Re: [Roll] [roll] #157 (draft-doi-roll-mpl-parameter-configuration): mpl-parameter-configuration-00 - Effect of inconsistent parameter set among nodes

Philip Levis <pal@cs.stanford.edu> Tue, 29 July 2014 17:25 UTC

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To: Routing Over Low power and Lossy networks <roll@ietf.org>
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Subject: Re: [Roll] [roll] #157 (draft-doi-roll-mpl-parameter-configuration): mpl-parameter-configuration-00 - Effect of inconsistent parameter set among nodes
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On May 26, 2014, at 2:01 AM, Yusuke DOI <yusuke.doi@toshiba.co.jp> wrote:

> Hi,
> 
> Thank you for summarizing issues on my draft.
> On #157, my understanding is as follows. If there are any issues you know, please let me know.
> 
> Inconsistent but valid parameter does not harm the network much (it may harm battery-powered nodes).

Given the L in RPL/MPL includes the term "low power", harming battery-powered nodes is kind of important. It's easy to try to ignore them because they're so challenging, but I'd strongly recommend against it.

> 
> Reason:
> 
> 1) If a node have smaller Imin/Imax, it takes more responsibility to repeat messages than surrounding nodes. The node will consume more power and the area will have higher traffic than expected until MPL parameters of the node is updated.

It's worth noting that transmission (in most link layers MPL/RPL care about) does not consume significantly more energy than reception. Therefore, one node that has a smaller Imin or Imax introduces an energy cost on *all* of the nodes around it.

A smaller Imin can double the latency of propagation over a single hop, as nodes, on receiving an update, receive multiple Trickle messages and so suppress for their first interval.

Depending on K, a difference in Imax can do more than just increase/reduce responsibility. It could actually lead to complete suppression. Consider the case where three nodes have Imax=t and one node has Imax=t+2 (so the actual time interval is 4 times greater). K<=3. The three nodes will almost always completely suppress the fourth node with the longer interval.

> 2) If a node have larger Imin/Imax, it takes less responsibility to send messages than surrounding nodes. As other nodes will send the message instead of the node with old parameters, effect for traffic load and e2e delay is negligible on dense mesh clusters. If the node with old parameters is on sparse area of a mesh network, larger Imin/Imax will cause larger e2e delay than new parameter's expectation untilthe node is updated.

You should disambiguate the effects of Imin and Imax. 

> 3) If a node have smaller K, it takes less responsibility to repeat messages than surrounding nodes. On dense network, the effect is negligible. On sparse network, messages pass through the node will have less reliability until the node is updated.

It also introduces uneven transmission load. A node with a higher K will transmit with a much higher probability that those with the lower K (within the logarithmic bounds introduced by packet loss).

I'd suggest re-reading 6206 (Section 6), which goes into greater detail than the bullet points above and is much more specific.

> 4) If a node have larger K, it will repeat almost all the messages. The area will have excessive traffic untile the node is updated.
> 

This is not true. Note that packet loss/imperfect duplicate suppression/uneven topologies often cause some nodes to receive more than K packets. Again, I'd suggest re-reading 6206.

Phil