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checksum calculation in ip

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Nandini Ganig

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I am implementing IPv6, communicating from pc to fpga
I know how to calculated checksum but the problem lies where i am seeing the capture protocol in Wireshark , there the checksum is zero but when i am seeing in logic analyzer the checksum is correct ( some number other than zero)

can anyone please tell me what to do while receiving and while transmitting about the checksum....

whether i must check the checksum while receiving also.

thanks in advance
Ganig
 

While transmitting u need to calculate the checksum and while receiving u need to verify the checksum.
I don't have an answer to ur wireshark vs LA query...
 
Many times running checksum calculations result in a "magic number" (i.e. a value of 0) when they compute the received checksum over the received data plus the checksum sent by the transmitter. Perhaps what Wieshark is displaying is the magic number of 0 which is the result of a good checksum computation.
 

I suspect that zero checksum is due to checksum offloading. The OS fills in zero, and trusts the NIC to do the checksum. Wireshark captures the packet just before it goes to the NIC and sees a checksum of zero. The NIC calculates the checksum and that goes on the wire. So any point after that sees the correct checksum. For some more info see: https://wiki.wireshark.org/CaptureSetup/Offloading
 
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mrflibble said:
I suspect that zero checksum is due to checksum offloading. The OS fills in zero, and trusts the NIC to do the checksum. Wireshark captures the packet just before it goes to the NIC and sees a checksum of zero. The NIC calculates the checksum and that goes on the wire. So any point after that sees the correct checksum. For some more info see: https://wiki.wireshark.org/CaptureSetup/Offloading
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That is useful to know, the testing I've done in the past involved network test equipment that generated IP packets and not a PC. Things like wireshark monitored the traffic between the test equipment and the DUT using a hub.
 

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