Subject: [Roll] RPL clarification questions

From: Gregory Detal <[email-masked]>
Date: Tue, 27 Apr 2010 14:41:35 +0200

Dear all,

I'm currently working on RPL for my PhD thesis and have read the last draft.
I came across two uncertainties:

1) What is the meaning of the DAG sequence number increment based on events?
I have some issues understanding section 12.1.4. It says that DAG sequence number can be incremented based on events triggering. However:
- I have never found any documentation on such events ? What are the events triggering a seqnum increment by an RPL node ? Are you thinking about cases where, for example, poisoning is happening in conjunction with a mobility event ?
- The DODAG root is the node in charge of incrementing this seqnum. How is it ensured that the node will be aware of the event triggering this operation, assuming that there is no more "connection" between the event location and the root itself ?	

2) How is the rank actually computed?
Could you confirm that a node advertises in the DIO message its own rank, and the direct children of that node in the DODAG compute their own rank as parent.rank + OF(parent.metric) ?

Best regards,
Gregory.

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From: Michael Richardson <[email-masked]>
Date: Tue, 27 Apr 2010 09:56:45 -0400

>>>>> "Gregory" == Gregory Detal <[email-masked]> writes:
    Gregory> 1) What is the meaning of the DAG sequence number increment
    Gregory> based on events?  I have some issues understanding section
    Gregory> 12.1.4. It says that DAG sequence number can be incremented
    Gregory> based on events triggering. However: - I have never found
    Gregory> any documentation on such events ? What are the events
    Gregory> triggering a seqnum increment by an RPL node ? Are you
    Gregory> thinking about cases where, for example, poisoning is
    Gregory> happening in conjunction with a mobility event ?  - The
    Gregory> DODAG root is the node in charge of incrementing this

Let's say the DODAG root looses it's (up)link.
It would send out new messages with a new sequence number, indicating it
is is no longer grounded.

    Gregory> 2) How is the rank actually computed?  Could you confirm
    Gregory> that a node advertises in the DIO message its own rank, and
    Gregory> the direct children of that node in the DODAG compute their
    Gregory> own rank as parent.rank + OF(parent.metric) ?

Yes.

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From: Gregory Detal <[email-masked]>
Date: Tue, 27 Apr 2010 16:21:00 +0200

Michael,

Thank you for your answer.

> is is no longer grounded.

Ok, so events are only linked to DODAG root and not any other RPL node ?

Gregory.

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From: Michael Richardson <[email-masked]>
Date: Tue, 27 Apr 2010 11:05:32 -0400

>> Let's say the DODAG root looses it's (up)link.  It would send out
    >> new messages with a new sequence number, indicating it is is no
    >> longer grounded.

    Gregory> Ok, so events are only linked to DODAG root and not any
    Gregory> other RPL node ?

I think that only the root can increment the sequence number.
I'm not sure what another node would do that had an "event"...

--

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From: Philip Levis <[email-masked]>
Date: Tue, 27 Apr 2010 08:09:31 -0700

On Apr 27, 2010, at 6:56 AM, Michael Richardson wrote:

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This is one example use.

More generally, incrementing the sequence number is a mechanism for the DODAG root to force the DODAG to reform. There are many reasons one might use this mechanism, and we don't want to specify them all. As nodes can't always move 100% freely within a DODAG (the max rank increase configuration parameter), sequence numbers allow RPL to serialize topology changes.

Phil