Ethereal Lab: TCP Version: 1.0 © 2005 J.F. Kurose, K.W. Ross. All Rights Reserved Computer Networking: A Top-down Approach Featuring the Internet, 3rd edition. In this lab, we’ll investigate the behavior of TCP 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 text.1 1. Capturing a bulk TCP transfer from your computer to a remote server Before beginning our exploration of TCP, we’ll need to use Ethereal 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 Ethereal during this time to obtain the trace of the TCP segments sent and received from your computer. Do the following: 1 All references to the text in this lab are to Computer Networking: A Top-down Approach Featuring the Internet, 3rd edition.• Start up your web browser. Go the http://gaia.cs.umass.edu/ethereal-labs/alice.txt and retrieve an ASCII copy of Alice in Wonderland. Store this file somewhere on your computer. • Next go to http://gaia.cs.umass.edu/ethereal-labs/TCP-ethereal-file1.html. • You should see a screen that looks like: • Use the Browse button in this form to enter the name of the file (full path name) on your computer containing Alice in Wonderland (or do so manually). Don’t yet press the “Upload alice.txt file” button. • Now start up Ethereal and begin packet capture (Capture->Start) and then press OK on the Ethereal Packet Capture Options screen (we’ll not need to select any options here). • Returning to your browser, press the “Upload alice.txt file” button to upload the file to the gaia.cs.umass.edu server. Once the file has been uploaded, a short congratulations message will be displayed in your browser window. • Stop Ethereal packet capture. Your Ethereal window should look similar to the window shown below.If you are unable to run Ethereal on a live network connection, you can download a packet trace file that was captured while following the steps above on one of the author’s computers2. You may well find it valuable to download this trace even if you’ve captured your own trace and use it, as well as your own trace, when you explore the questions below. 2. A first look at the captured trace Before analyzing the behavior of the TCP connection in detail, let’s take a high level view of the trace. • First, filter the packets displayed in the Ethereal window by entering “tcp” (lowercase, no quotes, and don’t forget to press return after entering!) into the display filter specification window towards the top of the Ethereal window. What you should see is series of TCP and HTTP messages between your computer and gaia.cs.umass.edu. You should see the initial three-way handshake containing a SYN 2 Download the zip file http://gaia.cs.umass.edu/ethereal-labs/ethereal-traces.zip and extract the file tcp-ethereal-trace-1. The traces in this zip file were collected by Ethereal running on one of the author’s computers, while performing the steps indicated in the Ethereal lab. Once you have downloaded the trace, you can load it into Ethereal and view the trace using the File pull down menu, choosing Open, and then selecting the tcp-ethereal-trace-1 trace file.message. You should see an HTTP POST message and a series of “HTTP Continuation” messages being sent from your computer to gaia.cs.umass.edu. Recall from our discussion in the earlier HTTP Ethereal lab, that is no such thing as an HTTP Continuation message – this is Ethereal’s way of indicating that there are multiple TCP segments being used to carry a single HTTP message. You should also see TCP ACK segments being returned from gaia.cs.umass.edu to your computer. Answer the following questions, by opening the Ethereal captured packet file tcp-ethereal-trace-1 in http://gaia.cs.umass.edu/ethereal-labs/ethereal-traces.zip (that is download the trace and open that trace in Ethereal; 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 printout 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 Ethereal” Lab if you’re uncertain about the Ethereal 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 Ethereal’s “listing of captured packets” window so that it shows information about the TCP segments containing the HTTP messages, rather