文本isp運(yùn)營(yíng)商系列qos22s06l

1、Congestion AvoidanceIntroducing Congestion AvoidanceBehavior of a TCP SenderSender sends “N” bytes (as much as credit allows)Start credit (window size) is small To avoid overloading network queuesIncreases credit exponentially To gauge network capabilityBehavior of a TCP ReceiverReceiver schedules a

2、n ACK on receipt of “next message.”TCP acknowledges the next segment it expects to receive, not the last segment it received.In the example, N+1 is blocked, so the receiver keeps acknowledging N+1 (the next segment it expects to receive).TCP Slow StartIf ACK acknowledges somethingUpdates credit and

3、sendsIf not, presumes it indicates a lost packetSends first unacknowledged message right awayHalves current credit (slows down)Increases slowly to gauge network throughputMultiple Drops in TCPIf multiple drops occur in the same session:Current TCPs wait for time-outSelective acknowledge may be a wor

4、karoundNew “fast retransmit” phase takes several round-trip times to recoverManaging Interface Congestionwith Tail DropRouter interfaces experience congestion when the output queue is full:Additional ing packets are tail-droppedDropped packets may cause significant application performance degradatio

5、nTail drop has significant drawbacksTail Drop LimitationsTail drop should be avoided because it contains significant flaws:TCP synchronizationTCP starvationNo differentiated dropTCP SynchronizationMultiple TCP sessions start at different times.TCP window sizes are increased.Tail drops cause many pac

6、kets of many sessions to be dropped at the same time.TCP sessions restart at the same time (synchronized).TCP Delay, Jitter and Starvation Constant high buffer usage (long queue) causes delay.More aggressive flows can cause other flows to starve.No differentiated dropping occurs.SummaryTCP sender se

7、nds as many bytes as credit allows. TCP receiver acknowledges the next segment it expects to receive.TCP uses windowing and the TCP slow-start mechanism as its means of controlling congestion.By default, routers resort to tail drop, relying on TCP congestion controls when queues e full. Tail drop causes significant issues including TCP