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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
Manfred Georg October 14th 2004 2
Overview
● TCP Reno● TCP Vegas● Fairness● Simulation● Conclusion
Manfred Georg October 14th 2004 3
TCP Reno
● Slow Start● Congestion Avoidance (AIMD)● Packet Loss● Window size and Network Buffering
Manfred Georg October 14th 2004 4
TCP Vegas
● Slow Start● Congestion Avoidence
– Round Trip Delay– Alpha and Beta
● Rerouting● Persistent Congestion
Manfred Georg October 14th 2004 5
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
Manfred Georg October 14th 2004 6
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
Manfred Georg October 14th 2004 7
Vegas Algorithm
● Diff = (Expected - Actual) BaseRTT
Manfred Georg October 14th 2004 8
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?
Manfred Georg October 14th 2004 9
Persistent Congestion
● New Connection Increases Congestion● BaseRTT updating
– Temporarily causes congestion– Other connections back off– All connections get accurate estimate of
BaseRTT
Manfred Georg October 14th 2004 10
Fairness
● Reno vs. Reno● Vegas vs. Vegas● Reno vs. Vegas
Manfred Georg October 14th 2004 11
Reno Fairness
D. Chiu and R. Jain,Journal of Computer Networks and ISDN Systems, 1989
Manfred Georg October 14th 2004 12
Vegas Fairnessz(t) = total backlog
ei(t) = ACK received rate
qi(t) = backlog in queue
wi(t) = window size
qi(t) between and ßα
Manfred Georg October 14th 2004 13
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
Manfred Georg October 14th 2004 14
Vegas Fairness
Fixed WindowStream
ß α
Manfred Georg October 14th 2004 15
Vegas Fairness
Fixed WindowStream
ß
α
Manfred Georg October 14th 2004 16
Vegas Fairness
Manfred Georg October 14th 2004 17
Vegas Fairness
80
Delay12.9ms
70
Delay18.9ms
Manfred Georg October 14th 2004 18
Vegas Fairness
110
Delay12.9ms
70
Delay36.9ms
Manfred Georg October 14th 2004 19
Vegas Fairness
160
Delay12.9ms
70
Delay66.9ms
Manfred Georg October 14th 2004 20
Vegas Fairness
400
Delay12.9ms
70
Delay186.9ms
Manfred Georg October 14th 2004 21
Simulation
● Vegas vs. Vegas● Reno vs. Reno● Reno vs. Vegas
Manfred Georg October 14th 2004 22
Vegas vs. Vegas
Manfred Georg October 14th 2004 23
Reno vs. Reno
Manfred Georg October 14th 2004 24
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)
Manfred Georg October 14th 2004 25
Reno vs. Vegas
Manfred Georg October 14th 2004 26
Reno vs. Vegas
Manfred Georg October 14th 2004 27
Conclusion
● Rerouting● Persistent Congestion● Vegas vs. Vegas Fairness● Reno vs. Reno Fairness● Vegas and Reno do not cooperate
Manfred Georg October 14th 2004 28
Thank You
'The time has come,' Jack Meier said,'To speak of many thing...'