Re: [Isis-wg] [spring] [OSPF] OSPFv2 Segment Routing Extensions ERO Extensions (would also effect OSPFv3 and IS-IS) - REPLY TO THIS ONE

Rob Shakir <rjs@rob.sh> Mon, 12 June 2017 13:08 UTC

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From: Rob Shakir <rjs@rob.sh>
Date: Mon, 12 Jun 2017 13:08:14 +0000
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To: bruno.decraene@orange.com, "spring@ietf.org" <spring@ietf.org>
Cc: "draft-ietf-ospf-segment-routing-extensions@ietf.org" <draft-ietf-ospf-segment-routing-extensions@ietf.org>, OSPF WG List <ospf@ietf.org>, "isis-wg@ietf.org" <isis-wg@ietf.org>, "idr-chairs@ietf.org" <idr-chairs@ietf.org>, Peter Psenak <ppsenak@cisco.com>
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Subject: Re: [Isis-wg] [spring] [OSPF] OSPFv2 Segment Routing Extensions ERO Extensions (would also effect OSPFv3 and IS-IS) - REPLY TO THIS ONE
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Bruno, SPRING,

I am aware of at least one implementation that makes heavy use of Binding
SIDs, so I do not think that this is something that we can remove from the
protocol specification. It seems to me that we have a number of cases that
continue to exist that make it useful to have them specified, particularly:

   - Binding of a SID to a deeper label stack to prevent there being large
   label stacks required on ingress. This is required due to limited push
   depth, and limited readable label depths for hashing.
   - Re-use of some other protocol's or network's forwarding path by a
   device that is imposing an SR label stack.

There is not an alternative construct that can be used for this purpose, so
we should not remove it.

In both of these cases there seems, to me, to be a use case for having the
information in the IGP in the case that an implementation computes TE paths
using cSPF, having binding SID information available to it (along with the
ERO) enables it to reduce the label stack depth by finding binding SIDs
that follow the same path as the computed ERO. Without the ERO (which might
not be an RSVP-TE ERO, but I believe that it how it was first envisaged)
how can the head-end of an TE path know what path the advertised Binding
SID takes? It's fine to punt this and say "the PCE in the sky will know" -
however, I believe SPRING's charter doesn't limit the technology to only
centralised computation of paths.

I don't believe current demand for this is a good reason to remove it from
the protocol specification - it is still somewhat early days for folks
deploying TE based on SR - where I think the Binding SID concept is most
important.

r.

On Mon, 12 Jun 2017 at 05:50 <bruno.decraene@orange.com> wrote:

> Hello SPRING WG,
>
> I'd like to encourage discussion on this thread.
>
> The related questions seem to be:
> - Binding SIDs:
>         -  Is there any implementation?
>         - Is it useful?
>         - Does it need to be specified?
>
> - Binding SIDs advertised in IGP:
>         -  Is there any implementation?
>          - Is it useful?
>         - Does it need to be specified?
>
> As of today, there seem to be multiple SPRING (related) document that make
> reference (define/use) to the Binding SIDs. e.g.
> - architecture
> https://tools.ietf.org/html/draft-ietf-spring-segment-routing-11#section-3.5.2
> - MPLS instantiation
> https://tools.ietf.org/html/draft-ietf-spring-segment-routing-mpls-08#section-2
> - non-protected paths
> https://tools.ietf.org/html/draft-litkowski-spring-non-protected-paths-01#section-3.3
> - SR policies:
> https://tools.ietf.org/html/draft-filsfils-spring-segment-routing-policy-00#section-7
>
>
> However, it also seems a priori possible to define Binding SIDs and not
> advertised them in the IGP. (e.g. by keeping them local to the PCE)
>
> On a side note, if the Binding SIDs are removed from the IGP, do they need
> to be removed from the BGP-LS extensions? [+IDR chairs]
>
> Thanks,
> Regards,
> --Bruno
>
> > From: OSPF [mailto:ospf-bounces@ietf.org] On Behalf Of Peter Psenak
> > Sent: Monday, June 12, 2017 10:18 AM
>  > To: OSPF WG List; spring@ietf.org; isis-wg@ietf.org
>  > Cc: draft-ietf-ospf-segment-routing-extensions@ietf.org
>  > Subject: Re: [OSPF] OSPFv2 Segment Routing Extensions ERO Extensions
> (would also effect
>  > OSPFv3 and IS-IS) - REPLY TO THIS ONE
>  >
>  > Hi,
>  >
>  > I would like to get some feedback on the usage of the SID/Label Binding
> TLV.
>  >
>  > Is there any implementation that uses SID/Label Binding TLV for
>  > advertising the SID/Label binding to a FEC as specified in section 6 of
>  > the draft-ietf-ospf-segment-routing-extensions-16 or section 2.4 of
>  > draft-ietf-isis-segment-routing-extensions-12?
>  >
>  > If not, do we see this as something we want to preserve in the IGP SR
>  > drafts?
>  >
>  > ISIS uses The SID/Label Binding TLV to advertise
>  > prefixes to SID/Label mappings, which is known to be supported by
>  > several implementations and that piece needs to be preserved.
>  >
>  > thanks,
>  > Peter
>  >
>  > On 09/06/17 19:04 , Peter Psenak wrote:
>  > > Acee,
>  > >
>  > > my question is whether we need the whole section 6 and the SID/Label
>  > > Binding Sub-TLV that it specifies. In OSPF Binding SID is not used for
>  > > SRMS advertisement like in ISIS.
>  > >
>  > > thanks,
>  > > Peter
>  > >
>  > >
>  > >
>  > > On 09/06/17 16:45 , Acee Lindem (acee) wrote:
>  > >> Corrected IS-IS WG alias – Please reply to this one.
>  > >> Thanks,
>  > >> Acee
>  > >>
>  > >> From: Acee Lindem <acee@cisco.com <mailto:acee@cisco.com>>
>  > >> Date: Friday, June 9, 2017 at 10:42 AM
>  > >> To: OSPF WG List <ospf@ietf.org <mailto:ospf@ietf.org>>,
>  > >> "spring@ietf.org <mailto:spring@ietf.org>" <spring@ietf.org
>  > >> <mailto:spring@ietf.org>>, "isis@ietf.org <mailto:isis@ietf.org>"
>  > >> <isis@ietf.org <mailto:isis@ietf.org>>
>  > >> Cc: "draft-ietf-ospf-segment-routing-extensions@ietf.org
>  > >> <mailto:draft-ietf-ospf-segment-routing-extensions@ietf.org>"
>  > >> <draft-ietf-ospf-segment-routing-extensions@ietf.org
>  > >> <mailto:draft-ietf-ospf-segment-routing-extensions@ietf.org>>
>  > >> Subject: OSPFv2 Segment Routing Extensions ERO Extensions (would also
>  > >> effect OSPFv3 and IS-IS)
>  > >>
>  > >>     Hi OSPF, ISIS, and SPRING WGs,
>  > >>
>  > >>     As part of the Alia’s AD review, she uncovered the fact that the
> ERO
>  > >>     extensions in 6.1 and 6.2 are specified as far as encoding but
> are
>  > >>     not specified as far as usage in any IGP or SPRING document. As
>  > >>     document shepherd,  my proposal is that they simply be removed
> since
>  > >>     they were incorporated as part of a draft merge and it appears
> that
>  > >>     no one has implemented them (other than parsing). We could also
>  > >>     deprecate types (4-8) in the OSPFv2 Extended Prefix LSA Sub-TLV
>  > >>     registry to delay usage of these code points for some time (or
>  > >>     indefinitely ;^).
>  > >>
>  > >>     Thanks,
>  > >>     Acee
>  > >>
>  > >
>  > > .
>  > >
>  >
>  > _______________________________________________
>  > OSPF mailing list
>  > OSPF@ietf.org
>  > https://www.ietf.org/mailman/listinfo/ospf
>
>
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