(a) (i) Define the terms capacity and latency as applied to communication channels; explaining whether there is a strict relationship between the capacity of a channel and its latency. [3 marks] (ii) Using a clear example explain how the latency of a channel can have a direct effect on the capacity of a higher-layer channel which uses it. [8 marks] (iii) Describe how the capacity of the higher-layer channel may be improved, without any change to the characteristics of the underlying channel. [3 marks] (iv) Describe in what circumstances such changes would provide only limited benefit. [2 marks] (b) The figure below illustrates, for a single TCP connection, changes in the advertised congestion window (CWND). (i) Indicate which phase of congestion control the TCP connection is in at G. (ii) Indicate which phase of congestion control the TCP connection is in at E. (iii) Describe the event that has occurred at D. (iv) Describe the event that has occurred at F. [1 mark each]

(a) The diagram below shows a TCP connection between Hosts HA and HB passing through networks with different values of Maximum Transmission Unit (MTU) shown. Version 4 of the Internet Protocol (IPv4) is in use. Router 1 Router 2 HA chooses a TCP segment size of 3000 bytes of data (TCP and IP headers are each 20 bytes in size). (i) Describe the way in which fragmentation takes place as HA sends data to HB. Include the arithmetic used to calculate fragment sizes. Explain the saving that may be made by HA choosing an optimal TCP segment size when sending 60KBytes of data. [8 marks] (ii) Briefly explain how the situation described in part (i) would be handled if Internet Protocol version 6 (IPv6) were used. [2 marks] (b) The formulae below are used in TCP implementations to compute a value for the retransmission time-out (R). R is an estimate of the round-trip time, M is the most recently measured round-trip measurement, = 0.875 and h = 0.25. D D + h(|M R| D) R R + (a )M R = R + 4D (i) How is M measured? [2 marks] (ii) Explain the principles behind the design of these formulae and the values h, and D. [8 marks]