Subject: [Roll] dt-roll-rpl: Gradient

From: Mukul Goyal <[email-masked]>
Date: Wed, 1 Jul 2009 10:27:32 -0500 (CDT)

Quoting from section 4.1 (the last paragraph)

"RPL is designed to survive and still operate, though in a somewhat
   degraded fashion, when confronted to such heterogeneity.  The key
   design point is that each node is solely responsible for setting the
   vector that it sources in the gradient, which is achieved by
   selecting its preferred parent.  As a result, the gradient is not
   broken if another node makes its decisions in as antagonistic
   fashion, though an end-to-end path might not fully achieve any of the
   optimizations that nodes along the way expect."

I am not sure what does the gradient mean and what does the following line mean

"each node is solely responsible for setting the vector that it sources in the gradient, which is achieved by selecting the preferred parent."

The draft needs to define gradient. And perhaps give an example of "the gradient is not broken if another node makes its decision in an atagonistic fashion".

Thanks
Mukul

---

From: "Pascal Thubert (pthubert)" <[email-masked]>
Date: Thu, 2 Jul 2009 14:17:20 +0200

Hi Mukul

We have changed language and terms a number of times and it appears
we're not fully settled:
-DAG has the inconvenience to miss the sibling path. 
-Gradient indicates that you select a preferred parent and orient a
vector towards it. But it fails to represent the alternate paths. 

Yet, Gradient represents a cool abstraction of how I see what the
protocol operation should be and I plead guilty for that text. In my
favorite mode, the router selects its absolute preferred parent by
whatever means (might include depth or not at all) and then selects a
depth that SHOULD be the depth of that parent +1. Exceptions would be
tolerated if for instance the node is short on battery and wish to
artificially avoid children by faking a infinite (or close to) depth.
Alt Parents are defined by lesser depth, siblings by same depth. Because
the depth might not be primary to select the preferred parent there
might be parents at a lesser depth than the preferred one.

At the moment, the draft enables to pick alternate parents that are
deeper than the most preferred.
As a result:
- there's no boundary to greediness (remember my drawings about this
issue)
- the depth represents the worst path as opposed to the preferred path
and looses value
- similarly, aggregating other metrics from multiple parents might also
lose all sense

I suppose that additional thinking will help resolve if the text should
stay as it stands or if we make it a bit more restrictive as I recommend
here. 

Suggestions / comments appreciated.

Pascal

>-----Original Message-----
>From: [email-masked] [mailto:[email-masked]] On Behalf Of
Mukul Goyal
>Sent: mercredi 1 juillet 2009 17:28
>To: roll
>Subject: [Roll] dt-roll-rpl: Gradient
>
>"each node is solely responsible for setting the vector that it sources
in the gradient, which is achieved by
>selecting the preferred parent."
>
>The draft needs to define gradient. And perhaps give an example of "the
gradient is not broken if another node
>makes its decision in an atagonistic fashion".
>
>Thanks
>Mukul
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