课程名称︰计算机网络
课程性质︰资工系必修
课程教师︰周承复
开课学院：电资学院
开课系所︰资工系
考试日期（年月日）︰
考试时限（分钟）：180分钟
试题 :
Final Exam
Computer Networks Fall 2022
Prof. Cheng-Fu Chou
Question 1: "Quickies' (25%) Answer each of the following questions briefly, i.e., in at most a few sentences.
a) (10%) Where can queueing occur in a router? Briefly explain the conditions that lead to such queueing.
b) (5%) Two hosts simultaneously send data through a link of capacity 1 Mbps. Host A generates data with a rate of 1Mbps and uses TCP. Host B uses UDP and transmits a 100bytes packet every Ims Which host will obtain higher throughput?
c) (10% Suppose a web server has 500 ongoing TCP connections. How many server-side sockets are used? How many server-side port numbers are used? Briefly (two sentences at most each) explain your answer.
Question 2: High Speed TCP (20%)
Consider TCP over long fat pipes.
a) (10%) Please derive the TCP throughput as a function of the loss rate (L), the round-trip time (RTT)
and the maximum segment size (MSS).
b)(10%) If we want to achieve average 20 Gbps throughput by using 1000-byte segment over a 100ms RTT connection, what is the average size of W (the average congestion window size)? What is the segment loss probability that Today's TCP congestion-control algorithm could tolerate?
Question 3: Congestion Control and TCP (30%)
a) (5%) What is the difference between congestion control and flow control?
b) (5%) It is said that a TCP connection "probes" the network path it uses for available bandwidth. What is meant by that?
c) Suppose that in TCP, the sender window is of size N, the base of the window is at sequence number x, and that the sender has just sent a complete window's worth of segments. Let RTT be the sender-to-receiver-to-sender round trip time, and let MSS be the segment size.
(i.)(10%) Is it possible that there are ACK segments in the receiver-to-sender channel for segments with sequence numbers lower than x? Justify your answer.
(ii.)(5%) Assuming no loss, what is the throughput (in packets/sec) of the sender-to-receiver connection?
(iii.)(5%) Suppose TCP is in its congestion avoidance phase. Assuming no loss, what will the window size be after the N segments are ACKed?
Question 4: A Distributed Transaction Processing System (25%)
Consider a distributed transaction processing system of a client and a remote server. The client receives transaction requests from local users. These transaction requests must be communicated to the server, which will execute the transaction request and return the result of the transaction request. You can think of a transaction as requesting or updating an account balance from the server database, and the response containing the resulting balance. The client and server communicate over a medium that
can lose and delay messages; the maximum delay in the medium is not known. The medium will not corrupt or reorder messages.
The client should receive requests from local users (via the event callbyuser(request) ) and return results to users (via the event returndatatouser(data) ) in the order in which the requests were generated. The server receives messages from the client via the messagefromclient (clientmsg) event,
executes a transaction via a call: result = execute(clientmsg) and sends messages to the client via the messagetoclient(servermsg) event, where clientmsg and servermsg are messages (that you define) sent from the client and server, respectively) The client receives messages from the server via the
messagefromserver(servermsg) event.
Metagotoserver (chienewsg)
Give a FSM description of the client and server. Describe the format of the messages sent from client-to-server and from server-to-client. Your protocol should be minimalist in the sense that it should not contain any functionality that is not strictly needed to meet the above requirements.
Question 5: TCP congestion control (homework review) (10%)
Assuming TCP Reno is the protocol experiencing the behavior shown below, answer the following questions.
a) Identify (1) the intervals of time when TCP slow start is operating. (2%) (2) the intervals of time when TCP congestion avoidance is operating. (2%)
b) What is the value of ssthresh at the 17th transmission round? (3%)
c) Suppose TCP Tahoe is used (instead of TCP Reno), and assume that triple duplicate ACKs are received at the 10th round. What is the value of ssthresh and how many packets sent out at the 16th round? (3%)
https://i.imgur.com/kFyic0m.jpg