Computer Networks

Practical Exercises #3

1. Use netscape (or Internet Explorer, mosaic or even lynx) to fetch an RFC - RFC854 (Telnet) might be a good one. You can do this by clicking on the URL given in the first slide of the lecture. You could also try using Unix-style anonymous FTP from the command line, as illustrated in the lecture. You could use either the ftp program and/or its enhanced version ncftp).

2. Look at the file /etc/services on a nearby Unix system: it defines the mapping between TCP port numbers and application protocols. Find the port numbers for echo, finger and chargen. Use telnet to connect to each of these ports. What do they do? Note that on some systems the telnet program accepts symbolic, or service, names on the command line. You might care to try it, eg:

   telnet redgum.bendigo.latrobe.edu.au finger
   

3. Try[1] using telnet to connect to another machine, say A, then invoke it on A to reach yet another machine, call it B. How does the combination of two telnets work? Any problems? What happens when you type the Escape character? Are <CR> (carriage return) and <LF> (line feed) characters handled properly?

4. On[2] one of the Unix systems, use FTP to copy a file from a machine to itself, and then from the machine to another machine on the same LAN. Do the data transfer rates surprise you? Note: be sure that you're not copying the file to or from an NFS-mounted directory - your home directory is NFS-mounted on the Indys, for example. If you're uncertain of what this means, feel free to ask.

5. Experiment[3] with FTP to see how fast you can transfer a file between two Unix systems on a LAN, when the network is busy and when it is idle. Explain the result. Compare the rates of transfer for FTP and NFS on the same LAN. Do the data transfer rates surprise you? You will need to use a utility such as /bin/time to time the speed of NFS transfers, and you also need to be aware of which files are NFS-mounted and which are stored locally.