Re: TCP/IP check sum

The short answer is two-fold:  first, that the TCP checksum is 
end-to-end, while the Ethernet checksum is only hop-by-hop, and second, 
that the TCP checksum detects different classes of errors, in 
particular those that occur on routers.

But all this is explained much more clearly in Stone and Partridge's 
SIGCOMM paper.  Have a look at http://citeseer.nj.nec.com/stone00when.html
for the abstract and pointers to the fuill text.

                --Steve Bellovin, http://www.research.att.com/~smb