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| 6.4 |  | Troubleshooting Complex Network Systems |
| 6.4.1 |  | Identifying complex transport layer problems |

As a network increases in complexity, the chances of network failure and sub optimal performance increase. Problems that might have remained undetected, in simpler networks, may combine to cause problems in a more complex one.

Complex network problems are usually caused by the combination of smaller problems, which by themselves appear to have no immediate impact on the network. To resolve these problems, develop a profile of the symptoms and use this as a starting point for troubleshooting activities.

Also keep in mind that some network problems can be triggered or made more obvious as a result of user behavior. For example, staff at a remote site might need to backup data to a central site every afternoon before going home causing the WAN link to become congested. This activity is likely to impact other users who are still working over the link. This particular problem may be solved by changing the work process (stop backing up across the WAN link), by provisioning more bandwidth, or possibly by implementing superior routing technologies such as Link Fragmentation and Interleaving to better utilize the link bandwidth. When the symptom is regular and predictable, it makes it easier to find the cause and solve the problem.

Some network problems are intermittent. These sorts of network problems occur with no obvious pattern and often just go away and reappear at will. Intermittent network problems are significantly more complicated to troubleshoot because the ability to collect solid information becomes increasingly difficult.

Again, intermittent problems may be caused by user behavior. For example, assume a user is loading a large file across the WAN. This obviously impacts the performance of the WAN link, generating a support call to the IT help desk. By the time the IT help desk has been made aware of the problem and investigates the issue, the file transfer has finished and the WAN link performance has returned to normal.

Intermittent problems can also be caused by the interaction of various technologies. Firewalls running dynamic NAT, configuring NAT for load balancing, and running parallel links between systems can all present intermittent problems in network communications. The key to solving these sorts of issues is in understanding the technologies involved.

Communication cannot be established between Host A and Host F because of misconfigured ACLs on both Router C and Router D. Using the traditional problem solving methodology of reversing changes before trying another possible solution would fail to resolve this problem.

In Figure , communication between Host B and FTP Server D fails intermittently. This time, the problem is caused by dynamic NAT timeouts being too finely tuned. When the Internet is not congested, communications are successful. Because the Internet intermittently gets congested, packets returning from FTP Server D occasionally take too long and the dynamic NAT connection on the router closes, breaking the connection.

In each of these situations, the network engineer would need to be able to recognize the misconfiguration as such, repair the configuration, and even though the problem is not immediately resolved, be confident that it was a contributing factor.