单选题 The network layer provides services to the transport layer. It can be based on either (71) . In both cases, its mainjob is (72) packets from the source to the destination.
In network layer, subnets 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, trafficshaping, resource (75) , and admission control. Approaches that have been designed forgood quality of service include integrated services (including RSVP), differentiated services, andMPLS.

【正确答案】 B

【答案解析】[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(virtual circuits or datagrams)，其主要工作是对源节点的数据包进行路由选择(routing)，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(lowering the throughput)降低。网络设计者试图通过良好的设计缓解网络拥塞，所使用的技术包括重传(retransmission)策略、缓冲策略和流控制等。
下一步的目标是试图达到设定的服务质量，而不仅仅是处理拥塞问题。可以使用的方法有客户端缓存、通信量整形、资源预留(reservation)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(lntServ)/资源预留协议(RSVP)、区分服务(DiffServ)和多协议标记交换(MPLS)等。