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Dynamic threshold Control of RED for establishing fairness among thousands of TCP connections Go Hasegawa Osaka University, Japan [email protected] 8th, Nov 2001 APSITT2001 1/15 Backgrounds Internet Router Buffer Tail-Drop Router Simple, easy implementation Discard arriving packets when buffer is full Bursty packet loss Poor performance Throughput fairness among connections RED (Random Early Detection) Discard incoming packets with a probability 8th, Nov 2001 APSITT2001 certain 2/15 RED (Random Early Detection) Probability is changed according to average queue length Avoid buffer overflow, keep queue length low Packet Discarding Probability Queue Length [packets] 80 １ maxth Queue Length 60 40 maxp 20 minth maxth Average Queue Length minth 0 100 Average Queue Length [packets] 8th, Nov 2001 APSITT2001 105 110 Time [sec] 115 120 3/15 Researches on RED Achieve better performance than Tail-Drop Throughput, Fairness among connections Difficult to set control parameters Depends on network condition, # of active connections, … 8th, Nov 2001 APSITT2001 4/15 Researches on RED (2) Many enhanced algorithm have been proposed Introducing new control parameters Parameter setting problem remains Focus only on throughput Small number of connections 8th, Nov 2001 APSITT2001 5/15 Objectives Evaluate fairness of RED with many connections Compare with Tail-Drop Propose new algorithm of RED Set threshold values dynamically Easy parameter setting Provide good fairness 8th, Nov 2001 APSITT2001 6/15 Network Model Router Buffer Size: B packets Sender Host 1 1.5Mbps, 4msec 100Mbps, 2msec Sender Host 2 Sender Host N Receiver Host N sender hosts transmit infinite size of data to receiver host by TCP Reno Evaluate fairness among mean throughputs of N TCP connections Measure: Fairness Index x f N 2 i 1 i N 2 i 1 i N x 8th, Nov 2001 0 (unfair) < f < 1 (completely fair) APSITT2001 7/15 Evaluation Result (1): B=1000 [packets] 1 Fairness Index 0.8 0.6 0.4 0.2 0 8th, Nov 2001 1 TailDrop RED maxth = 15 RED maxth = 800 10 100 1000 N: Number of TCP connections Tail-Drop shows degrades fairness when N is small Recommended parameter set of RED shows bad fairness especially when N is large RED with appropriate parameters shows good fairness regardless of N APSITT2001 8/15 Evaluation Result (2): B=10000 [packets] 1 Fairness Index 0.8 0.6 0.4 0.2 0 8th, Nov 2001 TailDrop RED maxth = 15 RED maxth = 2000 1 10 100 1000 N: Number of TCP connections Tail-Drop and RED with recommended parameter set cannot provide fairness Best setting of RED changes according to network condition APSITT2001 9/15 Fairness of RED Recommended parameter set cannot provide fairness when N is large Threshold value (maxth) should be set carefully to keep fairness Appropriate value will be changed by various factors 8th, Nov 2001 When average queue length becomes maxth, RED discards incoming packets in burst as Tail-Drop Bandwidth, Buffer size, # of connections, delay, … Static parameter setting is impossible APSITT2001 10/15 Proposed Algorithm: dt-RED Queue Length [packets] 4000 3000 2000 1000 maxth Queue Length Avg. QL 0 100 200 300 Time [sec] 8th, Nov 2001 400 500 Observe average queue length at regular intervals Change threshold values (maxth, minth) according to the average queue length Keep average queue length between maxth and minth Avoid bursty packet losses at the RED router buffer APSITT2001 11/15 Evaluation Result (3): B=1000 [packets] 1 Fairness Index 0.8 0.6 0.4 TailDrop RED max th = 15 0.2 RED maxth = 800 dt-RED 0 1 10 100 1000 Number of TCP connections 8th, Nov 2001 dt-RED can provide the best fairness among all algorithms APSITT2001 12/15 Evaluation Result (4): B=1000 [packets] 1 Fairness Index 0.8 0.6 0.4 0.2 0 8th, Nov 2001 TailDrop RED maxth=15 RED maxth=2000 dt-RED 1 10 100 1000 Number of TCP connections dt-RED shows good fairness regardless of the number of connections and buffer size APSITT2001 13/15 Characteristics of dt-RED Always provide the same fairness as original RED with best parameter set Automatically appropriately set threshold values Show good performance with one parameter set, regardless of network condition Remove the difficulty of parameter setting in original RED 8th, Nov 2001 APSITT2001 14/15 Conclusion Fairness of RED algorithm is largely dependent on control parameters Static parameter setting is difficult dt-RED can set parameters appropriately according to dynamic change of network condition 8th, Nov 2001 APSITT2001 15/15