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

Peter Psenak <ppsenak@cisco.com> Tue, 05 March 2019 15:28 UTC

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From: Peter Psenak <ppsenak@cisco.com>
To: Alexander Vainshtein <Alexander.Vainshtein@ecitele.com>
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Subject: Re: [Lsr] [spring] FlexAlgo and Global Adj-SIDs
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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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>
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