Page 150 - DCAP406_DCAP_207_Computer Networks

Unit 9: Network Layer

ARPAnet must use TCP/IP. The US military adopted TCP/IP as standard protocol in 1983 and Notes
recommended that all networks connected to the ARPAnet conform to the new standards.
The success of ARPAnet was more than the expectations of its own founders and TCP/IP
internetworking became widespread. As a result, new wide area networks (WAN) were created
in the USA and connected to ARPAnet using TCP/IP protocol. In turn, other networks in the rest
of the world, not necessarily based on the TCP/IP protocols, were added to the set of
interconnected networks. Computing facilities all over North America, Europe, Japan, and
other parts of the world are currently connected to the Internet via their own sub-networks,
constituting the world’s largest network. In 1990, ARPAnet was eliminated, and the Internet was
declared as the formal global network.
DARPA also funded a project to develop TCP/IP protocols for Berkeley UNIX on the VAX and
to distribute the developed codes free of charge with their UNIX operating system. The first
release of the Berkeley Software Distribution (BSD) to include the TCP/IP protocol set was made
available in 1983 (4.2BSD). This led to the spread of TCP/IP among universities and research
centers and has become the standard communications subsystem for all UNIX connectivity.
There are many updated versions of BSD code available. These are 4.3BSD (1986), 4.3BSD Tahoe
(1988), 4.3BSD Reno (1990) and 4.4BSD (1993).

9.5 Summary

The main role of the network layer is to accept packets from a source and deliver them to
a destination machine. The network layer provides services that should be independent of
the router technology. It shields the transport layer from the router network details and
facilitates, network addressing to be consistent across networks.
Services of the network layer are available in connection oriented and connectionless
modes. Connection-oriented services are useful only when the user wants to send a constant
stream of data down the line.

The routing algorithms that require selecting a path or route from many possible routes
in the network are part of the router software. They are of two basic types namely non-
adaptive or static and dynamic or adaptive. Selection of routing algorithms depends on
the minimum mean delay for the packets and number of hop before reaching to the
destination machine.
Link State Routing attempts to discover its neighbor and learn their network addresses
and enable the router to choose a shortest path. The hierarchal routing uses multiple
groups to route the packets. Broadcast and multicast routings are used to forward a single
packet to several recipients depending on whether they belong to broadcast or multicast
group.
The shortest path to each destination within the network is found by traversing the tree
and the most common shortest path first algorithm is the Dijkstra algorithm.
The distance vector algorithms is used to determine which path is the best path to each
destination based on the advertised details about the path and distance for each destination
which is maintained in a local database.

9.6 Keywords

Adaptive Algorithms: They are capable of changing their routing decisions to reflect changes in
the topology and the traffic and automatically update routing information when changes are
made to the network configuration.

LOVELY PROFESSIONAL UNIVERSITY 143