On Sun, 2007-Jan-21 09:32:10 -0500, Louis Mamakos wrote: >However, since it is a 1's complement checksum, there is a distinguished >value (all zero bits) that you could set the checksum field to that >wouldn't occur for a normal computed checksum.
That's a useful idea. > Since the presence of a >checksum is mandatory for TCP, you can use this trick and external >software could "know" that it's a distinguished value. Keep in mind that tcpdump/libpcap in contributed software so making local changes is frowned upon. >I suspect the overhead isn't the CPU instruction, but the memory >reference. If you manipulate the checksum field to the distinguished >value at the time the other fields are written, they they'll all get >written out to memory when the cache line is flushed. You've already >got to initialize the Urgent Pointer field, which is the other 16 bits >in the same 32 bit word. If you were really clever, you could make a >union to cover both fields and zero both out in the same store instruction. And given that a 16-bit update is no faster (and typically slower) than a 32-bit update on all supported architectures, this may even be faster. Feel free to come up with a patch. -- Peter Jeremy
pgpsNM4cAl6HT.pgp
Description: PGP signature
