[Roll] nsa extension comments

Rahul Jadhav <rahul.ietf@gmail.com> Sat, 07 December 2019 08:06 UTC

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From: Rahul Jadhav <rahul.ietf@gmail.com>
Date: Sat, 7 Dec 2019 16:05:45 +0800
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To: Routing Over Low power and Lossy networks <roll@ietf.org>
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Subject: [Roll] nsa extension comments
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Hello authors,

I have pushed my PR[1] to the corresponding roll-wg repo.  However, there
are other
points which require your attention. Following are my comments:

o Carrying TLVs in NSA
RFC 6551 explains that an NSA can be used to carry TLVs but it falls short
of
specifying the format for these TLVs. This document assumes the format of
8-bit
type and 8-bit length. This format is fine but future specifications will
have
to refer to this document if they got to add new TLVs. Thus the generic TLV
format should be made part of different section and an IANA registry needs
to
be created for the NSA TLV type.

o Preference in PS set
The document assumes that the parent addresses in the PS set have been
added in
the decreasing order of preference, without making it explicit. The PS IPv6
address(es) field requires an explanation in Section 4.

o DIO length
As per the draft, the DIO needs to carry NSA (Node State and Attributes)
metric
with PS (parent set) TLV. As per my understanding without compression, it
would
be a challenge to carry even two parent addresses in the PS. There has been
a
discussion about compression and as I remember we discussed doing it.
Regardless, I feel we should do it if we want this to be a practical
proposition.

o Section 3.4 Usage:
"For example, using different methods can be used to vary the transmission
reliability in each hop."
This statement implies that using different CA policy implementation can
"control" the transmission reliability. As I understand, the strict policy
is
the best policy (in terms of all performance metrics), the nodes will go for
other policy only when the strict policy cannot be used given their parent
sets.
The above statement implies that nodes may still go for a relaxed policy
when
strict can be used.

o Configuration parameters
The document specifies configuration parameters such as
a. PARENT_SET_SIZE ... Not defined
b. cur_ap_min_path_cost: How can the cur_ap_min_path_cost be equated with
PARENT_SWITCH_THRESHOLD as mentioned in section 3.2.2
c. MAX_PATH_COST: No definition for this.
The parameters warrant a rationale and a default value. Also more
importantly,
is it required that different nodes use the same value and if not what
could be
the impact.

o Terminology section
Terminology section should explain Alternative Parent, Parent Set, Preferred
Grand Parent.

o Section realignment
It is better to explain CA Strict/Medium/Relax policies before explaining
the
CAOF because as a reader one needs to be familiar with these policies before
understanding the OF.

Regards,
Rahul

[1] https://github.com/roll-wg/draft-ietf-roll-nsa-extension/pull/1