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

[Rishabh Parekh <[e-mail]>]

Bruno,
One clarification:

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>"
>
>
We did not want L12 to be mistaken as a SID and hence the "<R-SID2->L12>"
convention (indicating push R-SID2 and send on interface L12) as compared
to
"<N-SID6, R-SID6>" where both elements are SIDs.

-Rishabh

On Wed, Jul 15, 2020 at 2:04 PM Rishabh Parekh <[e-mail]> wrote:

> Bruno,
> Replies inline @ [RP]
>
>
>
>> [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.
>>
>> [RP] We are on the same page. An explicit interface can be identified in
> different ways.
>
>
>> [...]
>>
>> > >
>> > > -------
>> > >
>> > > " 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."
>>
>>
> [RP] I see you point. We shall re-word the text around this area in a
> future WG document (probably in rev 01).
>
>
>> 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>"
>>
>>
> [RP] Thanks for catching these. We will fix them in rev 00 of WG document.
>
>
>> Thanks,
>> Regards,
>> --Bruno
>>
>>
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