[Roll] How to react if Trickle Interval > Default Lifetime

[Cenk Gündogan <[e-mail]>]

Dear RPL Veterans,

I recently came across the question of how a RPL router may react,
if a parent's default lifetime (and unit -- specified in the DODAG Conf. 
Opt.)
is surpassed by the parent's continuously rising trickle interval?

Without any action from the children, I would assume that the upward link to
the parent would time out before the next DIO arrives to refresh the 
lifetime,
hence breaking the child -> parent relation.

Implementations seem to handle this case very differently..

* The RPL implementation of RIOT uses unicast DIS messages to request a 
one-shot DIO
from a parent that has a link which is about to expire.
* From what I could gather in [1], Contiki also seems to send unicast DIS
messages randomly in an interval of 30~55 seconds (Please correct me if 
I am wrong).
* In an off-list discussion with Michael, I learned that unstrung is 
e.g. handling
this case by not letting the Trickle interval exceed the default lifetime
(Again, please correct me if I am wrong).

I couldn't find any hints regarding this situation in RFC6550.
Is there actually an obvious way to handle this, but I am just not 
seeing it?

Best,
Cenk Gündoğan

[1] 
https://github.com/contiki-os/contiki/blob/master/core/net/rpl/rpl-timers.c