CSCI 356 / Spring 2015

Computer Networking

Practice: IP and Lower Layers

Q1. Ethernet and ARP

In the network shown below, nodes A through G are hosts, node H is an Ethernet hub, node S is an Ethernet switch. Host G acts as an IP-level router (i.e. a gateway) between the two subnets.

Subnet1 is allocated IP block 197.15.35.0/24, and Subnet2 is allocated IP block 200.40.18.192/26. All of the network interfaces in Subnet1 were manufactured by Intel. All of the network interfaces in Subnet2 were manufactured by Apple.

(a) Invent a plausible MAC address address for every network interface, host, or node that should have one.

(b) Invent a plausible IP address for every network interface, host, or node that should have one.

(c) Suppose a DNS resolver is running on Host D, and Host A wishes to make a DNS query. Host A must therefore send a UDP packet to Host D. Depending on the ARP and forwarding tables in the network, sending this DNS query can also require some additional packets (ARP queries, ARP replies, etc.) to be sent. Assume all ARP tables on all hosts are initially empty, and all forwarding tables on switches are also initially empty. Show, in some detail, all of the packets that are involved in getting A's DNS query to reach D. You should include all related ARP queries and replies. Describe what is contained in each packet (e.g. UDP source and dest ports, IP source and dest addresses, Ethernet MAC source and dest addresses, ARP information), which host sends each packet, and which hosts receive each packet. Say what each recipient does with each packet it receives.

Q2. Distance Vector Routing

Consider this network of IP routers. Assume all edges have cost 1, and the routers use a distance vector routing protocol.

(a) Show the contents of the initial routing table for router A at the start of the protocol.

(b) Suppose the following events happen in sequence: B advertises to all of its neighbors; then, after some time, C advertises to all of its neighbors; then later D advertises to all of its neighbors. Describe (or illustrate) the changes that router A will make to its routing table at each step as it receives each new advertisement. Note that A only directly receives two of the three advertisements that are sent. You will also have to reconstruct a little bit of what is happening on other nodes. Assume all advertisements arrive at their destinations and are processed immediately, and the advertisements listed above are the only ones sent..