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
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.
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__
#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
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];
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);
(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.
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.
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)",
RFC1962, June 1996.
[3] Rand, D., "PPP Reliable Transmission", RFC1663,
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
