Subject: [Lsr] Signaling next hop liveness

From: Robert Raszuk <[email-masked]>
Date: Sat, 20 Nov 2021 20:14:51 +0100

All,

With all the discussions about how to accomplish this I have one
fundamental doubt.

Let's take SR and ISIS or OSPF extensions for it (say RFC8667).

It seems that none of it works network wide if we instead of prefix
reachability start advertising summary routes from ABRs to
other areas/levels. That means no use for this there.

For MPLS LDP FECs already must be /32 so if we want to keep this at the
underlay level we have a way (and actually doing it today with ordered LDP
mode) and signal liveness network wide.

That means that the real practical use case has shrunk.

That said it may be still very useful and can be done with BGP recursion
when you have MPLS-over-UDP for your service tunneling.

Essentially what we are trying to race here with is BGP service routes
withdraws to speed up connectivity restoration time. Shouldn't we really
try to understand the issue (if there is one in the protocol which I
personally doubt) before we start investing essentially by building a
workaround in LSR sandbox ?

The ABR acting as state reflector directly to interested parties or via
their local relays (local area ABRs) sounds like a cool pub-sub option. But
that still only attempts to fix current proposals.

The fact that in my local area there are /32s does not really mean I can
reach those PEs from any remote sites. There can be fabric or LC data plane
bugs on the way, accidental ACLs,  broken SRH processors,
misconfigurations etc .. which can drop (all or worse some) of my packets
to a remote PE. Only inline data plane  telemetry and reaction to it will
allow me to choose a different path to a multihomed service. Again not a
problem for LSR WG I think.

Kind regards,
Robert

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From: Peter Psenak <[email-masked]>
Date: Mon, 22 Nov 2021 10:28:57 +0100

Robert,

On 20/11/2021 20:14, Robert Raszuk wrote:
> underlay level we have a way (and actually doing it today with 
> ordered LDP mode) and signal liveness network wide.
> 

time to look at SRv6, it does net require end-to-end LSP with host 
reachability - e.g. summarization is possible.

thanks,
Peter

> their local relays (local area ABRs) sounds like a cool pub-sub option. 
> But that still only attempts to fix current proposals.
> 
> reach those PEs from any remote sites. There can be fabric or LC data 
> plane bugs on the way, accidental ACLs,  broken SRH processors, 
> misconfigurations etc .. which can drop (all or worse some) of my 
> packets to a remote PE. Only inline data plane  telemetry and reaction 
> to it will allow me to choose a different path to a multihomed service. 
> Again not a problem for LSR WG I think.
> 
> Kind regards,
> Robert
> 
> 
> 
> 
> 
>

---

From: Robert Raszuk <[email-masked]>
Date: Mon, 22 Nov 2021 14:47:14 +0100

> reachability - e.g. summarization is possible.

And how do I know if remote PE is even SRv6 capable and for what behaviours
? How do I know its functions ? How about using EPE with adj SID on it ?
How will the ingress know all this ?

Via controller ? If so it can also know that the PE went down via
controller and we are done :).

Thx,
R

---

From: Peter Psenak <[email-masked]>
Date: Mon, 22 Nov 2021 14:52:24 +0100

On 22/11/2021 14:47, Robert Raszuk wrote:
> And how do I know if remote PE is even SRv6 capable and for what 
> behaviours ? How do I know its functions ? How about using EPE with adj 
> SID on it ? How will the ingress know all this ?

via BGP.

Peter