Chapter 10

10.1	In Figure 10.9 which of the routes came to `gateway` from the router `kpno` ?
A:	Thirteen of the routes came from `kpno:` all except 140.252.101.0 and 140.252.104.0, the other networks to which `gateway` is directly connected.
10.2	Assume a router has 30 routes to advertise using RIP, requiring one datagram with 25 routes and another with the remaining 5. What happens if once an hour the first datagram with 25 routes is lost?
A:	Sixty seconds will pass before the 25 routes advertised in the lost datagram are updated. This isn't a problem because RIP normally requires 3 minutes without an update before it declares a route dead.
10.3	The OSPF packet format has a checksum field, but the RIP packet does not. Why?
A:	RIP runs on top of UDP, and UDP provides an optional checksum for the data portion of the UDP datagram (Section 11.3). OSPF, however, runs on top of IP. The IP checksum covers only the IP header, so OSPF must add its own checksum field.
10.4	What effect does load balancing, as done by OSPF, have on a transport layer?
A:	Load balancing increases the chances of packets being delivered out of order, and possibly distorts the round-trip times calculated by the transport layer.
10.5	Read RFC 1058 for additional details on the implementation of RIP. In Figure 10.8 each router advertises only the routes that it provides, and none of the other routes that it learned about through the other router's broadcasts on the 140.252.1 network. What is this technique called?
A:	This is called simple split horizon.
10.6	In Section 3.4 we said there are more than 100 hosts on the 140.252.1 subnet in addition to the eight routers we show in Figure 10.7. What do these 100 hosts do with the eight broadcasts that arrive every 30 seconds (Figure 10.8)?
A:	In Figure 12.1 we show that each of the 100 hosts processes the broadcast UDP datagram through the device driver, IP layer, and UDP layer, where it'll finally be discarded when it's discovered that UDP port 520 is not in use.