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Revision 8 as of 2005-08-30 18:00:46
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Revision 9 as of 2006-04-10 11:12:50
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Editor: UlfLamping
Comment: offloading clarification
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By default and whenever possible Ethereal will verify whether the TCP checksum of a packet will be correct or not. By default and whenever possible Ethereal will verify whether the ["TCP"] checksum of a packet will be correct or not.
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The TCP checksum will only be tested for packets that have been fully captured, and thus for short packets, the checksum will never be verified. But then again, short packets will be ignored by the desegmentation engine anyway. The TCP checksum will only be tested for packets that have been fully captured, and thus for short packets, the checksum will not be verified. But then again, short packets will be ignored by the desegmentation engine anyway.
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However, there are other causes where you might see it.
If you capture on a ["Gigabit Ethernet"] ["NIC"], or on some slower Ethernet ["NIC"]s, you might see many such "errors" however. This is due to TCP Checksum offloading often being implemented on those ["NIC"]s and thus, for packets being transmitted by the machine, the checksum will not be calculated until the packet is sent out by the ["NIC"], long long after your capture tool intercepted the packet from the network stack.
=== TCP checksum offloading (lot's of checksum errors) ===

There are causes where you might see lot's of checksum errors.

If you capture on a recent Ethernet ["NIC"], you may see many such "checksum errors". This is due to TCP Checksum offloading often being implemented on those ["NIC"]s and thus, for packets being '''transmitted''' by the machine. The checksum will not be calculated until the packet is sent out by the ["NIC"] hardware, long long after your capture tool intercepted the packet from the network stack.

As this may be confusing and will prevent Ethereal from reassemble TCP segments it's a good idea to switch checksum verification off in these cases.
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TCP Checksum Verification

By default and whenever possible Ethereal will verify whether the ["TCP"] checksum of a packet will be correct or not. TCP packets that have invalid checksums will be marked as such with a warning in the information column in the summary pane and also, most important, if the checksum is BAD that tells ethereal that the packet is corrupted and it will NOT be included in any ["TCP Reassembly"]. I.e. these packets will be ignored by the ["TCP Reassembly"] engine and reassembly will not work.

The TCP checksum will only be tested for packets that have been fully captured, and thus for short packets, the checksum will not be verified. But then again, short packets will be ignored by the desegmentation engine anyway.

It should be VERY VERY rare with corrupted packets in todays networks unless you have a router or a switch with a bad RAM module with a sticky bit. Still, it should be VERY rare to see this for packets that actually are corrupted.

TCP checksum offloading (lot's of checksum errors)

There are causes where you might see lot's of checksum errors.

If you capture on a recent Ethernet ["NIC"], you may see many such "checksum errors". This is due to TCP Checksum offloading often being implemented on those ["NIC"]s and thus, for packets being transmitted by the machine. The checksum will not be calculated until the packet is sent out by the ["NIC"] hardware, long long after your capture tool intercepted the packet from the network stack.

As this may be confusing and will prevent Ethereal from reassemble TCP segments it's a good idea to switch checksum verification off in these cases.

To disable checking of the TCP checksum validity, go to the TCP preferences and untick the box for checksum verification

attachment:tcpchecksumchecking.jpg

Preference String

Check the validity of the TCP checksum when possible.

TCP_Checksum_Verification (last edited 2008-04-12 17:51:24 by localhost)