How does WRED achieve congestion avoidance?

Question

I understand what WRED is and what it does. I understand that once we exceed a certain threshold, it will start randomly dropping packets to prevent the queue from becoming full. I also understand that since it's weighted RED, it can be more strict towards low-priority traffic. This is all a basic explanation of course so I can save text and time.

My question is, WRED can drop packets more often from low-priority queues than from high-priority queues.

However, if we have two traffic classes therefore two queues - one for high-priority traffic and one for low-priority traffic, how would dropping something from the low-priority queue help the high-priority queue to avoid congestion? If they are two separate classes/queues with their own % of BW reserved.

Or in other words, I would understand this if low and high priority traffic were in the same queue and the lower-priority one would be dropped more often but they’re in separate queues, aren’t they? So dropping something from the low-prioriy queue wouldn’t really help the high-priority queue to "avoid congestion", or not?

Answer 1

WRED is per queue, and it helps to prevent TCP Global Synchronization by randomly dropping packets from a queue. Without it, multiple TCP streams can become synchronized such that they all detect congestion and back off at the same time, then synchronously get faster until they all again detect congestion. That can alternately fill and starve a queue.

By randomly dropping packets from a queue to prevent tail drop, the various TCP streams will even out more and not get synchronized.