Re: [spring] Understanding the replication draft

Rishabh Parekh <rishabhp@gmail.com> Wed, 01 July 2020 16:07 UTC

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From: Rishabh Parekh <rishabhp@gmail.com>
Date: Wed, 1 Jul 2020 09:06:46 -0700
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To: "Joel M. Halpern" <jmh@joelhalpern.com>
Cc: "spring@ietf.org" <spring@ietf.org>
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Subject: Re: [spring] Understanding the replication draft
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Joel,
Your request was not "lost", but it fell between the cracks :)

Anyway, responses inline.

On Mon, Jun 29, 2020 at 3:17 PM Joel M. Halpern <jmh@joelhalpern.com> wrote:
>
> I asked the authors a version of this question, but apparently my
> request got lost.
>
> For now, this is speaking as an individual.  And I sincerely hope that I
> am merely missing something obvious.
>
> I can not figure out from the current draft how the replication segment
> works in a SID (or label) stack.
> Is there an unstated requirement that the segment must be the last one
> in the stack?
> If not, how is a global SID after teh replication SID understood?

[RP] Replication SID does not need to be the last segment in the
stack. Although Section 2 of draft does not state this explicitly, If
there are other non-replication SIDs following the Replication SID,
the NEXT operation at a downstream node of the segment should process
those SIDs as normal.

> Or is a replication SID implicitly also a binding SID, replacing the
> rest of the stack no matter where it is in the stack?
>     In which case it is implicitly effectively last?

[RP] At a root or a Replication SID, when the active segment is a
Replication SID, it does act like a Binding SID in that it steers the
packet into the Replication segment towards downstream nodes. Note
that additional SIDs might be added on top of the Replication SID to
steer the packet from Root to a given downstream node. The Replication
SID will be at bottom of any such SIDs added to steer the packet, but
again it does not have to be the bottom most SID in the stack.

> Given taht a replication segment is qualified to a node, what happens if
> there is more than one in a stack?  Is it ignored when it hits a node it
> does not apply to?

[RP] On a given node, if an active SID in the stack is a Replication
SID that the node does not understand, it cannot process the packet.
This would be similar to any other kind of SID for which a node does
not have any state.
>
> Do I believe this can be made to work?  Yes.
> But I can not understand how the WG could adopt the work with its
> current lack of clarity.
> And this appears to me to be fundamental enough stuff that it can't be
> left to documents in other WGs.  It seems central to the definition and
> processing of replication SIDs.
>

[RP] Section 2 does specify behavior associated with Replication SID
at different nodes in terms of PUSH, CONTINUE or NEXT operations. If
it is not clear, we can enhance the text.
>
> Yours,
> Joel - speaking as a participant
>
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