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03_final_2 - gs 3?“ The wireless link often introduces...

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Unformatted text preview: gs 3;?“ The wireless link often introduces bit errors into the transmitted frames (m uch more so than a wireline link). Assume that the wireless linlc neither detects nor corrects hit errors. The wireless lint: and the wired linlr: between the router and the server have enough bandwidth so that there is only occasional congestion. Infect, it is more likely that a client packet will sufir from bit errors than be dropped at the router—to—server link due to congestion. 0 What mechanism does TCP use to detect bit errors? (4 points) TCP uses the Internet checksum to detect bit errors It The goal of the transport—layer error and congestion control in this scenario should he to (a) retransm it a packet if it is dropped at the receiver due to bit errors, provided that the sender—to—receiver path is not congested, and (b) reduce the sending rate if the packet is lost due to bufi’er overflow at a congested link, using an algorithm such as TCP’s slowstartfcongestion—avoidance (and increase the rate when the path is not congested}. How well does TCP meet these goals? Explain your answer. 6 points total. Not very well. {i— 2 points) The Internet checksum is a very weak form ofbit— error protection. It would be better to use a more powerful code such as CEO (cyclic redundancy check). (ii — 4 points) The problem with using TCP‘s congestion control in this setting is that TCP will decrease its sending rate whenever a packet is not acked. If there is a bit error then TCP will decrease its sending rate (due to lack of an ACE) even if the network is not congested. o What modifications would you make to YUP to achieve goals (a) and {b}? You can add any new messages or mechanisms that you want to TOP. You can describe your additions in English (you do not need to draw a FSM), but make sure there is enough detail to provide a clear statement of what you are proposing. 8 points. I would add a mechanism to TCP to allow it to explicitly indicate when a packet is received but in error, as opposed to lost. The easiestway to do this is to add a NARC!) that indicates that packet x was received, but had bit errors. A TOP sender receiving a NAKCX) would not have to decrease its sending rate, since the loss was not due to congestion. Itwould also be OK to add two kinds of NAK messages: NAK_bit_error(x) and NAK_lost_p acket(x}, where the latter is sent by the receiver as soon as a gap in the received sequence number space is detected. Question 3: Multiple access protocol: (18 points, 20 minutes) ...
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