Re: [spring] Understanding the replication draft

Rishabh Parekh <rishabhp@gmail.com> Tue, 14 July 2020 20:25 UTC

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From: Rishabh Parekh <rishabhp@gmail.com>
Date: Tue, 14 Jul 2020 13:25:01 -0700
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To: "Xiejingrong (Jingrong)" <xiejingrong@huawei.com>
Cc: Jeff Tantsura <jefftant.ietf@gmail.com>, Dhruv Dhody <dhruv.ietf@gmail.com>, "bruno.decraene@orange.com" <bruno.decraene@orange.com>, "spring@ietf.org" <spring@ietf.org>, Alexander Vainshtein <Alexander.Vainshtein@rbbn.com>
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Subject: Re: [spring] Understanding the replication draft
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Jingrong,
As I had explained to Jeff Tantsura earlier in the thread, with P2MP
services realized by P2MP trees, VPN label is NOT part of the label stack
since egress (leaf nodes of P2MP trees) of the P2MP service MAY have
different VPN labels for same context. For P2MP services, it is normal to
use the top transport label (Replication SID in this case) as implicit
context for the service. Of course, this implies one-to-one association
between P2MP transport and service.

There are techniques, using upstream label allocation or DCB (domain wide
context blocks), where the same VPN label can be assigned to the service on
all the egress nodes. In this case, the P2MP transports can be shared
across P2MP services (at a set of egress nodes) and the VPN label can
appear after the Replication SID as we have indicated in the PIM WG draft.
But, the VPN label is not a SID and, IMO, the statement "Replication SID
must be last SID...." still holds but it is not at the bottom of the stack.
We can clarify this when we publish the WG document.

-Rishabh

On Tue, Jul 14, 2020 at 4:55 AM Xiejingrong (Jingrong) <
xiejingrong@huawei.com> wrote:

> Hi
>
>
>
> The rev-04 says “The Replication SID MUST be the last SID (at the bottom
> of stack for SR-MPLS) in a packet that is steered out from a Replication
> node of a Replication Segment.”.
>
>
>
> I feel a little hard to understand ……
>
>
>
> My question is: Can an “MPLS packet” be carried over the P2MP policy ? say
> a packet like this: { Replication SID, (VPN Label, IPv4/IPv6 C-multicast
> packet) } ?
>
>
>
> In my understanding, with an imagined topology comprising of Replication
> Node R1/R2/R3/R4, the picture is like this:
>
>
>
> The Replication Node R2 receives such a packet (from R1), sees the
> Replication SID<R2> on the “Top” of the label stack,  performs the
> corresponding action of “replication”,  replicates the packet to downstream
> Replication Nodes R3 and R4, SWAPs the Replication SID<R2> to Replication
> SID<R3> and Replication SID<R4> respectively.
>
>
>
> The Replication Node R3 receives the packet { Replication SID<R3>, (VPN
> Label, IPv4/IPv6 C-multicast packet) }.
>
>
>
> The Replication Node R4 receives the packet { Replication SID<R4>, (VPN
> Label, IPv4/IPv6 C-multicast packet) }.
>
>
>
> Thanks
>
> Jingrong
>
>
>
> *From:* spring [mailto:spring-bounces@ietf.org] *On Behalf Of *Rishabh
> Parekh
> *Sent:* Saturday, July 11, 2020 4:46 AM
> *To:* Jeff Tantsura <jefftant.ietf@gmail.com>
> *Cc:* Dhruv Dhody <dhruv.ietf@gmail.com>om>; bruno.decraene@orange.com;
> spring@ietf.org; Alexander Vainshtein <Alexander.Vainshtein@rbbn.com>
> *Subject:* Re: [spring] Understanding the replication draft
>
>
>
> 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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