One document matched: draft-price-rohc-sigcomp-user-guide-01.txt
Differences from draft-price-rohc-sigcomp-user-guide-00.txt
Network Working Group Richard Price, Siemens/Roke Manor
INTERNET-DRAFT Abigail Surtees, Siemens/Roke Manor
Expires: May 2003 Mark A West, Siemens/Roke Manor
November 4, 2002
SigComp User Guide
<draft-price-rohc-sigcomp-user-guide-01.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
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 cite them other than as "work in progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/lid-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
This document is a submission of the IETF ROHC WG. Comments should be
directed to its mailing list, rohc@ietf.org.
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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Change history
Changes relative to <draft-price-rohc-sigcomp-user-guide-00.txt>:
1. Updated the assembly language based on mailing list discussions.
2. Added example compressed messages for each algorithm.
3. Added the assembled bytecode for each algorithm.
4. Fixed a bug in the code for LZSS.
Table of contents
1. Introduction..................................................2
2. Terminology...................................................3
3. Overview of the User Guide....................................3
4. UDVM assembly language........................................4
5. Compression algorithms........................................11
6. Additional SigComp mechanisms.................................29
7. Security considerations.......................................36
8. Acknowledgements..............................................36
9. Authors' addresses............................................36
10. Intellectual Property Right Considerations....................36
11. References....................................................37
Appendix A: UDVM bytecode for the compression algorithms..........38
1. Introduction
This document provides an informational guide for users of the
SigComp protocol [SIGCOMP]. 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 [DEFLATE] and LZW [LZW]. The bytecode for the
choice of compression algorithm is uploaded to the UDVM as part of
the compressed data.
The basic SigComp standard 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 describes how standard stream-
based compressors can be modified for robustness against lost and/or
misordered packets over an unreliable transport such as UDP.
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2. Terminology
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 RFC-2119 [RFC-2119].
3. 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 is available for
the corresponding decompressor.
This document provides bytecode for the following algorithms:
1. Simplified LZ77
2. LZSS
3. LZW
4. DEFLATE
5. LZJH
6. EPIC
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 over an unreliable
transport such as UDP.
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
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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.
4. 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.
4.1. 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] specifies the syntax of a
token:
token = (name / opcode / delimiter / dec / bin / hex)
name = (lowercase / "_") 1*(lowercase / digit / "_")
opcode = uppercase *(uppercase / digit / "-")
delimiter = "." / "!" / "$" / ":" / "(" / ")" / operator
dec = 1*(digit)
bin = "0b" 1*("0" / "1")
hex = "0x" 1*(hex_digit)
hex_digit = digit / %x41-46 / %x61-66
digit = %x30-39
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uppercase = %x41-5a
lowercase = %x61-7a
operator = "+" / "-" / "*" / "/" / "%" / "&" / "|" /
"^" / "~" / "<<" / ">>"
When parsing for tokens a longest match is applied, i.e. a token is
the longest string that matches the <token> rule specified above.
The syntax of whitespace and comments is specified by the following
ABNF:
ws = *(%x09 / %x0a / %x0d / %x20 / comment)
comment = ";" *(%x00-09 / %x0b-0c / %x0e-ff)
(%x0a / %x0d)
Whitespace that matches <ws> is skipped between tokens, but serves to
terminate the longest match for a token.
Comments are specified by the symbol ";" and are terminated by the
end of the line, for example:
LOAD (temp, 1) ; This is a comment.
Any other input is a syntax error.
When parsing on the lexical level the string of assembly should be
divided up into a list of successive tokens. The whitespace and
comments should also be deleted. The assembly should then be parsed
on the syntactic level as explained in Section 4.2.
4.2. Syntactic level
Once the string of assembly has been divided into tokens as per
Section 4.1, the next step is to convert the assembly into a string
of UDVM bytecode.
On a syntactic level a string of assembly consists of zero or more
instructions, directives or labels, each of which is itself built up
from one or more lexical tokens.
The following ABNF description specifies the syntax of the assembly
language. Note that the lexical parsing step is assumed to have been
carried out, so in particular the boundaries between tokens are
already known and the comments and whitespace have been deleted:
assembly = *(instruction / directive / label)
instruction = opcode ["(" operand *("," operand) ")"]
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operand = [["$"] expression]
; An empty operand is one that can be
; computed, typically a literal (#)
; operand that gives the number of
; operands.
; 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
; abbreviation for set(<name>, .)
directive = padding / data / set
; note that directive names are
; syntactically of category <name>; all
; directives are intended to syntactically
; match: name ["(" expression *(","
; expression) ")"]
padding = ("pad" / "align" / "at") "(" expression ")"
data = ("byte" / "word") "(" expression *(","
expression) ")"
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:
4.2.1. 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
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"*" 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 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.
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
integer value assigned to it. Section 4.2.3 explains how to assign
integer values to text names.
4.2.2. 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 [SIGCOMP].
Each operand consists of an expression which evaluates to an integer,
optionally preceded by the symbol "$". This symbol indicates that the
supplied integer value must be interpreted as the memory address at
which the operand value can be found rather than the actual operand
value itself.
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When converting each instruction operand to bytecode, the parser
first determines whether the instruction expects the operand to be a
literal, a reference, a multitype or an address. If the operand is a
literal then as per Figure 8 of SigComp, the parser inserts the
shortest bytecode capable of encoding the supplied operand value.
Since literal operands are used to indicate the total number of
operands for an instruction, it is possible to leave a literal
operand blank and allow its value to be inferred automatically by the
assembler. For example:
MULTILOAD (64, , 1, 2, 3, 4)
The missing operand should be given the value 4 because it is
followed by a total of 4 operands.
If the operand is a reference then as per Figure 9 of SigComp, the
parser inserts the shortest bytecode capable of encoding the supplied
memory address. Note that reference operands will always be preceded
by the symbol "$" 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 "$" is present. If so then as per Figure 10 of SigComp, it
inserts the shortest bytecode capable of encoding the supplied
integer as a memory address. If not then it inserts the inserts the
shortest bytecode capable of encoding the supplied integer as an
operand value.
If the operand is an address then the parser checks whether the
symbol "$" 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.
4.2.3. Directives
The assembly language provides a number of directives for evaluating
simple expressions, moving instructions to a particular memory
address and so on.
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).
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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. Note that if the bytecode is
already aligned to a multiple of n bytes then no padding bytes are
required.
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.
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.
4.2.4. 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, .).
4.3. 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
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beginning at Address 0 and then executed beginning from the first
instance of a UDVM instruction.
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. Note however that 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 undefined 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
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.
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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.
5. 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.
Section 5.1 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.
5.1. Simplified LZ77
This section describes how to implement a very simple compression
algorithm based on LZ77 [LZ77].
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
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
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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.
Note that the actual size of this circular buffer depends on the
total amount of memory available to the UDVM. The minimum size of the
UDVM memory is 1K, so the circular buffer will always contain at
least 512 bytes.
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)
: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)
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; 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
; "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, $decompressed_pointer)
; The byte string is also outputted onto the end of the decompressed
; message:
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OUTPUT ($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
0x016e 0001 0169 0001 0176 0001 0165 0001 0172 0002 0165 0001 010a
0x0001
The bytecode for the LZ77 decompressor can be uploaded as part of the
compressed message as specified in Section 4.3. 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:
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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 SigComp
for details of how state identifiers are derived).
5.2. LZSS
This section provides UDVM bytecode for the simple but effective LZSS
compression algorithm [LZSS].
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
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)
:index pad (2)
:length_value pad (2)
:old_pointer 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)
set (returned_parameters_location, 0)
align (64)
: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,
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)
:circular_buffer
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An example message compressed using the LZSS algorithm is given
below:
0x279a 0406 e378 b200 6074 1018 4ce6 1349 b842
5.3. LZW
This section provides UDVM bytecode for the well-known LZW
compression algorithm [LZW]. 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
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)
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: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:
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, $decompressed_pointer)
; The following instruction outputs the text string specified by the
; position/length pair:
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OUTPUT ($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)
: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
5.4. 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] 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. The bytecode supports only DEFLATE block type 01 (data compressed
with fixed Huffman codes).
Assembly for the DEFLATE decompressor is given below:
at (32)
:index pad (2)
:extra_length_bits pad (2)
:length_value pad (2)
:extra_distance_bits pad (2)
:distance_value pad (2)
at (42)
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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)
:length_table pad (116)
:distance_table pad (120)
set (returned_parameters_location, 0)
align (64)
: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)
:circular_buffer
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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 0000 00ff ff00
5.5. 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].
Assembly for the LZJH decompressor is given below:
at (32)
; 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
; 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)
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: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:
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)
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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, $length_value)
JUMP (prefix_after_codeword)
:string_extension
; The following code decompresses a Huffman-encoded string extension:
INPUT-HUFFMAN (index, !, 4, 1, 1, 1, 1, 2, 1, 3, 2, 1, 1, 1, 13, 3,
0, 7, 5)
COMPARE ($index, 13, continue, extra_bits, extra_bits)
:extra_bits
INPUT-BITS (max_extension_length, extra_extension_bits, !)
ADD ($index, $extra_extension_bits)
:continue
; The following code extends the most recently created codebook entry
; by the number of bits specified in the string extension:
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COPY-LITERAL ($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 ($index, 1, stepup_ordinal, stepup_codeword, 0)
:stepup_ordinal
ADD ($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)
:circular_buffer
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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
5.6. EPIC
This section provides bytecode for a version of the Efficient
Protocol Independent Compression (EPIC) scheme.
The basic EPIC scheme is designed to compress protocol headers such
as TCP/IP, but the underlying algorithm (known as Hierarchical
Huffman) can be applied to the compression of arbitrary data. In
particular the compression algorithm used by EPIC obtains a very high
compression ratio on data with a known structure, so it is ideally
suited for compressing the messages generated by SIP or other
signaling protocols.
Note however that in its basic form the EPIC algorithm does not have
the ability to detect and adapt to new patterns in the uncompressed
data; instead it relies on a fixed pre-programmed description of how
the protocol to be compressed is expected to behave.
The application messages encountered by SigComp will typically
contain segments of generic text that cannot be compressed using the
basic EPIC scheme. Fortunately however, EPIC can easily be upgraded
to cope with generic data by adding the ability to store a circular
buffer of previously received text strings as per LZ77 or DEFLATE.
The resulting hybrid algorithm offers the best of both worlds: a very
high compression ratio for the "well-behaved" parts of the
application message, and a good compression ratio even for the
portions of the message that cannot be pre-programmed into the
compression algorithm.
The following bytecode implements a decompressor for a hybrid of EPIC
and DEFLATE. The tables of compressed characters are generated using
the Hierarchical Huffman algorithm from EPIC, and are designed to
give a very high compression ratio for a typical SIP/SDP message. The
ability to store and retrieve text strings from a buffer of
previously received messages is taken from DEFLATE.
To illustrate the performance of the hybrid algorithm, the following
results have been generated for the call flow in Section 3.1.2 of
"SIP Call Flow Examples" [FLOWS]. Note that to improve the overall
compression ratio, all algorithms employ a static dictionary (see
Section 6.2) and the shared compression mechanism (see Section 6.5):
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Algorithm: Total compressed message size:
DEFLATE with static Huffman codes 660 bytes
DEFLATE with adaptive Huffman codes 625 bytes
EPIC/DEFLATE hybrid 560 bytes
The bytecode for the hybrid EPIC/DEFLATE algorithm is given below. A
compressor to generate messages for this algorithm can be adapted
from an ordinary DEFLATE compressor; the string matching rules should
be left unchanged but the tables of Huffman codes used by DEFLATE
should be replaced by those generated by the EPIC algorithm:
at (32)
: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)
: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,
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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,
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,
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0, 136, 247, 17,
0, 248, 319, 185,
1, 640, 1407, 257,
2, 5632, 6655, 1025,
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)
:circular_buffer
An example message compressed using the EPIC 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
6. Additional SigComp mechanisms
The following chapter covers the additional mechanisms that can be
employed by SigComp to improve the overall compression ratio;
including the acknowledgment of SigComp state over an unreliable
link, sharing state between two directions of a compressed message
flow etc.
When each of the compression algorithms described in Chapter 5 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
states that it has instructed the receiving endpoint to save.
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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 6.1).
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.
6.1. Acknowledging a state item
The simplest method for acknowledging a SigComp state item is to
employ a reliable transport layer such as TCP or SCTP. Alternatively,
over an unreliable transport such as UDP the SigComp feedback
mechanism can be used to acknowledge that a state item has been
successfully created at the receiving endpoint.
As explained in SigComp [SIGCOMP], 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 compression algorithms of
Chapter 5 by replacing "set (requested_feedback_location, 0)" with
the following piece of assembly:
:requested_feedback_location pad (1)
:requested_feedback_length pad (1)
:requested_feedback_field pad (12)
:hash_start pad (8)
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
5 at the ":end_of_message" label:
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: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.
6.2. 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 [SIP-DICT]. This dictionary is designed for use by a wide
range of compression algorithms including all of the ones covered in
Chapter 5.
In any of the compression algorithms of Chapter 5, the static
dictionary can be accessed by inserting the following instruction
after the ":initialize_memory" label:
STATE-ACCESS (dictionary_id, 6, 0, 0, 1024, 0)
The following line should also be inserted immediately after the END-
MESSAGE instruction:
: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
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contents of the SIP and SDP static dictionary can be found in the
relevant draft [SIP-DICT].
6.3. CRC checksum
Whilst the acknowledgement scheme of Section 6.1 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 [SIGCOMP].
6.4. 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.
The values of the following SigComp parameters can be announced using
the SigComp feedback mechanism:
cycles_per_bit
decompression_memory_size
state_memory_size
SigComp_version
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 |
+---+---+---+---+---+---+---+---+
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These fields can be reserved in any of the compression algorithms of
Chapter 5 by replacing "set (returned_parameters_location, 0)" with
the following piece of assembly:
:returned_parameters_location pad (1)
:returned_sigcomp_version pad (1)
When a SigComp message is successfully decompressed and saved as
state, the following bytecode announces to the receiving endpoint
that additional resources are available at the sending endpoint:
:end_of_message
LOAD (returned_parameters_location, N)
Note that the 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.
6.5. Shared compression
This section provides bytecode for implementing the SigComp shared
compression mechanism [EXTENDED]. 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 the Extended Operations document, two steps must be
taken to implement shared compression at an endpoint. Firstly, it is
necessary to announce to the remote endpoint that shared compression
is available. Conversely, if such an announcement is received from
the remote endpoint then the state created by shared compression can
be accessed to improve the overall compression ratio.
In order to announce that shared compression is available 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 |
+---+---+---+---+---+---+---+---+
Price et al. [Page 33]
INTERNET-DRAFT SigComp User Guide November 4, 2002
+---+---+---+---+---+---+---+---+
| |
: partial_state_identifier_1 :
| |
+---+---+---+---+---+---+---+---+
: :
+---+---+---+---+---+---+---+---+
| length_of_partial_state_ID_n |
+---+---+---+---+---+---+---+---+
| |
: partial_state_identifier_n :
| |
+---+---+---+---+---+---+---+---+
These fields can be reserved in any of the compression algorithms of
Chapter 5 by replacing "set (returned_parameters_location, 0)" with
the following piece of assembly:
:returned_parameters_location pad (1)
:returned_sigcomp_version pad (1)
:length_of_partial_state_id_a pad (1)
:partial_state_identifier_a pad (6)
:length_of_partial_state_id_b pad (1)
:partial_state_identifier_b pad (20)
:extended_flags pad (2)
:shared_state_id pad (6)
:padding pad (6)
:minimum_access_length pad (2)
:announcement_location pad (2)
:decompressed_start pad (2)
:decompressed_length pad (2)
:shared_hash_length pad (2)
In Figure 5 of the Extended Operations draft, an example SigComp
message format is provided to carry the shared compression
information between the two endpoints. This message format can be
decompressed at the receiving endpoint by inserting the following
assembly after the label ":decompress_sigcomp_message" in one of the
algorithms of Chapter 5:
:decompress_sigcomp_message
INPUT-BYTES (1, extended_flags, !)
COMPARE ($extended_flags, 32768, initialize_state_announcement,
access_shared_state, access_shared_state)
:access_shared_state
INPUT-BYTES (6, shared_state_id, !)
Price et al. [Page 34]
INTERNET-DRAFT SigComp User Guide November 4, 2002
STATE-ACCESS (shared_state_id, 6, 0, 0, $decompressed_start, 0)
:initialize_state_announcement
MULTILOAD (minimum_access_length, 4, 6, length_of_partial_state_id_a,
$decompressed_pointer, 5120)
COPY-LITERAL (padding, 8, $decompressed_pointer)
LSHIFT ($extended_flags, 1)
COMPARE ($extended_flags, 32768, algorithm_start,
announce_acked_state_id, announce_acked_state_id)
:announce_acked_state_id
LOAD (length_of_partial_state_id_a, 1536)
INPUT-BYTES (6, partial_state_identifier_a, !)
LOAD (announcement_location, length_of_partial_state_id_b)
:algorithm_start
Additionally, the following piece of assembly should be inserted
following the label ":end_of_message" in the chosen compression
algorithm:
:end_of_message
LSHIFT ($extended_flags, 1)
COMPARE ($extended_flags, 32768, end, announce_shared_state,
announce_shared_state)
:announce_shared_state
; The following instructions calculate the state identifier for the
; shared state:
COPY-LITERAL (decompressed_length, 1, $announcement_location)
set (buffer_size, (udvm_memory_size - circular_buffer))
MULTILOAD (decompressed_length, 2, 65528, $decompressed_pointer)
SUBTRACT ($shared_hash_length, $decompressed_start)
REMAINDER ($shared_hash_length, buffer_size)
ADD ($decompressed_length, $shared_hash_length)
LOAD ($decompressed_start, $decompressed_length)
SHA-1 ($decompressed_start, $shared_hash_length,
$announcement_location)
:end
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INTERNET-DRAFT SigComp User Guide November 4, 2002
7. Security considerations
This draft describes implementation options for the SigComp protocol
[SIGCOMP]. Consequently the security considerations for this draft
match those of SigComp.
8. Acknowledgements
Thanks to
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.
9. Authors' addresses
Richard Price Tel: +44 1794 833681
Email: richard.price@roke.co.uk
Abigail Surtees Tel: +44 1794 833131
Email: abigail.surtees@roke.co.uk
Mark A West Tel: +44 1794 833311
Email: mark.a.west@roke.co.uk
Roke Manor Research Ltd
Romsey, Hants, SO51 0ZN
United Kingdom
10. 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.
Price et al. [Page 36]
INTERNET-DRAFT SigComp User Guide November 4, 2002
11. References
[SIGCOMP] "Signaling Compression (SigComp)", R. Price et al,
<draft-ietf-rohc-sigcomp-07.txt>, June 2002
[EXTENDED] "SigComp - Extended Operations", Hannu et al,
<draft-ietf-rohc-sigcomp-extended-04.txt>, June 2002
[SIP-DICT] "The Session Initiation Protocol and Session Description
Protocol static dictionary for Signaling Compression",
Garcia et al, <draft-ietf-sipping-sigcomp-sip-dictionary-
05.txt>, November 2002
[FLOWS] "Session Initiation Protocol Basic Call Flow Examples",
A. Johnston et al, <draft-ietf-sipping-basic-call-flows-
01.txt>, October 2002
[RFC-2026] "The Internet Standards Process - Revision 3", Scott
Bradner, RFC 2026, Internet Engineering Task Force,
October 1996
[RFC-2119] "Key words for use in RFCs to Indicate Requirement
Levels", Scott Bradner, RFC 2119, Internet Engineering
Task Force, March 1997
[RFC-2234] "Augmented BNF for Syntax Specifications: ABNF",
D. Crocker and P. Overell, RFC 2234, Internet Engineering
Task Force, November 1997
[RFC-2326] "Real Time Streaming Protocol (RTSP)", H. Schulzrinne, A.
Rao and R. Lanphier, RFC 2326, Internet Engineering Task
Force, April 1998
[RFC-2543] "SIP: Session Initiation Protocol", Handley et al,
RFC 2543, Internet Engineering Task Force, March 1999
[LZ77] "A universal algorithm for sequential data compression",
J. Ziv and A. Lempel, IEEE 23:337-343, 1977
[LZSS] "Data Compression: Methods and Theory", J. Storer,
Computer Science Press, ISBN 0-88175-161-8, 1988
[LZW] "LZW Data Compression", Mark Nelson, Dr. Dobb's
Journal, October 1989
[DEFLATE] "DEFLATE Compressed Data Format Specification version
1.3", P. Deutsch, RFC 1951, Internet Engineering Task
Force, May 1996
[LZJH] "Data Compression Procedures", ITU-T Recommendation V.44,
November 2000
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Appendix A: UDVM bytecode for the compression algorithms
The following sections list the UDVM bytecode generated for each
compression algorithm of Chapter 5.
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. Simplified LZ77
0x0f86 0389 8d89 1588 8800 011c 0420 0d13 5051 2222 5051 16f5 2300
0x00bf c086 a08b 06
A.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
A.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.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.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
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INTERNET-DRAFT SigComp User Guide November 4, 2002
0xce16 9f57 1d01 24fa 1752 0107 0d9e c206 2501 169f 6506 2601 169f
0x7623 0000 bfc0 86a0 8e06
A.6. EPIC
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
0x00ff b001 0e24 6314 5150 2322 5250 169f 3b23 0000 bfc0 86a0 8906
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