Wireshark Lab: HTTP (Kurose and Ross)

This part of the exercises is based on the textbook Computer Networking -- A Topdown Approach by Kurose and Ross, 6th edition. The original assignment is listed at authors' website at http://gaia.cs.umass.edu/wireshark-labs/. The content is modified slightly for use in CSCI 363. Any mistakes are mine. Please send comments to me through xmeng@bucknell.edu.

"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

Having gotten our feet wet with the Wireshark packet sniffer in the introductory lab, were now ready to use Wireshark to investigate protocols in operation. In this lab, well explore several aspects of the HTTP protocol: the basic GET/response interaction, HTTP message formats, retrieving large HTML files, retrieving HTML files with embedded objects, and HTTP authentication and security. Before beginning these labs, you might want to review Section 2.2 of the text.1

  1. The Basic HTTP GET/response interaction

    Lets begin our exploration of HTTP by downloading a very simple HTML file - one that is very short, and contains no embedded objects. 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. ]"Jump back to the text."

    Your Wireshark window should look similar to the window shown in Figure 1. If you are unable to run Wireshark on a live network connection, you can download a packet trace that was created when the steps above were followed.2

    Figure 1: Wireshark Display after http://gaia.cs.umass.edu/wireshark-labs/HTTP-wireshark-file1.html has been retrieved by your browser

    The example in Figure 1 shows in the packet-listing window that two HTTP messages were captured: the GET message (from your browser to the gaia.cs.umass.edu web server) and the response message from the server to your browser. The packet-contents window shows details of the selected message (in this case the HTTP OK message, which is highlighted in the packet-listing window). Recall that since the HTTP message was carried inside a TCP segment, which was carried inside an IP datagram, which was carried within an Ethernet frame, Wireshark displays the Frame, Ethernet, IP, and TCP packet information as well. We want to minimize the amount of non-HTTP data displayed (were interested in HTTP here, and will be investigating these other protocols is later labs), so make sure the boxes at the far left of the Frame, Ethernet, IP and TCP information have a plus sign or a right-pointing triangle (which means there is hidden, undisplayed information), and the HTTP line has a minus sign or a down-pointing triangle (which means that all information about the HTTP message is displayed).

    2. [Download the zip file http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip and extract the file http-ethereal-trace-1. The traces in this zip file were collected by Wireshark running on one of the authors computers, while performing the steps indicated in the Wireshark lab. Once you have downloaded the trace, you can load it into Wireshark and view the trace using the File pull down menu, choosing Open, and then selecting the http-ethereal-trace-1 trace file. The resulting display should look similar to Figure 1. (The Wireshark user interface displays just a bit differently on different operating systems, and in different versions of Wireshark). (Note: You should ignore any HTTP GET and response for favicon.ico. If you see a reference to this file, it is your browser automatically asking the server if it (the server) has a small icon file that should be displayed next to the displayed URL in your browser. Well ignore references to this pesky file in this lab.). ]"Jump back to the text."

    By looking at the information in the HTTP GET and response messages, answer the following questions. When answering the following questions, you should print out the GET and response messages (see the introductory Wireshark lab for an explanation of how to do this) and indicate where in the message youve found the information that answers the following questions. When you hand in your assignment, annotate the output so that its clear where in the output youre getting the information for your answer (e.g., for our classes, we ask that students markup paper copies with a pen, or annotate electronic copies with text in a colored font).

    Answer the following questions:

    1. Is your browser running HTTP version 1.0 or 1.1? What version of HTTP is the server running?
    2. What languages (if any) does your browser indicate that it can accept to the server?
    3. What is the IP address of your computer? Of the gaia.cs.umass.edu server?
    4. What is the status code returned from the server to your browser?
    5. When was the HTML file that you are retrieving last modified at the server?
    6. How many bytes of content are being returned to your browser?
    7. By inspecting the raw data in the packet content window, do you see any headers within the data that are not displayed in the packet-listing window? If so, name one.

    In your answer to question 5 above, you might have been surprised to find that the document you just retrieved was last modified within a minute before you downloaded the document. Thats because (for this particular file), the gaia.cs.umass.edu server is setting the files last-modified time to be the current time, and is doing so once per minute. Thus, if you wait a minute between accesses, the file will appear to have been recently modified, and hence your browser will download a new copy of the document.

  2. The HTTP CONDITIONAL GET/response interaction

    Recall from Section 2.2.6 of the text, that most web browsers perform object caching and thus perform a conditional GET when retrieving an HTTP object. Before performing the steps below, make sure your browsers cache is empty. (To do this under Firefox, select the Menu from the top right corner, then select History, followed by Clear Recent History. To do it from Chrome, select the Menu from the top right corner, then select Moor Tools, the select Clear browsing data.) Now do the following:

    Answer the following questions:

    1. Inspect the contents of the first HTTP GET request from your browser to the server. Do you see an IF-MODIFIED-SINCE line in the HTTP GET?
    2. Inspect the contents of the server response. Did the server explicitly return the contents of the file? How can you tell?
    3. Now inspect the contents of the second HTTP GET request from your browser to the server. Do you see an IF-MODIFIED-SINCE: line in the HTTP GET? If so, what information follows the IF-MODIFIED-SINCE: header?
    4. What is the HTTP status code and phrase returned from the server in response to this second HTTP GET? Did the server explicitly return the contents of the file? Explain.
  3. Retrieving Long Documents

    In our examples thus far, the documents retrieved have been simple and short HTML files. Lets next see what happens when we download a long HTML file. Do the following:

    In the packet-listing window, you should see your HTTP GET message, followed by a multiple-packet TCP response to your HTTP GET request. This multiple-packet response deserves a bit of explanation. Recall from Section 2.2 (see Figure 2.9 in the text) that the HTTP response message consists of a status line, followed by header lines, followed by a blank line, followed by the entity body. In the case of our HTTP GET, the entity body in the response is the entire requested HTML file. In our case here, the HTML file is rather long, and at 4500 bytes is too large to fit in one TCP packet. The single HTTP response message is thus broken into several pieces by TCP, with each piece being contained within a separate TCP segment (see Figure 1.24 in the text). In recent versions of Wireshark, Wireshark indicates each TCP segment as a separate packet, and the fact that the single HTTP response was fragmented across multiple TCP packets is indicated by the "TCP segment of a reassembled PDU" in the Info column of the Wireshark display. Earlier versions of Wireshark used the "Continuation" phrase to indicated that the entire content of an HTTP message was broken across multiple TCP segments. We stress here that there is no "Continuation" message in HTTP!

    Answer the following questions:

    1. How many HTTP GET request messages did your browser send? Which packet number in the trace contains the GET message for the Bill or Rights?
    2. Which packet number in the trace contains the status code and phrase associated with the response to the HTTP GET request?
    3. What is the status code and phrase in the response?
    4. How many data-containing TCP segments were needed to carry the single HTTP response and the text of the Bill of Rights?
  4. HTML Documents with Embedded Objects

    Now that we've seen how Wireshark displays the captured packet traffic for large HTML files, we can look at what happens when your browser downloads a file with embedded objects, i.e., a file that includes other objects (in the example below, image files) that are stored on another server(s).

    Do the following:

    Answer the following questions:

    1. How many HTTP GET request messages did your browser send? To which Internet addresses were these GET requests sent?
    2. Can you tell whether your browser downloaded the two images serially, or whether they were downloaded from the two web sites in parallel? Explain.