0 delayed congestion response protocols thesis by sumitha bhandarkar under the guidance of dr. a. l....
TRANSCRIPT
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Delayed Congestion Response Protocols
Thesis By Sumitha BhandarkarUnder the Guidance of Dr. A. L. N. Reddy
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Layout of the presentation
• Introduction to “TCP-Friendliness”
• Motivation for Delayed Congestion Response Protocols
• Delayed Congestion Response Protocols
• Conclusions And Future Work
• Related Work
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Introduction to “TCP-Friendliness”
• Stability of internet depends on protocols that respond to congestion
• Congestion Control Algorithm of TCP results in drastic changes in sending rate
• When used with real-time audio/video application, this will cause drastic changes in user perceived quality
• UDP looks like a good alternative for such applications
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Introduction to “TCP-Friendliness” (contd.)
• Extensive use of UDP could result in
• Extreme unfairness to existing TCP applications
• Congestion collapse of the internet
• Lot of interest in new class of protocols called “TCP-Friendly” Protocols
• “TCP-Friendliness” indicates that the protocol chooses to send at a rate no higher than TCP under similar conditions of round trip delays and packet losses
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Introduction to “TCP-Friendliness” (contd.)
An analytical model for TCP was developed by J. Padhye et. al. which shows -
T = S
R 2p/3 + TRTO (3 3p/8) p ( 1 + 32p2)
T : Throughput
S : Packet Size
R : Round Trip Time
TRTO : Retransmission Timeout
p : Loss Rate
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Introduction to “TCP-Friendliness” (contd.)
• Simplified Throughput Equation
3/2T =
R * p
• In a very general sense a protocol which maintain the sending rate to at most some constant over the square root of the packet loss rate is said to be “TCP-friendly”.
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Motivation For Delayed Congestion Response Protocols
• Congestion in the network is notified to the protocol, in most cases, through packet drops.
• TCP as well as most of the TCP-friendly protocols reduce the sending rate once and as soon as allowed by protocol design, when a packet drop is noticed.
• By delaying congestion response by ‘’ RTTs, the transport protocol can provide application with an early warning regarding an impending reduction in sending rate.
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Motivation For Delayed Congestion Response Protocols (contd.)
• Smart applications can be designed to combine Early Notification with buffering techniques to provide smooth output.
• For applications that cannot take advantage of early notification, smooth sending rate can be provided by reducing the congestion window smoothly, during the period ‘’ after a packet drop.
• By studying the time scales over which we can delay the congestion response insight can be gained regarding the time scales for defining “TCP-Friendliness”.
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C w
n d
Delayed Congestion Response Protocols
• Protocols where response to congestion is deliberately delayed by ‘’ RTTs when a congestion is notified.
• Congestion control dynamics characterized by (f1(t), f2(t), , ).
f1(t)
f2(t)Pkt Drop
Time
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DCR-I
• f2(t) is an increasing function
• For simplicity of implementation and analysis we set
• f1(t) as an additive function.
• f2(t) = f1(t).
C w
n d
f1(t)
f2(t)Pkt Drop
Time
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DCR-I
• Thus we have,
f1(t) : wt+R wt + ; > 0
f2(t) : wt+R wt + ; > 0 , tdrop < t < tdrop +
wtdrop + * wtdrop + - ; < 1
wt is the congestion window at time tR is the RTT
, and are constants
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C w
n d
f1(t)
f2(t)Pkt Drop
Time
DCR-I
Steady State Analysis
A
Throughput = Number of packets between two successive drops
Time between two successive drops
t2+t1 t2
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DCR-I
Steady State Analysis (contd.)
( /2 ) ( 1 + ) / ( 1 - )
= R p
Comparing with the TCP-equation, the condition of TCP-Friendliness is :
3 ( 1 - ) =
( 1 + )
Infinite number of values can be chosen for and that satisfy the above condition . We chose = 1 and = 1/2 since it is makes DCR-I very similar to TCP-reno.
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DCR-I
Implementation
• Testing platform was ns-2
• Modifications were made to the existing TCP-reno.
• When congestion in the network was notified, the time-to-response was noted.
• Congestion window was continuously increased until the time-to-response, at which time it was cut down by half.
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DCR-D
• Primary aim of DCR-D is to provide smooth response.
• f2(t) is thus chosen to be a decreasing function and is set to1.
C w
n d
f1(t) f2(t)
Pkt Drop
Time
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DCR-D
•We have,
f1(t) : wt+R wt + ; > 0
f2(t) : wt+R * wt ; 0 < < 1, tdrop < t <tdrop +
wt is the congestion window at time tR is the RTT
, and are constants
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t2+
C w
n d
f1(t) f2(t)
Pkt Drop
Time
DCR-D
Steady State Analysis
Throughput = Number of packets between two successive drops
Time between two successive drops
N1 N2
t1 t2
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DCR-D
Steady State Analysis (contd.)
• Set K = , the factor by which the congestion window is decreased over the period of ‘’ RTTs
1
• =
R (2p(1-K)/(1+K) + p ([2(1-K)/lnK(1+K)] +
1))
• The above equation is TCP-Friendly if second term in the denominator is negligible
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DCR-D
Steady State Analysis (contd.)
• Condition for TCP-Friendliness is :
p 2 . (1-K) + 1 = 0lnK (1+K)
• For different values of K, the results of the above equation is given as follows -
K f(K)
0.9 0.0009
0.8 0.0041
0.7 0.0105
0.6 0.0212
0.5 0.0382
0.4 0.0646
0.3 0.1055
0.2 0.1716
0.1 0.2893
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DCR-D
Steady State Analysis (contd.)
• With K = 0.8, the throughput equation can be written as1
=
R (2p(1-K)/(1+K) + 0.0041* p)
• Second term in the denominator is negligible.
• Comparing the above equation with TCP equation we have,
3 ( 1-K) =
(1+K)
• With K = 0.8, = 0.333
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DCR-D
Implementation
• Testing platform was ns-2 with modifications made to the existing TCP-reno.
• Two modes of operation
• Increase mode : seeking bandwidth using f1(t)
• Decrease mode : reducing bandwidth using f2(t)
• On congestion notification start a delay timer for ‘’ RTTs and get into decrease mode.
• When the delay timer expires return to Increase mode.
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DCR-D
Implementation(contd.)
• While entering the decrease mode note down the target value of the congestion window to be achieved at the end of decrease mode.
• If packet is dropped in decrease mode,
• Reset the delay timer.
• Reduce the congestion window to its target value drastically.
• Set a new delay timer to take care of latest congestion.
• Note down the current target value.
Reasoning: Packet drop during decrease mode indicates high level of congestion. Thus drastic reduction in congestion window is required as compared to the smooth reduction using f2(t) .
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DCR-D
Congestion Window Evolution at High Droprates
C w
n d
TargetValue
Pkt Drop
Time
Pkt Drop
Original Timer New
Timer
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DCR-C
• f2(t) is an constant functionC
w n
d
f1(t)
Pkt Drop
Time
f2(t)
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DCR-C
• We have,
f1(t) : wt+R wt + ; > 0
f2(t) : wt+R wt ; tdrop < t <tdrop +
wtdrop + * wtdrop + - ; < 1
wt is the congestion window at time tR is the RTT
, and are constants
• Analysis shows this protocol cannot be TCP-Friendly. So simulations were not conducted for this protocol.
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Simulation Topology
R1
Src 1
R2
Sink 1
Sink 2
Sink n
Src 2
Src n
.
.
.
.
.
.
.
.
.
.
Bottleneck Link
B MbpsD ms
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Simulation ResultsDCR-I
Fairness Index at Different Droprates
Fairness Index of DCR-I at different droprates.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 5 10 15 20 25
tau (in RTT)
Fairn
ess
Inde
x
1.3 - 1.5% 3.4 - 3.7%
5.4 - 6.0% 7.8 - 8.5%
10.1 -10.8% 12.1 - 12.8%
13.8 - 14.4%
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Simulation Results DCR-I
Fairness Index at Different Buffer Sizes
Fairness Index of DCR-I different bufersizes.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 5 10 15 20 25
tau (in RTT)
Fairn
ess
Inde
x 0.5*Delay*BW (12.5 packets)
1*Delay*BW (25.0 packets)
3*Delay*BW (75.0 packets)
5*Delay*BW (125.0 packets)
8*Delay*BW (200.0 packets)
10*Delay*BW (250.0 packets)
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Simulation Results DCR-I
Fairness Index with mixed workloadFairness Index of DCR-I with mixed workload.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 5 10 15 20 25
tau (in RTT)
Fa
irn
ess
Ind
ex
1 Reno + 15 DCR-I
3 Reno + 13 DCR-I
6 Reno + 10 DCR-I
8 Reno + 8 DCR-I
10 Reno + 6 DCR-I
13 Reno + 3 DCR-I
15 Reno + 1 DCR-I
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Simulation Results DCR-I
Sample per-flow droprates for mixed workload
1 Reno + 15 DCR-I 8 Reno + 8 DCR-I 15 Reno + 1 DCR-I
tau TCP-renoperflow
droprate (%)
DCR-Iperflow
droprate (%)
TCP-renoperflow
droprate (%)
DCR-Iperflow
droprate (%)
TCP-renoperflow
droprate (%)
DCR-Iperflow
droprate (%)
2 1.669 2.381 1.755 1.719 1.632 1.330
4 1.662 3.060 1.739 1.667 1.607 1.256
6 1.719 3.101 1.711 1.676 1.618 1.067
8 1.608 3.294 1.787 1.707 1.657 1.209
10 1.646 3.391 1.886 1.699 1.642 1.343
15 1.365 3.300 1.749 1.729 1.639 1.063
20 1.253 3.117 1.849 1.755 1.630 1.288
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Simulation Results DCR-I
Fairness Index at Different Droprates with Drop Tail Router
Fairness Index of DCR-I at different droprates.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
0 5 10 15 20 25
tau (in RTT)
Fairn
ess
Inde
x
1.00% 2.7 - 3.0%
4.5 - 5.0% 6.2 - 6.6%
7.6 - 8.0% 8.8 - 9.3%
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Simulation Results DCR-I
Sample values of per-flow droprates (Drop Tail Router)
Bottleneck Link Droprate: 4.5 - 5.0%
tau reno perflowdroprate(%)
DCR-Iperflow
droprate(%)
linkdroprate(%)
2 5.013 4.286 4.5864 5.793 4.099 4.7486 5.639 4.206 4.7588 5.807 4.142 4.759
10 5.867 4.147 4.77415 6.127 4.181 4.86720 6.487 4.214 4.977
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Simulation Results
DCR-I
Effects of Clock Resolution
DCR-I Effects of TCP Clock Resolution.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 2 4 6 8 10 12
tau (in RTT)
Fairn
ess
Inde
x
100ms clk res
10ms clk res
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DCR-D (alpha = 0.333, K = 0.8 )Fairness Index at varying tau for Different Droprates.
0
0.2
0.4
0.6
0.8
1
1.2
0 2 4 6 8 10 12
tau (in RTT)
Fai
rnes
s In
dex
1.0 - 1.3% 2.4 - 3.1%
4.1 - 5.1% 6.2 - 7.5%
9.4 - 10.4% 11.5 - 12.7%
14.4 - 14.7%
Simulation Results
DCR-D
Fairness Index at Different Droprates
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DCR-D (alpha = 0.333, K = 0.8)Fairness Index at Varying tau for Different Buffer Sizes.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 2 4 6 8 10 12
tau (in RTT)
Fairn
ess
Inde
x
1 * Delay * BW (10.0 packets)
3 * Delay * BW (30.0 packets)
5 * Delay * BW (50.0 packets)
8 * Delay * BW (80.0 packets)
10 * Delay * BW (100.0 packets)
Simulation Results DCR-D
Fairness Index at Different Buffer Sizes
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DCR-D (alpha = 0.333, K = 0.8 )Fairness Index at varying tao with mixed workload
00.20.40.60.8
11.21.41.61.8
22.22.4
0 2 4 6 8 10 12
tau (in RTT)
Fa
irn
ess
Ind
ex
1 reno + 15 DCR-D
3 reno + 13 DCR-D
6 reno + 10 DCR-D
8 reno + 8 DCR-D
10 reno + 6 DCR-D
13 reno + 3 DCR-D
15 reno + 1 DCR-D
Simulation Results
DCR-D
Fairness Index with mixed workload
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Simulation Results DCR-D
Sample per-flow droprates for mixed workload
1 reno + 15 DCR-D 8 reno + 8 DCR-D 15 reno + 1 DCR-D
tau TCP-renoperflow
droprate (%)
DCR-Dperflow
droprate (%)
TCP-renoperflow
droprate (%)
DCR-Dperflow
droprate (%)
TCP-renoperflow
droprate (%)
DCR-Dperflow
droprate (%)
2 1.683 1.571 1.641 1.373 1.733 0.550
3 1.491 1.419 1.527 1.307 1.693 0.582
4 1.234 1.332 1.459 1.240 1.696 0.533
5 1.150 1.270 1.387 1.206 1.679 0.556
6 1.139 1.195 1.308 1.183 1.652 0.615
7 1.072 1.153 1.300 1.136 1.675 0.493
8 1.040 1.097 1.239 1.106 1.669 0.485
9 1.030 1.055 1.211 1.108 1.691 0.425
10 0.919 1.011 1.180 1.068 1.661 0.473
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Simulation Results DCR-D
Fairness Index at Different Droprates with Drop Tail Router
DCR-D (alpha = 0.333, K = 0.8)Fairness Index at varying tao for Different Droprates
(DropTail Router)
0
1
2
3
4
5
6
0 2 4 6 8 10 12
tau (in RTT)
Fa
irn
ess
Ind
ex
0.9 - 1.1% 2.0 - 2.6%
3.4 - 4.3% 4.5 - 5.7%
5.8 - 7.2% 7.8 - 9.2%
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Simulation Results DCR-D
Sample values of per-flow droprates (Drop Tail Router)Bottleneck Link Droprate: 4.5 - 5.7%
tau TCP-reno perflowdroprate (%)
DCR-D perflowdroprate (%)
linkdroprate (%)
2 9.569 4.028 5.6583 9.584 3.763 5.4674 9.723 3.252 5.1015 9.622 2.990 4.9506 9.539 2.878 4.8567 9.584 2.751 4.7578 9.896 2.614 4.7149 10.110 2.500 4.657
10 10.015 2.373 4.547
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Simulation Results DCR-D
Effects of Clock Resolution
DCR-D (alpha = 0.333, K = 0.8)Effects of TCP Clock Resolution.
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10 12
tau (in RTT)
Fa
irn
ess
Ind
ex
100ms clk res
10ms clk res
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Simulation Results DCR-D
Effects of Clock Resolution With Compensated
DCR-D with compensated alpha (alpha = 1.0, K = 0.8)Effects of TCP Clock Resolution.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 2 4 6 8 10 12
tau (in RTT)
Fa
irn
ess
Ind
ex
100ms clk res
10ms clk res
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Simulation Results Measure of smoothness
• Protocols used with real-time audio-video application require smooth sending rates
• Use coefficient of variance as a measure of smoothness
• Note throughput at intervals of time
• For each flow compute the cov as (standard deviation) / (mean) of these values
• For the protocol, the cov is the average cov of all the flows using that protocol.
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Simulation Results DCR-D
Coefficient of Variance at Different Droprates ( = 0.5s)
bottleneck link droprate (%) TCP-reno COV DCR-D COV1.7 0.4234 0.36282.1 0.4345 0.38982.5 0.4620 0.42773.1 0.4710 0.45704.1 0.4883 0.50204.9 0.5283 0.51715.8 0.5734 0.55416.9 0.6861 0.63598.2 0.8060 0.7243
12.0 0.8006 0.7243
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Simulation Results
DCR-D
Coefficient of Variance at Different values of (p = 0.1%)
delta (in seconds) TCP-reno COV DCR-D COV0.1 0.3725 0.25740.5 0.3184 0.17861 0.2961 0.17192 0.2581 0.16545 0.1960 0.1518
10 0.1511 0.132915 0.1242 0.1218
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Conclusions
In this thesis, we have,
• provided the general frame work for Delayed Congestion Response protocols.
• examined three cases in particular and shown through analysis and simulations that two of these can be TCP-friendly for a wide range of network parameters.
• Using DCR-D, shown that sending rate can be made smoother through the proper design of the function f2(t).
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Conclusions (contd.)
Regarding the TCP-Friendliness we have shown,
• 1/ p is necessary but not sufficient condition for determining TCP-Friendliness.
• TCP-Friendliness depends on the underlying buffer management scheme.
• TCP-Friendliness is affected by the type of workload on the system, even in the presence of an active buffer management scheme.
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Future Work
• Work needs to be done in synthesizing the general equations to provide proper guidelines for choosing the values of (f1(t), f2(t),, ).
• Substantial work needs to be still done to characterize TCP-Friendliness
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Questions ???
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Related Work
“ModelingTCP Throughput: A simple Model and its Empirical Validation” by J.Padhye, V. Firoiu, D.Towsley and J.Kurose.
• Developed an Analytical Model for TCP’s congestion control mechanism in terms of loss rate and RTT
• Captures the behavior of fast retransmit and the timeout mechanism.
• Evaluated using network traces obtained from the internet.
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Related Work (contd.)
“Equation-Based Congestion Control for Unicast Applications”by Sally Floyd, Mark Handley, Jitendra Padhye, and Joerg Widmer.
• Directly based on the TCP control Equation.
• Receiver provides feed back for RTT calculations.
• Receiver calculates loss event rate and feeds it back to sender.
• Sender takes care of RTT calculations, retransmission ( if required) and adapts the sending rate based on the equation.
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Related Work (contd.)
“Equation-Based Congestion Control for Unicast Applications”by Sally Floyd, Mark Handley, Jitendra Padhye, and Joerg Widmer.
• How to calculate the loss rate ?
•Instantaneous Values vary too much and are too noisy
• Averaged value could dampen the response to congestion.
• Limited History Weighted Average was used with history of previous 8 loss events out of which latest 4 were weighted heavily.
• Requires 4 to 8 round trip times to halve its sending rate in response to persistent congestion
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Related Work (contd.)
“Equation-Based Congestion Control for Unicast Applications”by Sally Floyd, Mark Handley, Jitendra Padhye, and Joerg Widmer.
• Shown to be TCP-Friendly over a wide range of parameters.
• Reduces variations in the sending rate compared to TCP.
• Loss rate calculations based on heuristics.
• Implementation is complex and requires modification of sender and receiver.
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Related Work (contd.)
“LDA+ TCP-Friendly Adaptation: A Measurement and Comparison Study” by Sisalem, D. and Wolisz, A.
• Also uses TCP control equations to compute sending rate, but at the application level.
• Uses Real-Time Protocol (RTP) for collecting loss and delay statistics.
• Shown via simulations and experiments on the public internet to be TCP-Friendly.
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Related Work (contd.)
“LDA+ TCP-Friendly Adaptation: A Measurement and Comparison Study” by Sisalem, D. and Wolisz, A.
• Application level solution which provides closed feedback requiring no implementation of separate transport layer protocol. Therefore, an attractive option.
• Depends on RTP messages to obtain loss rates, cannot change fast enough in case of rapid changes in the network.
• Receiver report uses 8 bits for loss information setting a limit of 0.002 for minimum loss rate.
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Related Work (contd.)“TCP-friendly Congestion Control for Real-time Streaming
Applications” by D. Bansal and H. Balakrishnan.
Congestion Control Equations written in the form of binomial equations
I : wt+R wt + / wtk ; > 0
D : wt+t wt - wtl ; 0 < < 1
I refers to increase in windowD refers to decrease in windowwt is the congestion window at time tR RTT
and constants
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Related Work (contd.)“TCP-friendly Congestion Control for Real-time Streaming
Applications” by D. Bansal and H. Balakrishnan.
• Covers the entire class of linear controls
• k= 0, l = 1 AIMD
• k= -1, l = 1 MIMD
• k= -1, l = 0 MIAD
• k= 0, l = 0 AIAD
• Steady State Analysis shows T 1/ p 1/(k + l + 1)
• This class of protocols will be TCP-Friendly when k + l = 1 and l 1 (called the k + l rule)
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Related Work (contd.)“TCP-friendly Congestion Control for Real-time Streaming
Applications” by D. Bansal and H. Balakrishnan.
• Simulations with two implementations namely Inverse Increase/Additive Decrease (k = 1, l = 0) and SQRT (k = 1/2, l = 1/2) were shown to be TCP-friendly
• Equations indicate that several TCP-Friendly protocols can be designed based on the application requirement, provided they follow the k+l rule.
• Important observation: TCP-Friendliness does not necessarily indicate TCP-Compatibility.
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DCR-I
Simulation Results (contd.)Effect of RED parameters
Fairness Index of DCR-I at different RED parameters (maxthresh = 75%)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 5 10 15 20 25
tau (in RTT)
Fairn
ess
Inde
x
(minth,maxth,pmax)=(3,22.5,0.1)
(minth,maxth,pmax)=(7.5,22.5,0.1)
(minth,maxth,pmax)=(15,22.5,0.1)
(minth,maxth,pmax)=(18,22.5,0.1)
(minth,maxth,pmax)=(21,22.5,0.1)
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DCR-I
Simulation Results (contd.)Effect of RED parameters (contd.)
Fairness Index of DCR-I with different RED parameters.(maxthresh = 100%)
0
0.20.4
0.60.8
1
1.21.4
1.6
1.82
2.22.4
2.6
0 5 10 15 20 25
Tau (in RTT)
Fairness In
dex
(minth,maxth,pmax)=(3,30,0.1)
(minth,maxth,pmax)=(7.5,30,0.1)(minth,maxth,pmax)=(15,30,0.1)
(minth,maxth,pmax)=(22.5,30,0.1)(minth,maxth,pmax)=(27,30,0.1)