课程名称︰计算机网路 课程性质︰大三必修 课程教师︰周承复 开课学院：电机资讯学院 开课系所︰资讯工程系 考试日期（年月日）︰2018.11.06 考试时限（分钟）：09:30~11:35 试题 : Question 1: Quickies (45%) Answer each of the following questions briefly, i.e., in at most a few sentences. a. (10%) What does it mean for a protocol to be stateful? What does it mean for a protocol to be stateless? Give an example of one stateful protocol and one stateless protocol. b. (10%) List four factors that contributes to the end-to-end dela in a packet- switched network. Which of these are constand and which of these depned on the load in the network? c. (10%) Suppose a web server has 500 ongoing TCP connecyions. How many server- side sockets are used? How many server-side ports are used? (Hint: remember the server implements fork() as introduced in the lecture) d. ( 5%) What is the purpos/use of UDP checksum? e. (10%) Suppose you would like to urgently deliver 50 terabtytes data from Boston to Los Angeles. You have a available 1Gbps dedicated link for data transfer. Would you prefer to transmit the data via this lin or instead use Fedex overnight delivery? Explain. Question 2: DNS service (15%) Suppose you open a startup company "sky" and want to set up your company network. Your network has the following servers: 1. DNS server: "dns1.sky.com" with IP "140.112.12.40" 2. Web server: "sky.com" with IPs "140.112.12.55" and "140.112.12.56". The web server also has a name as "www.sky.com" 3. Email server: "galaxy.sky.com" with IP "140.112.12.60" Your company's email address is "username @sky.com" a. ( 5%) What resource records (RRs) do you need to provide to the upper-level ".com" Registrar? b. (10%) What RRs do you need to put in your company's DNS server? Question 3: Web (20%) Consider an HTTP client that wants to retrieve a WWW document at a given URL. The IP address of the HTTP server is initially unknown. The WWW object at the URL has one embedded GIF image that resides at the same server as the original object. a. (10%) What transport and application layer protocols beside HTTP are needed in this scenario? b. (10%) Suppose that the time needed to contect and recieve a reply from any server (for any protocol) is RTT, and the time to trasmit the WWW object and GIF image is T. How much time (in RTT and T) is needed from when the user first enters the URL until the complete document is displayed? Assume that non- persistent HTTP us used. Consider delay of all protocols in your answer, not just those of HTTP. Question 4: Socket Programming ( 6%) Please fill in the correspinding system call (socket(), connect(), bind() ,accept(), send(), recv(), close()). https://i.imgur.com/l9vdp3o.png Question 5: Homework Review These problems are not the same as homework, which means that we have modified some description. Please be careful when answering. (12%) Suppose users share a 6Mbps link. Also suppose each user requires 100Kbps when transmitting, but each user transmits only 20% of the time. a. ( 3%) When circuit switching is used, how many users can be supported? b. ( 3%) For the remainder of this problem, suppose packet switching is used. Find the probability that a given user is transmitting. c. ( 3%) Suppose there are 120 users. Find the probability that at any given time, exactly n users are transmitting simultaneously. (Hint: Use the binomial distribution) d. ( 3%) Find the probability that there are 21 or more users transmitting simultaneously. (12%) True of False e. ( 3%) A user requests a Web page that consists of some text and three images . For this page, the client will send four request message and recieve four response messages. f. ( 3%) With non-persistent connection between browser and origin server, it is possible for a single TCP segment to carry to distinct HTTP request messages . g. ( 3%) HTTP response messages can have an empty message body. h. ( 3%) The date: header in the HTTP response message indicates when the object in the response was last modified. --

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