RE: [Tsvwg] [SCTP checksum problems]

["Douglas Otis" <dotis@sanlight.net>]

Lloyd,

I am aware of efforts to compare CRC with Alder-32.  CRC that is primarily
aimed at providing burst error detection but if while trying various
techniques, this modification may be interesting.

  #define BASE 65521
  unsigned s1 = 0x5555;
  unsigned s2 = 0;
  unsigned short dat_buf*

  while (length -= 2)
    {
    s1 += ntoh(*dat_buf++);  /* 16 bit summing */
    s2 += s1;
    if (s2 >= BASE)          /* Adler modulo for s2 only  */
      s2 -= BASE;
    }
  return (s2 << 16) | (s1 & 0xffff);

This would exercise more bits for small packets, improve burst error
sensitivity and trade a modulo function for a network to host swap in some
cases.  It seems to become a comparison against burst errors vs. missing
segment and stuck bit sensitivity.

The alternative code for CRC would look something like this.

Here is an example using a 256 entry (512 byte) table.

  unsigned char* dat_buf;
  unsigned long crc_syn = 0xffffffff;

  while(length--)
    crc_syn = (crc_syn >> 8) ^ crc32_table[(crc_syn & 0xff) ^ *dat_buf++];

  return (crc_syn ^ 0xffffffff);


Doug