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Re: [Roll] draft-rahul-roll-rpl-observations-00 Section 2.1: Wear leveling

Rahul Arvind Jadhav <rahul.jadhav@huawei.com> Mon, 07 May 2018 10:57 UTC

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From: Rahul Arvind Jadhav <rahul.jadhav@huawei.com>
To: Routing Over Low power and Lossy networks <roll@ietf.org>
Thread-Topic: [Roll] draft-rahul-roll-rpl-observations-00 Section 2.1: Wear leveling
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Date: Mon, 07 May 2018 10:57:08 +0000
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Subject: Re: [Roll] draft-rahul-roll-rpl-observations-00 Section 2.1: Wear leveling
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Hi Michael,

That's a good point you made about DTSN not been a lollipop counter (this was clearly misinterpreted by us and others in implementations). Section 7 explains "Sequence Counters" but DAO Trigger Sequence Number (DTSN) is not considered as sequence counter in that section!!

Please find my response inline ... 



-----Original Message-----
From: Roll [mailto:roll-bounces@ietf.org] On Behalf Of Michael Richardson
Sent: 04 May 2018 01:15
To: Routing Over Low power and Lossy networks <roll@ietf.org>
Subject: Re: [Roll] draft-rahul-roll-rpl-observations-00 Section 2.1: Wear leveling


Pascal Thubert (pthubert) <pthubert@cisco.com> wrote:
    > There might be a confusion with the DODAG version. DTSN is sourced at
    > each node, individually.

I'm not saying it has to be synchronized, but rather it can always be > than parent. If one does that, then there is no bootstrap problem.

Isn't that the non-storing case anyway?

[RJ] Even in non-storing case the DTSN is not derived from the parent's DTSN as per the spec. 

DODAG version would always be the same.

I looked again, and I don't see anything about DTSN being a lollipop counter that starts at 240, and wraps at 239.  That would, I think make it clear when a node has booted, and maybe that solves the problem.

[RJ] Very interesting point. We assumed that DTSN is a lollipop counter in our implementation. But looks like it is not. Section 7 explicitly states that only DAOSequence, DODAGVersionNumber, and Path Sequence are lollipop counters ... Looks like we read the spec wrong, also Contiki (including ng) has got it wrong (i.e. uses lollipop counter for DTSN). RIOT (rightly as per spec) does not assume lollipop counter for DTSN.
I think DTSN needs to be a lollipop counter which alleviates the problem but still does not completely avoid the persistent storage (discussed in other mail in same thread). This point should be added to the draft. 


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]               Never tell me the odds!                 | ipv6 mesh networks [ 
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