MPD mark probebility denominator

[devang_etcom]

in RED we use one parameter called MPD we can configure MPD for different flow of traffice as per MPD the percentage of packet drop, Right!!! but of which percentage of traffice is drop???

reply me

regards

devang

[ekiriakos]

Hi,

MPD determines how aggressively packets are droped. The lower the number the more aggressive the algorithm is.

Here is how it works. With RED you define a min and max Thresholds for the flow. These numbers mean the min and mux number of packets in the queue.

Say for example you had minTH=15 and maxTH=40 with a MPD=10. This means that for a queue_size<15 RED is inactive. For queue_size between 15-40 RED is active and the closer you are to 40 the higher the probability for a packet to be dropped. For example if the queue_size is (15+40)/2 packets (i.e middle of the drop probability curve (if my maths are correct), then 1 out of every 20 packets are droped. At queue_size 40 1 out of every 10 packets are droped. When you exceed 40 you have tail drop.

Now you can see that if MPD=1, then in the middle of the curve you would drop 1 out of every 2 packets...

HTH

E.

[devang_etcom]

thanks for reply,

but what ever i know is if you configure your MPD is 10 then 1/10 means 10% of packets are drop from new arriving packet right but how it will calculate 10% from which quantity??? and if my MPD is 1 then 1/1 means 100% packet drop....please clear me it if i am wrong and if u have good link then please send me...

regards

devang

[pkhatri]

Hi Devang,

The "quantity" you are interested in is the packets hitting the queue. So if you have an MPD of 10, then 10% of packets will be dropped if the queue fill level is such that the maximum drop probability applies. Assuming that this is the case, the router will drop every 10th packet that hits the queue (this is just an example). So packets 1-9 will make it through but packet 10 will be dropped. Packets 11-19 will make it through but packet 20 will be dropped. This is not entirely accurate since the dropping is a bit more random but this is probably a good way of understanding it...

Hope that helps - pls rate the post if it does.

Paresh

[devang_etcom]

hi pareshbhai,

it means it drop the packet which arrived at que and can u give me some source detail to clear more and more

regards

devang

[ekiriakos]

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos_c/fqcprt3/qcfconav.htm

Start with this document. The router works out the average queue size based on the formula shown in the document. The packet discard probability will depend on the average queue size, min/max ths and MPD. See also Fig.9

Rgds

E.