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WB-RTO: A Window-Based Retransmission Timeout

WB-RTO: A Window-Based Retransmission Timeout. Ioannis Psaras, Vassilis Tsaoussidis Demokritos University of Thrace, Xanthi, Greece. Motivation and Contribution. We observe that retransmission scheduling affects transmission scheduling WB-RTO results in: 50% less retransmitted pkts

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WB-RTO: A Window-Based Retransmission Timeout

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  1. WB-RTO: A Window-Based Retransmission Timeout Ioannis Psaras, Vassilis Tsaoussidis Demokritos University of Thrace, Xanthi, Greece

  2. Motivation and Contribution • We observe that retransmission scheduling affects transmission scheduling • WB-RTO results in: • 50% less retransmitted pkts • Higher Goodput, and • Better Fairness than TCP-RTO TCP-RTO WB-RTO COMputer NETworks Group (COMNET)

  3. Motivation-Contribution • Perspective: • When contention increases, the timeout becomes the scheduler for the link. • Motivation: • When contention is high, all flows measure similar RTTs. • TCP-RTO should not be solely based on RTT measurements. • Congestion events cause retransmission synchronization. • Algorithm: • Approximation of the current level of network contention • Estimation of the contribution of each flow to congestion • Allowance for asynchronous retransmissions when timeout happens. COMputer NETworks Group (COMNET)

  4. Motivation • Queuing Policy: DropTail • DBP = Buffer Size = 10 pkts • 5 participating flows • 1500 sec total simulation time • Flows ideal rate = 2 pkts/wnd • We trace: Seqno progress, RTT, RTO Sequence Number RTT (in secs) Dumbbell Network Topology COMputer NETworks Group (COMNET)

  5. Motivation • For the TCP-RTO performance, we observe: • RTT stabilization • Similar timeout values • ~3000 retransmitted packets in 1500 seconds • Un-Fairness • We investigate the impact of retransmission synchronization on system behavior (e.g. overhead) RTO (in secs) TCP Performance COMputer NETworks Group (COMNET)

  6. Outline of the presentation • The current Retransmission Timeout Algorithm • Recent Related Work • The proposed algorithm: WB-RTO • The algorithm • Expected Behavior • Evaluation Plan • Experimental Results COMputer NETworks Group (COMNET)

  7. The current Retransmission Timeout Algorithm • Upon each ACK arrival, the sender: • Calculates the RTT Variation: • Updates the expected RTT prior to calculating the timeout: • Calculates the Retransmission Timeout value: COMputer NETworks Group (COMNET)

  8. Recent Related Work • EifelRTOAlgorithm • Uses the timestamp option to detect a spurious timeout. • ForwardRTO Algorithm • Uses the first 3 ACKs after the timeout to decide if the timeout was spurious or not. • Peak-Hopper RTO Algorithm • Uses 2 timers: one is aggressive and one is conservative. Each time it decides which one to follow. • CA-RTO: A Contention-Adaptive RTO • Integrates a contention-adaptive parameter and introduces random retransmission scheduling. COMputer NETworks Group (COMNET)

  9. The 3 stages of WB-RTO • 1. Contribution to Congestion • penalty charge • 2. Estimation of Contention • determine the scale of possible values • 3. Calculation of Timeout COMputer NETworks Group (COMNET)

  10. Window-Based RTO (1/4): Proportional Timeout • Estimation of the contribution of the flow to congestion: c = f(cwnd , max cwnd ) • Compare the current cwnd_ with the max_cwnd_: • If cwnd_ < max_cwnd_ / 2, • c = 1: minimal charge • If max_cwnd_ / 2 < cwnd_ < (3/4)* max_cwnd_, • c = 1,5: medium charge • If (3/4)* max_cwnd_ < cwnd_ < max_cwnd_, • c = 2: major charge COMputer NETworks Group (COMNET)

  11. Window-Based RTO (2/4): Contention Estimation • Flow classification according to its cwnd_ history (awnd_): ai = g(awnd , Thresholdi) where, awnd=average window, Thresholds 1 to 4 represent different levels of network contention: • Threshold 1 corresponds to very high contention • Threshold 4 corresponds to low contention • Example: 1. awnd_ < 5: a1 = 10 2. 5 < awnd_ < 10: a2 = 5 3. 10 < awnd_ < 30: a3 = 3 4. 30 < awnd_ < 50: a4 = 2 COMputer NETworks Group (COMNET)

  12. Adjust the scale COMputer NETworks Group (COMNET)

  13. Window-Based RTO (3/4): Timeout Adjustment • Calculation of the Window-Based RTO: WB − RTO = random(rtt, c × ai) or WB − RTO = random(rtt, f(cwnd , max cwnd ) × g(awnd , Thresholdi)) • rtt, to avoid timeout expiration prior to the estimated RTT measurement • Parameter c captures the contribution of the flow to congestion • Parameter a approximates the current level of flow contention • Randomization guarantees asynchronous retransmission attempts COMputer NETworks Group (COMNET)

  14. Window-Based RTO (4/4): Expected Behavior • High penalties result in high timeout values. • As awnd_ increases timeout settles to smaller values. So, • Large windows do not always mean large timeout values. WB-RTO vs awnd_ COMputer NETworks Group (COMNET)

  15. Performance Evaluation Plan • WB-RTO is implemented in TCP-Reno • Evaluation Scenarios • Motivation Part Scenario • Scenario 1: Standard/Proposed Parameters • Scenario 2: Modified Parameters COMputer NETworks Group (COMNET)

  16. An Important Note… • WB-RTO does not improve the Goodput performance of TCP significantly • We focus on concurrent Retransmissions hence • we pay more attention on the combination of the retransmission effort and the Goodput performance, rather than on the Goodput performance alone. COMputer NETworks Group (COMNET)

  17. Scenario 1 • Queuing Policy: DropTail • Bottleneck BW = 10Mbps • Bottleneck Delay = 10ms • Buffer Size = 50 pkts • 1500 sec total simulation time Goodput (in B/s) Retransmitted Packets COMputer NETworks Group (COMNET)

  18. Scenario 1 • Contention grows->WB-RTO allows for better multiplexing • Behavior Captured by fairness index • More flows are getting service Fairness Goodput per Flow (B/s) COMputer NETworks Group (COMNET)

  19. Scenario 2 • Queuing Policy: DropTail • Bottleneck BW = 10Mbps • Bottleneck Delay = 10ms • Buffer Size = 50 pkts • 1500 sec total simulation time • Scale now different Goodput (in B/s) Retransmitted Packets COMputer NETworks Group (COMNET)

  20. Scenario 2 • Fairness initially drops when the scale is not adjusted appropriately Fairness Goodput per Flow (B/s) COMputer NETworks Group (COMNET)

  21. Interaction with AQM (i.e. RED) (1) • Topology: Dumbbell • Queuing Policy: RED • DBP = Buffer Size = 40 pkts • min_thresh = 4 pkts • max_thresh = 12 pkts • 1500 sec total simulation time Goodput (in B/s) Retransmitted Packets Number of Timeouts COMputer NETworks Group (COMNET)

  22. Interaction with AQM (i.e. RED) (2) • We observe: • Similar Goodput • Significant difference in Retransmission Effort (50%) • WB-RTO results in 66% less timeout expirations • TCP-RTO causes inefficient queue utilization • The average queue length always overcomes the max_thresh, when using TCP-RTO TCP-RTO WB-RTO COMputer NETworks Group (COMNET)

  23. Satellite Scenario (1) • Topology: Cross-Traffic • Bottleneck Queuing Policy: RED • The rest of the buffers use DT • bw_bottleneck = 20Mbps • bw_delay = 300ms • Buffer Size = 200 pkts • min_thresh = 20 pkts • max_thresh = 60 pkts • 150 sec total simulation time • PER = 0,0001 • 3 blackouts on the backbone link No blackout After 3 blackouts COMputer NETworks Group (COMNET)

  24. Satellite Scenario (2) • We observe that: • TCP-RTO interprets the blackout as a congestion signal • WB-RTO does not extend the timeout, due to low contention and hence exploits bandwidth faster • TCP still waits for the extended timeout to expire, while • WB-RTO resumes transmission immediately TCP-RTO WB-RTO COMputer NETworks Group (COMNET)

  25. Traffic Diversity (Mice and Elephants) (1) • Topology: Dumbbell • Bottleneck Queuing Policy: RED • bw_bottleneck = 10Mbps • bw_delay = 30ms • Buffer Size = 40 pkts Goodput (KB/s) Goodput per flow (KB/s) Retransmitted Packets COMputer NETworks Group (COMNET)

  26. Traffic Diversity (Mice and Elephants) (2) • We observe: • Simultaneous timeout events for TCP-RTO • All flows timeout during the Slow-Start • Flows 7-9 timeout simultaneously 10 times during the experiment • Short flows: 83 vs 50 timeouts • Long flows: 43 vs 12 timeouts • We conclude that: • most of the timeouts are spurious • WB-RTO achieves an important goal: it reduces the number of timeouts TCP-RTO WB-RTO COMputer NETworks Group (COMNET)

  27. Conclusions • RTT measurements cannot always reflect the level of network contention • TCP-RTO should not be solely based on RTT samples • A contention-aware RTO proves to be more efficient, since it is aware of current network conditions. • A randomization factor in the RTO schedules retransmissions in a fairer manner • WB-RTO cancels some of TCP miss-responses with non-congestion errors COMputer NETworks Group (COMNET)

  28. Other References [1] “CA-RTO: A Contention-Adaptive Retransmission Timeout for TCP”, Ioannis Psaras, Vassilis Tsaoussidis, IEEE ICCCN 05 [2] “Why TCP Timers (still) Don’t Work Well”, Ioannis Psaras, Vassilis Tsaoussidis, Computer Networks (COMNET), Elsevier Science, to appear 2007 http://comnet.ee.duth.gr/comnet/ COMputer NETworks Group (COMNET)

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