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TCP/IP Checksum Offload Issues

Posted on 2012-03-14
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Hello Guru’s

I have been troubleshooting some random performance issues regarding network throughput etc. and found something interesting.  While using the wireshark I’m noticing high amount of errors - outgoing packets marked black with red Text and the note [incorrect, should be xxxx (maybe caused by "TCP checksum offload"?)].

I have noticed that I can make some changes  - Control Panel->Network and Internet Connections->Network Connections, right click the connection to change and choose 'Properties'. Press the 'Configure...' button, choose the 'Advanced' tab to see or modify the "Offload Transmit TCP Checksum" and "Offload Receive TCP Checksum" values.

However, my Dell 2950 using a Broadcom BCM5708C NetXtreme II GigE driver with
IPv4 Checksum Offload (Tc/Rx enabled) . I would like to know what this really does? Why is this causing these check sum errors? Is there a repercussion of disabling this feature or not? I should also note that we are running on GIG backbone if that matters or not.

Thanks in advance.
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Question by:Littlemorris
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mat1458 earned 1500 total points
ID: 37724162
If you use TCP Offload you let a processor in the NIC perform the checksum calculation. So a software like Wireshark that runs in the OS does not see the correct checksum. A high number of checksum errors today is rather rare, in my opinion it can be sourced by the way you capture.

Do you capture the entire packet? Otherwise the checksum checking does not make much sense. There are some hints about the errors you are seeing in the Wireshark Wiki:
http://wiki.wireshark.org/TCP_Checksum_Verification
http://www.wireshark.org/docs/wsug_html_chunked/ChAdvChecksums.html
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