【71-75】a transport layer protocol usually has several responsibilities .one is to createaprocess-to-process communication;UDP uses （71）numbers to accomplish this.another responsibilityisto provide control mechanisms at the transpt level.UDP does this task at a very minimallevel.thereis no flow control mechanism and there is no（72） for received packet. UDP，however ，does provideerrorcontrol to some extent.if UDP detects an error in the received packet，it will silentlydropit.thetransport layer also provides a connection mechanism for the processes.the（73）must be abletosendstreams of data to the transport layer.it is the responsibility of the transport layer at（74）stationto make the connection with the receiver.chop the stream into transportable units，numberthem，andsend them one by one.it is the responsibility of th transorort layer at the receivingendtowaituntil all the different units belong to th same process have arrived，check and passthosethatare（75） free ，and deliver them to the receiving process as a stream.
