11111111|11111111|11100011|0 [25]        1ffffc6 [25]
         (232) |11111111|11111111|11100011|1 [25]        1ffffc7 [25]
         (233) |11111111|11111111|11100100|0 [25]        1ffffc8 [25]
         (234) |11111111|11111111|11100100|1 [25]        1ffffc9 [25]
         (235) |11111111|11111111|11100101|0 [25]        1ffffca [25]
         (236) |11111111|11111111|11100101|1 [25]        1ffffcb [25]



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         (237) |11111111|11111111|11100110|0 [25]        1ffffcc [25]
         (238) |11111111|11111111|11100110|1 [25]        1ffffcd [25]
         (239) |11111111|11111111|11100111|0 [25]        1ffffce [25]
         (240) |11111111|11111111|11100111|1 [25]        1ffffcf [25]
         (241) |11111111|11111111|11101000|0 [25]        1ffffd0 [25]
         (242) |11111111|11111111|11101000|1 [25]        1ffffd1 [25]
         (243) |11111111|11111111|11101001|0 [25]        1ffffd2 [25]
         (244) |11111111|11111111|11101001|1 [25]        1ffffd3 [25]
         (245) |11111111|11111111|11101010|0 [25]        1ffffd4 [25]
         (246) |11111111|11111111|11101010|1 [25]        1ffffd5 [25]
         (247) |11111111|11111111|11101011|0 [25]        1ffffd6 [25]
         (248) |11111111|11111111|11101011|1 [25]        1ffffd7 [25]
         (249) |11111111|11111111|11101100|0 [25]        1ffffd8 [25]
         (250) |11111111|11111111|11101100|1 [25]        1ffffd9 [25]
         (251) |11111111|11111111|11101101|0 [25]        1ffffda [25]
         (252) |11111111|11111111|11101101|1 [25]        1ffffdb [25]
         (253) |11111111|11111111|11101110|0 [25]        1ffffdc [25]
         (254) |11111111|11111111|11101110|1 [25]        1ffffdd [25]
         (255) |11111111|11111111|11101111|0 [25]        1ffffde [25]
         (256) |10010 [5]                                     12 [5]



19.  Normative References

   [CANON]    Schwartz, E. S. and Kallick, B., "Generating a canonical
              prefix encoding", Comm. ACM, 7,3 (March 1964), pp 166-
              169 .

   [HUFF]     Huffman, D. A., "A Method for the Construction of Minimim
              Redundancy Codes", Proceedings of the Institute of Radio
              Engineers, September 1952, Volume 40, Number 9, pp. 1098-
              1101 .

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

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

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








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Author's Address

   Roberto Peon
   Google, Inc

   Email: fenix@google.com













































Peon                   Expires September 19, 2013              [Page 39]


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