TRANSPORT LEVEL
The next three layers of the OSI reference model
make up the transport level, also known as the subnet.
The transport level defines the software protocols
necessary to exchange data on the network. The three
layers of the transport level are the network layer, the
transport layer, and the session layer.
Network Layer
The network layer decides which physical
pathway the data should take based on network
conditions, priorities of service, and other factors.
Software on the network interface card must build the
data packet, so the network layer can recognize and
route the data to the correct destination address. It
relieves the upper layers of the need to know anything
about the data transmission and switching
technologies used to connect the systems. It is
responsible for establishing, maintaining, and
terminating connections across the intervening
communications facility as follows:
Addresses messages
Sets up the path between communicating
nodes on possibly different networks
Routes messages among networks
Is concerned with the sequence delivery of
data packets
Controls congestion if too many packets are
on the network
Translates logical addresses or names into
physical addresses
Has accounting functions to count packets or
bits sent by users to produce billing
information
Transport Layer
duplications. It relieves higher layer protocols from
any concern with the transportation of data between
them as follows:
Message segmentation. Accepts data from the
session layer, splits it up into smaller units,
and passes the units down to the network layer
E s t a b l i s h e s a n d d e l e t e s h o s t - t o - h o st
connections across the network
Multiplexes several message streams onto one
channel and keeps track of which message
belongs to which connection
Provides reliable end-to-end delivery with
acknowledgment
Provides end-to-end flow control and window
management
Session Layer
The session layer allows users on different
machines to establish sessions between one another.
It performs the functions that enable two or more
applications to communicate across the network,
performing security, name recognition, logging,
administration, and other similar functions. Unlike
the network layer, this layer is dealing with the
programs in each machine to establish conversations
between them as follows:
Allows two applications processes on
different machines to establish, use, and
terminate a connection (or session)
Performs synchronization between end-user
tasks by placing checkpoints in the data
stream so that if the network fails, only the
data after the last checkpoint has to be
retransmitted
Provides dialogue control (who speaks, when,
how long, and so on)
The transport layer makes sure data units are
delivered error-free, in sequence, without losses or
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