[Lsr] Robert Wilton's No Objection on draft-ietf-ospf-te-link-attr-reuse-15: (with COMMENT)

Robert Wilton via Datatracker <noreply@ietf.org> Tue, 23 June 2020 10:42 UTC

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Subject: [Lsr] Robert Wilton's No Objection on draft-ietf-ospf-te-link-attr-reuse-15: (with COMMENT)
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Robert Wilton has entered the following ballot position for
draft-ietf-ospf-te-link-attr-reuse-15: No Objection

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COMMENT:
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Discuss cleared.  Thank you for addressing my comments.

Two possible nits in section 5:

   If the same attribute is advertised in more than single ASLA sub-TLVs
   with the application listed in the Application Bit Masks, the
   application SHOULD use the first instance of advertisement and ignore
   any subsequent advertisements of that attribute.

Propose changing "single" to "one".

   If link attributes are advertised associated with zero length
   Application Identifier Bit Masks for both standard applications and
   user defined applications, then any Standard Application and/or any
   User Defined Application is permitted to use that set of link
   attributes.

Propose changing "associated with" to "with associated" or just "with"

Thanks,
Rob

Previous discuss comments:

I found parts of this document hard to understand, but I'm not familiar with
the specifics of the protocols.

This discuss is in the vein of "I think that folks might struggle to implement
this correctly/consistently".   In particular I had some questions/concerns
about section 5 which, if clarified, would probably help this document.

In Section 5:

   The ASLA sub-TLV is an optional sub-TLV and can appear multiple times
   in the OSPFv2 Extended Link TLV and OSPFv3 Router-Link TLV.  The ASLA
   sub-TLV MUST be used for advertisement of the link attributes listed
   at the end on this section if these are advertised inside OSPFv2
   Extended Link TLV and OSPFv3 Router-Link TLV.  It has the following
   format:

I think that it would be useful to clarify when/why the ASLA sub-TLV can be
included multiple times.  I.e. when different applications want to control
different link attributes.

   Standard Application Identifier Bits are defined/sent starting with
   Bit 0.  Undefined bits which are transmitted MUST be transmitted as 0
   and MUST be ignored on receipt.  Bits that are not transmitted MUST
   be treated as if they are set to 0 on receipt.  Bits that are not
   supported by an implementation MUST be ignored on receipt.

It was not clear to me what it means if the SABM (or UDABM) fields are entirely
empty.  This paragraph states that they are treated as if they are 0, but
sections 8 and 11 imply that if the field is omitted then it acts as if all
applications are allowed.  Section 12.2 implies that if the field is omitted
then it is as if all applications are allowed unless there there is another
ASLA with the given application bit set, in which case it is treated as being a
0 again.  I think that this document would be helped if the specific behaviour
was defined in section 5, retaining the justification/clarification in the
subsequent sections.

It is also not entirely clear to me exactly how the bits are encoded on the
wire.  My assumption is that if bit 0 is set, then this would sent the highest
bit of the first byte.  E.g. 0x80?  Is that correct?  If not, then I think that
the document needs more text, if so, then an example of the encoding may still
aid readability.

   User Defined Application Identifier Bits have no relationship to
   Standard Application Identifier Bits and are not managed by IANA or
   any other standards body.  It is recommended that bits are used
   starting with Bit 0 so as to minimize the number of octets required
   to advertise all UDAs.

Doesn't this need more constraints to ensure easy interop (i.e. bits default to
0).  Otherwise, it would seem that anyone is allowed to put any value in this
field that they like that could harm interop, or otherwise it might be tricky
to compare a 4 byte UDABM to an 8 byte UDABM?

   This document defines the initial set of link attributes that MUST
   use the ASLA sub-TLV if advertised in the OSPFv2 Extended Link TLV or
   in the OSPFv3 Router-Link TLV.  Documents which define new link
   attributes MUST state whether the new attributes support application
   specific values and as such MUST be advertised in an ASLA sub-TLV.
   The link attributes that MUST be advertised in ASLA sub-TLVs are:

I think that I get what this means, but I find the last two sentences slightly
jarring given than the ASLA TLV is optional.  Perhaps predicate both of these
constraints with "(if supproted)".  E.g., something like,

 Documents which define new link
 attributes MUST state whether the new attributes support application
 specific values and as such MUST be advertised in an ASLA sub-TLV (if
 supported). The link attributes that MUST be advertised in ASLA sub-TLVs (if
 supported) are: