Subject: Re: [Roll] [6lowpan] ND-10 Status

From: "Pascal Thubert (pthubert)" <[email-masked]>
Date: Thu, 8 Jul 2010 13:11:08 +0200

Hi Erik

> > http://tools.ietf.org/html/draft-ietf-roll-rpl-10#section-6
> 
> Seems like the non-deployed OSPFv1 used lollipop, but since that was
still
> subject to the S1 < S2 < S3 < S1 issue, this was removed in OSPFv2.
> 
> That might be an issue for Roll; I haven't looked at it closely.
> 
> In any case, OSPFv1 includes the feature where using a number on the
stick
> of the lollipop will result in other routers telling you the last
sequence
> number they they have for your LSA. The message flow for 6lowpan-nd
> doesn't have such a mechanism, and given that we don't require
flooding all
> the information to all the routers, it isn't clear that we can
actually add such a
> thing to 6lowpan-nd. (Without requiring that the AROs be flooded to
all of
> the 6LRs.)
> 
> Thus I think there is some work needed before we can consider applying
> lollipop or any other sequence numbers to 6lowpan-nd.

[Pascal] Let's do it then. You'll find that RPL:

1) uses simple increments (by 1 I mean)
2) emulates OSPFv2 in that it uses the odd space as a straight part.
3) avoids the S1<s2<s3<S1 pb with a window of consistency check

Sequences are comparable if they are not too far apart, and when that's 
so the higher number wins.

If we are still wrong, then the stale information will win until the
correct 
information eventually catches up, which happens sooner if the window
of consistency is smaller.

RPL has a lifetime associates to most resources and the idea is that the

lifetime should be smaller than the time it takes to increments the
seqnb 
outside the window of consistency. Sadly the lifetime is not there for
the 
DIO version nb, but that's another story and probably an issue.

Neither RPL nor BR is synchronizing dbs. They are merely trying to
identify
the freshest information. the consequence of being wrong is a temporary 
outage for the resource associated to the seqnb.

> 
> > [Pascal] This is basically what the HA does in MIP.  This has an
added
> > value for the HA to be able to ensure bidir reachability by using a
> > new seq num as a challenge, though it is not officially used in the
> > protocol.
> 
> I HA in MIP is a centralized solution.
> A distributed protocol is q very different beast.
> 

[Pascal] Exactly. So getting the previously used seq nb from the wrong
node in the distributed world is probably the wrong idea :) RPL did not 
takes that path.

Pascal