计算机网络期中考试试题_2013_答案.doc

中山大学软件学院级软件工程专业《》期试题(考试形式：闭 卷 考试时间: 2小时) 《中山大学授予学士学位工作细则》第六条 考试作弊不授予学士学位 What’s the difference between HTTP and FTP? HTTP是带内连接，而FTP是带外连接；（3 points） HTTP是无状态的，而FTP是有状态的（在会话期间需要ID密码）。(3 points) 端口号、协议命令、返回码(一点一分，最多两分) (8 points) What are the changes (or advantages) of IPv6, compare to IPv4? /*(a).header format helps speed processing/forwarding (3 points) (b).header changes to facilitate QoS (3 points)*/ 1.扩大地址容量（128bits）2.简单高效的首部（各种其它解释） 3.不允许分片重组；4.取消校验和。 答1点3分，2点5分，3点7分，4点8分。 4．(8 points) How can UDP detect bit error for a received segment? Please describe such mechanism. /*使用checksum（1分） sender: checksum is addition (1’s complement sum) of segment contents. puts checksum value into UDP checksum field（3分） receiver:compute checksum of received segment. check if computed checksum equals checksum field value（3分） note:when adding numbers, a carryout from the most significant bit needs to be added to the result（1分）*/ 检查和（2分） 发送方：求和（溢出要回卷），补1运算得到checksum(3分) 接收方：所有字段求和（包括checksum），不全为1则有bit-error(3分) (10 points) Consider an e-commerce site that wants to keep a purchase record for each of its customers. Describe how this can be done with cookies. a.When the user first visits the site, the site returns a cookie number. This cookie number is stored on the user’s host and is managed by the browser. b. During each subsequent visit (and purchase), the browser sends the cookie number back to the site. Thus the site knows when this user (more precisely, this browser) is visiting the site. (6 points) In our rdt protocols, why did we need to introduce timers? To handle losses in the channel.(3 points) if not received within the duration of the timer for the packet, the packet (or its ACK or NACK) is assumed to have been lost.(3 points) (9 points) Install and compile the Java programs TCPClient and UDPClient on one host and TCPServer and UDPServer on another host. Suppose you run TCPClient before you run TCPServer. What happens? Why? Suppose you run UDPClient before y