The same general troubleshooting process used to isolate problems at the
lower layers can be used to isolate problems at the application layer. The
ideas stay the same, but the technological focus has shifted to involve things
such as refused or timed out connections, access lists, and DNS issues.
Application layer problems prevent services from being provided to
application programs. A problem at the application layer can result in
unreachable or unusable resources when the physical, data link, network, and
transport layers are functional. It is possible to have full network
connectivity but the application simply cannot provide data.
Another type of problem at the application layer occurs when the physical,
data link, network, and transport layers are functional, but the data transfer
and requests for network services from a single network service or application
do not meet the normal expectations of a user.
A problem at the
application layer may cause users to complain that the network or the
particular application that they are working with is sluggish or slower than
usual when transferring data or requesting network services.
Lab
Exercise: Isolating Problems at the Transport and Application Layers
After completing this lab, the student will be able to analyze user-feedback
and end-system data to decide which OSI layer to begin isolating problems.