Overview of Computer Networks

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operation; you should see the new socket listed.17

4.1.7 What Makes a Connection Unique

Several socket programs might be running concurrently on the same machine. They may even all be accepting

messages from the same remote machine. So, when a message arrives, how can the OS tell which

program it should be routed to? The answer is that different socket programs are distinguished by their port

numbers. The ftpd service is on port 21, for instance. A communication between two machines must be

defined by five pieces of information:

• protocol (TCP, UDP, etc.)

• server IP address

• server socket port number

• client IP address

• client (ephemeral) socket port number

The server, for example, may be involved in several TCP transactions sent from the same machine, and thus

the server needs to have some way of distinguishing between them. And similarly, when the server sends

messages back to a client, the TCP system at the client's machine has to have some way of determining

which program the server's message is intended for.

Say for example, there are two users currently on machine X, one using ftp to port 21 at machine Y, and

another using telnet to port 23 at machine Z. But the programs being run by the two users will also have

ephemeral ports at their own machines, say 2592 and 5009, temporarily assigned to them by the TCP

system at machine X. The ftp program will inform the FTP server at machine Y about this 2592 number

when it first connects to Y, and when the server sends back to X it will use this number. The TCP system at

X will see this number, and route the message to the ftp program accordingly. Similar statements hold for

telnet, etc. And even if the two users had both been using ftp, their two different ephemeral port numbers

would distinguish them from each other.

4.2 Sample TCP/IP Application: NFS

As mentioned earlier, NFS allows machines to share files, in a manner transparent to the user. NFS uses the

UDP transport protocol.18

When I ran the df command on the ACS machine taco, the output included a line

rosarita:/usr/pkg 2077470 1556279 313444 83% /usr/pkg

17You may also some "UNIX" sockets, which don't involve networks.

18NFS actually uses Remote Procedure Call (RPC), which in turn uses UDP. RPC is just what the name implies: A program can

actually call a function-with parameters-which will execute on another machine. We have earlier referred to UDP's "unreliable"

nature, which means RPC must take its own reliability measures.

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This says that the directory /usr/pkg on the machine rosarita has been mounted on taco, in a directory of

the same name, /usr/pkg (the name need not have been the same). This directory contains a number of

utilities, such as the GNU C compiler, gcc, in the file /usr/pkg/gnu/bin/gcc. Thus for instance the user can