The network layer provides services to the transport layer.It can be based on either（ ）. In both cases, its main job is （ ）packets from the source to the destination.
In network layer, subnets can easily become congested, increasing the delay and （ ）for packets.Network designers attempt to avoid congestion by proper design.Techniques include （ ） policy, caching, flow control, and more.
The next step beyond just dealing with congestion is to actually try to achieve a promisedquality of service.The methods that can be used for this include buffering at the client, trafficshaping, resource （ ）, and admission control.Approaches that have been designed forgood quality of service include integrated services （including RSVP）, differentiated services, and MPLS.
问题1选项
答案： A、B、A、B、D。
网络层为传输层提供服务，它基于虚电路或数据报方式，其主要工作是对源节点的包进行路由选择，转发到目的节点。
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、缓冲策略和流控制等。
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、资源预留和接纳控制等。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。
