Re: [OPSAWG] AD review of draft-ietf-opsawg-large-flow-load-balancing (draft response)

[Benoit Claise <[e-mail]>]

On 14/04/2014 19:09, Anoop Ghanwani wrote:
> Hi Benoit,
>
> I will work on the editorials shortly and I'm removing those from the 
> discussion.   See below:
>
>
> On Mon, Apr 14, 2014 at 5:28 AM, Benoit Claise <[e-mail] 
> <mailto:[e-mail]>> wrote:
>
>     Hi Anoop,
>
>     Thanks for the new draft version.
>     I removed some of the points
>>
>>
>>     On Tue, Feb 18, 2014 at 7:55 AM, Benoit Claise <[e-mail]
>>     <mailto:[e-mail]>> wrote:
>>
>>         -
>>
>>             A number of routers support sampling techniques such as sFlow [sFlow-
>>             v5, sFlow-LAG], PSAMP [RFC 5475] and NetFlow Sampling [RFC 3954].
>>             For the purpose of large flow identification, sampling must be
>>             enabled on all of the egress ports in the router where such
>>             measurements are desired.
>>
>>         I don't understand the second sentence.
>>         One way to read this is: sampling must be _enabled _on all of
>>         the egress ports where such measurements are desired.
>>             Ok, this is an obvious statement. If the measurements are
>>         desired, enable them
>>
>>
>>     Yes,
>>
>>         Or maybe you want to say: _sampling _must be enabled on all
>>         of the egress ports where such measurements are desired.
>>             This is a false statement: if you have the choice between
>>         sampling and non sampling, use non sampling measurements.
>>         Or maybe you want to say: sampling must be enabled on _all
>>         _of the egress ports where such measurements are desired.
>>             This is a false statement: if I have ECMP on 2 links, and
>>         only one of them can't do non sampling, then we should not force
>>             sampling on both links.
>>         You see, I'm confused.
>>
>>         You miss a couple of key messages:
>>         - if unsampled measurements are available, use those.
>>         - egress means where LAG/ECMP are enabled (this is important
>>         for the paragraph starting with "If egress sampling is not
>>         available, ingress sampling can suffice since the central
>>         management entity use")
>>
>>
>>     We were not intending to discuss a mix sampling and non-sampling
>>     interfaces in the same router, but this is a reasonable point and
>>     it will be clarified (i.e. we will state that it's possible to
>>     mix sampled and non sampled interfaces as long as the function of
>>     large flow detection/identification can be performed).
>     You're still missing the point that unsampled measurements is
>     better than sampled ones.
>
>
> We do point this out in Section 4.3.4.
> http://tools.ietf.org/html/draft-ietf-opsawg-large-flow-load-balancing-10#section-4.3.4
> >>>
>          As link speeds get higher, sampling rates are typically reduced
>          to keep the number of samples manageable which places a lower
>          bound on the detection time.  With automatic hardware
>          recognition, large flows can be detected in shorter windows on
>          higher link speeds since every packet is accounted for in
>          hardware [NDTM  <http://tools.ietf.org/html/draft-ietf-opsawg-large-flow-load-balancing-10#ref-NDTM>].
> >>>
I've seen that, but why do you equate automatic _hardware _recognition 
to unsampled measurements.
Whether it's done in hardware of software is orthogonal.

>
>
>     Is this what you mean by:
>
>         It is possible that a router may have line cards that support a
>         sampling technique while other line cards support automatic hardware
>         detection of large flows.
>
>     It's not very clear.
>
>
> No, this does not address your point.  This is talking about the case 
> where line cards have different capabilities, rather than a line card 
> that supports both.
>
> Since we already have the advantages and disadvantages listed in 
> 4.3.4, do you still see a need for explicitly mentioning that 
> automatic hardware detection is to be preferred over sampling if both 
> are available?
>
> We did debate the point about accuracy quite a bit among the authors. 
>  The question is -- does that level of accuracy really matter for the 
> large flow case?
Maybe not (for the details: http://dl.acm.org/citation.cfm?id=1791959), 
but I don't understand why you want to limit this mechanism to sampling 
only. Simply telling that sampled data could be good enough, but if you 
have unsampled data, you will get a better accuracy.
> Since we are dealing with flows that need to consume a certain percent 
> of the link bandwidth, sampling, if configured correctly,
And you don't go in the details of "sampling, if configured correctly"...

Regards, B.
> will catch anything that is important.
>
> Anoop
>
>