Exercises

8.1	What can happen if an IP implementation decrements the incoming TTL and then tests for equal to 0?
8.2	How does `traceroute` calculate the RTT? Compare this to the RTT calculation done by `ping.`
8.3	(This exercise and the next one are based on actual problems determined when `traceroute` was being developed, and are from comments in the `traceroute` source code.) Assume there are three routers (R1, R2, and R3) between the source and destination and that the middle router (R2) decrements the TTL but incorrectly forwards the IP datagram when the incoming TTL was 1. Describe what happens. How can you see this occur when running `traceroute?`
8.4	Again assume there are three routers between the source and destination. This time the destination host has a bug whereby it always uses the incoming TTL as the outgoing TTL of an ICMP message. Describe what happens and how you would see this.
8.5	We can run `tcpdump` on the SLIP link between `sun` and `netb` when running the example from Figure 8.8. If we specify the `-v` option we can see the TTL value of the returned ICMP messages. Doing this shows the incoming TTL from `netb` to be 255, from `butch` it's 253, from `Gabby` it's 252, and from `enssl42.UT.westnet.net` it's 249. Does this give any additional information about whether there really are some missing routers?
8.6	Both SunOS and SVR4 provide a version of `ping` with a `-l` option that provides a loose source route. The manual pages state that it's intended to be used with the `-R` option (which specifies the record route option). If you have access to either of these systems, try these two options together. What's happening? If you can watch the datagrams with `tcpdump,` describe what's going on.
8.7	Compare the ways `ping` and `traceroute` handle multiple instances of the client on the same host .
8.8	Compare the ways `ping` and `traceroute` measure the round-trip time .
8.9	We said `traceroute` picks the starting UDP destination port number at 33435 and increments this by one for each packet sent. In Section 1.9 we said ephemeral port numbers are normally between 1024 and 5000, making it unlikely that Traceroute's destination port is in use on the destination host. Is this still true under Solaris 2.2? ( Hint: Read Section E.4.)
8.10	Read RFC 1393 [Malkin 1993b] for a proposed alternative way of determining the path to a destination. What are its advantages and disadvantages?