关于无连接的通信，下面的描述中正确的是( )。
- A.由于为每一个分组独立地建立和释放逻辑连接，所以无连接的通信不适合传送大量的数据
- B.由于通信对方和通信线路都是预设的，所以在通信过程中无须任何有关连接的操作
- C.目标的地址信息被加在每个发送的分组
- D.无连接的通信协议UDP不能运行在电路交换或租用专线网络上
正确答案及解析
正确答案
解析
面向连接的方式功能强大，允许流量控制、差错控制以及顺序交付等。无连接的服务是不可靠的服务，无法许诺不会出现的交付和重复的差错，但这种协议代价很小，更适应于某些服务，比如内部的数据采集、向外的数据分发、请求一响应，以及实时应用等。因此在运输层既有面向连接的位置，也有无连接的用武之地。每一个分组独立地建立和释放逻辑连接，也适合传送大量的数据。
无连接的服务的通信线路不都是预设的。无连接的服务需要将目标地址信息加在每个发送的分组上，便于每个分组路由到达目的地。UDP在电路交换或租用专线网络上也能运行。
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