The Higher OSI Layers

There are three more layers in the OSI protocol, but these vary greatly according to the particular application being used. We won’t discuss them beyond these mentions.

The Session Layer

Layer 5, the session layer, is where ongoing interactions between applications happen. The data is couched in terms of things an application might understand (e.g., cookies for a Web browser). This is also the layer where check-pointing (i.e., saving work finished so far) happens. At this layer, we’d discuss things like RPC, SQL, or NetBIOS.

The Presentation Layer

The presentation layer converts data between formats and ensures standard encodings are used to present the information to the application. This layer is all about file formats: ASCII, EBCDIC, JPEG, GIF, and HTML, to name just a few.

The Application Layer

The top layer, layer 7, is totally the province of the application(s) involved in processing messages. Two techs talking about this layer would be swapping stories about application protocols, like FTP, DNS, SMTP, or NFS. Almost nothing that happens at this layer – except for throughput estimates or fouled daemon code – filters into designing or debugging networks.