The network layer provides services to the transport layer. It can be based on either___20___. In both cases, its main job is ___21___ packets from the source to the destination.

In network layer, subnets can easily become congested, ncreasing the delay and ___22___ for packets. Network designers attempt to avoid congestion by proper design. Techniques include___23___ 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___24___, and admission control. Approaches that have been designed for good quality of service include integrated services (including RSVP), differentiated services, and MPLS.

23（）

A. abandonment

B. retransmission

C. checksum

D. synchronism

备用水泵应该具备什么排水能力？