Re: [spring] Understanding the replication draft

Rishabh Parekh <> Wed, 01 July 2020 22:55 UTC

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From: Rishabh Parekh <>
Date: Wed, 1 Jul 2020 15:55:29 -0700
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To: Robert Raszuk <>
Cc: Alexander Vainshtein <>, Joel Halpern Direct <>, "" <>
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Subject: Re: [spring] Understanding the replication draft
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I agree with Robert. Building MDTs for services is covered in a
separate draft in BESS:


On Wed, Jul 1, 2020 at 2:56 PM Robert Raszuk <> wrote:
> Hi Alexander,
> Do we really know how to build MDTs as source routed chains ?
> If that is the goal here IMHO it deserves a whole separate document ....
> In general when we replicate we take a packet and copy it to interfaces A, B ... Z. That's a local rule I presume executed for flows containing R-SID-n. But we just can't send the packet with identical list of SIDs from that point on any outgoing interface as results will be rather poor.
> We need custom list for each outbound interface and that is something I am not seeing in the draft. Till this is explained I would keep thinking that building MDTs should be more of as control plane task.
> Thx.
> R.
> On Wed, Jul 1, 2020 at 9:42 PM Alexander Vainshtein <> wrote:
>> Robert, Rishabh and all,
>> I concur with Robert but would like to add that Option A woild effectively eliminate the possibility of building MDTs with more than one level using Replication Segments.
>> Which is probably not the intent of the draft.
>> My 2c.
>> Get Outlook for Android
>> ________________________________
>> From: spring <> on behalf of Robert Raszuk <>
>> Sent: Wednesday, July 1, 2020, 22:27
>> To: Rishabh Parekh
>> Cc: Joel Halpern Direct;
>> Subject: Re: [spring] Understanding the replication draft
>> 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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