计算机网络第四版习题答案(中文版)(28)

计算机网络第四版习题答案(中文版)

第 5 章 网络层

5-1 Give two example computer applications for which connection-oriented service is appropriate.

Now give two examples for which connectionless service is best.

答：文件传送、远程登录和视频点播需要面向连接的服务。另一方面，信用卡验证和其他销售点终端、电子资金转移，及许多形式远程数据库访问生来具有无连接性质，在一个方向上传送查询，在另一个方向上返回应答。

5-2 Are there any circumstances when connection-oriented service will (or at least should) deliver

packets out of order? Explain.

答：有。中断信号应该跳过在它前面的数据，进行不遵从顺序的投递。典型的例子是当一个终端用户键入退出（或kill）健时。由退出信号产生的分组应该立即发送，并且应该跳过当前队列中排在前面等待程序处理的任何数据（即已经键入但尚未被程序读取的数据）。

5-3 Datagram subnets route each packet as a separate unit, independent of all others.

Virtual-circuit subnets do not have to do this, since each data packet follows a predetermined route. Does this observation mean that virtual-circuit subnets do not need the capability to route isolated

packets from an arbitrary source to an arbitrary destination? Explain your answer.

答：不对。为了从任意源到任意目的地，为连接建立的分组选择路由，虚电路网络肯定需要这一能力。

5-5 Consider the following design problem concerning implementation of virtual-circuit service. If virtual circuits are used internal to the subnet, each data packet must have a 3-byte header and each router must tie up 8 bytes of storage for circuit identification. If datagrams are used internally, 15-byte headers are needed but no router table space is required. Transmission capacity costs 1 cent per 106 bytes, per hop. Very fast router memory can be purchased for 1 cent per byte and is depreciated over two years, assuming a 40-hour business week. The statistically average session runs for 1000 sec, in which time 200 packets are transmitted. The mean packet requires four hops.