Review_for_Chapter_2,川大计网部分习题答案第二章

Chapter 2 Review QeustionsR1,R3,R4,R5,R10,R11,R13,R15,R16,R20,R21,R22,R27,R28R1. List five nonproprietary Internet applications and the application-layer protocols that they use.Answer:The Web: HTTP; file transfer: FTP; remote login: Telnet; Network News: NNTP;e-mail: SMTP.R3. What is the difference between network architecture and application architecture?Answer:Network architecture refers to the organization of the communication process into layers (e.g., the five-layer Internet architecture). Application architecture, on the other hand, is designed by an application developer and dictates the broad structure of the application (e.g., client-server or P2P)•R4What information is used by a process running on one host to identify a process running on another host?Answer:The IP address of the destination host and the port number of the destination socket.•R5For a P2P file-sharing application, do you agree with the statement, “There is no notion of client and server sides of a communication session”? Why or why not?Answer:No. As stated in the text, all communication sessions have a client side and a server side. In a P2P file-sharing application, the peer that is receiving a file is typically the client and the peer that is sending the file is typically the server.•R10Recall that TCP can be enhanced with SSL to provide process-to-process security services, including encryption. Does SSL operate at the transport layer or the application layer? If the application developer wants TCP to be enhanced with SSL, what does the developer have to do ?Answer:SSL operates at the application layer. The SSL socket takes unencrypted data from the application layer, encrypts it and then passes it to the TCP socket. If the application developer wants TCP to be enhanced with SSL, she has to include the SSL code in the application.•R11What is meant by a handshaking protocol?Answer:A protocol uses handshaking if the two communicating entities first exchange control packets before sending data to each other. SMTP uses handshaking at the application layer whereas HTTP does not.•R13Telnet into a Web server and send a multiline request message. Include in the request message the If-modified-since: header line to force a response message with the 304 Not Modified status code.Answer:Issued the following command (in Windows command prompt) followed by the HTTP GET message to the “”web server:> telnet 80Since the index.html page in this web server was not modified since Fri, 18 May 2007 09:23:34 GMT, the following output was displayed when the above commands were issued on Sat, 19 May 2007. Note that the first 4 lines are the GET message and header lines input by the user and the next 4 lines (starting from HTTP/1.1 304 Not Modified) is the response from the web server.•R15Suppose Alice, with a Web-based e-mail account (such as Hotmail or gmail), sends a message to Bob, who accesses his mail from his mail server using POP3. Discuss how the message gets from Alice’s host to Bob’s host. Be sure to list the series of application-layer protocols that are used to move the message between the two hosts.Answer:Message is sent from Alice’s host to her mail server over HTTP. Alice’s mail server then sends the message to Bob’s mail server over SMTP. Bob then transfers the message from his mail server to his host over POP3.•R16Print out the header of an e-mail message you have recently received. How many Received: header lines are there? Analyze each of the header lines in the message.Answer: skipped•R20Consider a new peer Alice who joins BitTorrent without possessing any chunks. Without any chunks, she cannot become a top-four uploader for any of the other peers, since she has nothing to upload. How then will Alice get her first chunk?Answer:Alice will get her first chunk as a result of she being selected by one of her neighbors as a result of an “optimistic unchoke,”for sending out chunks to her.•R21In BitTorrent, suppose Alice provides chunks to Bob throughout a 30-second interval. Will Bob necessarily return the favor and provide chunks to Alice in this same interval? Why or why not?Answer:It is not necessary that Bob will also provide chunks to Alice. Alice has to be in the top 4 neighbors of Bob for Bob to send out chunks to her; this might not occur even if Alice is provides chunks to Bob throughout a 30-second interval.•R22What is an overlay network? Does it include routers? What are the edges in the overlay network? How is the query-flooding overlay network created and maintained?Answer:The overlay network in a P2P file sharing systemconsists of the nodes participating in the file sharingsystem and the logical links between the nodes. There is a logical link (an “edge”in graph theory terms) from node A to node B if there is a semi-permanent TCPconnection between A and B. An overlay network does not include routers. With Gnutella, when a node wants to join the Gnutella network, it first discovers the IPaddress of one or more nodes already in the network. It then sends join messages to these nodes. When the node receives confirmations, it becomes a member of the of Gnutella network. Nodes maintain their logical links with periodic refresh messages.•R27For the client-server application over TCP described in section 2.7, why must the server program be executed before the client program? For the client-server application over UDP described in Section 2.8, why may the client program be executed before the server program?Answer:For the TCP application, as soon as the client is executed, it attempts to initiate a TCP connection with the server. If the TCP server is not running, then the client will fail to make a connection. For the UDP application, the client does not initiate connections (or attempt to communicate with the UDP server) immediately upon execution•R28The UDP server described in Section 2.8 needed only one socket, where the TCP server described in Section 2.7 needed two sockets. Why? If the TCP server were to support n simultaneous connections, each from a different client host, how many sockets would the TCP server need?Answer:With the UDP server, there is no welcoming socket, and all data from different clients enters the server through this one socket. With the TCP server, there is awelcoming socket, and each time a client initiates aconnection to the server, a new socket is created. Thus, to support n simultaneous connections, the serverwould need n+1 sockets.ProblemsP1,P4,P5,P7,P8,P16,P24,P25•P1True or false?a. A user requests a Web page that consists of some textand three images. For this page, the client will sendone request message and receive four responsemessages.b.Two distinct Web pages (for example,/research.html and/student.html)can be sent over the same persistent connection.c.With nonpersistent connection between browser andorigin server, it is possible for a single TCP segment to carry two distinct HTTP request messages.d.The Date: header in the HTTP response messageindicates when the object in the response was lastmodified.Answer:a) Fb) Tc) Fd) F•P4Obtain the HTTP/1.1 specification (RFC2616). Answer the following questions: a.Explain the mechanism used forsignaling between the client and serverto indicate that a persistent connection is being closed. Can the client, theserver ,or both signal the close of aconnection?b.What encryption services are providedby HTTP?Answer:a) Persistent connections are discussed in section 8 ofRFC 2616 (the real goal of this question was to getyou to retrieve and read an RFC). Sections 8.1.2 and8.1.2.1 of the RFC indicate that either the client or theserver can indicate to the other that it is going to close the persistent connection. It does so by including theincluding the connection-token "close" in theConnection-header field of the http request/reply.b) HTTP does not provide any encryption services.•P5Read RFC 959 for FTP. List all of theclient commands that are supported by the RFC.Answer:Access control commands:USER, PASS, ACT, CWD, CDUP, SMNT, REIN, QUIT.Transfer parameter commands:PORT, PASV, TYPE STRU, MODE.Service commands:RETR, STOR, STOU, APPE, ALLO, REST, RNFR, RNTO, ABOR, DELE,RMD, MRD, PWD, LIST, NLST, SITE, SYST, STAT, HELP, NOOP.•P7Suppose within your Web browser you click on a link to obtain a Web page. The IP address for the associated URL is not cached in your local host, so a DNS lookup is necessary to obtain the IP address. Suppose that nDNS servers are visited before your host receives theIP address from DNS; the successive visits incur andRTTof RTT1,…RTTn. Further suppose that the Webpage associated with the link contains exactly one object, consisting of a small amount of HTML text. LetRTT0denote the RTT between the local host and theserver containing the object. Assuming zero transmission time of the object, how much time elapses from when the client clicks on the link until the client receives the object?Answer:The total amount of time to get the IPaddress iselapses to set up the Once the IP address is known, RTTTCP connection and another RTTelapses to requestand receive the small object. The total response time is•P8Referring to Problem P7,suppose theHTML file references three very smallobjects on the same server. Neglecting transmission times, how much timeelapses witha.Non-persistent HTTP with no parallelTCP connections?b.Non-persistent HTTP with parallelconnections?c.Persistent HTTP?Answer: a)b)c)•P16Consider distributing a file of F=5 Gbits to N peers. The server has an upload rateof us =20Mbps, and each peer has adownload rate of di =1Mbps and anupload rate of u. For N=10,100, and1,000 and u=100Kbps, 250Kbps,and 500Kbps,prepare a chart giving the minimum distribution time for each of the combinations of N and u for both client-server distribution and P2P distribution.Answer:For calculating the minimum distribution time for client-server distribution, we use the following formula:Similarly, for calculating the minimum distribution time for P2P distribution, we use the following formula:•P24Suppose that in UDPClient.java we replace the LineDatagramSocket clientSocket= new DatagramSocket();withDatagramSocket clientSocket= new DatagramSocket(5432);Will it become necessary to changeUDPServer.java? What are the port numbers for the sockets in UDPClient and UDPServer?What were they before making this change?Answer:•P25Install and compile the Java ProgramsTCPClient and UDPCLient on one hostand TCPServer and UDPServer onanother host.a.Suppose you run TCPClient before yourun TCPServer. What happens? Why?b.Suppose you run UDPCLient before yourun UDPServer. What happens? Why?c.What happens if you use different portnumbers for the client and server sides?Answer:a)If you run TCPClient first, then the client will attempt to make a TCP connection with a non-existent server process.A TCP connection will not be made.b) UDPClient doesn't establish a TCP connection with the server. Thus, everythingshould work fine if you first run UDPClient, then run UDPServer, and then type some input into the keyboard.c) If you use different port numbers, then the client will attempt to establish a TCP connection with the wrong process or a non-existent process. Errors will occur.。