The lab consists of a number of tasks to be solved and questions to be answered. The lab should be solved individually, but collaboration is allowed as long as proper credit is given in the report.

Some questions lack all the information needed in order to answer them. In those cases, you need to consider how to make a suitable assumption in order to solve the problem. Any assumptions you make need to be carefully presented in your report so that it is possible to understand how you argue. You need to convince the teacher that you understand the questions and the solutions/answers!

Commands

There are two programs that are needed to complete the assignment. They are listed in the table below, together with information on how to run the programs in different operating system environments. In Windows, you first have to start the command line shell (usually under programs -> accessories). If you are not already familiar with these programs, you are strongly adviced to study the manual pages (UNIX, Linux, MacOS X) or the help files (Windows).

Program name	UNIX-lab	Linux	MacOS X	Windows 2000/XP
traceroute	/usr/sbin/traceroute	traceroute	traceroute	tracert (1)
ifconfig	/usr/sbin/ifconfig	ifconfig	ifconfig	ipconfig

(1) The tracert program in Windows can not be run from within large parts of the student network.

Tasks

1. Figure out which way messages take when sent over the Internet between a computer in Uppsala and a computer in Luleå (e.g., www.ltu.se).
   • If you would run a cable from Uppsala to Luleå, what is the shortest possible time it can take to get a response from a computer in Luleå?
   • What is the measured time over the Internet?
   • What is the reason for the difference?
2. Log in to one of the IT-institutions UNIX-servers.
   • What are the IP-addresses and MTUs of the network interfaces on the machine you have logged on to?
   • One of the network interfaces have a much higher MTU than the others. How is this possible?
3. The students Pelle and Lisa each have Internet connections in their student appartments. Pelle has an 8 Mbit/s connetion, while Lisa only has a 2 Mbit/s connection. Pelle wants to send a 50 MB large file to Lisa.
   • How long does the file transfer take if the file is sent as one single large message? A switch between Pelle and Lisa uses a store-and-forward strategy.
   • How long does the file transfer take if the file is split up in a number of 1500 Byte messages, which each have 60 Bytes reserved for protocol headers?
4. You acquire a mail certificate from Thawte, registered on your student email address of the form foba1234@student.uu.se. When you later send a signed email to your friend from your student mail account it appears that the signature does not work. Why? How do you fix it?
5. Lisa and Pelle decides to exchange top-secret love letters via email, which they encrypt using a polyalphabetic cipher. You happen to get your hands on some ciphertext copies of the letters in the form of separate text files. Since you are extremely curious about what the letters contain, you decide to try and recreate the plaintext in the letters. Which attack method is best suited for this task, and how do you perform the actual attack? Note!: In reality, it is not acceptable to try and recreate the plaintext of an email just because it uses an insufficient protection mechanism.
6. www.aftonbladet.se is (unfortunately) one of the most popular webpages in Sweden. Classify the content on the webpage into different categories, e.g., real/actual content, relevant pictures, animations, advertisements, etc.
   • What do you pay for the advertisements and other unwanted information if you read the page using a 3G phone that charges you per MB? (You do not have a advertising filter in your webbrowser, or similar).
   • Propose a technique that might reduce this cost, without actually removing any advertisements (it is, by all means, necessary to finance the webpage!).

Hand-in instructions

Produce a written report with the answers to the questions, along with how you arrived at your solutions. Hand in the report with a cover page (as instructed on the lab page), in post box 73, floor 4, house 1, before the published deadline. It is not possible to hand in via email!

Marking

For maximum points on a task, it is required that you do not only have the correct answer, but also that you describe how you arrived at it. Smaller errors are only noted, two noted errors give 1% reduction. Large errors can give 1% reduction immediately. Even if you have arrived at the correct answer to a question, you can be given point reductions in case your solution is obscure or overly lengthy.