● A transport layer protocol usually has several responsibilities.One is to create a process-to-process communication;UDP uses__(71)__numbers to accomplish this. Another  responsibility is to provide control mechanisms at the transport level. UDP does this task at a very minimal level. There is no flow control mechanism and there is no__(72)__for received packet. UDP，however， does provide error control to some extent. If UDP detects an error in the received packet， it will silently drop it.
The transport layer also provides a connection mechanism for the processes. The __(73)__ must be able to send streams of data to the transport layer.It is the responsibility of the transport layer at__(74)__station to make the connection with the receiver， chop the stream into transportable units，number them， and send them one by one. It is the responsibility of the transport layer at the receiving end to wait until all the different units belonging to the same process have arrived， check and pass those that are__(75)__free， and deliver them to the receiving process as a stream.
 

（71）A．hop   

B．port   

C．route   

D．packet

（72）A．connection   

B．window 

C．acknowledgement 

D．destination

（73）A．jobs   

B．processes   

C．programs 

D．users

（74）A．sending 

B．routing   

C．switching 

D．receiving

（75）A．call 

B．state   

C．cost 

D．error