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Versions: (draft-price-rohc-sigcomp-user-guide) 00 01 02 03 04 RFC 4464

Robust Header Compression                                     A. Surtees
Internet-Draft                                                   M. West
Expires: April 1, 2006                       Siemens/Roke Manor Research
                                                      September 28, 2005


                          SigComp Users' Guide
               draft-ietf-rohc-sigcomp-user-guide-03.txt

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document provides an informational guide for users of the
   SigComp protocol.  The aim of the document is to assist users when
   making SigComp implementation decisions; for example the choice of
   compression algorithm and the level of robustness against lost or
   misordered packets.






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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Overview of the User Guide . . . . . . . . . . . . . . . . . .  3
   3.  UDVM assembly language . . . . . . . . . . . . . . . . . . . .  4
     3.1   Lexical level  . . . . . . . . . . . . . . . . . . . . . .  4
     3.2   Syntactic level  . . . . . . . . . . . . . . . . . . . . .  5
       3.2.1   Expressions  . . . . . . . . . . . . . . . . . . . . .  7
       3.2.2   Instructions . . . . . . . . . . . . . . . . . . . . .  8
       3.2.3   Directives . . . . . . . . . . . . . . . . . . . . . .  9
       3.2.4   Labels . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.3   Uploading the bytecode to the UDVM . . . . . . . . . . . . 10
   4.  Compression algorithms . . . . . . . . . . . . . . . . . . . . 12
     4.1   Well-known Compression Algorithms  . . . . . . . . . . . . 12
       4.1.1   Simplified LZ77  . . . . . . . . . . . . . . . . . . . 12
       4.1.2   LZSS . . . . . . . . . . . . . . . . . . . . . . . . . 16
       4.1.3   LZW  . . . . . . . . . . . . . . . . . . . . . . . . . 18
       4.1.4   DEFLATE  . . . . . . . . . . . . . . . . . . . . . . . 21
       4.1.5   LZJH . . . . . . . . . . . . . . . . . . . . . . . . . 24
     4.2   Adapted Algorithms . . . . . . . . . . . . . . . . . . . . 28
       4.2.1   Modified DEFLATE . . . . . . . . . . . . . . . . . . . 28
   5.  Additional SigComp mechanisms  . . . . . . . . . . . . . . . . 31
     5.1   Acknowledging a state item . . . . . . . . . . . . . . . . 32
     5.2   Static dictionary  . . . . . . . . . . . . . . . . . . . . 33
     5.3   CRC checksum . . . . . . . . . . . . . . . . . . . . . . . 34
     5.4   Announcing additional resources  . . . . . . . . . . . . . 34
     5.5   Shared compression . . . . . . . . . . . . . . . . . . . . 36
   6.  Security considerations  . . . . . . . . . . . . . . . . . . . 37
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 37
   8.  Intellectual Property Right Considerations . . . . . . . . . . 37
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 39
   A.  UDVM bytecode for the compression algorithms . . . . . . . . . 39
     A.1   Well-known Algorithms  . . . . . . . . . . . . . . . . . . 39
       A.1.1   Simplified LZ77  . . . . . . . . . . . . . . . . . . . 39
       A.1.2   LZSS . . . . . . . . . . . . . . . . . . . . . . . . . 39
       A.1.3   LZW  . . . . . . . . . . . . . . . . . . . . . . . . . 40
       A.1.4   DEFLATE  . . . . . . . . . . . . . . . . . . . . . . . 40
       A.1.5   LZJH . . . . . . . . . . . . . . . . . . . . . . . . . 40
     A.2   Adapted Algorithms . . . . . . . . . . . . . . . . . . . . 40
       A.2.1   Modified DEFLATE . . . . . . . . . . . . . . . . . . . 40
       Intellectual Property and Copyright Statements . . . . . . . . 42









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1.  Introduction

   This document provides an informational guide for users of the
   SigComp protocol RFC-3320 [4].  The idea behind SigComp is to
   standardize a Universal Decompressor Virtual Machine (UDVM) that can
   be programmed to understand the output of many well-known compressors
   including DEFLATE [10] and LZW [9].  The bytecode for the choice of
   compression algorithm is uploaded to the UDVM as part of the
   compressed data.

   The basic SigComp RFC describes the actions that an endpoint must
   take upon receiving a SigComp message.  However the entity
   responsible for generating new SigComp messages (the SigComp
   compressor) is left as an implementation decision; any compressor can
   be used provided that it generates SigComp messages that can be
   successfully decompressed by the receiving endpoint.

   This document offers a number of different compressors that can be
   used by the SigComp protocol.  It also gives examples of how to use
   some of the mechanisms (such as acknowledgements) described in RFC
   3321 [5].

2.  Overview of the User Guide

   When implementing a SigComp compressor the first step is to choose a
   compression algorithm that can encode the application messages into a
   (hopefully) smaller form.  Since SigComp can upload bytecode for new
   algorithms to the receiving endpoint, arbitrary compression
   algorithms can be supported provided that bytecode has been written
   for the corresponding decompressor.

   This document provides example bytecode for the following algorithms:
   1.  Simplified LZ77
   2.  LZSS
   3.  LZW
   4.  DEFLATE
   5.  LZJH

   Any of the above algorithms may be useful depending on the desired
   compression ratio, processing and memory requirements, code size,
   implementation complexity and Intellectual Property (IPR)
   considerations.

   As well as encoding the application messages using the chosen
   algorithm, the SigComp compressor is responsible for ensuring that
   messages can be correctly decompressed even if packets are lost or
   misordered during transmission.  The SigComp feedback mechanism can
   be used to acknowledge successful decompression at the remote



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   endpoint.

   The following robustness techniques and other mechanisms specific to
   the SigComp environment are covered in this document:
   1.  Acknowledgements using the SigComp feedback mechanism
   2.  Static dictionary
   3.  CRC checksum
   4.  Announcing additional resources
   5.  Shared compression

   Any or all of the above mechanisms can be implemented in conjunction
   with the chosen compression algorithm.  A subroutine of UDVM bytecode
   is provided for each of the mechanisms; these subroutines can be
   added to the bytecode for one of the basic compression algorithms.

3.  UDVM assembly language

   Writing UDVM programs directly in bytecode would be a daunting task,
   so a simple assembly language is provided to facilitate the creation
   of new decompression algorithms.  The assembly language includes
   mnemonic codes for each of the UDVM instructions, as well as simple
   directives for evaluating integer expressions, padding the bytecode
   and so forth.

   The syntax of the UDVM assembly language uses the customary two-level
   description technique, partitioning the grammar into a lexical and a
   syntactical level.

3.1  Lexical level

   On a lexical level, a string of assembly consists of zero or more
   tokens optionally separated by whitespace.  Each token can be a text
   name, an instruction opcode, a delimiter, or an integer (specified as
   decimal, binary or hex).

   The following ABNF description RFC-2234 [3] specifies the syntax of a
   token:














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   token            =     (name / opcode / delimiter / dec / bin / hex)
   name             =     (lowercase / "_") 0*(lowercase / digit / "_")
   opcode           =     uppercase *(uppercase / digit / "-")
   delimiter        =     "." / "," / "!" / "$" / ":" / "(" / ")" /
                          operator
   dec              =     1*(digit)
   bin              =     "0b" 1*("0" / "1")
   hex              =     "0x" 1*(hex_digit)
   hex_digit        =     digit / %x41-46 / %x61-66
   digit            =     %x30-39
   uppercase        =     %x41-5a
   lowercase        =     %x61-7a
   operator         =     "+" / "-" / "*" / "/" / "%" / "&" / "|" /
                          "^" / "~" / "<<" / ">>"

   When parsing for tokens a longest match is applied, i.e. a token is
   the longest string that matches the <token> rule specified above.

   The syntax of whitespace and comments is specified by the following
   ABNF:


   ws               =     *(%x09 / %x0a / %x0d / %x20 / comment)
   comment          =     ";" *(%x00-09 / %x0b-0c / %x0e-ff)
                          (%x0a / %x0d)

   Whitespace that matches <ws> is skipped between tokens, but serves to
   terminate the longest match for a token.

   Comments are specified by the symbol ";" and are terminated by the
   end of the line, for example:

   LOAD (temp, 1)             ; This is a comment.

   Any other input is a syntax error.

   When parsing on the lexical level the string of assembly should be
   divided up into a list of successive tokens.  The whitespace and
   comments should also be deleted.  The assembly should then be parsed
   on the syntactic level as explained in Section 3.2.

3.2  Syntactic level

   Once the string of assembly has been divided into tokens as per
   Section 3.1, the next step is to convert the assembly into a string
   of UDVM bytecode.

   On a syntactic level, a string of assembly consists of zero or more



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   instructions, directives or labels, each of which is itself built up
   from one or more lexical tokens.

   The following ABNF description specifies the syntax of the assembly
   language.  Note that the lexical parsing step is assumed to have been
   carried out, so in particular the boundaries between tokens are
   already known and the comments and whitespace have been deleted:


   assembly          =    *(instruction / directive / label)
   instruction       =    opcode ["(" operand *("," operand) ")"]
   operand           =    [["$"] expression]
                              ; Operands can be left blank if they can
                              ; be automatically inferred by the
                              ; compiler, e.g. a literal operand
                              ; that specifies the total number of
                              ; operands for the instruction.
                              ; When "$" is prepended to an operand,
                              ; the corresponding integer is an
                              ; address rather than the actual operand
                              ; value. This symbol is mandatory for
                              ; reference operands, optional for
                              ; multitypes and addresses, and
                              ; disallowed for literals.
   label             =    ":" name
   directive         =    padding / data / set / readonly /
                          unknown_directive
   unknown_directive =    name ["(" expression *("," expression) ")"]
                              ; The parser can ignore unknown
                              ; directives.  The resulting bytecode
                              ; may or may not generate the expected
                              ; results.
   padding           =    ("pad" / "align" / "at") "(" expression ")"
   data              =    ("byte" / "word") "(" expression *(","
                          expression) ")"
   readonly          =    "readonly" "(" "0" / "1" ")"
   set               =    "set" "(" name "," expression ")"
   expression        =    value / "(" expression operator expression ")"
   value             =    dec / bin / hex / name / "." / "!"
                              ; "." is the location of this
                              ; instruction/directive, whereas "!" is
                              ; the location of the closest
                              ; DECOMPRESSION-FAILURE

   The following sections define how to convert the instructions, labels
   and directives into UDVM bytecode:





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3.2.1  Expressions

   The operand values needed by particular instructions or directives
   can be given in the form of expressions.  An expression can include
   one or more values specified as decimal, binary or hex (binary values
   are preceded by "0b" and hex values are preceded by "0x").  The
   expression may also include one or more of the following operators:


          "+"    Addition
          "-"    Subtraction
          "*"    Multiplication
          "/"    Integer division
          "%"    Modulo arithmetic (a%b := a modulo b)
          "&"    Binary AND
          "|"    Binary OR
          "^"    Binary XOR
          "~"    Binary XNOR
          "<<"   Binary LSHIFT
          ">>"   Binary RSHIFT

   The operands for each operator must always be surrounded by
   parentheses so that the order in which the operators should be
   evaluated is clear.  For example:

   ((1 + (2 * 3)) & (0xabcd - 0b00101010)) gives the result 3.

   Expressions can also include the special values "." and "!".  When
   the symbol "." is encountered, it is replaced by the location in the
   bytecode of the current instruction/directive.  When the symbol "!"
   is encountered it is replaced by the location in the bytecode of the
   closest DECOMPRESSION-FAILURE instruction (i.e. the closest zero
   byte).  This can be useful when writing UDVM instructions that call a
   decompression failure, for example:

   INPUT-BYTES (1, temp, !)

   The above instruction causes a decompression failure to occur if it
   tries to input data from beyond the end of the compressed message.

   N.B. When using "!" in the assembly language to generate bytecode,
   care must be taken to ensure that the address of the zero used at
   bytecode generation time will still contain zero when the bytecode is
   run.  The readonly directive (see Section 3.2.3) can be used to do
   this.

   It is also possible to assign integer values to text names: when a
   text name is encountered in an expression it is replaced by the



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   integer value assigned to it.  Section 3.2.3 explains how to assign
   integer values to text names.

3.2.2  Instructions

   A UDVM instruction is specified by the instruction opcode followed by
   zero or more operands.  The instruction operands are enclosed in
   parentheses and separated by commas, for example:

   ADD (3, 4)

   When generating the bytecode the parser should replace the
   instruction opcode with the corresponding 1-byte value as per Figure
   11 of SigComp [4].

   Each operand consists of an expression which evaluates to an integer,
   optionally preceded by the symbol "$".  This symbol indicates that
   the supplied integer value must be interpreted as the memory address
   at which the operand value can be found, rather than the actual
   operand value itself.

   When converting each instruction operand to bytecode, the parser
   first determines whether the instruction expects the operand to be a
   literal, a reference, a multitype or an address.  If the operand is a
   literal then as per Figure 8 of SigComp, the parser inserts bytecode
   (usually the shortest) capable of encoding the supplied operand
   value.

   Since literal operands are used to indicate the total number of
   operands for an instruction, it is possible to leave a literal
   operand blank and allow its value to be inferred automatically by the
   assembler.  For example:

   MULTILOAD (64, , 1, 2, 3, 4)

   The missing operand should be given the value 4 because it is
   followed by a total of 4 operands.

   If the operand is a reference then as per Figure 9 of SigComp, the
   parser inserts bytecode (ususally the shortest) capable of encoding
   the supplied memory address.  Note that reference operands will
   always be preceded by the symbol "$" in assembly because they always
   encode memory addresses rather than actual operand values.

   If the operand is a multitype then the parser first checks whether
   the symbol "$" is present.  If so then, as per Figure 10 of SigComp,
   it inserts bytecode (usually the shortest) capable of encoding the
   supplied integer as a memory address.  If not then, as per Figure 10



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   of SigComp, it inserts bytecode (usually the shortest) that encodes
   the supplied integer as an operand value.

   If the operand is an address then the parser checks whether the
   symbol "$" is present.  If so then the supplied integer is encoded as
   a memory address, just as for the multitype instruction above.  If
   not then the byte position of the opcode is subtracted from the
   supplied integer modulo 16, and the result is encoded as an operand
   value as per Figure 10 of SigComp.

   The length of the resulting bytecode is dependent on the parser in
   use.  There can be several correct and useable representations of the
   same instruction.

3.2.3  Directives

   The assembly language provides a number of directives for evaluating
   expressions, moving instructions to a particular memory address etc.

   The directives "pad", "align" and "at" can be used to add padding to
   the bytecode.

   The directive "pad (n)" appends n consecutive padding bytes to the
   bytecode.  The actual value of the padding bytes is unimportant, so
   when the bytecode is uploaded to the UDVM the padding bytes can be
   set to the initial values contained in the UDVM memory (this helps to
   reduce the size of a SigComp message).

   The directive "align (n)" appends the minimum number of padding bytes
   to the bytecode such that the total bytecode generated so far is
   aligned to a multiple of n bytes.  If the bytecode is already aligned
   to a multiple of n bytes then no padding bytes are added.

   The directive "at (n)" appends enough padding bytes to the bytecode
   such that the total bytecode generated so far is exactly n bytes.  If
   more than n bytes have already been generated before the "at"
   directive is encountered then the assembly code contains an error.

   The directives "byte" and "word" can be used to add specific data
   strings to the bytecode.

   The directive "byte (n[0],..., n[k-1])" appends k consecutive bytes
   to the bytecode.  The byte string is supplied as expressions which
   evaluate to give integers n[0],..., n[k-1] from 0 to 255.

   The directive "word (n[0],..., n[k-1])" appends k consecutive 2-byte
   words to the bytecode.  The word string is supplied as expressions
   which evaluate to give integers n[0],..., n[k-1] from 0 to 65535.



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   The directive "set (name, n)" assigns an integer value n to a
   specified text name.  The integer value can be supplied in the form
   of an expression.

   The directive "readonly (n)" where n is 0 or 1 can be used to
   indicate that an area of memory could be changed (0) or will not be
   changed (1) during the execution of the UDVM.  This directive could
   be used, for example, in conjunction with "!" to ensure that the
   address of the zero used will still contain zero when the bytecode is
   executed.  If no readonly directive is used, then any address
   containing zero can be used by "!" (i.e. by default there is assumed
   to be readonly(1) directive at address 0) and it is up to the author
   of the assembly code to ensure that the address in question will
   still contain zero when the bytecode is executed.  If the readonly
   directive is used, then bytes between a readonly (0) and readonly (1)
   pair are NOT to be used by "!".  When a readonly directive has been
   used, the bytes obey that directive from that address to either
   another readonly directive or the end of UDVM memory, whichever comes
   first.

3.2.4  Labels

   A label is a special directive used to assign memory addresses to
   text names.

   Labels are specified by giving a single colon followed by the text
   name to be defined.  The (absolute) position of the byte immediately
   following the label is evaluated and assigned to the text name.  For
   example:


   :start

   LOAD (temp, 1)



   Since the label "start" occurs at the beginning of the bytecode, it
   is assigned the integer value 0.

   Note that writing the label ":name" has exactly the same behavior as
   writing the directive "set (name, .)".

3.3  Uploading the bytecode to the UDVM

   Once the parser has converted a string of assembly into the
   corresponding bytecode, it must be copied to the UDVM memory
   beginning at Address 0 and then executed beginning from the first



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   UDVM instruction in the bytecode.

   SigComp provides the following message format for uploading bytecode
   to the UDVM:


     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1   1   1   1   1 | T |   0   |
   +---+---+---+---+---+---+---+---+
   |                               |
   :    returned feedback item     :  if T = 1
   |                               |
   +---+---+---+---+---+---+---+---+
   |           code_len            |
   +---+---+---+---+---+---+---+---+
   |   code_len    |  destination  |
   +---+---+---+---+---+---+---+---+
   |                               |
   :    uploaded UDVM bytecode     :
   |                               |
   +---+---+---+---+---+---+---+---+
   |                               |
   :   remaining SigComp message   :
   |                               |
   +---+---+---+---+---+---+---+---+



   The destination field should be set to the memory address of the
   first UDVM instruction.  Note that if this address cannot be
   represented by the destination field then the bytecode cannot be
   uploaded to the UDVM using the standard SigComp header.  In
   particular, the memory address of the first UDVM instruction must
   always be a multiple of 64 bytes or the standard SigComp header
   cannot be used.  Of course, there may be other ways to upload the
   bytecode to the UDVM, such as retrieving the bytecode directly via
   the INPUT-BYTES instruction.

   Additionally, all memory addresses between Address 0 and Address 31
   inclusive are initialized to endpoint-specific values by the UDVM, so
   they must be specified as padding in the bytecode or the standard
   SigComp header cannot be used.  Memory addresses from Address 32 to
   Address (destination - 1) inclusive are initialized to 0, so they
   must be specified either as padding or as 0s if the bytecode is to be
   successfully uploaded using the standard SigComp header.

   The code_len field should be set to the smallest value such that all



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   memory addresses beginning at Address (destination + code_len) are
   either padding, set to 0 by the bytecode, or are beyond the total
   length of the bytecode.

   The "uploaded UDVM bytecode" should be set to contain the segment of
   bytecode that lies between Address (destination) and Address
   (destination + code_len - 1) inclusive.

4.  Compression algorithms

   This chapter describes a number of compression algorithms that can be
   used by a SigComp compressor.  In each case the document provides
   UDVM bytecode for the corresponding decompression algorithm, which
   can be uploaded to the receiving endpoint as part of a SigComp
   message.  Each algorithm (as written in this chapter) assumes that
   there is 16K decompression memory size, 16 cycles per bit and 8K
   state memory size.  Decompression will succeed with a smaller value
   for state memory size, however, the full state will not be created.

   Section 4.1.1 covers a simple algorithm in some detail, including the
   steps required to compress and decompress a SigComp message.  The
   remaining sections cover well-known compression algorithms that can
   be adapted for use in SigComp with minimal modification.

4.1  Well-known Compression Algorithms

4.1.1  Simplified LZ77

   This section describes how to implement a very simple compression
   algorithm based on LZ77 [7].

   A compressed message generated by the simplified LZ77 scheme consists
   of a sequence of 4-byte characters, where each character contains a
   2-byte position value followed by a 2-byte length value.  Each pair
   of integers identifies a byte string in the UDVM memory; when
   concatenated these byte strings form the decompressed message.

   When implementing a bytecode decompressor for the simplified LZ77
   scheme, the UDVM memory is partitioned into five distinct areas as
   shown below:


   0             64          128        256          512
   | scratch-pad | variables | bytecode | dictionary | circular buffer |
   +-------------+-----------+----------+------------+-----------------+
    <-----------> <---------> <--------> <----------> <--------------->
       64 bytes     64 bytes   128 bytes   256 bytes      512+ bytes




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   The first 128 bytes are used to hold the 2-byte variables needed by
   the LZ77 decompressor.  Within this memory the first 64 bytes is used
   as a scratch-pad, holding the 2-byte variables that can be discarded
   between SigComp messages.  In contrast the next 64 bytes (and in fact
   all of the UDVM memory starting from Address 64) should be saved
   after decompressing a SigComp message to improve the compression
   ratio of subsequent messages.

   The bytecode for the LZ77 decompressor is stored beginning at Address
   128.  A total of 128 bytes are reserved for the bytecode although the
   LZ77 decompressor requires less; this allows room for adding
   additional features to the decompressor at a later stage.

   The next 256 bytes are initialized by the bytecode to contain the
   integers 0 to 255 inclusive.  The purpose of this memory area is to
   provide a dictionary of all possible uncompressed characters; this is
   important to ensure that the compressor can always generate a
   sequence of position/length pairs that encode a given message.  For
   example, a byte with value 0x41 (corresponding to the ASCII character
   "A") can be found at Address 0x0141 of the UDVM memory, so the
   compressed character 0x0141 0001 will decompress to give this ASCII
   character.  Note that encoding each byte in the application message
   as a separate 4-byte compressed character is not recommended,
   however, as the resulting "compressed" message is four times as large
   as the original uncompressed message.

   The compression ratio of LZ77 is improved by the remaining UDVM
   memory, which is used to store a history buffer containing the
   previously decompressed messages.  Compressed characters can point to
   strings that have previously been decompressed and stored in the
   buffer; so the overall compression ratio of the LZ77 algorithm
   improves as the decompressor "learns" more text strings and is able
   to encode longer strings using a single compressed character.  The
   buffer is circular, so older messages are overwritten by new data
   when the buffer becomes full.

   The steps required to implement an LZ77 compressor and decompressor
   are similar, although compression is more processor-intensive as it
   requires a searching operation to be performed.  Assembly for the
   simplified LZ77 decompressor is given below:



   ; Variables that do not need to be stored after decompressing each
   ; SigComp message are stored here:

   at (32)




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   :index                          pad (2)
   :length_value                   pad (2)

   at (42)

   set (requested_feedback_location, 0)

   ; The UDVM registers must be stored beginning at Address 64:

   at (64)

   ; Variables that should be stored after decompressing a message are
   ; stored here. These variables will form part of the SigComp state
   ; item created by the bytecode:

   :byte_copy_left                 pad (2)
   :byte_copy_right                pad (2)
   :decompressed_pointer           pad (2)

   set (returned_parameters_location, 0)

   align (64)

   :initialize_memory

   set (udvm_memory_size, 8192)
   set (state_length, (udvm_memory_size - 64))

   ; The UDVM registers byte_copy_left and byte_copy_right are set to
   ; indicate the bounds of the circular buffer in the UDVM memory. A
   ; variable decompressed_pointer is also created and set pointing to
   ; the start of the circular buffer:

   MULTILOAD (64, 3, circular_buffer, udvm_memory_size, circular_buffer)

   ; The "dictionary" area of the UDVM memory is initialized to contain
   ; the values 0 to 255 inclusive:

   MEMSET (static_dictionary, 256, 0, 1)

   :decompress_sigcomp_message

   :next_character

   ; The next character in the compressed message is read by the UDVM
   ; and the position and length integers are stored in the variables
   ; position_value and length_value respectively. If no more
   ; compressed data is available the decompressor jumps to the



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   ; "end_of_message" subroutine:

   INPUT-BYTES (4, index, end_of_message)

   ; The position_value and length_value point to a byte string in the
   ; UDVM memory, which is copied into the circular buffer at the
   ; position specified by decompressed_pointer. This allows the string
   ; to be referenced by later characters in the compressed message:

   COPY-LITERAL ($index, $length_value, $decompressed_pointer)

   ; The byte string is also outputted onto the end of the decompressed
   ; message:

   OUTPUT ($index, $length_value)

   ; The decompressor jumps back to consider the next character in the
   ; compressed message:

   JUMP (next_character)

   :end_of_message

   ; The decompressor saves the UDVM memory and halts:

   END-MESSAGE (requested_feedback_location,
   returned_parameters_location, state_length, 64,
   decompress_sigcomp_message, 6, 0)

   at (256)

   ; Memory for the dictionary and the circular buffer are reserved by
   ; the following statements:

   :static_dictionary              pad (256)
   :circular_buffer



   The task of an LZ77 compressor is simply to discover a sequence of 4-
   byte compressed characters which the above bytecode will decompress
   to give the desired application message.  As an example, a message
   compressed using the simplified LZ77 algorithm is given below:

   0x0154 0001 0168 0001 0165 0001 0120 0001 0152 0001 0165 0001 0173
   0x0002 0161 0001 0175 0001 0172 0001 0161 0001 016e 0001 0174 0001
   0x0120 0001 0161 0001 020d 0002 0174 0001 0201 0003 0145 0001 016e
   0x0001 0164 0001 0120 0001 016f 0001 0166 0001 0211 0005 0155 0001



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   0x016e 0001 0169 0001 0176 0001 0165 0001 0172 0002 0165 0001 010a
   0x0001

   The uncompressed message is "The Restaurant at the End of the
   Universe\n".

   The bytecode for the LZ77 decompressor can be uploaded as part of the
   compressed message as specified in Section 3.3.  However, in order to
   improve the overall compression ratio it is important to avoid
   uploading bytecode in every compressed message.  For this reason
   SigComp allows the UDVM to save an area of its memory as a state item
   between compressed messages.  Once a state item has been created it
   can be retrieved by sending the corresponding state identifier using
   the following SigComp message format:


     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1   1   1   1   1 | T |   1   |
   +---+---+---+---+---+---+---+---+
   |                               |
   :    returned feedback item     :  if T = 1
   |                               |
   +---+---+---+---+---+---+---+---+
   |                               |
   :   partial state identifier    :
   |                               |
   +---+---+---+---+---+---+---+---+
   |                               |
   :   remaining SigComp message   :
   |                               |
   +---+---+---+---+---+---+---+---+



   The partial_state_identifier field must contain the first 6 bytes of
   the state identifier for the state item to be accessed (see [4] for
   details of how state identifiers are derived).

4.1.2  LZSS

   This section provides UDVM bytecode for the simple but effective LZSS
   compression algorithm [8].

   The principal improvement offered by LZSS over LZ77 is that each
   compressed character begins with a 1-bit indicator flag to specify
   whether the character is a literal or an offset/length pair.  A
   literal value is simply a single uncompressed byte that is appended



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   directly to the decompressed message.

   An offset/length pair contains a 12-bit offset value from 1 to 4096
   inclusive, followed by a 4-bit length value from 3 to 18 inclusive.
   Taken together these values specify one of the previously received

   text strings in the circular buffer, which is then appended to the
   end of the decompressed message.

   Assembly for an LZSS decompressor is given below:


   at (32)
   readonly (0)

   :index                          pad (2)
   :length_value                   pad (2)
   :old_pointer                    pad (2)

   at (42)

   set (requested_feedback_location, 0)

   at (64)

   :byte_copy_left                 pad (2)
   :byte_copy_right                pad (2)
   :input_bit_order                pad (2)
   :decompressed_pointer           pad (2)

   set (returned_parameters_location, 0)

   align (64)
   readonly (1)

   :initialize_memory

   set (udvm_memory_size, 8192)
   set (state_length, (udvm_memory_size - 64))

   MULTILOAD (64, 4, circular_buffer, udvm_memory_size, 0,
   circular_buffer)

   :decompress_sigcomp_message

   :next_character

   INPUT-HUFFMAN (index, end_of_message, 2, 9, 0, 255, 16384, 4, 4096,



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   8191, 1)
   COMPARE ($index, 8192, length, end_of_message, literal)

   :literal

   set (index_lsb, (index + 1))

   OUTPUT (index_lsb, 1)
   COPY-LITERAL (index_lsb, 1, $decompressed_pointer)
   JUMP (next_character)

   :length

   INPUT-BITS (4, length_value, !)
   ADD ($length_value, 3)
   LOAD (old_pointer, $decompressed_pointer)
   COPY-OFFSET ($index, $length_value, $decompressed_pointer)
   OUTPUT ($old_pointer, $length_value)
   JUMP (next_character)

   :end_of_message

   END-MESSAGE (requested_feedback_location,
   returned_parameters_location, state_length, 64,
   decompress_sigcomp_message, 6, 0)

   readonly (0)
   :circular_buffer


   An example message compressed using the LZSS algorithm is given
   below:

   0x279a 0406 e378 b200 6074 1018 4ce6 1349 b842

   The uncompressed message is "Oh no, not again!".

4.1.3  LZW

   This section provides UDVM bytecode for the well-known LZW
   compression algorithm LZW [9].  This algorithm is used in a number of
   standards including the GIF image format.

   LZW compression operates in a similar manner to LZ77 in that it
   maintains a circular buffer of previously received decompressed data,
   and each compressed character references exactly one byte string from
   the circular buffer.  However, LZW also maintains a "codebook"
   containing 1024 position/length pairs that point to byte strings



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   which LZW believes are most likely to occur in the uncompressed data.

   The byte strings stored in the LZW codebook can be referenced by
   sending a single 10-bit value from 0 to 1023 inclusive.  The UDVM
   extracts the corresponding text string from the codebook and appends
   it to the end of the decompressed message.  It then creates a new
   codebook entry containing the current text string plus the next
   character to occur in the decompressed message.

   Assembly for an LZW decompressor is given below:


   at (32)

   :length_value                   pad (2)
   :position_value                 pad (2)
   :index                          pad (2)

   at (42)

   set (requested_feedback_location, 0)

   at (64)

   :byte_copy_left                 pad (2)
   :byte_copy_right                pad (2)
   :input_bit_order                pad (2)

   :codebook_next                  pad (2)
   :current_length                 pad (2)
   :decompressed_pointer           pad (2)

   set (returned_parameters_location, 0)

   align (64)

   :initialize_memory

   set (udvm_memory_size, 8192)
   set (state_length, (udvm_memory_size - 64))

   MULTILOAD (64, 6, circular_buffer, udvm_memory_size, 0, codebook, 1,
   static_dictionary)

   :initialize_codebook

   ; The following instructions are used to initialize the first 256
   ; entries in the LZW codebook with single ASCII characters:



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   set (index_lsb, (index + 1))
   set (current_length_lsb, (current_length + 1))

   COPY-LITERAL (current_length_lsb, 3, $codebook_next)
   COPY-LITERAL (index_lsb, 1, $decompressed_pointer)
   ADD ($index, 1)
   COMPARE ($index, 256, initialize_codebook, next_character, 0)

   :decompress_sigcomp_message

   :next_character

   ; The following INPUT-BITS instruction extracts 10 bits from the
   ; compressed message:

   INPUT-BITS (10, index, end_of_message)

   ; The following instructions interpret the received bits as an index
   ; into the LZW codebook, and extract the corresponding
   ; position/length pair:

   set (length_value_lsb, (length_value + 1))

   MULTIPLY ($index, 3)
   ADD ($index, codebook)
   COPY ($index, 3, length_value_lsb)

   ; The following instructions append the selected text string to the
   ; circular buffer and create a new codebook entry pointing to this
   ; text string:

   LOAD (current_length, 1)
   ADD ($current_length, $length_value)
   COPY-LITERAL (current_length_lsb, 3, $codebook_next)
   COPY-LITERAL ($position_value, $length_value, $decompressed_pointer)

   ; The following instruction outputs the text string specified by the
   ; position/length pair:

   OUTPUT ($position_value, $length_value)
   JUMP (next_character)

   :end_of_message

   END-MESSAGE (requested_feedback_location,
   returned_parameters_location, state_length, 64,
   decompress_sigcomp_message, 6, 0)




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   :static_dictionary              pad (256)
   :circular_buffer

   at (4492)

   :codebook


   An example message compressed using the LZW algorithm is given below:
   0x14c6 f080 6c1b c6e1 9c20 1846 e190 201d 0684 206b 1cc2 0198 6f1c
   0x9071 b06c 42c6 8195 111a 4731 a021 02bf f0

   The uncompressed message is "So long and thanks for all the fish!\n".

4.1.4  DEFLATE

   This section provides UDVM bytecode for the DEFLATE compression
   algorithm.  DEFLATE is the algorithm used in the well-known "gzip"
   file format.

   The following bytecode will decompress the DEFLATE compressed data
   format DEFLATE [10] with the following modifications:
   1.  The DEFLATE compressed data format separates blocks of compressed
       data by transmitting 7 consecutive zero bits.  Each SigComp
       message is assumed to contain a separate block of compressed
       data, so the end-of-block bits are implicit and do not need to be
       transmitted at the end of a SigComp message.
   2.  This bytecode supports only DEFLATE block type 01 (data
       compressed with fixed Huffman codes).

   Assembly for the DEFLATE decompressor is given below:


   at (32)
   readonly (0)

   :index                          pad (2)
   :extra_length_bits              pad (2)
   :length_value                   pad (2)
   :extra_distance_bits            pad (2)
   :distance_value                 pad (2)

   at (42)

   set (requested_feedback_location, 0)

   at (64)




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   :byte_copy_left                 pad (2)
   :byte_copy_right                pad (2)
   :input_bit_order                pad (2)
   :decompressed_pointer           pad (2)

   :length_table                   pad (116)
   :distance_table                 pad (120)

   set (returned_parameters_location, 0)

   align (64)

   readonly (1)
   :initialize_memory

   set (udvm_memory_size, 8192)
   set (state_length, (udvm_memory_size - 64))
   set (length_table_start, (((length_table - 4) + 65536) / 4))
   set (length_table_mid, (length_table_start + 24))
   set (distance_table_start, (distance_table / 4))

   MULTILOAD (64, 122, circular_buffer, udvm_memory_size, 5,
   circular_buffer,

   0,       3,       0,       4,       0,       5,
   0,       6,       0,       7,       0,       8,
   0,       9,       0,       10,      1,       11,
   1,       13,      1,       15,      1,       17,
   2,       19,      2,       23,      2,       27,
   2,       31,      3,       35,      3,       43,
   3,       51,      3,       59,      4,       67,
   4,       83,      4,       99,      4,       115,
   5,       131,     5,       163,     5,       195,
   5,       227,     0,       258,

   0,       1,       0,       2,       0,       3,
   0,       4,       1,       5,       1,       7,
   2,       9,       2,       13,      3,       17,
   3,       25,      4,       33,      4,       49,
   5,       65,      5,       97,      6,       129,
   6,       193,     7,       257,     7,       385,
   8,       513,     8,       769,     9,       1025,
   9,       1537,    10,      2049,    10,      3073,
   11,      4097,    11,      6145,    12,      8193,
   12,      12289,   13,      16385,   13,      24577)

   :decompress_sigcomp_message




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   INPUT-BITS (3, extra_length_bits, !)

   :next_character

   INPUT-HUFFMAN (index, end_of_message, 4,
       7, 0, 23, length_table_start,
       1, 48, 191, 0,
       0, 192, 199, length_table_mid,
       1, 400, 511, 144)
   COMPARE ($index, length_table_start, literal, end_of_message,
   length_distance)

   :literal

   set (index_lsb, (index + 1))

   OUTPUT (index_lsb, 1)
   COPY-LITERAL (index_lsb, 1, $decompressed_pointer)
   JUMP (next_character)

   :length_distance

   ; this is the length part

   MULTIPLY ($index, 4)
   COPY ($index, 4, extra_length_bits)
   INPUT-BITS ($extra_length_bits, extra_length_bits, !)
   ADD ($length_value, $extra_length_bits)

   ; this is the distance part

   INPUT-HUFFMAN (index, !, 1, 5, 0, 31, distance_table_start)
   MULTIPLY ($index, 4)
   COPY ($index, 4, extra_distance_bits)

   INPUT-BITS ($extra_distance_bits, extra_distance_bits, !)
   ADD ($distance_value, $extra_distance_bits)
   LOAD (index, $decompressed_pointer)
   COPY-OFFSET ($distance_value, $length_value, $decompressed_pointer)
   OUTPUT ($index, $length_value)
   JUMP (next_character)

   :end_of_message

   END-MESSAGE (requested_feedback_location,
   returned_parameters_location, state_length, 64,
   decompress_sigcomp_message, 6, 0)




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   readonly (0)
   :circular_buffer


   An example message compressed using the DEFLATE algorithm is given
   below:

   0xf3c9 4c4b d551 28c9 4855 08cd cb2c 4b2d 2a4e 5548 cc4b 5170 0532
   0x2b4b 3232 f3d2 b900

   The uncompressed message is "Life, the Universe and Everything\n".

4.1.5  LZJH

   This section provides UDVM bytecode for the LZJH compression
   algorithm.  LZJH is the algorithm adopted by the International
   Telecommunication Union (ITU-T) Recommendation V.44 LZJH [11].

   Assembly for the LZJH decompressor is given below:


   at (32)
   readonly (0)

   ; The following 2-byte variables are stored in the scratch-pad memory
   ; area because they do not need to be saved after decompressing a
   ; SigComp message:

   :length_value                   pad (2)
   :position_value                 pad (2)
   :index                          pad (2)
   :extra_extension_bits           pad (2)
   :codebook_old                   pad (2)

   at (42)

   set (requested_feedback_location, 0)

   at (64)

   ; UDVM_registers

   :byte_copy_left                 pad (2)
   :byte_copy_right                pad (2)

   :input_bit_order                pad (2)

   ; The following 2-byte variables are saved as state after



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   ; decompressing a SigComp message:

   :current_length                 pad (2)
   :decompressed_pointer           pad (2)
   :ordinal_length                 pad (2)
   :codeword_length                pad (2)
   :codebook_next                  pad (2)

   set (returned_parameters_location, 0)

   align (64)
   readonly (1)

   :initialize_memory

   ; The following constants can be adjusted to configure the LZJH
   ; decompressor. The current settings are as recommended in the V.44
   ; specification (given that a total of 8K UDVM memory is available):

   set (udvm_memory_size, 8192)   ; sets the total memory for LZJH
   set (max_extension_length, 8)  ; sets the maximum string extension
   set (min_ordinal_length, 7)    ; sets the minimum ordinal length
   set (min_codeword_length, 6)   ; sets the minimum codeword length

   set (codebook_start, 4492)
   set (first_codeword, (codebook_start - 12))
   set (state_length, (udvm_memory_size - 64))

   MULTILOAD (64, 8, circular_buffer, udvm_memory_size, 7, 0,
   circular_buffer, min_ordinal_length, min_codeword_length,
   codebook_start)

   :decompress_sigcomp_message

   :standard_prefix

   ; The following code decompresses the standard 1-bit LZJH prefix
   ; which specifies whether the next character is an ordinal or a
   ; codeword/control value:

   INPUT-BITS (1, index, end_of_message)
   COMPARE ($index, 1, ordinal, codeword_control, codeword_control)

   :prefix_after_codeword

   ; The following code decompresses the special LZJH prefix that only
   ; occurs after a codeword. It specifies whether the next character is
   ; an ordinal, a codeword/control value, or a string extension:



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   INPUT-HUFFMAN (index, end_of_message, 2, 1, 1, 1, 2, 1, 0, 1, 0)
   COMPARE ($index, 1, ordinal, string_extension, codeword_control)

   :ordinal

   ; The following code decompresses an ordinal character, and creates
   ; a new codebook entry consisting of the ordinal character plus the
   ; next character to be decompressed:

   set (index_lsb, (index + 1))
   set (current_length_lsb, (current_length + 1))

   INPUT-BITS ($ordinal_length, index, !)
   OUTPUT (index_lsb, 1)
   LOAD (current_length, 2)
   COPY-LITERAL (current_length_lsb, 3, $codebook_next)
   COPY-LITERAL (index_lsb, 1, $decompressed_pointer)
   JUMP (standard_prefix)

   :codeword_control

   ; The following code decompresses a codeword/control value:

   INPUT-BITS ($codeword_length, index, !)
   COMPARE ($index, 3, control_code, initialize_memory, codeword)

   :codeword

   ; The following code interprets a codeword as an index into the LZJH
   ; codebook. It extracts the position/length pair from the specified
   ; codebook entry; the position/length pair points to a byte string
   ; in the circular buffer which is then copied to the end of the
   ; decompressed message. The code also creates a new codebook entry
   ; consisting of the byte string plus the next character to be
   ; decompressed:

   set (length_value_lsb, (length_value + 1))

   MULTIPLY ($index, 3)
   ADD ($index, first_codeword)
   COPY ($index, 3, length_value_lsb)
   LOAD (current_length, 1)
   ADD ($current_length, $length_value)
   LOAD (codebook_old, $codebook_next)
   COPY-LITERAL (current_length_lsb, 3, $codebook_next)
   COPY-LITERAL ($position_value, $length_value, $decompressed_pointer)
   OUTPUT ($position_value, $length_value)
   JUMP (prefix_after_codeword)



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   :string_extension

   ; The following code decompresses a Huffman-encoded string extension:

   INPUT-HUFFMAN (index, !, 4, 1, 1, 1, 1, 2, 1, 3, 2, 1, 1, 1, 13, 3,
   0, 7, 5)
   COMPARE ($index, 13, continue, extra_bits, extra_bits)

   :extra_bits

   INPUT-BITS (max_extension_length, extra_extension_bits, !)
   ADD ($index, $extra_extension_bits)

   :continue

   ; The following code extends the most recently created codebook entry
   ; by the number of bits specified in the string extension:

   COPY-LITERAL ($position_value, $length_value, $position_value)
   COPY-LITERAL ($position_value, $index, $decompressed_pointer)
   OUTPUT ($position_value, $index)
   ADD ($index, $length_value)
   COPY (index_lsb, 1, $codebook_old)
   JUMP (standard_prefix)

   :control_code

   ; The code can handle all of the control characters in V.44 except
   ; for ETM (Enter Transparent Mode), which is not required for
   ; message-based protocols such as SigComp.

   COMPARE ($index, 1, !, flush, stepup)

   :flush

   ; The FLUSH control character jumps to the beginning of the next
   ; complete byte in the compressed message:

   INPUT-BYTES (0, 0, 0)
   JUMP (standard_prefix)

   :stepup

   ; The STEPUP control character increases the number of bits used to
   ; encode an ordinal value or a codeword:

   INPUT-BITS (1, index, !)
   COMPARE ($index, 1, stepup_ordinal, stepup_codeword, 0)



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   :stepup_ordinal

   ADD ($ordinal_length, 1)
   JUMP (ordinal)

   :stepup_codeword

   ADD ($codeword_length, 1)
   JUMP (codeword_control)

   :end_of_message

   END-MESSAGE (requested_feedback_location,
   returned_parameters_location, state_length, 64,
   decompress_sigcomp_message, 6, 0)

   readonly (0)
   :circular_buffer



   An example message compressed using the LZJH algorithm is given
   below:

   0x5c09 e6e0 cadc c8d2 dcce 40c2 40f2 cac2 e440 c825 c840 ccde 29e8
   0xc2f0 40e0 eae4 e0de e6ca e65c 1403

   The uncompressed message is "...spending a year dead for tax
   purposes.\n".

4.2  Adapted Algorithms

4.2.1  Modified DEFLATE

   Alternative algorithms can also be used with SigComp.  This section
   shows a modified version of the DEFLATE [10] algorithm.  The two-
   stage encoding of DEFLATE is replaced by a single step with a
   discrete Huffman code for each symbol.  The literal/length symbol
   probabilities are dependent upon whether the previously symbol was a
   literal or a match.  Bit-handling is also simpler, in that all bits
   are input using the INPUT-HUFFMAN instruction and the value of the H
   bit does not change so all bits are input, read and interpreted in
   the same order.

   Assembly for the algorithm is given below.  String matching rules are
   the same as for the other LZ-based algorithms, with the alternative
   encoding of the literals and length/distance pairs.




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   at (32)
   readonly (0)

   :index                          pad (2)
   :distance_value                 pad (2)
   :old_pointer                    pad (2)

   at (42)

   set (requested_feedback_location, 0)

   at (64)

   :byte_copy_left                 pad (2)
   :byte_copy_right                pad (2)
   :input_bit_order                pad (2)
   :decompressed_pointer           pad (2)

   set (returned_parameters_location, 0)

   at (128)
   readonly (1)

   :initialize_memory

   set (udvm_memory_size, 8192)
   set (state_length, (udvm_memory_size - 64))

   MULTILOAD (64, 4, circular_buffer, udvm_memory_size, 0,
   circular_buffer)

   :decompress_sigcomp_message

   :character_after_literal

   INPUT-HUFFMAN (index, end_of_message, 16,
       5, 0, 11, 46,
       0, 12, 12, 256,
       1, 26, 32, 257,
       1, 66, 68, 32,
       0, 69, 94, 97,
       0, 95, 102, 264,
       0, 103, 103, 511,
       2, 416, 426, 35,
       0, 427, 465, 58,
       0, 466, 481, 272,
       1, 964, 995, 288,
       3, 7968, 7988, 123,



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       0, 7989, 8115, 384,
       1, 16232, 16263, 0,
       0, 16264, 16327, 320,
       1, 32656, 32767, 144)

   COMPARE ($index, 256, literal, distance, distance)

   :character_after_match

   INPUT-HUFFMAN (index, end_of_message, 16,
       4, 0, 0, 511,
       1, 2, 9, 256,
       1, 20, 22, 32,
       0, 23, 30, 264,
       1, 62, 73, 46,
       0, 74, 89, 272,
       2, 360, 385, 97,
       0, 386, 417, 288,
       1, 836, 874, 58,
       0, 875, 938, 320,
       1, 1878, 1888, 35,
       0, 1889, 2015, 384,
       1, 4032, 4052, 123,
       1, 8106, 8137, 0,
       1, 16276, 16379, 144,
       1, 32760, 32767, 248)

   COMPARE ($index, 256, literal, distance, distance)

   :literal

   set (index_lsb, (index + 1))

   OUTPUT (index_lsb, 1)
   COPY-LITERAL (index_lsb, 1, $decompressed_pointer)
   JUMP (character_after_literal)

   :distance

   SUBTRACT ($index, 253)
   INPUT-HUFFMAN (distance_value, !, 9,
       9, 0, 7, 9,
       0, 8, 63, 129,
       1, 128, 135, 1,
       0, 136, 247, 17,
       0, 248, 319, 185,
       1, 640, 1407, 257,
       2, 5632, 6655, 1025,



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       1, 13312, 15359, 2049,
       2, 61440, 65535, 4097)

   LOAD (old_pointer, $decompressed_pointer)
   COPY-OFFSET ($distance_value, $index, $decompressed_pointer)
   OUTPUT ($old_pointer, $index)
   JUMP (character_after_match)

   :end_of_message

   END-MESSAGE (requested_feedback_location,
   returned_parameters_location, state_length, 64,
   decompress_sigcomp_message, 6, 0)

   readonly (0)
   :circular_buffer


   An example message compressed using the modified DEFLATE algorithm is
   given below:

   0xd956 b132 cd68 5424 c5a9 6215 8a70 a64d af0a 5499 3621 509b 3e4c
   0x28b4 a145 b362 653a d0a6 498b 5a6d 2970 ac4c 930a a4ca 74a4 c268
   0x0c

   The uncompressed message is "Arthur leapt to his feet like an author
   hearing the phone ring".

5.  Additional SigComp mechanisms

   The following chapter covers the additional mechanisms that can be
   employed by SigComp to improve the overall compression ratio;
   including the use of acknowledgements, dictionaries and sharing state
   between two directions of a compressed message flow.

   When each of the compression algorithms described in Chapter 4 has
   successfully decompressed the current SigComp message, the contents
   of the UDVM memory are saved as a SigComp state item.  Subsequent
   messages can access this state item by uploading the correct state
   identifier to the receiving endpoint, which avoids the need to upload
   the bytecode for the compression algorithm on a per-message basis.
   However, before a state item can be accessed the compressor must
   first ensure that it is available at the receiving endpoint.

   For each SigComp compartment, the receiving endpoint maintains a list
   of currently available states (where the total amount of state saved
   does not exceed the state_memory_size for the compartment).  The
   SigComp compressor should maintain a similar list containing the



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   states that it has instructed the receiving endpoint to save.

   As well as tracking the list of state items that it has saved at the
   remote endpoint, the compressor also maintains a flag for each state
   item indicating whether the state can safely be accessed or not.
   State items should not be accessed until they have been acknowledged
   (e.g. by using the SigComp feedback mechanism as per Section 5.1).

   State items are deleted from the list when the total
   state_memory_size for the compartment is used up by states of a
   higher priority.  The SigComp compressor should not attempt to access
   any state items that have been deleted in this manner, as they may no
   longer be available at the receiving endpoint.

5.1  Acknowledging a state item

   SigComp [4] defines a feedback mechanism to allow the compressor to
   request feedback from the decompressor, to give the compressor
   indication that a message has been received, correctly decompressed
   and state storage attempted.  (Note, this mechanism cannot convey the
   success or failure of individual state creation requests.)  In order
   to invoke the feedback mechanism the following fields must be
   reserved in the UDVM memory:


     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   |     reserved      | Q | S | I |  requested_feedback_location
   +---+---+---+---+---+---+---+---+
   | 1 | requested_feedback_length |  if Q = 1
   +---+---+---+---+---+---+---+---+
   |                               |
   :   requested_feedback_field    :  if Q = 1
   |                               |
   +---+---+---+---+---+---+---+---+



   These fields can be reserved in any of the algorithms of Chapter 4 by
   replacing the line "set (requested_feedback_location, 0)" with the
   following assembly:


   :requested_feedback_location    pad (1)
   :requested_feedback_length      pad (1)
   :requested_feedback_field       pad (12)
   :hash_start                     pad (8)




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   When a SigComp message is successfully decompressed and saved as
   state, the following bytecode instructs the receiving endpoint to
   return the first 6 bytes of the corresponding state identifier.  The
   bytecode can be added to any of the compression algorithms of Chapter
   4 immediately following the ":end_of_message" label:


   :end_of_message

   set (hash_length, (state_length + 8))

   LOAD (requested_feedback_location, 1158)
   MULTILOAD (hash_start, 4, state_length, 64,
   decompress_sigcomp_message, 6)
   SHA-1 (hash_start, hash_length, requested_feedback_field)


   The receiving endpoint then returns the state identifier in the
   "returned feedback field" of the next SigComp message to be
   transmitted in the reverse direction.

   When the state identifier is returned, the compressor can set the
   availability flag for the corresponding state to 1.

5.2  Static dictionary

   Certain protocols that can be compressed using SigComp offer a fixed,
   mandatory state item known as a static dictionary.  This dictionary
   contains a number of text strings that commonly occur in messages
   generated by the protocol in question.  The overall compression ratio
   can often be improved by accessing the text phrases from this static
   dictionary rather than by uploading them as part of the compressed
   message.

   As an example, a static dictionary is provided for the protocols SIP
   and SDP RFC-3485 [6].  This dictionary is designed for use by a wide
   range of compression algorithms including all of the ones covered in
   Chapter 4.

   In any of the compression algorithms of Chapter 4, the static
   dictionary can be accessed by inserting the following instruction
   immediately after the ":initialize_memory" label:

   STATE-ACCESS (dictionary_id, 6, 0, 0, 1024, 0)

   The following lines should also be inserted immediately after the
   END-MESSAGE instruction:




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   :dictionary_id

   byte (0xfb, 0xe5, 0x07, 0xdf, 0xe5, 0xe6)


   The text strings contained in the static dictionary can then be
   accessed in exactly the same manner as the text strings from
   previously decompressed messages (see Section 5.1 for further
   details).

   Note that in some cases it is sufficient to only load part of the
   static dictionary into the UDVM memory.  Further information on the
   contents of the SIP and SDP static dictionary can be found in the
   relevant document RFC-3485 [6].

5.3  CRC checksum

   Whilst the acknowledgement scheme of Section 5.1 is designed to
   ensure that SigComp does not propagate errors introduced by the
   underlying transport layer, in some cases it may be useful to add an
   extra CRC check over the UDVM memory.  For example, if the transport
   layer fails to discard a damaged SigComp message then a CRC check can
   ensure that the corresponding decompressed message is not forwarded
   to the application.

   If an additional CRC check is required then the following bytecode
   can be inserted after the ":end_of_message" label:



   INPUT-BYTES (2, index, !)
   CRC ($index, 64, state_length, !)


   The bytecode extracts a 2-byte CRC from the end of the SigComp
   message and compares it with a CRC calculated over the UDVM memory.
   Decompression failure occurs if the two CRC values do not match.

   A definition of the CRC polynomial used by the CRC instruction can be
   found in SigComp [4].

5.4  Announcing additional resources

   If a particular endpoint is able to offer more processing or memory
   resources than the mandatory minimum, the SigComp feedback mechanism
   can be used to announce that these resources are available to the
   remote endpoint.  This may help to improve the overall compression
   ratio between the two endpoints.



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   Additionally, if an endpoint has any pieces of state that may be
   useful for the remote endpoint to reference, it can advertise the
   identifiers for the states.  The remote endpoint can then make use of
   any that it also knows about (i.e. knows the contents of) e.g. a
   dictionary or shared mode state (see Section 5.5).

   The values of the following SigComp parameters can be announced using
   the SigComp advertisement mechanism:

      cycles_per_bit
      decompression_memory_size
      state_memory_size>
      SigComp_version
      state identifiers

   As explained in SigComp, in order to announce the values of these
   parameters the following fields must be reserved in the UDVM memory:


     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   |  cpb  |    dms    |    sms    |  returned_parameters_location
   +---+---+---+---+---+---+---+---+
   |        SigComp_version        |
   +---+---+---+---+---+---+---+---+
   | length_of_partial_state_ID_1  |
   +---+---+---+---+---+---+---+---+
   |                               |
   :  partial_state_identifier_1   :
   |                               |
   +---+---+---+---+---+---+---+---+
           :               :
   +---+---+---+---+---+---+---+---+
   | length_of_partial_state_ID_n  |
   +---+---+---+---+---+---+---+---+
   |                               |
   :  partial_state_identifier_n   :
   |                               |
   +---+---+---+---+---+---+---+---+


   These fields can be reserved in any of the algorithms of Chapter 4 by
   replacing the line "set (returned_parameters_location, 0)" with the
   following piece of assembly:







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   :adverts_len                    pad (1)
   :adverts_len_lsb                pad (1)
   :returned_parameters_location   pad (1)
   :returned_sigcomp_version       pad (1)
   :state_ids                      pad (x)

   where x is enough space for the number state identifiers that the
   endpoint wishes to advertise.


   When a SigComp message is successfully decompressed and saved as
   state, the following bytecode announces to the receiving endpoint
   that additional resources and pieces of state are available at the
   sending endpoint:


   :end_of_message

   LOAD (returned_parameters_location, N)
   INPUT-BYTES (1, adverts_len_lsb, done)
   INPUT-BYTES ($adverts_len, state_ids, done)
   :done


   Note that the integer value "N" should be set equal to the amount of
   resources available at the sending endpoint.  N should be expressed
   as a 2-byte integer with the most significant bits corresponding to
   the cycles_per_bit parameter and the least significant bits
   corresponding to the SigComp_version parameter.

   The length of the state identifiers, followed by the state
   identifiers in the format shown are appended to the end of the
   compressed message.

5.5  Shared compression

   This section provides bytecode for implementing the SigComp shared
   compression mechanism RFC-3321 [5].  If two endpoints A and B are
   communicating via SigComp, shared compression allows the messages
   sent from Endpoint A to Endpoint B to be compressed relative to the
   messages sent from Endpoint B to Endpoint A (and vice versa).  This
   may improve the overall compression ratio by reducing the need to
   transmit the same information in both directions.

   As described in RFC-3321 [5], two steps must be taken to implement
   shared compression at an endpoint.

   Firstly, it is necessary to announce to the remote endpoint that



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   shared compression is available.  This is done by announcing the
   state identifier as an available piece of state.  This can be done
   using the returned_parameters_location announcement as in
   Section 5.4.

   Secondly, assuming that such an announcement is received from the
   remote endpoint, then the state created by shared compression needs
   to be accessed by the message sent in the opposite direction.  This
   can be done in a similar way to accessing the static dictionary (see
   Section Section 5.2), but using the appropriate state identifier, for
   example, by using the INPUT-BYTES instruction as below:


   :shared_state_id        pad (6)

   :access_shared_state

   INPUT-BYTES (6, shared_state_id, !)
   STATE-ACCESS (shared_state_id, 6, 0, 0, $decompressed_start, 0)



6.  Security considerations

   This document describes implementation options for the SigComp
   protocol [4].  Consequently the security considerations for this
   document match those of SigComp.

7.  Acknowledgements

   Thanks to Richard Price, Carsten Bormann, Adam Roach, Lawrence
   Conroy, Christian Schmidt, Max Riegel, Lars-Erik Jonsson, Jonathan
   Rosenberg, Stefan Forsgren, Krister Svanbro, Miguel Garcia,
   Christopher Clanton, Khiem Le, Ka Cheong Leung, Zoltan Barczikay for
   valuable input and review.

   Special thanks to Pekka Pessi and Cristian Constantin who served as
   committed working group document reviewers.

8.  Intellectual Property Right Considerations

   The IETF has been notified of intellectual property rights claimed in
   regard to some or all of the specification contained in this
   document.  For more information consult the online list of claimed
   rights.






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9.  References

   [1]   Johnston, A., Donovan, S., Sparks, R., Cunningham, C., and K.
         Summers, "Session Initiation Protocol (SIP) Basic Call Flow
         Examples", RFC 3665, December 2003.

   [2]   Bradner, S., "The Internet Standards Process -- Revision 3",
         RFC 3667, February 2004.

   [3]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 2234, November 1997.

   [4]   Price, R., Borman, C., Christoffersson, J., Hannu, H., Liu, Z.,
         and J. Rosenberg, "Signaling Compression (SigComp)", RFC 3320,
         January 2003.

   [5]   Hannu, H., Christoffersson, J., Forsgren, S., Leung, K., Liu,
         Z., and R. Price, "Signaling Compression (SigComp) - Extended
         Operations", RFC 3321, January 2003.

   [6]   Garcia-Martin, M., Borman, C., Ott, J., Price, R., and A.
         Roach, "The Session Initiation Protocol (SIP) and Session
         Description Protocol (SDP) Static Dictionary for Signaling
         Compression (SigComp)", RFC 3485, February 2003.

   [7]   Ziv, J. and A. Lempel, "A universal algorithm for sequential
         data compression", IEEE 23:337-343, 1977.

   [8]   Storer, J., "Data Compression: Methods and Theory", Computer
         Science Press ISBN 0-88175-161-8, 1998.

   [9]   Nelson, M., "LZW Data Compression", Dr Dobb's Journal,
         October 1989.

   [10]  Deutsch, P., "DEFLATE Compressed Data Format Specification
         version 1.3", RFC 1951, May 1996.

   [11]  "Data Compression Procedures", ITU-T Recommendation V.44,
         November 2000.












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Authors' Addresses

   Abigail Surtees
   Siemens/Roke Manor Research
   Roke Manor Research Ltd.
   Romsey, Hants  SO51 0ZN
   UK

   Phone: +44 (0)1794 833131
   Email: abigail.surtees@roke.co.uk
   URI:   http://www.roke.co.uk


   Mark A. West
   Siemens/Roke Manor Research
   Roke Manor Research Ltd.
   Romsey, Hants  SO51 0ZN
   UK

   Phone: +44 (0)1794 833311
   Email: mark.a.west@roke.co.uk
   URI:   http://www.roke.co.uk

Appendix A.  UDVM bytecode for the compression algorithms

   The following sections list the UDVM bytecode generated for each
   compression algorithm of Section 4.

   Note that the different assemblers can output different bytecode for
   the same piece of assembly code, so a valid assembler can produce
   results different from those presented below.  However, the following
   bytecode should always generate the same decompressed messages on any
   UDVM.

A.1  Well-known Algorithms

A.1.1  Simplified LZ77

   0x0f86 0389 8d89 1588 8800 011c 0420 0d13 5051 2222 5051 16f5 2300
   0x00bf c086 a08b 06

A.1.2  LZSS

   0x0f86 04a0 c48d 00a0 c41e 2031 0209 00a0 ff8e 048c bfff 0117 508d
   0x0f23 0622 2101 1321 0123 16e5 1d04 22e8 0611 030e 2463 1450 5123
   0x2252 5116 9fd2 2300 00bf c086 a089 06





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A.1.3  LZW

   0x0f86 06a1 ce8d 00b1 8f01 a0ce 13a0 4903 2313 2501 2506 1201 1752
   0x88f4 079f 681d 0a24 2508 1203 0612 b18f 1252 0321 0ea0 4801 0624
   0x5013 a049 0323 1351 5025 2251 5016 9fde 2300 00bf c086 a09f 06

A.1.4  DEFLATE

   0x0f86 7aa2 528d 05a2 5200 0300 0400 0500 0600 0700 0800 0900 0a01
   0x0b01 0d01 0f01 1102 1302 1702 1b02 1f03 2303 2b03 3303 3b04 a043
   0x04a0 5304 a063 04a0 7305 a083 05a0 a305 a0c3 05a0 e300 a102 0001
   0x0002 0003 0004 0105 0107 0209 020d 0311 0319 0421 0431 05a0 4105
   0xa061 06a0 8106 a0c1 07a1 0107 a181 08a2 0108 a301 09a4 0109 a601
   0x0aa8 010a ac01 0bb0 010b b801 0c80 2001 0c80 3001 0d80 4001 0d80
   0x6001 1d03 229f b41e 20a0 6504 0700 1780 4011 0130 a0bf 0000 a0c0
   0xa0c7 8040 2901 a190 a1ff a090 1750 8040 1109 a046 1322 2101 1321
   0x0123 169f d108 1004 1250 0422 1d51 229f d706 1251 1e20 9fcf 0105
   0x001f 2f08 1004 1250 0426 1d53 26f6 0614 530e 2063 1454 5223 2250
   0x5216 9f9e 2300 00bf c086 a1de 06

A.1.5  LZJH

   0x0f86 08a1 5b8d 0700 a15b 0706 b18f 1d01 24a0 c317 5201 1a31 311e
   0x24a0 b802 0101 0102 0100 0100 1752 0107 a04e 1e1d 6524 f822 2501
   0x0ea0 4602 13a0 4703 2713 2501 2416 9fcd 1d66 24e1 1752 03a0 639f
   0xb808 0812 0306 12b1 8312 5203 210e a046 0106 2350 0e28 6713 a047
   0x0327 1351 5024 2251 5016 9fa8 1e24 9fb1 0401 0101 0102 0103 0201
   0x0101 0d03 0007 0517 520d 0d06 061d 0826 f706 1253 1351 5011 1351
   0x5224 2251 5206 1250 1225 0154 169f 6617 5201 9fdb 070f 1c00 009e
   0xce16 9f57 1d01 24fa 1752 0107 0d9e c206 2501 169f 6506 2601 169f
   0x7623 0000 bfc0 86a0 8e06

A.2  Adapted Algorithms

A.2.1  Modified DEFLATE

   0x0f86 04a1 d38d 00a1 d31e 20a1 4010 0500 0b2e 000c 0c88 011a 20a1
   0x0101 a042 a044 2000 a045 a05e a061 00a0 5fa0 66a1 0800 a067 a067
   0xa1ff 02a1 a0a1 aa23 00a1 aba1 d13a 00a1 d2a1 e1a1 1001 a3c4 a3e3
   0xa120 03bf 20bf 34a0 7b00 bf35 bfb3 a180 0180 3f68 803f 8700 0080
   0x3f88 803f c7a1 4001 807f 9080 7fff a090 1750 88a0 79a0 83a0 831e
   0x20a0 c810 0400 00a1 ff01 0209 8801 1416 2000 171e a108 013e a049
   0x2e00 a04a a059 a110 02a1 68a1 81a0 6100 a182 a1a1 a120 01a3 44a3
   0x6a3a 00a3 6ba3 aaa1 4001 a756 a760 2300 a761 a7df a180 01af c0af
   0xd4a0 7b01 bfaa bfc9 0001 803f 9480 3ffb a090 0180 7ff8 807f ffa0
   0xf817 5088 0610 1022 2101 1321 0123 169f 1107 10a0 fd1e 229f d909
   0x0900 0709 0008 3fa0 8101 87a0 8701 00a0 88a0 f711 00a0 f8a1 3fa0
   0xb901 a280 a57f a101 02b6 00b9 ffa4 0101 8034 0080 3bff a801 0290



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   0x00ff b001 0e24 6314 5150 2322 5250 169f 3b23 0000 bfc0 86a0 8906


















































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Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
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   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.


Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Copyright Statement

   Copyright (C) The Internet Society (2005).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

This Internet-Draft will expire on April 1, 2006.


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