Congestion

Dr. Abdulaziz Almulhem

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Congestion

It occurs when network resources are becoming scarce

High demand

Over utilized

Offered load exceeds throughput

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Taxonomy

Router-Centric vs. Host-CentricRouters takes responsibility for forwarding

decisions.Host observes the network behavior and make

decisions.

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Taxonomy

Reservation-Based vs. Feedback-Based

Resources are allocated based on request

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Congestion Control

Proactive vs. ReactiveBackpressureChoke packets Implicit congestion controlExplicit congestion controlPolicing

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Taxonomy

Window-Based vs. Rate-BasedSame terminology applies to flow control;

however congestion control deals with resource allocation.

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TCP Congestion Control

Congestion control in TCP/IP is complex and difficult, because: IP is connectionless: no way to provision

congestionTCP is end-to-end: deduces indirectly

presence of congestionTCP entities cannot cooperate to maintain

certain level of flow (greedy way)

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TCP Flow Control

The rate at which TCP source can send data is determined by the rate at which acknowledgements are received. Rate of Ack arrival is determined by the bottleneck

in the roundtrip path.

TCP automatically senses the network bottleneck and regulates its flow accordingly TCP’s self-clocking

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Network Congestion

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Destination Congestion

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Improving Performance

Retransmission timer management:The value has a critical effect on TCP

reaction to congestion:RTT variance estimationExponential RTO backoffKarn’s algorithm

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Improving Performance

Window managementManaging the size of the send window has

a critical effect on TCPSlow startDynamic window sizing on congestionFast retransmitFast recovery

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Slow start

To avoid reaching congestion levels quickly, the window size is incremented progressively.

Upon receiving an Ack for a specific segment, the window is incremented by 2.

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Dynamic Window Sizing

Slow start is effective for initializing a connectionThis can grow infinitely however at some point in time resources will fill and a segment will get lost (congestion)What to do?

Reset window to 1 and start all over (not practical, congestion may repeat)

Caution could be possible

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Congestion detection

Congestion recovery

Congestion avoidance

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Fast Retransmit

Instead of reacting to lost segments by setting the window to 1Ack for missing segment will be received;

however sender will continue sending subsequent segments.

If three duplicate Acks are received, that segment is retransmitted regardless of time out.

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Fast Recovery

Since duplicate Ack are getting through fairly regularly, then there is no point to set window to 1.Better to cut window to half Proceed with increasing window size

linearly (congestion avoidance)

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