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End-to-End QoS Network Design by Christina Hattingh, Tim Szigeti - CCIE No. 9794

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Chapter 6. Congestion-Avoidance Tools

This chapter includes the following topics:

  • Random Early Detection

  • Weighted Random Early Detection

  • DSCP-Based Weighted Random Early Detection

  • Explicit congestion notification

Congestion-avoidance mechanisms, such as Weighted Random Early Detection (WRED), are complementary to (and dependent on) queuing algorithms. Queuing/scheduling algorithms manage the front of a queue, whereas congestion-avoidance mechanisms manage the tail of a queue.

Congestion-avoidance QoS tools are designed to handle TCP-based data traffic. TCP has built-in flow-control mechanisms that operate by increasing the transmission rates of traffic flows (even though bounded by buffers and window sizes) until packet loss occurs. At this point, TCP ...

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