This example is an actual case that was solved by protocol analysis. The problem was reported as an occasional FTP failure with the error message:
netout: Option not supported by protocol 421 Service not available, remote server has closed connection
Only one user reported the problem, and it occurred only when transferring large files from a workstation to the central computer, via our FDDI backbone network.
We obtained the user’s data file and were able to duplicate the problem from other workstations, but only when we transferred the file to the same central system via the backbone network. Figure 11-7 graphically summarizes the tests we ran to duplicate the problem.
Figure 11-7. FTP test summary
We notified all users of the problem. In response, we received reports that others had also experienced it, but again only when transferring to the central system and only when transferring via the backbone. They had not reported it because they rarely saw it. But the additional reports gave us some evidence that the problem did not relate to any recent network changes.
Because the problem had been duplicated on other systems, it probably was not a configuration problem on the user’s system. The FTP failure could also be avoided if the backbone routers and the central system did not interact. So we concentrated our attention on those systems. We checked the routing tables ...