Re: [Isis-wg] Conflicting MS entries

<bruno.decraene@orange.com> Wed, 17 June 2015 14:54 UTC

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From: <bruno.decraene@orange.com>
To: "Stefano Previdi (sprevidi)" <sprevidi@cisco.com>
Thread-Topic: Conflicting MS entries
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Date: Wed, 17 Jun 2015 14:53:55 +0000
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Subject: Re: [Isis-wg] Conflicting MS entries
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Hi Stefano,

Thanks for initiating discussion on this point.

1) I don't feel that this is a IS-IS encoding specific issue, but rather a SPRING consideration, so I'm adding spring.
Eventually this seems related to draft-filsfils-spring-segment-routing-ldp-interop

<chair hat off>

2) From a service provider standpoint, i.e. from a functional standpoint, I think what we should try to handle this as nicely as possible. Indeed, having conflicting configurations between 2 nodes may happen, and even the most clever implementation may not avoid such configuration mistake. So in such case, dropping traffic for Mapping Server SID seems a significant network wide disruption.

3) As a contributor to spring, from a protocol standpoint, this looks a priori easier than BGP error handling, so we should probably be able to do at least as good.
It seems easier since:
- the state of the speaker is not relevant as we don't need to send spring traffic or spring signaling to it.
- all receivers receive the same information which is syntaxally correct

So to have a coherent network state, it would seem sufficient to specify a behavior on the receiving side.

4) Going into more details, and referring to St├ęphane analysis:
	 1) I don't really the issue. From a forwarding standpoint, looks like we can simply program multiple SIDs in the FIB.
	 2) Seems more complex to me. A priori we need to define a tie-breaker based on received MS entries. We seem to all agree that local only mapping must be disregarded/less preferred. According to you, this is the easy part, so looks good.
	3) Is already specified in the ISIS document: https://tools.ietf.org/html/draft-ietf-isis-segment-routing-extensions-04#section-2.4.5

So it looks like the remaining work is to define a tie-breaker.

Thanks,
Bruno

> From: Isis-wg [mailto:isis-wg-bounces@ietf.org] On Behalf Of Stefano Previdi (sprevidi)
> 
> <removing the long recipient list and adding isis-wg> Background
> information about this issue is on the original email here below.
> 
> 
> Hi Stephane,
> 
> On Jun 17, 2015, at 10:42 AM, <stephane.litkowski@orange.com> wrote:
> > Hi Stefano,
> >
> > As discussed in Dallas, there may be no ideal option but let's try to find
> one which may try to preserve the transport service. So I'm not in favor of
> option 1.
> > There may be three kinds of overlap :
> > 1) prefix overlap with different indexes, example :
> > 	192.0.2.0/32 range 10 index 100
> > 	192.0.2.5/32 range 1 index 2000
> > 	192.0.2.5/32 range 2 index 3000
> > 	In this case 192.0.2.5/32 is associated with three different indexes
> >
> > 2) index overlap, example :
> > 	192.0.2.0/32 range 10 index 100
> > 	198.15.0.0/32 range 100 index 100
> > 	In this case, different prefixes will use the same index.
> >
> > 3) overlap between binding TLV and prefix entry
> > 	192.0.2.0/32 range 10 index 100 learned from MS
> > 	192.0.2.5/32 index 200 learned from TLV135
> > 	In this case 192.0.2.5/32 is associated with two different indexes
> > learned by two different ways
> 
> 
> very good points above and that illustrate even better the complexity of the
> scheme we would need to define in order to cope with all possible cases.
> 
> 
> > 2) must be solved as we manage conflicting SIDs associated with
> connected prefixes. It's a similar case, two different prefixes having the
> same SID.
> 
> 
> yes, this is the easy one. Again, no real solution here but as you pointed out,
> you may still want to preserve part of the reachability.
> 
> >
> > 1)3) If we think about tiebreaker, I'm not sure that preferring local MS
> entries over remote MS entries is a good idea as it may lead to
> inconsistencies while the target may be to have everyone taking the same
> decision.  In contrary, I would make prefer prefix SID mappings over MS
> entries.
> 
> 
> yes, I agree. The point was what to prefer between a received MS entry vs. a
> locally defined MS entry. But as you aid, if we prefer anything "local" we
> can't rely on network-wide consistent behavior.
> 
> s.
> 
> 
> > To compare MS entries, it may be possible to consider something like
> lowest range and then lowest index for example.
> >
> > -----Original Message-----
> > From: Stefano Previdi (sprevidi) [mailto:sprevidi@cisco.com]
> > Sent: Tuesday, June 16, 2015 19:15
> > To: Clarence Filsfils (cfilsfil); Ahmed Bashandy (bashandy); Hannes
> > Gredler; LITKOWSKI Stephane SCE/IBNF; DECRAENE Bruno IMT/OLN; Jeff
> > Tantsura; Les Ginsberg (ginsberg); Wim (Wim) Henderickx; Steven Luong
> > (sluong)
> > Subject: Conflicting MS entries
> >
> > Co-authors and Contributors,
> >
> > I'd like to close on the conflicting MS entries issues.
> >
> > The issue is related to what a receiver should do when receiving to or
> more conflicting MS mappings.
> >
> > So far we agreed that there's no way we can determine which one is good
> or bad and therefore whatever action we mandate it will create disruption.
> The choices are:
> >
> > 1. ignore all conflicting entries
> > 2. select one of the conflicting entry based on whatever breaking tie algo.
> >
> > Obviously, option1 is the easiest knowing that no other option would allow
> to fix the conflict anyway.
> > Option2 may be appealing but in such case we would have to consider all
> corner cases such as:
> > . what about a conflict between locally configured mapping entry and
> received ones ?
> > . what about conflicting entries received by the same router ? which one to
> select ?
> >
> > I think it is preferable to ignore all conflicting entries and log and error
> message. We discussed about this in Dallas but despite the agreement on
> the absence of a cure we decided, at that time, to let the operators to give
> feedback on the proposal.
> >
> > Is there anything that has changed ?
> >
> > Also, let me know if you'd prefer to bring the issue to the mailing list
> before reaching agreement among co-authors.
> >
> > Thanks.
> > s.
> >
> >
> >
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