计算机网络实验3Wireshark Lab TCPWord文档下载推荐.docx

1、 First, filter the packets displayed in the Wireshark window by entering “tcp” （lowercase, no quotes, and dont forget to press return after entering!） into the display filter specification window towards the top of the Wireshark window.QUESTIONS1. What is the IP address and TCP port number used by t

2、he client computer （source） that is transferring the file to gaia.cs.umass.edu? To answer this question, its probably easiest to select an HTTP message and explore the details of the TCP packet used to carry this HTTP message, using the “details of the selected packet header window”. The IP address

3、is 192.168.1.102. The TCP port number is 1161.2. What is the IP address of gaia.cs.umass.edu? On what port number is it sending and receiving TCP segments for this connection?The IP address is 128.119.245.12. The TCP port number is 80.3. What is the IP address and TCP port number used by your client

4、 computer （source） to transfer the file to gaia.cs.umass.edu?The IP address is 172.18.40.131. The TCP port number is 51458.4. What is the sequence number of the TCP SYN segment that is used to initiate the TCP connection between the client computer and gaia.cs.umass.edu? What is it in the segment th

5、at identifies the segment as a SYN segment? The sequence number of the TCP SYN segment is 0. The SYN flag is set to 1 identifies the segment as a SYN segment.5. What is the sequence number of the SYNACK segment sent by gaia.cs.umass.edu to the client computer in reply to the SYN? What is the value o

6、f the ACKnowledgement field in the SYNACK segment? How did gaia.cs.umass.edu determine that value? What is it in the segment that identifies the segment as a SYNACK segment?The sequence number of the SYNACK segment sent by gaia.cs.umass.edu is 0. The value of the Acknowledgement field in the SYNACK

7、segment is 1. The value is adding 1 to the initial sequence number of SYN segment. The SYN flag and Acknowledgement flag in the segment are set to 1 identifies the segment as a SYNACK segment.6. What is the sequence number of the TCP segment containing the HTTP POST command? Note that in order to fi

8、nd the POST command, youll need to dig into the packet content field at the bottom of the Wireshark window, looking for a segment with a “POST” within its DATA field. The sequence number of the TCP segment containing the HTTP POST command is 1.7. Consider the TCP segment containing the HTTP POST as

9、the first segment in the TCP connection. What are the sequence numbers of the first six segments in the TCP connection （including the segment containing the HTTP POST）? At what time was each segment sent? When was the ACK for each segment received? Given the difference between when each TCP segment

10、was sent, and when its acknowledgement was received, what is the RTT value for each of the six segments? What is the EstimatedRTT value （see page 249 in text） after the receipt of each ACK? Assume that the value of the EstimatedRTT is equal to the measured RTT for the first segment, and then is comp

11、uted using the EstimatedRTT equation on page 249 for all subsequent segments. Note: Wireshark has a nice feature that allows you to plot the RTT for each of the TCP segments sent. Select a TCP segment in the “listing of captured packets” window that is being sent from the client to the gaia.cs.umass

12、.edu server. Then select: Statistics-TCP Stream Graph- Round Trip Time Graph. We can get table 1: Table 1NumberTime （s）Seq NoACK No40.026477150.04173756660.05393770.054026202680.05469348690.077294100.0774054946110.0781576406120.124085130.1241857866140.169118150.217299160.267802Then, we can get table

13、 2 form table 1. Table 2NOSend Time （s）Ack Time （s）RTT （s）0.0274620.03555730.0700590.1144280.1398940.189645EstimatedRTT after the receipt of the ACK of segment 1:EstimatedRTT=0.02746 sEstimatedRTT after the receipt of the ACK of segment 2:EstimatedRTT=0.875*0.02746+0.125*0.035557=0.0285 sEstimatedRT

14、T after the receipt of the ACK of segment 3:EstimatedRTT=0.875*0.0285+0.125*0.070059=0.0337 sEstimatedRTT after the receipt of the ACK of segment 4:EstimatedRTT=0.875*0.0337+0.125*0.114428=0.0438 sEstimatedRTT after the receipt of the ACK of segment 5:EstimatedRTT=0.875*0.0438+0.125*0.139894=0.0558

15、sEstimatedRTT after the receipt of the ACK of segment 6:EstimatedRTT=0.875*0.0558+0.125*0.189645=0.0725 s8. What is the length of each of the first six TCP segments?According to Table 1, we can get that the first TCP segments length is 565 bytes. The other 5 is 1460 bytes. 9. What is the minimum amo

16、unt of available buffer space advertised at the received for the entire trace? Does the lack of receiver buffer space ever throttle the sender?The minimum amount of available buffer space advertised at the received for the entire trace is 5840 bytes. We can see that the sender is never throttle beca

17、use of the lack of receiver buffer space.10. Are there any retransmitted segments in the trace file? What did you check for （in the trace） in order to answer this question?There arent any retransmitted segments in the trace file. We can check the Time-Sequence-Graph （Stevens）. In the graph, there ar

18、ent two dots in the same y-line.11. How much data does the receiver typically acknowledge in an ACK? Can you identify cases where the receiver is ACKing every other received segment （see Table 3.2 on page 257 in the text）. The receiver typically acknowledge 1460 bytes in an ACK.80th is ACKing every

19、other received segment. It acks 76th and 77th two segment.12. What is the throughput （bytes transferred per unit time） for the TCP connection? Explain how you calculated this value.The last segments sequence number is 164091. So the total data is 164091-1=164090 bytes. The whole time is 5.455830 0.0

20、26477 = 5.429353 s. So the throughput is 164090/5.429353=30.222 KB/sec. Select a TCP segment in the Wiresharks “listing of captured-packets” window. Then select the menu :TCP Stream Graph- Time-Sequence-Graph（Stevens）. You should see a plot that looks similar to the following plot, which was created

21、 from the captured packets in the packet trace tcp-etherealtrace-1 in http:/gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip 13. Use the Time-Sequence-Graph（Stevens） plotting tool to view the sequence number versus time plot of segments being sent from the client to the gaia.cs.umass.edu server

22、. Can you identify where TCPs slowstart phase begins and ends, and where congestion avoidance takes over? Comment on ways in which the measured data differs from the idealized behavior of TCP that weve studied in the text. TCP slow start begins at the start of the connection. So when the HTTP POST s

23、egment is sent out, the slow start begins. But if we want to identify where TCPs slow start phase ends and where congestion avoidance takes over, we have to know the value of congestion window size of this sender. Sadly, we cant get the value directly. All we can do is to estimate it by the amount o

24、f data without acknowledgement. Because we know that LastByteSend LastbyteAcked = min CongWin, RcvWin and the RcvWin is large enough. But even we get the lower bound of the TCP window size, it is still hard to identify where TCPs slow start phase ends and where congestion avoidance takes over. After

25、 all, the amount of data without acknowledgement doesnt equal to the value of congestion window size. The idealized behavior of TCP that weve studied in the text: TCP sender will try to send more data. When it get an ACK, it will send more data. But once congestion happened, the Congwin will drop do

26、wn to a half. But in practice, TCP behavior also depends on the application. For example, in some web applications, the web objects size is very small. So before the end of slow start, the transmission is over. Then we will get a long delay and a small throughput. 14. Answer each of two questions above for the trace that you have gathered when you transferred a file from your computer to gaia.cs.umass.edu.= min CongWin, RcvWin and the RcvWin is large enough. But even we get the lower bound of the TCP window size, it is still hard to identify wher