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CPSC 471-05 Assignment

Introduction

At the heart of the communication involving a browser and a server, RFC 2616 defines the fun-damental protocol that governs interactions between internet devices. The result was known as the HTTP protocol. In this assignment, we will capture HTTP traffic using Wireshark packet sniffer and examine various HTTP header information and their applications in details. 

HTTP provides a very important feature that enables browser caching mechanism. The results are improved user’s experiences, significant reduction in communication link bandwidth re quirement and expenses. We will observe HTTP caching mechanism interactions in details.

Finally, the benefit of network architectural design layering approach becomes clear when we develop internet-enabled applications that seamlessly exchange data between host devices across the internet using socket programming.

Required Softwares

Submission

Reminders:

File Size Limitation

Submission Grace Period

Note: The assignment due date and time, and the assignment closing date and time are clearly shown on Canvas. Once the assignment is closed you can no longer submit.

What To Submit

The zip file shall contain all below listed files and must follow the naming convention as shown:

where xx is your first name and last name initials.

Hints: Use the standard compression utility already available from your PC Operating System to produce the zip file. That way the submitted zip file can be opened for grading using standar Windows OS built-in uncompress utility.

Part 1: Analyzing HTTP Messages using Wireshark

Part 1a: Basic HTTP GET/response interaction

Very Important notes on keeping captured files small:

Your Wireshark window should look similar to the window shown in Figure 1.

Figure 1: Wireshark Display after http://gaia.cs.umass.edu/wireshark-labs/ HTTP-wiresharkfile1.html has been retrieved by your browser.

The example in Figure 1 shows in the packet-listing window that two HTTP messages were cap-tured: 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, 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, not displayed information), and the HTTP line has a minus sign or a down-pointing trian

(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. We’ll ignore references to this pesky file.)

Part 1a Items To Submit

In the PDF file report, create a section called Part 1a – Basic HTTP GET/response interaction. Include answers for Part 1a questions below.

By looking at the information in the HTTP GET and response messages, answer the following questions.

When answering each question, you should include the screen capture of the GET and response messages and graphically circle or highlight to indicate where within the message the requested information came from.

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. That’s because (for this particular file), the gaia.cs.umass.edu server is setting the file’s last-modified time to be the current time and is doing so once per minute. Thus, if you wait a minute

Part 1b: The HTTP CONDITIONAL GET/response interaction

• Start up your web browser, and make sure your browser’s cache was cleared, as dis cussed above.
• Begin Wireshark capture
• Enter the following URL into your browser http://gaia.cs.umass.edu/wireshark-labs/HTTP-wireshark-file2.html Your browser should display a very simple five-line HTML file.
• Quickly enter the same URL into your browser again (or simply select the refresh button on your browser)
• Stop Wireshark capture and enter “http” in the display-filter-specification window, so that only captured HTTP messages will be displayed later in the packet-listing window. Save the Wireshark capture in file xxP1b.pcapng.

Part 1b Items To Submit

In the PDF file report, Create a section called Part 1b: The HTTP CONDITIONAL GET/response interaction. Include answers for Part 1b questions below:

Part 1c: Retrieving Long Documents

Part 1c Items To Submit

Include the Wireshark capture file xxP1c.pcapng in the submission zip file.

In the PDF file report, Create a section called Part 1c: Retrieving Long Documents. Include an swers for Part 1c questions below.

Part 1d: HTML Documents with Embedded Objects

• Start up your web browser, and make sure your browser’s cache is cleared
• Begin Wireshark capture
• Enter the following URL into your browser http://gaia.cs.umass.edu/wireshark-labs/HTTP-wireshark-file4.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 in dicated web sites. Our publisher’s logo is retrieved from the gaia.cs.umass.edu web site. The image of the cover for our 5th edition (one of our favorite covers) is stored at the caite.cs.umass.edu server. (These are two different web servers inside cs.umass.edu).
• Stop Wireshark capture and enter “http” in the display-filter-specification window, so that only captured HTTP messages will be displayed. Save the capture in file xxP1d.pcapng

Part 1d Items To Submit

In the PDF file report, Create a section called Part 1d: HTML Documents with Embedded Ob jects. Include answers for Part 1d questions below.

Part 1e: HTTP Authentication

Now let’s examine the Wireshark output. You might want to first read up on HTTP authentication by reviewing the easy-to-read material on “HTTP Access Authentication Framework” at http://frontier.userland.com/stories/storyReader$2159

Part 1e Items To Submit

In the PDF file report, Create a section called Part 1e: HTTP Authentication. Include answers for Part 1e questions below.

The username (wireshark-students) and password (network) that you entered are encoded in the string of characters (d2lyZXNoYXJrLXN0dWRlbnRzOm5ldHdvcms=) following the “Authorization: Basic” header in the client’s HTTP GET message. While it may appear that your username and password are encrypted, they are simply encoded in a format known as Base64 format. The username and password are not encrypted! To see this, go to http://www.motobit.com/util/base64-decoder-encoder.asp and enter the base64-encoded string d2lyZXNoYXJrLXN0dWRlbnRz and decode. Voila! You have translated from Base64 encoding to ASCII encoding, and thus should see your username! To view the password, enter the remainder of the string Om5ldHdvcms= and press decode. Since anyone can download a tool like Wireshark and sniff packets (not just their own) passing by their network adaptor, and any one can translate from Base64 to ASCII (you just did it!), it should be clear to you that simple passwords on WWW sites are not secure unless additional measures are taken.

Part 2: Socket Programming

Part 2a: UDP Pinger with No Delay and No Loss

In this portion, you will learn the basics of socket programming for UDP in Python. You will learn how to send and receive datagram packets using UDP sockets and, how to set a proper socket timeout. Throughout the lab, you will gain familiarity with a Ping application and its usefulness in computing statistics such as packet loss rate.

You will first study a simple Internet ping server written in the Python and implement a corresponding client. The functionality provided by these programs is like the functionality provided by standard ping programs available in modern operating systems. However, these programs use a simpler protocol, UDP, rather than the standard Internet Control Message Protocol (ICMP) to communicate with each other. The ping protocol allows a client machine to send a packet ofdata to a remote machine, and have the remote machine return the data back to the client unchanged (an action referred to as echoing). Among other uses, the ping protocol allows hosts to determine round-trip times to other machines.

You are given the complete code for the Ping server in the next subsection. Your task is to write the UDP Ping client.

Server Code

Client Code

You task is to implement the client program as explained below.

The client should send a specified number of pings to the server. Because UDP is an unreliable protocol, a packet sent from the client to the server may be lost in the network, or vice versa. For this reason, the client cannot wait indefinitely for a reply to a ping message. You should get the client wait up to one second for a reply; if no reply is received within one second, your client program should assume that the packet was lost during transmission across the network. You will need to look up the Python documentation to find out how to set the timeout value on a datagram socket.

Requirements for Client Code

Ping Message Format

ping_number starts at 1 and progresses to total number of pings for each successive ping message sent by the client, and time is the time when the client sends the message.

For example: The following is a sample display from the client program for student Alice.

Notes: the blue text represents the response from the server upon receiving the ping message (which is the message the server received from the client). The other texts are from the client itself.

Part 2a Items To Submit

In the PDF file report, create a section called Part 2a – UDP Pinger with No Delay and No Loss. Include the followings:

a. Include run-time screen captures for a sequence consists of 10 pings

Refer to the Appendix section on the last page for samples and required data format.

Part 2b: UDP Pinger with Delays
Delays

Part 2b Items To Submit

In the PDF file report, create a section called Part 2b – UDP Pinger No Loss, with Delays. Include the followings:

(2) Explain how to run your code, i.e., command line and any applicable parameter(s) 

a. Include run-time screen captures for a sequence consists of 10 pings

Refer to the Appendix section on the last page for samples and required data format.

Part 2c: UDP Pinger with Delays and Packet Losses
Packet Loss Injection

Hint: Create a variable which holds a randomized integer to determine whether a particular in coming packet is lost or not.

Part 2c Items To Submit

In the PDF file report, Create a section called Part 2c – UDP Pinger with Delays and Packet Losses. Include the followings:

(1) Describe the operation of your UDP Ping Server and explain how it simulates delays between 10ms and 20ms, with up to 10% packet losses.

(2) Explain how to run your code, i.e., command line and any applicable parameter(s)

a. Include run-time screen captures for a sequence consists of 50 pings

Refer to the Appendix section on the last page for samples and required data format.

Appendix

Part 2a: Sequence of 10 Pings with No Delay and No Packet Loss

Part 2b: Sequence of 10 Pings with 10ms-20ms Delays and No Packet Loss

Part 2c: Sequence of 50 Pings with 10ms-20ms Delays and up to 10% Packet Loss