Re: [spring] draft-voyer-spring-sr-replication-segment-03

bruno.decraene@orange.com Wed, 15 July 2020 18:02 UTC

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From: <bruno.decraene@orange.com>
To: Rishabh Parekh <rishabhp@gmail.com>
CC: "draft-voyer-spring-sr-replication-segment@ietf.org" <draft-voyer-spring-sr-replication-segment@ietf.org>, "spring@ietf.org" <spring@ietf.org>
Thread-Topic: [spring] draft-voyer-spring-sr-replication-segment-03
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Date: Wed, 15 Jul 2020 18:02:20 +0000
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Subject: Re: [spring] draft-voyer-spring-sr-replication-segment-03
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Rishabh,

Thanks for answers and new -04
Please see inline [Bruno]

Still speaking as an individual WG member.


> -----Original Message-----
> From: spring [mailto:spring-bounces@ietf.org] On Behalf Of Rishabh Parekh
> Sent: Thursday, July 9, 2020 1:28 AM
> To: DECRAENE Bruno TGI/OLN <bruno.decraene@orange.com>
> Cc: draft-voyer-spring-sr-replication-segment@ietf.org; spring@ietf.org
> Subject: Re: [spring] draft-voyer-spring-sr-replication-segment-03
> 
> Bruno,
> Replies inline
> 
> > ---
> >
> > "A SR Replication segment allows a packet to be replicated from a
> replication node to downstream nodes."
> >
> > May be adding "Each downstream node is reached by using a unicast
> segment or SR policy."
> >
> > (otherwise, the building of a multicast tree seem possibly within the scope)
> >
> >
> If the downstream node is directly connected, we don't need a unicast
> segment or SR policy to reach that node, just an interface to that
> node is sufficient.

[Bruno] Agreed, in this specific case, you don't need the SID on the _packet_. 
- Still the PCE/configuration CLI/YANG need a way to identify this interface and it could be via a SID (e.g. both local adjacency SID and global nodes SID (assuming PHP) seems to work, depending on needs)
- It seems to me that SR Policy draft is in the same situation, and they still call this an SR Policy.
But that's a detail.
 
[...]
 
> >
> > -------
> >
> > " o  When the Active Segment [RFC8402] is the Replication SID.  In this
> >
> >       case, the operation for a replicated copy is CONTINUE."
> >
> >
> >
> > "CONTINUE" would mean that the segment is not a local segment.
> >
> > So the document really needs to clarify whether the replication
> SID/segment is a local segment, or a global segment, or something new to be
> defined..
> >
> >
> The CONTINUE operation just captures the label swap for each
> replication, with just the Downstream Replication SID in the simplest
> case.

"CONTINUE: the active segment is not completed; hence, it remains
   active.  In SR-MPLS, the CONTINUE operation is implemented as a SWAP
   of the top label [RFC3031].  In SRv6, this is the plain IPv6
   forwarding action of a regular IPv6 packet according to its
   destination address."
https://tools.ietf.org/html/rfc8402#section-2

1) SR terminology
Given that I think that we agreed that the replication segment is a local segment (i.e. local to one node), I don't see how it could continue once used. As we used it on the single node which understand it, it needs to be terminated (NEXT).

2) Data plane
2.a) SR-MPLS
I don't see a label swap. I see a pop of the BSID/replication SID and a push of the list of SID in the SR-policy. As described in  https://tools.ietf.org/html/draft-ietf-spring-segment-routing-policy-08#section-8.3
"Let us assume that headend H has a valid SR Policy P of Segment-List
   <S1, S2, S3> and BSID B.

   When H receives a packet K with label stack <B, L2, L3>, H pops B and
   pushes <S1, S2, S3> and forwards the resulting packet according to
   SID S1."

2.b) SRv6
Quoting RFC 8402 (above): "In SRv6, this is the plain IPv6
   forwarding action of a regular IPv6 packet according to its
   destination address"
I don't see how this can translate in a specific SR action (here "replication" on the replication node. ) But since there is no SRv6 specific text in the draft, it's hard to guess.


Note that my understanding seems to match your new text/illustration in appendix: there is no CONTINUE operation on a replication SID. Only NEXT operation:
" R2, as Leaf, performs NEXT operation, pops R-SID2 label and delivers the payload."
"R6, as Leaf, performs NEXT operation, pops R-SID1 label and delivers the payload." 
"R7, as Leaf, performs NEXT operation, pops R-SID7 label and delivers the payload."

BTW, there may be two typos in the draft:
"R6, as Leaf, performs NEXT operation, pops R-SID1 label and delivers the payload."
:s/R-SID1/R-SID6

OLD:    Replication State:
                R2: <R-SID1->L12>"

NEW:    Replication State:
                R2: <R-SID2, L12>"
 
Thanks,
Regards,
--Bruno

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