Better to know some
... than all
Network Sub layers
To better address the internetworking problem, the network layer is further subdivided into three sub layers. This subdivision is intended to isolate the functions of internetworking from the functions of intranetworking. The latter is handled by the bottom sublayer, and the former by the upper two sublayers. IWUs typically need to implement all three sub layers. Depending on the packet switching mode employed by the subnets, three situations are possible:
1. All subnets use virtual circuits. In this case a network level virtual circuit protocol is used for internetworking. The IWUs perform the protocol conversion between the subnets' network level protocols and the internetworking protocol.
2. All subnets use datagram's. In this case a network level datagram protocol is used for internetworking. The IWUs perform the protocol conversion between the subnets' network level protocols and the internetworking protocol.
3. Some subnets use virtual circuits, some datagram's. This situation has no easy solution. A practical way of addressing it is to use either a network level virtual circuit protocol or a network level datagram protocol for internetworking and require all participating subnets and IWUs that do not support that mode to implement both modes.
Given that the subnets may use different network architectures and different protocols, many incompatibilities need to be overcome by the IWUs and gateways, including the following:
* Different types of service and network user interfaces
* Different message formats
* Different addressing schemes
* Different packet switching modes
* Different routing methods
* Different error handling methods
* Different security measures