Computer Networking
Revision
Dr Sandra I. Woolley
Recommended Text
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Communication Networks
 A. Leon-Garcia & I. Widjaja
 McGraw-Hill
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(2nd Edition printed Aug
2003)
2
Cryptography Text
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Relevant as background to the
cryptography covered in the
course.
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Not required for revision.
3
Course Content (and location in course text)
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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.
4
Revision Pointers
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Lecture slide material is assessable.
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Work through examples.
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Additional example questions, in the form of exam questions, are
provided on the following slides.
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Remember to read the question carefully and then check again
when you finish that you have responded to all parts.
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You will need a scientific calculator.
5
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.
6
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.
7
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
255.255.240.0. What is the maximum number of hosts per subnet?
(c) Use the generator polynomial, G(x)=x4+x+1, to encode the data
sequence, 1101011011, and show your working clearly.
8
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, Yx (mod P), for DiffieHellman-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.
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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) 4094 12bits 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 11010110111110
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 nonunique 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.
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Thank you and good luck