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

["Les Ginsberg (ginsberg)" <[e-mail]>]

Alexey -

The document states in Section 2:

" The Router CAPABILITY TLV is OPTIONAL.  As specified in Section 3,
   more than one Router CAPABILITY TLV from the same source MAY be
   present."

This clearly allows multiple CAPABILITY TLVs from the same source to be present. Why? Because the basic unit of advertisement in IS-IS is a TLV - in this case one which is intended to include multiple sub-TLVs. As the value field of a  TLV is limited in size to 255 bytes if a router has more than 255 bytes of CAPABILITY information to advertise then multiple TLVs will be required. This isn't a "trick" - it is fundamental to the operation of IS-IS. 

It is unclear to me how you could interpret the above text as meaning that multiple CAPABILITY TLVs from the same source are either disallowed or unexpected.

I appreciate that you may not be an expert in IS-IS and some fundamentals may be unfamiliar to you. That is why the document provides references to the base specifications which provide these definitions.  A new implementer of the protocol could not write an implementation solely based on this document - and it is not reasonable to request that this document be written without dependencies on the base specifications.

   Les

> -----Original Message-----
> From: Alexey Melnikov [mailto:[e-mail]]
> Sent: Thursday, August 18, 2016 2:59 AM
> To: Les Ginsberg (ginsberg); The IESG
> Cc: [e-mail]; Christian Hopps; [e-mail];
> [e-mail]
> Subject: Re: Alexey Melnikov's Discuss on draft-ietf-isis-rfc4971bis-03: (with
> DISCUSS and COMMENT)
> 
> 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.