Random Early Detection (RED) manages congestion proactively by dropping a percentage of packets after a queue passes a threshold, before the queue becomes full. This reduces tail drop and helps prevent TCP global synchronization, where many hosts reduce their sending windows simultaneously. section 10.6.2
Use RED with these operational boundaries in mind:
- Drop packets from only some flows before the queue fills, allowing affected hosts to reduce their TCP windows and helping lower overall traffic and queue size. section 10.6.2
- Distinguish RED from Weighted Random Early Detection (WRED): WRED adds control over which packets are dropped, including decisions based on AF drop precedence. section 10.6.2
- Avoid RED and WRED for loss-sensitive traffic; the LLQ priority queue is specifically identified as an unsuitable location. section 10.6.2
- Apply the intended QoS treatment at every device because QoS behavior, including handling of markings, is per-hop rather than automatically network-wide. section 10.6.2
