Strange TCP checksum problem with several interconnected networks

[brescia at Bbn-Unix.ARPA]

From:  Mike Brescia <brescia at Bbn-Unix.ARPA>

Rick,

My first reaction is to suspect IP source routing.  Are you using SR to get
through to nets that are not known to the world?

The feature of the TCP checksum that enters in here is that it is invariant
as source routing is followed.  The TCP pseudoheader used in the checksum
has the ultimate destination rather than the first IP destination in it.

Mike