Re: [spring] Understanding the replication draft

Dhruv Dhody <dhruv.ietf@gmail.com> Tue, 07 July 2020 10:02 UTC

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From: Dhruv Dhody <dhruv.ietf@gmail.com>
Date: Tue, 7 Jul 2020 15:31:38 +0530
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To: bruno.decraene@orange.com
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Subject: Re: [spring] Understanding the replication draft
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Hi Bruno,

Yes, thanks! I was just making sure that we have an agreement that PIM
is the right place to define SR P2MP Policy.

What is missing in the spring I-D is some very high level discussion
in terms of architecture on how replication segment and SR P2MP policy
come together. The current I-D tries to define a replication segment
as an independent entity that could be used on its own but it makes it
difficult to visualize without some high level text or an example.

Thanks!
Dhruv

On Tue, Jul 7, 2020 at 3:12 PM <bruno.decraene@orange.com> wrote:
>
> Hi Dhruv,
>
> > -----Original Message-----
> > From: spring [mailto:spring-bounces@ietf.org] On Behalf Of Dhruv Dhody
> > Subject: Re: [spring] Understanding the replication draft
> >
> > Hi WG,
> >
> > Reading the I-D and based on the discussion on this thread I believe
> > more description is required. As Joel pointed out, clarity on what is
> > legal/illegal (or out of scope for now) is needed.
>
> I leave this for the authors to reply.
>
> > I have no strong opinion if that needs to be done before or after adoption!
> >
> > I do not follow PIM closely and just realized that the SR P2MP Policy
> > in the PIM I-D [1] is adopted by the PIM WG, but not yet posted [2].
> > I wanted to make sure that it has been decided that PIM is the right
> > place to define SR P2MP policy (discussed either in the WG and/or
> > between chairs/AD)?
>
> draft-voyer-pim-sr-p2mp-policy-00 is to be worked in the PIM WG.
> The PIM WG is waiting for the SPRING WG to adopt draft-voyer-spring-sr-replication-segment, since the -pim- document has a dependency on the -spring- document.
>
> Quoting the PIM chairs: "We have solid support to adopt this draft. [...] We are waiting to hear back from the spring chairs as to the wg status of the local replication draft of which your draft is dependent. So please hold off on submitting the new draft-ietf-pim-sr-p2mp-policy until they give us the green light."
>
> Does this answer your question?
>
> --Bruno
>
> > Thanks!
> > Dhruv
> >
> > [1] https://datatracker.ietf.org/doc/draft-voyer-pim-sr-p2mp-policy/
> > [2]
> > https://mailarchive.ietf.org/arch/msg/pim/YyF7I02aaRtpZngf7uTno69IB90/
> >
> >
> >
> > On Thu, Jul 2, 2020 at 12:05 PM Rishabh Parekh <rishabhp@gmail.com>
> > wrote:
> > >
> > > Jeff,
> > > Your explanation of distinct Transport SIDs and service labels (which
> > > appear at BoS) applies to Point-to-Point services. Same model can be
> > > applied when a Point-to-Multipoint service is realized by one
> > > end-to-end replication segment; "Downstream Replication SID" as
> > > specified in draft is effectively the service label at a specific
> > > downstream node of a Replication segment with Transport SIDs imposed
> > > on top to take replicated packet to that node.
> > >
> > > However, when a Point-to-Multipoint service is over replication
> > > segments stitched together to form a P2MP tree, as described in PIM WG
> > > draft, this model no longer holds since all service egress nodes would
> > > have to agree on one common service label. So  P2MP services
> > > implicitly map the P2MP transport label (Replication SID at BoS in
> > > this case) to the P2MP service. Of course, this implies one-to-one
> > > association between a P2MP service and a P2MP transport. There are
> > > techniques to share one P2MP transport across different P2MP services
> > > using either upstream assigned label or a global context from
> > > "Domain-wide Common Block"
> > > (draft-ietf-bess-mvpn-evpn-aggregation-label). These and other gory
> > > details are described in Section 2 of PIM WG draft and to some extent
> > > in BESS MVPN-EVPN draft.
> > >
> > > -Rishabh
> > >
> > > On Wed, Jul 1, 2020 at 5:50 PM Jeff Tantsura <jefftant.ietf@gmail.com>
> > wrote:
> > > >
> > > > Rishabh,
> > > >
> > > > Transport SID with a service on top can’t be a BoS label, there’s s service
> > label below, since a service is associated with a particular node, there would
> > be at least a N-SID associated with the service node.
> > > > It seems like B-SID behavior is the correct one, when R-SID is looked up
> > and popped, it would yield: replication  (as programmed by a controller, since
> > there’s no state) + new label stack associated with the new, post
> > replication/branching path that is imposed on top of existing label stack, so
> > service label ( + optionally more transport labels) are preserved.
> > > >
> > > > Cheers,
> > > > Jeff
> > > > On Jul 1, 2020, 12:40 PM -0700, Rishabh Parekh <rishabhp@gmail.com>om>,
> > wrote:
> > > >
> > > > Robert,
> > > >
> > > > A) Firmly state that replication SID MUST be the last one on the stack
> > > > B) Instead of real SID after the replication SID provide a binding SID which
> > locally will be mapped to a different SID list imposed to each replicated flow.
> > > >
> > > >
> > > > We would be fine with A), but we don't want to exclude possibility of
> > > > something like what you describe in B.
> > > >
> > > > -Rishabh
> > > >
> > > > On Wed, Jul 1, 2020 at 12:27 PM Robert Raszuk <robert@raszuk.net>
> > wrote:
> > > >
> > > >
> > > > Hi Rishabh,
> > > >
> > > > Of course, care must be
> > > > taken to avoid the "explosion" as you describe it. G-SID-2 has to map
> > > > to a unique node; for example, it may be an Anycast-SID that takes
> > > > packet to distinct nodes from each of the downstream node, or the
> > > > downstream nodes can be border nodes connecting to other segment
> > > > routing domains where G-SID-2 resolves to distinct nodes in each
> > > > domain.
> > > >
> > > >
> > > > I think you are stretching it too thin.
> > > >
> > > > See even if G-SID-2 is anycast SID you have zero assurance that physical
> > nodes packets will land on would be at all diverse.
> > > >
> > > > Likewise crossing domains yet providing identical global SID now to be a
> > different node in each such domain to me is not a realistic example.
> > > >
> > > > I think we have two options:
> > > >
> > > > A) Firmly state that replication SID MUST be the last one on the stack
> > > >
> > > > B) Instead of real SID after the replication SID provide a binding SID which
> > locally will be mapped to a different SID list imposed to each replicated flow.
> > > >
> > > > What is currently in the draft seems to be very counterintuitive and IMHO
> > will result in operational difficulties.
> > > >
> > > > Thx a lot,
> > > > R.
> > > >
> > > >
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