RE: [Tsvwg] [SCTP checksum problems]

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

All,

A few corrections to this post.

   #define BASE 65521
   unsigned short 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 );

The 32 bit CRC table for an 8 bit lookup would be 1k byte in size and not
512 bytes.

> 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