EE 122: Congestion Control The Sequel

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EE 122 Congestion Control The Sequel

• October 1, 2003

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Quick Review

• Slow-Start cwnd upon every new ACK
• Congestion avoidance AIMD if cwnd gt ssthresh
• ACK cwnd cwnd 1/cwnd
• Drop ssthresh cwnd/2 and cwnd1
• Fast Recovery
• duplicate ACKS cwndcwnd/2
• Timeout cwnd1

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TCP Flavors

• TCP-Tahoe
• cwnd 1 whenever drop is detected
• TCP-Reno
• cwnd 1 on timeout
• cwnd cwnd/2 on dupack
• TCP-newReno
• TCP-Reno improved fast recovery
• TCP-Vegas, TCP-SACK

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TCP Vegas

• Improved timeout mechanism
• Decrease cwnd only for losses sent at current
  rate
• avoids reducing rate twice
• Congestion avoidance phase
• compare Actual rate (A) to Expected rate (E)
• if E-A gt ?, decrease cwnd linearly
• if E-A lt ?, increase cwnd linearly
• rate measurements delay measurements
• see textbook for details!

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TCP-SACK

• SACK Selective Acknowledgements
• ACK packets identify exactly which packets have
  arrived
• Makes recovery from multiple losses much easier

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Standards?

• How can all these algorithms coexist?
• Dont we need a single, uniform standard?
• What happens if Im using Reno and you are using
  Tahoe, and we try to communicate?

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Equation-Based CC

• Simple scenario
• assume a drop every kth RTT (for some large k)
• w, w1, w2, ...wk-1 DROP (wk-1)/2,
  (wk-1)/21,...
• Observations
• In steady state w (wk-1)/2 so wk-1
• Average window 1.5(k-1)
• Total packets between drops 1.5k(k-1)
• Drop probability p 1/1.5k(k-1)
• Throughput T (1/RTT)sqrt(3/2p)

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Equation-Based CC

• Idea
• Forget complicated increase/decrease algorithms
• Use this equation T(p) directly!
• Approach
• measure drop rate (dont need ACKs for this)
• send drop rate p to source
• source sends at rate T(p)
• Good for streaming audio/video that cant
  tolerate the high variability of TCPs sending
  rate

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Question!

• Why use the TCP equation?
• Why not use any equation for T(p)?

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Cheating

• Three main ways to cheat
• increasing cwnd faster than 1 per RTT
• using large initial cwnd
• Opening many connections

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Increasing cwnd Faster
y
C
x increases by 2 per RTT y increases by 1 per RTT
Limit rates x 2y
x
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Increasing cwnd Faster
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Larger Initial cwnd
x starts SS with cwnd 4 y starts SS with cwnd
1
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Open Many Connections

• Assume
• A starts 10 connections to B
• D starts 1 connection to E
• Each connection gets about the same throughput
• Then A gets 10 times more throughput than D

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Cheating and Game Theory
D ? Increases by 1 Increases by 5
A Increases by 1 Increases by 5
(x, y)
22, 22 10, 35
35, 10 15, 15

• Too aggressive
• Losses
• Throughput falls

Individual incentives cheating pays Social
incentives better off without cheating Classic
PD resolution depends on accountability
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Lossy Links

• TCP assumes that all losses are due to congestion
• What happens when the link is lossy?
• Recall that Tput 1/sqrt(p) where p is loss
  prob.
• This applies even for non-congestion losses

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Example
p 0
p 1
p 10