Network Performance and
Quality of Service
8. Congestion Control in Data Networks
Introduction
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Congestion occurs when number of
packets transmitted approaches
network capacity
Objective of congestion control:
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keep the number of packets that are
entering/within the network below the
level at which performance drops off
dramatically
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Queuing Theory
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Data network is a network of queues
If at any queue …
arrival rate > transmission rate
then, queue size grows without bound
and packet delay goes to infinity
General rule of thumb …
Design point: ρ = Lλ/R < .8
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At Saturation Point …
Two Possible Strategies at Node …
 Discard any incoming packet if no
buffer available
 Exercise flow control over neighbors
over neighbors
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May cause congestion to propagate
throughout network
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Queues in Nodes and Networks
(delay propagation)
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Ideal Performance
Infinite Buffers, No Congestion Control Overheads
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No variable overhead for packet
transmission or congestion control
Throughput increases with offered load
up to full capacity
Packet delay increases with offered load
approaching infinity at full capacity
Power = throughput / delay, or a measure
of the balance between throughput and
delay
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Higher throughput results in higher delay
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Ideal Network Utilization
Load:
Ts = L/R
Power:
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Ratio of Normalized
Throughput to Delay
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Practical Performance
Finite Buffers, Congestion Control Overheads
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Non-zero packet processing overhead
With no congestion control, increased
load eventually causes moderate
congestion: throughput increases at
slower rate than load
Further increased load causes packet
delays to increase and eventually
throughput to drop to zero
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Effects of Congestion (if not controlled)
What’s happening
here?
• buffers fill
• packets discarded
• sources re-transmit
• routers generate
more traffic to
update paths
• good packets resent
• delays propagate
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Common Congestion Control
Mechanisms
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Backpressure
Choke packet
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Implicit congestion signaling
Explicit congestion signaling
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Congestion Control
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Backpressure
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Choke packet
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Specific message back to source
e.g., ICMP Source Quench
Implicit congestion signaling
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Congested node requests sender to reduce rate
Useful only on a logical connection basis
Requires hop-by-hop flow control mechanism
Source detects congestion from transmission delays
and lost packets and reduces flow
Explicit congestion signaling
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Network alerts the end systems to growing congestion
End systems take steps to reduce offered load. 11
Explicit congestion signaling
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Direction
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Categories
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Backward
Forward
Binary
Credit-based
rate-based
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Traffic Management in Congested
Network – Some Considerations
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Fairness
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Quality of Service (QoS)
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Various flows should “suffer” equally
Last-in-first-discarded may not be fair
Flows treated differently, based on need
Voice, video: delay sensitive, loss insensitive
File transfer, mail: delay insensitive, loss sensitive
Interactive computing: delay and loss sensitive
Reservations
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Policing: excess traffic discarded or handled on
best-effort basis
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