Question 3 [10 Marks]. Some people think TCP with congestion control can ensure the fairness among TCP connections, while other people believe TCP cannot ensure the fairness among end nodes. Which one do you support and why?
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Congestion control is a mechanism within TCP that helps manage the amount of data entering the network to prevent congestion and ensure efficient data transmission. Show more…
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QUESTION 3 (14 marks) Answer the following questions concerning network utilisation and congestion. a) Explain the difference between flow control and congestion control, with reference to that part of an end-to-end telecommunication system that is running out of resources in each case. [2 marks] b) A "stop and wait" network transport protocol is used to transport 500 byte packets (including headers) at 100 Mbit/s over a network link that is 100 km long. Assuming the propagation speed is 0.7c, calculate the link utilisation assuming zero errors. (The speed of light, c, is 3 x 10⁸ m/s.) [3 marks] c) Instead of the "stop and wait" protocol in b) above, consider use of a sliding window protocol using the same packet size, transmission rate, network bit rate and distance between sender and receiver. What is the minimum length (in bits) of the sequence number that would be necessary to support this protocol? Explain the reasoning behind your answer. [4 marks] d) Despite what you have calculated in part c), suppose the sequence number that is actually used is 4 bits long. What is the maximum link utilisation in the case of zero errors? Explain how you obtained your answer. [3 marks] e) Explain any difference in the delay sensitivity between the following two audiovisual streaming applications: i. A video-on-demand service such as Netflix ii. A video calling application such as Facetime. By directly relating your explanation to their delay requirements, explain the suitability of each service to use UDP or TCP Transport Protocols. [2 marks]
Adi S.
1. The size of the TCP rwnd only reduces as more and more segments are received by the receiver from the sender. 2. Support Host A is sending Host B a large file over a TCP connection. The number of unacknowledged bytes that A sends cannot exceed the size of the receive buffer. 3. Suppose Host A is sending Host B a large file over a TCP connection. If the sequence number for a segment of this connection is m, then the sequence number for the subsequent segment will be m+n, where n is the size (in bytes) of the mth segment. 4. Suppose Host A is sending Host B a large file over a TCP connection. If the sequence number for a segment of this connection is m, then the sequence number in the acknowledgement from B to A would be m+1. 5. Suppose Host A sends one segment with sequence number 38 and 4 bytes of data over a TCP connection to Host B. In this same segment, the acknowledgement number is necessarily 42. 6. One of the problems with the Go-back-N algorithm is that a single packet error can cause it to retransmit a larger number of packets, many unnecessarily. 7. Both Go-back-N and Selective Repeat can be implemented using a single timer at the sender side. 8. By default, TCP uses cumulative acknowledgements. 9. With TCP, if sampleRTT values have a lot of variation, TimeoutInterval will be smaller. 10. With TCP, the timer at any time can be thought of as being associated with the oldest acknowledged segment. 11. A congested network results in packets being dropped at routers due to buffer overflow. 12. In the slow start phase of TCP congestion control, the congestion window is doubled after every acknowledgement. 13. In the congestion avoidance phase of TCP congestion control, the congestion window is increased by 1 MSS every RTT. 14. It is possible for a sender to receive an ACK for a packet that falls outside of its current window in the Go-back-N protocol. 15. It is possible for a sender to receive an ACK for a packet that falls outside of its current window in the Selective Repeat protocol.
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