Putting your answer together with the one from Chuck :
I understand that if the tcpdump program runs on the same host as the one which is sending
the packets, it may not be able to correctly see the TCP checksum, since it captures the
packet before it goes out on the network, and it is the NIC which calculates and inserts
the TCP checksum just before the packet is sent over the network.
Right ?
But is this the case here ?
Where is/was the tcpdump program run, which captured these packets, as compared to the
"client" and "server" systems ?
Asankha C. Perera wrote:
Something puzzles me since your first post : ...
What is this "TCP CHECKSUM INCORRECT" thing ?
This is the output of some protocol analyser thing, right ?
Yes, its a capture from tcpdump, analyzed by wireshark
So it is totally independent of Tomcat or whatever.
This packet is one that comes from whatever your client is, toward
Tomcat.
Why does it show that message ? And if that message can be believed,
is it then not normal that the protocol stack which receives that
(bad) TCP packet would reject it, and break the connection ?
I guess this is normal. I did a quick search and came across the following:
http://www.ethereal.com/lists/ethereal-dev/200406/msg00090.html
http://stackoverflow.com/questions/667848/java-socket-tcp-checksum-incorrect
http://wiki.wireshark.org/TCP_Checksum_Verification
This trace is from a EC2 node
ubuntu@ip-10-202-99-31:~/configs$ ethtool -k eth0
Offload parameters for eth0:
rx-checksumming: on
tx-checksumming: on
scatter-gather: on
tcp-segmentation-offload: on
udp-fragmentation-offload: off
generic-segmentation-offload: off
generic-receive-offload: off
large-receive-offload: off
ntuple-filters: off
receive-hashing: off
thanks
asankha
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