COM-208: Computer Networks - Homework 6

Transcription

1 COM-208: Computer Networks - Homework 6. (P22) Suppose you are interested in detecting the number of hosts behind a NAT. You observe that the IP layer stamps an identification number sequentially on each IP packet. The identification number of the first IP packet generated by a host is a random number, and the identification numbers of the subsequent IP packets are sequentially assigned. Assume all IP packets generated by hosts behind the NAT are sent to the outside world. (a) Based on this observation, and assuming you can sniff all packets sent by the NAT to the outside, can you outline a simple technique that detects the number of unique hosts behind a NAT? (b) If the identification numbers are not sequentially assigned but randomly assigned, would your technique work? Figure : Topology for Problem (P27) Consider the network shown in Figure. Using Dijkstra s algorithm, and showing your work using a table, do the following: (a) Compute the shortest path from x to all network nodes. (b) Compute the shortest path from v to all network nodes. (c) Compute the shortest path from t to all network nodes.

2 Figure 2: Topology for Problem (P28) Consider the network in Figure 2, and assume that each node initially only knows the costs to each of its neighbors. Execute the distance-vector algorithm step-by-step and show the distance table entries at node z. 4. (P29) Consider a general topology (that is, not a specific network topology) and a synchronous version of the distance-vector algorithm (i.e.,, in each step, all nodes compute their distance tables and then exchange tables at the same time). Suppose that at each iteration, a node exchanges its distance vectors with its neighbors and receives their distance vectors. Suppose that the algorithm begins with each node knowing only the costs to its immediate neighbors. What is the maximum number of iterations required (from when the algorithm is run from the first time) before the distributed algorithm converges? 2

3 5. (Final exam 202) Consider the network topology and the associated cost of the links in Figure 3. The nodes in the network run Bellman-Ford with poisoned reverse. Suppose the algorithm has run for some time and has converged to the correct least-cost paths. Table contains the reachability information from each node to node A (e.g., from node D we can reach node A through node C with a cost 3). Now suppose that link A B goes down. C A B D 2 E Figure 3: Network setup for Problem 5. (a) Describe the first 6 steps of the algorithm. For each step, show the reachability information from each node to node A (i.e., the next hop and cost of the least-cost path). (b) Will the algorithm converge to the correct least-cost path values? If yes, in how many steps? (c) Propose a simple way to make the algorithm converge faster. Route to node A from node B from node C from node D from node E step 0 via A 2 via B 3 via C 3 via B step step 2 step 3 step 4 step 5 step 6 Table : The least-cost path to node A. Each column contains the information tored on one node. E.g., the first column contains the information stored on node B - how node B can reach node A. 3

4 Figure 4: Topology for Problem (P37) Consider the network shown in Figure 4 and suppose the following: AS3 and AS2 use OSPF as their intra-as routing protocol. AS and AS4 use RIP as their intra-as routing protocol. ibgp and ebgp are used for the inter-as routing protocol. There is no phsical link between AS2 and AS4. For each of the following routers (i) state from which protocol each router learns about prefix x (e.g., OSPF, RIP, ebgp or ibgp), (ii) indicate from which router the information was received. (a) Router 3c. (b) Router 3a. (c) Router c. (d) Router d. 7. (P38) Referring back to Problem 6, consider router d in AS, and its adjacent links, l and l 2. Once router d learns about prefix x it will put entry (x, L) in its forwarding table. which means that all packets with destination IP inside prefix x, will be forwarded via output link L (L is equal to either l or l 2 ). For each of the following scenarios, you need to explain what the value of L is going to be (either l or l 2 ). In each case, you should briefly justify your answers (one sentence). (a) There is no physical link between AS2 and AS4. (b) There is a physical link between AS2 and AS4. Thus, router d learns that prefix x is accessible via AS2 as well as AS3. (c) Now suppose there is another AS, called AS5 (not shown in the figure), which is located on the path between AS2 and AS4. i.e., AS2 and AS4 are no longer connected. 4

5 8. (P39) Consider the network in Figure 5. Each ISP consists of one AS. Figure 5: Topology for Problem 8. ISP B provides national backbone service to regional ISP A. ISP C provides national backbone service to regional ISP D. B and C peer with each other in two places using BGP. Now consider traffic going from A to D. B would prefer to hand that traffic over to C on the West Coast (so that C would have to absorb the cost of carrying the traffic cross-country) C would prefer to get the traffic via its East Coast peering point with B (so that B would have to carry the traffic across the country). As a network administrator of ISP C, what BGP mechanism would you use in order to make ISP B to hand over A-to-D traffic at its East-Coast peering point? To answer this question, you will need to dig into information about BGP provided in the book. W B A V C Figure 6: Problem 9 topology 9. (P42) Consider that the ISPs in Figure 6 have the following routing relationships between them: Stub network V is a customer of ISP A. B and C have a peering relationship between them. A is a customer of both B and C. Suppose that A would like to make it that: All traffic destined to W should only come via B. All traffic destined to V should come from either B or C. (a) Which AS routes should A advertise to B and C? (b) Which AS routes does C receive? 5