FAST TCP

Anwis DasAjay Gulati

Slides adapted from : IETF presentation slidesLink: http://netlab.caltech.edu/FAST/index.html

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 + 0.01

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 Ui

They determine equilibrium and stability

Different congestion measure pi Loss probability (Reno, HSTCP, STCP) Queueing delay (Vegas, FAST)

Generic flow level dynamics!

windowadjustment

controlgain

flow levelgoal=

Problem: binary signal

TCP

oscillation

Solution: multibit signal

FAST

stabilized

FAST TCP Dynamics

Generic equation:

windowadjustment

controlgain

flow levelgoal=

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 xi (proportional fairness)

Dynamic sharing: 3 flowsFAST Linux

HSTCP STCP

Steady throughput

FAST Linux

throughput

loss

queue

STCPHSTCP

Dynamic sharing on Dummynet capacity = 800Mbps delay=120ms 14 flows iperf throughput Linux 2.4.x (HSTCP: UCL)

30min

FAST Linux

throughput

loss

queue

STCPHSTCP

30min

HSTCP

Aggregate throughput

small window800pkts

largewindow

8000

Dummynet: cap = 800Mbps; delay = 50-200ms; #flows = 1-14; 29 expts

Fairness

Jain’s index

HST

CP ~

Ren

oDummynet: cap = 800Mbps; delay = 50-200ms; #flows = 1-14; 29 expts

Stability

Dummynet: cap = 800Mbps; delay = 50-200ms; #flows = 1-14; 29 expts

stable indiverse

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!”