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We have two scanners in our branch office which upload images to the head office via FTP. In the last week, both scanners have started delivering a lot of corrupted images.

I suspect that the problem may be with the WAN link, and that TCP might not be detecting/correcting all errors. Is there any software for Windows that allows me to test the integrity of the connection by sending packets with an embedded CRC?

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2 Answers 2

up vote 2 down vote accepted

Run Wireshark on the file server. Collect traffic. Analyze.

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This worked perfectly. Since starting Wireshark this morning, I've already detected several bad TCP checksums. –  Nic Mar 11 '10 at 21:55

I worked at a company that manufactured network equipment. We would stress test lines by sending as much UDP traffic as we could across it and measuring what got dropped. That really is only feasible if you directly control both ends of the link. Otherwise you're testing the path between the two endpoints. If you want to rule out the WAN link as the problem, I would suggest you first:

  1. Setup FTP to another machine on the LAN, if that is corrupted, you know it's not the WAN link but rather the scanner. If it is clear, that tells you the LAN is not the culprit. Next setup FTP to another system elsewhere on the internet. (At your ISP if you can swing it)
  2. If it's corrupted again, that's an indicator of the WAN, if it's clean, it points to further out.

TCP checksum failures doesn't make much sense as TCP is designed to fail if the checksum fails.

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