Computer Networks

Tutorial #14

1. What information can the network manager obtain from the ping command? Give at least 3 answers. What does ping actually do?

2. What does the traceroute command do? How does it do it?

3. The ASN.1 APPLICATION-specific data types of SNMP mostly use the sub range feature of ASN.1, which we didn't actually mention in our introductory ASN.1 lecture. What is the significance of the sub range values for the SNMP ASN.1 APPLICATION types Counter and Gauge?

4. The following are some (highly edited) entries from the standard Unix MIB definition, in file /etc/mib.txt.

   ipForwarding OBJECT-TYPE ::= { ip 1 }
   icmpInEchos OBJECT-TYPE ::= { icmp 8 }
   tcpMaxConn OBJECT-TYPE ::= { tcp 4 }
   

   What are the numeric OBJECT IDENTIFIERS of the objects ipForwarding, icmpInEchoes and tcpMaxConn.

5. (Extension Question) One interesting aspect of the ASN.1/BER is the way in which OBJECT IDENTIFIERS are encoded for transmission. In general, the integers which specify the OBJECT IDENTIFIER are simply encoded in BER as a SEQUENCE of single byte values. However, the first two integers (let's call them a and b) are encoded in a compact form, taking only a single byte, of the form 40a + b. So, for the Internet, the first two integers are 1.3, therefore they are encoded as the single byte value 43. Can you imagine a reason why this is done? What does it say about the values of these first two integers?

6. (Extension Question) Marshall T. Rose^[1] espouses the fundamental axiom of network management, which is: The impact of adding management to managed nodes must be minimal, reflecting a lowest common denominator. Discuss.

Practical Exercises:

• Use the ping^[2] command to check reachability of various hosts at Bendigo and elsewhere. Now investigate the command line options of ping. If you're not on campus, there are versions of ping for virtually all combinations of hardware and operating system -- see what you can find on the various 'net archives.

• The traceroute command is available on ironbark/redgum and the Indys, as well as for PCs and Macs. Use traceroute (judiciously - see its documentation). You might also be interested to have a look at ntping on the SGI systems (Indys, ironbark -- try ntping -trace 30 hostname).

• Give yourself a shell on one of the Unix systems and try netstat. Try a few of its command line options. The most interesting option is netstat -C. Can you make sense of the display, and the various options?