Not necessarily. If the receiver doesn't send any data then there's
nothing for the sender to ACK so the ACK # stays the same. If there is
any two way data though you're correct that you'd need state. For that
reason I probably would prefer overloading checksum and urgent if this
needed to be done.
i very much hate to bring up the possibility, but it might be better
to explicitly label flows and recover necessary forwarding state based
on the label, rather than exploit a series of special cases to cram a
few bits here and there in the original packet
of course managing this state (distributed forwarding cache) is a
pretty severe pain pain in the ass
I'd argue that we should ignore the problem for now, but keep it in
mind. Doing it the "right" way (ie an MPLS-style forwarder) is
reasonable if this constitutes a problem that is of reserach interest to
us. Doing it the "wrong" way (hacking a few bits here and there) is
reasonable if we just need to eliminate fragmentation bias for our
measurements.