Effect of Algorithms that Improve Fairness of TCP Congestion Avoidance on Performance of Slow links

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Effect of Algorithms that Improve Fairness of TCP
Congestion Avoidance on Performance of Slow links
and Long Thin NetworksVenkatesh Obanaik,
Lillykutty Jacob, A L Ananda,Center for Internet
Research,National University of Singapore.

• 
  ICCCN 2002
• 
  Date 14th October 2002

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Contents

• Issues with fairness
• Unfairness of TCP Congestion Avoidance
• Algorithms that improve the fairness of TCP
  congestion avoidance
• Issue addressed by the paper
• What effect do the fairness algorithms have
  on the performance of slow links and LTNs?
• Possible solutions

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Fairness

• What is fairness?
• If N TCP sessions share a bottleneck
  link, then each session should get 1/N of the
  link capacity.

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Unfairness of TCP Congestion Avoidance

• During congestion avoidance phase, cwnd cwnd
  1/cwnd, on receipt of every ACK.
• TCP sender increases cwnd by utmost 1 segment
  after each RTT.
• Connections with long RTT open up their windows
  relatively slower than connections with short
  RTT.
• Short RTT connections get more than a fair share
  of bottleneck link capacity.

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Algorithms that Improve fairness of TCP
congestion Avoidance
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Constant Rate Policy

• Say, r is the average RTT of the connection.
• During congestion avoidance, the cwnd is
  increased at the rate of approximately 1 segment
  every r seconds.
• Throughput 1/r segments/s in every r seconds
• Rate of increase in throughput is 1/r2
  segments/s2
• CR policy suggested an increase of cr2
  segments i.e, cwnd cwnd (cr2) / cwnd.
• Issues with proper choice of c

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Increase By K Policy

• Describes the standard algorithm as Increase by
  1 policy.
• Hence IBK implies that, cwnd should be
  increased by utmost K segments after every RTT.
• Designed for long RTT connections to increase
  their throughput without co-operation from other
  connections.
• To be enabled selectively on long RTT
  connections.

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CANIT (Congestion Avoidance Normalized Interval
of Time)

• Addresses the problem in a slightly different
  perspective.
• In the standard congestion avoidance policy, the
  cwnd is increased on the arrival of an ACK
  packet.
• In short RTT connections ACKs arrive relatively
  faster than long RTT connections.
• Increase the cwnd of all connections by same
  amount in a specified time interval (NIT)
• cwnd cwnd (RTT)/(NIT) (1/cwnd)

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Implicit assumptions in the fairness algorithms
and their effects

• Long RTT of the connection is due to the presence
  of a long pipe accompanied by bandwidth
• ( Presence of Long thin
  Networks not considered )
• Bottleneck is in the core of the network
• ( Presence of Slow access
  links not considered )
• Slow links/LTNs only offer a long RTT but not
  high bandwidth
• TCP sender usually unaware of the network path,
  will cause increased probing in a quest for
  non-existent bandwidth

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Simulation Study

• Commonly used test configuration in similar
  studies conducted previously.
• Bandwidth and delay for router R2 and last-hop
  router chosen based on a similar study (RFC2416)
• MTU size chosen as 296 (according to
  recommendations of RFC 3150)
• Limited buffer size on last-hop router (RFC 3150
  RFC 2757)

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Behavior of IBK,CANIT and CR policies on
connections traversing Slow links/LTNs.
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Variation of congestion window when fairness
algorithms are enabled

• Amount of increase in cwnd caused by each
  arriving ACK when different policies are enabled.
• CANIT (RTT/ NIT) (1/cwnd)
• (optimum value for NIT
  30ms)
• CR (cRTT2) / cwnd
• (Value of c 10)
• IBK K/cwnd
• (Value of K 4)

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Variation of cwnd on slow link connection

• CANIT Vs
  Standard Algorithm
  IBK Vs
  Standard Algorithm

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Variation of cwnd on LTN connection

• CANIT,CR Vs Standard Algorithm
  IBK Vs
  Standard Algorithm

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Effect on Performance of Slow links and Long Thin
Networks

• Slow links and LTNs are last-hop links connecting
  end-user to the high speed internet
• Limited buffer size on last-hop router (ISPs),
  sometimes as low as a buffer of 3 packets
• Increased last-hop router buffer overflows seen
  when the fairness algorithms are enabled on slow
  link/LTN connections
• Losses very high, especially in the fairness
  algorithms that consider RTT as a parameter

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Slow link connection Goodput and Losses

• Goodput Vs QueueSize
  
  Loss Vs QueueSize

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LTN connection Goodput and Losses

• Goodput Vs QueueSize
  
  Loss Vs QueueSize

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Possible solutions

• Increasing last-hop router buffer size
• Improvement in goodput of connections seen
  but it also increases the queuing delay and
  thereby the RTT

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Possible Solutions

• Advertising a smaller receive window for slow
  link/LTN connections (RFC 3150)
• Amount of data injected into the
• network reduced, so are the losses.
• Only slight improvement seen in
• goodput.
• The sender is limited from
• transmitting new segments during
• fast recovery phase.

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• Inappropriate to apply the fairness algorithms to
  slow link/LTN connections.

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Possible Solutions

• Selectively disabling the algorithms on slow
  link/LTN connections.
• Relatively high goodput and very low losses.
• Since competing connections have alternate
  fairness algorithms enabled.

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Conclusions

• Fairness algorithms harm the slow link / LTN
  connection.
• Inappropriate to apply the fairness algorithms to
  slow link/LTN connections.
• We recommend selectively disabling the policy on
  slow links/ LTNs
• The bottleneck is not in the core but is in the
  access links.

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Future Work

• Implement mechanisms to disable fairness
  algorithms
• Test the implementation on the testbed
• Repeat the experiments on the testbed
• Repeat tests on the real network (internet2 link)