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| 7.1 |  | Troubleshooting the Application Layer |
| 7.1.1 |  | Overview |

The primary responsibility of the upper layers of the OSI model is to provide services such as e-mail, file transfer, and data transport. Application layer problems result when data is not delivered to the destination or network performance degrades to a level where productivity is affected.

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.

Problem isolation is vital to successfully troubleshoot any problem. Merely isolating the problem will not bring the types of changes necessary to return network functions to the documented baseline. To meet the troubleshooting objective of resolving the problem, use the tools and resources that are provided to correctly configure the properties of a properly functioning network.

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 Activity 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.