15110 Fall 2011 [Cortina/von Ronne]

Written Homework 10 - due Friday, November 18 in class

Reading Assignment

Read chapter 5 and Appendix A of the book Blown To Bits.



  1. (1 pt) Explain the main differences between circuit switching and packet switching. Which is used on the Internet? Why?

  2. (1 pt) Using the original IPv4 addressing scheme, a computer gets the IP address .

    1. What type of address is this: class A, class B, or class C? Why?

    2. Based on your answer from part (a), which numbers of the IP address will all computers in this organization share? Explain.

  3. (2 pts) The Internet is based on a number of different communication protocols.

    1. What is a communication protocol? What aspects of communication does it regulate?

    2. Which aspect of communication is regulated by TCP's requiement that...
      1. ... a source port, a destination port, a sequence number, and the actual data occur at specific locations within a TCP packet.
      2. ... the sequence number indicates the order in which the packets should be re-assembled.
      3. ... the receiver should send an acknowledgment after it receives each packet.

    3. Real Time Protocol (which streams voice/video data) is normally layered above UDP rather than TCP. What characteristics of TCP make it less desirable than UDP for streaming voice/video traffic?

    4. Suppose the Jet Propulsion laboratory develops a new protocol describing how frames containing data can be transmitted over radio waves directly between a satellite orbiting earth and one orbiting mars. To which of the layers in the four-layer internet model would this protocol belong? Briefly explain your reasoning.

  4. (2 pts) While discussing RSA encryption, we looked at some simpler encryption algorithms that lasted for many years. For each of these problems, show your work for maximum credit.

    1. Encode the message PACKETSWITCHING using a Caesar cipher with a shift of 8.

    2. Encode the message INTERNETPROTOCOL using a Vigenere cipher (as discussed in class) with a key of EMAIL.

    3. Decode the message RNXXNXXNUUNWNAJW that was encoded using a Caesar cipher. What is the shift that was used?

    4. Decode the message CIABIRWIQCEEO that was encoded using a Vigenere cipher. HINT: The key is in this sentence.

  5. (2 pts) Consider a public key encryption system using RSA encryption that starts with two prime numbers p = 103 and q = 337.

    1. Compute the public key pair (e, n) and the private key pair (d, n) for this system. Select the smallest value for e that will work, and then select the smallest value for d that will work given your value for e. Show your work.

    2. Consider the numerical message 15110 that is to be transmitted. What is the encrypted message that should be transmitted using this system? Show your work.

    3. Verify that the receiver can decode the message from part (b) using the private key pair. Show your work.

    4. Why is RSA considered secure? Explain briefly.

    You may use irb to help you with the large computations for problem 5.

  6. (1 pt) Based on your reading in Blown To Bits, Appendix A, answer the following questions:

    1. Suppose an ISP company starts a service to sell movie downloads. As part of this new venture, the company examines packets being sent to its users and slows down a user's connection if they detect packets from a competing movie provider. Does this violate the principle of net neutrality? Why or why not?

    2. If you have ten computers at home all connected to the Internet via your ISP, how many unique IP addresses do you get? Briefly explain how traffic is routed to each computer.

  7. (1 pt) Based on your reading in Blown To Bits, Chapter 5, you learned that today's encryption methods allow anyone to encrypt email and other data securely before being sent. The U.S. Government was very concerned about this type of technology since terrorists could use it to communicate without revealing their messages. What did the U.S. Government try to do in the 1990s to control this situation, and why did they give up trying to regulate encryption technology in the 2000s, even after the attacks of 9-11?