rfc1978.txt

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           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];



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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);



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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.





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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.





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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: [e-mail]

Author's Address

   Questions about this memo can also be directed to:

   Dave Rand
   Novell, Inc.
   2180 Fortune Drive
   San Jose, CA  95131

   [phone]
   EMail: [e-mail]



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