tailieunhanh - Internetworking with TCP/IP- P60

Internetworking with TCP/IP- P60: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | Sec. Resource Reservation And Quality Of Service 549 that adding per-flow QoS is infeasible because routers will make the system both expensive and slow. The QoS controversy has produced many proposals implementations and experiments. Although it operates without QoS the Internet is already used to send audio. Technologies like ATM that were derived from the telephone system model provide QoS guarantees for each individual connection. Finally in Chapter 7 we learned that the IETF adopted a conservative differentiated services approach that divides traffic into separate QoS classes. The differentiated services scheme sacrifices fine grain control for less complex forwarding. QoS Utilization And Capacity The debate over QoS is reminiscent of earlier debates on resource allocation such as those waged over operating system policies for memory allocation and processor scheduling. The central issue is utilization when a network has sufficient resources for all traffic QoS constraints are unnecessary when traffic exceeds network capacity no QoS system can satisfy all users demands. That is a network with 1 utilization does not need QoS and a network with 101 utilization will fail under any QoS. In effect proponents who argue for QoS mechanisms assert that complex QoS mechanisms achieve two goals. First by dividing the existing resources among more users they make the system more fair . Second by shaping the traffic from each user they allow the network to run at higher utilization without danger of collapse. One of the major arguments against complicated QoS mechanisms arises from improvements in the performance of underlying networks. Network capacity has increased dramatically. As long as rapid increases in capacity continue QoS mechanisms merely represent unnecessary overhead. However if demand rises more rapidly than capacity QoS may become an economic issue by associating higher prices with higher levels of service ISPs can use cost to ration capacity. .