Why not centralize DNS（英文复习试题）.docVIP

Chapter2 PROBLEMSMY OWN ANSWERS Why not centralize DNS? ?single point of failure 单点故障 ?traffic volume 通信容量 ?distant centralized database 远距离和集中式数据库 ?maintenance 维护 UDP service: - unreliable data transfer between sending and receiving process - does not provide: connection setup, reliability, flow control, congestion control, timing, throughput guarantee, or security Q: why bother? Why is there a UDP? 1.因为实时应用通常可以忍受一定的数据丢失，同时有最低速率的要求，用UDP实现的可能性要大于TCP协议实现的可能性。 2.选择使用UDP协议可以避开TCP协议的拥塞控制机制和分组开销。 Whats optimistically unchoke in P2P networks? 在P2P中，对等方A每隔30秒要随机选择新的对换伙伴（假定为B）并向其发送块，因为A向B发送数据，所以A可能成为B的前4为上载者之一，这样B就会向A发送数据，只要发送速率足够高，就可能成为A的前4为上载者之一。这样，两个对等方都能以趋于满意的速率上载。 Why Web caching? Whats its advantage? PPT 2-40 大大减少对客户机请求的响应时间； 大大减少一个机构的内部网与因特网接入链路上的通信量； 3.让较差的内容提供者能更有效的分发内容； How do they connect to each other when both Alice and Bob are behind “NATs? Alice和Bob向系统注册时，会分别被指派到一个非NAT的中继； 通过中继初始化对等方之间的会话； 对等方即可通过中继越过NATs进行交流。 Whats the goal of web caches (proxy server), how it works? 1.大大减少对客户机请求的响应时间； 2.大大减少一个机构的内部网与因特网接入链路上的通信量； 当客户端请求某对象时，向web缓存请求该对象，web缓存查看本地是否有该对象的拷贝，如有则返回该对象；否则发送请求获取该对象的报文，接受该对象时，先在本地存储该对象的拷贝，再向客户机的浏览器发送对象的拷贝。 Users write programs that run on (different) end systems, communicate over network. No need to write software for network-core devices. True No need to write software for network-core devices. network-core devices do not run user applications.? applications on end systems allows for rapid app development, propagation. A user requests a web page that consists of some text and 4 images. For this page, the client will send 5 HTTP request messages and 5 HTTP response messages. Whats a Torrent in BitTorrent? How many torrent does Alice need to join when she want to download 3 files. 参与一个特定文件分发的所有对等方的集合成为一个洪流。3个。 Whats the POP3 modes and their weakness? PPT 2-60 POP3通常配置为“下载并删除”或者“下载并保留”方式。前者的缺点是使得拥有多个客户机的Bob无法在另一台客户机重新读取在某一台客户机已读过的邮件。另外，POP3协议的会话是不携带状态的。 Many Web sites use cookies. List the four components. many Web sites use cookies four compon