1. Fast TCP
Matt Weaver
CS622 Fall 2007

2. FAST TCP: Motivation, Architecture, Algorithms, Performance
David X. Wei, Student Member, IEEE, Cheng Jin, Steven H. Low, Senior Member, IEEE, and Sanjay Hegde
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FAST TCP: Motivation, Architecture, Algorithms, Performance

3. FAST TCP: Motivation, Architecture, Algorithms, Performance
Abstract
FAST TCP is a congestion control algorithm that attempts to solve the problems of congestion control.
This paper covers:
The algorithm itself.
Performance metrics
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4. FAST TCP: Motivation, Architecture, Algorithms, Performance
Background
“Congestion control is a distributed algorithm to share network resources among competing users.”
A difficult problem to solve...
Resource needs vary, depending on time of day.
Available resources is usually static.
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5. FAST TCP: Motivation, Architecture, Algorithms, Performance
FAST is a recursive acronym:
FAST AQM Scalable TCP
AQM: Active Queue Management
TCP: Transmission Control Protocol (duh)
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6. FAST TCP: Motivation, Architecture, Algorithms, Performance
Current Issues
As congestion is monitored, current algorithms slow down monitoring as packets are dropped, the average sending rate depends on low loss probability.
High data transmission rates are required for low loss.
Usually lower than WiFi can support.
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7. FAST TCP: Motivation, Architecture, Algorithms, Performance
Solution
FAST TCP uses queues to store a constant number of packets.
If too few packets are queued, the sending rate increases.
If too few, the rate decreases.
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8. FAST TCP: Motivation, Architecture, Algorithms, Performance
Congestion Control
Current TCP congestion control algorithm (aka Reno).
At the packet level, linear increase by one packet per roundtrip time (RTT) is too slow, and multiplicative decrease per loss event is too drastic.
At the flow level, maintaining large average congestion windows requires an extremely small equilibrium loss probability.
At the packet level, oscillation in congestion window is unavoidable because TCP uses a binary congestion signal (packet loss).
At the flow level, the dynamics is unstable, leading to severe oscillations that can only be reduced by the accurate estimation of packet loss probability and a stable design of the flow dynamics.
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9. FAST TCP: Motivation, Architecture, Algorithms, Performance
Motivations
Two levels of design:
The flow level (macroscopic) covers:
QoS
Stability
etc
Packet level (microscopic) covers:
The same goals, but focused on end to end.
Reno suffered because higher level control was considered after the micro level.
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10. FAST TCP: Motivation, Architecture, Algorithms, Performance
Calculations
Congestion and utility:
U calculates utility for each stakeholder (user) at a given flow.
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11. FAST TCP: Motivation, Architecture, Algorithms, Performance
Calculations
Equilibrium (FAST):
Equilibrium (Reno):
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12. FAST TCP: Motivation, Architecture, Algorithms, Performance
Calculations
“A key departure of our model from those in the literature is that we assume that a source’s send rate, defined as xi(t) :=wi(t)=Ti(t), cannot exceed the throughput it receives. “
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13. FAST TCP: Motivation, Architecture, Algorithms, Performance
Dynamic Structure
The weakness of current schemes versus FAST is shown for large window sizes.
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14. FAST TCP: Motivation, Architecture, Algorithms, Performance
Equilibrium
Equilibrium measures congestion consistency.
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15. FAST TCP: Motivation, Architecture, Algorithms, Performance
Differences
Though all of the aforementioned algorithms look different at the packet level, they actually have similar structures at the flow and equilibrium levels.
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16. FAST TCP: Motivation, Architecture, Algorithms, Performance
Performance Test 1
To test performance, packet data is pushed through a semi-articial network.
Identical sender and receiver boxes, running dummynet on FreeBSD.
Emulated router.
Dummynet running:
Paths with RTTs of 50, 100, 150, and 200ms.
Second path with a bottleneck capacity of 8M/s and a buffer size of 2,000 packets shared by all the delay pipes.
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17. FAST TCP: Motivation, Architecture, Algorithms, Performance
Results
Dynamic state I:
Small flows, large windows
Dynamic state II:
Larger flows
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18. FAST TCP: Motivation, Architecture, Algorithms, Performance
Dynamic State I
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19. FAST TCP: Motivation, Architecture, Algorithms, Performance
Throughput kbps
Queue(avg) # of pkts
FAST vs Reno I
Sec
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20. FAST TCP: Motivation, Architecture, Algorithms, Performance
Dynamic State II
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21. FAST TCP: Motivation, Architecture, Algorithms, Performance
Dynamic State II
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22. FAST TCP: Motivation, Architecture, Algorithms, Performance
FAST vs Reno II
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23. FAST TCP: Motivation, Architecture, Algorithms, Performance
BIC
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24. FAST TCP: Motivation, Architecture, Algorithms, Performance
Performance Test 2
Dummynet tests are limited to a single bottleneck and the same protocols.
NS-2 Simulation run in lab:
Same algorithm.
Noise added to eliminate phase artifacts.
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25. FAST TCP: Motivation, Architecture, Algorithms, Performance
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28. FAST TCP: Motivation, Architecture, Algorithms, Performance
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29. FAST TCP: Motivation, Architecture, Algorithms, Performance
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31. FAST TCP: Motivation, Architecture, Algorithms, Performance
Conclusion
Because of the use of queues, FAST TCP can handle lower transmission rates.
The paper also covers some simulated scenarios (too lengthy to cover properly here).
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32. FAST TCP: Motivation, Architecture, Algorithms, Performance
Caveat Emptor
Possibly biased research:
Jin Cheng, Steve Low, and David Wei (the authors) patented and market the FAST TCP algorithm.
FAST TCP implementation sold as FastSoft Aria (a 1 U rack mountable hardware solution).
Ao Tang proposed that these measurements were somewhat misleading in another paper.
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33. FAST TCP: Motivation, Architecture, Algorithms, Performance
Other CC Algorithms
BIC TCP
Compound TCP
CUBIC
H-TCP
High Speed TCP
HSTCP-LP
Hybla
New Reno
Tahoe
TCP-Illinois
TCP-LP
TCP-SACK
TCP-Veno
Westwood
Westwood+
XCP
YeAH-TCP
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34. FAST TCP: Motivation, Architecture, Algorithms, Performance
Related Work
Tang, A., Wang, J., Low, S. H., and Chiang, M Equilibrium of heterogeneous congestion control: existence and uniqueness. IEEE/ACM Trans. Netw. 15, 4 (Aug. 2007), DOI=
Ma, J., Ruutu, J., and Wu, J An enhanced TCP mechanism—fast-TCP in IP networks with wireless links. Wirel. Netw. 6, 5 (Nov. 2000), DOI=
Gu, Y., Hong, X., and Grossman, R. L Experiences in Design and Implementation of a High Performance Transport Protocol. In Proceedings of the 2004 ACM/IEEE Conference on Supercomputing (November , 2004). Conference on High Performance Networking and Computing. IEEE Computer Society, Washington, DC, 22. DOI=
Grieco, L. A. and Mascolo, S Performance evaluation and comparison of Westwood+, New Reno, and Vegas TCP congestion control. SIGCOMM Comput. Commun. Rev. 34, 2 (Apr. 2004), DOI=
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