Re: [spring] SRv6 PSP use case

["Darren Dukes (ddukes)" <[e-mail]>]

Hi Joel, what you describe was also described by Dan Voyer and Jingrong previously.  You’ve added some signalling color but otherwise the same.
If you’ve read https://tools.ietf.org/html/draft-filsfils-spring-srv6-net-pgm-illustration-00#section-2.4
you can see how SRv6 is used for an L3VPN without an SRH present.

Combine that with the TE description with PSP here https://tools.ietf.org/html/draft-filsfils-spring-srv6-net-pgm-illustration-00#section-2.8.1

Now you should be able to see how to put this together.

The WG decided illustrations such as this belong in the illustration draft.
I believe the WG requested that be split from draft-filsfils-spring-srv6-network-programming draft before adoption.
That was over a year ago.

Darren


On Mar 4, 2020, at 3:41 PM, Joel M. Halpern <[e-mail]<mailto:[e-mail]>> 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.
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

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