Chapter Questions
Suppose a router discovers it is about to forward an IP datagram back over the same network interface on which the datagram arrived. What should it do? Why?
After reading RFC 823 and RFC 1812, explain what an Internet core router (i.e., one with complete routing information) should do in the situation described in the previous question.
How can routers in a core system use default routes to send all illegal datagrams to a specific machine?
Imagine that a manager accidentally misconfigures a router to advertise that it has direct connections to six specific networks when it does not. How can other routers that receive the advertisement protect themselves from invalid advertisements while still accepting other updates from "untrusted" routers?
Which ICMP messages does a router generate?
Assume a router is using unreliable transport for delivery. How can the router determine whether a designated neighbor's status is $u p$ or down? (Hint: consult RFC 823 to find out how the original core system solved the problem.)
Suppose two routers each advertise the same cost, $k$, to reach a given network, $N$. Describe the circumstances under which forwarding through one of them may take fewer total hops than forwarding through the other one.
How does a router know whether an incoming datagram carries a routing update message?
Consider the distance-vector update shown in Figure 12.5 carefully. For each item updated in the table, give the reason why the router will perform the update.
Consider the use of sequence numbers to ensure that two routers do not become confused when datagrams are duplicated, delayed, or delivered out of order. How should initial sequence numbers be selected? Why?