BIC Control for Fast Long-Distance Networks paper written by Injong Rhee, Lisong Xu & Khaled Harfoush (2004) Presented by Jonathan di Costanzo (2009/02/18)

Outline Motivation ◦Origin of the problem ◦Response Function of TCP Existing protocols BIC-TCP Conclusion 2/23

Origin of the problem – TCP strengh Extremely reliable (ACK notification) Many high-speed networks (>10Gbps) 90% of datatransmission 3/2 3 NL SURFnet GENEVA UK SuperJANET4 ABILENE ESNET CALREN It GARR-B GEANT NewYork Fr Renater STAR-TAP STARLIGHT

Origin of the problem – TCP Weakness 4/23 The instantaneous throughput of TCP is controlled by a variable cwnd, TCP transmits approximately a cwnd number of packets per RTT (Round-Trip Time). Time (RTT)Slow startCongestion avoidance Packet loss cwnd Packet loss TCP cwnd = cwnd + 1 cwnd = cwnd * (1-1/2)

Origin of the problem – TCP Weakness 5 Packet loss Time (RTT)Congestion avoidance Packet loss cwnd Slow start Packet loss A TCP connection with 1250-Byte packet size and 100ms RTT is running over a 10Gbps link (assuming no other connections, and no buffers at routers) 100,00010Gbps 50,0005Gbps 1.4 hours TCP big decrease slow increase

Response Function 6/23 Response 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. R : Average Throughput MSS: Packet Size RTT: Round-Trip Time P : Packet Loss Probability Response Function of TCP is :

Response Function 7 10Gbps requires a packet loss rate of , or correspondingly a link bit error rate of at most , Assuming 1250-Byte packet size, and 100ms RTT

Outline Motivation Existing protocols ◦AIMD ◦HSTCP ◦STCP BIC-TCP Conclusion 8/23

High-Speed Protocols A new protocol is needed ◦More aggressive about increasing transmission speeds (scalability) ◦Able to work simultaneously with TCP Advanced TCP ◦AIMD (Additive Increase Multiplicative Decrease) ◦HSTCP (High-Speed TCP) ◦STCP (Scalable TCP) 9/23

AIMD 10 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 1/2 Packet loss Time (RTT)Slow startCongestion avoidance Packet loss cwnd Packet loss cwnd = cwnd + 1 cwnd = cwnd + 32 cwnd = cwnd * (1-1/2) cwnd = cwnd * (1-1/8) TCP

AIMD – Response Function 11 TCP: AIMD: The throughput of AIMD is always about 13 times larger than that of TCP

AIMD – Response Function 12 Bandwidth Scalable Bandwidth Scalability The ability to achieve 10Gbps with a reasonable packet loss probability NOT TCP Friendly TCP-Friendliness The ability to share bandwidth with TCP connections on low-speed networks

STCP & HSTCP – RTT unfairness RTT unfairness between 2 flows Numerical definition : = ratio between the average throughputs 13/23

STCP 14 STCP adaptively increases cwnd, and decreases cwnd by 1/8. Packet loss Time (RTT)Slow startCongestion avoidance Packet loss cwnd Packet loss cwnd = cwnd + 1 cwnd = cwnd *cwnd cwnd = cwnd * (1-1/2) cwnd = cwnd * (1-1/8) TCP

HSTCP 15 HSTCP adaptively increases cwnd, and adaptively decreases cwnd. The larger the cwnd, the larger the increment, and the smaller the decrement. Packet loss Time (RTT)Slow startCongestion avoidance Packet loss cwnd Packet loss cwnd = cwnd * (1-1/2) cwnd = cwnd * (1-dec(cwnd)) cwnd = cwnd + 1 cwnd = cwnd + inc(cwnd) TCP

STCP & HSTCP – Response Functions 16 HSTCP: STCP: Bandwidth Scalable TCP Friendly HSTCP and STCP are both bandwidth scalable and TCP friendly

Outline Motivation Existing protocols BIC-TCP ◦RTT fairness ◦Protocol design ◦Binary increase search ◦Response Function / RTT fairness Conclusion 17/23

RTT fairness ◦Differents session  different RTT  We want a fair bandwidth allocation ◦RTT fairness index = throughout ratio of two flows with ≠ RTTs 18

RTT fairness 19 For a protocol with the following response function, where c and d are protocol-related constants. The RTT Fairness Index (or the throughput ratio of two flows) networks is For low-speed networks For high-speed networks ◦Low speed networks  same RTT fairness ◦High speed networks  same RTT fairness

RTT fairness General response function ◦  RTT fairness 20

RTT fairness  Throughout ratio of two flows on a 2.5Gbps Link  When the network capacity increases, the RTT fairness become worse 21 Inverse RTT Ratio136 AIMD HSTCP STCP best worse

STCP & HSTCP – RTT unfairness 22/23 80ms 160ms

Protocol Design 23 TCP Fairness Scalability, RTT Fairness

24 BIC adaptively increase cwnd, and decrease cwnd by 1/8 Packet loss Time (RTT)Slow startCongestion avoidance Packet loss cwnd Packet loss cwnd = cwnd + 1 cwnd = cwnd + f(cwnd, history) cwnd = cwnd * (1-1/2) cwnd = cwnd * (1-1/8) TCP

Binary increase search ◦The problem is to know how to adjust the cwnd  Throughput (cwnd/RTT) vs. Available bandwidth ◦Linear search ? 25 Available Bandwidth

Binary increase search ◦The problem is to know how to adjust the cwnd  Throughput (cwnd/RTT) vs. Available bandwidth ◦Binary search ? 26 Smin Smax Wmax Wmin inc = (Wmin+Wmax)/2 – cwnd Smin > inc > Smax cwnd = cwnd + inc

Binary increase search 27

Response Function 28 Bandwidth scalability RTT Fairness TCP-Friendliness

RTT fairness 29 Inverse RTT Ratio136 BIC11238 AIMD1622 HSTCP STCP Throughput ratio of two flows with different RTTs on a 2.5Gbps link

Outline Motivation Existing protocols BIC-TCP Conclusion 30/23

Conclusion Quick summary 31 AIMDHSTCPSTCPBIC Scalability  TCP-Friendliness  RTT Fairness 

Conclusion Further works : CUBIC 32

Any questions ? 33/23