CS455 Computer Communications and Networking

Answer the following questions [100pt]

1. [18pt] The following figure shows 7 interconnected ASes: A, B, C, V, W, X and Y. AS A, B and C are provided networks (e.g., backbone ASes) and V, W, X and Y are their customer networks (e.g., access networks). These ASes use BGP to set up the routes between them. Note that an example BGP route “AS2-AS3-AS4” sent from AS2 to its neighbor AS1 notifies AS1 that AS2 can reach AS4 via AS3.

Suppose that A would like to have the traffic destined to W to come from B only (not C), and the

traffic destined to V from C only (not B). Answer the following questions.

a. What route(s) does A advertise to B?

b. What route(s) does A advertise to C?

c. Suppose that B and C have a peering relationship, what are the routes received by B and

C respectively (pertaining to W or V)?

2. [18pt] Consider the network shown below. Here H1, H2, H3 and H4 are hubs, S1 and S2 and switches and R is a router connected to the Internet.

Assume that ARP tables at all nodes are up-to-date and the routing table at R is also up-to-date.

Switch S1 has an up-to-date switching table, however, switch S2’s forwarding table is empty.

Assume node A sends an IP packet (encapsulated in a MAC frame) to node L. Answer if the following is true or false and provide explanation of your answer. And please correct the following statement if it is incorrect.

a. Both node G and H receives the frame.

b. When an IP packet is sent from A to L, the source MAC address in the frame is A’s MAC address and destination MAC address is L’s MAC address.

c. When the frame is forwarded from A to R, the source IP is A’s IP address, and the destination IP address is R’s IP address (R’s interface connected to S1)

3. [18pt] Suppose nodes A, B, C and D are connected on an 800 kbps link in a line A---B---C---D. The propagation delay between the neighboring nodes (A and B, B and C and C and D) is 5 ms.

Assume nodes use CSMA/CD and exponential backoff is disabled. Consider the following cases and answer if they are possible or not. Explain your answer and show calculations. Note that each of the following cases are independent (not occurring one after the other) and frame size is 1000

bytes.

a. Assume that node A starts transmitting a frame at t=0ms. Now, node D wants to transmit a frame at t = 14ms. Will node D be able to start the transmission of the frame? Will there be a collision?

b. Assume that node A starts transmitting a frame at t=5ms. Node D starts transmitting a frame at t=19ms. Will there be a collision? If yes, who will (A or D) will detect the collision first and when will they detect the collision respectively?

c. Assume that node A starts transmitting a frame to node B at t=6ms and node D starts transmitting a frame to node C at t=0ms. Can these two frames be received at their intended receivers without collision? Why?