From: Carlos G Mendioroz (tron@huapi.ba.ar)
Date: Sun May 23 2004 - 19:05:00 GMT-3
>>Yes, once you start queuing, you keep queueing (you are not supposed to
>>reorder traffic in traffic shaping) and then transmit from the queue to
>>the tx-ring as much as Bc per Tc. Eventually you either empty the queue
>>and fall back to initial strategy or you start tail-dropping excess
>>traffic that does not fit your shaping buffer.
>
>
> That makes sense. Until I come across a better explanation, this is the one
> I'll stick to . Thanks.
>
>
>>All shaping metrics are done based on Tc. Tc is calculated from rate
>>and Bc. Then there is that shape to average or peak. I don't really know
>>what the difference between shaping to average with be = 0 and shaping
>>to peak would be though, but it comes handy to easily define different
>>shaping schemes. (AFAIK, shaping to peak with given (Bc,Be) is the same
>>as shaping to average with (Bc+Be,0).
>
>
> If Be is 0 then the shaping is as aggressive as possible -- at no point
> would the real rate be greater than Bc in a Tc period of time.
Yes, and that's what you get by shaping to peak too.
> If Be is set
> such that the total bucket size is equal to the peak rate divided by Tc,
> then you are allowing an occassional burst to go to maximum capacity of the
> interface.
I don't follow what you mean here.
Bucket size will be Bc + Be, but only Bc applies to "refill every Tc".
Peak rate is something that only makes sense when you shape to peak
(i.e. an analogy to Be=0) or else is something very lousy.
If you have a huge Be, then peak rate will only be limited by your
access rate. AFAIK, there's no way to limit peak rate different from CIR
for longer than a Tc.
>
>
>>One thing that seems not clear is that Be does not apply to a Tc, but
>>instead is the size of your "overrun" quota. That quota is
>>"rechargeable" so to speak, as long and you drop under cir.
>
>
> In other words shaping is a "save now, spend more later" approach. Which is
> differeent from traffic policing -- "spend more now, pay it back later".
>
Don't follow you here either. Shaping and policing are quite different
animals, even though their implementation use quite simmilar tools.
Cisco tends to explain them based on simmilarities, and that sometimes
generates some confusion.
I like to think of shaping as what you do not to be policed. With same
parameters, a shaped traffic will not be dropped/marked by a policer.
In that sense, "save now" is just "have up to Be available for later
use", and "spend more now" is "use that saved Be". Same thing...
-- Carlos G Mendioroz <tron@huapi.ba.ar> LW7 EQI Argentina
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