I have a couple of customers that have been complaining of issues on their circuits, an issue that causes them to have problems with large file transfers. The only noteworthy problems in their data streams seem to be TCP Dup Acks – I’ve seen as many as sixty, or over a hundred, in file transfers of 100 MB test files. However, as near as I can determine, these errors are being introduced in the Internet, outside of our network (the customers use VPNs over internet circuits with major carriers for these file transfers).

As I said, we’ve tested our own network thoroughly, but I’m at a loss as to where to go with this issue. Obviously, telling the customer, “It’s not our fault” is unacceptable, as that doesn’t move them any closer to error-free file transfers. On the other hand, I’m not sure where to tell the carriers’ help desk technicians to look for the source of this issue. Has anyone seen this before on Internet circuits, and is there some way I can use Wireshark to help pinpoint the issue more specifically than telling the carrier, “It’s in your cloud”?