Re: [LOOPS] draft-li-tsvwg-loops-problem-opportunities-05 - why do traffic engineering at the overlay as well?

[Carsten Bormann <[e-mail]>]

Hi Gorry,

On 2020-07-07, at 17:28, Gorry Fairhurst <[e-mail]> wrote:
> 
> I have a few quetsions about then problem draft:
> 
> Section 3.4 provides a use-case, which I would like to understand better:
> 
> “By continuously measuring the delay of path segments
>    and use them as metrics for path selection, when the number of
>    overlay nodes is sufficiently large, there is a high chance that a
>    better path could be found".

Multipath operation is out of scope for the LOOPS activity that we are presenting at the IETF108 BOF.  Multipath is a valid use case for in-network recovery, but I would expect we would want to combine the considerable knowledge gained with MPTCP and various other multipathing approaches (e.g., MPDCCP) before even defining the outline of a standardizable solution.

(Running LOOPS on a segment that is internally multipathed, or within a single segment contributing to a multipath arrangement, should be considerably simpler.)

> - This seems in part to rely upon measuring and the choosing which “disjoint segments” to forward over. A previous discussion asked how this would operate when the network itself implemented methods such as BGP and ECMP to do some form of traffic engineerinfg, and whether this introduced additional level of automatic re-routing would in some way conflict?

For LOOPS at IETF108, we are trying to avoid control loops fighting against each other.  For people who enjoy those fights, we can always ask to add it later.

> - Is this decision within the network a good idea for stable oeprations? Is this a decision better made at the edge?

I’m not sure we know the answer to that.  A vantage point within the network may have better data, both with respect to the speed of reaction (shorter RTTs for a partial path) and with respect to the aggregated traffic volume providing better measurements.  But you said “at the edge”, which is in the network…
In any case, getting this right, over a wide range of operating points, would be quite an accomplishment.

Grüße, Carsten