A case with no SRH is clearly not a case for PSP.
With regard to the case for PSP, as I said in my note I am concerned
that if we want to support this specific case, then there are
restrictions on deployment and operation that need to be called out.
For example, a path compute engine for example needs to know that it not
only can not use such nodes as .END without prior PSP, but that it can
not include them in transit paths.
Yours,
Joel
On 3/4/2020 5:01 PM, Darren Dukes (ddukes) wrote:
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 <j...@joelhalpern.com
<mailto:j...@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.
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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