●The network layer provides services to the transport layer.It can be based on either__(71)__.In both cases，its main job is __(72)__packets from the source to the destination.
In network layer，sub nets can easily become congested，increasing the delay and

●The network layer provides services to the transport layer.It can be based on either__(71)__.In both cases，its main job is __(72)__packets from the source to the destination.
In network layer，sub nets can easily become congested，increasing the delay and __(73)__for packets.Network designers attempt to avoid congestion by proper design.Techniques include __(74)__ policy，caching，flow control，and more.
The next step beyond just dealing with congestion is to actually try to achieve a promised quality of service.The methods that can be used for this include buffering at the client，traffic shaping，resource __(75)__，and admission control.Approaches that have been designed for good quality of service include integrated services（including RSVP），differentiated services，and MPLS.
（71）A．virtual circuits or data grams
B．TCP or UDP
C．TCP or IP
D．IP or ARP
（72）A．dealing with
B．routing
C．sending
D．receiving
（73）A．lowering the throughput
B．lowering the correctness
C．lowering the effectiveness
D．lowering the preciseness
（74）A．abandonment
B．retransmission
C．checksum
D．synchronism
（75）A．distribution
B．guarantee
C．scheme
D．reservation