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CLI-based Configuration Guide - QoS

AR100-S, AR110-S, AR120-S, AR150-S, AR160-S, AR200-S, AR1200-S, AR2200-S, and AR3200-S V200R009

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Congestion Avoidance

Congestion Avoidance

Congestion avoidance is a mechanism used to control service flows. A system configured with congestion avoidance monitors network resource usage such as queues and memory buffers. When congestion occurs or aggravates, the system starts to discard packets.

Congestion avoidance uses tail drop and WRED to discard packets.

  • Traditional tail drop policy

    The traditional packet drop policy uses the tail drop method. When the length of a queue reaches the maximum value, all the packets last added to the queue (at the tail of the queue) are discarded.

    This packet drop policy may cause global TCP synchronization. As a result, TCP connections cannot be set up. The three colors represent three TCP connections. When packets from multiple TCP connections are discarded, these TCP connections enter the congestion avoidance and slow start state. Traffic reduces, and then reaches the peak. The volume of traffic varies greatly.

    Figure 4-1  Tail drop policy

  • WRED

    To avoid global TCP synchronization, Random Early Detection (RED) is used. The RED mechanism randomly discards packets so that the transmission speed of multiple TCP connections is not reduced simultaneously. In this manner, global TCP synchronization is prevented. The rate of TCP traffic and network traffic becomes stable.

    Figure 4-2  RED

    The device provides Weighted Random Early Detection (WRED) based on RED technology. WRED discards packets in queues based on DSCP priorities or IP priorities. The upper drop threshold, lower drop threshold, and drop probability can be set for each priority. When the length of a queue is smaller than the lower drop threshold, no packets are discarded. When the length of a queue exceeds the upper drop threshold, all new packets in the queue are discarded. When the length of a queue is between the upper drop threshold and the lower drop threshold, new packets are discarded randomly. A longer queue means higher drop probability, but the drop probability has a maximum value.


    LAN-side subcards do not support WRED.

Updated: 2019-05-17

Document ID: EDOC1000174115

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