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The TCP/IP Guide  9  The Open System Interconnection (OSI) Reference Model       9  OSI Reference Model Layers

Transport Layer (Layer 4) 1 2 Presentation Layer (Layer 6)

Session Layer Functions

As I have mentioned in a few places in this Guide, the boundaries between layers start to get very fuzzy once you get to the session layer, which makes it hard to categorize what exactly belongs at layer 5. Some technologies really span layers 5 through 7, and especially in the world of TCP/IP, it is not common to identify protocols that are specific to the OSI session layer.

The term “session” is somewhat vague, and this means that there is sometimes disagreement on the specific functions that belong at the session layer, or even whether certain protocols belong at the session layer or not. To add to this potential confusion, there is the matter of differentiating between a “connection” and a “session”. Connections are normally the province of layer four and layer three, yet a Transmission Control Protocol (TCP) connection, for example, can persist for a long time. The longevity of TCP connections makes them hard to distinguish from “sessions” (and in fact there are some people who feel that the TCP/IP host-to-host transport layer really straddles OSI layers four and five).

Transport Layer (Layer 4) 1 2 Presentation Layer (Layer 6)