5:20 O routers build their forwarding tables. This turns out to be a complex topic, divided into routing within single organizations and ISPs 13 Routing-Update Algorithms and routing between organizations 14 Large-Scale IP Routing. 10.6 Exercises But before that, in the next chapter, we compare IPv4 with IPv6, now twenty years old but still seeing limited adoption. The biggest issue fixed by IPv6 is IPv4's lack of address space, but there are also several other less dramatic improvements. (a). What will happen if A now sends a unicast repeat ARP query for B? (b). What will happen if A now sends a broadcast repeat ARP query for B? 4G Exercises are given fractional (floating point) numbers, to allow for interpolation of new exercises. 1.0. In 10.2 Address Resolution Protocol: ARP it was stated that, in newer implementations, repeat ARP queries about a timed out entry are first sent unicast, in order to reduce broadcast traffic. Suppose host A uses ARP to retrieve B's LAN address (MAC address). A short time later, B changes its LAN address, either through a hardware swap or through software reconfiguration. 244 57% 2.0. Suppose A broadcasts an ARP query who-has B?", receives B's response, and proceeds to send B a regular IPv4 packet. If B now wishes to reply, why is it likely that A will already be present in B's ARP cache? Identify a circumstance under which this can fail. 10 IPv4 Companion Protocols An Introduction to Computer Networks, Release 2.0.5 = 3.0. Suppose A broadcasts an ARP request who-has B, but inadvertently lists the physical address of another machine C instead of its own (that is, A's ARP query has IP src A, but LANsrc = C). What will happen? Will A receive a reply? Will any other hosts on the LAN be able to send to A? What entries will be made in the ARP caches on A, B and C? 4.0. Suppose host A connects to the Internet via Wi-Fi. The default router is Rw. Host A now begins exchanging packets with a remote host B: A sends to B, B replies, etc. The exact form of the connection does not matter, except that TCP may not work. ||| (a). You now plug in A's Ethernet cable. The Ethernet port is assumed to be on a different subnet from the Wi-Fi (so that the strong and weak end-system models of 10.2.5 ARP and multihomed hosts do not play a role here). Assume A automatically selects the new Ethernet connection as its default route, with router R. What happens to the original connection to A? Can packets still travel back and forth? Does the return address used for either direction change? (b). You now disconnect A's Wi-Fi interface, leaving the Ethernet interface connected. What happens now to the connection to B? Hint: to what IP address are the packets from B being sent? O See also 13 Routing-Update Algorithms exercise 16.0, and 18 TCP Issues and Alternatives exercise 5.0.