I writing to this list because the TCP workgroup was shutdown a while ago.
We have a compatibility problem between two third party implementations of
the TCP checksum.
The problem concerns the representation of zero, which has two 1-s
complement representations (0000 and ffff).
We don't have source to either stack but I managed to deduce the problem
from looking at a packet trace. The receiver drops all datagrams containing
a TCP with a TCP checksum of ffff.
The receiver appears to be following the checksum procedure in the RFC
literally - ie zero checksum, recalculate and check that the result is the
complement of what the sender sent. The problem is that the sender and
receiver don't agree about zero and hence the datagram is dropped silently.
My view is that the receiver is in error; it should be checking the special
case of 0.
All the receiver code I have seen doesn't work this way. The normal
approach is to calculate the1-s complement sum with checksum in place and
check that this is zero. The methods I konw always return just one
representation for zero, hence there is no special case.
Any thoughts?
Thanks,
Chris
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