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Computer Communication & Networks

Lecture 25

Transport Layer: Congestion Control

http://web.uettaxila.edu.pk/CMS/coeCCNbsSp09/index.asp

Waleed Ejazwaleed.ejaz@uettaxila.edu.pk

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Transport Layer

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Transport Layer Topics to CoverUDP, TCP, STCP

Congestion Control

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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.

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Question:

Is it possible to avoid/reduce congestion by increasing buffer size?

Congestion in packet switching networks

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NO

Buffer is larger, then waiting time is longer. It willcause time out and retransmit packets, thus more

congestion. If buffer size is infinite, then packet can delay forever.

Congestion control is a very hard problem

Congestion in packet switching networks

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Packet delay and throughput as functions of load

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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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Example

To better understand the concept of congestion control, let us give an example: Congestion Control in TCP.

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Slow Start, Exponential Increase In the slow-start algorithm, the size of the

congestion window increases exponentially until it reaches a threshold.

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Congestion Avoidance: Additive Increase In the congestion avoidance algorithm, the

size of the congestion window increases additively until congestion is detected.

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Congestion Example

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Quiz 5 (1)

Use the Bellman Ford algorithm to find the shortest paths from all other nodes to node 3.

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Quiz 5 (2)

Apply Dijkstra’s algorithm to the following network to generate a shortest-path-tree for node E, and build a routing table for node E.