The basic design issue of the network layer includes the services provided to the transport layer, routing of packets through the subnet, congestion control, and connection of multiple networks together(internetworking).
The network layer provides services to the transport layer at the network layer/transport layer interface. Often, this interface has great importance due to another reason – It frequently is the interface between the carrier and the customer, that is, the boundary of the subnet.
Carrier often consists of the control of the protocols and interface up to including the network layer. its function is to deliver packets given to it by its customers. Due to this reason, this interface must be especially well defined. Network layer services have been designed with the following goals-
The transport layer should be shielded from the number, type, and topology of the routers present.
Services should be independent of the router technology.
Network addresses made available to the transport layer should use a uniform numbering plan, even across LANs and WANs.
With these goals, designers of the network layer have a lot of freedom in writing detailed specifications of the services to be provided to the transport layer.
This freedom degenerates into a raging battle between two warring factions. The center of the discussion is, whether the network layer should provide connectionless service or connection-oriented service.
According to one camp (represented by the international community) , argues that the router’s job is moving packets around and nothing else. According to them, the subnet is inherently unreliable, no matter how it is designed.
Thus, the hosts should accept the fact that the network is unreliable and perform error control and flow control themselves.
This thinking leads quickly to the conclusion that the network service should be connectionless, with primitives SEND PACKET and RECEIVE PACKET and little else.
Particularly no packet ordering and flow control should be done, since the hosts are going to do that anyway, and there is usually little to be gained by doing it twice.
In addition, each packet must carry the full destination address, because each packet transmitted is carried independently of its predecessors, if any.
On the other hand, the argument of the other camp is that the subnet should provide a reliable, connection-oriented service. In their view, quality of service is the main factor and quality of service is very difficult to obtain without connections in the subnet, especially for real-time traffic such as video and voice.
The best example of these camps is the internet and ATM. The Internet provides connectionless network layer service and ATM networks provide connection-oriented network layer service. But, it is interesting to notice that as the quality of service guarantees are becoming more and more important, the internet is evolving.
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