A transport layer protocol usually has several responsibilities. One is to create a process-to-processcommunication UDP uses ______ numbers to accomplish this. Another responsibility is to provide controlmechanisms at the transport level, UDP does this task at a very minimal level. There is no flow control mechanismand there is no ______ for received packet. UDP, however, does provide error control to some extent. If UDPdetects an error in the received packet, it will silently drop it.
The transport layer also provides a connection mechanism for the processes. The ______ must be able to sendstreams of data to the transport layer. It is the responsibility of the transport layer at ______ station to make theconnection with the receiver chop the stream into transportable units, number them, and send them one by one. Itis the responsibility of the transport layer at the receiving end to wait until all the different units belonging to thesame process have arrived, check and pass those that are ______ free, and deliver them to the receiving process asa stream.
A．call
B．state
C．cost
D．error
参考答案：D
解析：
[分析]： 传输层协议通常有多种责任。一个是创建进程间通信，UDP使用端口数来完成这项工作。另一个责任是在传输层提供控制机制，UDP以一个最低的级别完成这项工作，无流控制机制和接收包的应答机制。但是，UDP在一些范围上提供了差错控制。如果UDP在接收包中检测到了错误，它将静静地丢弃它。
传输层同样为进程提供了一个连接机制。这个进程必须能发送数据流到传输层。而发送站传输层的责任就是与接收站建立连接，将流放入传输单元，并编号，然后一个接一个地发送。接收站传输层的责任是等待一个相同进程中的不同的数据单元的到达，检查它们，将错误单元丢弃，并且以流的形式传递到接收进程中。