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Supplements-ComputerNetworking-ATopDownApproach-7th-ed
Supplements material of Computer Networking: A Top-Down Approach, 7th ed. (By J.F. Kurose and K.W. Ross), including wireshark labs guides and powerpoint slides.
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 //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.
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:- Start up your web browser.
- Start up the Wireshark packet sniffer, as described in the Introductory lab (but don't yet begin packet capture). Enter "http" (just the letters, not the quotation marks) in the display-filter-specification window, so that only captured HTTP messages will be displayed later in the packet-listing window. (Were only interested in the HTTP protocol here, and don't want to see the clutter of all captured packets). Click the Apply button to the right.
- Wait a bit more than one minute (well see why shortly), and then begin Wireshark packet capture.
- Enter the following to your browser //gaia.cs.umass.edu/wireshark-labs/HTTP-wireshark-file1.html Your browser should display the very simple, one-line HTML file.
- Stop Wireshark packet capture.
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. ]
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.
Figure 1: Wireshark Display after //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 //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.). ]
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: - Is your browser running HTTP version 1.0 or 1.1? What version of HTTP is the server running?
- What languages (if any) does your browser indicate that it can accept to the server?
- What is the IP address of your computer? Of the gaia.cs.umass.edu server?
- What is the status code returned from the server to your browser?
- When was the HTML file that you are retrieving last modified at the server?
- How many bytes of content are being returned to your browser?
- 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.
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:
- Start up your web browser, and make sure your browsers cache is cleared, as discussed above.
- Start up the Wireshark packet sniffer. Enter the following URL into your browser //gaia.cs.umass.edu/wireshark-labs/HTTP-wireshark-file2.html Your browser should display a short HTML file with two images. These two images are referenced in the base HTML file. That is, the images themselves are not contained in the HTML; instead the URLs for the images are contained in the downloaded HTML file. As discussed in the textbook, your browser will have to retrieve these logos from the indicated web sites. Our publishers logo is retrieved from the www.aw-bc.com web site. The image of the cover for our 5th edition (one of our favorite covers) is stored at the manic.cs.umass.edu server.
How did Gaia cs Umass Edu determine that value?
The value of the ACKnowledgement field in the SYNACK segment is determined by gaia.cs.umass.edu by adding 1 to the initial sequence number of SYN segment from the client computer (i.e. the sequence number of the SYN segment initiated by the client computer is 0.).
Did the server explicitly return the contents of the file Wireshark?
The server did explicitly return the contents of the file. Wireshark includes a section titled “Line-Based Text Data” which shows what the server sent back to my browser which is specifically what the website showed when I brought it up on my browser.
When your browser sends the HTTP GET message for the second time what new field is included in the HTTP GET message?
The new field that is now included is the authorization field. This is included because we sent the server a username and password along with our request stating that we were authorized to receive the page.
What is the IP address of your computer of the Gaia cs Umass Edu server?
According to above figure, the IP address of gaia.cs.umass.edu is 128.119. 245.12 and the TCP port number is 80.