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Chapter 10. Congestion Control in Data Networks and Internets. Introduction. Packet–switched networks get congested! Congestion occurs when the number of packets transmitted approaches network capacity Objective of congestion control :

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Chapter 10

Chapter 10

Congestion Control in Data Networks and Internets


Introduction
Introduction

  • Packet–switched networks get congested!

  • Congestion occurs when the number of packets transmitted approaches network capacity

  • Objective of congestion control:

    • keep the number of packets that are entering/within the network below the level at which performance drops off dramatically

Chapter 10: Congestion Control


Queuing theory
Queuing Theory

  • Recall from Chapter 8 that a data network is a network of queues

  • If arrival rate at any queue > transmission rate from the node then queue size grows without bound and packet delay goes to infinity

  • Rule of Thumb Design Point:  = L/R < .8 *

Chapter 10: Congestion Control


Input output queues at a node
Input & Output Queues at a Node

Ts = L/R

Ts = L/R

Nodal

Processing

Ts = L/R

Chapter 10: Congestion Control


At saturation point
At Saturation Point

Two Possible Strategies at Node:

  • Discard any incoming packet if no buffer space is available

  • Exercise flow control over neighbors

    • May cause congestion to propagate throughout network

Chapter 10: Congestion Control



Jackson s theorem application in packet switched networks

Internal load:

 =  i

where:

 = total on all links in network

i = load on link i

L = total number of links

L

i=1

  • Note:

  • Internal > offered load

  • Average length for all paths:

  • E[number of links in path] = /

  • Average number of items waiting

  • and being served in link i: ri = i Tri

  • Average delay of packets sent

  • through the network is:

  • T = 

  • where: M is average packet length and

  • Ri is the data rate on link i

External load, offered to network:

 =   jk

where:

 = total workload in packets/sec

jk = workload between source j

and destination k

N = total number of (external)

sources and destinations

N N

j=1 k=2

Mi

Ri - Mi

1

L

i=1

Jackson’s Theorem - Application in Packet Switched Networks

Packet Switched

Network

Notice: As any i increases,

total delay increases.

Chapter 10: Congestion Control


Ideal performance
Ideal Performance

  • I.e., infinite buffers, 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

    • Higher throughput results in higher delay

Chapter 10: Congestion Control


Ideal network utilization
Ideal Network Utilization

Load:

Ts = L/R

Power: relationship between

Normalized Throughput and Delay

Chapter 10: Congestion Control


Practical performance
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

Chapter 10: Congestion Control


Effects of congestion

What’s happening here?

Effects of Congestion

  • buffers fill

  • packets discarded

  • sources re-transmit

  • routers generate more traffic to update paths

  • good packets resent

  • delays propagate

Chapter 10: Congestion Control


Common congestion control mechanisms
Common Congestion Control Mechanisms

Chapter 10: Congestion Control


Congestion control
Congestion Control

  • Backpressure

    • Request from destination to source to reduce rate

    • Useful only on a logical connection basis

    • Requires hop-by-hop flow control mechanism

  • Policing

    • Measuring and restricting packets as they enter the network

  • Choke packet

    • Specific message back to source

    • E.g., ICMP Source Quench

  • Implicit congestion signaling

    • Source detects congestion from transmission delays and lost packets and reduces flow

Chapter 10: Congestion Control


Explicit congestion signaling
Explicit congestion signaling

  • Direction

    • Backward

    • Forward

  • Categories

    • Binary

    • Credit-based

    • Rate-based

Chapter 10: Congestion Control


Traffic management in congested network some considerations
Traffic Management in Congested Network – Some Considerations

  • Fairness

    • Various flows should “suffer” equally

    • Last-in-first-discarded may not be fair

  • Quality of Service (QoS)

    • 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

    • Policing: excess traffic discarded or handled on best-effort basis

Chapter 10: Congestion Control


Frame relay congestion control
Frame Relay Congestion Control Considerations

  • Minimize frame discard

  • Maintain QoS (per-connection bandwidth)

  • 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

Chapter 10: Congestion Control


Frame relay techniques

more Considerations

more

more

Frame Relay Techniques

Chapter 10: Congestion Control


Congestion avoidance with explicit signaling
Congestion Avoidance with Explicit Signaling Considerations

Two general strategies considered:

  • Hypothesis 1: Congestion always occurs slowly, almost always at egress nodes

    • forward explicit congestion avoidance

  • Hypothesis 2: Congestion grows very quickly in internal nodes and requires quick action

    • backward explicit congestion avoidance

Chapter 10: Congestion Control


Congestion control becn fecn
Congestion Control: BECN/FECN Considerations

Chapter 10: Congestion Control


Fr 2 bits for explicit signaling
FR - 2 Bits for Explicit Signaling Considerations

  • 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

Chapter 10: Congestion Control


Explicit signaling response
Explicit Signaling Response Considerations

  • Network Response

    • each frame handler monitors its queuing behavior and takes action

    • use FECN/BECN bits

    • some/all connections notified of congestion

  • User (end-system) Response

    • receipt of BECN/FECN bits in frame

    • BECN at sender: reduce transmission rate

    • FECN at receiver: notify peer (via LAPF or higher layer) to restrict flow

Chapter 10: Congestion Control


Frame relay traffic rate management parameters
Frame Relay Traffic Rate Management Parameters Considerations

  • Committed Information Rate (CIR)

    • Average data rate in bits/second that the network agrees to support for a connection

  • Data Rate of User Access Channel (Access Rate)

    • Fixed rate link between user and network (for network access)

  • Committed Burst Size (Bc)

    • Maximum data over an interval agreed to by network

  • Excess Burst Size (Be)

    • Maximum data, above Bc, over an interval that network will attempt to transfer

Chapter 10: Congestion Control


Committed information rate cir operation

Current rate at which user is sending over the channel Considerations

Average data rate (bps) committed to the user by the Frame Relay network.

Maximum data rate over time period allowed for this connection by the Frame Relay network.

Maximum line speed of connection to Frame Relay network

(i.e., peak data rate)

Committed Information Rate (CIR) Operation

Be

Bc

  • CIRi,j  AccessRatej

    i

Chapter 10: Congestion Control


Frame relay traffic rate management parameters1

CIR = bps Considerations

Bc

T

Frame Relay Traffic Rate Management Parameters

Max.

Rate

Chapter 10: Congestion Control


Relationship of congestion parameters
Relationship of Congestion Parameters Considerations

Bc

CIR

Note that T =

From ITU-T I.370

Chapter 10: Congestion Control


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