RE: [PCN] Architecture draft - probing section & general updates.

["Anna Charny (acharny)" <[e-mail]>]

Hi Lars,

On the low agfgregation point, please see below: 

> -----Original Message-----
> From: Lars Eggert [mailto:[e-mail]] 
> Sent: Friday, October 19, 2007 8:58 AM
> To: Anna Charny (acharny)
> Cc: Hancock, Robert; [e-mail]; [e-mail]
> Subject: Re: [PCN] Architecture draft - probing section & 
> general updates.
> 
> On 2007-10-16, at 21:55, ext Anna Charny (acharny) wrote:
> > Yes, Robert's is a fair concern to which no obvious solution is in 
> > sight. Different equipment might use different algorithms and might 
> > use different fields for ECMP load-balancing under different 
> > circumstances.
> > IMHO this is a killer argument of why the use of probing for 
> > discovering the state of ECMP paths should not be considered within 
> > the scope of PCN WG.
> 
> Agreed.
> 
> > There remains a question of whether probing can/should be 
> considerd to 
> > probe the path regardless of the ECMP issue.  I see most of 
> its value 
> > in flash crowd situations in combination with low ingress-egress 
> > agregation.
> 
> I note that scenarios with low aggregation aren't in scope of the
> charter:
> 
>    The initial scope of the PCN WG is restricted by the following
>    assumptions:
> ...
>    (C) the number of flows across any potential aggregation bottleneck
>    is sufficiently large for stateless, statistical mechanisms
>    to be effective

Actually,  I did not mean low aggregation *at the bottleneck*, which is
what the charter seems to restrict.  Rather I meant the case when the
bottleneck has high aggregation, but traffic on that bottleneck comes
from a large number of ingress-egress pairs, each having very low
aggregation levels.  I believe technically the charter does put any
explicit restrictions on the scope for ingress-egress aggregation. 

Perhaps the WG should consider whether it is reasonable to impose
restrictions on the *ingress-egress* aggregation levels as well.  An
argument can be made that in practice a large number of ingress-egress
pairs may only have a few flows, even when the bottleneck aggregations
are large. 

The decision on whether low ingress-egress aggregation level is in scope
seems to be important for choosing among the various approaches proposed
to the WG, as some of them are substantially more sensitive to the low
ingress-egress aggregations than others (e.g. single marking does not
perform well at very low levels of aggregation, as we showed at the last
meeting).  

Perhaps an explicit discussion on the assumptions regarding the expected
levels of ingress-egress aggregations is needed on the list? 

Towards that discussion, my personal view is that in the long range
ignoring low levels of ingress-egress aggregation levels will severely
limit the viability/usefulness of the technology.  However, perhaps as
an initial step it would be OK to assume moderate to high aggregation
levels, as long as a clear path is visible on how to address low
aggregations in the future within the scope of defined behaviors.  But I
think it is important to have a clear consensus on this point. 

Anna 

> 
> Lars
> 


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