n is set up in advance before any packets are sent. The packet originator sends a call request to its serving node, which sets up a route in advance to the desired destination. All packets of a particular message traverse this route, and each packet of the message contains a virtual circuit identifier (logical channel number) along with the packet data. At any given time each station can have more than one virtual circuit to any other station in the network. With virtual circuits, routing decisions are made in advance. Another routing method is called the datagram. Datagram service used optimal routing on a packet-by-packet basis, usually over diverse routes. It is that ideal packet switching service described above. With the datagram approach ad hoc decisions are made for each packet at each node. There is no call-setup phase with datagrams as there is with virtual connections. Virtual connections are advantageous for high community-of-interest 12.3 TCP/IP AND RELATED PROTOCOLS connectivities, whereas datagram service is advantageous for low community-of-interest traffic relations. Datagram service is more reliable because traffic can be alternately routed around network congestion points. Virtual circuits are fixed-routed for a particular call. Call- setup time at each node is eliminated on a packet basis as with the virtual connection technique. One packet switching protocol allows the possibility of setting up permanent virtual connections (PVCs) and is network-assigned. This latter alternative is economically viable for very high traffic intensity relations; otherwise these permanently assigned logical channels will have long dormant periods. 12.3 TCP/IP AND RELATED PROTOCOLS 12.3.1 Background and Scope The transmission control protocol/Internet protocol (TCP/IP) family was developed for the ARPANET (Advanced Research Projects Agency Network). ARPANET was one of the first large advanced packet-switched networks. It was initially designed and operated to interconnect the very large university and industrial d