Re: [Roll] I-D Action: draft-ietf-roll-p2p-rpl-10 comments

Federico Consoli <admin@ipv6it.org> Sat, 05 May 2012 16:31 UTC

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Date: Sat, 05 May 2012 18:31:39 +0200
From: Federico Consoli <admin@ipv6it.org>
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Subject: Re: [Roll] I-D Action: draft-ietf-roll-p2p-rpl-10 comments
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Il 05/05/2012 18.05, Mukul Goyal ha scritto:
> [Federico]
>> Yes but even if you got a small Imin value, network congestion may be
> only in a small initial period because Imin is incremented at each
> submission.
>
> [Mukul]
> In smart home environment, you care a lot about extra DIO packets (to save on battery). So, even though network congestion might be for a small time duration during one particular route discovery, the overall life time of a device might take a big hit over the course of multiple discoveries over time.
> [Federico]
>> IMHO I think that K=1 is too conservative.
> [Mukul]
> It is indeed conservative and that is because smart home needs it to be conservative. And smart home is the application environment that cares most about using the default configuration option in order to save on bytes (and hence battery).
>
> [Federico]
>> The situation that I describedcan be seen from another point of view.
> Suppose now that A and B got about 15-20 neighbor.
> A got a rank of 500
> B got a rank of 3000
>
> A suppresses DIOs for the reason described previously so node B will
> continue to use a bad path for a very long time because it does "not
> know" that node A "exists". I think that K should be at least =2 or more.
>
> [Mukul]
> Continue using bad path? The route discovery is still going on! In the scenario you described, B probably will take a long time to be "discovered" or perhaps not be discovered at all within the specified DAG lifetime.
[Federico]
Exactly, do not you think that the fact that node B take a long time to 
discover node Ais a problem? Suppose that node B wants to communicate 
with a neighbor of node A, it could lead to sending many unnecessary DIOs.

>
> Thanks
> Mukul
>
>
>
>> On the other hand, if Imin/K are set less conservatively, you might see too many DIOs generated. There is no silver bullet. No single setting would work well in all situations. We do provide some guidance on how to set trickle timer in Section 9.2 of the draft. The trickle parameter values in default configuration option are a conservative estimate based on the desire to avoid too many DIOs. If these settings are not OK for a particular deployment, well then the default configuration option should not be used. The connectivity variance you described is indeed a tricky situation and would require a tradeoff with the need to avoid generating too many DIOs. I imagine one possible solution could be to allow a node to set its k based on its position (rank) in the DAG (k increases as you move closer to the edge). But, again there might be cases where this (or any other fix) wont work.
>>
>> Thanks
>> Mukul
>>
>> ----- Original Message -----
>> From: "Federico Consoli"<admin@ipv6it.org>
>> To: roll@ietf.org
>> Sent: Saturday, May 5, 2012 4:04:09 AM
>> Subject: [Roll]  I-D Action: draft-ietf-roll-p2p-rpl-10 comments
>>
>> Hi,
>> I disagree with the default value of trickle timer, especially for
>> Imin=6 and K=1.
>> If you got a dense network it could lead to a very long time to build DAG.
>>
>> Eg. Suppose you got 2 node A, B.
>> Node A got about 15-20 neighbor
>> Node B got only A as a neighbor.
>>
>> When node A receive its first DIO it resets trickle to Imin and it
>> starts trickle timer. Since A got a lot of neighbor its probably receive
>> another consistent DIO so it will suppress the DIO message and it will
>> increase its Imin to 7. So if node A suppress 6-8 times its DIO node B
>> will receive it after a long time. Finally if the link A-B got an ETX of
>> 2 or 3 (or more) this time doubles.
>>
>> These situations are not uncommon, especially in the routers at the edge
>> of dense networks.
>>
>


-- 
Regards
Consoli Federico