Feng Gang National Laboratory of Communication,UESTC Aug 2017 Ver 1.4 Example RTT estimation RTT:gaia.cs.umass.edu to fantasia.eurecom.fr 350 300 250 200 150 100 1 8 15 22 29 36 43 50 5764 71 78 85 92 99 106 time(seconnds) -SampleRTT--Estimated RTT 2616009:Network Traffic Engineering 3:TCP Traffic Control Page.6
2616009: Network Traffic Engineering Feng Gang National Laboratory of Communication, UESTC Aug 2017 Ver 1.4 3: TCP Traffic Control Page.6 Example RTT estimation RTT: gaia.cs.umass.edu to fantasia.eurecom.fr 100 150 200 250 300 350 1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106 time (seconnds) RTT (milliseconds) SampleRTT Estimated RTT
Feng Gang National Laboratory of Communication,UESTC Aug 2017 Ver 1.4 TCP Round Trip Time and Timeout EstimatedRTT (1-x)*EstimatedRTT x*SampleRTT Exponential weighted moving average influence of given sample decreases exponentially fast typical value of x:0.1 Setting the timeout EstimtedRTT plus "safety margin" large variation in EstimatedRTT -larger safety margin Timeout EstimatedRTT 4*Deviation Deviation (1-x)*Deviation x*|SampleRTT-EstimatedRTT 2616009:Network Traffic Engineering 3:TCP Traffic Control Page.7
2616009: Network Traffic Engineering Feng Gang National Laboratory of Communication, UESTC Aug 2017 Ver 1.4 3: TCP Traffic Control Page.7 TCP Round Trip Time and Timeout EstimatedRTT = (1-x)*EstimatedRTT + x*SampleRTT Exponential weighted moving average influence of given sample decreases exponentially fast typical value of x: 0.1 Setting the timeout • EstimtedRTT plus “safety margin” • large variation in EstimatedRTT -> larger safety margin Timeout = EstimatedRTT + 4*Deviation Deviation = (1-x)*Deviation + x*|SampleRTT-EstimatedRTT|
Feng Gang National Laboratory of Communication,UESTC Aug 2017 Ver 1.4 TCP Congestion Control Mechanisms that limit amount of data in the network Avoid overrun of internal network buffers Network-wide problem Often phrased as global optimization problem A control system with delayed feedback Implementation today Algorithms in TCP 2616009:Network Traffic Engineering 3:TCP Traffic Control Page.8
2616009: Network Traffic Engineering Feng Gang National Laboratory of Communication, UESTC Aug 2017 Ver 1.4 3: TCP Traffic Control Page.8 TCP Congestion Control • Mechanisms that limit amount of data in the network - Avoid overrun of internal network buffers • Network-wide problem - Often phrased as global optimization problem - A control system with delayed feedback • Implementation today - Algorithms in TCP
Feng Gang National Laboratory of Communication,UESTC Aug 2017 Ver 1.4 Congestion Collapse Decrease of network efficiency under load efficiency utilization of bandwidth Waste resources on useless or undelivered data ·Network layer load->drops,>1 fragment dropped ·Transport layer retransmit too many times no congestion control avoidance 2616009:Network Traffic Engineering 3:TCP Traffic Control Page.9
2616009: Network Traffic Engineering Feng Gang National Laboratory of Communication, UESTC Aug 2017 Ver 1.4 3: TCP Traffic Control Page.9 Congestion Collapse • Decrease of network efficiency under load - efficiency = utilization of bandwidth • Waste resources on useless or undelivered data • Network layer - loaddrops, 1 fragment dropped • Transport layer - retransmit too many times - no congestion control / avoidance
Feng Gang National Laboratory of Communication,UESTC Aug 2017 Ver 1.4 Congestion Collapse (cont'd) Penalized 90%loss ignores loss bw wasted Waste resources on undelivered data A flow sends data at a high rate despite loss Its packets consume bandwidth at earlier links,only to be dropped at a later link 2616009:Network Traffic Engineering 3:TCP Traffic Control Page.10
2616009: Network Traffic Engineering Feng Gang National Laboratory of Communication, UESTC Aug 2017 Ver 1.4 3: TCP Traffic Control Page.10 Congestion Collapse (cont’d) • Waste resources on undelivered data • A flow sends data at a high rate despite loss • Its packets consume bandwidth at earlier links, only to be dropped at a later link 90% loss bw wasted ignores loss Penalized