In a similar story, in my TAC support days (I supported the PIX firewall) I got an odd case: an FTP transfer of a particular file always stopped for a customer.

In a similar story, in my TAC support days (I supported the PIX firewall) I got an odd case: an FTP transfer of a particular file always stopped for a customer. At the same place. No other files were affected. I knew that there was nothing to interfere with an active data transfer in a PIX but nonetheless. I asked him to take a sniffer trace outside the PIX. Indeed, one TCP segment just kept getting dropped and retransmitted. I extracted the data from that particular segment, zipped it as a file, and asked him to try transferring this, much shorter file. It failed. And this was when I spotted that each time we’d attempt to transfer the file, the CRC errors on the receiving PIX interface incremented by one. Replacing the cable connecting to that interface had alleviated the problem. Whether it was a cable quality or a cable just not plugged correctly, will remain a mystery, but that case was helpful to always remember that somewhere low enough everything digital is still analog. And yes, having layers in the architecture is very good.