盘算机网络:自顶向下 第一章 课后习题参考答案(官方文档中文解析)
前言对于知识点的掌握不能仅仅是知晓，能够加以应用才能熟练掌握，计算机网络：自顶向下这本书中为读者提供了符合的习题来帮助我们练习。接下来我将和大家一起解读官方答案文档（英文），并给出我所理解的中文翻译(英语好的读者发起直接观看官方英语文档，部分题目笔者能力有限无法解答)。
Chapter 1 Problems
Problem 1
英文：
There is no single right answer to this question. Many protocols would do the trick. Here’s a simple answer below:
Messages from ATM machine to Server
Msg namepurposeHELO Let server know that there is a card in the ATM machine and ATM card transmits user ID to ServerPASSWD User enters PIN, which is sent to serverBALANCEUser requests balanceWITHDRAWL User asks to withdraw moneyBYEuser all doneMessages from Server to ATM machine (display)
Msg namepurposePASSWDAsk user for PIN (password)OKlast requested operation (PASSWD, WITHDRAWL)ERRlast requested operation (PASSWD, WITHDRAWL) in ERRORAMOUNT sent in response to BALANCE requestBYEuser done, display welcome screen at ATMCorrect operation:
clientserverHELO (userid)--------------> (check if valid userid) (check password) check if enough $ to cover withdrawl (check if valid userid) (check password)BALANCE --------------> (检查是用户否正当)（请输入暗码） (检查暗码)暗码正确（请求取款）（输入取款金额） （检查余额是否足够）（账户余额足够）（服务竣事）（服务竣事） (检查是用户否正当)请输入暗码 (检查暗码)暗码正确（请求取款）（输入取款金额） （检查余额是否足够）（无金钱支出）BYE -------------->服务竣事服务竣事
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