Last Call Review of draft-ietf-isis-te-app-13
review-ietf-isis-te-app-13-tsvart-lc-rose-2020-05-20-00

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This document describes a mechanism for specifying per-application attributes
for link state routing using IS-IS. It proposes no routing functionality
changes to applications making use of such attributes, only enabling the
ability to choose which attributes in an advertisement apply to a particular
application, and consequently I see no specifically transport-related issues in
this draft.

Nonetheless, I have a few comments.

* Update language?

There are several places in which it is possible that normative language in
prior RFCs is revised. For example, in section 6.1, the last paragraph states:

  New applications which future documents define to make use of the
  advertisements defined in this document MUST NOT make use of legacy
  advertisements.

This looks like it was written in such a way as to avoid requiring such
updates, but it would be good to confirm that there is no normative language in
older documents that would conflict with this requirement.

* Encoding

In section 4.1, the bit masks are defined with a 7 bit length field for which
only 4 bits are useful (values 0-8). It may make sense to keep the 3 high order
bits as "reserved" and set to zero, but either way it would be nice to
understand the justification for whatever design decision is made.

You go to some length to save space (e.g., a zero-length SABM means "all
applications") but always include an octet for UDABM length, which I presume
will be zero outside the lab in most cases. How much does an extra octet cost?
If it's a lot, you could use the three high-order bits to represent the first
three applications (RSVP-TE, SRTE, and LFA) and save yourself an octet until a
fourth application appears.

For that matter, how likely are you to get to 256 standardized applications
under IS-IS?

The fallback from application-specific advertisements to legacy advertisements
is not entirely clear. It sounds like:

 - An application is to use the legacy advertisements if there is at least one
 application specific advertisement for that application with L=1, in which
 case *all* advertisements for that application must also have L=1. - An
 application is to use the application-specific advertisements if there is at
 least one application specific advertisement for that application with L=0, in
 which case *all* advertisements for that application must also have L=0, *and*
 that application is to ignore all legacy advertisements.

In effect, use of legacy advertisements vs. app-specific advertisements is
all-or-nothing. If that's the case, wouldn't a top-level TLV specifying a
legacy mask for applications be more compact, less redundant, and further
reduce the overall number/size of advertisements?

Anyway, I'm out of my element, so feel free to ignore these comments if I'm way
off the mark.