Computer Networking Revision Dr Sandra I. Woolley

2 Recommended Text  Communication Networks  A. Leon-Garcia & I. Widjaja  McGraw-Hill  (2 nd Edition printed Aug 2003)

3 Cryptography Text  Relevant as background to the cryptography covered in the course.  Not required for revision.

4 Course Content (and location in course text)  Introduction to the course and computer networks - Chapter 1  Layered architectures - Chapter 2  LANs and medium access control: Chapter 6  Error control coding - Chapter 3 - section 8  Packet switching - Chapter 7  TCP/IP - Chapter 8  Network Quality-of-Service (QoS)  Queueing - Appendix A  Cryptography - Chapter 11 and (optionally) Simon Singh's, “The Code Book”. The 3 page handout is available on the course web page.  Network management and security- An overview of Appendix B and sections of Chapter 11 plus security case study examples (from Stallings). Chapter 11 contains more details on security protocols such as IPsec and secure sockets which are not covered and are not part of the assessed material.

5 Revision Pointers  Lecture slide material is assessable.  Working through past exam paper questions is good exam practice. Please see questions and solutions on the course web page (not hardcopy handouts).  Additional example questions, in the form of exam questions, are provided on the following slides.

6 Examples 1. (a) Briefly explain the differences between TCP and UDP and their different roles in the Internet. (b) With reference to packet header contents and router treatment of packets, summarise important differences between IPv4 and IPv6. (c) Describe how PGP public-key encryption employs RSA and IDEA algorithms and how keys can be used to ensure both privacy and authentication. (d) Using the RSA algorithm and showing your working, encrypt and decrypt the message M=8 with p=5, q=9, e=3.

7 Examples - continued 2. (a) Provide a brief definition and description for each of the following i) SNMP ii) Dijkstra's algorithm (b) Explain how the Bellman-Ford algorithm works. Explain the problems this algorithm can have when a break between network nodes occurs and describe the modifications which avoid this problem. (b) Briefly compare and contrast ALOHA and slotted ALOHA. (c) Briefly describe and contrast differentiated service and integrated service approaches to network quality of service.

8 Examples - continued 3. (a) i) With the aid of suitable diagrams, describe the OSI and TCP/IP layer models. Summarise the function of each layer. ii) Briefly comment on why the TCP/IP and not the OSI architecture was employed on the Internet. (b) i) Explain the limitations inherent in the original classful IP addressing scheme. ii) A class B network on the Internet has a subnet mask of What is the maximum number of hosts per subnet? (c) Use the generator polynomial, G(x)=x 4 +x+1, to encode the data sequence, , and show your working clearly.

9 Examples - continued 4. (a) Provide a brief definition and description for each of the following i) IGP and EGP ii) ARP iii) Little's formula (b) Summarise how traceroute works. (c) Alice and Bob agree a one-way function, Y x (mod P), for Diffie- Hellman-Merkle key exchange, with Y=5 and P=11. Alice privately selects the value A=3 and Bob privately selects the value B=4. Compute the values a and b which they exchange and use them to generate the secret key, showing that they both arrive at the same value.

10 Numerical Solutions to Examples Numerical solutions to questions: 1 d) C=17 and d=11 As shown in the handout... 17^11 (mod45) can be written as (17^8x17^2x17^1)(mod45)... remember to ignore zeroes... =(19x17)(mod45) = 8. I have added a note below about this question. 3 b) iii) bits available - 2 hosts (all 0's and all 1's addresses are reserved) 3 c) The check bits are “1110” so the encoded data is The check bits are appended to the data bits to produce the protected codeword. 4 check bits were expected because the generator polynomial has power 4. An aside (i.e., interesting but not necessary for revision)... question 1 d) is a flawed example. q is not prime (which it should be for RSA). We discuss this briefly in class. The question works fine as a simple example for us to work through BUT when our values are not prime we get non- unique solutions for d. d is the private key and it should be very difficult to extract, so multiple solutions are undesirable in practice. An example of another solution for d in this example is 43 since 3x43 (mod32) = 129 (mod32) = 1.

Thank you and good luck