Re: [Roll] NSA PS-set metric/constraint

"Georgios Z. Papadopoulos" <georgios.papadopoulos@imt-atlantique.fr> Tue, 11 February 2020 10:25 UTC

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From: "Georgios Z. Papadopoulos" <georgios.papadopoulos@imt-atlantique.fr>
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Date: Tue, 11 Feb 2020 11:25:30 +0100
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To: Rahul Jadhav <rahul.ietf@gmail.com>, Routing Over Low power and Lossy networks <roll@ietf.org>
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Subject: Re: [Roll] NSA PS-set metric/constraint
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Dear Rahul and all,

Once again, thank you for spotting this issue. Great catch! 
We are happy with any of the following solutions : 

- We can indeed tie the PS set metric to CAOF specifically in the draft.
Furthermore, the policies in CAOF are extensible (we only specify examples), so potentially another policy could make a different use of PS as part of CAOF.

- We can indeed explain the issue in the draft so that the implementers decide on their own how to handle it.
Additionally, since the whole RPL instance uses the same OF, we would expect that if an OF which understands the TLV is used, then all the nodes will know how to process it.
So, this issue should not be very common.

- Another option would be to add an exception to the "all TLVs need to be propagated” rule from RFC 6551 specifically for the PS TLV.
The exception would specify that the PS TLV would be dropped if not understood.


Best regards,
Georgios and Aris


> On Feb 11, 2020, at 04:32, Rahul Jadhav <rahul.ietf@gmail.com> wrote:
> 
> Hello Authors of NSA extn and all,
> 
> One more question in the context:
> 
> The draft adds new routing metrics/constraints (PS set) nested as part
> of the NSA parent metric.
> 
> My question is with regards to what happens if this routing metric is
> used outside of the CAOF. Any metrics/constraints could be used by any
> OF and thus PS set metric defined by the draft can be used by other
> OF.
> 
> RFC 6551 says that if the metric is not understood by the node and if
> it is a 6LR then it may not process it but it has to propagate it.
> Unlike other metrics, the PS set metric in this case contains
> link-local values that cannot be used beyond link-local. As such upon
> propagation, any downstream 6LR that understands the PS set metric
> would use the information and get impacted adversely.
> 
> Should we tie the PS set metric to CAOF specifically in the draft, in
> which case this problem won't be there? Or we can specify this issue
> as it is for the readers to understand the problem if the PS set is
> used outside of the CAOF. Either way works for me.
> 
> Best,
> Rahul
> 
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