[已注销]对《计算机网络》的笔记(3)

计算机网络
  • 书名: 计算机网络
  • 作者: Andrew S. Tanenbaum
  • 副标题: 第4版
  • 页数: 743
  • 出版社: 清华大学出版社
  • 出版年: 2004-8-1
  • 第374页
    The Story of Goldilocks and the Three Bears
    Once upon a time, there was a little girl named Goldilocks. She went for a walk in the forest. Pretty soon, she came upon a house. She knocked and, when no one answered, she walked right in.
    At the table in the kitchen, there were three bowls of porridge. Goldilocks was hungry. She tasted the porridge from the first bowl.
    "This porridge is too hot!" she exclaimed.
    So, she tasted the porridge from the second bowl.
    "This porridge is too cold," she said
    So, she tasted the last bowl of porridge.
    "Ahhh, this porridge is just right," she said happily and she ate it all up.
    After she'd eaten the three bears' breakfasts she decided she was feeling a little tired. So, she walked into the living room where she saw three chairs. Goldilocks sat in the first chair to rest her feet.
    "This chair is too big!" she exclaimed.
    So she sat in the second chair.
    "This chair is too big, too!" she whined.
    So she tried the last and smallest chair.
    "Ahhh, this chair is just right," she sighed. But just as she settled down into the chair to rest, it broke into pieces!
    Goldilocks was very tired by this time, so she went upstairs to the bedroom. She lay down in the first bed, but it was too hard. Then she lay in the second bed, but it was too soft. Then she lay down in the third bed and it was just right. Goldilocks fell asleep.
    As she was sleeping, the three bears came home.
    "Someone's been eating my porridge," growled the Papa bear.
    "Someone's been eating my porridge," said the Mama bear.
    "Someone's been eating my porridge and they ate it all up!" cried the Baby bear.
    "Someone's been sitting in my chair," growled the Papa bear.
    "Someone's been sitting in my chair," said the Mama bear.
    "Someone's been sitting in my chair and they've broken it all to pieces," cried the Baby bear.
    They decided to look around some more and when they got upstairs to the bedroom, Papa bear growled, "Someone's been sleeping in my bed,"
    "Someone's been sleeping in my bed, too" said the Mama bear
    "Someone's been sleeping in my bed and she's still there!" exclaimed Baby bear.
    Just then, Goldilocks woke up and saw the three bears. She screamed, "Help!" And she jumped up and ran out of the room. Goldilocks ran down the stairs, opened the door, and ran away into the forest. And she never returned to the home of the three bears.
    A类地址太多,C类地址又不够用,B类刚刚好。
    2011-06-02 16:17:15 回应
  • 第200页
    所有的多路访问协议(multiple access protocol),ALOHA也好,CSMA也好,都是在试图解决一个问题:在非集中广播式网络如何解决冲突问题?即在一个会议中,如何保持所有与会者可以有秩序的发言,而不产生冲突。
    ALOHA的策略属于“想说就说,想唱就唱”,即与会者一想到什么马上说出来(憋在肚子里难受),即使除了他以外还有其他人也在发言,他也会毫无顾及地把话说完(但他心里清楚这次的发言受到了干扰,所以稍后会重讲。)
    CSMA(carrier sense multiple access)则相对比较文明,当一个人有话要说时,他会先听会议上有没有人在说话,如果没有,他才会站起来发言。
    但是CSMA有一种特殊情况:当两个人同时发现会议处在无人发言的阶段,他们两人将会同时发言。这时冲突将会产生,如果两个人意识到这点以后马上都会立刻停下来(反正也是浪费口水),等待一个随机的时间再尝试站起来。这就是所谓的CSMA/CD(CSMA with collision detection)
    以上两个协议都有可能发生冲突(同一时刻有两个人以上的声音回荡在会议厅)
    为了避免冲突的发生,可以在发言前进行一轮举手表决,这便是所有无冲突协议的核心思想。
    位图协议(bit map)就是利用一个竞争时槽,想发言的人把标志位置1(举手),这样下一轮谁想发言、谁不想发言就一目了然了。
    2011-06-02 16:21:31 回应
  • 第155页
    协议模拟器的修改
    http://www.chengyichao.info/a-true-story-of-tanenbaums-protocol-simulator/
    2011-06-08 21:06:14 回应

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