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In the Data Link layer, we dealt with machines which are adjacent to one another. Even in the Medium Access sublayer where we have a broadcast network, we still restricted to machines which are one hop apart. Now we get on to how to get a packet from a source machine to a destination machine which is more than one hop away. The issues we deal with is how each intermediate machine is suppose to handle moving the packet along.
Internetworking is a really important point in having the Network Layer services. The whole point of having a global network like " the Internet" is to allow different LANs to share information, eg. a machine in our lab running IEEE 802.3 Ethernet and some other machine in Melbourne running FDDI. With all intermediate routers support Network Layer services, these machines can send and receive information with each other.
We have already seen some internetworking-type services when we talked about bridges in the last topic. In some LAN architectures (eg. IEEE 802), these services are defined in the medium access control sublayer.
The issue of having a virtual circuit or a datagram network is very similar to the circuit-switching vs packet-switching issue we discussed in Topic 2. It is, however, different from the connection-oriented vs connectionless service issue. In connection-oriented vs connection service, we are talking about whether the sender and receiver needs to negotiate and set-up (set configuration, allocate devices and resources, etc) a connection before sending data packets, and to relinquish the connection (set configuration back to default, de-allocate resources, etc) when finished. In virtual circuit vs datagram, we are talking about whether to set-up a route (and route ONLY) before sending packets.
Figure 5-2 p344.
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