Manfred Georg October 14th 2004

1

TCP Reno vs. Vegas

Paper by:

Jeonghoon MoRichard LaVenkat AnantharamJean Walrand

INFOCOM 1999

Manfred Georg

Presented by:

Discussion Leader:

Jack Meier

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Overview

● TCP Reno● TCP Vegas● Fairness● Simulation● Conclusion

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TCP Reno

● Slow Start● Congestion Avoidance (AIMD)● Packet Loss● Window size and Network Buffering

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TCP Vegas

● Slow Start● Congestion Avoidence

– Round Trip Delay– Alpha and Beta

● Rerouting● Persistent Congestion

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Vegas Algorithm

● Expected = CWND / BaseRTT● Actual = CWND / RTT● Diff = (Expected - Actual) BaseRTT

CWNDBaseRTTRTT

ExpectedActualDiff

= 60 packets= 20 ms= 30 ms

= 3000 packets/s= 2000 packets/s= 20 packets

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Vegas Algorithm

● Expected = CWND / BaseRTT● Actual = CWND / RTT● Diff = (Expected - Actual) BaseRTT

CWND =

CWND +1

CWND – 1

CWND

if Diff < alpha

if Diff > beta

otherwise

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Vegas Algorithm

● Diff = (Expected - Actual) BaseRTT

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Rerouting Vegas

● No Explicit Feedback● Propagation Delay Changes

– Shorter Delay Detected– Longer Delay Interpreted as Congestion

● Occasionally update BaseRTT– BaseRTT = min RTT since last update– If (RTT < BaseRTT) update BaseRTT = RTT

● Cause Persistent Congestion?

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Persistent Congestion

● New Connection Increases Congestion● BaseRTT updating

– Temporarily causes congestion– Other connections back off– All connections get accurate estimate of

BaseRTT

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Fairness

● Reno vs. Reno● Vegas vs. Vegas● Reno vs. Vegas

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Reno Fairness

D. Chiu and R. Jain,Journal of Computer Networks and ISDN Systems, 1989

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Vegas Fairnessz(t) = total backlog

ei(t) = ACK received rate

qi(t) = backlog in queue

wi(t) = window size

qi(t) between and ßα

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Vegas Fairnessz(t) = total backlog

ei(t) = ACK received rate

qi(t) = backlog in queue

wi(t) = window size

wi(t) = q

i(t) + e

i(t) d

i )

ei(t) = w

i(t) / (z

i(t) + d

i)

e

1(t) + e

2(t) = 1

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Vegas Fairness

Fixed WindowStream

ß α

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Vegas Fairness

Fixed WindowStream

ß

α

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Vegas Fairness

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Vegas Fairness

80

Delay12.9ms

70

Delay18.9ms

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Vegas Fairness

110

Delay12.9ms

70

Delay36.9ms

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Vegas Fairness

160

Delay12.9ms

70

Delay66.9ms

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Vegas Fairness

400

Delay12.9ms

70

Delay186.9ms

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Simulation

● Vegas vs. Vegas● Reno vs. Reno● Reno vs. Vegas

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Vegas vs. Vegas

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Reno vs. Reno

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Reno vs. Vegas

● Vegas less than ß packets buffered– k

● Reno fills the rest (half the time)– (B-k)/2

● Vegas to Reno Throughput– 2k/(B-k)

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Reno vs. Vegas

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Reno vs. Vegas

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Conclusion

● Rerouting● Persistent Congestion● Vegas vs. Vegas Fairness● Reno vs. Reno Fairness● Vegas and Reno do not cooperate

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Thank You

'The time has come,' Jack Meier said,'To speak of many thing...'