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.
20（）
A. virtual circuits or datagrams
B. TCP or UDP
C. TCP or IP
D. IP or ARP
参考答案：A
解析：
网络层为传输层提供服务，它基于虚电路或数据报方式，其主要工作是对源结点的包进行路由选择，转发到目的结点。
在网络层，通信子网很容易出现拥塞，导致包的延迟增加、吞吐率降低。网络设计者试图通过良好的设计避免拥塞，所使用的技术包括重传策略、缓冲策略、流控制等。
仅仅处理拥塞是不够的，下一步的目标是试图达到设定的服务质量。可以使用的方法有客户端缓存、通信量整形、资源预留、接纳控制等。已提出的、较好的服务质量控制方法有集成服务(包括RSVP)、区分服务和MPLS。