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

alexey,

i second Les' observation. the TLV-adding semantics of the router-CAP TLV (and other TLVs like IP prefix TLV) are well understood  - even by junior implementers and IMO do not need explicitly be mentioned. Furthermore historically we just did document deviations ( if any)  from that rule of thumb.

/hannes

> On 18 Aug 2016, at 18:48, Les Ginsberg (ginsberg) <ginsberg@cisco.com> wrote:
> 
> 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:aamelnikov@fastmail.fm]
>> Sent: Thursday, August 18, 2016 2:59 AM
>> To: Les Ginsberg (ginsberg); The IESG
>> Cc: draft-ietf-isis-rfc4971bis@ietf.org; Christian Hopps; isis-chairs@ietf.org;
>> isis-wg@ietf.org
>> 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.
> 
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