Link Layer

  1. Kurose & Ross, Chapter 6, Problem P5.

    Consider the 5-bit generator, G = 10011, and suppose that D has the value 1010101010. What is the value of R?

  2. Kurose & Ross, Chapter 6, Problem P15.

    Consider the figure shown below.


    a. Consider sending an IP datagram from Host E to Host F. Will Host E ask router R1 to help forward the datagram? Why? In the Ethernet frame containing the IP datagram, what are the source and destination IP and MAC addresses?

    b. Suppose E would like to send an IP datagram to B, and assume that E's ARP cache does not contain B's MAC address. Will E perform an ARP query to find B's MAC address? Why? In the Ehternet frame (containing the IP datagram destined to B) that is delivered to router R1, what are the source and destination IP and MAC addresses?

    c. Suppose Host A would like to send an IP datagram to Host B, and neither A's ARP cache contains B's MAC address nor does B's ARP cache contain A's MAC address. Further suppose that the switch S1's forwarding table contains entries for Host B and router R1 only. Thus, A will broadcast an ARP request message. What actions will switch S1 perform once it receives the ARP request message? Will router R1 also receive this ARP request message? If so, will R1 forward the message to Subnet 3? Once Host B receives this ARP request message, it will send back to Host A an ARP response message. But will it send an ARP query message to ask for A's MAC address? Why? What will switch S1 do once it receives an ARP response message from Host B?

  3. Kurose & Ross, Chapter 6, Problem P27

    In this problem, we explore the use of small packets for Voice-over-IP applications. One of the drawbacks of a small packet size is that a large fraction of link bandwidth is consumed by overhead bytes. To this end, suppose that the packet consists of P bytes and 5 bytes of header.

    a. Consider sending a digitally encoded voice source directly. Suppose the source is encoded at a constant rate of 128 kbps. Assume each packet is entirely filled before the source sends the packet into the network. The time required to fill the packet is the packetization delay. In terms of L, determine the packetization delay in milliseconds.

    b. Packetization delays greater than 20 msec can cause a noticeable and unpleasant echo. Determine the packetization delay for L = 1,500 bytes (roughly corresponding to a maximum-sized Ethernet packet) and for L = 50 (corresponding to an ATM packet).

    c. Calculate the store-and-forward delay at a single switch for a link rate of R = 622 Mbps for L = 1,500 bytes, and for L = 50 bytes.

    d. Comment on the advantages of using a small packet size.

  4. Kurose & Ross, Chapter 6, Problem P29

    Consider the MPLS network shown below, and suppose that routers R5 and R6 are now MPLS enabled.


    Suppose that we want to perform traffic engineering so that packets from R6 destined for A are switched to A via R6-R4-R3-R1, and packets from R5 destined for A are switched via R5-R4-R2-R1. Show the MPLS tables in R5 and R6, as well as the modified table in R4, that would make this possible.

  5. Kurose & Ross, Chapter 6, Problem P32

    Consider the data center network with hierarchical topology in the figure below.


    Suppose now there are 80 pairs of flows, with ten flows between the first and ninth rack, ten flows between the second and tenth rack, and so on. Further suppose that all links in the network are 10 Gbps, except for the links between hosts and TOR switches, which are 1 Gbps.

    a. Each flow has the same data rate; determine the maximum rate of a flow.

    b. For the same traffic pattern, determine the maximum rate of a flow for the highly interconnected topology shown below.

    c. Now suppose there is a similar traffic pattern, but involving 20 hosts on each rack and 160 pairs of flows. Determine the maximum flow rates for the two topologies.