Re: [spring] Understanding the replication draft

Rishabh Parekh <rishabhp@gmail.com> Fri, 10 July 2020 20:45 UTC

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From: Rishabh Parekh <rishabhp@gmail.com>
Date: Fri, 10 Jul 2020 13:45:41 -0700
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To: Jeff Tantsura <jefftant.ietf@gmail.com>
Cc: Dhruv Dhody <dhruv.ietf@gmail.com>, "bruno.decraene@orange.com" <bruno.decraene@orange.com>, Alexander Vainshtein <Alexander.Vainshtein@rbbn.com>, "spring@ietf.org" <spring@ietf.org>
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Subject: Re: [spring] Understanding the replication draft
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All,
We have posted revision 04
<https://tools.ietf.org/html/draft-voyer-spring-sr-replication-segment-04>
of the draft. This addresses comments on this thread and adds an example
illustrating replication segment. Diff from rev 03:
https://tinyurl.com/y96ueb9h

Some WG  members also wanted to understand how Replication segments are
stitched together to form a P2MP tree based on SR P2MP policy in the PIM
draft. We have posted revision 02
<https://tools.ietf.org/html/draft-voyer-pim-sr-p2mp-policy-02> of PIM P2MP
policy draft that adds examples of how these things tie together. I hope
that helps clarify the relationship. Diff from rev 01:
https://tinyurl.com/yban2jp6

Please review,
-Rishabh


On Tue, Jul 7, 2020 at 2:16 PM Jeff Tantsura <jefftant.ietf@gmail.com>
wrote:

> Thanks Rishabh!
> +1 to the above comments and looking forward to the updated draft.
>
> Cheers,
> Jeff
> On Jul 7, 2020, 7:35 AM -0700, Alexander Vainshtein <
> Alexander.Vainshtein@rbbn.com>gt;, wrote:
>
> Dhruv, Bruno and all,
> Regarding the statement " 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" - I cannot agree more.
>
> My 2c,
> Sasha
>
> Office: +972-39266302
> Cell: +972-549266302
> Email: Alexander.Vainshtein@ecitele.com
>
>
> -----Original Message-----
> From: spring <spring-bounces@ietf.org> On Behalf Of Dhruv Dhody
> Sent: Tuesday, July 7, 2020 1:02 PM
> To: bruno.decraene@orange.com
> Cc: spring@ietf.org
> Subject: Re: [spring] Understanding the replication draft
>
> 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://clicktime.symantec.com/36sxrNPFssiaB8jE9mzzZDd6H2?u=https%3A
> > > %2F%2Fdatatracker.ietf.org%2Fdoc%2Fdraft-voyer-pim-sr-p2mp-policy%2F
> > > [2]
> > > https://clicktime.symantec.com/3LtBxbxRK3NkuKYc3FC7rr46H2?u=https%3A
> > > %2F%2Fmailarchive.ietf.org%2Farch%2Fmsg%2Fpim%2FYyF7I02aaRtpZngf7uTn
> > > o69IB90%2F
> > >
> > >
> > >
> > > 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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