Re: [Pce] Benjamin Kaduk's Discuss on draft-ietf-pce-stateful-path-protection-10: (with DISCUSS and COMMENT)

[Mahend Negi <[e-mail]>]

Hi Ben/Dhruv,

Thanks for the review and clarification. Comments are incorporated in the
working copy (links below).

Working copy:
https://github.com/mahendnegi/ietf/blob/master/pce/path-protection/iesg/draft-ietf-pce-stateful-path-protection-11.txt
Diff:
https://github.com/mahendnegi/ietf/commit/c51efb2b025d545e066b9b13de6d84de48d1a03c

Please do share your opinion.

Thanks,
Mahendra


On Fri, Sep 20, 2019 at 2:33 AM Benjamin Kaduk <[e-mail]> wrote:

> Hi Dhruv,
>
> On Wed, Sep 18, 2019 at 11:45:23AM +0530, Dhruv Dhody wrote:
> > Hi Ben,
> >
> > Thanks for your review, let me start the discussion and the authors
> > can interject as needed.
>
> Thanks; it's a great start.
>
> > On Wed, Sep 18, 2019 at 5:14 AM Benjamin Kaduk via Datatracker
> > <[e-mail]> wrote:
> > >
> > > Benjamin Kaduk has entered the following ballot position for
> > > draft-ietf-pce-stateful-path-protection-10: Discuss
> > >
> > > When responding, please keep the subject line intact and reply to all
> > > email addresses included in the To and CC lines. (Feel free to cut this
> > > introductory paragraph, however.)
> > >
> > >
> > > Please refer to
> https://www.ietf.org/iesg/statement/discuss-criteria.html
> > > for more information about IESG DISCUSS and COMMENT positions.
> > >
> > >
> > > The document, along with other ballot positions, can be found here:
> > >
> https://datatracker.ietf.org/doc/draft-ietf-pce-stateful-path-protection/
> > >
> > >
> > >
> > > ----------------------------------------------------------------------
> > > DISCUSS:
> > > ----------------------------------------------------------------------
> > >
> > > (1) draft-ietf-pce-association-group notes that "PCEP extensions that
> define
> > > a new association type should clarify the relationship between the SVEC
> > > object and the association type, if any".  Where do we do so for the
> > > path protection association type?
> > >
> >
> > This is discussed in
> >
> https://tools.ietf.org/html/draft-ietf-pce-association-diversity-10#section-5.3
> ,
> > where it is more relevant. This draft includes the relationship with
> > diversity association in Section 5.
>
> A naive reading of the linked section would be that it applies only to the
> "Disjointness Association Type" defined in that document.  I'm not in a
> position to claim that the reasoning in that document does not apply to
> this one, but I think we should make a more clear link between the Path
> Protection ASsociation Type and the discussion in that document.
>
> > > (2) Section 3.2 says:
> > >
> > >       Protection Type (PT): 6 bits - This field is as defined in
> > >       Section 14.1 of [RFC4872] to indicate the LSP protection type in
> > >       use.
> > >
> > > There doesn't seem to be a registry created by RFC 4872 to track these
> > > PT values, so I assume that the way to allocate a new value is
> > > "standards-track RFC that Updates: RFC 4872".  Is that also the way to
> > > allocate new PT values for PPAG usage?  How would someone updating RFC
> > > 4872 to allocate a new type know to consider/document whether it
> applies
> > > for PPAG usage?
> > >
> >
> > That's true. Maybe we can add this text - "Any type already defined or
> > that could be defined in the future for use in the RSVP-TE PROTECTION
> > object is acceptable in this TLV unless explicitly stated otherwise."
>
> To be honest, that's probably good enough in practice, since it seems
> somewhat unlikely that the last bit is ever going to get allocated (though,
> I guess technically the field could be converted to an integer instead of
> flags).  I do still have to wonder how the reader would know *where* it
> would be stated otherwise, and how the author of a document defining a new
> type would know to consider our use case as well as the RFC 4872 use case.
>
> > > (3) In Section 4.3:
> > >
> > >    A PCE can create/update working and protection LSPs independently.
> > >    As specified in [I-D.ietf-pce-association-group], Association Groups
> > >    can be created by both the PCE and the PCC.  Further, a PCE can
> > >
> > > The requirement that source, destination, and tunnel ID of all LSPs
> > > within a PPAG MUST be the same is new to this document, so I think we
> > > need to specify error handling for when attempts to update LSPs
> > > independently would violate that invariant (presumably in Section 4.5).
> > >
> >
> > In the existing error case in Section 4.5 we could make it "add or
> update".
>
> Wording it right might be a little finicky, but that's fine in general.
>
> > > (4) In Section 6.3:
> > >
> > >                                       IANA is requested to allocate new
> > >    error values within the "PCEP-ERROR Object Error Types and Values"
> > >    sub-registry of the PCEP Numbers registry, as follows:
> > >
> > > The following table only lists Error-values.  What Error-type(s) should
> > > they be associated with?
> > >
> >
> > It is mentioned in the text, but worth making it explicit in the table
> as well.
> >
> > > (5) We don't say which objects the PPAG TLV can appear in.  (Section
> 3.2
> > > says "[t]he Path Protection Association TLV is an optional TLV for use
> > > with the Path Protection Association Type", but it's hard to interpret
> > > that as meaning "for use [only] with the ASSOCIATION object defined in
> > > draft-ietf-pce-association-group", especially since there is a "path
> > > protection association type" already (and it's a codepoint in the
> > > "ASSOCIATION Type Field" registry).
> > >
> >
> > We can update to - "The Path Protection Association TLV is an optional
> > TLV for use in the ASSOCIATION Object with the Path Protection
> > Association Type."
>
> Thanks!
>
> > > (6) I'm not sure if a change to the document is needed here, but
> perhaps
> > > some discussion is in order: we say that a given LSP can belong to more
> > > than one PPAG, but only allow one PPAG TLV per [some context that
> > > remains unclear; see (5)].  I don't have a good handle for whether
> these
> > > two requirements are potentially in conflict: that is, whether a single
> > > PPAG TLV would have to specify the flags that apply for both PPAGs that
> > > a given LSP is a member of, or if the containing objects serve to scope
> > > the PPAG TLV flags' interpretation.
> > >
> >
> > When a given LSP belong to more than one PPAG, we expect to have
> > multiple ASSOCIATION Object for each PPAG with each of them having a
> > single PPAG TLV.
>
> Sounds good, and quite natural given (5).
>
> > > (7) Do the Protection Type fields of the PPAG TLV in the various LSPs
> > > that are members of the same PPAG need to match, in the same way that
> > > the source/destination/tunnel-ID do?
> > >
> >
> > Yes. Authors should make that explicit.
>
> Thanks
>
> > >
> > > ----------------------------------------------------------------------
> > > COMMENT:
> > > ----------------------------------------------------------------------
> > >
> > > Section 1
> > >
> > >    This document specifies a stateful PCEP extension to associate two
> or
> > >    more LSPs for the purpose of setting up path protection.  The
> > >    proposed extension covers the following scenarios:
> > >
> > > nit: after publication, it doesn't really seem like a *proposed*
> > > extension anymore.
> > >
> > > Section 3.1
> > >
> > >    LSPs are not associated by listing the other LSPs with which they
> > >    interact, but rather by making them belong to an association group
> > >    referred to as "Path Protection Association Group" (PPAG) in this
> > >    document.  [...]
> > >
> > > The first 2/3 of this sentence is a (true) generic statement about all
> > > association groups, which exist outside of this document in a generic
> > > fashion.  I strongly suggest rewording this to make clear that the PPAG
> > > is the specific association (group) type used for this document's
> > > functionality but that other association groups are possible.  The
> > > current wording implies that the PPAG is the only association possible,
> > > and it is just given a special name for this document's purposes.
> > >
> > >    [I-D.ietf-pce-association-group] specifies the mechanism for the
> > >    capability advertisement of the Association types supported by a
> PCEP
> > >    speaker by defining a ASSOC-Type-List TLV to be carried within an
> > >    OPEN object.  This capability exchange for the Association type
> > >    described in this document (i.e.  Path Protection Association Type)
> > >    MAY be done before using the policy association, i.e., the PCEP
> > >    speaker MAY include the Path Protection Association Type (TBD1) in
> > >    the ASSOC-Type-List TLV before using the PPAG in the PCEP messages.
> > >
> > > Why is this only MAY and not MUST?
> > >
> >
> > In [I-D.ietf-pce-association-group] we explicitly did not mandate the
> > capability exchange for all association types and let each type decide
> > for itself. This association type fits into where the capability
> > exchange is optional. When the capability exchange was added in
> > [I-D.ietf-pce-association-group], there was a desire from the WG to
> > keep it optional for already defined/implemented protection
> > association type. Also this is a basic feature in path computation and
> > it is expected to be supported universally. In a corner case there is
> > an error handling for unknown/unsupported association-type to take
> > care of it.
>
> Okay, thanks for giving me the backstory.
>
> > >    This Association type is dynamic in nature and created by the PCC or
> > >
> > > nit: is it the type that is dynamic or the associations of that type?
> > >
> >
> > It is the type. [I-D.ietf-pce-association-group] has dynamic or
> > operator-configured association, this one is dynamic.
>
> Ah, of course, I had forgotten about that division of configuration.
>
> > >       Protecting (P): 1 bit - This bit is as defined in Section 14.1 of
> > >       [RFC4872] to indicate if the LSP is working or protection LSP.
> > >
> > >       Secondary (S): 1 bit - This bit is as defined in Section 14.1 of
> > >       [RFC4872] to indicate if the LSP is primary or secondary LSP.
> The
> > >       S flag is ignored if the P flag is not set.
> > >
> > > Just to check my understanding (no change expected to result): if P is
> > > zero, the LSP is working, and thus we know that it's primary since it's
> > > actively carrying traffic?
> > >
> >
> > Yes, Working LSPs are always primary.
> >
> >
> > > Section 3.2
> > >
> > > nit(?) The section heading does not match the name of the TLV requested
> > > in the IANA considerations, nor do we mention here that "PPAG TLV" is
> > > synonymous with it.
> > >
> > >    If the TLV is missing, it is considered that the LSP is the working
> > >    LSP (i.e. as if P bit is unset).
> > >
> > > Is this conditional on the LSP being part of a PPAG, or does it hold
> > > generically as well?
> > >
> >
> > IMHO when part of a PPAG i.e. association object but no TLV.
>
> It may be worth making the text explicit about it.
>
> > > Section 4.5
> > >
> > >    As per the processing rules specified in section 5.4 of
> > >    [I-D.ietf-pce-association-group], if a PCEP speaker does not support
> > >
> > > There is no Section 5.4 in draft-ietf-pce-association-group-10;
> > > presumably this was supposed to be Section 6.4?
> > >
> > >    A given LSP MAY belong to more than one PPAG.  If there is a
> conflict
> > >
> > > While I'm not arguing to remove this option, I'm also a bit confused at
> > > how useful having a single LSP be in multiple PPAGs woud be, given the
> > > need for source/destination/tunnel-ID to match.
> > >
> >
> > One working LSP could have multiple protection LSPs with each
> > belonging to a different association.
> >
> > >    When the protection type is set to 1+1 or 1:N with N=1, there MUST
> be
> > >    only one working LSP and one protection LSP within a PPAG.  If a
> PCEP
> > >    speaker attempts to add another working/protection LSP, the PCEP
> peer
> > >    MUST send PCErr with Error-Type 26 (Association Error)
> > >    [I-D.ietf-pce-association-group] and Error-Value TBD4 (Attempt to
> add
> > >    another working/protection LSP for Path Protection Association).
> > >
> > > This seems to prevent a scenario where there's a need to change the
> > > protection LSP and a desire to do so without removing the protected
> > > nature of the working LSP.  Unless it's presumed that this could always
> > > be done by updating the protection LSP and it would never be necessary
> > > to create a new one?  (Similarly for 1:N, though maybe less severe.)
> > >
> >
> > One can remove the protection LSP and keep the association intact. The
> > error will not be triggered.
> > Also, PCEP messages are per LSP, when the first LSP is reported with
> > association, it is bound to be the only LSP in the association.
> > So the error in this section are focused only when one adds "another".
>
> I don't think that the points you cover above are what I was concerned
> about -- I was thinking more along the lines of a "make-before-break"
> scheme for changing what LSP is the protection LSP for a given working LSP.
> Maybe given the timescales involved it's not an issue in practice, though.
>
> > Maybe we can add "..at maximum.." to make it clear?
>
> (I don't think that's needed.)
>
> Thanks!
>
> -Ben
>
> > > Section 5
> > >
> > >    [RFC4872] introduces the concept and mechanisms to support the
> > >    association of one LSP to another LSP across different RSVP -
> Traffic
> > >    Engineering (RSVP-TE) sessions using ASSOCIATION and PROTECTION
> > >    object.  The information in the PPAG TLV in PCEP as received from
> the
> > >    PCE, is used to trigger the signalling of working LSP and protection
> > >    LSP, with the Path Protection Association Flags mapped to the
> > >    corresponding fields in the PROTECTION Object in RSVP-TE.
> > >
> > > Just to check my understanding: this paragraph is saying that the
> > > contents of the PPAG TLV received by the PCC is used as input to
> > > populating the PROTECTION object that corresponds to the protection
> > > group?
> > >
> >
> > Yes.
> >
> > > Section 10.2
> > >
> > > I think RFCs 7525 and 8253 need to be normative (and please cite RFC
> > > 7525 as BCP 195).
> > >
> > >
> >
> > Thanks!
> > Dhruv
>