Re: [Isis-wg] Alexey Melnikov's Discuss on draft-ietf-isis-rfc4971bis-03: (with DISCUSS and COMMENT)

[Alia Atlas <[e-mail]>]

Hi Alexey,

On Thu, Aug 18, 2016 at 5:58 AM, Alexey Melnikov <[e-mail]>
wrote:

> Hi Les,
>
> On Tue, Aug 16, 2016, at 05:56 PM, Les Ginsberg (ginsberg) wrote:
> > Alexey -
>
>  [snip]
> > > ----------------------------------------------------------------------
> > > DISCUSS:
> > > ----------------------------------------------------------------------
> > >
> > > I would like to get clarification on the following points before
> recommending
> > > approval of this document:
> > >
> > > 1) How do multiple CAPABILITY TLVs from the same source treated, if
> they
> > > have the same S and D flags, but different subTLV? Are the cumulative?
> Or
> > > this is not allowed?
> > > I am sorry if I missed where this was described, let me know if I did.
> >
> > [Les:] Section 2 states:
> >
> > " The Router CAPABILITY TLV is OPTIONAL.  As specified in Section 3,
> >    more than one Router CAPABILITY TLV from the same source MAY be
> >    present."
> >
> > The only problematic case is when you have two TLVs from the same source
> > which have different settings for the same attribute i.e. same sub-TLV
> > but different values. This case is discussed in detail in Section 3.
> > Different sub-TLVs presents no issue of course.
> > Note this text is unchanged from the existing RFC.
>
> What happens in real world if there are two CAPABILITY TLVs which
> contain different attributes? Are they treated as cumulative (i.e. this
> is a nice trick to overcome CAPABILITY TLV length limit), does the
> second CAPABILITY  TLV value always overrides earlier instances of the
> CAPABILITY TLV?
>
> I think the document should state what happens. If there is no
> consistent behaviour in this area, the document should says so as well.
>

First, with IS-IS flooding, there is no defined order in which different
TLVs can be
assumed to be received.  It isn't possible to do a "first-in" or "last-in"
tie-breaker
because the TLVs can be received in different order at different routers.

Second, the entire content of the CAPABILITY TLV is a set of flags,
currently
used solely to control how the TLV is distributed between IS-IS level 1 and
IS-IS
level 2, and optional sub-TLVs.

It would be possible for additional of the reserved flags to be, in theory,
allocated
in which case they would also represent an "attribute" of the CAPABILITY
TLV.
Currently, only sub-TLVs have been defined; each individually may contain
multiple
pieces of information.  What is considered "the same attribute" could vary
by sub-TLV
or be the whole sub-TLV.  For instance, in RFC 4972, "IS-IS TE-MESH-GROUP
sub-TLV may contain one or more mesh-group entries"

The document already specifies "Where a receiving system has two copies of
a CAPABILITY TLV from the
   same system that have different settings for a given attribute, the
   procedure used to choose which copy shall be used is undefined."

Regards,
Alia