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．sending

B．routing

C．switching

D．receiving

BRI接口中的两个B通道可以（）

A、分别作为两个物理接口的备份接口

B、分别作为两个逻辑通道的备份接口

C、同时作为一个物理接口的两个备份接口

D、一个作为物理接口的备份接口，一个作为逻辑通道的接口