Re: [Lsr] [spring] FlexAlgo and Global Adj-SIDs

Hi Sasha,

On 05/03/2019 17:28 , Alexander Vainshtein wrote:
> Peter,
> Lots of thanks for a prompt and very encouraging response.
>
> Do you think that the new Algo specific Adj-SID sub-TLV could be added to the current IS-IS segment Routing Extensions draft, or should be handled in a small dedicated document?

it would have to be a new draft, existing ISIS SR draft is on its way to 
become an RFC and we do not want to return it back to the WG due to this.

thanks,
Peter

>
> Regards, and lots of thanks in advance,
> Sasha
>
> Office: +972-39266302
> Cell:      +972-549266302
> Email:   Alexander.Vainshtein@ecitele.com
>
> -----Original Message-----
> From: Peter Psenak <ppsenak@cisco.com>
> Sent: Tuesday, March 5, 2019 5:28 PM
> To: Alexander Vainshtein <Alexander.Vainshtein@ecitele.com>
> Cc: lsr@ietf.org; spring@ietf.org; draft-ietf-isis-segment-routing-extensions.all@ietf.org
> Subject: Re: [spring] FlexAlgo and Global Adj-SIDs
>
> Hi Sasha,
>
> On 02/03/2019 18:57 , Alexander Vainshtein wrote:
>> Peter,
>> Lots of thanks for a prompt and hivhly informative response.
>>
>> It seems that per-FlexAlgo Adj-SIDs can be useful even if they are local.
>> The relevant use case could be a protected local Adj-SID that is used
>> in a SR-TE LSP that has been set up with some constraints in mind.
>> These constraints would be preserved when the protected adjacency in
>> question is UP, but could be violated when it fails and the shortest
>> path to the node at the remote end of adjacency is used.
>
> yes, this is a possible use case. This would require a new Algo specific Adj-SID sub-TLV to be defined. This could be done.
>
> thanks,
> Peter
>
>
>
>>
>> Local protected Adj-SIDs for sure have bern implemented, so this looks
>> like a more relevant use case than global adjacencies to me.
>>
>> What do you think?
>>
>> Regards, and lots of thanks in advance,
>>
>> Thumb typed by Sasha Vainshtein
>>
>> ----------------------------------------------------------------------
>> --
>> *From:* Peter Psenak <ppsenak@cisco.com>
>> *Sent:* Thursday, February 28, 2019 9:56:49 PM
>> *To:* Alexander Vainshtein;
>> draft-ietf-isis-segment-routing-extensions@ietf.org
>> *Cc:* lsr@ietf.org; spring@ietf.org
>> *Subject:* Re: [spring] FlexAlgo and Global Adj-SIDs
>>
>> Hi Alexander,
>>
>> On 28/02/2019 19:19 , Alexander Vainshtein wrote:
>>> Dear colleagues,
>>>
>>> I have a question regarding global Adj-SIDs in
>>> draft-ietf-isis-segment-routing-extensions.
>>>
>>>
>>>
>>> Section 3.4 of RFC 8402 <https://tools.ietf.org/html/rfc8402>
>>> defines definition and handling of global Adj-SIDs.
>>>
>>> The relevant text is given below:
>>>
>>>
>>>
>>> <quote>
>>>
>>>    Similarly, when using a global Adj-SID, a packet injected anywhere
>>>
>>>    within the SR domain with a segment list {SNL}, where SNL is a
>>> global
>>>
>>>    Adj-SID attached by node N to its adjacency over link L, will be
>>>
>>>    forwarded along the shortest path to Nand then be switched by N,
>>>
>>>    without any IP shortest-path consideration, towards link L.
>>>
>>> ...
>>>
>>>    The use of global Adj-SID allows to reduce the size of the segment
>>> list
>>>
>>>    When expressing a path at the cost of additional state (i.e., the
>>> global
>>>
>>>    Adj-SID will be inserted by all routers within the area in their
>>>
>>>    forwarding table).
>>>
>>> <end quote>
>>>
>>>
>>>
>>> The definition of the Adjacency Segment Identifier Sub-TLV in Section
>>> 2.2.1 of the draft matches the behavior defined in RFC 8402, i.e., it
>>> allows advertisement of global Adj-SIDs.
>>>
>>> These advertisements can be associated with a specific IGP adjacency
>>> and, in multi-topology scenarios, a specific topology. But it is not
>>> associated with any specific algorithm since the default algorithm
>>> for reaching the advertising node is implicitly assumed in full
>>> alignment with RFC 8402.
>>>
>>>
>>>
>>> My question is about the situation in which multiple algorithms are
>>> supported by the routers in the SR domain, so that each node
>>> advertises a dedicated Node SID for each of these algorithm.
>>>
>>> In this scenario, the operator can set up a SR-TE LSP that meets
>>> specific constraints (incorporated in one of these algorithms, see
>>> IGP Flexible Algorithm
>>> <https://tools.ietf.org/html/draft-ietf-lsr-flex-algo-01> draft) by
>>> using Node SIDs that have been advertised with the corresponding
>>> algorithm and local Adj-SIDs. However, global Adj-SIDs cannot be used
>>> (e.g., for reducing the label stack depth) in this situation.
>>>
>>> I wonder if the possibility to advertise global Adj-SID (that can be
>>> considered as replacing a combination of the Node SID of the
>>> advertising node and one of its local Adj-SIDs) as associated with a
>>> specific algorithm has ever been considered?
>>
>> to be honest, it has been not.
>>
>> The usage of the global Adj-SID has not been widely considered due to
>> the additional forwarding entries it requires on rest of the routers
>> in the area. Not sure if there are implementations of the global
>> ADj-SID out there.
>>
>> If you believe this is important, we can certainly addressed that in a
>> small draft.
>>
>> thanks,
>> Peter
>>
>>
>>
>>
>>>
>>>
>>>
>>> Regards, and lots of thanks in advance,
>>>
>>> Sasha
>>>
>>>
>>>
>>> Office: +972-39266302
>>>
>>> Cell:      +972-549266302
>>>
>>> Email:   Alexander.Vainshtein@ecitele.com
>>>
>>>
>>>
>>>
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