FAST v3 - Rice University Department of Computer Science
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Transcript FAST v3 - Rice University Department of Computer Science
FAST TCP
Anwis Das
Ajay Gulati
Slides adapted from : IETF presentation slides
Link: 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
Reno TCP
Packet level
W
W + 1/W
Loss: W
W – 0.5W
ACK:
Flow level dynamics
pkts
TCP Variant Performance
Reno: AIMD (1, 0.5)
ACK:
W W + 1/W
Loss: W W – 0.5W
HSTCP: AIMD (a(w), b(w))
ACK:
W W + a(w)/W
Loss: W W – b(w)W
STCP: MIMD (1/100, 1/8)
ACK:
W W + 0.01
Loss: W W – 0.125W
Flow level:
Reno, HSTCP, STCP, FAST
Generic flow level dynamics!
window
adjustment
=
control
gain
flow level
goal
Different gain k and utility Ui
They determine equilibrium and stability
Different congestion measure pi
Loss probability (Reno, HSTCP, STCP)
Queueing delay (Vegas, FAST)
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 a
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: ai log xi (proportional fairness)
FAST
Linux
Dynamic sharing: 3 flows
Steady throughput
HSTCP
STCP
queue
FAST
loss
Linux
throughput
30min
Dynamic sharing on Dummynet
capacity = 800Mbps
HSTCP
delay=120ms
14 flows
iperf throughput
Linux 2.4.x (HSTCP: UCL)
STCP
queue
FAST
loss
Linux
throughput
HSTCP
HSTCP
30min
STCP
Aggregate throughput
large
window
8000
small
window
800pkts
Dummynet: cap = 800Mbps; delay = 50-200ms; #flows = 1-14; 29 expts
Fairness
Jain’s index
Dummynet: cap = 800Mbps; delay = 50-200ms; #flows = 1-14; 29 expts
Stability
stable in
diverse
scenarios
Dummynet: cap = 800Mbps; delay = 50-200ms; #flows = 1-14; 29 expts
Open issues
1 BaseRTT estimation
route changes, dynamic sharing
does not upset stability
2 Small network buffer
at least like TCP
adapt a 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!”