The session layer
allows users on different machines to establish sessions between them.
A session allows ordinary data transport, as does the transport layer,
but it also provides some enhanced services useful in a some applications.
A session
might be used to allow a user to log into
a remote time-sharing system or to transfer a file between two machines.
One of the services of the session layer is
to manage dialogue control. Sessions can allow traffic to go in both directions
at the same time, or in only one direction at a time. If traffic can only
go one way at a time, the session layer can help keep track of whose turn
it is.
A related session service is token management.
For some protocols, it is essential that both sides do not attempt the
same operation at the same time. To manage these activities, the session
layer provides tokens that can be exchanged. Only the side holding the
token may perform the critical operation.
Another session service is synchronization.
Consider the problems that might occur when trying to do a two-hour file
transfer between two machines on a network with a 1 hour mean time between
crashes. After each transfer was aborted, the whole
transfer would have to start over again, and
would probably fail again with the next network crash. To eliminate this
problem, the session layer provides a way to insert checkpoints into the
data stream, so that after a crash, only the data after the last
checkpoint has to be repeated.
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