課程名稱︰計算機網路 課程性質︰必修 課程教師︰周承復 開課學院：電資 開課系所︰資工 考試日期（年月日）︰2015/11/12 考試時限（分鐘）：120 試題 : Computer Network 2015 Mid-Term Exam Question: "Basic of Computer Networks and the Internet" Please fill into the blanks (15%) a) The amount of time required to push all of a packet's bits into a link is called ______. b) The Internet's application layer includes many protocols, such as the ______ protocol (which provides for Web document request and transfer), ______ (which provides for the transfer of e-mail messages), and ______ (which provides for the transfer of files between two end systems). c) How many IP Protocols are there in the Internet? ______ d) An application-layer process sends messages into, and receives messages from, the network through a software interface called a ______. This is also referred to as an API which stands for ______. e) Internet protocols are defined in ______. f) In order to deal with the issue of scale, the DNS uses a large number of servers, organized in a ______ fashion and distributed around the world. No single DNS server has all of the mappings for all of the hosts in the Internet. Instead, the mappings are distributed across the DNS servers. To a first approximantion, there are three classes of DNS servers-- ______ DNS servers, ______ DNS servers, and ______ DNS servers. g) In a DNS server, a resource record is a four-tuple that contains the following fields: (Name, Value, Type, TTL) The meaning of Name and Value depend on Type: * If Type = A, then Name is a ______ and Value is the ______ for the host-name. Thus, a Type A record provides the standard hostname-to-IP address mapping. As an example, (relay1.bar.foo.com,145.37.93.126,A) is a Type A record. * If Type = ______, then Name is a domain and Value is the host-name of an authoritative DNS server that knows how to obtain the IP addresses for hosts in the domain. This record is used to route DNS queries further along in the query chain. Answer each of the following questions briefly, i.e., in at most a few sentecnes a) (5%)(True or false) In a circuit switched network, all data belonging to a connection traverses the same route, whereas in a packet switched network, different packets belonging to the same connection may take different routes. b) (5%)(True or false) An IP header contains a source port and a destination port number thar are each 2 bytes long. c) (5%)(True or false) Sequence numbers are necessary to tolerate losses, but are not needed to tolerate bit errors alone. d) (5%)(True or false) Describe the use of the "If-Modified-Since" header in the HTTP protocol. e) (10%) NTU Computer Center hires you as the p2p expert to reduce its rising bandwidth costs due the rising use of BitTorrent. How would you address their problem? f) (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 embadded GIF image that resides at the same server as the orgiginal object. a. What transport application layer protocols besides http are needed in this scenario? b. Suppose that the time needed to contact and receive a reply from any server (for any protocols) is RTT, and the time to transmint 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 is used. Consider the delays of all protocols in your answer, not just thoss of http. Question 2: "DHT": Distributed Hash Table (a) (20%) Consider a circular Distributed Hash Table(DHT) with node identifiers in the range[0:15]. Suppose there are seven peers with identifiers 1, 4, 7, 9, 11, 13 and 15. (i) Suppose that the following (key,value) pairs should be stored in the DHT: (2,0), (6,5), (8,13) and (14,7). Which peers will store which (key,value) pairs? Fill in the table below. (key,value) Identifier of the responsible peer (2,0) (6,5) (8,13) (14,7) (ii) Suppose that peer 7 learns that peer 9 has left the DHT. How does peer 7 update its successor state information? Which peer is now its first successor? Its second successor? (b) (10%) Explain how DHT improves search performance from O(n) to O(log n), where n is the total number hosts in DHT method. Question: A reliable data transfer protocol (25%) Suppose we have two network entities, A and B. B has a supply of data messages that will be sent to A according to the following conventions. When A gets 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 corrupted) in the A-to-B channel; D messages, once sent, are always delivered correctly. The delay along both channels is unknown and variable. Assume no reordering occurs. Design (give an FSM description of) a protocol that incorporates the apporpriate 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. Question: Homework Review: These problems are not the same as homeworks, which means that we have modified some description. Please be careful when answering. (a) (15%) Consider two hosts, X and Y connected by a single link of rate A bps. Suppose that the two hosts are seperated by B meters, and suppose the propagation speed along the link is C meters/sec. Host X is to send a packet of size D bits to Host Y. (1) (5%) Ignoring processing and queing delays, obtain an expression for the end-to-end delaye in terms of A, B, C, and D. Remember to write the unit. (2) (5%) Suppose propagation delay is greater than transmission delay. At time t = transmission delay, where is the first bit of the packet? (3) (5%) Suppose link rate A = 8 Mbps, distance B = 120 lm, and packet size D = 30 bytes. Find the propagation speed C so that propagation delay equals transmission delay. Remember to write the unit. (b) (15%) Answer true or false for each statement, and briefly explain your answer if it's flase. (1) (5%) A user requests a Web page that consists of some text and four images. For this page, the client will send four request messages and receive four respond messages. (2) (5%) Two distinct Web pages (for example, www.mit.edu/research.html and www.mit.edu/students.html) can be sent over the same nonpersistent connection. (3) (5%) With persistent connections between browser and origin server, it is possible for a single TCP segment to carry two distinct HTTP request messages. --

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