习题课(3)

a. The segments sent from A to S.

b. The segments sent from B to S. c. The segments sent from S to A. d. The segments sent from S to B.

e. If A and B are different hosts, is it possible that the source port number in the segments from A to S is the same as that from B to S? f. How about if they are the same host?

11. Consider Figure 3.5. What are the source and destination port values in the segments flowing

from the server back to the clients' processes? What are the IP addresses in the

network-layer datagram carrying the transport-layer segments?

12. Consider the GBN protocol with a sender window size of 3 and a sequence number range of

1,024. Suppose that at time r, the next in-order packet that the receiver is expecting has a sequence number of t. Assume that the medium does not reorder messages. Answer the following questions:

a. What are the possible sets of sequence numbers inside the sender's window at time t? Justify your answer.

b. What are all possible values of the ACK field in all possible messages currently propagating back to the sender at time r? Justify your answer.

13. Answer true or false to the following questions and briefly justify your answer:

a. With the SR protocol, it is possible for the sender to receive an ACK for a packet that falls outside of its current window

b. With GBN, it is possible for the sender to receive an ACK for a packet that falls outside of its current window.

c. The alternating-bit protocol is the same as the SR protocol with a sender and receiver window size of 1.

d. The alternating-bit protocol is the same as the GBN protocol with a sender and receiver window size of 1.

14. Host A and B are communicating over a TCP connection, and Host B has already received

from A all bytes up through byte 358. Suppose Host A then sends two segments to Host B back-to-back. The first and second segments contain 50 and 80 bytes of data, respectively. In

the first segment, the sequence number is 359, the source port number is 1028, and the destination port number is 80. Host B sends an acknowledgement whenever it receives a segment from Host A.

a. In the second segment sent from Host A to B, what are the sequence number, source port number, and destination port number?

b. If the first segment arrives before the second segment, in the acknowledgement of the first arriving segment, what is the acknowledgment number, the source port number, and the destination port number?

c. If the second segment arrives before the first segment, in the acknowledgement of the first arriving segment, what is the acknowledgment number?

d. Suppose the two segments sent by A arrive in order at B. The first acknowledgement is lost and the second acknowledgement arrives after the first timeout interval. Draw a timing diagram, showing these segments and all other segments and acknowledgements sent. (Assume there is no additional packet loss.) For each segment in your figure, provide the

sequence number and the number of bytes of data; for each acknowledgement that you add, provide the acknowledgement number.

15. Host A and B are directly connected with a 200 Mbps link. There is one TCP connection

between the two hosts, and Host A is sending to Host B an enormous file over this connection. Host A can send application data into the link at 100 Mbps but Host B can read

out of its TCP receive buffer at a maximum rate of 50 Mbps. Describe the effect of TCP flow control.

16. In Section 3.5.4, we saw that TCP waits until it has received three duplicate ACKs before

performing a fast retransmit. Why do you think the TCP designers chose not to perform a fast

retransmit after the first duplicate ACK for a segment is received? 17. Consider the following plot of TCP window size as a function of time.

Assuming TCP Reno is the protocol experiencing the behavior shown above, answer the following questions. In all cases, you should provide a short discussion justifying your answer. a. Identify the intervals of time when TCP slow start is operating.

b. Identify the intervals of time when TCP congestion avoidance is operating.

c. After the 16th transmission round, is segment loss detected by a triple duplicate ACK or by a timeout?

d. After the 22nd transmission round, is segment loss detected by a triple duplicate ACK or by a timeout?

e. What is the initial value of Threshold at the first transmission round? f. What is the value of Threshold at the 18th transmission round? g. What is the value of Threshold at the 24th transmission round?

h. During what transmission round is the 70th segment sent?

i. Assuming a packet loss is detected after the 26th round by the receipt of a triple duplicate ACK, what will be the values of the congestion window size and of Threshold?

18. Host A is sending an enormous file to Host B over a TCP connection. Over this connection

there is never any packet loss and the timers never expire. Denote the transmission rate of

the link connecting Host A to the Internet by R bps. Suppose that the process in Host A is capable of sending data into its TCP socket at a rate S bps, where S = 10*R. Further suppose that the TCP receive buffer is large enough to hold the entire file, and the send buffer can hold only one percent of the file. What would prevent the process in Host A from continuously passing data to its TCP socket at rate S bps? TCP flow control? TCP congestion control? Or something else? Elaborate.

Chapter 4

1. What is the difference between routing and forwarding?

2. What are the two most important network-layer functions in a datagram network? What are

the three most important network-layer functions in a virtual-circuit network? 3. Do the routers in both datagram networks and virtual-circuit networks use forwarding tables?

If so, describe the forwarding tables for both classes of networks. 4. Describe how packet loss can occur at input ports. Describe how packet loss at input ports

can be eliminated (without using infinite buffers). 5. Describe how packet loss can occur at output ports.

6. What is HOL blocking? Does it occur in input ports or output ports? 7. What is the 32-bit binary equivalent of the IP address 223.1.3.27?

8. Do routers have IP addresses? If so, how many?

9. Is it necessary that every autonomous system use the same intra-AS routing algorithm? Why or why not?

10. Consider Figure 4.35. Starting with the original table in D, suppose that D receives from A

the following advertisement:

Will the table in D change? If so how?

11. Why are different inter-AS and intra-AS protocols used in the Internet?

12. Consider a datagram network using 32-bit host addresses. Suppose a router has five links,

numbered 0 through 4, and packets are to be forwarded to the link interfaces as follows:

Destination Address Range Link Interface

11100000 00000000 00000000 00000000

Through 0 11100000 00000000 11111111 11111111

11100000 00000001 00000000 00000000

Through 1 11100000 00000001 11111111 11111111

11100000 00000010 00000000 00000000

Through 2

11100000 11111111 11111111 11111111

11100001 00000000 00000000 00000000

Through 3

11100001 11111111 11111111 11111111

Otherwise 4

a. Provide a forwarding table that has four 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 11100000 10101101 11000011 00111100

11100001 10000000 00010001 01110111

13. Consider a datagram network using 4 bit host addresses. Suppose a router uses longest prefix

matching and has the following forwarding table: Prefix Match 00 01 10 11 Interface 0 1 2 3 For each of the four interfaces, give the associated range of destination host addresses and the number of addresses in the range. （14、15、16可选讲）

14. 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

220.2.240/120. Also suppose that Subnet 1 is required to support up to 2000 interfaces, and Subnets 2 and 3 are each required to support up to 1000 interfaces. Provide three network addresses (of the form a.b.c.d/x) that satisfy these constraints.

15. 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. Consider the topology shown in Figure 4.17. Denote the three subnets with hosts (starting

clockwise at l2:fi)) 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 from214.97.254/23; 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.

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