Chapter 10 Congestion Control in Data Networks1 Congestion Control in Data Networks and Internets COMP5416 Chapter 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 (  )

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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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8 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

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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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Chapter 10 Congestion Control in Data Networks 12 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

Chapter 10 Congestion Control in Data Networks 13 Explicit congestion signaling Direction Backward Forward Categories Binary Credit-based rate-based

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Chapter 10 Congestion Control in Data Networks 15 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

Chapter 10 Congestion Control in Data Networks 16 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)

Chapter 10 Congestion Control in Data Networks 17 Frame Relay Stack

Chapter 10 Congestion Control in Data Networks 18 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

Chapter 10 Congestion Control in Data Networks 19 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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Chapter 10 Congestion Control in Data Networks 21 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 (B c ) Maximum data over one interval agreed to by network Excess Burst Size (B e ) Maximum data over one interval that network will attempt

Chapter 10 Congestion Control in Data Networks 22 Figure 10.6

Chapter 10 Congestion Control in Data Networks 23 Figure 10.7 B C – committed burst size B e – excess burst size

Chapter 10 Congestion Control in Data Networks 24 Summary Congestion control and traffic management are required to ensure acceptable network performance Frame relay has simple schemes to handle traffic Next: TCP traffic management & control Traffic Mgmt Congestion Control