Re: [spring] Draft for Node protection of intermediate nodes in SR Paths

[Shraddha Hegde <[e-mail]>]

Sasha,

We are in agreement on separating the trigger from the protection mechanism.

> In any case I think that it woyld make sense to separate the protection scheme proposed in the draft from specific triggers for its activation >similar to how this has been done in MPLS Egress Protection Framework draft.

I'll add text in the next revision for this.

Rgds
Shraddha


From: Alexander Vainshtein <[e-mail]>
Sent: Monday, December 2, 2019 12:24 PM
To: Shraddha Hegde <[e-mail]>
Cc: [e-mail]; [e-mail]; [e-mail]; Robert Raszuk <[e-mail]>
Subject: Re: [spring] Draft for Node protection of intermediate nodes in SR Paths

Shraddha,
Lots of thanks for athe tesponse.

I probably did not express myself clearly enough. I will try to fix thst now, and I apologise in advance for a long email.

I have not been speaking about end-yo-end protecyion, only about local protection against failure of an intermediate (a.k.a. pinned) node of an SR path and, specifically, triggers for such protection. This context has been actually defined by Robert in his original comment.

To the best of my understanding, Robert's concern was that failure of the link beteeen the pinned node of a SR path and its adjacency (the penultimate node of the Segment represented by the Node SID of the pinned node) is not a good enough indication of the pinned node failure.

I agree with this statement even if my understanding of a good indication differs from Robert's:
- I think that it is not sufficiently specific and therefore could result in flapping (local node protection activated and then released)
-Robert's concern, to the best of my understanding, was that it could miss some failures (e.g. the Fabric failure).

Therefore I have suggested two possibilities for more specific and more rrliabke detection of failure of the pinned node by its adjacency:

1. Run a multi-hop IP BFD session between the peniltimate node ans the pinned ones using prefixes acting as Node SIDs of this pair.  This wiuld ignore link failures but locally detect such node failurs as power-down or crash.

2.  Run S-BFD sessions to all other adjacencies of the pinned node using in each case a list of two SIDs: the protected Adj-SID to the pinned node followed by tge Node SID of the other adjacency, ans declare pinned node failure when all these sessions fail. This would again ignore failure of the link between the penultimate node and the pinned node but detect various real failures of the pinned node, e.g. failure of its Fabric.

In any case I think that it woyld make sense to separate the protection scheme proposed in the draft from specific triggers for its activation similar to how this has been done in MPLS Egress Protection Framework draft.

My 2c.






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________________________________
From: Shraddha Hegde <[e-mail]<mailto:[e-mail]>>
Sent: Monday, December 2, 2019, 06:10
To: Alexander Vainshtein; Robert Raszuk
Cc: [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Subject: RE: [spring] Draft for Node protection of intermediate nodes in SR Paths


Robert/Sasha,


S-BFD based mechanism is  head-end triggered protection. It is not a local protection.
S-BFD mechanism is orthogonal to the mechanism described in this draft and an operator can
choose what kind of protection makes more sense to his/her network.

In many cases, node-protecting backup path will be different from link-protecting/SRLG protecting backup path.
If you really want to use link-protecting backup path when link fails and node protecting backup path when node fails,
You will have to download both link protecting and node-protecting backup paths in FIB and detect which
failure really happened and have the ability in hardware to use appropriate backup path. None of these
is in the scope of this document.

Rgds
Shraddha


From: Alexander Vainshtein <[e-mail]<mailto:[e-mail]>>
Sent: Saturday, November 23, 2019 8:15 PM
To: Robert Raszuk <[e-mail]<mailto:[e-mail]>>; Shraddha Hegde <[e-mail]<mailto:[e-mail]>>
Cc: [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Subject: Re: [spring] Draft for Node protection of intermediate nodes in SR Paths

Robert,
On the second thought, for the purpose of this draft (i.e. in the scope of SR) it is possible to implement your suggestion by running S-BFD sessions between R7 (as the initiator) and each other adjacency of R8  (acting as Reflectors) of a SR policy with list of two SIDs:
- protected adjacency between R7 and R8
- Node SID of the specific "other" adjacency  of R8.

If all these sessions fail, R7 can reliably consider R8 as failed.

I am not sure this would be much better than multi-hop IP BFD, and it looks much more complicated to me.


What do you think?




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________________________________
From: Alexander Vainshtein <[e-mail]<mailto:[e-mail]>>
Sent: Saturday, November 23, 2019, 13:15
To: Robert Raszuk; Shraddha Hegde
Cc: [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Subject: Re: [spring] Draft for Node protection of intermediate nodes in SR Paths

Robert,
Lots of thanks for a prompt response.

I respectfully disagree with your statement that BFD implementation  is usually offloaded to the HW of the ingress line card.  I do not think this can wor for MH BFD sessions because the ingress and egress line cards are not known in advance and change with the routing changes
A good  multi-hop BFD implementation should be ready to overcome this.. There are many ways to achieve that. A naive implementation that runs in SW of the control card is also possible of course. And they would sensd and receive packets

My 2c.
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________________________________
From: Robert Raszuk <[e-mail]<mailto:[e-mail]>>
Sent: Saturday, November 23, 2019, 12:37
To: Alexander Vainshtein; Shraddha Hegde
Cc: [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Subject: Re: [spring] Draft for Node protection of intermediate nodes in SR Paths

Hi Sasha,

On the surface your suggestion may look cool - but if you zoom in - I do not think it will work in practice.

See - one of the biggest value of BFD is its offload to line card's hardware. And in most cases it is ingress line card to the box. So if you instruct such hardware to respond to SID address loopback you still did not gain much in terms of detection router's fabric failures, remote LC failure or control plane issues which could soon result in box failure. The catalogue of router failures is of course much more colorful.

If you ask BFD to be responded by RP/RE it no longer has the BFD advantage.

IMHO the best way to detect node failure is actually to send the probes *across* the node under test to its peers.

The way I would think of establishing such m-hop sessions would be fully automated with one knob per IGP adj. ex: "bfd detect-node-failure [max N]" where local BFD subsystem would create N sessions to IGP peers of the node we are to protect. LSDB has those peers so no new protocol extension is needed, perhaps even no new IETF draft is required :). N would be the limit of such sessions in case the node under protection has say 10s of peers. Default could be perhaps even 1.

Thx,
Robert.


On Sat, Nov 23, 2019 at 10:00 AM Alexander Vainshtein <[e-mail]<mailto:[e-mail]>> wrote:
Shraddha, Robert and all,
Regarding Robert's question:
I wonder if multi-hop IP BFD session with addresses used as /32 (or /128) prefixes serving as Nose SIDs of R8 and R7 respectively could be used as such a trigger by R7? Such a session would not respond to link failures, and I find it problematic to imagine a scenario when it would be kept UP in the case of a real node failure.

Of course such a session would have to be slow enough not to react to link failures. But it still couks be much faster than IGP conversion IMHO.

My 2c,
Sasha

Such


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________________________________
From: spring <[e-mail]<mailto:[e-mail]>> on behalf of Robert Raszuk <[e-mail]<mailto:[e-mail]>>
Sent: Friday, November 22, 2019, 11:22
To: Shraddha Hegde
Cc: [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Subject: Re: [spring] Draft for Node protection of intermediate nodes in SR Paths

Hi Shraddha,

I have one question to the document.

As you know the critical element for the effective protection of any scheme is the failure detection. On that your draft seems to have just one little paragraph:


   Note that R7 activates the node-protecting backup path when it

   detects that the link to R8 has failed.  R7 does not know that node

   R8 has actually failed.  However, the node-protecting backup path is

   computed assuming that the failure of the link to R8 implies that R8

   has failed.

Well IMO this is not enough. Specifically there can be a lot of types of node failure when link is still up. Moreover there can be even running BFD across the link just fine when say fabric failure occurs at R8.

While this is not solely issue with this draft, it is our common IETF failure to provide correct means of detecting end to end path or fragments of path failures (I am specifically not calling them segment here :).

For example I propose that to effectively detect R8 failure as node failure which is the topic of your proposal a mechanism is clearly defined and includes bi-dir data plane probes send between R7-R9, R3-R7, R4-R7, R4-R9, R3-R9

Many thx,
Robert.


On Fri, Nov 22, 2019 at 4:38 AM Shraddha Hegde <shraddha=[e-mail]..org<mailto:[e-mail]>> wrote:
WG,

This is the draft I pointed out that talks about solutions for providing node-protection.
It covers Anycast case as well as keeping forwarding plane longer.
https://tools.ietf.org/html/draft-hegde-spring-node-protection-for-sr-te-paths-05<https://urldefense.com/v3/__https:/clicktime.symantec.com/3HvrzHXwAou2JruETj6jcyF6H2?u=https*3A*2F*2Furldefense.com*2Fv3*2F__https*3A*2Fclicktime.symantec.com*2F375SW6TBGPi2mN7V9YeVWGg6H2*3Fu*3Dhttps*2A3A*2A2F*2A2Ftools.ietf.org*2A2Fhtml*2A2Fdraft-hegde-spring-node-protection-for-sr-te-paths-05__*3BJSUlJSU*218WoA6RjC81c*21Xo-D7e5MfUeTOyV17KUflgSI002KCmsw_EjxLc9pxA6sJ1EbrioDE0Vmgg0xmj_C*24__;JSUlJSUlJSUlJSUlJSUlJSUlJQ!8WoA6RjC81c!QAhYaA0qhoUZ3yxQWl05Ap12CMR-J-RTL_O_d_wKElC5ktZrPekfTDhLH9RfbOqT$>

Review and comments solicited.

Rgds
Shraddha

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