On Wed, Mar 07, 2001 at 09:40:34AM -0500, Louis A. Mamakos wrote:
>
> > > So that the same logic applies to TCP packets as well. Currently, we
> > > can send a TCP packet with a checksum of 0, which is legal. Of possible
> > > interest is that Linux doesn't do this; they alwyas send a non-zero
> > > checksum in the TCP case, if a checksum was computed.
> > >
> > Hmm, but why would we do this for TCP? This violates RFC 793.
> > AFAIK, only UDP checksums are special.
>
> 0x0000 and 0xFFFF are both 16-bit 1's complement representations of
> zero, so you could send either and still have the remote TCP validate
> the checksum.
>
Louis, I recall our long discussion on the topic. It depends on how
remote end verifies the checksum. It it uses the value in checksum
field, then it is irrelevant. If it recomputes the checksum from
scratch, it does matter, and they won't match if you replace 0x0000
computed checksum by 0xffff.
And after I read Stevens, I still think they are the different values,
and you can't receive zero for a two's complement sum if at least one
item is non-zero. Therefore, checksum computed on any valid IP packet
is guaranteed to be non-0xffff. That's why UDP uses zero to indicate
no-checksum, and requires to replace 0xffff by 0.
Cheers,
--
Ruslan Ermilov Oracle Developer/DBA,
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