CCIP-v2 2 pptQOS22S06L01

1、Congestion Avoidance,Introducing Congestion Avoidance,Behavior of a TCP Sender,Sender sends “N” bytes (as much as credit allows) Start credit (window size) is small To avoid overloading network queues Increases credit exponentially To gauge network capability,Behavior of a TCP Receiver,Receiver sche

2、dules an 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 Start,If ACK acknowledges something Updates

3、 credit and sends If not, presumes it indicates a lost packet Sends first unacknowledged message right away Halves current credit (slows down) Increases slowly to gauge network throughput,Multiple Drops in TCP,If multiple drops occur in the same session: Current TCPs wait for time-out Selective ackn

4、owledge may be a workaround New “fast retransmit” phase takes several round-trip times to recover,Managing Interface Congestionwith Tail Drop,Router interfaces experience congestion when the output queue is full: Additional incoming packets are tail-dropped Dropped packets may cause significant appl

5、ication performance degradation Tail drop has significant drawbacks,Tail Drop Limitations,Tail drop should be avoided because it contains significant flaws: TCP synchronization TCP starvation No differentiated drop,TCP Synchronization,Multiple TCP sessions start at different times. TCP window sizes

6、are increased. Tail drops cause many packets 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 differe

7、ntiated dropping occurs.,Summary,TCP sender sends 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 become full. Tail drop causes significant issues including T