An Operating Systems Vade Mecum

Distributed operating systems 3113.3 Long-haul networksThese protocols were initially designed for long-haul networks, although they have beenused since in local-area networks as well. A long-haul network is a collection of widelyscattered computers connected by a common communication network following a set ofprotocols. The Arpanet, mentioned earlier, is a good example of such a network. It ispart of the Arpa Internet, which also includes MilNet (military sites), SatNet (satellitelinks), and many local networks. There are at least 2000 computers in the Internet.Routing of messages is automatic, based on a hierarchical addressing scheme and a backboneof relay computers. Messages are divided into packets, each of which is routedseparately.The computer on which this text was created is a member of a local-area network,part of the Arpa Internet, that connects it to other local research and instructionalmachines. The machine is thereby connected as well to CSNET, BITNET, and theUUCP network. CSNET is an organization connecting academic computer sciencedepartments. It uses both Arpanet and PhoneNet, which has a central computer thatphones member sites on a regular basis to collect and deliver messages. BITNET connectsover 1500 computers worldwide using dedicated leased lines. Routing of messagesis performed automatically, using manually maintained files distributed from a centralsite. The UUCP network is is a community of perhaps 10,000 computers that use theUUCP (Unix-to-Unix copy) protocol, usually over telephone connections. It is by nomeans restricted to Unix sites. Routing of messages is often explicitly specified by thesource computer.Networks can be connected together my means of gateway computers that resideon two or more networks. They relay packets from one network to the other, modifyingthe format when necessary to accommodate the different protocols. They must sometimesdivide packets in order to obey packet-length requirements. A single message maygo through several gateways as it makes its way from a local area network throughseveral long-haul networks until it reaches its final destination.Long-haul networks often provide a number of services, most important of whichis the ability for users to mail messages to each other across the country. Naming conventionsfor mail addresses are not standardized across networks, but recent attempts tounify addresses have been helpful. A more general way to transfer information is bytransferring files from one machine to another across the network.Another service is remote logon, which allows a user to log on to a machineacross the network. Every character typed by such a user is typically transmitted throughthe network to the distant machine, echoed there, and sent back to the local machine. Ofcourse, most installations require that users who want to log on to their machines musthave accounts (and passwords) for those machines.Long-haul networks also provide information services that allow one to obtaininformation about users of any machine on the network. For example, the Finger serverwe have been discussing is available on many machines and allows remote requestsabout the local user community. Some machines maintain databases of informationabout the entire user community of the network. These databases may be queried fromany machine in the network. Information often includes mail addresses (both computer