Chapter 10 high speed networks

Chapter 10 Congestion Control in Data Networks and Internets
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Chapter 10

Congestion Control in Data
Networks and Internets
Chapter 10 Congestion Control in Data Networks and Internets
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Introduction
 Congestion occurs when number of
packets transmitted approaches network
capacity
 Objective of congestion control:
– keep number of packets below level at which
performance drops off dramatically
Chapter 10 Congestion Control in Data Networks and Internets
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Queuing Theory
 Data network is a network of queues
 If arrival rate > transmission rate
then queue size grows without bound and
packet delay goes to infinity
Chapter 10 Congestion Control in Data Networks and Internets
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Figure 10.1
Chapter 10 Congestion Control in Data Networks and Internets
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At Saturation Point, 2 Strategies
 Discard any incoming packet if no buffer
available
 Saturated node exercises flow control over
neighbors
– May cause congestion to propagate throughout
network
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Figure 10.2
Chapter 10 Congestion Control in Data Networks and Internets
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Ideal Performance
 I.e., infinite buffers, no overhead for
packet transmission or congestion control
 Throughput increases with offered load
until full capacity
 Packet delay increases with offered load
approaching infinity at full capacity
 Power = throughput / delay
 Higher throughput results in higher delay
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Figure 10.3
Chapter 10 Congestion Control in Data Networks and Internets
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Practical Performance
 I.e., finite buffers, 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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Figure 10.4
Chapter 10 Congestion Control in Data Networks and Internets
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Congestion Control
 Backpressure
– Request from destination to source to reduce
rate
– Choke packet: ICMP Source Quench
 Implicit congestion signaling
– Source detects congestion from transmission
delays and discarded packets and reduces flow
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Explicit congestion signaling
 Direction
– Backward
– Forward
 Categories
– Binary
– Credit-based
– rate-based
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Traffic Management
 Fairness
– Last-in-first-discarded may not be fair
 Quality of Service
– Voice, video: delay sensitive, loss insensitive
– File transfer, mail: delay insensitive, loss sensitive
– Interactive computing: delay and loss sensitive
 Reservations
– Policing: excess traffic discarded or handled on best-
effort basis
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Figure 10.5
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Frame Relay Congestion Control
 Minimize frame size
 Maintain QoS
 Minimize monopolization of network
 Simple to implement, little overhead
 Minimal additional network traffic
 Resources distributed fairly
 Limit spread of congestion
 Operate effectively regardless of flow
 Have minimum impact other systems in network
 Minimize variance in QoS
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Table 10.1
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Traffic Rate Management
 Committed Information Rate (CIR)
– Rate that network agrees to support
 Aggregate of CIRs < capacity
– For node and user-network interface (access)
 Committed Burst Size
– Maximum data over one interval agreed to by network
 Excess Burst Size
– Maximum data over one interval that network will
attempt
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Figure 10.6
Chapter 10 Congestion Control in Data Networks and Internets
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Figure 10.7
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Congestion Avoidance with Explicit
Signaling
2 strategies
 Congestion always occurred slowly,
almost always at egress nodes
– forward explicit congestion avoidance
 Congestion grew very quickly in internal
nodes and required quick action
– backward explicit congestion avoidance

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2 Bits for Explicit Signaling
 Forward Explicit Congestion Notification
– For traffic in same direction as received frame
– This frame has encountered congestion
 Backward Explicit Congestion Notification
– For traffic in opposite direction of received
frame
– Frames transmitted may encounter congestion