Cdt

up2thetime

Hey everyone,

Wondering if someone could help me out with the Congestive Discard Threshold.

I am using the QoS book and looking at the explanation on Pg 281.

For example, suppose we have 4 dynamically created flow based queues.
Supposed Queue 2 and Queue 3 are completely filled, meaning their CDT has been reached.
If a new packet (packetX) arrives which should be placed into Queue 2, and there is a higher SN packet
in Queue 3, then the higher SN packet from Queue 3 will be discarded... then packetX will be placed into Queue 2.

What I don't understand is, how does dropping a packet from a different queue free up space in another queue.
How would dropping a higher SN packet from Queue 3 allow us to enqueue packetX in Queue 2 since Queue 2 is at CDT and cannot hold anymore.

Any help would be appreciated! Thanks!

pitviper

That sounds about right – I guess the key is to think of the queue sizes being *somewhat* dynamic with the CDT not necessarily being the absolute ceiling (unless of course the congestion is all of the same type of flow).