Page 141 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Computer Networks/Networks

Notes 9.1 Network Layer Design Issues

The network layer design issues include the service provided to the transport layer, routing of
packets through the subnet, congestion control, and connection of multiple networks together,
etc. The design issues of network layer are given as below:
It is the purpose of network layer to provide seamless services to different users connected
to different networks, therefore, the services provided should be independent of the
underlying technology. In other words, users availing the service need not to bother of
the physical implementation of the network for transmitting their messages. It should be
able to provide interoperability among variety of networks in operation and provided by
different vendors. Hence, the design of the layer should not restrict the uses from
connecting to networks of different technologies.

The transport layer at the host machine should not need to know as to how the
communication link with destination machine is established. Hence, it should be shielded
from the number, type and different topologies of the subnets that it uses.

There should be some uniform addressing scheme for network addresses.
There are two different types of communication links. They are connection oriented and
connectionless.

Connection Oriented Services: In connection-oriented service, each packet is associated with a
source/destination connection. These packets are routed along the same path, known as a virtual
circuit. Thus, it provides end-to-end connection to the user for reliable data transfer. It delivers
data in order without duplication or missing data and therefore, does not congest the
communication channel and the buffer of the receiving machine. The host machine requests a
connection to communicate and closes the connection after transmission of the data. A telephone
communication is an example of a connection-oriented service. In the connection-oriented service,
the user engages the bandwidth and other resources of the network for duration of the connection
and therefore bound to pay more. This service also keeps network resources engaged even
when there is no communication during the connection. It is found to be efficient to send a
constant stream of data down the line. If user wishes to send only a packet or two of data, then
the cost of setting up the connection is enormously high and most of the time the line will
remain idle and wasting bandwidth and resources of the network. Apparently, connection-
oriented services are useful when the user has a constant stream of data to transmit.
Connectionless Service: In connectionless service, a router treats each packet individually. The
packets are routed through different paths through the network according to the decisions made
by routers. In connectionless service, the network or communication channel does not guarantee
delivery of data from host machine to destination machine. The data to be transmitted is broken
into packets. These independent packets are called datagrams in analogy with telegrams. The
packets contain the address of the destination machine. A connectionless service is equivalent to
the postal system. In postal system, a letter is put in an envelope that contains the address of the
destination. It is then put in a letterbox. The letter finally delivers at the destination through
postal network. However, it does not guarantee to arrive at the addressee’s letterbox. Similarly,
in connectionless service packets of data containing address are transmitted with a hope that it
will finally reach to the destination after bouncing forth and back in the communication network.
The connectionless service as compared to connection-oriented services appears to offer a
drawback in terms of unreliable delivery of data but the probability of loss of packet is quite
low. Many applications have their own error detection, flow and congestion control mechanism
at a higher level of layers in the protocol stack i.e. transport layer at either host or destination
machine or at both ends.

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