Re: 3560 Policer Burst Value

Regarding method 1, if you look at traditional CAR based policing on a
router, that is the recommended way to calculate burst on the docCD and is
pretty well known. The way I have always understood it is that it would
allow bursts up to 1.5x the CIR for short periods of time.

On Thu, Sep 15, 2011 at 7:25 PM, Carlos G Mendioroz <tron_at_huapi.ba.ar>wrote:

> Joe,
> the method 1 makes litle sense to me. This is assigning 1 whole second of
> burst (actually, 1.5). Why would you do that ?
>
> Here's what I believe to be fundamental:
> -you are trying to police a given rate. As you are monitoring, and you have
> the delta to the previous packet, if your traffic was even spaced
> the rate is all you need. You can tell if a packet is going over the rate
> without further info.
> -this is hardly a reality, and jitter will happen, and bursts will appear.
> That is a bunch of bytes that exceed the mean rate if taken without some
> consideration. This is somehow related to how much time
> the link can delay the delivery of some byte, piling the following
> and arriving all together at access rate for some time.
> Method 2, I guess, is an estimation of a high mark for that delay.
> Multiply by 2 just as safety.
> -In the end, I doubt that if you pick a larger than needed value, it
> will affect anything too much. This is not memory allocation, just a
> safety not to drop on bursting. Any long time rate excess will be dropped
> no matter what.
>
> -Carlos
>
>
> Joe Astorino @ 15/09/2011 19:31 -0300 dixit:
>
> I am having a real hard time finding good information on this topic for
>> use
>> in the real world. In the lab, we would usually just configure the burst
>> size we are told on a Cat 3560. I have done a LOT of reading on it, and
>> there are a lot of conflicting stories with regards to this.
>>
>> Basically, I am trying to find out how to calculate an optimal burst value
>> on a 3560 QoS policy doing policing. As you probably know the syntax
>> looks
>> like this:
>>
>> police [rate in bits/s] [burst size in bytes]. Remember, this is policing
>> not shaping so the classic shaping formula of tc = bc/cir has no relevance
>> here mainly because the token refresh rate is not based on a static set
>> amount of time. The burst size is actually the size of the token bucket
>> itself in bytes, not a rate of any kind and it is filled as a function of
>> the policed rate and the packet arrival rate. The refill rate of the
>> bucket
>> is not based on a static amount of time like in FRTS for example. It
>> basically says "how long was it since the last packet...multiply that
>> times
>> the policed rate, and divide by 8 to give me bytes". In other words it
>> pro-rates the tokens. Makes sense.
>>
>> Anyways...I have found 2 sort of "methods" to calculating this, but they
>> are
>> so far off from one another I am not quite sure which one to use in the
>> real
>> world.
>>
>> Method 1: The classic CAR formula we see on routers: (rate * 1.5) / 8.
>> This basically gives you 1.5x the policed rate, and converts it to bytes.
>> Makes sense.
>> Method 2: 2x the amount of traffic sent during a single RTT.
>>
>> In my case, I am trying to police a video conferencing endpoint to 3Mbps
>> so
>> by method 1 that gives me a burst size of 562,500 bytes. Using method 2,
>> let's just say I have an average RTT of 100ms. That method would yield a
>> burst size of 75,000 bytes. That is a HUGE difference
>>
>> This came about because the video endpoint was dropping frames. I noticed
>> the policed rate in the policy was 3,000,000 but the burst size was 8000
>> bytes (the lowest possible value). When I changed the burst based on a
>> 100ms RTT and the above formula the problem went away, but now I am having
>> doubts on the proper value to use here.
>>
>> Does anybody have any insight on how to actually calculate this properly?
>>
>>
> --
> Carlos G Mendioroz <tron_at_huapi.ba.ar> LW7 EQI Argentina
>

-- 
Regards,
Joe Astorino
CCIE #24347
Blog: http://astorinonetworks.com
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