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Given the 2 supplied capture files ...

I used Tracewrangler to restore the original IP's, namely => and => to more easily compare them and to be able to refer to the same IP addresses in both capture files.

The captures were apparently taken with a snaplen of 54 bytes, which is a bit unfortunate as we don't have full frames for checksum verification. That said, I focused on a single packet, namely the 1st retransmission of the 1460 byte segment being sent from to with sequence # 3020828.

In the snd.pcapng file, this is frame #58376; in the recv.pcapng file, this is frame #13878. Comparing these 2 frames, the only differences are:

  • TTL: It is 128 in snd.pcapng and 127 in recv.pcapng. This TTL difference was already noted by Packet_vlad.
  • TCP Checksum: It is 0x3f4f (unverified) in snd.pcapng and 0x31fa (unverified) in recv.pcapng.

Unfortunately, the TCP checksums can't be verified because of the snaplen used. That said, the TCP Checksums should be identical since TTL isn't included as part of the 96-bit pseudo header that goes into the TCP Checksum algorithm. The TCP checksums would/could differ though if:

  • There was some NAT'ing going on, in which case the IP addresses could have changed. Is this the case?
  • TCP Checksum Offloading is being done on the sender side, in which case 0x3f4f might not be the actual TCP checksum that was calculated and transmitted. If this could be the case, then it would be better to capture outside of the host.

It might be worthwhile to capture both sides again, but without applying a snaplen.

In any case, IF the TCP checksum is wrong when the TCP segment arrives at its destination, then this would explain why the receiver never ACK's it.