Re: [spring] SRv6 PSP use case

Hi Joel,

On Thu, 5 Mar 2020, 07:42 Joel M. Halpern, <jmh@joelhalpern.com> wrote:

> I think I have now inferred what the intended use case is for PSP.  I
> really wish folks had stated it in full and explicitly, rather than
> implicitly a piece at a time, on the list.
>
> As noted below after the explanation, I think that supporting this use
> case does require some explanations somewhere.  And given that the
> support is in terms of PSP, I guess the NP draft is the place to put the
> caveats.
>
> As far as I can tell, the use case is as follows.
> The operator has devices, that they reasonably wish to continue to use.
>

My understanding is that the operator wants to extend their SRv6 control
plane domain onto PEs, past the edge of their current SRv6 forwarding plane
domain, because they don't currently have budget available to upgrade the
forwarding plane in those PEs.

This situation would be temporary until there is budget to upgrade the PEs'
forwarding planes. I'd think this situation is likely to last no more than
2 to 3 years in a big operator. The more successful SR is, the quicker the
upgrades are likely to be.

So this is effectively a permanent IETF technical workaround for a
temporary financial problem.

In 5 to 10 years time it is unlikely anybody would have a need or want PSP
in this scenario, because all SRv6 control and SRv6 forwarding planes in
all SR enabled networks would be congruent.

Regards,
Mark.

These devices can support encapsulation and decapsulation with
> sufficiently arbitrary content.
> These devices comply with the RFC 8200 requirement for ignoring routing
> headers by punting those to the slow path.  With significant performance
> penalty.
>    --  Presumably, these devices have some form of protection to prevent
> this slow-pathing from becoming a DoS on the other necessary control
> functions.  I don't think that protection is an SRv6 or NP problem.  But
> it is necessary.
>
> Thus, the SRv6 designers want to be able to use these devices as part of
> the SRv6 domain, strictly at entry and exit.  They use PSP as a way to
> avoid hitting the slow path on decapsulate.  (Presumably because the
> check that punts the packet to the slow path is before the check that
> says "decapsulate".  And it probably should be in that order.)
>
> In order to support this, the authors have also pretended that maximum
> SID depth is meaningful for a thing that is not a stack, and that 0
> means "no SRH permitted".  While an interesting stretch on the routing
> protocol semantics, it is not SPRING's problem.
>
> The fact that these nodes can not be SRv6 end nodes other than as
> terminal nodes with a prior node that advertised PSP SID(s) and where
> those PSP SIDs are used on any path that terminates at these end nodes
> is important.  It probably should be called out.  It would have helped a
> number of the examples that were discussed on the list.
>
> There is another implication that needs to be stated explicitly.  And I
> do not know how the necessary property can be indicated.  These nodes
> MUST NOT be transit nodes in an SRv6 path.
>
> Having parsed the use case, I would note that the topological
> constraints are pretty severe.  the operator must ensure that there are
> PSP processing nodes sufficiently close to these edge nodes that they do
> not destroy the traffic engineering properties in order to achieve the
> ingress / egress utilization.
>
> If all of this had been stated explicitly, I think we could have had a
> clear discussion of teh costs and benefits.
>
> Yours,
> Joel
>
> _______________________________________________
> spring mailing list
> spring@ietf.org
> https://www.ietf.org/mailman/listinfo/spring
>