Congestion Control Algorithms

7/29/2019 Congestion Control Algorithms

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2/8/2013 Congestion Control Algorithms 1

Congestion ControlAlgorithms

Packets Sent

PacketsDelivered

Perfect

Desirable

Congested

Maximumcarryingcapacity ofsubnet

Congestion is a state of excessive accumulation or overfilling or overcrowding.


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Factors responsible forcongestion

Packet arrival rate exceeds the outgoing link

capacity.

Insufficient memory to store arriving packets

Bursty traffic

Slow processor

Congestion tends to feed upon itself & become

worse.


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Difference between CongestionControl & Flow Control

Congestion control has to make sure that

subnet is able to carry the offered load.

Flow control relates to point-to-point traffic

between a given sender & a receiver.


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Principles Of CongestionControl

Control Theory

Open Loop Control Closed Loop Control


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Open Loop Control

Open loop control attempts to solve theproblem by making sure it doesnt occur inthe first place.

Tools: When to accept a new packet.

Deciding when to discard packets & which

ones. Making scheduling decisions at various

points in the network.


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Closed Loop Control

Its based on the concept of feedback loop.

When applied to congestion control:

1. Monitors the system to detect when &where congestion occurs.

2. Pass this information to the places where

action can be taken.3. Adjust system operation to correct the

problem.


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Techniques

Several techniques can be employed.

These include:

Choke packets

Load shedding

Traffic shaping

The first 2 deal with congestion detectionand recovery. The last one deals with

congestion avoidance.


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Traffic Shaping

Forcing the packets to be transmitted in a

more predictable rate.

Traffic policing:

Monitoring the traffic flow.


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The Leaky Bucket Algorithm


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The Token Bucket Algorithm


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Leaky Bucket vs Token Bucket LB discards packets; TB does not. TB discards

tokens.

With TB, a packet can only be transmitted if there areenough tokens to cover its length in bytes.

LB sends packets at an average rate. TB allows forlarge bursts to be sent faster by speeding up theoutput.

TB allows saving up tokens (permissions) to sendlarge bursts. LB does not allow saving.


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Choke Packets

A more direct way of telling the source to

slow down.

A choke packet is a control packet

generated at a congested node and

transmitted to restrict traffic flow.

The source, on receiving the choke packet

must reduce its transmission rate by a

certain percentage.


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unew =auold ( 1-a ) f

U Utilization of its output lines.A a constant, how fast the router forgets the recent

history.

F instantaneous line utilization


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Weighted Fair Queuing

2019151161

161272

83

171394

1814105

A

B

C

D

E

O

Packet Finishing time

C 8

B 16

D 17

E 18

A 20

A router with 5 packets

queued for line O

Finishing times for 5

packets

C


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Choke Packets that affects only the source

B C

A D

E F

B C

A D

E F

Choke

B C

A D

E FChoke

B C

A D

E F

Choke

B C

A D

E FReduced

Flow

B C

A D

E FFlow is

still at

maximum

rateB C

A D

E F

Flow is

reduced


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Hop-by-hop Choke Packet

B C

A D

E FHeavy Flow

B C

A D

E F

Choke

B C

A D

E FChoke Reduced

Flow

B C

A D

E F

Choke

B C

A D

E F

a)

b)

c)

d)

e)


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Load Shedding

Load Shedding is a way in which the

routers when flooded with packets that it

cannot handle, just throws them away.


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Jitter Control

In this technique, the packets which are

ahead of schedule get slowed down &

packets that are behind schedule get

speeded up.


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Congestion Control For

Multicasting

RSVP--- Resource reSerVation Protocol

1 2

A

D

G

J

B C

E F

H I

K L

3 4 5

Sender

s

Receivers

Congestion Control Algorithms

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