RE: SRLG usage in the IGP Flexible Algorithm draft

[Alexander Vainshtein <[e-mail]>]

Peter,

Lots of thanks for a prompt response.



My reading of your response is as following:

There are two different ways in which SRLG information can be used with Flexible Algorithms:

1.       In a context of a single Flexible Algorithm, it can be used for computation of backup paths (e.g., as described in the TI-LFA draft). This usage does not require association of any specific SRLG with the given Flexible Algorithm.

2.       In the context of multiple Flexible Algorithms it can be used for creating disjoint sets of paths by pruning the links belonging to a specific SRLG from the topology on which a specific Flexible Algorithm computes its paths. This usage:

a.       Extends the definition of the topology used by a given Flexible Algorithm that can be defined using Admin groups (a.k.a. affinities)

b.       Facilitates usage of already deployed SRLG configurations where such configuration have been in place

c.       Requires explicit association of a given Flexible Algorithm with a specific set of SRLG as defined in the current Flex Algo draft.

The two usages mentioned above are strictly orthogonal.



If the understanding above is correct, it makes to me sense to add the corresponding clarifying text to the draft.



Regards,

Sasha



Office: [phone]

Cell:      [phone]

Email:   [e-mail]



-----Original Message-----
From: Peter Psenak <[e-mail]>
Sent: Friday, May 1, 2020 12:57 PM
To: Alexander Vainshtein <[e-mail]>; [e-mail]; [e-mail]; [e-mail]; [e-mail]
Cc: [e-mail]; [e-mail]; [e-mail]; Michael Gorokhovsky <[e-mail]>; Ron Sdayoor <[e-mail]>
Subject: Re: SRLG usage in the IGP Flexible Algorithm draft



Alexander,



On 30/04/2020 17:21, Alexander Vainshtein wrote:

> Hi all,

>

> I have a question about the proposed usage of SRLG in the IGP Flexible

> Algorithm <https://clicktime.symantec.com/3Wn8dBNRoEXXTU1kJz6RrsR6H2?u=https%3A%2F%2Ftools.ietf.org%2Fhtml%2Fdraft-ietf-lsr-flex-algo-07> draft.

>

> This usage is defined Section 12 of the draft with the reference to

> the SRLG exclude rule as following:

>

>

>

>        2.  Check if any exclude SRLG rule is part of the

> Flex-Algorithm

>

>        definition.  If such exclude rule exists, check if the link is

>

>        part of any SRLG that is also part of the SRLG exclude rule.

> If

>

>        the link is part of such SRLG, the link MUST be pruned from the

>

>        computation.

>

> This looks effectively undistinguishable from the usage of the exclude

> Admin groups rule as described in the same Section 12 of the draft:

>

>        1.  Check if any exclude rule is part of the Flex-Algorithm

>

>        definition.  If such exclude rule exists, check if any color

> that

>

>        is part of the exclude rule is also set on the link.  If such a

>

>        color is set, the link MUST be pruned from the computation.

>

>  From my POV, with such a definition, there is no need in the

> dedicated “Exclude SRLG” rule as part of the specification of the

> Flexible Algorithm, since such the SRLG Exclude rule can be replaced

> with a matching Exclude All rule  using Admin groups.



there is one very important point. Maintaining the affinities is operationally complex. Some networks have already deployed SRLGs. If SRLG exclude with flex-algo gives people what they want, asking them to deploy affinities would be redundant. That's the main point.



Simple use case:



I have two SRLGs in the network. For some specific traffic I want to send the data via two independent streams. I want to avoid single SRLG failure to affect both streams. I define two flex-algos, each one excluding one SRLG. No need to define extra affinities. This is btw not an academical example, this has been requested by real users.





>

> I also think that such a usage of SRLG does not fit the needs of the

> TI-LFA

> <https://clicktime.symantec.com/3GhKPQ2JAgebW7vLidLjXRx6H2?u=https%3A%

> 2F%2Ftools.ietf.org%2Fhtml%2Fdraft-ietf-rtgwg-segment-routing-ti-lfa-0

> 3> draft that considers an SRLG as a resource that fails when any of

> the links/nodes comprising it fails. E.g., it says in Section 2:

>

>     The Point of Local Repair (PLR), S, needs to find a node Q (a

> repair

>

>     node) that is capable of safely forwarding the traffic to a

>

>     destination D affected by the failure of the protected link L, a

> set

>

>     of links including L (SRLG), or the node F itself.  The PLR also

>

>     needs to find a way to reach Q without being affected by the

>

>     convergence state of the nodes over the paths it wants to use to

>

>     reach Q: the PLR needs a loop-free path to reach Q.



I see no conflict between the LFA draft and flex-algo one. If you do, please be precise where the conflict is.



>

> To me this suggests that SRLGs are only relevant when computing backup

> paths for specific failures, e.g., an LFA for failure of a link hat

> belongs to a specific SRLG must be computed in the topology from which

> all the links belonging to the same SRLG are pruned. This

> understanding matches RFC 4090 that states in Section 6.2 “Procedures

> for Backup Path

> Computation”:



SRLG is a mechanism to express fate share. I see no reason why SRLG could not be used for other than LFA purposes.



thanks,

Peter





>

>        - The backup LSP cannot traverse the downstream node and/or

> link

>

>          whose failure is being protected against.  Note that if the

> PLR

>

>          is the penultimate hop, node protection is not possible, and

>

>          only the downstream link can be avoided. The backup path may

> be

>

>          computed to be SRLG disjoint from the downstream node and/or

>

>          link being avoided.

>

> If SRLGs are only relevant for computation of backup paths, it is not

> clear to me if they should be part of the definition of a specific

> Flexible Algorithm.

>

> What, if anything, did I miss?

>

> Regards, and lots of thanks in advance,

>

> Sasha

>

> Office: [phone]

>

> Cell:      [phone]

>

> Email:   [e-mail]<mailto:[e-mail]>

>

>

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