At 02:05 PM 7/16/2003 -0700, Karl Auerbach wrote:
The last time I saw a comparision of checksum algorithm strengths was back
in the OSI days when the IP checksum was compared to the OSI Fletcher
checksum (my memory is that the IP checksum came in second.)
um, well, it was certainly behind the Fletcher checksum; the IP and TCP
checksums are trivial to beat - you can literally swap any two 16 bit words
without fear. The Fletcher checksum isn't as strong as a CRC, but I did see
its strength compared positively with a CRC. The XNS checksum (one's
complement sum, like IP, but with rotation) fell somewhere in between.
As I understood the discussion, it wasn't so much about getting a better
error check as it was adding FEC, however. Even a 32 bit CRC loses its
value above 10^5 bits, and we're talking about 10^5 *bytes*.
At 04:34 PM 7/16/2003 -0400, Bill Strahm wrote:
Why, oh WHY would I want to receive a known corrupted packet ?
usually it is something about it taking multiple seconds for data to arrive
with a very high BER, and including FEC in the application data that would
allow the system to recover enough useful information to make it
worthwhile. Think interplanetary space. There is also discussion of 64K
MTUs in some sectors, which I tend to think is misguided (I understand
reasons for larger packets, or at least I think I do, but I think the
trade-offs don't justify them).