high speed network

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Chapter 10Chapter 10
Congestion Control in Data
Networks and Internets

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IntroductionIntroduction
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

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Queuing TheoryQueuing 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

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Figure 10.1Figure 10.1

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At Saturation Point, 2 StrategiesAt 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 10.2Figure 10.2

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Ideal PerformanceIdeal 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 10.3Figure 10.3

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Practical PerformancePractical 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 10.4Figure 10.4

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Congestion ControlCongestion 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 signalingExplicit congestion signaling
Direction
– Backward
– Forward
Categories
– Binary
– Credit-based
– rate-based

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Traffic ManagementTraffic 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 10.5Figure 10.5

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Frame Relay Congestion ControlFrame 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 10.1Table 10.1

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Traffic Rate ManagementTraffic 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 10.6Figure 10.6

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Figure 10.7Figure 10.7

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Congestion Avoidance with ExplicitCongestion Avoidance with Explicit
SignalingSignaling
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 Signaling2 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