12.1. Networking Primer

Mail (paper and electronic) and telephones are two distinct forms of communication. A telephone conversation is connection-oriented , because the caller and the called "own" the line (have a continuous link) until the end of the conversation. Connection-oriented communication guarantees message delivery, preserves the order in which messages are sent, and allows a stream of data to be sent. Mail, in contrast, is a connectionless mode of transfer, which transports information in packets (or datagrams) and gives no guarantees about message delivery and the order in which the packets are received. It has a higher overhead because each packet identifies its sender and the intended receiver; in contrast, a connection-oriented conversation proceeds without further ado, once the parties have identified themselves. Computer networks offer you a similar choice of connection versus connectionless mode of data transfer. It must be mentioned that there are connectionless protocols such as reliable UDP that do offer guaranteed delivery and sequence integrity.

The networking world assigns each computer an internet address, also called an IP address (short for Internet Protocol), a sequence of four bytes typically written in a dot sequence, like this: 192.23.34.1. (This will change with IPv6, because the world is fast running out of four-byte IP addresses.) Just as you have convenient phone aliases such as 1-800-FLOWERS, computers are often given unique aliases, such as www.yahoo.com. Now, many programs can run on one machine, and it is not enough to deliver a message to the machine: it has to be handed over to the appropriate application program running on that machine. A program can ask for one or more ports to be opened, the equivalent of a private mailbox or telephone extension. To send a message to a program, you need its full address: its machine name and the port on which it is listening. Standard applications such as ftp, telnet, and mail actually come in pairs; for example, the ftp program you use talks to a counterpart server program called ftpd (ftp daemon) on the remote computer. Such server programs listen on standard port numbers; when you type www.yahoo.com on your web browser, the browser automatically connects to port 80 on that machine, where it assumes the corresponding web server to be listening. Port numbers 1-1024 are reserved for standard, well-known Internet applications. Many platforms reserve the name "localhost" (and the address 127.0.0.1) to mean the machine on which the program is running.