Network Layer

  1. Kurose & Ross, Chapter 4, Problem P9.

    Consider the switch shown below. Suppose that all datagrams have the same fixed length, that the switch operates in a slotted, synchronous manner, and that in one time slot a datagram can be transferred from an input port to an output port. The switch fabric is a crossbar so that at most one datagram can be transferred to a given output port in a time slot, but different output ports can receive datagrams from different input ports in a single time slot. What is the minimal number of time slots needd to transfer the packets shown from input ports to their output pots, assuming any input queue scheduling order you want (i.e., it need not have HOL blocking)? What is the largest number of slots needed, assuming the worst-case scheduling order you can devise, assuming that a non-empty input queue is never idle?

    router

  2. Kurose & Ross, Chapter 4, Problem P10.

    Consider a datagram network using 32-bit host addresses. Suppose a router has four links, numbered 0 through 3, and packets are to be forwarded to the link interfaces as follows:

    router

    a. Provide a forwarding table that has five entries, uses longest prefix matching, and forwards packets to the correct link interfaces.

    b. Describe how your forwarding table determines the appropriate link interface for datagrams with destination addresses:

    11001000 10010001 01010001 01010101
    11100001 01000000 11000011 00111100
    11100001 10000000 00010001 01110111

  3. Kurose & Ross, Chapter 4, Problem P13.

    Consider a router that interconnects three subnets: Subnet 1, Subnet 2, and Subnet 3. Suppose all of the interfaces in each of these three subnets are required to have the prefix 223.1.17/24. Also suppose that Subnet 1 is required to support at least 62 interfaces, Subnet 2 is to support at least 95 interfaces, and Subnet 3 is to support at least 16 interfaces. Provide three network addresses (of the form a.b.c.d/x) that satisfy these constraints.

  4. Kurose & Ross, Chapter 4, Problem P19.

    Consider sending a 2400-byte datagram into a link that has an MTU of 700 bytes. Suppose the original datagram is stamped with the identification number 422. How many fragments are generated? What are the values in the various fields in the IP datagram(s) generated related to fragmentation?

  5. Kurose & Ross, Chapter 4, Problem P31.

    Consider the three-node topology shown below. Rather than having the link costs shown in the figure, the link costs are c(x,y) = 3, c(y,z) = 6, c(z,x) = 4. Compute the distance tables after the initialization step and after each iteration of a synchronous version of the distance-vector algorithm.

  6. Kurose & Ross, Chapter 4, Problem P38.

    Consider the network shown below, with BGP running as the inter-AS routing protocol. Initially suppose there is no physical link between AS2 and AS4.

    network

    Suppose router $1d$ learns about $x$. It will put an entry (x,I) in its forwarding table.

    a. Will $I$ be equal to $I_1$ or $I_2$ for this entry? Explain why in one sentence.

    b. Now suppose that there is a physical link between AS2 and AS4, shown by the dotted line. Suppose router $1d$ learns that $x$ is accessible via AS2 as well as via AS3. Will $I$ be set to $I_1$ or $I_2$? Explain why in one sentence.

    c. Now suppose there is another AS, called AS5, which lies on the path between AS2 and AS4 (not shown in diagram). Suppose router 1d learns that x is accessible via AS2 AS5 AS4 as well as via AS3 AS4. Will $I$ be set to $I_1$ or $I_2$ ? Explain why in one sentence.

  7. Kurose & Ross, Chapter 4, Problem P39.

    Consider the following network.

    network

    ISP B provides national backbone service to regional ISP A. ISP C provides national backbone service to regional ISP D. Each ISP consists of one AS. B and C peer with each other in two places using BGP. 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), while C would prefer to get the traffic via its East Coast peering point with B (so that B would have carried the traffic across the country). What BGP mechanism might C use, so that B would hand over A-to-D traffic at its East Coast peering point? To answer this question, you will not need to dig into the BGP specification.

  8. Kurose & Ross, Chapter 4, Problem P47.

    Consider the topology shown below. Suppose that all links have unit cost and that node E is the broadcast source. Using arrows like those shown in the figure, indicate links over which packets will be forwarded using RPF, and links over which packets will not be forwarded, given that node E is the source.

    network

  9. Kurose & Ross, Chapter 4, Problem P49.

    Consider the topology shown below, and suppose that each link has unit cost.

    network

    Suppose node C is chosen as the center in a center-based multicast routing algorithm. Assuming that each attached router uses its least-cost path to node C to send join messages to C, draw the resulting center-based routing tree. Is the resulting tree a minimum-cost tree? Justify your answer.

  10. Kurose & Ross, Chapter 4, Problem P55.

    What is the size of the multicast address space? Suppose now that two multicast groups randomly choose a multicast address. What is the probability that they choose the same address? Suppose now that 1,000 multicast groups are ongoing at the same time and choose their multicast group addresses at random. What is the probability that they interfere with each other? Hint: use an approximation for the birthday problem.