1 TCP Vegas: New Techniques for Congestion Detection and Avoidance Lawrence S. Brakmo Sean W. O’Malley Larry L. Peterson Department of Computer Science University of Arizona Tucson, AZ Presented By Khandoker Nadim Parvez Computer Science University of Calgary

2 Outline Problem Statement / Motivation Background: TCP/TCP Reno TCP Vegas[1] –Modified Congestion Avoidance –Aggressive Retransmission –Aggressive Window Adaptation –Modified Slow-Start Simulation Results Vegas Revisit[2] –Effectiveness of Vegas Techniques Conclusion

3 Problem Statement / Motivation Problems of TCP (Reno) –TCP needs packet loss to infer available bandwidth –TCP reduces transmission rate in response to each packet loss Our motivation –To design a proactive TCP To design a TCP that will be able to track the bottleneck bandwidth without incurring packet loss –To response to a loss event, not each packet loss Multiple loss in a single window should be considered as a single loss event and hence should reduce the transmission rate only one once –To have a more aggressive TCP when we have proactive mechanism retransmission and transmission should be more aggressive

4 Outline Problem Statement / Motivation Background: TCP/TCP Reno TCP Vegas –Modified Congestion Avoidance –Aggressive Retransmission –Aggressive Window Adaptation –Modified Slow-Start Simulation Results Vegas Revisit –Effectiveness of Vegas Techniques Conclusion

5 Background: TCP Reliable ordered delivery –Reliability achieved by means of retransmissions if necessary Cumulative acknowledgements –An acks acknowledges all contiguously received data End-to-end semantics –Acknowledgements sent to TCP sender confirm delivery of data received by TCP receiver Implements Congestion Control –Slow-Start –Congestion Avoidance

6 TCP: Congestion Control Slow-Start Phase: When cwnd<ssthresh. cwnd increases exponentially,i.e. by 1 for each ACK. Congestion Avoidance Phase: When cwnd>ssthresh. Cwnd increases linearly, i.e., by 1 in each RTT. Transmission (without loss): –At 0 sec, cwnd=1 & speed= 1KB/S –At 1 sec, cwnd=2 & speed= 2KB/S –At 2 sec, cwnd=4 & speed= 4KB/S –At 3 sec, cwnd=8 & speed= 8KB/S –At 4 sec, cwnd=9 & speed= 9KB/S –At 5 sec, cwnd=10 & speed=10KB/S Slow start Congestion avoidance Slow start threshold (ssthresh=8) Receiver Window Sink TCP ACKACK 500ms, 20KB/S Source

7 TCP Reno Slow-Start Congestion Avoidance Fast Retransmit –Retransmits the loss packet when it receives 3 dupacks Fast Recovery –Enters into fast recovery with reception of 3 dupacks Sets ssthresh=cwnd/2 Sets cwnd=cwnd/2+3 –Exits fast recovery when receives the acknowledgement of the lost packet –Increases the congestion window by one for each received dupack Packet Sequence: Window

8 Outline Problem Statement / Motivation Background: TCP/TCP Reno TCP Vegas –Modified Congestion Avoidance –Aggressive Retransmission –Aggressive Window Adaptation –Modified Slow-Start Simulation Results Vegas Revisit –Effectiveness of Vegas Techniques Conclusion

9 TCP Vegas Developed over TCP Reno Modified Congestion Avoidance Aggressive Retransmission (use fine grained timer) –With dupacks –With partial acks Aggressive Congestion Window Adaptation –With recovery –With multiple loss –Initial setting Modified Slow-Start

10 Vegas: Modified Congestion Avoidance Vegas Calculates (Once per RTT):  Expected Throughput=WindowSize/BaseRTT  Actual Throughput= ActualSentAmount /RTT Static Parameters:   = 1 pkts/RTT   = 3 pkts/RTT

11 Vegas: Modified Congestion Avoidance Expected Throughput   Throughput Actual Throughput (  Tx Unchanged) (  Decrease Tx) (  Increase Tx)

12 Vegas: Modified Congestion Avoidance

13 Vegas: Modified Congestion Avoidance TCP transmission rate = cwnd/RTT TCP takes congestion window updating decision once per RTT The decision is applied throughput the next RTT for each received ACK as follows: Increase Tx Rate (Expected-Actual>  ) : :  cwnd = cwnd + 1/cwnd Decrease Tx Rate (Expected-Actual<  ) :  cwnd = cwnd - 1/cwnd Tx Rate Unchanged (  <Expected-Actual<  ) : :  cwnd = cwnd

14 Vegas: Aggressive Retransmission With dupacks –When Vegas receives the first dupack or the second dupacks, it checks the fine grained timer expiry –If timer expirers, it retransmits immediately With partial acks –For the first two partial acks, Vegas checks whether fine grained timer expires –If timer expires, it retransmits immediately Packet Sequence: Window

15 Vegas: Aggressive cwnd Updating With recovery –Reduce cwnd by one quarter instead of half when it enters into recovery With multiple loss –In case of multiple segment loss from a single window, it reduces the cwnd only once With Initial setting –cwnd is set to 2 instead of 1

16 Vegas: Modified Slow-Start Vegas Calculates (in every alternate RTT):  Expected Throughput=WindowSize/BaseRTT  Actual Throughput= ActualSentAmount /RTT Static Parameters:   = 1 pkts/RTT

17 Vegas: Modified Slow-Start Expected Throughput  Throughput Actual Throughput (  Switch to CA) (  Continue SS)

18 Vegas: Modified Slow-Start TCP keeps the congestion window fixed in every other RTT and it measures the throughput On every next RTT, it does the followings: Continue SS (Expected-Actual<  ) :  Exponential Increase.  cwnd = cwnd + 1 for each ACK, that is,  Cwnd = 2 * cwnd for each RTT Switch to CA (Expected-Actual>  ) :  Set ssthresh=cwnd  Follow the rules of CA

19 Outline Problem Statement / Motivation Background: TCP/TCP Reno TCP Vegas –Modified Congestion Avoidance –Aggressive Retransmission –Aggressive Window Adaptation –Modified Slow-Start Simulation Results Vegas Revisit –Effectiveness of Vegas Techniques Conclusion

20 Simulation Topology Target TCP runs from 1a to 1b Background traffic runs from 3a to 3b

21 TCP Reno with no Background Traffic

22 TCP Vegas with no Background Traffic

23 Vegas with Background Traffic

24 Vegas: Simulation Result

25 Vegas: Internet Experiments

26 Outline Problem Statement / Motivation Background: TCP/TCP Reno TCP Vegas –Modified Congestion Avoidance –Aggressive Retransmission –Aggressive Window Adaptation –Modified Slow-Start Simulation Results Vegas Revisit –Effectiveness of Vegas Techniques Conclusion

27 TCP Vegas Revisit: 3 Mechanisms Congestion Avoidance(CA) –Modified Congestion Avoidance Congestion Recovery(REC) –Aggressive Retransmission (use fine grained timer) With dupacks With partial acks –Aggressive Congestion Window Updating With recovery With multiple loss Slow-Start(SS) –Modified Slow-Start –Initial cwnd

28 Percentage of Throughput Variation LOW BACKGROUND TRAFFIC RENO BG.VEGAS BG. SS CA REC

29 Percentage of Throughput Variation HIGH BACKGROUND TRAFFIC RENO BG.VEGAS BG. SS CA REC

30 Effectiveness of Vegas Mechanisms Congestion Avoidance(CA) –Modified Congestion Avoidance Congestion Recovery(REC) –Aggressive Retransmission (use fine grained timer) With dupacks With partial acks (Retransmission with 1 st /2 nd partial ack) –Aggressive Congestion Window Updating With recovery (Reduction of cwnd by quarter) With multiple loss Slow-Start(SS) –Modified Slow-Start –Initial cwnd

31 Vegas CA: Condition of Increment

32 Vegas: Problems of CA [3] Unfair Treatment of Old Connections –Old connections have smaller baseRTT –Vegas decreases Tx Rate if –Vegas increases Tx Rate if Persistent Congestion

33 Outline Problem Statement / Motivation Background: TCP/TCP Reno TCP Vegas –Modified Congestion Avoidance –Aggressive Retransmission –Aggressive Window Adaptation –Modified Slow-Start Simulation Results Vegas Revisit –Effectiveness of Vegas Techniques Conclusion

34 Vegas: Conclusion Less fluctuation –Less fluctuation in bottleneck queue –Less fluctuation in send rate Enhanced Throughput Better Utilization of bottleneck capacity Unfair treatment of old connection Ineffectiveness of congestion avoidance

35 Questions ?

36 THANKS

37 Bibliography [1] Lawrence S. Brakmo and Larry L. Peterson, “TCP Vegas: End to end congestion avoidance on a global internet”, IEEE Journal on Selected Areas in Communication, 13(8): , October [2] U. Hengartner, J. Bolliger, and Th. Gross, "TCP Vegas Revisited," in Proc. of IEEE Infocom '2000, March [3] Mo, R. J. La, V. Anantharam, and J. Walrand, “Analysis and comparison of TCP Reno and Vegas”, Proc. of IEEE INFOCOM'99, New York, NY, March 1999.