Re: [Lsr] Using IS-IS Multi-Topology (MT) for Segment Routing basedVirtual Transport Network

Hi Les,

On 29/03/2020 17:57, Les Ginsberg (ginsberg) wrote:
> Peter/Aijun -
> 
> We are in agreement.
> I never said that an L2 Bundle member could/should be associated with a specific MTID.

ok, but that is what the authors of the draft-xie-lsr-isis-sr-vtn-mt are 
proposing. That's the reason why I was opposing that idea.
> 
> It is the L3 adjacency that has the MTID association.
> If we were going to support MTID in TLV 25 the correct place to put it would be as part of the " Parent L3 Neighbor Descriptor".

sure, although it looks redundant to me, given that the same L3 link is 
already advertised in other TLV which can have MTID in it.

thanks,
Peter
> 
>     Les
> 
> 
>> -----Original Message-----
>> From: Lsr <lsr-bounces@ietf.org> On Behalf Of Aijun Wang
>> Sent: Sunday, March 29, 2020 3:46 AM
>> To: Peter Psenak <ppsenak=40cisco.com@dmarc.ietf.org>
>> Cc: peng.shaofu@zte.com.cn; Dongjie (Jimmy) <jie.dong@huawei.com>;
>> xiechf@chinatelecom.cn; Les Ginsberg (ginsberg)
>> <ginsberg=40cisco.com@dmarc.ietf.org>; lsr@ietf.org; Tony Przygienda
>> <tonysietf@gmail.com>
>> Subject: Re: [Lsr] Using IS-IS Multi-Topology (MT) for Segment Routing
>> basedVirtual Transport Network
>>
>> I have the same consideration as Peter.
>>
>> Aijun Wang
>> China Telecom
>>
>>> On Mar 29, 2020, at 17:10, Peter Psenak
>> <ppsenak=40cisco.com@dmarc.ietf.org> wrote:
>>>
>>> Hi Les,
>>>
>>>> On 29/03/2020 00:00, Les Ginsberg (ginsberg) wrote:
>>>> Tony –
>>>> There are a few misunderstandings in your post.
>>>> Let me try to correct them.
>>>> RFC 8668 defines a new top-level TLV (25). This is NOT a sub-TLV(sic) of
>> TLVs 22,23,141,222,223.
>>>> Conceptually, it could have been defined as a sub-TLV, but because of the
>> limited length of a TLV in IS-IS (255 octets) and the likelihood that advertising
>> L2Bundle member attributes would consume a significant amount of space,
>> we decided to use a new top-level TLV which references the associated L3
>> adjacency advertised in TLV 22. This means TLV22 advertisements are not
>> impacted by the presence of TLV 25. In particular, the use of TLV 25 does not
>> lead to multiple TLV 22 advertisements for the same adjacency, which we
>> thought was a desirable outcome.
>>>> The equivalent OSPF draft (https://tools.ietf.org/html/draft-ketant-lsr-
>> ospf-l2bundles-01 ) takes the sub-TLV approach, but since OSPF has a 16 bit
>> length for TLVs it does not face the same encoding issues.
>>>> I fully agree with you that an L2 bundle is a Layer 3 construct – and as such
>> can be associated with any Layer 3 MTID in theory. However, when writing
>> RFC 8668 we did not consider that there was a use case for topology specific
>> link attributes. The current encoding does not support MTID.
>>>
>>> RFC 8668 defines a TLV to advertise attributes of the individual L2 link
>> members. While the TLV itself can be considered as L3 construct (and have
>> MT associated with it), the individual L2 Bundle Member Links inside this TLV
>> are not L3 constructs IMHO and should never have a MTID associated with
>> them. The ask here is to advertise different MTID for each individual L2
>> bundle member link.
>>>
>>> MTID is used to construct a topology. I do not see how a L2 bundle link
>> member would ever become part of the L3 topology.
>>>
>>> my 2c,
>>> Peter
>>>
>>>
>>>> At this point, if we needed to extend RFC 8668 to support MTID we would
>> need to allocate an additional TLV code point that included MTID – similar to
>> the distinction between TLV 22 and TLV 222.
>>>> HTH
>>>>     Les
>>>> *From:* Lsr <lsr-bounces@ietf.org> *On Behalf Of * Tony Przygienda
>>>> *Sent:* Saturday, March 28, 2020 10:25 AM
>>>> *To:* peng.shaofu@
>>
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> 
> 