Re: [Pce] [Gen-art] Genart last call review of draft-ietf-pce-association-diversity-10

Alissa Cooper <alissa@cooperw.in> Wed, 30 October 2019 15:39 UTC

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From: Alissa Cooper <alissa@cooperw.in>
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Date: Wed, 30 Oct 2019 10:39:25 -0500
Cc: General Area Review Team <gen-art@ietf.org>, pce@ietf.org, draft-ietf-pce-association-diversity.all@ietf.org, last-call@ietf.org
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To: Dale Worley <worley@ariadne.com>
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Subject: Re: [Pce] [Gen-art] Genart last call review of draft-ietf-pce-association-diversity-10
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Thanks Dale. I entered a DISCUSS ballot to chat about a couple of your major points below and requested a response to the rest of your review.

Alissa


> On Oct 12, 2019, at 8:04 PM, Dale Worley via Datatracker <noreply@ietf.org>; wrote:
> 
> Reviewer: Dale Worley
> Review result: Ready with Issues
> 
> I am the assigned Gen-ART reviewer for this draft.  The General Area
> Review Team (Gen-ART) reviews all IETF documents being processed by
> the IESG for the IETF Chair.  Please treat these comments just like
> any other last call comments.
> 
> For more information, please see the FAQ at
> 
> <https://trac.ietf.org/trac/gen/wiki/GenArtfaq>;.
> 
> Document:  draft-ietf-pce-association-diversity-10
> Reviewer:  Dale R. Worley
> Review Date:  2019-10-12
> IETF LC End Date:  2019-10-18
> IESG Telechat date:  not known
> 
> Summary:
> 
>       This draft is on the right track but has open issues, described
>       in the review.
> 
> There are various exposition issues, as detailed below.
> 
> There are some minor technical issues about reporting of error
> conditions, the significance of the P bit in DISJOINTNESS-STATUS-TLV,
> the detailed definition of the new objective functions, etc. as
> detailed below.
> 
> But it seems like a number of complex cases haven't been clearly
> specified:  (1) the effect of the P bit if multiple LSPs have P=1, (2)
> the interaction of SVEC and disjoint-groups, and (3) the interaction
> of partially overlapping disjoint-groups and/or SVEC sets.
> 
> Major issues:
> 
> The relationship of this mechanism with SVEC seems to be important but
> is not clearly stated.  The relevant sections of the text seem to be:
> section 4 para 2, section 5.3, and section 5.4 from "[RFC5440] uses
> SVEC diversity flag" on.  I think that they need to be pulled into one
> section.  Then it will be possible to have a good description of the
> interaction with SVEC.
> 
> At the end of section 5.6 is the revelation that (1) the
> DISJOINTNESS-CONFIGURATION-TLV is not attached to a group (in some
> sense), but instead a separate copy of it is attached to every LSP in
> the group, (2) these copies must match in the group IDs they carry and
> also the T, S, N, and L flags in them must agree, (3) (I suspect) any
> OF-code must agree, and (4) the P flags in them may differ.  This
> information is needed as background for a number of parts of the text
> to make sense, particularly the description of the P bit in section
> 5.2 and section 5.5.  I recommend that the last para of section 5.5 be
> moved to section 5.1 or the beginning of section 5.2, so the reader is
> aware that the descriptions of each LSP carry separate, and not
> necessarily identical, DISJOINTNESS-CONFIGURATION-TLVs.
> 
> The path computation effects of the P bit are described in the "P"
> item in section 5.2 and section 5.5.  But the descriptions are
> unclear, or perhaps they presume that there are only two LSPs in the
> group.  I think the intended meaning is that all of the LSPs in the
> group with P=1 are computed first, and then with those LSPs fixed, the
> LSPs in the group with P=0 are computed.  This will cause
> shortest-path constraints (and other objective functions) to be
> optimized on the P=1 LSPs, and those paths will not be de-optimized by
> competition from the other paths.  This should probably be pulled out
> of the description of the "P" in its TLV and put into a separate
> paragraph.
> 
> Minor issues:
> 
> Nits/editorial comments: 
> 
> In general, check the uses of "could" and "would".  I think "would" is
> sometimes used to mean "... in that situation, the XXX MUST ...", and
> "could" is sometimes used where "MAY" would be clearer.
> 
> The running title is given as "ASSOC-DISJOINT".  I think "PCEP
> Diversity Constraint Signaling" would be clearer.
> 
> Abstract
> 
>   The proposed extension allows a Path Computation Client (PCC) to
>   advertise to a PCE that a particular LSP belongs to a
>   disjoint-group, thus the PCE knows that ...
> 
> It would be clearer if "a disjoint-group" was amplified to "a
> particular disjoint-group".
> 
> Table of Contents
> 
> There are a few section titles in which important words aren't
> capitalized ("functions" in section 5.4), or unimportant words are
> capitalized ("On" in sections 8.5 and 8.6).
> 
> The title of section 8.4 is a verb phrase, compared to the titles of
> the other subsections of section 8.  I suggest "Verification of
> Correct Operations" or maybe "Correct Operation Verification".
> 
> Perhaps change the title of Appendix A to just "Contributors", since
> there is no other enumeration of contributors.
> 
> 1.  Introduction
> 
>   This document specifies a PCEP extension to signal that a particular
>   group of LSPs should use diverse paths including the requested type
> , ---------------------------------------^
>   of diversity.
> 
> Add a comma as indicated, to make it clear that "including" modifies
> "signal" rather than "paths".
> 
>   This document specifies a PCEP extension to signal that a particular
>   group of LSPs should use diverse paths including the requested type
>   of diversity.
> 
> This implies that the "group" here is an instance of the association
> "grouping" of the preceding paragraph, but the Intro doesn't state
> that this document's grouping is an instance of the generic grouping
> mechanism.  Perhaps
> 
>   This document specifies a PCEP grouping to signal that ...
> 
> 3.  Motivation
> 
>   A customer may request
>   that the operator provide two end-to-end disjoint paths across the
>   IP/MPLS core.  
> 
> I suggest "the operator's IP/MPLS core".
> 
> [There are various places where the document seems to be phrased from
> the point of view of the network vendor rather than the point of view
> of the endpoint/user.  The IETF convention is to focus on the endpoint
> or user.]
> 
>   The customer may use those paths as primary/backup or
>   active/active.
> 
> This phrasing uses an adjective as a noun.  I prefer:
> 
>   The customer may use these paths with a primary/backup or an
>   active/active configuration.
> 
> --
> 
>   Different level of disjointness may be offered:
> 
> Perhaps "Different levels of disjointness can be defined:"
> 
> 4.  Applicability
> 
>   In the figure above, let us consider that the customer wants to have
>   two disjoint paths between CE1/CE2 and CE3/CE4.
> 
> I think that to make this accurate, you need to say "two disjoint
> paths, one between CE1 and CE2 and one between CE3 and CE4."
> 
> This section seems disorganized.  In particular, 2nd and 3rd paras
> don't seem to add much.  Para 2 should be relocated to the SVEC
> discussion (see "Major issues).  Para 3 seems to have two sentences of
> background information (I think it's duplicated in section 1) and one
> sentence that essentially states that this document defines a
> "diversity group" of LSPs which is a type of the generalized "LSP
> group".
> 
>   Using PCEP, the PCC could indicate that a disjoint path computation
>   is required, such indication should include disjointness parameters
>   such as the type of disjointness, the disjoint group identifiers, and
>   any customization parameters according to the configured local
>   policy.  As mentioned previously, the extension described in
>   [I-D.ietf-pce-association-group] is well suited to associate a set of
>   LSPs with a particular disjoint-group.
> 
> This touches on the right topics, but doesn't really connect them.
> Also, don't use "could" for an action that this document defines how
> to do.  You want to say something like
> 
>   Using the extension to PCEP defined in this document, the PCC uses
>   the [I-D.ietf-pce-association-group] extension to indicate that a
>   group of LSPs are required to be disjoint; such indication should
>   include disjointness parameters such as the type of disjointness,
>   the disjoint group identifiers, and any customization parameters
>   according to the configured local policy.
> 
> --
> 
>     Figure 2 - Sample use-cases for carrying disjoint-group over PCEP
>                                  session
> 
> In this figure, there seems to be a conflict in notation:  Case 1 uses
> "PCC" seemingly in the same role for which Case 2 uses "PE 1" and "PE
> 3".  The captions of both cases refer to "PCC".
> 
> Also, the Case 1 and Case 2 captions run into each other.
> 
>   Using the disjoint-group within a PCEP messages may have two purpose:
> 
> Better would be "The disjoint-group within PCEP messages is used for:"
> 
> 5.1.  Association Group
> 
>   As per [I-D.ietf-pce-association-group], LSPs are associated with
>   other LSPs with which they interact by adding them to a common
>   association group.  
> 
> To be exact, you want to say "by adding all of them"; by default
> "them" refers only to the "LSPs" that is the subject of the sentence.
> 
>   The Association parameters, as described in
>   [I-D.ietf-pce-association-group] as the combination of the mandatory
>   fields Association type, Association ID and Association Source in the
>   ASSOCIATION object, that uniquely identify the association group
>   belonging to this association.  If the optional TLVs - Global
>   Association Source or Extended Association ID - are included, then
>   they are included in combination with mandatory fields to uniquely
>   identify the association group.
> 
> Use either "field" or "TLV" consistently (unless the TLVs really are
> different from the "fields").
> 
> This is awkward.  Better would be
> 
>   As described in [I-D.ietf-pce-association-group], the association
>   group is uniquely identified by the combination of these fields in
>   the ASSOCIATION object:  Association Type, Association ID,
>   Association Source, and (if present) Global Association Source or
>   Extended Association ID.
> 
>   ... This document defines a new ...
> 
> Start a new para with this sentence.
> 
>   o  Association type = TBD1 ("Disjointness Association Type") for
>      Disjoint Association Group (DAG).
> 
> This seems redundant, unless there exists another type that is also
> associated with Disjoint Association Group (DAG).  Otherwise, you can
> probably use
> 
>   o  Association type = TBD1 Disjoint Association Type
> 
> --
> 
>   This capability exchange for the association type
>   described in this document (i.e.  Disjointness Association Type) MUST
>   be done before using the disjointness association.
> 
> This can be simplified to
> 
>   This capability exchange for Disjointness Association Type MUST
>   be done before using the disjointness association.
> 
> There seem to be multiple uses of "Disjoint Association Group (DAG)"
> and "Disjointness Association Type (TBD1)", each of which states the
> equivalence of two things.  You should be able to use one instance of
> each.  Also, it's probably worth putting "Disjointness Association
> Type" into section 2.
> 
>   This association type is considered to be both dynamic and operator-
>   configured in nature.
> 
> It might be worth providing references for the meaning of "dynamic"
> and "operator-configured".
> 
>   A disjoint group can have two or more LSPs, [...]
> 
> The significance of this clause to the rest of the sentence is
> unclear; it seems obvious that a disjoint group needs at least two
> members before it is significant.  However, as a matter of protocol,
> can a group that has only one LSP be established?
> 
>   The disjoint status is informed to the PCC.
> 
> This seems to be describing how to report to the PCC that the
> computation was not done as requested.  I think you want to provide
> more details here, or a pointer to them.  In any case, "disjoint
> status" isn't really a clear statement that the error condition will
> be reported.
> 
>   Associating a particular LSP to multiple disjoint groups is
>   authorized from a protocol perspective, however there is no assurance
>   that the PCE will be able to compute properly the multi-disjointness
>   constraint.
> 
> You want to state *much* more clearly what might happen.  This starts
> with "A particular LSP MAY be associated to multiple disjoint groups,
> but in that case, the PCE MAY ..."  What are the outer boundaries of
> the results that the PCE MAY return?  Is the PCE required to provide
> any particular error indication?
> 
> 5.2.  Disjoint TLVs
> 
> See "Major issues" regarding explaining that there is a separate copy
> of DISJOINTNESS-CONFIGURATION-TLV for each LSP, and they needn't all
> be the same.  All of the constraints described in the last paragraph
> of section 5.5 should be moved to here, or perhaps section 5.1.
> 
>    0                   1                   2                   3
>    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
>   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>   |         Type = TBD2           |            Length             |
>   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>   |                 Flags                               |T|P|S|N|L|
>   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> 
> "Flags" should be "Reserved".
> 
>      *  P (Shortest path) bit: when set, this indicates that the
>         computed path of the LSP SHOULD satisfy all the constraints and
>         objective functions first without considering the diversity
>         constraint.  ...
> 
> See "Major issues" for providing a separate paragraph to explain the
> semantics of the "P" bit.
> 
>      *  T (Strict disjointness) bit: when set, if disjoint paths cannot
>         be found, PCE SHOULD return no path for LSPs that could not be
>         be disjoint.  When unset, the PCE is allowed to relax
>         disjointness by either applying a requested objective function
>         (cf.  Section 5.4 below) or using any other behavior if no
>         objective function is requested (e.g. using a lower disjoint
>         type (link instead of node) or fully relaxing disjointness
>         constraint).  Further see Section 5.6 for details.
> 
> Instead of "or using any other behavior if no objective function is
> requested" it would be more conventional to say "or, if no objective
> function is requested, using local policy".
> 
>   The DISJOINTNESS-STATUS-TLV uses the same format as the DISJOINTNESS-
>   CONFIGURATION-TLV with a different type TBD3 (in the TLV).  The L, N,
>   and S flags are set based if the computed path meet the disjointness
>   criteria.  
> 
> Should be "... are set if the respective disjointness criterion was
> requested and the computed paths meet it"
> 
>   The P flag is set to indicate that the computed path is
>   the shortest
> 
> Surely "the shortest" needs further explanation -- the shortest path
> that meets what constraints?  I suspect that this just reflects the P
> flag in the CONFIGURATION-TLV -- is there a situation where it would
> not?
> 
>   and the T flag has no meaning in the DISJOINTNESS-
>   STATUS-TLV and MUST NOT be set while sending and ignored on receipt.
> 
> This clause should be a separate sentence.
> 
>   Any new flag defined for the DISJOINTNESS-CONFIGURATION-TLV is be
>   automatically applicable to the DISJOINTNESS-STATUS-TLV.
> 
> This can't be correct, as there is no defined way to automatically
> apply a new flag to DISJOINTNESS-STATUS-TLV.  (The previous paragraph
> defines three separate semantics for flags in DISJOINTNESS-STATUS-TLV!)
> But you can say:
> 
>   Any document defining a new flag for the
>   DISJOINTNESS-CONFIGURATION-TLV automatically defines a new flag
>   with the same name and in the same location in
>   DISJOINTNESS-STATUS-TLV; the semantics of the flag in
>   DISJOINTNESS-STATUS-TLV MUST be specified in the document that
>   specifies the flag in DISJOINTNESS-CONFIGURATION-TLV.
> 
> 5.3.  Relationship to SVEC
> 
>   The PCE would consider both the objects
> 
> I think you mean MUST instead of "would".
> 
>   In case no such path is possible (or the
>   constraints are incompatible), the PCE MUST send a path computation
>   reply (PCRep) with a NO-PATH object indicating path computation
>   failure as per [RFC5440].
> 
> Adding "(or the constraints are incompatible)" to "no such path is
> possible" is redundant!
> 
> This discussion is not clear to me.  Partly it is because I'm not
> familiar with SVEC.  But I suspect that "synchronization of a set of
> path computation requests" means that "the computations have to be
> done together" and "dependent requests" means something similar to the
> constraints between computed paths specified by the
> DISJOINTNESS-CONFIGURATION-TLV.
> 
> Is it possible to provide a comparison between the requirements
> specified by SVEC and the requirements specified by
> DISJOINTNESS-CONFIGURATION-TLV?
> 
> Also, is there any specific relationship between the set of LSPs in an
> SVEC and the set of LSPs in a disjoint-group?  What if multiple SVECs
> and disjoint-groups overlap in a complex way?
> 
> 5.4.  Disjointness Objective functions
> 
>   To minimize the common shared resources (Node, Link or SRLG) between
>   a set of paths during path computation three new OF-codes are
>   proposed:
> 
> Actually, the new codes are to allow specification of minimization of
> shared resources.  Also, an RFC doesn't just proposed the codes, it
> specifies them.  So this is better phrased:
> 
>   Three new OF-codes allow specification of minimization of common
>   shared resources (Node, Link or SRLG) between a set of paths during
>   path computation:
> 
> --
> 
>   MSL
> 
>   * Name:  Minimize the number of shared (common) Links.
> 
>   * Objective Function Code:  TBD4
> 
>   * Description:  Find a set of paths such that it passes through the
>      least number of shared (common) links.
> 
>   MSS
> 
>   * Name:  Minimize the number of shared (common) SRLGs.
> 
>   * Objective Function Code:  TBD5
> 
>   * Description:  Find a set of paths such that it passes through the
>      least number of shared (common) SRLGs.
> 
>   MSN
> 
>   * Name:  Minimize the number of shared (common) Nodes.
> 
>   * Objective Function Code:  TBD6
> 
>   * Description:  Find a set of paths such that it passes through the
>      least number of shared (common) nodes.
> 
> The use of "it" in the above definitions is unclear, since the paths
> are "they".
> 
> There are a number of similar but different ways that "the number of
> shared" entities can be defined.  E.g., if four LSPs pass through a
> single node, does that count as 1 (a single node is shared), 4 (four
> LSPs pass through a node that is shared), 6 (six pairs of LSPs
> conflict, in that the paths in the pair share a node)?  That is, are
> we counting (and thus minimizing) shared entities, the number of LSPs
> that violate the requested constraint, or the number of pairs of LSPs
> that overlap in an unacceptable way?
> 
>   If the OF-list TLV is included in the Association Object, the OF-code
>   inside the OF Object MUST include one of the disjoint OFs defined in
>   this document.  If this condition is not met, the PCEP speaker MUST
>   respond with a PCErr message with Error-Type=10 (Reception of an
>   invalid object) and Error-Value=TBD9 (Incompatible OF code).
> 
> I find it surprising that you've defined a specific error code for an
> unexpected OF-code, but you don't apply that error code to the
> situation where there are multiple OF-codes.  Instead, you have all
> but the first silently ignored.
> 
> There's also the intersection case where the first OF-code is
> legitimate, but the second OF-code is not one in this set.  Does that
> get an "Incompatible OF code" error, or is the second one just
> ignored?
> 
> I think it's cleaner to change TBD9 to "Incompatible OF-list for
> disjoint-group" and require it for all of these cases.
> 
>   [RFC5440] uses SVEC diversity flag for node, link or SRLG to describe
>   the potential disjointness between the set of path computation
>   requests used in PCEP protocol.
> 
> This paragraph seems very important, but completely out of place here.
> Might it fit better in section 5.3?
> 
>   This document defines three new OF-codes to maximize diversity as
>   much as possible, in other words, minimize the common shared
>   resources (Node, Link or SRLG) between a set of paths.
> 
> This paragraph seems redundant after the detailed discussion of these
> codes.
> 
> The remaining paragraphs of this section seem like they should go in
> section 5.3.
> 
> 5.5.  P Flag Considerations
> 
> See above queries about the P flag.
> 
>   As mentioned in Section 5.2, the P flag (when set) indicates that the
>   computed path of the LSP SHOULD satisfies all constraints and
>   objective functions first without considering the diversity
>   constraint.
> 
> There are complexities that don't seem to be addressed.  E.g., if
> there are two LSPs with P=1 and two with P=0, I would expect that the
> first two would be computed with full consideration of the
> disjointness constraint between them.  So "without considering the
> diversity constraint" really isn't a good description of what is
> intended.
> 
> 5.6.  Disjointness Computation Issues
> 
>   Also, in case of network event leading to an impossible
>   strict disjointness, the PCE MUST send a PCUpd message ...
> 
> I think you want to start a new paragraph here and say
> 
>   In case of a network event which makes it impossible to satisfy the
>   constraints, the PCE MUST send a PCUpd message ...
> 
> --
> 
>   The
>   actual level of disjointness computed by the PCE ...
> 
> More correct to say
> 
>   The
>   actual level of disjointness of the paths computed by the PCE ...
> 
> --
> 
>   While the DISJOINTNESS-CONFIGURATION-TLV defines the
>   expected level of disjointness required by configuration, the
>   DISJOINTNESS-STATUS-TLV defines the actual level of disjointness
>   computed.
> 
> Better to s/expected/desired/ and s/actual/achieved/.
> 
>   There are some cases where the PCE may need to completely relax the
>   disjointness constraint in order to provide a path to all the LSPs
>   that are part of the association.  A mechanism that allows the PCE to
>   fully relax a constraint is considered by the authors as more global
>   to PCEP rather than linked to the disjointness use case.  As a
>   consequence, it is considered as out of scope of the document.
> 
> This para doesn't seem to make any difference.  As long as the T bit
> is clear and the local policy allows complete relaxation of the
> requested disjointness, this case is covered within the mechanisms of
> this document.  And this case will be reported in a straightforward
> way by a DISJOINTNESS-STATUS-TLV with all bits clear.
> 
> 6.  Security Considerations
> 
>   This document defines one new type for association ...
> 
> Better "This document defines one new PCEP association type ..."
> 
> After para 1, you probably want to add this case: "A spurious LSP can
> have flags that are inconsistent with those of the legitimate LSPs of
> the group and thus cause LSP allocation for the legitimate LSPs to
> fail with an error."
> 
> 
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