Network Layer

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The main functions of network layer is to provide
end-to-end addressing, routing and to provide a connectionless
datagram delivery of packets. One of the main use of
this connectionless service is to serve the transport layer above
it. There had also been suggestions that network layer provides
connection-oriented service. But it’s been pretty much decided
that connectionless service is what network layer provides. It is
therefore, how the layer is defined in OSI and in TCP/IP.

By connectionless we
mean that no prior connection setup is required in order
to send the packets and different packets may follow
entirely different routes through the network. Because
of this, different data units, which are
called datagrams in the network layer terminology,
may arrive at the destination out of order. Some of which may
even be lost. It is not the function of network layer to
reorder the datagrams or to recover the lost packets.

On the other hands, connection-oriented applications
require the call being set up before the actual connections.
In this case, the service provided will guarantee the orderly
deliver of packets, lost packets will have to be retransmitted.

It is possible to use connectionless service provided by
the network layer, such as IP,
to implement connection-oriented services such as TCP,
and to use connection-oriented service provided by data link
layer, such as ATM, to implement connectionless service, such
as IP. This has become an especially important issue given
the adoption of ATM standards and the popularity of existing TCP/IP.
As we will see later that the distinction between connection-oriented
and connectionless can be quite confusing depending on which layer
we are dealing with.