chapter 10 congestion control in data networks1 congestion control in data networks and internets...
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Chapter 10 Congestion Control in Data Networks
1
Congestion Control in Data Networks and Internets
COMP5416Chapter 10
Chapter 10 Congestion Control in Data Networks 2
Review Performance and QoS are key design
requirements for networks Greater PC computing power, distributed
applications, multimedia contents driven the needs for higher capacity LANs (i.e. Gigabit Ethernet) and WANs (ATM & WDM)
Key to design is ability to model and estimate performance metrics
Has profound effects on network configurations and protocol design
Queueing analysis and simulations are some tools Key to monitor networks in (near) congestion:
Need to device congestion and traffic management tools
Chapter 10 Congestion Control in Data Networks 3
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 4
Queuing Theory Data network is a network of
queues If arrival rate > transmission rate
(>) queue size grows without bound
and packet delay goes to infinity ()
Chapter 10 Congestion Control in Data Networks 6
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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Ideal Network Performance I.e., infinite buffers and 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
Chapter 10 Congestion Control in Data Networks 10
Practical Performance I.e., finite buffers and 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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Congestion Control Approaches
Backpressure Request from destination to source to
reduce rate Choke packet: ICMP Source Quench
Implicit congestion signalling 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 Issues Fairness
Last-in-first-discarded (i.e. drop-tail) may not be fair
Quality of Service (QoS) – provision of service differentiation 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
TM CC
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Example: Frame Relay A high-performance WAN protocol that
operates at the physical and data link layers of the OSI reference model
It provides connection-oriented link layer communication Connection exists between each pair of
devices and are associated with a connection identifier (DLCI)
Two categories of virtual connections: switched virtual circuits (SVCs) permanent virtual circuits (PVCs)
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Frame Relay Congestion Control Implements a simple congestion-notification
mechanisms (i.e. binary) rather than explicit, per-virtual-circuit flow control
Flow control left to higher-layer protocols FR uses two congestion-notification mechanisms:
Forward-explicit congestion notification (FECN) Backward-explicit congestion notification (BECN)
Each is controlled by a one bit in FR frame header Header also contains a Discard Eligibility (DE) bit
which is used to identify less important frames that can be dropped during periods of congestion
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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
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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 Committed Burst Size (Bc)
Maximum data over one interval agreed to by network
Excess Burst Size (Be) Maximum data over one interval that
network will attempt
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Figure 10.7
BC – committed burst size
Be – excess burst size