CUBIC Marcos Vieira Agenda n n Brief Introduction

CUBIC Marcos Vieira

Agenda n n Brief Introduction of CUBIC Prehistory of CUBIC q q n n Standard TCP BIC CUBIC Conclusion 2

Brief Introduction n CUBIC is a less aggressive and more systematic derivative of BIC, in which the window is a cubic function of time since the last congestion event, with the inflection point set to the window prior to the event. 3

Why do we need CUBIC-TCP? n Compares to: q q Standard TCP BIC-TCP 4

Standard TCP n Underutilization of the bandwidth in High-Speed Network 1 Link Utilization 0. 9 155622 0. 8 0. 7 Cannot fully utilize the huge capacity of highspeed networks! 2500 0. 6 5000 0. 5 0. 4 10000 0. 3 0 2000 4000 6000 Link Capacity (Mbps) 8000 10000 NS-2 Simulation (100 sec) l Link Capacity = 155 Mbps, 622 Mbps, 2. 5 Gbps, 10 Gbps, l Drop-Tail Routers, 0. 1 BDP Buffer l 5 TCP Connections, 100 ms RTT, 1000 -Byte Packet Size Presentation: "Congestion Control on High-Speed Networks”, Injong Rhee, Lisong Xu, Slide 6 5

Standard TCP n Low window size resilience to packet loss in High-Speed Network 1. 4 hours Packet loss cwnd 1. 4 hours Packet loss 100, 000 50, 000 Slow start 1. 4 hours TCP Packet loss 10 Gbps Slow Increase cwnd = cwnd + 1 Fast Decrease cwnd = cwnd * 0. 5 Congestion avoidance 5 Gbps Time (RTT) Presentation: "Congestion Control on High-Speed Networks”, Injong Rhee, Lisong Xu, Slide 7 6

Why BIC? n n Existing schemes have a severe RTT unfairness problem RTT unfairness for high-speed networks occurs distinctly with drop tail routers for flows with large congestion windows where packet loss can be highly synchronized. 7

BIC n “Binary Increase Congestion Control (BIC) for Fast Long. Distance Networks”, Lisong Xu, Khaled Harfoush, and Injong Rhee, IEEE INFOCOM 2004 8

BIC Algorithm n If cwnd < low_window, normal TCP: q Else, BIC ACK received n q n cwnd = cwnd + 1 Enter recovery n cwnd = cwnd * 0. 5 9

BIC Algorithm n ACK received q If cwnd < Wmax n q Recovery q cwnd += (Wmax – cwnd) / 2 Else n n If cwnd < Wmax n q cwnd += cwnd - Wmax Else n q Wmax = cwnd * (1 – ß / 2) Wmax = cwnd *= 1 - ß Smin <= cwnd && cwnd <=Smax 10

BIC with no lost Smin 256 224 192 cwnd 160 Linear Search 128 96 64 32 Smax Binary Search with Smax and Smin 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Time (RTT) 11

Why CUBIC? n n n Window control of BIC is so complex! BIC’s growth function can still be too aggressive for TCP, especially under short RTT or low speed networks. BIC still has room for improving TCP-friendliness and RTTfairness! 12

Window Growth Function CUBIC "CUBIC: A New TCP-Friendly High-Speed TCP Variant", Injong Rhee, and Lisong Xu 13

RTT fairness n n CUBIC is RTT fair. How? Its window growth function is defined in real-time so that its growth will be independent of RTT. t: elapsed time from the last window reduction W Different RTT flows will have the same t after a synchronized packet loss. 14

CUBIC Algorithm n ACK received q q C is a scaling factor t is the elapsed time from the last window reduction cwnd cannot be window less Wmax is the size than just before the last window reduction K to is keep updated at the time as the growth same as ofrate lastthelost event standard TCP in short RTT networks. n Recovery q Update K with: q Update Wmax with: q β is a constant multiplication decrease factor 15

CUBIC window curves with competing flows (NS simulation in a network with 500 Mbps and 100 ms RTT), C = 0. 4, β = 0. 8. 16

Stability 4 flows of a high-speed TCP variant over a long-RTT network path (~220 ms) 4 flows of long-term TCP-SACK flows over a short-RTT path (~20 ms) "CUBIC: A New TCP-Friendly High-Speed TCP Variant", Injong Rhee, and Lisong Xu 17

Coefficient of Variation (Co. V) n n n “There is no well-defined metric of stability. ” “Often the Co. V of transmission rates are used to depict stability. ” “For a less satisfactory measure, we plotted the Co. V of throughput. ” "CUBIC: A New TCP-Friendly High-Speed TCP Variant", Injong Rhee, and Lisong Xu 18

Co. V - 20% BDP "CUBIC: A New TCP-Friendly High-Speed TCP Variant", Injong Rhee, and Lisong Xu 19

Co. V - 200% BDP "CUBIC: A New TCP-Friendly High-Speed TCP Variant", Injong Rhee, and Lisong Xu 20

Thanks