BIC & CUBIC

CUBIC

BIC & CUBICAhmed El-HassanyCISC856: CISC 856 TCP/IP and Upper Layer ProtocolsSlides adopted from: Injong Rhee, Lisong Xu

AgendaWhat?Why?How?Can we do better?Source: TCP/IP Protocol Suite 4th3Congestion example

AIMD (Additive Increase Multiplicative Decrease)AIMD increases cwnd by a larger number, say 32, instead of 1 per RTT.After a packet loss, AIMD decreases cwnd by 1/8, instead of 1/24Packet lossTime (RTT)Slow startCongestion avoidancePacket lossPacket losscwndPacket losscwnd = cwnd + 1

cwnd = cwnd + 32cwnd = cwnd * (1-1/2)

cwnd = cwnd * (1-1/8)TCP4WhatHow much time is needed increase cwnd of a 10Gbps from half utilization to full utilization?1500-byte PDU100 ms RTT

Full utilization cwnd = 10Gbps/1500byte ~=83333Half utilization cwnd = 83333/2 = 41666.5Remember cwnd is increased by 1 for each RTT41667 RTT is needed to fully utilized the link41667 RTT * 100ms(RTT time) = 69.44minutesSLOWWhat

x msy1 msy2 msz1 msz2 msy1 + x + z1 >> y2 + x + z2UnfairDefinitionsBandwidth-Delay Product: Maximum amount of data on the network circuit at any given timeLinks capacity (bytes/sec)* end-to-end delay (sec)TCP Fairness: a new protocol receive no larger share of the network than a comparable TCP flow.TCP-friendliness: defines whether a protocol is being fair to TCP.WhatTCP slowly increases its congestion window (cwnd) for every RTT.TCP reduces cwnd by half at a lost event.

Response Function of TCPResponse function of TCP is the average throughput of a TCP connection in terms of the packet loss probability, the packet size, and the round-trip time.9

R : Average ThroughputMSS: Packet SizeRTT: Round-Trip TimeP : Packet Loss ProbabilityResponse Function of TCP is :J. Padhye, V. Firoio, D. Towsley, J. Kurose, "Modeling TCP Throughput: a Simple Model and its Empirical Validation", Proceedings of SIGCOMM 989Usually, we study the performance of TCP using its response function, which is defined as the average throughput of TCP in terms of packet loss probability.

Here is the response function of TCP. In this function, MSS is the packet size, RTT is the round trip time, and p is the packet loss probability. From this function, we can see that the throughput of TCP depends on the packet size, the round-trip time, and the packet loss probability. In order to increase the throughput of TCP, we can either increase the packet size, or decrease the round trip time, or decrease the packet loss probability. Usually for a TCP connection, the packet size and round-trip time are fixed. Therefore, the throughput of a TCP connection mainly depends on the packet loss probability.Why (Response Function of TCP)

1010Gbps requires a packet loss rate of 10-10, or correspondingly a link bit error rate of at most 10-14, which is an unrealistic (or at least hard) requirement for current networksUsing Log-Log scale, Assuming 1250-Byte packet size, and 100ms RTT10In this figure, we show how the throughput changes as we vary the packet loss probability. Note that the figure is shown in log-log scale. We can see that as the packet loss probability decreases, the TCP throughput increases. Assume the packet size is 1000 Bytes, and the RTT is 100ms, then TCP achieves the 10Gbps throughput when the packet loss probability is around 10-10.which corresponds to a link BER of 10-14. which is an unrealistic requirement for current networks. That is, in the current networks, it is impossible for TCP to achieve 10Gbps throughput.A Search ProblemA Search ProblemBIC consider the increase part of congestion avoidance as a search problem, in which a connection looks for the available bandwidth by comparing its current throughput with the available bandwidth, and adjusting cwnd accordingly. 11How does TCP find the available bandwidth?Linear searchwhile (no packet loss){cwnd++;}Q: How to compare R with A?R = current throughput = cwnd/RTTA = available bandwidthA: Check for packet lossesNo packet loss: R A11Linear Search

12Available Bandwidth12BIC: Binary Search with Smax and Smin13BIC - Binary searchwhile (Wmin Smax) inc = Smax;else if (inc < Smin) inc = Smin;cwnd = cwnd + inc;if (no packet losses) Wmin = cwnd;else break;}Wmax: Max WindowWmin: Min WindowSmax: Max IncrementSmin: Min Increment13Binary Search with Smax and Smin

14SminSmaxWmaxWminAvailable Bandwidth14Setting Smax

15Response Function of BIC on high-speed networks

Bandwidth scalability of BIC depends only on SmaxRTT Fairness of BIC on high-speed networks is the same as that of AIMDBandwidth scalabilitySetting Smin

16Response Function of BIC on low-speed networks

TCP-friendliness of BIC depends only on SminTCP friendlinessResponse Functions

17Bandwidth scalabilityRTT FairnessTCP-Friendliness17Finally, here is the response function of BIC. Which has a good scalability, a good TCP-friendliness, and a good RTT Fairness in high-speed networks.In Summary BIC function18

BIC overall performs very well in evaluation of advanced TCP stacks on fast long-distance production.

BIC (also HSTCP & STCP) growth function can be still aggressive for TCP especially under short RTTs or low speed networks.Thinking Out of The BoxMake window size growth function independent from RTT.Use the elapsed real-time since the last loss event.GoalsTCP friendlyRTT fairCo-existing flows with different RTTs are treated fairlyEfficient use of available bandwidthThe CUBIC function20

where C is a scaling factor, t is the elapsed time from the last window reduction, and is a constant multiplication decrease factoraccelerateaccelerateslow downCUBIC AlgorithmIf (received ACK && state == cong avoid)Compute Wcubic(t+RTT).If cwnd < WTCP CUBIC in TCP mode If cwnd < Wmax CUBIC in concave regionIf cwnd > Wmax CUBIC in convex region TCP ModeDetailed derivation of this equation is in Section 3.3 in CUBIC paper

Concave RegionDetailed derivation of this equation is in Section 3.4 in CUBIC paper

Convex Regioncwnd > WmaxNew bandwidth might be availableUse the same window growth function.

Detailed derivation of this equation is in Section 3.5 in CUBIC paper

Packet Loss EventIf cwnd < Wmax and fast_convergenceWmax = cwnd * ((2-)/2)Else:Wmax = cwnd

ssthread = cwnd = cwnd*(1-)Testbed (Dummynet) Setup26Router 1

Sender 1

Sender 2

Background TrafficGenerator 1

Router 2

Receiver

Background TrafficGenerator 2

FreeBSD

Linux

Bottleneck Point : 800 MbpsSetting RTT for each path between Senders and Receiver

RTT for Background Traffic : Exponential Distribution (Next Slide)1 Gbps linkBackground Traffic Generation (Next Slide)Background Traffic Generation (Next Slide)High-Speed TCP Variants :e.g. CUBIC, BIC, FAST, HSTCP, STCPHigh-Speed TCPor TCP SACK27

TCP Friendliness on short RTT - 5ms80 Mbps200 MbpsBackground trafficLink Utilization (%) Dummynet Testbed : RTT 5ms & 800 Mbps, 100% router buffer of the BDP with 80 ~ 200 Mbps background trafficTCP Friendliness (cont.)TCP Friendliness (cont.)28

TCP Friendliness on short RTT - 10ms80 Mbps200 MbpsBackground trafficLink Utilization (%) Dummynet Testbed : RTT 10ms & 800 Mbps, 100% router buffer of the BDP with 80 ~ 200 Mbps background trafficTCP Friendliness (cont.)29

TCP Friendliness on long RTT - 100ms80 Mbps200 MbpsBackground trafficLink Utilization (%) Dummynet Testbed : RTT 100ms & 800 Mbps, 100% router buffer of the BDP with 80 ~ 200 Mbps background trafficTCP Friendliness (cont.)30

TCP Friendliness on long RTT - 200ms80 Mbps200 MbpsBackground trafficLink Utilization (%) Dummynet Testbed : RTT 200ms & 800 Mbps, 100% router buffer of the BDP with 80 ~ 200 Mbps background trafficRTT Fairness31 Dummynet testbed : RTT 40, 120, 240 ms & 800 Mbps, Router buffer: 50% of the BDP with 200 Mbps background traffic

Can We do better??ReferencesSangtae Ha, Injong Rhee, and Lisong Xu. CUBIC: a new TCP-friendly high-speed TCP variant. SIGOPS Oper. Syst. Rev. 42, 5 (July 2008)

Lisong Xu, Khaled Harfoush, and Injong Rhee,Binary Increase Congestion Control for Fast, Long Distance Networks,Infocom,IEEE, 2004

Chart1338.98314.4378.1868.4530.93138.9475.9354.84478.71182.9577.9364.41456.37198.8779.0165.75562.25111.6372.3153.810000212.79185.03161.07241.11334.06124139.6202.35348.2198.99138.91213.89263.99159.49138.23238.29357.7109.26149.57183.48

High-SpeedTCPBackgroundUnused

Sheet150% queue size5ms200ms100ms10msCUBIC235.8588622.5350.1225.85438.6250.20.9575911790.09183673470.06200803210.7146529563BIC436.4574.9631.2591142.576.629.270.70.32653528870.13324056360.046261090.1196277496FAST457.7?131.2528.9104.2647.4535.8125.50.227660039304.08384146340.237284931HSTCP323.7450.3570.7496176.8188.583.4123.50.5461847390.41860981570.14613632380.2489919355STCP487.6525600.6623.6128?11.845.20.26251025430.00190476190.01964701960.0724823605CUBIC235.8225.80.960.06BIC436.4142.50.330.04FAST457.7104.20.234.08HSTCP323.7176.80.550.15STCP487.61280.260.01CUBIC622.538.6BIC631.229.2FAST131.2535.8HSTCP570.783.4STCP600.611.8100 % queue5ms10ms100ms200msCUBICHigh-Speed338.98433.61604.7557.68Ttcp314.43231.8356.2888.14Bgtraffic78.1876.8174.2568.25Unused68.457.7564.7685.93BICHigh-Speed530.93612.94652.63532.99Ttcp138.9455.6819.05130.17Bgtraffic75.9375.7168.1968.08Unused54.8455.6760.1368.76FASTHigh-Speed478.71367.2711.3510.28Ttcp182.95299.66655.18658.4Bgtraffic77.9378.8875.0269.78Unused64.4154.1958.4561.53HSTCPHigh-Speed456.37544.5639.09529.14Ttcp198.87120.2227.04134.69Bgtraffic79.0179.6371.4670.27Unused65.7555.6562.465.89STCPHigh-Speed562.25636.15571.66495.37Ttcp111.6334.7113.4418.98Bgtraffic72.3174.4669.9460.3Unused53.8154.69144.95225.35High-SpeedTCPBackgroundUnused5msCUBIC338.98314.4378.1868.4BIC530.93138.9475.9354.84FAST478.71182.9577.9364.41HSTCP456.37198.8779.0165.75STCP562.25111.6372.3153.8110msCUBIC433.61231.8376.8157.75BIC612.9455.6875.7155.67FAST367.27299.6678.8854.19HSTCP544.5120.2279.6355.65STCP636.1534.7174.4654.69100msCUBIC604.756.2874.2564.76BIC652.6319.0568.1960.13FAST11.35655.1875.0258.45HSTCP639.0927.0471.4662.4STCP571.6613.4469.94144.95CUBIC557.6888.1468.2585.93200msBIC532.99130.1768.0868.76FAST10.28658.469.7861.53HSTCP529.14134.6970.2765.89STCP495.3718.9860.3225.35

Sheet2200Mbps50% queue5msHigh-SpeedTCPBackgroundUnusedCUBIC176.33139.73174.52309.42BIC305.62118.81133.76241.81FAST366.0296.46117.17220.36HSTCP204.28128.56177290.17STCP327.52113.07149.06210.3410msHigh-SpeedTCPBackgroundUnusedCUBIC229.62182.34203.12184.92BIC423.0285.68199.2592.06FAST444.0572.16160.94122.86HSTCP318.52129.66197.66154.15STCP456.5572.15192.0279.28100msHigh-SpeedTCPBackgroundUnusedCUBIC495.4449.6191.6263.35BIC531.0128.54180.559.95FAST73.33459.52201.6265.53HSTCP489.9147.63203.159.36STCP517.7428.47179.274.6200msHigh-SpeedTCPBackgroundUnusedCUBIC512.2551.98166.4369.34BIC477.3673.66184.1264.85FAST131.5396.13209.3463.02HSTCP500.2647.25187.5264.97STCP551.114.74135.498.77100% queueHigh-SpeedTCPBackgroundUnused5msCUBIC212.79185.03161.07241.11BIC334.06124139.6202.35FAST348.2198.99138.91213.89HSTCP263.99159.49138.23238.29STCP357.7109.26149.57183.48CUBIC258.42173.37184.18184.0410msBIC377.7568.49177.42176.34FAST313.09135.84171.4179.67HSTCP324.52108.72183.66182.1STCP408.1550.51165.43175.89CUBIC390.5460.46168.16180.84100msBIC434.2827.89161.56176.27FAST12.43546.9556.54184.08HSTCP428.7624.76169.12177.35STCP431.089.7159.2200.01CUBIC402.1454.31154.45189.1200msBIC404.1455.2154.1186.57FAST7.7458.7157.33176.28HSTCP371.9178.81164.56184.72STCP393.3611.51131.76263.37

Sheet35msHigh-SpeedTCPBackgroundUnusedCUBIC338.98314.4378.1868.4BIC530.93138.9475.9354.84FAST478.71182.9577.9364.41HSTCP456.37198.8779.0165.75STCP562.25111.6372.3153.81High-SpeedTCPBackgroundUnusedCUBIC212.79185.03161.07241.11BIC334.06124139.6202.35FAST348.2198.99138.91213.89HSTCP263.99159.49138.23238.29STCP357.7109.26149.57183.4810msHigh-SpeedTCPBackgroundUnusedCUBIC433.61231.8376.8157.75BIC612.9455.6875.7155.67FAST367.27299.6678.8854.19HSTCP544.5120.2279.6355.65STCP636.1534.7174.4654.69CUBIC258.42173.37184.18184.04BIC377.7568.49177.42176.34FAST313.09135.84171.4179.67HSTCP324.52108.72183.66182.1STCP408.1550.51165.43175.89100msHigh-SpeedTCPBackgroundUnusedCUBIC604.756.2874.2564.76BIC652.6319.0568.1960.13FAST11.35655.1875.0258.45HSTCP639.0927.0471.4662.4STCP571.6613.4469.94144.95CUBIC390.5460.46168.16180.84BIC434.2827.89161.56176.27FAST12.43546.9556.54184.08HSTCP428.7624.76169.12177.35STCP431.089.7159.2200.01200msHigh-SpeedTCPBackgroundUnusedCUBIC557.6888.1468.2585.93BIC532.99130.1768.0868.76FAST10.28658.469.7861.53HSTCP529.14134.6970.2765.89STCP495.3718.9860.3225.35CUBIC402.1454.31154.45189.1BIC404.1455.2154.1186.57FAST7.7458.7157.33176.28HSTCP371.9178.81164.56184.72STCP393.3611.51131.76263.37

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BIC & CUBIC

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