复旦大学：《计算机网络 Computer Networking》课程教学资源_考试样卷

1. Explain the similarities and differences among bridges, switches and routers 2. Explain how BGP and RIP/OSPF co-work together in the Internet. What's the difference between BGP and the theoretical distance-vector algorithm? 3. Explain the advantages and disadvantages of global addressing, source routing and virtual circuits. Illustrate the reason to choose global addressing in Internet 4. If you have an opportunity to design the Internet from scratch, describe the design philosophy and detailed steps 5. Calculate the latency( from first bit sent to last bit received for the following) a) 10-Mbps Ethernet with a single store-and-forward switch in the path, and a packet size of 5000 bits. Assume that each link introduces a propagation delay of 10us, and that the switch begins retransmitting immediately after it has finished receiving the packet b) Same as a) but with three switches c) Same as a)but assume the switch implements"cut-through "switching: It is able to begin retransmitting the packet after the first 200 bits have been received 6. Show the Nrz, Manchester and nRZI encodings for the following bit sequence: 1000100111 Assume that the nrzi signal starts out loy 7. Suppose the round-trip propagation delay for Ethernet is 46.4 us. This yields a minimum packet size of 512 bits(464 bits corresponding to propagation delay +48 bits of jam signal) a)What happens to the minimum packet size if the delay time is held constant, and the signaling rate rises to 100 Mbps? o) What are the drawbacks to so large a minimum packet size? c)If compatibility were not an issue, how might the specifications be written so as to permit a smaller minimum packet size? 8. Let A and b be two stations attempting to transmit on an ethernet. Each has a steady queue of frames ready to send. As frames will be numbered Al, A2, and so on, and Bs similarly. Let T-51.2 us be the exponential backoff base unit. Suppose A and B simultaneously attempt to send frame 1, collide, and happen to choose backoff times of OXT and 1xT, respectively meaning a wins the race and transmit al while b waits. At the end of this transmission b will attempt to retransmit bl while A will attempt to transmit A2. These first attempts will collide, but now a backs off for either OXT or lxt, while b backs off for time equal to one of a) Give the probability that A wins this second backoff race immediately after this first b) Suppose A wins this second backoff race. A transmits A3, and when it is finished, A and B collide again as a tries to transmit a4 and b tries once more to transmit bl. give the probability that a wins this third backoff race immediately after the first collision c)Give a reasonable lower bound for the probability that a wins all the remaining backoff races d) What then happens to the frame Bl? 9. Suppose that a TCP message that contains 2048 bytes of data and 20 bytes of TCP header is passed to IP for delivery across two networks of the Internet. The first network uses 14-byte headers and has an MTU of 1024 bytes; the second uses 8-byte headers with an MTU of 512 bytes. Each network's MTU gives the total packet size that may be sent, including the network header. Give the sizes and offsets of the sequence of fragments delivered to the

1. Explain the similarities and differences among bridges, switches and routers. 2. Explain how BGP and RIP/OSPF co-work together in the Internet. What's the difference between BGP and the theoretical distance-vector algorithm? 3. Explain the advantages and disadvantages of global addressing, source routing and virtual circuits. Illustrate the reason to choose global addressing in Internet. 4. If you have an opportunity to design the Internet from scratch, describe the design philosophy and detailed steps. 5. Calculate the latency (from first bit sent to last bit received for the following): a) 10-Mbps Ethernet with a single store-and-forward switch in the path, and a packet size of 5000 bits. Assume that each link introduces a propagation delay of 10us, and that the switch begins retransmitting immediately after it has finished receiving the packet. b) Same as a) but with three switches. c) Same as a) but assume the switch implements “cut-through” switching: It is able to begin retransmitting the packet after the first 200 bits have been received. 6. Show the NRZ, Manchester and NRZI encodings for the following bit sequence: 1000100111. Assume that the NRZI signal starts out low. 7. Suppose the round-trip propagation delay for Ethernet is 46.4 us. This yields a minimum packet size of 512 bits (464 bits corresponding to propagation delay+48 bits of jam signal). a) What happens to the minimum packet size if the delay time is held constant, and the signaling rate rises to 100 Mbps? b) What are the drawbacks to so large a minimum packet size? c) If compatibility were not an issue, how might the specifications be written so as to permit a smaller minimum packet size? 8. Let A and B be two stations attempting to transmit on an Ethernet. Each has a steady queue of frames ready to send. A’s frames will be numbered A1, A2, and so on, and B’s similarly. Let T=51.2 us be the exponential backoff base unit. Suppose A and B simultaneously attempt to send frame 1, collide, and happen to choose backoff times of 0×T and 1×T, respectively, meaning A wins the race and transmit A1 while B waits. At the end of this transmission, B will attempt to retransmit B1 while A will attempt to transmit A2. These first attempts will collide, but now A backs off for either 0×T or 1×T, while B backs off for time equal to one of 0×T, …, 3×T. a) Give the probability that A wins this second backoff race immediately after this first collision. b) Suppose A wins this second backoff race. A transmits A3, and when it is finished, A and B collide again as A tries to transmit A4 and B tries once more to transmit B1. Give the probability that A wins this third backoff race immediately after the first collision. c) Give a reasonable lower bound for the probability that A wins all the remaining backoff races. d) What then happens to the frame B1? 9. Suppose that a TCP message that contains 2048 bytes of data and 20 bytes of TCP header is passed to IP for delivery across two networks of the Internet. The first network uses 14-byte headers and has an MTU of 1024 bytes; the second uses 8-byte headers with an MTU of 512 bytes. Each network’s MTU gives the total packet size that may be sent, including the network header. Give the sizes and offsets of the sequence of fragments delivered to the

network layer at the destination host. Assume all IP headers are 20 bytes 10. Suppose an IP implementation adheres literally to the following algorithm on receipt of a If( Ethernet address for D is in ARP cache) (Send Pi i send out an ARP query for D Put P into a queue until the response comes back a) If the IP layer receives a burst of packets destined for D, how might this algorithm waste resources unnecessarily? b)Sketch an improved version 11. For the network given in Fig. 1, given global distance-vector tables like those of Tables 4.5 and 4 8 when a) Each node knows only the distances to its immediate neighbors b) Each node has reported the information it had in the preceding step to its immediate c) Step b) happens a second time Suppose the forwarding tables are all established and then the C-E link fails. Give d) The tables of A, B, d and F after C and E have reported the news e) The tables of A and d after their next mutual exchange f The table of C after A exchanges with it 2 12. For the network given in Fig. 1, show how the link-state algorithm builds the routing table for node d 13. USing winpcap(packet capture tool) to send an ARP packet to a destination host and the return result of packet length is 60 bytes. But the real ARP length is 42 bytes according to the ARP format shown in Fig 4.7, including 14-byte Ethernet header. If the ARP packet is sent to a virtual machine(such as VMVaRe)the winpcap return result is also 42 bytes. What is the ARP packet length on earth? Why?

network layer at the destination host. Assume all IP headers are 20 bytes. 10. Suppose an IP implementation adheres literally to the following algorithm on receipt of a packet, P, destined for IP address D: If (Ethernet address for D is in ARP cache) {Send P} Else {send out an ARP query for D Put P into a queue until the response comes back} a) If the IP layer receives a burst of packets destined for D, how might this algorithm waste resources unnecessarily? b) Sketch an improved version. 11. For the network given in Fig.1, given global distance-vector tables like those of Tables 4.5 and 4.8 when a) Each node knows only the distances to its immediate neighbors. b) Each node has reported the information it had in the preceding step to its immediate neighbors. c) Step b) happens a second time. Suppose the forwarding tables are all established and then the C-E link fails. Give d) The tables of A, B, D and F after C and E have reported the news. e) The tables of A and D after their next mutual exchange. f) The table of C after A exchanges with it. A B D E C F 3 8 2 2 1 6 Fig.1 12. For the network given in Fig.1, show how the link-state algorithm builds the routing table for node D. 13. Using winpcap (packet capture tool) to send an ARP packet to a destination host and the return result of packet length is 60 bytes. But the real ARP length is 42 bytes according to the ARP format shown in Fig.4.7, including 14-byte Ethernet header. If the ARP packet is sent to a virtual machine (such as VMVARE) the winpcap return result is also 42 bytes. What is the ARP packet length on earth? Why?