[mat-wg] Atlas UDMs and Probe-Assignments

Hi Gert,

On 2013.07.25. 14:08, Gert Doering wrote:
> Hi,
> 
> I'm wondering about something, maybe one of you can enlighten me here.
> 
> I have set up a few UDMs (... to eventually get out of Atlas what we
> had with TTM).  A small one with only 10 probes is now showing that
> actually only 7 probes return data - two more have returned data at
> some point in the past - "Jul 06, 06:11" in one case - while the 10th
> assigned problem seems to have never sent anything.
> 
> (This is UDM 1002000, the "dead" probe is #2760 in AS 3292).
> 
> The question coming from this is: how do probes get assigned to UDMs, 
> and will dead probes get replaced eventually?  Or is the assignment
> done statically at the beginning of the UDM?

The scheduler selects probes when the UDM is started. This is a one-off
operation, and takes into account the UDM specification, probe settings
and a few other inputs in order to figure out which probes to invite into
the measurement.

> The reason why this is relevant: if I am going to set up UDMs to
> replace what TTM does, the idea is to have a reasonable high number of
> probes, say "50" or "100", and define our Internet reliability as "more
> than 2/3rd of the UDM probes have been answered without loss", or
> something like that - taking into account that somewhere on the
> Internet, things will always be broken, but that's not our
> responsibility.

Right; the probes are all around the 'net, in all sorts of locations, so
it's unlikely that all of them will be successful in your measurement.

> This falls apart of probes deteriorate after a given time, and a given 
> UDM measurement will use "less and less probes" (so in the extreme, of 
> the 50 initial probes, 30 might be "dead probe" and thus the result is 
> always "0% reliability, more than 50% of the probes can not reach us"),
>  instead of the system keeping the requested number of probes by
> replacing dead ones...

Indeed, it's reasonable to expect that over some time period a subset of
the probes will no longer participate in your measurement (or in any other
measurements).

We have a feature on our backlog to help this: defining and using "low
thresholds" for the number of participating probes (you can already set
and see this on the UI, but it doesn't really do anything yet). The idea
is that you can ask for X probes, and also say that if the number falls
below Y then "something" should happen. We imagine that the "something"
can be:
* notify me
* stop the measurement
* stop the measurement and schedule a new one with the same parameters
* try to swap out non-functioning probes with new ones
* (or something else)

I expect that we can start working on this feature this year.

Hope this helps,
Robert

> Gert Doering -- NetMaster
>