计算机网络 Top down 部分课后题答案.doc

1. Design and describe an application-level protocol to be used between an automatic teller machine and a bank’s centralized computer. You protocol should allow a user’s card and password to be verified, the account balance (which is maintained at the centralized computer) to be queried, and an account withdrawal to be made (that is, money disbursed to the user). Your protocol entities should be able to handle the all-too-common case in which there is not enough money in the account to cover the withdrawal. Specify your protocol by listing the messages exchanged and the action taken by the automatic teller machine or the bank’s centralized computer on transmission and receipt of messages. Sketch the operation of your protocol for the case of a simple withdrawal with no errors, using a diagram similar to that in Figure 1.2. Explicitly state the assumption made by your protocol about underlying end-to-end transport service. Answers: 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 money: HELO (userid) (check if valid userid) PASSWD PASSWD passwd (check password) OK (password is OK) BALANCE AMOUNT amt WITHDRAWL