计网-第三章作业

Chapter 3

注：括弧中标题号为第四版教材中对应的习题号

1. (R14)Suppose HostA sends two TCP segments back to back to Host B over a TCPconnection.

The first segment has sequence number 90; the second hassequence number 110. a. How much data is in the first segment?

b.Suppose that the first segment is lost but the second segment arrives at B.In the acknowledgment that Host B sends to Host A, what will be theacknowledgment number? 答：

a．[90,109]=20bytes

b．ack number=90，对第一个报文段确认

2. (R15)True or false?

a. The size of the TCP RcvWindow never changes throughout the durationof the connection. b. suppose Host A is sending Host B a large file over a TCP connection. Thenumber of unacknowledged bytes that A sends cannot exceed the size ofthe receive buffer.

c. HostA is sending Host B a large file over a TCP connection. Assume HostB has no data to send Host A. Host B will not send acknowledgments toHost A because Host B cannot piggyback the acknowledgment on data.

d. The TCP segment has a field in its header for RcvWindow.

e. Suppose Host A is sending a large file to Host B over a TCP connection. Ifthe sequence number for a segment of this connection is m, then thesequence number for the subsequent segment will necessarily be m + 1.

f. Suppose that the last SampleRTT in a TCP connection is equal to 1 sec.The current value of TimeoutInterval for the connection will necessarilybe>=1 sec.

g.Suppose Host A sends one segment with sequence number 38 and 4 bytesof data over a TCP connection to Host B. In this same segment theacknowledgment number is necessarily 42. 答： a.F b.T

c.F：即使没有数据传送，也会进行单独确认 d.T

e.F：按字节编号，不按报文段编号 f.F

g.F：B->A的确认号不一定为38+4=42

3. (R17)True or false? Consider congestion control in TCP. When the timer expires atthe sender,

the threshold is set to one half of its previous value. 答：

F：应为当前拥塞窗口的一半，而不是阈值的一半。

4. (P3)UDP and TCP use 1s complement for their checksums. Suppose you have the following

three 8-bit bytes: 01101010, 01001111, 01110011. What is the 1s complement of the sum of these 8-bit byte? (Note that although UDP and TCP use 16-bit words in computing the

checksum, for this problem you are being asked to consider 8-bit sums. ) Show all work.. Why is it that UDP takes complement of the sum; that is, why not just use the sum? With the 1s complement scheme, how does the receiver detect errors? Is it possible that a 1-bit error will go undetected? How about a 2-bit error? 答：

01101010+01001111=11000101, 11000101+01110011=00010001

取反为11101110。为了发现错误，接收端增加4个字组（3个原始的，1个取反后的），如果总数包含0，即有错误。所有的一位错误会发现，但两位错误有可能不会被发现。

5. (P7)Draw the FSM for the receiver side of protocol rdt3.0.

答：

6. (P13)Consider a reliable data transfer protocol that uses only negative acknowledgements.

Suppose the sender sends data only infrequently. Would a NAK-only protocol be preferable to a protocol to that uses ACKs? Why? Now suppose the sender has a lot of data to send and the end-to-end connection experiences few losses. In this second case, would a NAK-only protocol be preferable to a protocol that uses ACKs? Why? 答：

在仅使用NAK的协议中，只有当接收到分组x+1时才能检测到分组x的丢失。如果传输x和传输x+1之间有很长的延时，那么在此协议中修复分组x需要很长的时间；如果要发送大量的数据，在仅有NAK的协议中修复速度很快；如果错误很少，那么NAK只偶尔发送ACK，则会明显减少反馈时间。

7. (P14)Consider the cross-country example shown in Figure 3.17. How big would the window

size have to be for the channel utilization to be greater than 80 percent?

答：

U=n×L/R/(RTT+L/R)≥80% n≥3001

8. (P19)Answer true or false to the following questions and briefly justify youranswer:

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

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

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

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

答：

a. T：在t0时刻发送方窗口3发送包1,2,3；在t1时刻接收方接收ACKs1,2,3；在t2时刻

发送方延时并重新发送1,2,3；在t3时刻接收方接收包并重新发送确认1,2,3；在t4时刻发送方接收接收方在t1时刻发送的ACKs并进入窗口4,5,6；在t5时刻发送方接收接收方在t2时刻发送的ACKs1,2,3。这些ACKs在窗口之外。 b. T：见a c. T

d. T：在窗口1时，SR,GBN,the alternating bit protocol 在功能上是一样的，窗口1会自

动排除有可能无序的包。

9. (P23)Consider transferring an enormous file of L bytes from Host A to Host B. Assume an

MSS of 1,460 bytes.

a. What is the maximum value of L such that TCP sequence numbers are not exhausted?

Recall that the TCP sequence number fields has 4 bytes.

b. For the L you obtain in (a), find how long it takes to transmit the file. Assume that a total

of 66 bytes of transport, network, and data-link header are added to each segment before the resulting packet is sent our over a 100 Mbps link. Ignore flow control and congestion control so A can pump out the segments back to back and continuously. 答：

a. TCP序号范围为4bytes，LMAX=232bytes b. 传输速度=155Mbps，每段加66bytes大小的头，共分段：232bytes/1460bytes=2941758

段；头大小和=2941758×66=194156028bytes；总共需传输194156028+232bytes=4489123324bytes=35912986592bits的数据；用10Mbps的速度传输则时间为3591s。

10. (P34)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 shortdiscussion 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 tripleduplicate ACK or by a timeout?

d. After the 22nd transmission round, is segment loss detected by a tripleduplicate 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 atriple duplicate ACK, what will be the values of the congestion windowsize and of Threshold? 答：

a. 运行TCP慢启动的时间间隔是[1,6]和[23,26]； b. 运行TCP避免拥塞时的时间间隔是[1,6]和[17,22]；

c. 在第16个传输周期后，通过3个冗余ACK能够检测到一个报文段丢失。如果有一

个超时，拥塞窗口尺寸将减小为1.

d. 在第22个传输周期后，因为超时能够检测到一个报文段丢失，因此拥塞窗口的尺寸

被设置为1。

e. Threshold的初始值设置为32，因为在这个窗口尺寸是慢启动停止，避免拥塞开始。 f. 当检测到报文段丢失时，threshold被设置为拥塞窗口值的一半。当在第16个周期检

测到丢失时，拥塞窗口的大小是42，因此在第18个传输周期时threshold值为21。 g. 当检测到报文段丢失时，threshold被设置为拥塞窗口值的一半。当在第22个周期检

测到丢失时，拥塞窗口的大小是26，因此在第24个传输周期时threshold值为13。

h. 在第1个传输周期内，报文段1被传送；在第2个传输周期发送报文段2-3；在第3

个传输周期发送报文段4-7，在第4个传输周期发送8-15；在第5个传输周期发送16-31；在第6个传输周期发送32-63；在第7个传输周期发送64-96；因此，报文段70在第7个传送周期内发送。

i. 当丢失出现时拥塞窗口和threshold的值被设置为目前拥塞窗口长度8的一半。因此

新的拥塞窗口和threshold的值为4.

11. (P38)Host A is sending an enormous file to Host B over a TCP connection. Overthis

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 byR bps. Suppose that the process in Host A is capable of sending data into itsTCP socket at a rateS bps, where S = 10*R. Further suppose that the TCPreceive buffer is large enough to hold the entire file, and the send buffer canhold only one percent of the file. What would prevent the process in HostAfrom continuously passing data to its TCP socket at rate S bps? TCP flowcontrol? TCP congestion control? Or something else? Elaborate. 答：

在这个问题中，接收机没有溢出的危险，因为接收机的接收缓冲区可以承载整个文件。同样的，因为在计时器计时完毕前，没有丢失和回复确认，TCP不会限制发送方。但是，在端系统A上的进程不会一直传输数据给套接字，因为发送缓冲区会很快被填满，一旦发送缓冲区满，那么进程就会以平均速率传输数据。

h. 在第1个传输周期内，报文段1被传送；在第2个传输周期发送报文段2-3；在第3

个传输周期发送报文段4-7，在第4个传输周期发送8-15；在第5个传输周期发送16-31；在第6个传输周期发送32-63；在第7个传输周期发送64-96；因此，报文段70在第7个传送周期内发送。

i. 当丢失出现时拥塞窗口和threshold的值被设置为目前拥塞窗口长度8的一半。因此

新的拥塞窗口和threshold的值为4.

11. (P38)Host A is sending an enormous file to Host B over a TCP connection. Overthis

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 byR bps. Suppose that the process in Host A is capable of sending data into itsTCP socket at a rateS bps, where S = 10*R. Further suppose that the TCPreceive buffer is large enough to hold the entire file, and the send buffer canhold only one percent of the file. What would prevent the process in HostAfrom continuously passing data to its TCP socket at rate S bps? TCP flowcontrol? TCP congestion control? Or something else? Elaborate. 答：

在这个问题中，接收机没有溢出的危险，因为接收机的接收缓冲区可以承载整个文件。同样的，因为在计时器计时完毕前，没有丢失和回复确认，TCP不会限制发送方。但是，在端系统A上的进程不会一直传输数据给套接字，因为发送缓冲区会很快被填满，一旦发送缓冲区满，那么进程就会以平均速率传输数据。

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