计算机网络原理习题讲解(5)

15. （P14，答案P14）Consider a subnet with prefix 101.101.101.64/26. Give an example of one

IP address (of form xxx.xxx.xxx.xxx) that can be assigned to this network. Suppose an ISP owns the block of addresses of the form 101.101.128/17. Suppose it wants to create four subnets from this block, with each block having the same number of IP addresses. What are the prefixes (of form a.b.c.d/x) for the four subnets? 16. （P15，答案P15，好题，分得很细致）Consider the topology shown in Figure 4.17. Denote

the three subnets with hosts (starting clockwise at l2:00) as Networks A, B, and C. Denote the subnets without hosts as Networks D, E, and F.

a. Assign network addresses to each of these six subnets, with the following constraints: All addresses must be allocated from 214.97.254/23; Subnet A should have enough addresses to support 250 interfaces; Subnet B should have enough addresses to support 120 interfaces; and Subnet C should have enough addresses to support 120 interfaces. Of course, subnets D, E and F should each be able to support two interfaces. For each subnet, the assignment should take the form a.b.c.d/x or a.b.c.d/x - e.f.g.h/y.

b. Using your answer to part (a), provide the forwarding tables (using longest prefix matching) for each of the three routers. 17. （P22，答案P22）Consider the following network. With the indicated link costs, use Dijkstra's

shortest-path algorithm to compute the shortest path from x to all network nodes. Show how the algorithm works by computing a table similar to Table 4.3.

18. （P24，答案P24）Consider the network shown below, and assume that each node initially

knows the costs to each of its neighbors. Consider the distance-vector algorithm and show the distance table entries at node z.

19. （P25，答案P26）Consider the network fragment shown below. x has only two attached

neighbors, w and y. w has a minimum-cost path to destination u (not shown) of 5,and y has a minimum-cost path to u of 6. The complete paths from w and y to u (and between w and y) are not shown. All link costs in the network have strictly positive integer values.

a. Give x's distance vector for destinations w, y, and u.

b. Give a link-cost change for either c(x,w) or c(x,y) such that x will inform its neighbors of a new minimum-cost path to u as a result of executing the distance-vector algorithm.

c. Give a link-cost change for either c(x,w) or c(x,y) such that x will not inform its neighbors of a new minimum-cost path to u as a result of executing the distance-vector algorithm.

Chapter 5

1. （R1，参考答案R2）What are some of the possible services that a link-layer protocol can

offer to the network layer? Which of these link-layer services have corresponding services in IP? In TCP?

2. （R2，参考答案R1）If all the links in the Internet were to provide reliable delivery service,

would the TCP reliable delivery service be redundant? Why or why not? 3. （R7，参考答案R8）Suppose nodes A, B, and C each attach to the same broadcast LAN

(through their adapters). If A sends thousands of IP datagrams to B with each encapsulating frame addressed to the MAC address of B, will C's adapter process these frames? If so, will C's adapter pass the IP datagrams in these frames to the network layer C? How would your answers change if A sends frames with the MAC broadcast address? 4. （R8，参考答案R7）How big is the MAC address space? The IPv4 address space? The IPv6

address space?

5. （R9，参考答案R9）Why is an ARP query sent within a broadcast frame? Why is an ARP

response sent within a frame with a specific destination MAC address? 6. （R11，参考答案R13）In CSMA/CD, after the fifth collision, what is the probability that a

node chooses K = 4? The result K = 4 corresponds to a delay of how many seconds on a 10 Mbps Ethernet? 7. （P1，参考答案P1，值不一致）Suppose the information content of a packet is the bit pattern

1110110010001010 and an even parity scheme is being used. What would the value of the field containing the parity bits be for the case of a two-dimensional parity scheme? Your answer should be such that a minimum-length checksum field is used. 8. （P5，参考答案P5）Consider the 4-bit generator, G, shown in Figure 5.8, and suppose that D

has the value 11111010. What is the value of R?

9. （P6，参考答案P6）Consider the previous problem, but suppose that D has the value a. 10010001. b. 10100011. c. 01010101 10. （P12，参考答案P12）Consider three LANs interconnected by two routers, as shown in

Figure 5.38.

a. Redraw the diagram to include adapters.

b. Assign IP addresses to all of the interfaces. For Subnet I use addresses of the form 111.111.111.xxx; for Subnet 2 uses addresses of the form 122.122.122.xxx; and for Subnet 3 use addresses of the form 133.133.133.xxx. c. Assign MAC addresses to all of the adapters.

d. Consider sending an IP datagram from Host A to Host F. Suppose all of the ARP tables are up to date. Enumerate all the steps, as done for the single-router example in Section 5.4.2. e. Repeat (d), now assuming that the ARP table in the sending host is empty(and the other tables are up to date).

11. （P15，参考答案P16）Suppose nodes A and B are on the same 10 Mbps Ethernet bus, and

the propagation delay between the two nodes is 225 bit times. Suppose A and B send frames at the same time, the frames collide, and then A and B choose different values of K in the CSMA/CD algorithm. Assuming no other nodes are active, can the retransmissions from A and B collide? For our purposes, it suffices to work out the following example. Suppose A and B begin transmission at t = 0 bit times. They both detect collisions at t = 225 bit times. They

finish transmitting a jam signal at t = 225 + 48 = 273 bit times. Suppose KA?0 and

KB?1, At what time does B schedule its retransmission? At what time does A begin

transmission? (Note: The nodes must wait for an idle channel after returning to Step 2-see protocol.) At what time does A's signal reach B? Does B refrain from transmitting at its scheduled time?

12. （P16，参考答案P15）Suppose nodes A and B are on the same 10 Mbps Ethernet bus, and

the propagation delay between the two nodes is 225 bit times. Suppose node A begins transmitting a frame and, before it finishes, node B begins transmitting a frame. Can A finish transmitting before it detects that B has transmitted? Why or why not? If the answer is yes, then A incorrectly believes that its frame was successfully transmitted without a collision. Hint: Suppose at time t = 0 bit times, A begins transmitting a frame. In the worst case, A transmits a minimum-sized frame of 512 + 64 bit times. So A would finish transmitting the frame at t = 512 + 64 bit times. Thus, the answer is no, if B's signal reaches A before bit time t

= 512 + 64 bits. In the worst case, when does B's signal reach A?

13. （P19，参考答案P19）Suppose two nodes, A and B, are attached to opposite ends of a 900

m cable, and that they each have one frame of 1,000 bits (including all headers and preambles) to send to each other. Both nodes attempt to transmit at time t = 0. Suppose there are four repeaters between A and B, each inserting a 20-bit delay. Assume the transmission rate is 10Mbps, and CSMA/CD with backoff intervals of multiples of 512 bits is used. After the first collision, A draws K = 0 and B draws K = 1 in the exponential backoff protocol. Ignore the jam signal and the 96-bit time delay.

a. What is the one-way propagation delay (including repeater delays) between A and B in seconds? Assume that the signal propagation speed is 2?10 m/sec.

b. At what time (in seconds) is A s packet completely delivered at B?

c. Now suppose that only A has a packet to send and that the repeaters are replaced with switches. Suppose that each switch has a 20-bit processing delay in addition to a store-and-forward delay. At what time, in seconds, is A’s packet delivered at B?

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