网络工程专业英语课件ch3 Transport Layer.ppt

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Kurose and Ross forgot to say anything about wrapping the carry and adding it to low order bit * * * * * * * * * * * * * * * * * * * * * * * Transport Layer 3-* TCP Slow Start When connection begins, CongWin = 1 MSS Example: MSS = 500 bytes RTT = 200 msec initial rate = 20 kbps available bandwidth may be MSS/RTT desirable to quickly ramp up to respectable rate When connection begins, increase rate exponentially fast until first loss event Transport Layer 3-* TCP Slow Start (more) When connection begins, increase rate exponentially until first loss event: double CongWin every RTT done by incrementing CongWin for every ACK received Summary: initial rate is slow but ramps up exponentially fast Host A one segment RTT Host B time two segments four segments Transport Layer 3-* Refinement After 3 dup ACKs: CongWin is cut in half window then grows linearly But after timeout event: CongWin instead set to 1 MSS; window then grows exponentially to a threshold, then grows linearly 3 dup ACKs indicates network capable of delivering some segments timeout before 3 dup ACKs is “more alarming” Philosophy: Transport Layer 3-* Refinement (more) Q: When should the exponential increase switch to linear? A: When CongWin gets to 1/2 of its value before timeout. Implementation: Variable Threshold At loss event, Threshold is set to 1/2 of CongWin just before loss event Transport Layer 3-* Summary: TCP Congestion Control When CongWin is below Threshold, sender in slow-start phase, window grows exponentially. When CongWin is above Threshold, sender is in congestion-avoidance phase, window grows linearly. When a triple duplicate ACK occurs, Threshold set to CongWin/2 and CongWin set to Threshold. When timeout occurs, Threshold set to CongWin/2 and CongWin is set to 1 MSS. Transport Layer 3-* TCP sender congestion control Event State TCP Sender Action Commentary ACK receipt for previous