課程名稱︰計算機網路 課程性質︰資訊系大三必修 課程教師︰周承復 開課學院：電資學院 開課系所︰資訊系 考試日期（年月日）︰102/10 考試時限（分鐘）：120 是否需發放獎勵金：是 （如未明確表示，則不予發放） 試題 : Question1:"Quickies"(15%) Answer each of the following question briefly, i.e., in at most a few sentence a)(5%)Please explain what is head-of-the-line blocking in a router. b)(5%)TCP's current EstimatedRTT is 100ms. Upon receipt of next sample, the EstimatedRTT is 5ms. Is this possible?If so,when.If not, why not? c)(5%)What is meant by network-assited congestion control? Give an example of a congestion control protocol that take this approach. Does TCP take this approach? Explain your answer. Question2: A reliable data transfer protocol(25%) Suppose we have two network entities, A and B. B has a suply of data messages that will be sent to A according to the following conventions. When A gets a request from the layer above to get the next data(D) message from B, A must send a request(R) message to B on the A-to-B channel. Only when B receives an R message can it send a data(D) message back to A on the B-to-A channel. A should deliver exactly one copy of each D message to the layer above. R messages can be lost (but not corrupt) in the A-to-B channel; D message, once sent, are always delivered correctly. The delay along both channel is unknown and variable. Assume no reordering occurs. Design (give an FSM description of) a protocol that incorporate the appropriate mechanisms to compensate for the loss-prone A-to-B channel and implements message passing to the layer above at entity A, as discussed above. Use only those mechanisms that are absolutely necessary. Use a pencil or a rough sheet before finalizing your answer. Qusetion3:Routing Algorithms(30%) Consider the network in the figure below. The numbers on links between the nodes represent the costs corresponding to these links.Assume that nodes initially know only the costs to their neighbors. D--(5)--E--(3)--A | | | (2) (9) | | | | C--(1)--B---(7)-- a)(10%)Using the distance-vector algorithm, show the distance tables at node E Assume that the algorithm works in a synchronous manner, where all nodes simultaneously receive distance vectors from their neighbors, compute their new distance vectors, and inform their neighbors if their distance vectors have changed. b)(5%)Create a routing loop between the nodes B and C by changing the cost of the link between the nodes C and D. What is the minimum change in link cost that creates the routing loop? What is this problem alternatively called? c)(5%)How does the distance-vector-like protocol solve this problem? If the distance-vector-like protocol was used for routing in the above network , what is the finite number that would play the rule of (無限大)? d)(5%)If the link-state-like protocol was used in the above network, how would it handle the routing loop? How do nodes learn the link costs in the link- state-like protocol? e)(5%)Is there any other method to solve this problem? Question4:Transport Layer(20%) Consider the following plot of TCP window size as a function of time for two TCP connection A and B. In this problem we will suppose that both TCP senders are sending large files. We also assume that the packet loss events are independent in connection A and B. a)(5%)Considering the above value of congestion window(CongWin) for these connections, can we identify the type of TCP connections(Reno or Tahoe)that have been used by connection A and B? Justify your answers. b)(5%)What are the values of the Threshold parameter between the 1st and the 14th transmission rounds for each connection? c)(5%)At the 12th transmission round for connection A, is segment loss detected by a triple duplicate ACK or by timeout? Justfy your answer. d)(5%)Assume that the segment size is 1460 bytes and that a total of 87600 bytes have been successfully transmitted over connection A before the 13th transmission round. At which transmission round the cumulative amount of successful transmitted data is equal to 163520 bytes? Again we assume that there is neither timeout nor dupACK after the 13th transmission round. (附圖為折線圖) Question5:(20%) Compare GBN, SR, and TCP(no delayed ack).Assume that the timeout values for all 3 protocols are sufficiently long such that 5 consecutive data segment and their corresponding ACKs can be received (if not lost in the channel) by the receiveing host(Host B) and the 2nd segment(sent from A)is lost.In the end , all 5 data segment have been correctly received by host B. a)(15%)How many segments has lost A sent in total and how many acks has host B sent in total? What are their sequence numbers?Answer this question for all 3 protocols. b)(5%)If the timeout values for all three protocols are much longer than 5 RTT ,then which protocol successfully delivers all 5 data segments in shortest time interval? Question6:(10%) Consider the network shown below. Suppose AS3 and AS2 are running OSPF for their intra-AS routing protocol. Suppose AS1 and AS4 are running RIP for their intra-AS routing protocol. Suppose eBGP and iBGP are used for the inter-AS routing protocol. Initially suppose there is no physical link between AS2 &AS4 (a) Router 3a learns about prefix X from which routing protocol? (b) Router 3c learns about X from which routing protocol? (c) Router 1d learns about X from which routing protocol? (d) Router 1c learns about X form which routing protocol? (圖參考作業) --

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