Network Working Group D. Rand
Request for Comments: 1978 Novell
Category: Informational August 1996
PPP Predictor Compression Protocol
Status of This Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
The Point-to-Point Protocol (PPP) [1] provides a standard method of
encapsulating multiple protocol datagrams over point-to-point links.
The PPP Compression Control Protocol [2] provides a method for
transporting multi-protocol datagrams over PPP encapsulated links.
This document describes the use of the Predictor data compression
algorithm for compressing PPP encapsulated packets.
Table of Contents
1. Introduction ...................................... 1
2. Licensing ......................................... 2
3. Predictor Packets ................................. 2
3.1 Predictor theory ............................ 2
3.2 Encapsulation for Predictor type 1 .......... 7
3.3 Encapsulation for Predictor type 2 .......... 8
4. Configuration Option Format ....................... 9
SECURITY CONSIDERATIONS .................................. 9
REFERENCES ............................................... 9
ACKNOWLEDGEMENTS ......................................... 9
CHAIR'S ADDRESS .......................................... 9
AUTHOR'S ADDRESS ......................................... 9
1. Introduction
Predictor is a high speed compression algorithm, available without
license fees. The compression ratio obtained using predictor is not
as good as other compression algorithms, but it remains one of the
fastest algorithms available.
Note that although care has been taken to ensure that the following
code does not infringe any patents, there is no assurance that it is
Rand Informational [Page 1]
RFC 1978 Predictor Protocol August 1996 not covered by a patent. 2. Licensing There are no license fees or costs associated with using the Predictor algorithm. Use the following code at your own risk. 3. Predictor Packets Before any Predictor packets may be communicated, PPP must reach the Network-Layer Protocol phase, and the Compression Control Protocol must reach the Opened state. Exactly one Predictor datagram is encapsulated in the PPP Information field, where the PPP Protocol field indicates type hex 00FD (compressed datagram). The maximum length of the Predictor datagram transmitted over a PPP link is the same as the maximum length of the Information field of a PPP encapsulated packet. Prior to compression, the uncompressed data begins with the PPP Protocol number. This value MAY be compressed when Protocol-Field- Compression is negotiated. PPP Link Control Protocol packets MUST NOT be send within compressed data. 3.1. Predictor theory Predictor works by filling a guess table with values, based on the hash of the previous characters seen. Since we are either emitting the source data, or depending on the guess table, we add a flag bit for every byte of input, telling the decompressor if it should retrieve the byte from the compressed data stream, or the guess table. Blocking the input into groups of 8 characters means that we don't have to bit-insert the compressed output - a flag byte preceeds every 8 bytes of compressed data. Each bit of the flag byte corresponds to one byte of reconstructed data. Take the source file: 000000 4141 4141 4141 410a 4141 4141 4141 410a AAAAAAA.AAAAAAA. 000010 4141 4141 4141 410a 4141 4141 4141 410a AAAAAAA.AAAAAAA. 000020 4142 4142 4142 410a 4241 4241 4241 420a ABABABA.BABABAB. 000030 7878 7878 7878 780a xxxxxxx. Rand Informational [Page 2]
RFC 1978 Predictor Protocol August 1996 Compressing the above data yields the following: 000000 6041 4141 4141 0a60 4141 4141 410a 6f41 `AAAAA.`AAAAA.oA 000010 0a6f 410a 4142 4142 4142 0a60 4241 4241 .oA.ABABAB.`BABA 000020 420a 6078 7878 7878 0a B.`xxxxx. Reading the above data says: flag = 0x60 - 2 bytes in this block were guessed correctly, 5 and 6. Reconstructed data is: 0 1 2 3 4 5 6 7 File: A A A A A Guess table: A A flag = 0x60 - 2 bytes in this block were guessed correctly, 5 and 6. Reconstructed data is: 0 1 2 3 4 5 6 7 File: A A A A A Guess table: A A flag = 0x6f - 6 bytes in this block were guessed correctly, 0-3, 5 and 6. Reconstructed data is: 0 1 2 3 4 5 6 7 File: A Guess table: A A A A A A flag = 0x6f - 6 bytes in this block were guessed correctly, 0-3, 5 and 6. Reconstructed data is: 0 1 2 3 4 5 6 7 File: A Guess table: A A A A A A flag = 0x41 - 2 bytes in this block were guessed correctly, 0 and 6. Reconstructed data is: 0 1 2 3 4 5 6 7 File: B A B A B Guess table: A A flag = 0x60 - 2 bytes in this block were guessed correctly, 5 and 6. Reconstructed data is: 0 1 2 3 4 5 6 7 File: B A B A B Guess table: A B flag = 0x60 - 2 bytes in this block were guessed correctly, 5 and 6 Reconstructed data is: 0 1 2 3 4 5 6 7 File: x x x x x Guess table: x x And now, on to the source - note that it has been modified to work with a split block. If your data stream can't be split within a block (e.g., compressing packets), then the code dealing with "final", and the memcpy are not required. You can detect this situation (or errors, for that matter) by observing that the flag byte indicates that more data is required from the compressed data stream, but you are out of data (len in decompress is <= 0). It *is* ok if len == 0, and flags indicate guess table usage. #include <stdio.h> #ifdef __STDC__ Rand Informational [Page 3]
RFC 1978 Predictor Protocol August 1996 #include <stdlib.h> #endif #include <string.h> /* * pred.c -- Test program for Dave Rand's rendition of the * predictor algorithm * Updated by: iand@labtam.labtam.oz.au (Ian Donaldson) * Updated by: Carsten Bormann <cabo@cs.tu-berlin.de> * Original : Dave Rand <dlr@bungi.com>/<dave_rand@novell.com> */ /* The following hash code is the heart of the algorithm: * It builds a sliding hash sum of the previous 3-and-a-bit * characters which will be used to index the guess table. * A better hash function would result in additional compression, * at the expense of time. */ #define HASH(x) Hash = (Hash << 4) ^ (x) static unsigned short int Hash; static unsigned char GuessTable[65536]; static int compress(source, dest, len) unsigned char *source, *dest; int len; { int i, bitmask; unsigned char *flagdest, flags, *orgdest; orgdest = dest; while (len) { flagdest = dest++; flags = 0; /* All guess wrong initially */ for (bitmask=1, i=0; i < 8 && len; i++, bitmask <<= 1) { if (GuessTable[Hash] == *source) { flags |= bitmask; /* Guess was right - don't output */ } else { GuessTable[Hash] = *source; *dest++ = *source; /* Guess wrong, output char */ } HASH(*source++);len--; } *flagdest = flags; } return(dest - orgdest); } static int Rand Informational [Page 4]
RFC 1978 Predictor Protocol August 1996 decompress(source, dest, lenp, final) unsigned char *source, *dest; int *lenp, final; { int i, bitmask; unsigned char flags, *orgdest; int len = *lenp; orgdest = dest; while (len >= 9) { flags = *source++; for (i=0, bitmask = 1; i < 8; i++, bitmask <<= 1) { if (flags & bitmask) { *dest = GuessTable[Hash]; /* Guess correct */ } else { GuessTable[Hash] = *source; /* Guess wrong */ *dest = *source++; /* Read from source */ len--; } HASH(*dest++); } len--; } while (final && len) { flags = *source++; len--; for (i=0, bitmask = 1; i < 8; i++, bitmask <<= 1) { if (flags & bitmask) { *dest = GuessTable[Hash]; /* Guess correct */ } else { if (!len) break; /* we seem to be really done -- cabo */ GuessTable[Hash] = *source; /* Guess wrong */ *dest = *source++; /* Read from source */ len--; } HASH(*dest++); } } *lenp = len; return(dest - orgdest); } #define SIZ1 8192 static void compress_file(f) FILE *f; { char bufp[SIZ1]; char bufc[SIZ1/8*9+9]; Rand Informational [Page 5]
RFC 1978 Predictor Protocol August 1996 int len1, len2; while ((len1 = fread(bufp, 1, SIZ1, f)) > 0) { len2 = compress((unsigned char *)bufp, (unsigned char *)bufc, len1); (void) fwrite(bufc, 1, len2, stdout); } } static void decompress_file(f) FILE *f; { char bufp[SIZ1+9]; char bufc[SIZ1*9+9]; int len1, len2, len3; len1 = 0; while ((len3 = fread(bufp+len1, 1, SIZ1, f)) > 0) { len1 += len3; len3 = len1; len2 = decompress((unsigned char *)bufp, (unsigned char *)bufc, &len1, 0); (void) fwrite(bufc, 1, len2, stdout); (void) memcpy(bufp, bufp+len3-len1, len1); } len2 = decompress((unsigned char *)bufp, (unsigned char *)bufc, &len1, 1); (void) fwrite(bufc, 1, len2, stdout); } int main(ac, av) int ac; char** av; { char **p = av+1; int dflag = 0; for (; --ac > 0; p++) { if (!strcmp(*p, "-d")) dflag = 1; else if (!strcmp(*p, "-")) (dflag?decompress_file:compress_file)(stdin); else { FILE *f = fopen(*p, "r"); if (!f) { perror(*p); exit(1); } (dflag?decompress_file:compress_file)(f); Rand Informational [Page 6]
RFC 1978 Predictor Protocol August 1996 (void) fclose(f); } } return(0); } 3.2. Encapsulation for Predictor type 1 The correct encapsulation for type 1 compression is the protocol type, 1 bit indicating if the data is compressed or not, 15 bits of the uncompressed data length in octets, compressed data, and uncompressed CRC-16 of the two octets of unsigned length in network byte order, followed by the original, uncompressed data packet. 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | CCP Protocol Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |*| Uncompressed length (octets)| * is compressed flag +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1 means data is compressed | Compressed data... | 0 means data is not compressed +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | CRC - 16 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The CCP Protocol Identifier that starts the packet is always 0xfd. If PPP Protocol field compression has not be negotiated, it MUST be a 16-bit field. The Compressed data is the Protocol Identifier and the Info fields of the original PPP packet described in [1], but not the Address, Control, FCS, or Flag. The CCP Protocol field MAY be compressed as described in [1], regardless of whether the Protocol field of the CCP Protocol Identifier is compressed or whether PPP Protocol field compression has been negotiated. It is not required that any of the fields land on an even word boundary - the compressed data may be of any length. If during the decode procedure, the CRC-16 does not match the decoded frame, it means that the compress or decompress process has become desyncronized. This will happen as a result of a frame being lost in transit if LAPB is not used. In this case, a new configure-request must be sent, and the CCP will drop out of the open state. Upon receipt of the configure-ack, the predictor tables are cleared to zero, and compression can be resumed without data loss. Rand Informational [Page 7]
RFC 1978 Predictor Protocol August 1996 3.3. Encapsulation for Predictor type 2 The correct encapsulation for type 2 compression is the protocol type, followed by the data stream. Within the data stream is the current frame length (uncompressed), compressed data, and uncompressed CRC-16 of the two octets of unsigned length in network byte order, followed by the original, uncompressed data. The data stream may be broken at any convenient place for encapsulation purposes. With type 2 encapsulation, LAPB is almost essential for correct delivery. 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | CCP Protocol Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |*| Uncompressed length (octets)| * is compressed flag +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1 means data is compressed | Compressed data... | 0 means data is not compressed +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | CRC-16 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |*| Uncompressed length (octets)| * is compressed flag +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... The CCP Protocol Identifier that starts the packet is always 0xfd. If PPP Protocol field compression has not be negotiated, it MUST be a 16-bit field. The Compressed data is the Protocol Identifier and the Info fields of the original PPP packet described in [1], but not the Address, Control, FCS, or Flag. The CCP Protocol field MAY be compressed as described in [1], regardless of whether the Protocol field of the CCP Protocol Identifier is compressed or whether PPP Protocol field compression It is not required that any field land on an even word boundary - the compressed data may be of any length. If during the decode procedure, the CRC-16 does not match the decoded frame, it means that the compress or decompress process has become desyncronized. This will happen as a result of a frame being lost in transit if LAPB is not used. In this case, a new configure-request must be sent, and the CCP will drop out of the open state. Upon receipt of the configure-ack, the predictor tables are cleared to zero, and compression can be resumed without data loss. Rand Informational [Page 8]
RFC 1978 Predictor Protocol August 1996 4. Configuration Option Format There are no options for Predictor type one or two. Security Considerations Security issues are not discussed in this memo. References [1] Simpson, W., "The Point-to-Point Protocol", STD 51, RFC 1661, July 1994. [2] Rand, D., "The PPP Compression Control Protocol (CCP)", RFC 1962, June 1996. [3] Rand, D., "PPP Reliable Transmission", RFC 1663, July 1994. Acknowledgments The predictor algorithm was originally implemented by Timo Raita, at the ACM SIG Conference, New Orleans, 1987. Bill Simpson helped with the document formatting. Chair's Address The working group can be contacted via the current chair: Karl Fox Ascend Communications 3518 Riverside Drive, Suite 101 Columbus, Ohio 43221 EMail: karl@ascend.com Author's Address Questions about this memo can also be directed to: Dave Rand Novell, Inc. 2180 Fortune Drive San Jose, CA 95131 +1 408 321-1259 EMail: dave_rand@novell.com Rand Informational [Page 9]
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