Congestion Control and Fairness Models

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Congestion Control and Fairness Models

• Nick FeamsterCS 4251 Computer Networking
  IISpring 2008

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Internet Pipes?

• How should you control the faucet?
• Too fast sink overflows
• Too slow what happens?
• Goals
• Fill the bucket as quickly as possible
• Avoid overflowing the sink
• Solution watch the sink

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Congestion
10 Mbps
1.5 Mbps
100 Mbps

• Different sources compete for resources inside
  network
• Why is it a problem?
• Sources are unaware of current state of resource
• Sources are unaware of each other
• Manifestations
• Lost packets (buffer overflow at routers)
• Long delays (queuing in router buffers)
• Can result in throughput less than bottleneck
  link (1.5Mbps for the above topology) ? a.k.a.
  congestion collapse

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Causes Costs of Congestion
Q What happens as rate increases?

• Four senders multihop paths
• Timeout/retransmit

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Causes Costs of Congestion

• When packet dropped, any upstream transmission
  capacity used for that packet was wasted!

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Congestion Collapse

• Definition Increase in network load results in
  decrease of useful work done
• Many possible causes
• Spurious retransmissions of packets still in
  flight
• Classical congestion collapse
• How can this happen with packet conservation?
  RTT increases!
• Solution better timers and TCP congestion
  control
• Undelivered packets
• Packets consume resources and are dropped
  elsewhere in network
• Solution congestion control for ALL traffic

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Congestion Control and Avoidance

• A mechanism that
• Uses network resources efficiently
• Preserves fair network resource allocation
• Prevents or avoids collapse
• Congestion collapse is not just a theory
• Has been frequently observed in many networks

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Congestion Control Approaches

• Two broad approaches

• End-end congestion control
• No explicit feedback from network
• Congestion inferred from end-system observed
  loss, delay
• Approach taken by TCP

• Network-assisted congestion control
• Routers provide feedback to end systems
• Single bit indicating congestion (SNA, DECbit,
  TCP/IP ECN, ATM)
• Explicit rate sender should send at
• Problem makes routers complicated

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Example TCP Congestion Control

• Very simple mechanisms in network
• FIFO scheduling with shared buffer pool
• Feedback through packet drops
• TCP interprets packet drops as signs of
  congestion and slows down
• This is an assumption packet drops are not a
  sign of congestion in all networks
• E.g. wireless networks
• Periodically probes the network to check whether
  more bandwidth has become available.

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Objectives

• Simple router behavior
• Distributed operation
• Efficiency X Sxi(t)
• Solution leads to high network utilization
• Fairness (Sxi)2/n(Sxi2)
• What are the important properties of this
  function?
• Convergence control system must be stable

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End-to-End Congestion Control

• Increase algorithm
• Sender must test the network to determine
  whether or not the network can sustain a higher
  rate
• Decrease algorithm
• Senders react to congestion to achieve optimal
  loss rates, delays, sending rates

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Two Approaches

• Window-based
• Sender uses ACKs from receiver to clock
  transmission of new data
• Rate-based
• Sender monitors loss rate and uses timer to
  modulate the transmission rate
• Actually need a burst rate and a burst size

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Phase Plots

• What are desirable properties?
• What if flows are not equal?

Fairness Line
Overload
User 2s Allocation x2
Optimal point
Underutilization
Efficiency Line
User 1s Allocation x1
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Basic Control Model

• Reduce speed when congestion is perceived
• How is congestion signaled?
• Either mark or drop packets
• How much to reduce?
• Increase speed otherwise
• Probe for available bandwidth how?

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Linear Control

• Many different possibilities for reaction to
  congestion and probing
• Examine simple linear controls
• Window(t 1) a b Window(t)
• Different ai/bi for increase and ad/bd for
  decrease
• Supports various reaction to signals
• Increase/decrease additively
• Increased/decrease multiplicatively
• Which of the four combinations is optimal?

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Phase Plots

• Simple way to visualize behavior of competing
  connections over time

User 2s Allocation x2
User 1s Allocation x1
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Additive Increase/Decrease

• Both X1 and X2 increase/ decrease by the same
  amount over time
• Additive increase improves fairness and additive
  decrease reduces fairness

Fairness Line
T1
User 2s Allocation x2
T0
Efficiency Line
User 1s Allocation x1
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Multiplicative Increase/Decrease

• Both X1 and X2 increase by the same factor over
  time
• Extension from origin constant fairness

Fairness Line
T1
User 2s Allocation x2
T0
Efficiency Line
User 1s Allocation x1
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Convergence to Efficiency
Fairness Line
xH
User 2s Allocation x2
Efficiency Line
User 1s Allocation x1
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Distributed Convergence to Efficiency
agt0 bgt1
a0
b1
Fairness Line
alt0 bgt1
xH
agt0 blt1
User 2s Allocation x2
alt0 blt1
Efficiency Line
User 1s Allocation x1
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Convergence to Fairness
Fairness Line
xH
User 2s Allocation x2
xH
Efficiency Line
User 1s Allocation x1
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Convergence to Efficiency and Fairness

• Intersection of valid regions
• For decrease a0 b lt 1

Fairness Line
xH
User 2s Allocation x2
xH
Efficiency Line
User 1s Allocation x1
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Approach

• Constraints limit us to AIMD
• Can have multiplicative term in increase(MAIMD)
• AIMD moves towards optimal point

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Results

• Assuming syncrhonized feedback (i.e., congestion
  is signalled to all connections sharing a
  bottleneck)
• Additive increase improves fairness and
  efficiency
• Multiplicative decrease moves the system towards
  efficiency without altering fairness
• In contrast
• Additive decrease reduces fairness
• MIMD does not ever improve fairness

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AIMD

• Distributed, fair and efficient
• Packet loss is seen as sign of congestion and
  results in a multiplicative rate decrease
• Factor of 2
• TCP periodically probes for available bandwidth
  by increasing its rate

Rate
Time
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Implementation

• Operating system timers are very coarse how to
  pace packets out smoothly?
• Implemented using a congestion window that limits
  how much data can be in the network.
• TCP also keeps track of how much data is in
  transit
• Data can only be sent when the amount of
  outstanding data is less than the congestion
  window.
• The amount of outstanding data is increased on a
  send and decreased on ack
• (last sent last acked) lt congestion window
• Window limited by both congestion and buffering
• Senders maximum window Min (advertised window,
  cwnd)

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Congestion Avoidance

• If loss occurs when cwnd W
• Network can handle 0.5W W segments
• Set cwnd to 0.5W (multiplicative decrease)
• Upon receiving ACK
• Increase cwnd by (1 packet)/cwnd
• What is 1 packet? ? 1 MSS worth of bytes
• After cwnd packets have passed by ? approximately
  increase of 1 MSS
• Implements AIMD

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Example Sequence Number Plot
Sequence No
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Throughput vs. Loss Rate

• To the first order, throughput is proportional to
  1/sqrt(loss rate)
• TCP friendliness
• Consider following diagram to derive throughput

How many packets between periods of packet
loss?(arithmetic series)Compute loss rate from
thisThroughput avg rate / RTT