Dawson Terry - Linux Network Administrator Guide, Second Edition стр 10.

TCP identifies the end points of such a connection by the IP addresses of the two hosts involved and the number of a port on each host. Ports may be viewed as attachment points for network connections. If we are to strain the telephone example a little more, and you imagine that cities are like hosts, one might compare IP addresses to area codes (where numbers map to cities), and port numbers to local codes (where numbers map to individual people's telephones). An individual host may support many different services, each distinguished by its own port number.

In the rlogin example, the client application (rlogin) opens a port on erdos and connects to port 513 on quark , to which the rlogind server is known to listen. This action establishes a TCP connection. Using this connection, rlogind performs the authorization procedure and then spawns the shell. The shell's standard input and output are redirected to the TCP connection, so that anything you type to rlogin on your machine will be passed through the TCP stream and be given to the shell as standard input.

The User Datagram Protocol

Assume you want to request a small amount of data from a database server. It takes at least three datagrams to establish a TCP connection, another three to send and confirm a small amount of data each way, and another three to close the connection. UDP provides us with a means of using only two datagrams to achieve almost the same result. UDP is said to be connectionless, and it doesn't require us to establish and close a session. We simply put our data into a datagram and send it to the server; the server formulates its reply, puts the data into a datagram addressed back to us, and transmits it back. While this is both faster and more efficient than TCP for simple transactions, UDP was not designed to deal with datagram loss. It is up to the application, a name server for example, to take care of this.

More on Ports

open on many different machines, but on each machine only one process can open a port at any one time.

An important property of ports is that once a connection has been established between the client and the server, another copy of the server may attach to the server port and listen for more clients. This property permits, for instance, several concurrent remote logins to the same host, all using the same port 513. TCP is able to tell these connections from one another because they all come from different ports or hosts. For example, if you log in twice to quark from erdos , the first rlogin client will use the local port 1023, and the second one will use port 1022. Both, however, will connect to the same port 513 on quark . The two connections will be distinguished by use of the port numbers used at erdos .

This example shows the use of ports as rendezvous points, where a client contacts a specific port to obtain a specific service. In order for a client to know the proper port number, an agreement has to be reached between the administrators of both systems on the assignment of these numbers. For services that are widely used, such as rlogin, these numbers have to be administered centrally. This is done by the IETF (Internet Engineering Task Force), which regularly releases an RFC titled Assigned Numbers (RFC-1700). It describes, among other things, the port numbers assigned to well-known services. Linux uses a file called /etc/services that maps service names to numbers.

It is worth noting that although both TCP and UDP connections rely on ports, these numbers do not conflict. This means that TCP port 513, for example, is different from UDP port 513. In fact, these ports serve as access points for two different services, namely rlogin (TCP) and rwho (UDP).

The Socket Library

In Linux, the socket library is part of the standard libc C library. It supports the AF_INET and AF_INET6 sockets for TCP/IP and AF_UNIX for Unix domain sockets. It also supports AF_IPX for Novell's network protocols, AF_X25 for the X.25 network protocol, AF_ATMPVC and AF_ATMSVC for the ATM network protocol and AF_AX25 , AF_NETROM , and AF_ROSE sockets for Amateur Radio protocol support. Other protocol families are being developed and will be added in time.

UUCP Networks

UUCP was first developed by Bell Laboratories in 1977 for communication between their Unix development sites. In mid-1978, this network already connected over 80 sites. It was running email as an application, as well as remote printing. However, the system's central use was in distributing new software and bug fixes. Today, UUCP is not confined solely to the Unix environment. There are free and commercial ports available for a variety of platforms, including AmigaOS, DOS, and Atari's TOS.

One of the main disadvantages of UUCP networks is that they operate in batches. Rather than having a permanent connection established between hosts,