Behavior of TCP

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Welcome back to the Lab Programme!

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 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. 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:
Start up your web browser. Go the and retrieve an ASCII copy of Alice in Wonderland. Store this file somewhere on your computer. •Next go to •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 Wireshark and begin packet capture (Capture-> Options) and then press OK on the Wireshark 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 server. Once the file has been uploaded, a short congratulations message will be displayed in your browser window. •Stop Wireshark packet capture. Your Wireshark window should look similar to the window shown 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 Wireshark 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 Wireshark window. What you should see is series of TCP and HTTP messages between your computer and You should see the initial three-way handshake containing a SYN message. You should see an HTTP POST message and a series of “HTTP Continuation” messages being sent from your computer to 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. You should also see TCP ACK segments being returned from to your computer. Answer the following questions, by opening the Wireshark captured packet file tcp-ethereal-trace-1 in (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...
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