[alto] Shepherd review for draft-ietf-alto-cost-mode-01

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Dear WG and authors of draft-ietf-alto-cost-mode,

I am posting this review of draft-ietf-alto-cost-mode-01 to the mailing list, as
part of my shepherd write-up. Any comments and feedback are more than welcome!

Best,
Kai

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This draft extends the base ALTO protocol (RFC 7285) by relaxing the constraint
on valid cost mode values and introducing a new IANA (sub-)registry to document
new cost mode values. The motivation is clear and the proposed mechanism
is clean. Most comments raised in Call for Adoption and WGLC are addressed in
the latest revision except Dhruv's comment [1] on giving more detailed
specifications of the contents in IANA registry. There are two remaining
comments and I think the draft is ready for publication once they are addressed.

Comments:

Section 3.1, last paragraph: The paragraph says 

   Future documents that define a new cost mode SHOULD indicate whether
   that new cost mode applies to all or a subset of cost metrics.

In that case, it seems to me that the default behavior should be specified in case
the applicability of the new cost mode is not indicated. Either the "SHOULD" keyword
is replaced by "MUST" or an additional sentence is required, e.g.,

NEW:
    If not explicitly specified, the new cost mode applies to all cost metrics.

Section 4:

I also agree with Dhruv's comment that the contents of the "ALTO Cost Modes"
registry should be better specified. While the initial entries set good examples
of how to register a new cost mode, it can still be helpful if the format and
content of each field are specified in more details, e.g., using similar
specifications in Sections 14.2 and 14.3 of RFC 7285 (as suggested by Dhruv).

I also suggest renaming the "Specification" field to "Intended Semantics", to be
consistent with other ALTO registries (in RFC 7285 and in the unified property draft).

[1] https://mailarchive.ietf.org/arch/msg/alto/B1agkfVtdu7tsad2-MzErQXMk44/