Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...

The OSI Model 77
Layer 5: Session
The session layer is responsible for establishing, monitoring, and terminating
sessions, using the virtual circuits established by the transport layer. Its
characteristics and functions are as follows:
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It is responsible for putting header information into data packets to indicate
where the message begins and ends.
It performs synchronization between the sender’s session layer and the
receiver’s session layer. The use of acknowledgement messages (ACKs) helps
coordinate transfer of data at the session layer.
It controls whether the communications within a session are sent as full
duplex or half duplex messages.
It establishes a connection between two processes. A process is a defined task
related to an application.
The session layer is responsible for setting up the connection between an
application process on one computer and an application process on another
computer, after the transport layer has established the connection between
the two machines.
Protocols that operate at the session layer include Windows Sockets
(the Winsock interface) and the Network Basic Input/Output interface
(NetBIOS).
Layer 6: Presentation
In the presentation layer, data translation is the primary activity performed. The
sender’s application passes data down to the presentation layer, where it is put
into a common format. When the data is received on the other end, the presentation
layer changes the data from the common format back into a format that
is useable by the application. Protocol translation, the conversion of data from
one protocol to another so that it can be exchanged between computers that use
different platforms or operating systems, takes place here. Its characteristics and
functions are as follows:
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Gateway services operate at this layer. Gateways are connection points
between networks that use different platforms or applications.
It is responsible for data compression, which is used to minimize the actual
number of bits that must be transmitted on the network media to the
receiver.
It is responsible for data encryption and decryption.
Layer 7: Application
The application layer is the point at which the user application program interacts
with the network. This layer of the OSI model should not be confused with
the application itself. Application processes are initiated within a user application
and then the data created by that process are handed to the application layer of the