Lect-16 - Transport Protocols Congestion Avoidance ECE/CS 438 Fall 2011 Slides mostly from Peterson Davies ECE/CS 438 1 Congestion Avoidance

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ECE/CS 438 1 Transport Protocols – Congestion Avoidance ECE/CS 438 Fall 2011 Slides mostly from Peterson & Davies
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Congestion Avoidance Mechanism n It is important to understand that TCP s strategy is to control congestion once it happens, as opposed to trying to avoid congestion in the first place. n In fact, TCP repeatedly increases the load it imposes on the network in an effort to find the point at which congestion occurs, and then it backs off from this point. n An appealing alternative, but one that has not yet been widely adopted, is to predict when congestion is about to happen and then to reduce the rate at which hosts send data just before packets start being discarded. n We call such a strategy congestion avoidance, to distinguish it from congestion control.
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Congestion Avoidance Mechanism n DEC Bit ¡ The first mechanismwas developed for use on the Digital Network Architecture (DNA), a connectionless network with a connection-oriented transport protocol. ¡ This mechanism could, therefore, also be applied to TCP and IP. ¡ As noted above, the idea here is to more evenly split the responsibility for congestion control between the routers and the end nodes.
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Congestion Avoidance Mechanism n DEC Bit ¡ Each router monitors the load it is experiencing and explicitly notifies the end nodes when congestion is about to occur. ¡ This notification is implemented by setting a binary congestion bit in the packets that flow through the router; hence the name DECbit. ¡ The destination host then copies this congestion bit into the ACK it sends back to the source. ¡ Finally, the source adjusts its sending rate so as to avoid congestion
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Congestion Avoidance Mechanism n DEC Bit ¡ A single congestion bit is added to the packet header. A router sets this bit in a packet if its average queue length is greater than or equal to 1 at the time the packet arrives. ¡ This average queue length is measured over a time interval that spans the last busy+idle cycle, plus the current busy cycle.
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Congestion Avoidance Mechanism n DEC Bit ¡ Essentially, the router calculates the area under the curve and divides this value by the time interval to compute the average queue length. ¡
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This note was uploaded on 02/08/2012 for the course ECE 438 taught by Professor Luo during the Fall '08 term at University of Illinois, Urbana Champaign.

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Lect-16 - Transport Protocols Congestion Avoidance ECE/CS 438 Fall 2011 Slides mostly from Peterson Davies ECE/CS 438 1 Congestion Avoidance

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