2.2.2 Networks

The constituent networks of the Internet system are required to provide only packet (connectionless) transport. According to the IP service specification, datagrams can be delivered out of order, be lost or duplicated, and/or contain errors.

For reasonable performance of the protocols that use IP (e.g., TCP), the loss rate of the network should be very low. In networks providing connection-oriented service, the extra reliability provided by virtual circuits enhances the end-end robustness of the system, but is not necessary for Internet operation.

Constituent networks may generally be divided into two classes:

• Local-Area Networks (LANs) LANs may have a variety of designs. LANs normally cover a small geographical area (e.g., a single building or plant site) and provide high bandwidth with low delays. LANs may be passive (similar to Ethernet) or they may be active (such as ATM).

• Wide-Area Networks (WANs) Geographically dispersed hosts and LANs are interconnected by wide-area networks, also called long-haul networks. These networks may have a complex internal structure of lines and packet-switches, or they may be as simple as point-to-point lines.