Re: [Roll] [roll] #132: draft-ietf-roll-trickle-mcast-04 - Clarify scope value of 3 - subnet-local

"Ralph Droms (rdroms)" <rdroms@cisco.com> Tue, 15 October 2013 22:33 UTC

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From: "Ralph Droms (rdroms)" <rdroms@cisco.com>
To: Routing Over Low power and Lossy networks <roll@ietf.org>
Thread-Topic: [Roll] [roll] #132: draft-ietf-roll-trickle-mcast-04 - Clarify scope value of 3 - subnet-local
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Date: Tue, 15 Oct 2013 22:33:30 +0000
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Cc: "mariainesrobles@gmail.com" <mariainesrobles@gmail.com>, "Jonathan Hui (johui)" <johui@cisco.com>
Subject: Re: [Roll] [roll] #132: draft-ietf-roll-trickle-mcast-04 - Clarify scope value of 3 - subnet-local
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On Oct 15, 2013, at 1:01 PM 10/15/13, Richard Kelsey <richard.kelsey@silabs.com> wrote:

>> Date: Tue, 15 Oct 2013 13:17:35 -0400
>> From: Kerry Lynn <kerlyn@ieee.org>
>> 
>> Link-local scope is essentially defined by a combination of link layer
>> connectivity and routing behavior.  I believe we need something similar
>> in order to automatically define scope 0x03.  To the extent that we need
>> scope 0x03 at all, I think it's to emulate classic link-local behavior in a
>> mesh.  There are only so many ways that meshes can be distinguished:
>> either by location, frequency, or code diversity (assuming any two of
>> three overlap).  In Ralph's example, he assumes location and frequency
>> overlap.  In Michael's example, I suspect he assumes location and PAN
>> ID overlap.
>> 
>> Finally, we may need to constrain Ralph's definition a bit further and
>> define an 802.15.4  scope 0x03 zone as a set of interfaces that share
>> a common PAN coordinator and PAN ID.
>> 
>> -K-
> 
> I think you have to fall back on routing behavior.  Consider a
> mixed-PHY mesh, such as a a combination of 802.15.4 and PLC
> links.  If the routing layer considers it all one mesh, then it
> is, regardless of whether or not there are any 802.15.4 nodes in
> direct communication using the same PAN ID.

OK, but I don't think there is a good way to associate a specific MPL datagram with a DODAG ID, so a node can determine which scope 3 that MPL datagram is associated with.

And, using DODAG ID tightly couples MPL with RPL, when MPL as currently defined can be used without RPL.

I think scope 4 is a much better candidate for the "composite" subnet use case, while scope 3 is good for a single physical mesh.

- Ralph

>  On the other hand,
> the routing layer could take the same physical setup and treat it
> as distinct 802.15.4 meshes with PLC interconnects, in which case
> you would get multiple 0x03 scopes.
> 
>  I shall not today attempt further to define 0x03 scope; and
>  perhaps I could never succeed in intelligibly doing so. But I
>  know it when I see it.
>                         (paraphrasing Justice Potter Stewart)
> 
> -Richard
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