[6tisch] SF0 - Cell reclamation by RPL parent

"S.V.R.Anand" <anand@ece.iisc.ernet.in> Mon, 07 March 2016 10:37 UTC

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To: "Prof. Diego Dujovne" <diego.dujovne@mail.udp.cl>
From: "S.V.R.Anand" <anand@ece.iisc.ernet.in>
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Date: Mon, 07 Mar 2016 16:06:56 +0530
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Subject: [6tisch] SF0 - Cell reclamation by RPL parent
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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 
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 
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 

- Referring to the following text in 4.2.5 of the 6tisch architecture 

   "...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.


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