Last Call Review of draft-ietf-softwire-iftunnel-04
review-ietf-softwire-iftunnel-04-tsvart-lc-black-2019-05-07-00

This document has been reviewed as part of the transport area review team's
ongoing effort to review key IETF documents. These comments were written
primarily for the transport area directors, but are copied to the document's
authors and WG to allow them to address any issues raised and also to the
IETF discussion list for information.

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review as part of the last-call comments they receive. Please always CC
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This draft defines a YANG module for tunnel types based on the MIB-2
tunnel type registry maintained by IANA.

My fundamental concern with this draft is that the MIB-2 tunnel type
registry is seriously incomplete and out of date, as there are a large
number of tunnel types that aren't included in that registry, e.g., IPsec
tunnel-mode AMT tunneling.  In its current form, that registry does not
appear to be a good starting point for specifying YANG management of
tunnels.     

A limited justification that I could envision for defining this YANG module
would be to use it for mechanical translations to YANG of existing MIBs
that use MIB-2 tunnel types - if that's the justification, then it would need
to be clearly stated in an applicability statement within this draft, and the
discussion of extension of this YANG module would need to be aligned with
that limited applicability. 

The proverbial "right thing to do" would be to update both the MIB-2 tunnel
type registry and this draft with all of the currently known tunnel types.
The references section of draft-ietf-tsvwg-rfc6040update-shim
(https://datatracker.ietf.org/doc/draft-ietf-tsvwg-rfc6040update-shim/)
may help in identifying tunnel protocols that should be included.

A minor concern involves the use of RFC 8085 as the reference for UDP
tunnels; while that's certainly better than the existing use of RFC 4087, due
to the extensive design guidance in RFC 8085, designers of UDP-encapsulated
tunnel protocols ought to be encouraged to register their protocols as separate
tunnel types (e.g., so the network operator has some idea of what the UDP
tunnel is actually being used for).  This draft ought to encourage tunnel
protocol designers to register their own tunnel types in preference to reuse
of the UDP tunnel type, including placing text in the IANA tunnel type
registry and this YANG module to encourage that course of action.