whut_计算机网络试卷答案.docVIP

一、 （1）The application developer can put reliable data transfer into the application layer protocol. This would require a significant amount of work and debugging, however. 应用程序开发者可以将可靠数据传输放到应用层协议中完成。但是这需要相当大的工作量和进行调试。 （2）反过来 An application developer may not want its application to use TCP’s congestion control, which can throttle the application’s sending rate at times of congestion. Often, designers of IP telephony and IP videoconference applications choose to run their applications over UDP because they want to avoid TCP’s congestion control. Also, some applications do not need the reliable data transfer provided by TCP. 应用程序开发者可能不想其应用程序使用TCP 的拥塞控制，因为这会在出现拥塞时降低应用程序的传输速率。通常，IP 电话和IP 视频会议应用程序的设计者选择让他们的应用程序运行在UDP 上，因为他们想要避免TCP 的拥塞控制。还有，一些应用不需要TCP 提供的可靠数据传输。 二、改为时序图 There is no single right answer to this question. Many protocols would do the trick. Heres a simple answer below: Messages from ATM machine to Server Msg name purpose -------- ------- HELO userid Let server know that there is a card in the ATM machine ATM card transmits user ID to Server PASSWD passwd User enters PIN, which is sent to server BALANCE User requests balance WITHDRAWL amount User asks to withdraw money BYE user all done Messages from Server to ATM machine (display) Msg name purpose -------- ------- PASSWD Ask user for PIN (password) OK last requested operation (PASSWD, WITHDRAWL) OK ERR last requested operation (PASSWD, WITHDRAWL)in ERROR AMOUNT amt sent in response to BALANCE request BYE user done, display welcome screen at ATM Correct operation: client server HELO (userid) -------------- (check if valid userid) ------------- PASSWD PASSWD passwd -------------- (check password) ------------- OK (password is OK) BALANCE -------------- ------------- AMOUNT amt WITHDRAWL amt -------------- check if enough $ to cover withdrawl ------------- OK ATM dispenses $ BYE -------------- ------------- BYE In situation when theres not enough mo