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计算机网络 Top down 部分课后题答案
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
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