Why does my capture contain packets with invalid checksums?
Posted by Francis Rammeloo on 23 June 2021 12:42 PM

When creating a packet capture (.pcap file) using the ByteBlower GUI, then it's possible that some packets will have an incorrect checksum in the capture. Wireshark typically marks those in red with the hint: "maybe caused by UDP checksum offload?".

That hint is actually correct. In some cases the ByteBlower makes use of checksum offloading. This means that the checksumming is done by the NIC hardware. Because the packets are stored in the capture before sending them to the NIC they still have the invalid checksum in the capture.

Currently checksum offloading is only used for Frame Blasting flows that have one of the following features enabled: latency, out-of-sequence, random size modifier, growing size modifier or field modifier. In all other cases the ByteBlower will perform the checksumming in software.

Remember that the actual packet sent on the network will have the correct checksum! You can verify this by capturing on the receiving interface. The incoming packets will should have a correct checksum. Unless, of course, something bad happened like packet corruption.

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