CA-RTO: A Contention- Adaptive Retransmission Timeout I. Psaras, V. Tsaoussidis, L. Mamatas Demokritos University of Thrace, Xanthi, Greece This study was presented in the International Conference on Computer Communications and Networks (ICCCN 2005)

COMputer NETworks Group (COMNET)2 Contributions of this work Our perspective: When contention increases, the timeout becomes the scheduler for the link Our observations: When contention increases, timeout decreases! Congestion events cause synchronization, at least to some extend. Our solutions: We integrate a contention adaptive parameter into the timeout algorithm  Unnecessary retransmissions are reduced We introduce randomness to avoid synchronization

COMputer NETworks Group (COMNET)3 Research Steps We investigated the timeout behavior towards various types of contention We observed that when contention increases, RTO undertakes the role of the transmission scheduler for the link We focused on scenarios with high contention RTT can not always capture efficiently the network conditions RTO is a bad scheduler for the link We fixed the timeout value to correspond to different contention levels We concluded that different levels of contention call for distinct timeout adjustments We ran simulation results

COMputer NETworks Group (COMNET)4 TCP Retransmission Timeout The timer is adaptive to varying delay The timeout is calculated every RTT Given a sample RTT measurement M and the history of average RTT A,  the distance from the average is measured: Diff = M – A,  the average is updated: A = A + gDiff (where g=0.125),  the RTT Deviation is calculated: Dev = Dev +d(|Diff| – Dev) (where d = 0.25)  and finally, the RTO value is adjusted to: RTO = A + 4D

COMputer NETworks Group (COMNET)5 Observations: Anomalies of the TCP Retransmission Timer SCENARIO Setup  Dumbbell Topology  DropTail: 50 pkts  BxD = 10 packets  Contention Increase  0 – 250s: 1 flow  250 – 500s: 100 flows

COMputer NETworks Group (COMNET)6 Contention Grows, Timeout Shrinks Contention Increase Scenario RTT RTO = A + 4D

COMputer NETworks Group (COMNET)7 Contention Grows, Timeout Shrinks Contention Increase Scenario RTO = A+4D

COMputer NETworks Group (COMNET)8 Contention Grows, Timeout Shrinks

COMputer NETworks Group (COMNET)9 Contention Grows, Timeout Shrinks In this case: 4D  0 Hence, RTO = A 1. Timeout Decreases instead of Increasing (there is no Deviation) 2. Smoothed RTT (A) does not differentiate between different flows 3. Synchronization is possible (although buffers are always full) 4. Fairness is not guaranteed

COMputer NETworks Group (COMNET)10 The Proposed Algorithm (CA-RTO) CA-RTO: RTO = A + 4D + c*p We incorporate contention:  c = 1/cwnd_  cont_diff_ = max_cwnd_ - cwnd_  cont_diff_ = cont_diff_ / 100 We introduce retransmission randomness:  p = Random(0, cont_diff_) CA-RTO: RTO = A + 4D + c*p

COMputer NETworks Group (COMNET)11 Behavior of the Proposed Algorithm (CA-RTO) The max_cwnd_ ever reached is 200pkts Step 1: c = 1/cwnd_ Step 2: cont_diff_ = max_cwnd_ - cwnd_ cont_diff = cont_diff_/100

COMputer NETworks Group (COMNET)12 Behavior of the Proposed Algorithm (CA-RTO) Step 3: p = Random(0, cont_diff_) Finally: CA-RTO = RTO + c*p

COMputer NETworks Group (COMNET)13 Behavior of the Proposed Algorithm (CA-RTO) Contention Increase Scenario CA-RTO = RTO+c*p

COMputer NETworks Group (COMNET)14 Behavior of the Proposed Algorithm (CA-RTO) Small congestion window: gives big value to parameter c c = 1/cwnd_ may result in big cont_diff_ cont_diff_ = max_cwnd_ - cwnd_ Big congestion window: gives small value to parameter c results in small cont_diff_ We do not want to affect RTO’s performance in low contention scenaria We try to capture high contention We get aware of dynamic network conditions, e.g. contention increase

COMputer NETworks Group (COMNET)15 Possible Further Enhancements The algorithm may “punish” flows with small windows by extending their RTO value (e.g. during startup)  maybe a parameter indicating the history of cwnd_ has to be integrated instead of the current cwnd_ The current mechanism used to capture contention may not be very accurate The randomization factor used to split flows in time can be further improved

COMputer NETworks Group (COMNET)16 Evaluation Methodology We target high contented links/networks We simulate large numbers of flows transmitting in low capacity channels Hence:  Fair-share is small  Flows are operating with small windows  Buffers are always full

COMputer NETworks Group (COMNET)17 Evaluation Methodology We use the dumbbell network topology Bandwidth x Delay = 10 or 100 packets We use both DropTail and RED queuing policies We implement CA-RTO in TCP- Reno and compare the two versions We measure Goodput, Throughput, Fairness and Number of Retransmitted Packets

COMputer NETworks Group (COMNET)18 Experimental Results Scenario 1 B x D = 10 packets Buffer size = 50 packets, DropTail Fairness Retransmitted Packets (up to 0.2 Index Points) (up to 4000 less retransmissions)

COMputer NETworks Group (COMNET)19 Experimental Results Scenario 1 Throughput (in Bps) Goodput (in Bps)

COMputer NETworks Group (COMNET)20 Experimental Results Scenario 2 B x D = 100 packets Buffer size = 100 packets, DropTail Fairness Retransmitted Packets (at least 0.15 Index Points) (25 % less retransmissions, 4500pkts)

COMputer NETworks Group (COMNET)21 Experimental Results Scenario 3 bw_bb = 100Mbps, B x D = 250 pkts Buffer size = 100 packets, DropTail CA-RTO affects TCP’s performance only if needed… Goodput

COMputer NETworks Group (COMNET)22 Experimental Results Scenario 4 B x D = 10 packets Buffer size = 50 packets, DropTail Packet Error Rate: 10% Fairness Index Retransmitted Packets

COMputer NETworks Group (COMNET)23 Conclusions Current RTO presents some behavioral anomalies when contention increases A Contention-Adaptive RTO proves to be more efficient in terms of successful retransmissions. That calls for further investigation of the energy potential of CA-RTO A Randomization Factor in the RTO schedules the participating flows in a more fair manner

COMputer NETworks Group (COMNET)24 CA-RTO: A Contention-Adaptive Retransmission Timeout Thank you!!