Re: [Pce] Benjamin Kaduk's Discuss on draft-ietf-pce-stateful-hpce-13: (with DISCUSS and COMMENT)

[Dhruv Dhody <[e-mail]>]

Hi Ben,

Snipping to the final discuss that is still open -


> > > ----------------------------------------------------------------------
> > > DISCUSS:
> > > ----------------------------------------------------------------------
> > >
> > > I think this should be pretty easy to resolve, though I'm not sure what
> > > the right way to do so it.
> > >
> > > Section 3 says:
> > >
> > >    [I-D.ietf-pce-hierarchy-extensions] defines the H-PCE Capability TLV
> > >    that is used in the Open message to advertise the H-PCE capability.
> > >    [RFC8231] defines the Stateful PCE Capability TLV used in the Open
> > >    message to indicate stateful support.  The presence of both TLVs in
> > >    an Open message indicates the support for stateful H-PCE operations
> > >    as described in this document.
> > >
> > > There is no normative reference relationship (in either direction)
> > > between draft-ietf-pce-hierarchy-extension and this document; I think
> > > that the use of the capability TLV to imply both sets of functionality
> > > implies some sort of normative relationship; we wouldn't want version
> > > skew between documents to induce breaking changes.  In particular, an
> > > implementation that already supports RFC 8231 and is implementing the
> > > hierarchy extensions would need to know to look at this document *and
> > > implement it*, or would unknowingly be noncompliant with this document
> > > and fail to interoperate with a peer that is compliant with this
> > > document.
> > >
> >
> > How about we add normative text for this -
> >
> >    [I-D.ietf-pce-hierarchy-extensions] defines the H-PCE Capability TLV
> >    that is used in the Open message to advertise the H-PCE capability.
> >    [RFC8231] defines the Stateful PCE Capability TLV used in the Open
> >    message to indicate stateful support. To indicates the support for
> >    stateful H-PCE operations described in this document, a PCEP speaker
> >    MUST include both TLVs in an Open message. It is RECOMMENDED that any
> >    implementation that supports stateful operations [RFC8231] and H-PCE
> >    [I-D.ietf-pce-hierarchy-extensions] would also implements the
> >    stateful H-PCE operations as described in this document.
> >
> > This would be true in most deployments/implementations of C-PCE and
> > P-PCE that are also stateful!
>
> This does remove the problematic normative requirement on implementations
> of other documents, but I'm not sure if it does what's needed for the
> interactions across documents.  Specifically, what will happen if two peers
> both support/advertise stateful PCE and H-PCE but only one implements
> stateful HPCE? Will there be a clean error handling at runtime and
> degredation to one or the other, or will there be messy errors?  If the
> latter, then I don't think we can just have a RECOMMENDED relationship.
>

The assumption was that any implementation that claims to support
stateful and H-PCE on a particular session would also support Stateful
H-PCE and this document just describes the interaction between these
two features as an informational document.

But, lets take a case where PCC and P-PCE support stateful H-PCE but
the C-PCE does not. PCC would send stateful message to C-PCE and C-PCE
would not further propagate them.

I further did a mental exercise for PCC -> C-PCE -> P-PCE and assumed
all support stateful and H-PCE extension but what happens when any
PCEP speaker does not support stateful H-PCE but the peer assumes that
it does. On further PCEP message exchange, the messages may not get
further propagated and thus at worse would not lead to the stateful
H-PCE based 'parent' control of the LSP. This is something any peer
should be prepared for anyways.

The "clean" solution would be to add a new flag; but then we also need
to move this a standards track and loose the claim that this is just a
combination of existing protocol extensions.

Thanks!
Dhruv