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Tuesday, July 21, 2009

Quick Tech Tip - Connectionless Internetworking

Connectionless internetworking follows the datagram model. In this model, the network layer offers the transport layer the ability to inject datagrams into the subnet. There is no notion of a virtual circuit at all in the network layer. This model does not require all packets belonging to one connection to traverse the same sequence of gateways.
A routing decision is made separately for each packet, possibly depending on the traffic at the moment the packet is sent. This strategy can use multiple routes and thus achieve a higher bandwidth than the concatenated virtual circuit model.

Connectionless Internetworking

Datagrams are constructed and sent in the usual way. Routing decisions are made on a packet by packet basis so that we cannot even guarantee that all the packets in a message will be sent over the same set of networks. Since each of the networks can have different properties the processing that the packets receive will be different depending on the route they took.
Since each network will have its own network layer protocol we cannot simply transfer network layer packets across the routers. One possibility is to try and convert from one protocol to another but this is not very successful for much the same set of reasons that converting between the different frame types of the 802 Ethernets was difficult.
A major issue in transferring from one network to another is that of addressing. In general different networks use different addressing schemes. One possibility would be to assign every host an address for every sort of network but apart from being inefficient (lots of addresses would never be used) it would also require a huge translation table to be kept.
What IP (Internet Protocol) attempts to do is to define a universal packet which can be carried across all networks. Of course others have also had this idea so there are several ‘universal’ schemes and these have to be dealt with as well.
A major advantage of the datagram approach is that it can be used over subnets that do not use virtual circuits inside. Many LANs, mobile networks and even some WANs fall into this category. When an internet includes one of these, serious problem occurs if the networking strategy is based on virtual circuits.

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