《计算机网络 Computer Networking》课程课程实验讲稿（Wireshark Lab，6th edition）Analyze TCP with Wireshark

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Wireshark Lab: TCP v6.0 Supplement to Computer Networking: A Top-Down Approach, 6th ed., J.F. Kurose and K.W. Ross “Tell me and I forget. Show me and I remember. Involve me and I understand.” Chinese proverb © 2005-21012, J.F Kurose and K.W. Ross, All Rights Reserved In this lab, we’ll investigate the behavior of the celebrated TCP protocol in detail. We’ll do so by analyzing a trace of the TCP segments sent and received in transferring a 150KB file (containing the text of Lewis Carrol’s Alice’s Adventures in Wonderland) from your computer to a remote server. We’ll study TCP’s use of sequence and acknowledgement numbers for providing reliable data transfer; we’ll see TCP’s congestion control algorithm – slow start and congestion avoidance – in action; and we’ll look at TCP’s receiver-advertised flow control mechanism. We’ll also briefly consider TCP connection setup and we’ll investigate the performance (throughput and round-trip time) of the TCP connection between your computer and the server. Before beginning this lab, you’ll probably want to review sections 3.5 and 3.7 in the text1 . 1. Capturing a bulk TCP transfer from your computer to a remote server Before beginning our exploration of TCP, we’ll need to use Wireshark to obtain a packet trace of the TCP transfer of a file from your computer to a remote server. You’ll do so by accessing a Web page that will allow you to enter the name of a file stored on your computer (which contains the ASCII text of Alice in Wonderland), and then transfer the file to a Web server using the HTTP POST method (see section 2.2.3 in the text). We’re using the POST method rather than the GET method as we’d like to transfer a large amount of data from your computer to another computer. Of course, we’ll be running Wireshark during this time to obtain the trace of the TCP segments sent and received from your computer. Do the following: 1 References to figures and sections are for the 6th edition of our text, Computer Networks, A Top-down Approach, 6th ed., J.F. Kurose and K.W. Ross, Addison-Wesley/Pearson, 2012

Wireshark you are using.you might see a series of"HTTP Continuation"messages being sent from your computer to gaia.cs.umass.edu.Recall from our discussion in the earlier HTTP Wireshark lab,that is no such thing as an HTTP Continuation message-this is Wireshark's way of indicating that there are multiple TCP segments being used to carry a single HTTP message.In more recent versions of Wireshark,you'll see"[TCP segment of a reassembled PDU]"in the Info column of the Wireshark display to indicate that this TCP segment contained data that belonged to an upper layer protocol message(in our case here.HTTP).You should also see TCP ACK segments being returned from gaia.cs.umass.edu to your computer. Answer the following by openin ng the Wireshark captured packet file rk-l download t trace and open that ote 2 vhen answ g a question you s n a printout of the packet(s) that you used to answer the question asked nnotate the printo to ain your answer.To print a packet,use File->Print,choose Selected packet only,choose Packer summary line,and select the minimum amount of packet detail that you need to answer the question. 1.What is the IP address and TCP port number used by the client computer(source) that is transferring the file to gaia.cs.umass.edu?To answer this question,it's 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"(refer to Figure 2 in the"Getting Started with Wireshark"Lab if you're uncertain about the Wireshark windows. 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? If you have been able to create your own trace,answer the following question 3.What is the IP address and TCP port nur sed by your client computer (source)to transfer the file to gaia.cs.umass.edu? Since this lab is about TCP rather than HTPlet's change Wireshark's"of captured pac ow so that shows information ab out the TCPsegments redohs selectae Then uncheck the HP bos and ITTP messages,rather th an about the HTTP mess o have select OK.You should now see a Wireshark window that looks like: you've found the an colored pen)noting what you found in what

Wireshark you are using, you might see a series of “HTTP Continuation” messages being sent from your computer to gaia.cs.umass.edu. Recall from our discussion in the earlier HTTP Wireshark lab, that is no such thing as an HTTP Continuation message – this is Wireshark’s way of indicating that there are multiple TCP segments being used to carry a single HTTP message. In more recent versions of Wireshark, you’ll see “[TCP segment of a reassembled PDU]” in the Info column of the Wireshark display to indicate that this TCP segment contained data that belonged to an upper layer protocol message (in our case here, HTTP). You should also see TCP ACK segments being returned from gaia.cs.umass.edu to your computer. Answer the following questions, by opening the Wireshark captured packet file tcpethereal-trace-1 in http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip (that is download the trace and open that trace in Wireshark; see footnote 2). Whenever possible, when answering a question you should hand in a printout of the packet(s) within the trace that you used to answer the question asked. Annotate the printout3 to explain your answer. To print a packet, use File->Print, choose Selected packet only, choose Packet summary line, and select the minimum amount of packet detail that you need to answer the question. 1. What is the IP address and TCP port number used by the client computer (source) that is transferring the file to gaia.cs.umass.edu? To answer this question, it’s 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” (refer to Figure 2 in the “Getting Started with Wireshark” Lab if you’re uncertain about the Wireshark windows. 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? If you have been able to create your own trace, answer the following question: 3. What is the IP address and TCP port number used by your client computer (source) to transfer the file to gaia.cs.umass.edu? Since this lab is about TCP rather than HTTP, let’s change Wireshark’s “listing of captured packets” window so that it shows information about the TCP segments containing the HTTP messages, rather than about the HTTP messages. To have Wireshark do this, select Analyze->Enabled Protocols. Then uncheck the HTTP box and select OK. You should now see a Wireshark window that looks like: 3 What do we mean by “annotate”? If you hand in a paper copy, please highlight where in the printout you’ve found the answer and add some text (preferably with a colored pen) noting what you found in what you ‘ve highlight. If you hand in an electronic copy, it would be great if you could also highlight and annotate

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 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 Section 3.5.3, page 239 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 computed using the EstimatedRTT equation on page 239 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.edu server. Then select: Statistics->TCP Stream Graph- >Round Trip Time Graph. 8. What is the length of each of the first six TCP segments?4 9. What is the minimum amount of available buffer space advertised at the received for the entire trace? Does the lack of receiver buffer space ever throttle the sender? 10. Are there any retransmitted segments in the trace file? What did you check for (in the trace) in order to answer this question? 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 247 in the text). 12. What is the throughput (bytes transferred per unit time) for the TCP connection? Explain how you calculated this value. 4 The TCP segments in the tcp-ethereal-trace-1 trace file are all less that 1460 bytes. This is because the computer on which the trace was gathered has an Ethernet card that limits the length of the maximum IP packet to 1500 bytes (40 bytes of TCP/IP header data and 1460 bytes of TCP payload). This 1500 byte value is the standard maximum length allowed by Ethernet. If your trace indicates a TCP length greater than 1500 bytes, and your computer is using an Ethernet connection, then Wireshark is reporting the wrong TCP segment length; it will likely also show only one large TCP segment rather than multiple smaller segments. Your computer is indeed probably sending multiple smaller segments, as indicated by the ACKs it receives. This inconsistency in reported segment lengths is due to the interaction between the Ethernet driver and the Wireshark software. We recommend that if you have this inconsistency, that you perform this lab using the provided trace file

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