FAST TCP Anwis Das Ajay Gulati Slides adapted from : IETF presentation slides Link:

Background  Last time we saw reliable communication requires a transport level protocol  But user’s are selfish, leading to congestion  Van Jacobson solved the problem by adapting TCP Slow Start Congestion Avoidance Accurate Estimation of RTT, etc  Is the solution still valid considering today’s technology??

Hypothesis  TCP Reno is a good solution for low speed networks but not a viable solution for high-speed networks Too conservative, not stable, and requires extremely small equilibrium loss probability  There exists an equation that governs the flow of all variants of TCP FAST provides a solution to this equation and shows that it is stable and reaches equilibrium quickly

Philosophy of Reno  Packet level Designed and implemented first  Flow level Understood afterwards  Flow level dynamics determines Equilibrium: performance, fairness Stability  Design flow level equilibrium & stability  Implement flow level goals at packet level

Problems with TCP Reno  Equilibrium problem Packet level: AI too slow, MI too drastic Flow level: required loss probability too small  Dynamic problem Packet level: must oscillate on binary signal Flow level: unstable at large window

TCP Flow Dynamics ACK: W  W + 1/W Loss: W  W – 0.5W  Packet level  Flow level dynamics Reno TCP pkts

TCP Variant Performance ACK: W  W + 1/W Loss: W  W – 0.5W  Reno: AIMD (1, 0.5) ACK: W  W + a(w)/W Loss: W  W – b(w)W ACK: W  W Loss: W  W – 0.125W  HSTCP: AIMD (a(w), b(w))  STCP : MIMD (1/100, 1/8)

Flow level: Reno, HSTCP, STCP, FAST  Different gain  and utility U i They determine equilibrium and stability  Different congestion measure p i Loss probability (Reno, HSTCP, STCP) Queueing delay (Vegas, FAST)  Generic flow level dynamics! window adjustment control gain flow level goal =

Problem: binary signal TCP oscillation

Solution: multibit signal FAST stabilized

FAST TCP Dynamics  Generic equation: window adjustment control gain flow level goal =  FAST TCP equation:  Can map generic equation to FAST TCP equation and understand how FAST TCP stabilizes faster than other TCP variants

Window control algorithm  Full utilization regardless of bandwidth-delay product  Globally stable exponential convergence  Fairness weighted proportional fairness parameter 

Window control algorithm Theorem (Jin, Wei, Low ‘03) In absence of delay  Mapping from w(t) to w(t+1) is contraction  Global exponential convergence  Full utilization after finite time  Utility function:  i log x i (proportional fairness)

Dynamic sharing: 3 flows FASTLinux HSTCPSTCP Steady throughput

Dynamic sharing Dynamic sharing on Dummynet  capacity = 800Mbps  delay=120ms  3 flows  iperf throughput  Linux 2.4.x (HSTCP: UCL)

FASTLinux throughput loss queue STCPHSTCP Dynamic sharing on Dummynet  capacity = 800Mbps  delay=120ms  14 flows  iperf throughput  Linux 2.4.x (HSTCP: UCL) 30min

FASTLinux throughput loss queue STCPHSTCP 30min HSTCP

Aggregate throughput small window 800pkts large window 8000 Dummynet: cap = 800Mbps; delay = ms; #flows = 1-14; 29 expts

Fairness Jain’s index HSTCP ~ Reno Dummynet: cap = 800Mbps; delay = ms; #flows = 1-14; 29 expts

Stability Dummynet: cap = 800Mbps; delay = ms; #flows = 1-14; 29 expts stable in diverse scenarios

Open issues  1 BaseRTT estimation route changes, dynamic sharing does not upset stability  2 Small network buffer at least like TCP adapt on slow timescale, but how?  3 TCP-friendliness friendly at least at small window tunable, but how to tune?  4 Reverse path congestion should react? rare for large transfer?  5 Did not compare to Vegas Future plans

What people on the street are saying about FAST TCP…  PC Magazine“ The fastest hardware in the world will not do you much good if your software can't take advantage of it.  Business Week “Connections that leave broadband in the dust”  Steven Low “This is OK for driving slowly in a parking lot. But on the open road you need to be able to look further ahead: That's what we are doing with Fast TCP.”  Future Zone “ Scientists have developed a new data transfer protocol for the Internet fast enough to download a full-length DVD movie in less than five seconds”

And Of Course…  Anwis and Ajay (Rice CS) – “FAST TCP Rocks!!! We can’t wait to use it!”