Network Working Group Richard Price, Siemens/Roke Manor INTERNET-DRAFT Abigail Surtees, Siemens/Roke Manor Expires: December 2002 Mark A West, Siemens/Roke Manor June 24, 2002 SigComp User Guide 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. Price et al. [Page 1] INTERNET-DRAFT SigComp User Guide June 24, 2002 Table of contents 1. Introduction..................................................2 2. Terminology...................................................2 3. Overview of the User Guide....................................2 4. Mnemonic language.............................................4 5. Compression algorithms........................................10 6. Additional SigComp mechanisms.................................28 7. Security considerations.......................................35 8. Acknowledgements..............................................35 9. Authors' addresses............................................35 10. Intellectual Property Right Considerations....................35 11. References....................................................36 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. 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]. 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 Price et al. [Page 2] INTERNET-DRAFT SigComp User Guide June 24, 2002 algorithms can be supported provided that bytecode is available for the corresponding decompression algorithm. 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 4. Announcing additional resources 5. Shared compression Any or all of the above mechanisms can be implemented in conjunction with each compression algorithm. A subroutine of UDVM bytecode is provided for each of the mechanisms; these subroutines can be added to the bytecode for any of the basic compression algorithms. Price et al. [Page 3] INTERNET-DRAFT SigComp User Guide June 24, 2002 4. Mnemonic language Writing UDVM programs directly in bytecode would be a daunting task, so a simple mnemonic language is provided to facilitate the creation of new decompression algorithms. The mnemonic language allows the UDVM instructions and their operands to be specified as text names as well as integer values. A string of mnemonic code consists of one or more primitives separated by whitespace. Each primitive can be one instance of a comment, instruction, label or padding, as illustrated by the following ABNF description [RFC-2234]: mnemonic_code = primitive *(ws primitive) primitive = comment | instruction | label | padding ws = 1*(%x09 | %x0A | %x0D | %x20) A parser for the mnemonic language scans for each primitive in turn and converts it into the corresponding UDVM bytecode. The resulting string of bytes can then be uploaded to the UDVM as explained in Section 4.4. Comments are specified by the symbol ";" and are terminated by the end of the line: comment = ";" *(%x00-09 | %x0B-0C | %x0E-FF) For example: ; This is a comment. Comments are deleted when converting the mnemonic code into bytecode. The following sections define how to convert the remaining types of primitive into bytecode. 4.1. Instructions A UDVM instruction is specified by the instruction opcode followed by zero or more operands: instruction = opcode [ws "(" operand_list ")"] operand_list = operand *("," ws operand) The instruction operands are enclosed in parentheses and separated by commas, for example: ADD (3, 4) Price et al. [Page 4] INTERNET-DRAFT SigComp User Guide June 24, 2002 In the mnemonic language instruction opcodes are expressed in uppercase text: when generating the bytecode the parser should replace this text with the corresponding 1-byte value as per Figure 11 of SigComp. opcode = uppercase *(uppercase | digit | "-") uppercase = %x41-5A digit = %x30-39 Instruction operands are expressed either as integer values or as text names. In the latter case the text name is replaced by its corresponding integer value (assigned as per Section 4.2). An operand may also be preceded by the symbol "$", which indicates that the supplied integer value must be interpreted as the memory address at which the operand value can be found rather than the actual operand value itself. operand = ["$"] value value = integer | text_name integer = decimal | binary | hex decimal = 1*(digit) binary = "0b" 1*("0" | "1") hex = "0h" 1*(hex_digit) hex_digit = digit | %x41-46 | %x61-66 text_name = 1*(lowercase | "_") lowercase = %x61-7A 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. 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 "$" in the mnemonic code because they always encode memory addresses rather than actual operand values. Price et al. [Page 5] INTERNET-DRAFT SigComp User Guide June 24, 2002 If the operand is a multitype then the parser first checks whether the symbol "$" 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. Labels As described in Section 4.1 the mnemonic language allows instruction operands to be specified as text names as well as integer values. Each text name should be assigned a corresponding integer value by means of a label. The mnemonic language defines a label to be a single colon followed by the text name to be defined and optionally an expression: label = ":" text_name [ws "=" ws expression] If an expression is present then it is evaluated and the resulting integer is assigned to the text string. Acceptable expressions include a single value, or two values separated by one of the four basic arithmetic operators. Moreover each value can be an integer or an instance of a text name to which a value has previously been assigned: expression = value | (value ws operator ws value) operator = "+" | "-" | "*" | "/" If no expression is present then the (absolute) position of the byte immediately following the label is evaluated and assigned to the text string. 4.3. Padding The mnemonic language also provides the ability to pad bytecode, which is useful for reserving areas of the UDVM memory for variables and other data. Three types of padding are provided by the mnemonic language: padding = padding_bytes | alignment | byte_string Price et al. [Page 6] INTERNET-DRAFT SigComp User Guide June 24, 2002 The statement ".pad n" appends n consecutive padding bytes to the bytecode. The actual value of the padding bytes is unimportant; when the bytecode is uploaded to the UDVM the padding bytes will be set to the initial values contained in the UDVM memory. padding_bytes = ".pad" ws integer The statement ".align n" appends sufficient padding bytes to the bytecode such that the total bytecode generated so far is a multiple of n bytes. alignment = ".align" ws integer The statement ".byte" appends a specified byte string to the bytecode. The byte string is supplied as integers from 0 to 255, separated by whitespace. The string of integers is terminated by the end of a line. byte_string = ".byte" 1*((%x09 | %x20) integer) 4.4. Uploading the bytecode to the UDVM Once the parser has converted a string of mnemonic code into the corresponding bytecode, it must be copied to the UDVM memory 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 : | | +---+---+---+---+---+---+---+---+ Price et al. [Page 7] INTERNET-DRAFT SigComp User Guide June 24, 2002 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. 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. 4.5. ABNF description of mnemonic language The following is a complete ABNF [RFC-2234] description of the mnemonic language: mnemonic_code = primitive *(ws primitive) primitive = comment | instruction | label | padding ws = 1*(%x09 | %x0A | %x0D | %x20) comment = ";" *(%x00-09 | %x0B-0C | %x0E-FF) ; Comments are indicated by a semicolon ; and continue to the end of the line. instruction = opcode *(ws operand) opcode = uppercase *(uppercase | digit | "-") ; The list of instruction opcodes and ; their corresponding 1-byte values is ; given in Figure 11 of SigComp. Price et al. [Page 8] INTERNET-DRAFT SigComp User Guide June 24, 2002 operand = ["$"] value ; When a "$" symbol is appended to the ; front of an instruction operand then the ; corresponding integer must be encoded as ; a memory address rather than as the ; actual operand value. This symbol is ; mandatory for reference operands, ; optional for multitypes and addresses, ; and disallowed for literals. value = integer | text_name integer = decimal | binary | hex ; Instruction operands can be given in the ; form of integers. They are converted ; into the shortest bytecode capable of ; representing the integer by the rules of ; Section 8.5 of SigComp. decimal = 1*(digit) binary = "0b" 1*("0" | "1") hex = "0h" 1*(hex_digit) hex_digit = digit | %x41-46 | %x61-66 digit = %x30-39 text_name = 1*(lowercase | "_") ; Instruction parameters can also be given ; in the form of lowercase names. These ; names are assigned an integer value by ; means of a label. uppercase = %x41-5A lowercase = %x61-7A label = ":" text_name [ws "=" ws expression] ; Label names are given as a colon ; followed by lowercase text. They are ; deleted when converting the mnemonic ; code to bytecode. expression = value | (value ws operator ws value) Price et al. [Page 9] INTERNET-DRAFT SigComp User Guide June 24, 2002 operator = "+" | "-" | "*" | "/" padding = padding_bytes | alignment | byte_string padding_bytes = ".pad" ws integer alignment = ".align" ws integer byte_string = ".byte" 1*((%x09 | %x20) integer) 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 the compressed data. 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 Price et al. [Page 10] INTERNET-DRAFT SigComp User Guide June 24, 2002 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 0x01410001 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. The mnemonic code for the simplified LZ77 decompressor is given below: ; Variables that do not need to be stored after decompressing each ; SigComp message are stored here: .pad 32 :index .pad 2 :length_value .pad 2 .align 42 Price et al. [Page 11] INTERNET-DRAFT SigComp User Guide June 24, 2002 :requested_feedback_location = 0 ; The UDVM registers must be stored beginning at Address 64: .align 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 :returned_parameters_location = 0 .align 64 :initialize_memory :udvm_memory_size = 8192 :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 Price et al. [Page 12] INTERNET-DRAFT SigComp User Guide June 24, 2002 ; 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) :fail DECOMPRESSION-FAILURE .align 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: 0x01 54 00 01 01 68 00 01 01 65 00 01 01 20 00 01 01 52 00 01 0x01 65 00 01 01 73 00 02 01 61 00 01 01 75 00 01 01 72 00 01 0x01 61 00 01 01 6e 00 01 01 74 00 01 01 20 00 01 01 61 00 01 0x02 0d 00 02 01 74 00 01 02 01 00 03 01 45 00 01 01 6e 00 01 0x01 64 00 01 01 20 00 01 01 6f 00 01 01 66 00 01 02 11 00 05 0x01 55 00 01 01 6e 00 01 01 69 00 01 01 76 00 01 01 65 00 01 0x01 72 00 02 01 65 00 01 01 0a 00 01 The bytecode for the LZ77 decompressor can be uploaded as part of the compressed message as specified in Section 4.4. However, in order to improve the overall compression ratio it is important to avoid uploading bytecode in every compressed message. For this reason Price et al. [Page 13] INTERNET-DRAFT SigComp User Guide June 24, 2002 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 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. Mnemonic code for an LZSS decompressor is given below: .pad 32 :index .pad 2 :length_value .pad 2 :old_pointer .pad 2 Price et al. [Page 14] INTERNET-DRAFT SigComp User Guide June 24, 2002 .align 42 :requested_feedback_location = 0 .align 64 :byte_copy_left .pad 2 :byte_copy_right .pad 2 :input_bit_order .pad 2 :decompressed_pointer .pad 2 :returned_parameters_location = 0 .align 64 :initialize_memory :udvm_memory_size = 8192 :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, 0, 8191, 1) COMPARE ($index, 8192, length, end_of_message, literal) :literal :index_lsb = index + 1 OUTPUT (index_lsb, 1) COPY-LITERAL (index_lsb, 1, $decompressed_pointer) JUMP (next_character) :length INPUT-BITS (4, length_value, fail) ADD ($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 Price et al. [Page 15] INTERNET-DRAFT SigComp User Guide June 24, 2002 END-MESSAGE (requested_feedback_location, returned_parameters_location, state_length, 64, decompress_sigcomp_message, 6, 0) :fail DECOMPRESSION-FAILURE :circular_buffer 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. Mnemonic code for an LZW decompressor is given below: .pad 32 :length_value .pad 2 :position_value .pad 2 :index .pad 2 .align 42 :requested_feedback_location = 0 .align 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 Price et al. [Page 16] INTERNET-DRAFT SigComp User Guide June 24, 2002 :returned_parameters_location = 0 .align 64 :initialize_memory :udvm_memory_size = 8192 :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: :index_lsb = index + 1 :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 ($index, 256, initialize_codebook, next_character, 0) :decompress_sigcomp_message :next_character ; The following INPUT-BITS instruction extracts 10 bits from the ; compressed message: INPUT-BITS (10, index, end_of_message) ; The following instructions interpret the received bits as an index ; into the LZW codebook, and extract the corresponding ; position/length pair: :length_value_lsb = length_value + 1 MULTIPLY ($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, $codebook_next) Price et al. [Page 17] INTERNET-DRAFT SigComp User Guide June 24, 2002 COPY-LITERAL ($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) :static_dictionary .pad 256 :circular_buffer .align 4879 :codebook An example message compressed using the LZW algorithm is given below: 0x14 c6 f0 80 6c 1b c6 e1 9c 20 18 46 e1 90 20 1d 06 84 20 6b 1c c2 0x01 98 6f 1c 90 71 b0 6c 42 c6 81 95 11 1a 47 31 a0 21 02 bf 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). Mnemonic code for the DEFLATE decompressor is given below: .pad 32 :index .pad 2 :extra_length_bits .pad 2 Price et al. [Page 18] INTERNET-DRAFT SigComp User Guide June 24, 2002 :length_value .pad 2 :extra_distance_bits .pad 2 :distance_value .pad 2 .align 42 :requested_feedback_location = 0 .align 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 :returned_parameters_location = 0 .align 64 :initialize_memory :udvm_memory_size = 8192 :state_length = udvm_memory_size - 64 :table_one = length_table - 4 :table_two = table_one + 65536 :length_table_start = table_two / 4 :length_table_mid = length_table_start + 24 :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, Price et al. [Page 19] INTERNET-DRAFT SigComp User Guide June 24, 2002 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 INPUT-BITS (3, extra_length_bits, fail) :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 :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, fail) ADD ($length_value, $extra_length_bits) ; this is the distance part INPUT-BITS (5, index, fail) ADD ($index, distance_table_start) MULTIPLY ($index, 4) COPY ($index, 4, extra_distance_bits) INPUT-BITS ($extra_distance_bits, extra_distance_bits, fail) ADD ($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 Price et al. [Page 20] INTERNET-DRAFT SigComp User Guide June 24, 2002 END-MESSAGE (requested_feedback_location, returned_parameters_location, state_length, 64, decompress_sigcomp_message, 6, 0) :fail DECOMPRESSION-FAILURE :circular_buffer An example message compressed using the DEFLATE algorithm is given below: 0xf3 c9 4c 4b d5 51 28 c9 48 55 08 cd cb 2c 4b 2d 2a 4e 55 48 cc 4b 0x51 70 05 32 2b 4b 32 32 f3 d2 b9 00 00 00 00 ff ff 00 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]. Mnemonic code for the LZJH decompressor is given below: .pad 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 .align 42 :requested_feedback_location = 0 .align 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 Price et al. [Page 21] INTERNET-DRAFT SigComp User Guide June 24, 2002 ; decompressing a SigComp message: :current_length .pad 2 :decompressed_pointer .pad 2 :ordinal_length .pad 2 :codeword_length .pad 2 :codebook_next .pad 2 :returned_parameters_location = 0 .align 64 :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): :udvm_memory_size = 8192 ; sets the total memory for LZJH :max_extension_length = 8 ; sets the maximum string extension length :min_ordinal_length = 7 ; sets the minimum ordinal length :min_codeword_length = 6 ; sets the minimum codeword length :codebook_start = 4879 :first_codeword = codebook_start - 12 :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, fail, 2, 1, 1, 1, 2, 1, 0, 1, 0) COMPARE ($index, 1, ordinal, string_extension, codeword_control) Price et al. [Page 22] INTERNET-DRAFT SigComp User Guide June 24, 2002 :ordinal ; The following code decompresses an ordinal character, and creates ; a new codebook entry consisting of the ordinal character plus the ; next character to be decompressed: :index_lsb = index + 1 :current_length_lsb = current_length + 1 INPUT-BITS ($ordinal_length, index, fail) 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, fail) COMPARE ($index, 3, control_code, initialize_memory, codeword) :codeword ; The following code interprets a codeword as an index into the LZJH ; codebook. It extracts the position/length pair from the specified ; codebook entry; the position/length pair points to a byte string ; in the circular buffer which is then copied to the end of the ; decompressed message. The code also creates a new codebook entry ; consisting of the byte string plus the next character to be ; decompressed: :length_value_lsb = length_value + 1 MULTIPLY ($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, $length_value) JUMP (prefix_after_codeword) :string_extension ; The following code decompresses a Huffman-encoded string extension: Price et al. [Page 23] INTERNET-DRAFT SigComp User Guide June 24, 2002 INPUT-HUFFMAN (index, fail, 4, 1, 1, 1, 1, 2, 1, 3, 2, 1, 1, 1, 13, 3, 0, 7, 5) COMPARE ($index, 13, continue, extra_bits, extra_bits) :extra_bits INPUT-BITS (max_extension_length, extra_extension_bits, fail) 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, fail, 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, fail) COMPARE ($index, 1, stepup_ordinal, stepup_codeword, 0) :stepup_ordinal ADD ($ordinal_length, 1) JUMP (ordinal) :stepup_codeword Price et al. [Page 24] INTERNET-DRAFT SigComp User Guide June 24, 2002 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) :fail DECOMPRESSION-FAILURE :circular_buffer 5.6. EPIC This section provides bytecode for a version of the Efficient Protocol Independent Compression (EPIC) scheme [EPIC]. The basic EPIC scheme [EPIC] 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. Price et al. [Page 25] INTERNET-DRAFT SigComp User Guide June 24, 2002 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): 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: .pad 32 :index .pad 2 :distance_value .pad 2 :old_pointer .pad 2 .align 42 :requested_feedback_location = 0 .align 64 :byte_copy_left .pad 2 :byte_copy_right .pad 2 :input_bit_order .pad 2 :decompressed_pointer .pad 2 :returned_parameters_location = 0 .align 64 :initialize_memory :udvm_memory_size = 8192 :state_length = udvm_memory_size - 64 MULTILOAD (64, 4, circular_buffer, udvm_memory_size, 0, circular_buffer) :decompress_sigcomp_message :character_after_literal Price et al. [Page 26] INTERNET-DRAFT SigComp User Guide June 24, 2002 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, 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 :index_lsb = index + 1 OUTPUT (index_lsb, 1) COPY-LITERAL (index_lsb, 1, $decompressed_pointer) JUMP (character_after_literal) :distance Price et al. [Page 27] INTERNET-DRAFT SigComp User Guide June 24, 2002 SUBTRACT ($index, 253) INPUT-HUFFMAN (distance_value, fail, 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, 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) :fail DECOMPRESSION-FAILURE :circular_buffer 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 Price et al. [Page 28] INTERNET-DRAFT SigComp User Guide June 24, 2002 SigComp compressor should maintain a similar list containing the 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 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 the label ":requested_feedback_location = 0" with the following piece of mnemonic code: :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: Price et al. [Page 29] INTERNET-DRAFT SigComp User Guide June 24, 2002 :end_of_message :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 0xc7 0xb6 0x11 0x50 0x61 0x44 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 draft [SIP-DICT]. Price et al. [Page 30] INTERNET-DRAFT SigComp User Guide June 24, 2002 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, fail) CRC ($index, 64, state_length, fail) The bytecode extracts a 2-byte CRC checksum 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 | +---+---+---+---+---+---+---+---+ Price et al. [Page 31] INTERNET-DRAFT SigComp User Guide June 24, 2002 These fields can be reserved in any of the compression algorithms of Chapter 5 by replacing the label ":returned_parameters_location = 0" with the following piece of mnemonic code: :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 | +---+---+---+---+---+---+---+---+ Price et al. [Page 32] INTERNET-DRAFT SigComp User Guide June 24, 2002 +---+---+---+---+---+---+---+---+ | 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 compression algorithms of Chapter 5 by replacing the label ":returned_parameters_location = 0" with the following piece of mnemonic code: :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 mnemonic code after the label ":decompress_sigcomp_message" in one of the algorithms of Chapter 5: :decompress_sigcomp_message INPUT-BYTES (1, extended_flags, fail) COMPARE ($extended_flags, 32768, initialize_state_announcement, access_shared_state, access_shared_state) Price et al. [Page 33] INTERNET-DRAFT SigComp User Guide June 24, 2002 :access_shared_state INPUT-BYTES (6, shared_state_id, fail) 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 ($extended_flags, 32768, algorithm_start, announce_acked_state_id, announce_acked_state_id) :announce_acked_state_id LOAD (length_of_partial_state_id_a, 1536) INPUT-BYTES (6, partial_state_identifier_a, fail) LOAD (announcement_location, length_of_partial_state_id_b) :algorithm_start Additionally, the following piece of mnemonic code should be inserted following the label ":end_of_message" in the chosen compression algorithm: :end_of_message LSHIFT ($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) :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 Price et al. [Page 34] INTERNET-DRAFT SigComp User Guide June 24, 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 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 35] INTERNET-DRAFT SigComp User Guide June 24, 2002 11. References [SIGCOMP] "Signaling Compression (SigComp)", R. Price et al., , June 2002 [EXTENDED] "SigComp - Extended Operations", Hannu et al., , June 2002 [SIP-DICT] "The Session Initiation Protocol (SIP) and Session Description Protocol (SDP) static dictionary for Signaling Compression (SigComp)", Garcia et al., , June 2002 [FLOWS] "SIP Call Flow Examples", A. Johnston et al., "draft-ietf-sipping-call-flows-00.txt", February 2002 [RFC-2026] "The Internet Standards Process - Revision 3", Scott Bradner, Internet Engineering Task Force, October 1996 [RFC-2119] "Key words for use in RFCs to Indicate Requirement Levels", Scott Bradner, Internet Engineering Task Force, March 1997 [RFC-2234] "Augmented BNF for Syntax Specifications: ABNF", D. Crocker and P. Overell, RFC 2234, November 1997 [RFC-2326] "Real Time Streaming Protocol (RTSP)", H. Schulzrinne, A. Rao and R. Lanphier, RFC 2326, 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", RFC 1951, P. Deutsch, May 1996 [LZJH] "Data Compression Procedures", ITU-T Recommendation V.44, November 2000 [EPIC] "Enhanced TCP/IP Compression for ROHC", R. Price et al., , February 2002 Price et al. [Page 36]