Computer Networking Revision

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Computer NetworkingRevision

• Dr Sandra I. Woolley

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Recommended Text

• Communication Networks
• A. Leon-Garcia I. Widjaja
• McGraw-Hill
• (2nd Edition printed Aug 2003)

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Cryptography Text

• Relevant as background to the cryptography
  covered in the course.
• Not required for revision.

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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.

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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.

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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 M8 with
  p5, q9, e3.

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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.

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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)x4x1,
  to encode the data sequence, 1101011011, and show
  your working clearly.

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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 Diffie-Hellman-Merkle key exchange,
  with Y5 and P11. Alice privately selects the
  value A3 and Bob privately selects the value
  B4. 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) C17
• and d11
• As shown in the handout ... 1711 (mod45) can be
  written as (178x172x171)(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
  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.

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Thank you and good luck