Re: MDP .- WRED

From: Petr Lapukhov (petr@internetworkexpert.com)
Date: Mon Jul 03 2006 - 03:45:31 ART

Oh, I see your point now :)

Now, let's take a look how it works:

MDP is just a *probablity* of drop, when you reach the upper limit.

RED tracks "smoothed" average queue depth (AQD). Queue is
FIFO, so scheduler basically just takes the first packet and sends
it, _or_ drops it, depending on AQD and thresholds.

When AQD reaches low-threshold, you have drop probability of zero.
That is, no packets are dropped here, first packet in FIFO queue
is always sent.

When AQD grows from low to high threshold, drop probability is
linearly interpolated:

drop_prob = (AQD-min_thresh)/(max_thresh-min_thresh)*MDP

Finally, when you reach high threshold, approx every 1 of MDP
packets in queue head will be dropped, and every other is sent.
Above that threshold, every packet is dropped by scheduler

You see, MDP is a value to calculate drop_prob, and make a
"decision" for scheduler - send upper packet in FIFO queue, or
drop it.

HTH 

-- 
Petr Lapukhov, CCIE #16379
petr@internetworkexpert.com

Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Outside US: 775-826-4344

2006/7/3, Victor Cappuccio <cvictor@protokolgroup.com>:
>
> Petr, that was very informative.
>
>
>
> But I'm looking for the formula here.
>
>
>
> So if the Theory Says that the MPD is the fraction of packets drops, how
> many packets are dropped using that Config that you posed ?  512 of 20?
>
> 0,0390625 when the Average Q Depth is 20?
>
> Thanks
>
>
>
>
>
>   _____
>
> De: petrsoft@gmail.com [mailto:petrsoft@gmail.com] En nombre de Petr
> Lapukhov
> Enviado el: Lunes, 03 de Julio de 2006 01:36 a.m.
> Para: Victor Cappuccio
> CC: Cisco certification
> Asunto: Re: MDP .- WRED
>
>
>
> Victor,
>
> It may be set separately for precedence or dscp value.
> There are some basic default, at the same time.
>
> e.g.
>
> Rack1R6(config-if)#random-detect
> Rack1R6(config-if)#random-detect precedence 0 10 20 512
>
> 0   - precedence value
> 10 - min thresh
> 20 - max thresh
> 512 - MPD
>
> --
> Petr Lapukhov, CCIE #16379
> petr@internetworkexpert.com
>
> Internetwork Expert, Inc.
> http://www.InternetworkExpert.com
> Toll Free: 877-224-8987
> Outside US: 775-826-4344
>
> 2006/7/3, Victor Cappuccio <cvictor@protokolgroup.com>:
>
> Hi guys,
>
> I just can not get what they are trying to say
>
> http://www.cisco.com/en/US/products/sw/iosswrel/ps1831/products_configuratio
>
> n_guide_chapter09186a00800c60d9.html#1000915
>
> The mark probability denominator is the fraction of packets dropped when
> the
> average queue depth is at the maximum threshold. For example, if the
> denominator is 512, one out of every 512 packets is dropped when the
> average
>
> queue is at the maximum threshold.
>
> But then the MAX-TH of the Q depth is set to what number?  512??
>
> I think that they forgot to tell that
>
> Opinions?
>
> Thanks
> Victor.-
>
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