Re: [6tisch] SF0 - Cell reclamation by RPL parent

[Thomas Watteyne <[e-mail]>]

Anand,

You are raising a good point. This would mean that a 6P transaction is
started not by the the sender, but the receiver.

I have added a slide in the set for this afternoon's call, as a
high-bandwidth discussion might be best approach here.

Thomas

On Mon, Mar 7, 2016 at 11:36 AM, S.V.R.Anand <[e-mail]>
wrote:

> 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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