WFQ Early dropping

Bosefus

Good afternoon all,

I have a question regarding WFQ early dropping. From what I can gather WFQ Early dropping occurs when the CDT for a given queue is reached.

The ONT book states that there is an exception, if there is another packet in a different queue with a higher sequence number, that packet is dropped instead of the packet with the lower sequence number whos queues CDT is full.

My question is, how does dropping a packet in one queue affect the CDT of another queue? I would think that this would just affect the WFQ Hold queue.

-Thanks!

networker050184

I think you are mixing WFQ and WRED together. WFQ does not drop packets, it just queues them until tail drop occurs. WRED randomly drops packets to prevent tail drop.

Bosefus

The ONT Exam cert guide states

" Each flow based queue within WFQ has a CDT. If a packet arrives and the hold queue is not full but the CDT of that packet flow has been reached, the packet is dropped. This is called WFQ Early Dropping. This has one exception, if a packet in another queue has a higher sequence number than the arriving packet, the packet with the higher sequence number is dropped instead"

Maybe the working of the book is throwing me off, any help would be appreciated. Thanks!

networker050184

Ok sorry, I was misunderstanding your question there.

The hold queue has a maximum number of packets it can hold which is the sum of all flow queues. If a flow queue is full but the hold queue is not full, it can drop a packet from a lower weighted queue and give the space to a higher weighted packet. So, dropping a packet will not affect the CDT of another queue, but will allow lower weighted packets to be dropped before higher weighted packets. Keep in mind this is still tail drop and not random dropping to prevent queues from filling.

Bosefus

So the flow queues size can change dynamically as need?


For example, if a high priority packet comes in and its flow queue CDT is reached, another queue with lower priority packets will drop a packet, which which free up more space for the priority's packet queue?

Thanks for helping me work with this concept.

networker050184

Yes that is my understanding of how it works.