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Session Layer (Layer 5)
(Page 2 of 2)
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).
Key Concept: The fifth layer in the OSI Reference Model layer is the session layer. As its name suggests, it is the layer intended to provide functions for establishing and managing sessions between software processes. Session layer technologies are often implemented as sets of software tools called application program interfaces (APIs), which provide a consistent set of services that allow programmers to develop networking applications without needing to worry about lower-level details of transport, addressing and delivery.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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