Congestion Control and Fairness Models

1
Congestion Control and Fairness Models

• Nick FeamsterCS 4251 Computer Networking
  IISpring 2008

2
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

3
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

4
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

5
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
6
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?

7
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?

8
Phase Plots

• Simple way to visualize behavior of competing
  connections over time

User 2s Allocation x2
User 1s Allocation x1
9
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
10
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
11
Convergence to Efficiency
Fairness Line
xH
User 2s Allocation x2
Efficiency Line
User 1s Allocation x1
12
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
13
Convergence to Fairness
Fairness Line
xH
User 2s Allocation x2
xH
Efficiency Line
User 1s Allocation x1
14
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
15
Approach

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

16
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

17
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
18
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)

32
19
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

20
Example Sequence Number Plot
Sequence No
21
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