Random Early Detection (RED)
- Router notices that queue is getting full and randomly drops packets to signal congestion
- Packet drop probability
- Drop probability increases as queue length increases
- Else, set drop probability to some function of average queue length
Properties of RED:
- Drops packets before queue full, in hopes of reducing the rates of some flows
- Drops packets in proportion to each flow’s rate
- High-rate flows selected more often
- Drops are spaced out in time to help desynchronize the TCP senders
- Tolerant of burstiness in traffic by basing decisions on average queue length
Problems with RED
- Hard to get tunable parameters just right
- How early to start dropping packets?
- What slope for increase in drop probability?
- What time scale for averaging queue length?
- Has to drop packet to give feedback, unlike ECN
- RED has mixed adoption in practice
- If parameters not set right, RED doesn’t help