The Network Layer
The Data Link Layer had the responsibility of
reliably transmitting frames across along a single wire (or
wireless, ...) link.
The Network Layer's responsibility is to get packets from an
actual source machine to a destination machine.
There may be many hops along the way.
The Network Layer is thus the lowest OSI Layer that has to deal with
The Network Layer must be aware of the immediate topology of its subnet
(its neighbours) to make a routing choice that 'makes progress',
avoids introducing congestion on links, and
avoids already congested links.
If the source and destination machine are on different networks
(either physically or politically),
we must support internetworking.
Network Layer Design Objectives
The design objectives of an effective Network Layer are :
- to be independent of processor/communication technology,
- to be independent of the number, type and topology of the subnets, and
- to provide a uniform addressing scheme for all hosts in the network.