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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Draft R. Friend 3 Expires in six months R. Monsour 4 Hi/fn, Inc. 5 November 18, 1997 7 IP Payload Compression Using LZS 8 10 Status of this Memo 12 This document is an Internet-Draft. Internet Drafts are working 13 documents of the Internet Engineering Task Force (IETF), its areas, 14 and its working Groups. Note that other groups may also distribute 15 working documents as Internet Drafts. 17 Internet-Drafts draft documents are valid for a maximum of six 18 months and may be updated, replaced, or obsolete by other documents 19 at any time. It is inappropriate to use Internet-Drafts as reference 20 material or to cite them other than as "work in progress." 22 To learn the current status of any Internet-Draft, please check the 23 "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow 24 Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), 25 munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or 26 ftp.isi.edu (US West Coast). 28 Distribution of this memo is unlimited. 30 It is intended that a future version of this draft be submitted to 31 the IESG for publication as an Informational RFC. 33 Abstract 35 This document describes a compression method based on the LZS 36 compression algorithm. This document defines the application of the 37 LZS algorithm to the IP Payload Compression Protocol [IPCOMP]. 38 [IPCOMP] defines a method for applying lossless compression to the 39 payloads of Internet Protocol datagrams. 41 Acknowledgments 43 The LZS details presented here are similar to those in PPP LZS-DCP 44 Compression Protocol (LZS-DCP), [RFC-1967]. 46 The author wishes to thank the participants of the IPPCP working 47 group mailing list whose discussion is currently active and is 48 working to generate the protocol specification for integrating 49 compression with IP. 51 Table of Contents 53 1. Introduction...................................................2 54 1.1 General....................................................2 55 1.2 Background of LZS Compression..............................2 56 1.3 Licensing..................................................3 57 1.4 Specification of Requirements..............................3 58 2. Compression Process............................................3 59 2.1 Compression History........................................3 60 2.2 Anti-expansion of Payload Data.............................3 61 2.3 Format of Compressed Datagram Payload......................3 62 2.4 Compression Encoding Format................................4 63 2.5 Padding....................................................5 64 3. Decompression Process..........................................5 65 4. Security Considerations........................................5 66 5. References.....................................................5 67 6. Authors Addresses..............................................7 68 7. Appendix: Compression Efficiency versus Datagram Size..........7 70 1. Introduction 72 1.1 General 74 This document is a submission to the IETF IP Payload Compression 75 Protocol (IPPCP) Working Group. Comments are solicited and should be 76 addressed to the working group mailing list (ippcp@external.cisco.com) 77 or to the editor. 79 This document specifies the application of LZS compression, a lossless 80 compression algorithm, to IP datagram payloads. This document is to 81 be used in conjunction with the IP Payload Compression Protocol 82 [IPCOMP]. This specification assumes a thorough understanding of 83 the IPComp protocol. 85 1.2 Background of LZS Compression 87 Starting with a sliding window compression history, similar to [LZ1], 88 Hi/fn developed a new, enhanced compression algorithm identified as 89 LZS. The LZS algorithm is a general purpose lossless compression 90 algorithm for use with a wide variety of data types. Its encoding 91 method is very efficient, providing compression for strings as short 92 as two octets in length. 94 The LZS algorithm uses a sliding window of 2,048 bytes. During 95 compression, redundant sequences of data are replaced with tokens that 96 represent those sequences. During decompression, the original 97 sequences are substituted for the tokens in such a way that the 98 original data is exactly recovered. LZS differs from lossy compression 99 algorithms, such as those often used for video compression, that do 100 not exactly reproduce the original data. 102 The details of LZS compression can be found in [ANSI94]. 104 The efficiency of the LZS algorithm depends on the degree of 105 redundancy in the original data. A table of compression ratios for 106 the [Calgary] Corpus file set is provided in the appendix in 107 Section 7. 109 1.3 Licensing 111 Hi/fn, Inc. holds patents on the LZS algorithm. Licenses for a 112 reference implementation are available for use in IPPCP, IPSec, TLS 113 and PPP applications at no cost. Source and object licenses are 114 available on a non-discriminatory basis. Hardware implementations are 115 also available. For more information, contact Hi/fn at the address 116 listed with the authors' addresses. 118 1.4 Specification of Requirements 120 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 121 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 122 document are to be interpreted as described in [RFC-2119]. 124 2. Compression Process 126 2.1 Compression History 128 The sender MUST reset the compression history prior to processing each 129 datagram's payload. This ensures that each datagram's payload can be 130 decompressed independently of any other, as is needed when datagrams 131 are received out of order. 133 The sender MUST flush the compressor each time it transmits a 134 compressed datagram. Flushing means that all data going into the 135 compressor is included in the output, i.e., no data is held back in 136 the hope of achieving better compression. Flushing is necessary to 137 prevent a datagram's data from spilling over into a later datagram. 139 2.2 Anti-expansion of Payload Data 141 The maximum expansion produced by the LZS algorithm is 12.5%. 143 If the size of a compressed IP datagram, including the Next Header, 144 Flags, and CPI fields, is not smaller than the size of the original 145 IP datagram, the IP datagram MUST be sent in the original non- 146 compressed form, as described in [IPCOMP]. 148 2.3 Format of Compressed Datagram Payload 150 The following is an example datagram that results when using LZS as 151 the compression algorithm for the IP Payload Control Protocol. Note 152 that the IP header has been omitted for clarity. 154 0 1 2 3 155 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 156 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 157 | Next Header | Flags | Compression Parameter Index | 158 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 159 | | 160 ~ Payload Data (variable) ~ 161 | | 162 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 164 The Next Header, Flags, Compression Parameter Index fields are all described in [IPCOMP]. 166 2.4 Compression Encoding Format 168 The input to the payload compression algorithm is an IP datagram 169 payload. The output of the algorithm is a new (and hopefully smaller) 170 payload. The output payload contains the input payload's data in 171 either compressed or uncompressed format. The input and output 172 payloads are each an integral number of bytes in length. 174 If the uncompressed form is used, the output payload is identical to 175 the input payload and the IPComp header is omitted. If the 176 compressed form is used, the output payload is prepended with the 177 IPComp header and encoded as defined in [ANSI94], which is repeated 178 here for informational purposes ONLY. 180 := [] 181 := 0 | 1 182 := (8-bit byte) 183 := 185 := 1 | (7-bit offset) 186 0 (11-bit offset) 187 := 110000000 189 := 1 | 0 191 := 192 00 = 2 1111 0110 = 14 193 01 = 3 1111 0111 = 15 194 10 = 4 1111 1000 = 16 195 1100 = 5 1111 1001 = 17 196 1101 = 6 1111 1010 = 18 197 1110 = 7 1111 1011 = 19 198 1111 0000 = 8 1111 1100 = 20 199 1111 0001 = 9 1111 1101 = 21 200 1111 0010 = 10 1111 1110 = 22 201 1111 0011 = 11 1111 1111 0000 = 23 202 1111 0100 = 12 1111 1111 0001 = 24 203 1111 0101 = 13 ... 205 2.5 Padding 207 A datagram payload compressed using LZS always ends with the last 208 compressed data byte (also known as the ), which is used 209 to disambiguate padding. This allows trailing bits as well as bytes 210 to be considered padding. 212 The size of a compressed payload MUST be in whole octet units. 214 3. Decompression Process 216 If the received datagram is compressed, the receiver MUST reset the 217 decompression history prior to processing the datagram. This ensures 218 that each datagram can be decompressed independently of any other, as 219 is needed when datagrams are received out of order. Following the 220 reset of the decompression history, the receiver decompresses the 221 Payload Data field according to the encoding specified in section 3.2 222 of [ANSI94]. 224 If the received datagram is not compressed, the receiver needs to 225 perform no decompression processing and the Payload Data field of the 226 datagram is ready for processing by the next protocol layer. 228 4. Security Considerations 230 IP payload compression potentially reduces the security of the 231 Internet, similar to the effects of IP encapsulation [RFC-2003]. For 232 example, IPComp makes it difficult for border routers to filter 233 datagrams based on header fields. In particular, the original value 234 of the Protocol field in the IP header is not located in its normal 235 positions within the datagram, and any transport-layer header fields 236 within the datagram, such as port numbers, are neither located in 237 their normal positions within the datagram nor presented in their 238 original values after compression. A filtering border router can 239 filter the datagram only if it shares the IPComp Association used for 240 the compression. To allow this sort of compression in environments in 241 which all packets need to be filtered (or at least accounted for), a 242 mechanism must be in place for the receiving node to securely 243 communicate the IPComp Association to the border router. This might, 244 more rarely, also apply to the IPComp Association used for outgoing 245 datagrams. 247 When IPComp is used in the context of IPSec, it is not believed to 248 have an effect on the underlying security functionality provide by 249 the IPSec protocol; i.e., the use of compression is not known to 250 degrade or alter the nature of the underlying security architecture 251 or the encryption technologies used to implement it. 253 5. References 255 [AH] Kent, S. and Atkinson, R., "IP Authentication Header", draft- 256 ietf-ipsec-auth-header-01.txt, Work in Progress, July 1997. 257 [ANSI94] American National Standards Institute, Inc., "Data 258 Compression Method for Information Systems," ANSI X3.241-1994, August 259 1994. 261 [Calgary] Text Compression Corpus, University of Calgary, available 262 at ftp://ftp.cpsc.ucalgary.ca/pub/projects/text.compression.corpus. 264 [IPCOMP] Shacham, A., "IP Payload Compression Protocol (IPComp)", 265 draft-ietf-ippcp-protocol-01.txt, Work in Progress, October 1997. 267 [LZ1] Lempel, A. and Ziv, J., "A Universal Algorithm for Sequential 268 Data Compression", IEEE Transactions On Information Theory, Vol. IT- 269 23, No. 3, May 1977. 271 [RFC-1962] Rand, D., "The PPP Compression Control Protocol (CCP)", 272 RFC-1962, June 1996. 274 [RFC-1967] K. Schneider, R. Friend, "PPP LZS-DCP Compression Protocol 275 (LZS-DCP)", RFC-1967, August, 1996. 277 [RFC-2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, 278 October 1996. 280 [RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate 281 Requirement Levels", RFC 2119, March 1997. 283 6. Authors Addresses 285 Robert Friend 286 Hi/fn Inc. 287 5973 Avenida Encinas 288 Suite 110 289 Carlsbad, CA 92008 290 Email: rfriend@hifn.com 292 Robert Monsour 293 Hi/fn Inc. 294 2105 Hamilton Avenue 295 Suite 230 296 San Jose, CA 95125 297 Email: rmonsour@hifn.com 299 7. Appendix: Compression Efficiency versus Datagram Size 301 The following table offers some guidance on the compression 302 efficiency that can be achieved as a function of datagram size. 303 Each entry in the table shows the compression ratio that was 304 achieved when LZS was applied to a test file using datagrams of a 305 specified size. 307 The test file was the University of Calgary Text Compression Corpus 308 [Calgary]. The Calgary Corpus consists of 18 files with a total 309 size (all files) of 3.278MB. 311 Datagram size,| 312 bytes | 64 128 256 512 1024 2048 4096 8192 16384 313 --------------|---------------------------------------------------- 314 Compression |1.18 1.28 1.43 1.58 1.74 1.91 2.04 2.11 2.14 315 ratio |