Session Layer (Layer 5)

Wednesday, 08 September 2010 09:41

The fifth layer in the OSI Reference Model is the session layer. As we proceed up the OSI layer stack from the bottom, the session layer is the first one where pretty much all practical matters related to the addressing, packaging and delivery of data are left behind—they are functions of layers four and below. It is the lowest of the three upper layers, which collectively are concerned mainly with software application issues and not with the details of network and internet implementation.

The name of this layer tells you much about what it is designed to do: to allow devices to establish and manage sessions. In general terms, a session is a persistent logical linking of two software application processes, to allow them to exchange data over a prolonged period of time. In some discussions, these sessions are called dialogs; they are roughly analogous to a telephone call made between two people.

Application Program Interfaces (APIs)

The primary job of session layer protocols is to provide the means necessary to set up, manage, and end sessions. In fact, in some ways, session layer software products are more sets of tools than specific protocols. These session-layer tools are normally provided to higher layer protocols through command sets often called application program interfaces or APIs.

Common APIs include NetBIOS, TCP/IP Sockets and Remote Procedure Calls (RPCs). They allow an application to accomplish certain high-level communications over the network easily, by using a standardized set of services. Most of these session-layer tools are of primary interest to the developers of application software. The programmers use the APIs to write software that is able to communicate using TCP/IP without having to know the implementation details of how TCP/IP works.

For example, the Sockets interface lies conceptually at layer five and is used by TCP/IP application programmers to create sessions between software programs over the Internet on the UNIX operating system. Windows Sockets similarly lets programmers create Windows software that is Internet-capable and able to interact easily with other software that uses that interface. (Strictly speaking, Sockets is not a protocol, but rather a programming method.)

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).