Hi Diego and others,
Since we are discussing about the message transactions in SF0, this mail
is with
respect to the RPL parent, the cell donor.
There are situations where the RPL parent might want to apply a cell
reclamation
and, depending on the context, a reallocation policy dynamically. This
results in one or more cells that
have been given to its children are reclaimed by the RPL parent due to
various
reasons as below.
- The child node is out of the network, or child and parent cannot reach
each other,
after the 6P add cell transaction is complete, thereby delete cell
operation is not
being transacted. Cell reclamation helps in "cell garbage" collection.
- As add cell requests arrive asynchronously from its children, an RPL
parent implementing certain fairness objective might re-appropriate
the cells
that have already alloted its children, especially during a resource
crunch.
- Referring to the following text in 4.2.5 of the 6tisch architecture
document,
"...Note that a PCE is expected to have precedence in the allocation,
so that an RPL parent
would only be able to obtain portions that are not in-use by the PCE."
It may be that, an RPL parent might have to relinquish its own cells if
needed by PCE any time. In such a scenario, the RPL parent may be
forced to
reclaim the cells given to its children.
While the first case is relatively less complex, the latter two cases are
problematic as these cases can lead to (i) potential race conditions, say
between PCE and SF0 operations, and (ii) a cascading effect across
several RPL
parents down the DODAG. Currently, there is no mechanism defined to address
this problem.
I suppose the above text extends beyond SF0.
In light of the above use cases and the associated problems, do you
think it is a
good idea to include an appropriate message in SF0 for reclaiming the
cells by
the RPL parent ? or are we inviting new set of problems by doing so ?
Will be happy to receive your inputs.
Anand
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