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Why does creating modifiers on UPD or TCP Headers cause the checksum to be invalid?


This issue does not always occur. Below are the situations when it could occur and how to fix.

  1. Creating a non-signature frame with the size set to auto.
Fix this by changing it back to a fixed frame size.
  1. Generating streams from pcap.
The default of this procedure sets the frame size to auto.
Setting to a fixed frame size alone will not fix this issue, it will also require to remove the last 2 bytes of the custom header.
The reason for this is because Spirent uses these two bytes at the end of the payload (before the FCS) to help calculate the UPD or TCP checksum.
The one issue remaining for this is that the very first stream of the pcap will always have a UDP checksum error.
See screen capture of pcap below shows the 1st frame has a UDP checksum error but the other 2 does not.
This particular pcap only had 3 frames. If the pcap had 100 frames, Spirent TestCenter will create 100 streamblocks and the 1st streamblock will have a UDP checksum error, the remaining 99 will be okay.
To fix this issue the first streamblock needs to be deleted and recreated.

Product : L2-3