This document discusses congestion control in computer networks. It begins by defining common traffic descriptors like average data rate and peak data rate. It then explains different traffic profiles such as constant bit rate and variable bit rate traffic. The document discusses how congestion occurs when network load exceeds capacity. It describes open-loop and closed-loop congestion control techniques, including choke packets and hop-by-hop choke packets. The document also discusses the Internet Control Message Protocol and how it is used for error reporting and source quenching to control congestion.

1. PRESENTED BY:
Arkadyuti Ranjan Das(roll no:107)

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CONTENTS
• Data traffic
• Congestion
Network performance
• Congestion Control
open-loop Congestion Control
Closed-loopCongestion Control
• Internet Control Message Protocol
Source Quench

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DATA TRAFFIC
The main focus of congestion control and quality of service is data traffic. In
congestion control we try to avoid traffic congestion. In quality of service, we
try to create an appropriate environment for the traffic. So, before talking
about congestion control, we discuss the data traffic itself.
 Traffic Descriptor
 Traffic Profiles

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 Traffic descriptors
traffic descriptors are qualitative values
that represent a data flow.
Average data rate
The average data rate is the number of bits sent during a period of a time , divided by the
number of a seconds in that period.
Peak data rate
The peak data rate defines the maximum rate of the traffic.
Maximum burst size
The Maximum burst size normally refers to the maximum length of time the traffic is
generated at the peak rate.

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 traffic profiles
constant bit rate
A constant bit rate has a data rate
that does not change . In this type
of flow, the average data rate and the
peak data rate are the same.
Variable bit rate
In theVariable bit rate category, the rate
of the data flow changes in time, with the changes smooth instead of sudden and sharp. In this
type of flow, the average data rate and the peak data rate are different.

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burst
In the burst data category, the data rate changes
suddenly in a very short time . It may jump from
zero, for 1 Mbps in a microseconds and vice
versa.The average bit rate and peak bit rate
are very different in this type of flow

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 CONGESTION
Congestion in a network may occur if the load on the network—the number of
packets sent to the network—is greater than the capacity of the network—the
number of packets a network can handle. Congestion control refers to the
mechanisms and techniques to control the congestion and keep the load below
the capacity.
Congestion happens on a freeway because any abnormality in the flow, such as
accident during rush hour, creates blockage.
Queues in a router

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 Network performance
Congestion control involves two factor that measure the performance of network : delay and
throughput.
delay versus load : When the load is much less then the capacity of the
network, the delay is at a minimum .When the load reaches the net-
work capacity, the delay increases sharply because we now need to add
the waiting time in the queues to the total delay .Then the delay
becomes infinite when the load is greater then the capacity . Delay has
negative effect on the load and consequently the congestion.

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Throughput versus load :Throughput in a network as the number
of packets passing through the network in a unit of time.When the
load is below the capacity of the network, the throughput increases
proportionally with the load .We expect the throughput to remain
constant after the load reaches the capacity, but instead the through-
put declines sharply .The reason is the discarding of packets by the
the routers.

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Congestion Control inTransport
Layer

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 Congestion control
Congestion control refers to techniques and mechanisms that can either prevent
congestion, before it happens, or remove congestion, after it has happened. In general, we
can divide congestion control mechanisms into two broad categories:
open-loop congestion control (prevention)
closed-loop congestion control (removal)

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 Open loop congestion control
In open loop congestion control, policies are applied to prevent congestion before it happens .
congestion control is handled by either the source or the destination.
Retransmission policy : Retransmission is sometimes unavoidable. If the sender feels that a
sent packet is lost or corrupted, the packet needs to be retransmitted. Retransmission in general
may increase congestion in the network. However, a good retransmission policy can prevent
congestion.
Window Policy :The type of window at the sender may also affect congestion.
Acknowledgment Policy : The acknowledgment policy imposed by the receiver may also
affect congestion. If the receiver does not acknowledge every packet it receives, it may slow down
the sender and help prevent congestion.

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Discarding Policy :A good discarding policy by the routers may prevent congestion and at the
same time may not harm the integrity of the transmission.
Admission Policy :An admission policy, which is a quality-of-service mechanism, can also
prevent congestion in virtual-circuit networks.

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 Closed-Loop Congestion Control
Closed-loop congestion control mechanisms try to alleviate congestion after it happens
Back-pressure: The technique of backpressure refers to a congestion control mechanism in
which a congested node stops receiving data from the immediate upstream node or nodes.This may
cause the upstream node or nodes to become congested, and they, in turn, reject data from their
upstream nodes or nodes.And so on. Backpressure is a node-to-node congestion control that starts
with a node and propagates, in the opposite direction of data flow, to the source.

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 Choke packet
A choke packet is In the choke packet method, the warning is from the router, which
has encountered congestion, to the source station directly .A packet sent by a node to
the source to inform it of congestion.The intermediate nodes through which the
packet has traveled are not warned.

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 Hop-by Hop Choke Packets
 Over a long distances or at the speeds choke packets are not very effective .
 A more efficient method is to send to choke packets hop-by-hop.
 This requires each hop to reduce its transmission even before the choke
packet arrive at the source.

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Congestion Control in Network
Layer

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 Internet Control Message Protocol(ICMP)
It is a supporting protocol in the Internet protocol. It is used by network devices to send
error messages and operational information indicating
ERROR REPORTING :
ICMP does not correct errors, it simply reports them.
ICMP always reports the message back to the source.
Not error correction, because there is no transmission of lost or damaged packets.

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Source Quench :
 Weak form of flow control
 When host sending packets gets source quench, it slows down
 If another source quench message arrives, slows down even more
 If source quench messages stop, slowly increases speed

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REFERENCES :
Links:
http://nptel.ac.in/courses/106105080/pdf/M7L5.pdf (8-9-2017 9:23a.m)
books:
data communication and networking 4th edition by Behrouz a forouzan

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Thank you