1. TCP Vegas: New Techniques for Congestion Detection and Avoidance
Niranjan Mysore, Radhika
Woo, Dong Hyuk

2. ECE4605 - Advanced Internetworking
Contents
TCP Vegas
Five techniques used in TCP Vegas
Simulation results
Results on the Internet
Critique
ECE Advanced Internetworking

3. ECE4605 - Advanced Internetworking
TCP Vegas
Alternative TCP implementation
Improve TCP’s congestion control mechanism
Interoperate with any other valid TCP implementation
Changes only in the sending side
Performance compared with Reno
Better throughput (40%~70%)
Less loss (20%~50%)
ECE Advanced Internetworking

4. Five Techniques used in TCP Vegas
More Accurate RTT Calculation
New Mechanism for Deciding to Retransmit
Modified Window Sizing on Congestion
Spike Suppression
Congestion Detection and Avoidance
ECE Advanced Internetworking

5. More Accurate RTT Calculation
Important for
Accuracy
Helps in Timely decision to retransmit dropped segement
Coarse Grained Timer no longer sufficient
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6. New Mechanism for Deciding Retransmit
Coarse Grained Timer
On receipt of ACKs
Duplicate ACK
ACK after recent retransmission
n Duplicate ACKs no longer necessary before retransmission
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7. Modified Window Sizing on Congestion
Congestion window decreased only if losses were seen due to current rates of transmission.
Only once per RTT interval
Why?
Sensitivity
Aggressive Bandwidth Utilization
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8. ECE4605 - Advanced Internetworking
Spike suppression
Ideally we should have evenly spaced ACKs.
So that the sending rate is also smooth.
Spikes - reasons
Large cumulative ACKs (When?)
ACK compression
Spikes a necessity? or cause of losses?
SegSpacing = RTT * MaxSeg / WindowSize
No of bytes on transit have to catch up with congestion window size
Turned off during Slow Start since we have exponentially increasing transmission rate
ECE Advanced Internetworking

9. Congestion Detection and Avoidance
Reno reacts to losses - but also induces losses.
Vegas philosophy - correct number of extra bytes in the network. (Why?)
Bytes in transit proportional to throughput and window size
BaseRTT
Expected Throughput (We assume we don’t overflow the connection.)
Actual Throughput
Thresholds - significance based on extra data
Steady congestion window (Does not oscillate every now and then)
Extra buffers occupied in the network.
ECE Advanced Internetworking

10. Modified Slow-Start with Congestion Detection
Doubling Congestion window - Doubling Actual Throughput
What happens when we over run the connection's bandwidth? What is the magnitude of loss?
It can increase if there is a burst due to another connection.
Vegas - Exponential Increase only once every other RTT
Fixed congestion Window allows valid comparison of Throughput.
Threshold Gamma
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11. TCP Reno with No Other Traffic
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12. TCP Vegas with No Other Traffic
Expected
Actual
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13. ECE4605 - Advanced Internetworking
Simulation Results
Network configuration
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14. Simulation Results (cont’d)
One-on-one experiment
=52
=44.8
=19.5
Reno/Vegas
Vegas
Reno
300 KB
1 MB
ECE Advanced Internetworking

15. Simulation Results (cont’d)
Experiments with background traffic
Throughput of 1 Mb traffic
Reno
Reno
tcplib
1 Mb
ECE Advanced Internetworking

16. Simulation Results (cont’d)
Throughput of background traffic
BG traffic throughput increases by 20%
tcplib
1 Mb
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17. Results on the Internet
From UA to NIH
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18. Results on the Internet (cont’d)
1MB transfer over the Internet
Effects of transfer size over the Internet
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19. ECE4605 - Advanced Internetworking
Critique
TCP Vegas competing with TCP Reno Connections
Rerouting
Persistent Congestion
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20. ECE4605 - Advanced Internetworking
Rerouting
BaseRTT
Longer Propagation Delay? Or congestion?
Backlogged Packets = W - r*d
To keep the packets in the switch buffers the same, W should increase.
ECE Advanced Internetworking

21. ECE4605 - Advanced Internetworking
Rerouting - Remedy
Estimate diff_estimate for K packets
Estimate BaseRTTestimate for N packets If
(BaseRTTestimate – BaseRTT) > (diff_estimate + min { d.BaseRTT, g })
for L consecutive times
Then
BaseRTT = BaseRTTestimate
cwnd = cwnd * BaseRTTestimate / BaseRTT + 1
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22. Persistent Congestion
N > 1 connections
Overestimate propagation delays
Network congestion?
Backlogged packets in queue?
Unfair - connections that start up later have greater flow rate
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23. Persistent Congestion - Remedy
RED?
All connections overestimate propagation delay due to congestion
Congestion causes them to reduce congestion widow size
Congestion reduces
New BaseRTT
ECE Advanced Internetworking

24. ECE4605 - Advanced Internetworking
References
TRANSMISSION CONTROL PROTOCOL (RFC793)
Issues in TCP Vegas
Richard J. La, Jean Walrand, and Venkat Anantharam
Department of Electrical Engineering and Computer Sciences, University of California, Berkeley
TCP Vegas Revisited
U. Hengartner, J. Bolliger and Th. Gross
Department Informatik ETH Zurich, School of Computer Sciences Carnegie Melon University
Analysis and Comparison of TCP Reno and Vegas
Jeonghoon MO, Richard J. La, Venkat Anantharam, and Jean Walrand
ECE Advanced Internetworking