Review for Chapter 3

Transcription

1 Review for Chapter 3

2 R3.Describle why an application developer might choose to run an application over UDP rather than TCP. Answer: An application developer may not want its application to use TCP s congestion control, which can throttle the application s sending rate at times of congestion. Often, designers of IP telephony and IP videoconference applications choose to run their applications over UDP because they want to avoid TCP s congestion control. Also, some applications do not need the reliable data transfer provided by TCP.

3 R5.Is it possible for an application to enjoy reliable data transfer even when the application runs over UDP? If so, how? Answer: Yes. The application developer can put reliable data transfer into the application layer protocol. This would require a significant amount of work and debugging, however.

4 R6.Consider a TCP connection between Host A and Host B. Suppose that the TCP segments traveling from Host A to Host B have source port number x and destination port number y. What are the source and destination port numbers for the segments traveling from Host B to Host A? Answer: Source port number y and destination port number x.

5 R7.Suppose a process in Host C has a UDP socket with port number 6789.Suppose both Host A and Host B each send a UDP segment to Host C with destination port number Will both of these segments be directed to the same socket at Host C? If so, how will the process at Host C know that these two segments originated from two different hosts? Answer: Yes, both segments will be directed to the same socket. For each received segment, at the socket interface, the operating system will provide the process with the IP addresses to determine the origins of the individual segments.

6 R8.Suppose that a Web sever runs in Host C on port 80.Suppose this Web server uses persistent connections, and is currently receiving requests from two different Hosts, A and B. Are all of the requests being sent through the same socket an Host C? If they are being passed through different sockets,do both of the sockets have port 80? Discuss and explain. Answer: For each persistent connection, the Web server creates a separate connection socket. Each connection socket is identified with a fourtuple: (source IP address, source port number, destination IP address, destination port number). When host C receives and IP datagram, it examines these four fields in the datagram/segment to determine to which socket it should pass the payload of the TCP segment. Thus, the requests from A and B pass through different sockets. The identifier for both of these sockets has 80 for the destination port; however, the identifiers for these sockets have different values for source IP addresses. Unlike UDP, when the transport layer passes a TCP segment s payload to the application process, it does not specify the source IP address, as this implicitly specified by the socket identifier.

7 R14. Suppose Host A sends two TCP segments back to back to Host B over a TCP connection. The first segment has sequence number 90;the second had sequence 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 the acknowledgment number? Answer: 20 bytes ack number = 90

8 R15. True or false? a. The size of the TCP RcvWindow never changes throughout the duration of the connection. b. Suppose Host A is sending Host B a large file over a TCP connection. The number of unacknowledged bytes that A sends cannot exceed the size of the receive buffer. c. Host A is sending Host B a large file over a TCP connection.assume Host B has no data to send Host A. Host B will not send acknowledgments to Host A because Host B cannot piggyback the acknowledgments on data d. The TCP segment has a field in its header for RcvWindow.

9 e. Suppose Host A is sending a large file to Host B over a TCP connection. If the sequence number for a segment of this connection is m, then the sequence number for the subsequent segment will necessarily be m+1. f. Suppose that the last samplertt in a TCP connection is equal to 1sec. The current value of Timeout Interval for the connection will necessarily be > or = 1 sec. g. Suppose Host A sends one segment with sequence number 38 and 4 bytes of data over a TCP connection to Host B. In this same segment the acknowledgment number id necessarily 42.

10 Answer: a) False b) true c) false d) true e) false f) false g) false

11 P1.Suppose Client A initiates a Telnet session with Server S. At about the same time,client B also initiates a Telnet session with Server S. Provide possible source and destination port numbers for 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. Hoe about if they are the same host?

12 Answer: e) Yes. f) No.

13 P5. a. Suppose you have the following 2 bytes: and What is the 1s complement of sum of these 2 bytes? b. Suppose you have the following 2 bytes: and What is the 1s complement of sum of these 2 bytes? c. For the bytes in part (a), give an example where one bit is flipped in each of the 2 bytes and yet the 1s complement doesn t change.

14 Answer : (a) Adding the two bytes gives Taking the one s complement gives (b) Adding the two bytes gives ; the one s complement gives (c) first byte = ; second byte =

15 P21. Consider the GBN and SR protocols. Suppose the sequence number space is of size k. what is the largest allowable sender window that will avoid the occurrence of problems such as that in Figure 3.27 for each of these protocols?

16 Answer:

17 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 field has 4 bytes. b. For the L you obtain in (a), find how long it take 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 out over a 100 Mbps link. Ignore flow control and congestion control so A can pimp out the segment back to back and continuously.

18 Answer:

19 P24. Host A and Host 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 ack 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 pot number? b. If the first segment arrives before the second segment, in the acknowledgment 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 acknowledgment 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 acknowledgment is lost and the second acknowledgment 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 acknowledgment that you add, provide the acknowledgment number.

20 Answer : a. In the second segment from Host A to B, the sequence number is 409, source port number is 1028 and destination port number is 80. b. If the first segment arrives before the second, in the acknowledgement of the first arriving segment, the acknowledgement number is 409, the source port number is 80 and the destination port number is c. If the second segment arrives before the first segment, in the acknowledgement of the first arriving segment, the acknowledgement number is 359, indicating that it is still waiting for bytes 359 and onwards.

21 P34. 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 slow start 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?

22 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 pkt 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?

23 a) TCP slowstart is operating in the intervals [1,6] and [23,26] b) TCP congestion advoidance is operating in the intervals [6,16] and [17,22] c) After the 16th transmission round, packet loss is recognized by a triple duplicate ACK. If there was a timeout, the congestion window size would have dropped to 1. d) After the 22nd transmission round, segment loss is detected due to timeout, and hence the congestion window size is set to 1. e) The threshold is initially 32, since it is at this window size that slowtart stops and congestion avoidance begins.

24 f) The threshold is set to half the value of the congestion window when packet loss is detected. When loss is detected during transmission round 16, the congestion windows size is 42. Hence the threshold is 21 during the 18th transmission round. g) The threshold is set to half the value of the congestion window when packet loss is detected. When loss is detected during transmission round 22, the congestion windows size is 26. Hence the threshold is 13 during the 24th transmission round. h) During the 1st transmission round, packet 1 is sent; packet 2 3 are sent in the 2nd transmission round; packets 4 7 are sent in the 3rd transmission round; packets 8 15 are sent in the 4th transmission round; packets16 31 are sent in the 5th transmission round; packets are sent in the 6th transmission round; packets are sent in the 7th transmission round. Thus packet 70 is sent in the 7th transmission round. i) The congestion window and threshold will be set to half the current value of the congestion window when the loss occurred. Thus the new values of the threshold and window will be 4.

25 P36.Consider sending a large file from a host to another over a TCP connection that has no loss. a. Suppose TCP uses AIMD for its congestion control without slow start. Assuming CongWin increases by 1 MSS every time a batch of ACKs is received and assuming approximately constant round trip times, how long does it take for CongWin to increase from 1MSS to 8 MSS (assuming no loss events)? b. What is the average throughout (in terms of MSS and RTT) for this connection up through time = 7 RTT?

26 Answer : It takes 1 RTT to increase CongWin to 2 MSS; 2 RTTs to increase to 3 MSS; 3 RTTs to increase to 4 MSS; 4 RTTs to increase to 5 MSS; and 7 RTTs to increase to 8 MSS. b) In the first RTT 1 MSS was sent; in the second RTT 2 MSS was sent; in the third RTT 3 MSS was sent; in the forth RTT 4 MSS was sent;... Thus, up to time 7 RTT, = 28 MSS were sent (and acknowledged). Thus, once can say that the average throughout up to time 7 RTT was (28 MSS)/(7RTT) = 4 MSS/RTT.

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28 bytes _ sent average _ rate duration 1 MSS 2MSS L W RTT L* W* RTT L W W W 3L 8 4 According to (1) and (2), thus MSS 1.22MSS average_rate 2 RTT L L * RTT 3 (1) (2)