Re: [Pce] Benjamin Kaduk's Discuss on draft-ietf-pce-wson-rwa-ext-11: (with DISCUSS and COMMENT)

[Leeyoung <[e-mail]>]

Hi Benjamin,



Thanks for your detailed comments which are very good. Please see inline for my response.  Attached is v.12 (which was just published) and below is the pointer for the diff file showing the revision has incorporated your comments as well as other IESG members’ comments.

https://www.ietf.org/rfcdiff?url2=draft-ietf-pce-wson-rwa-ext-12



Please let us know if you have further comments.



Thanks & Best regards,

Young



-----Original Message-----

From: Benjamin Kaduk [mailto:[e-mail]]

Sent: Saturday, February 2, 2019 9:39 PM

To: The IESG <[e-mail]>

Cc: [e-mail]; Daniele Ceccarelli <[e-mail]>; [e-mail]; [e-mail]; [e-mail]

Subject: Benjamin Kaduk's Discuss on draft-ietf-pce-wson-rwa-ext-11: (with DISCUSS and COMMENT)



Benjamin Kaduk has entered the following ballot position for

draft-ietf-pce-wson-rwa-ext-11: Discuss



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The document, along with other ballot positions, can be found here:

https://datatracker.ietf.org/doc/draft-ietf-pce-wson-rwa-ext/







----------------------------------------------------------------------

DISCUSS:

----------------------------------------------------------------------



I'm concerned that this is not sufficiently specified to be implementable in an interoperable fashion.  In particular, I'm concerned that there need to be some values allocated from IANA registries that are not currently mentioned in this document, and there are some potential subtleties surrounding data structure reuse that I'm not entirely sure about as well.



I include section-by-section comments in this DISCUSS section (populated by duplicating my COMMENT section and trimming; my apologies is a comment is duplicated in both ballot sections by mistake).



Section 4.1



   Additionally, given a range of potential labels to allocate, the

   request SHOULD convey the heuristic / mechanism to the allocation.



I can't tell which protocol interaction is being described here.



YL>> This is done by the Wavelength Selection TLV (see Section 4.2.) which is a part of the WA Object.





   <PCReq Message> ::= <Common Header>



                          [<svec-list>]



                          <request-list>



      Where:



         <request-list>::=<request>[<request-list>]



         <request>::= <RP>



                      <ENDPOINTS>



                      <WA>



                      [other optional objects...]



Is this intended to conform to any particular formal language, or is it an ad hoc description?  Where is <svec-list> defined?  (RFC 5440 spells it as "<END-POINTS>" and not "<ENDPOINTS>", BTW.)



YL>> This is per Section 6.4. of RFC 5440. I will make the reference to RFC 5440 to make it clearer.  The <svec-list> is defined in RFC 5440. I will correct with <END-POINTS>.



I would change the text right before the format as follows:



OLD:



The format of a PCReq message after incorporating the Wavelength Assignment (WA) object is as follows:



NEW:



The format of a PCReq message per [RFC5440] after incorporating the Wavelength Assignment (WA) object is as follows:









   If the WA object is present in the request, it MUST be encoded after

   the ENDPOINTS object as defined in [PCEP-GMPLS]. Orderings with

   respect to the other following objects are irrelevant.



The prose and the figure do not exactly match up in this regard (is WA optional or mandatory; does <WA> need to be the first of the optional objects?).



YL>> The WA object is mandatory in this document, which MUST be encoded right after <END-POINT> Object. “Orderings with respect to the other following objects are irrelevant” is meant to say “Orderings for the other optional objects are irrelevant”.



     . Wavelength Selection TLV (32 bits): See Section 4.2 for

        details.



Either this is a proper TLV, in which case it has 32 bits of tag and length plus an additional 32 bits of value, for 64 bits total, or it is not a TLV and comprises solely of the "value" field of the Wavelength Selection Sub-TLV.  Section 8.2 allocates a TLV type indicator for it, which suggests that the full TLV encoding is intended; where are the 32 bits for type and length reflected in this text and in the figure?



YL>> This is s full TLV. I will modify the figure to 64 bits.



Section 4.3



   The Wavelength Restriction Constraint TLV type is TBD3 (See Section

   8.3). This TLV MAY appear more than once to be able to specify

   multiple restrictions.



This is in conflict with the diagram in Section 4.1 (which does not appear to depict multiple occurrences).  It's also unclear that the stated reasoning applies, since the RBNF indicates that (<Link

Identifiers> <Wavelength Restriction>) can repeat, so the need for

Identifiers> multiple

TLVs is for different *action* (and count) rather than specifically for the wavelength restrictions.



YL>> You’re correct. I deleted “This TLV MAY appear more than once to be able to specify

   multiple restrictions.” from the sentence you listed above. I added a new sentence at

   the  end of RBNF.



  NEW:



   <Link Identifiers> and <Wavelength Restriction> fields together MAY

   appear more than once to be able to specify multiple restrictions.



Please note that wavelength restriction may be applied to a number of link identifiers.



How are future "Action" values to be defined?



YL>> I added “2-255 – for future use”.



   Various encoding errors are possible with this TLV (e.g., not

   exactly two link identifiers with the range case, unknown identifier

   types, no matching link for a given identifier, etc.). To indicate

   errors associated with this type, a new Error-Type (TBD8) and an

   Error-value (Error-value=3) MUST be defined so that the PCE MUST

   send a PCErr message with a PCEP-ERROR Object. See Section 5.1 for

   the details.



This normative language is not appropriate -- it in effect is only constraining the current document, so descriptive language of "a new error type is assigned" is more appropriate.



YL>> Agree.  s/MUST be defined/are assigned



What is the mechanism for extensibility of future Link Identifier sub-TLV types?  Should there be a registry?



YL>>  I am not sure if we need reserve for future interface type. Typical PCEP/GMPLS RFCs do not go beyond this three types and the unnumbered type is flexible enough to accommodate other types than IPv4 and IPv6.





Section 4.3.2



RFC 6205 says that the "Identifier" is a per-node assigned and scoped value that may change on a per-hop basis.  I don't see where our base label gets scoped to a node (just that it's part of a PCReq message which does not seem scoped to a node), so this seems problematic.



YL>> RFC 6205 is written from the perspective of GMPLS signaling which is on a per-hop basis. Here we are using the same encoding, but from the PCEP perspective. In the PCEP, PCC is the head-end node and the head-end node would apply the wavelength restriction to its out-going interfaces. If the restriction applies to globally, then GMPLS signaling will carry this information to sub-sequent nodes. If the restriction locally applies, then PCE-CC mechanism (PCE talks to each node) would allow the restriction to be applied to each node.  This draft was written the restriction would be applied globally (as a mechanism for limiting the tunable range of wavelength globally so that the head-end node can propagate this restriction to other node (via GMPLS signaling mechanism).



Section 4.4



   The END-POINTS type generalized endpoint is extended as follows:



   <endpoint-restrictions> ::= <LABEL-REQUEST>



                               <Wavelength Restriction Constraint>



                               [<signal-compatibility-restriction>...]



Where is the original <endpoint-restrictions> definition that is being updated?  (Why does this definition not include the <label-restriction-list> component from [PCEP-GMPLS]?)



YL>> <wavelength restriction constraint> is a type of <label-restriction-list>. Wavelength is a type of label.



Why is there no

Updates: relationship to reflect this extension?  Is <Wavelength Restriction Constraint> supposed to be the same TLV as defined in Section

4.3.2 without a separate containing PCEP object?



YL>> This draft is extending PCEP TLV for wavelength type of label. <Wavelength Restriction Constraint> is extending generic label field to indicate optical channel grid channel spacing, its identifiers, etc. (the second 4-octet field of 4.3.2).



Per [PCEP-GMPLS], <LABEL-REQUEST> is a TLV.  Does that not also mean that <Wavelength Restriction Constraint> and <signal-compatibility-restriction>

need to be (comprised of) sibling TLVs?  This document allocates a TLV type for Wavelength Restriction Constraint in Section 8.3, but the references to RFC 7581 for <Optical Interface Class List> and <Client Signal Information> seem to only be for the encoding of sub-TLVs, with sub-TLV values that live in the separate "Types for Subfields of WSON Resource Block Information"

registry and are in a colliding namespace.  Don't we need to allocate TLV values from the same place as <LABEL-REQUEST> (i.e., first-level PCEP TLVs) in order for this to be en/decodable?



YL>> This is PCEP where we don’t have WSON Resource Block Information level of TLV. We define a new WA Object in which to define Wavelength Selection TLV, Wavelength Restriction Constraint TLV, etc. while using the same encoding used in GMPLS routing/signaling but in a different context. Regarding the PCEP-GMPLS <Label-request> which is a top level TLV, here we are creating a new object WA in which to define other TLVs. The decision in the WG was to create a new object <WA> due to the different nature of wavelength assignment than general label assignment (there is no label swapping per se in the optical network as GMPLS case; the same wavelength has to be used across all interface of the WSON path) and in doing so, we put all other information field as TLV type.



Section 4.4.1



   The permitted sub-sub objects are the Optical Interface Class List

   and the Client Signal information whose encodings are described in

   Section 4.1 and Section 4.2 of [RFC7581], respectively.



Similarly to for the <endpoint-restrictions>, don't we need to allocate XRO Subobject values in order for these structures to be semantically en/decodable?



YL>> Yes, this is not clearly explained in the draft. Added are:



The XRO needs to support the new sub-object types:

                             Type                    Sub-sub object

                             TBD9                   Optical Interface Class List

                             TBD10                 Client Signal Information



The PCEP XRO registry will be added in the IANA Section.



Section 4.4.2



   This is supported by adding the sub-object "WSON Processing Hop

   Attribute TLV" defined for ERO in Section 4.2 [RFC7689] to the PCEP

   IRO object [RFC5440].



The referenced structure is defined as an RSVP-TE LSP attribute.

I cannot find any evidence that its usage in PCEP is defined, nor any TLV or subobject type allocated for its usage with PCEP.  (Is there some generic equivalence or mapping between (G)MPLS EROs and IROs and the PCEP analogues that I haven't encountered yet?)  Don't we need to allocate an IRO Subobject value for this usage in a PCEP IRO object?  Also, the WSON Processing Hop Attribute field is encoded as a sequence of sub-TLVs; if we want to reuse the same sub-TLVs from the existing usage, don't we need to document the linkage from the existing registry to the new usage somewhere?

How does the error handling translate to PCEP usage?

This seems rather underspecified.



YL>>  Correct. It is implicit. Added are:



The IRO needs to support the new sub-object types as defined in Section 4.2 [RFC7689] “WSON Processing Hop Attribute TLV” (TBD9) defined for ERO in Section 4.2 [RFC7689] to the PCEP IRO object [RFC5440]:

                             Type                    Sub-sub-object

                             TBD11                 Optical Interface Class List

                             TBD12                 Client Signal Information



The existing PCEP IRO will be added in the IANA Section to reflect the above addition.



Section 5



I'm very confused by the structure definition.  It claims to be the "TLV data", but also includes a type and length field so as to indicate that it is not just the data contents but the header as well.  But the length field is truncated by a bit for use as the 'M' flag -- how can we modify the outer TLV header in this way?!  Section 8.4 indicates that this type value is to be allocated from the "PCEP TLV Type Indicators" subregistry created by RFC 5440, that uses the full 2 bytes for the "length" field.



YL>> Good point. I will delete T/L field to make this TLV data as it is claimed while allocating one bit for M bit in the new 32 bit field.



Section 5.1



This section describes an Error-value=3 that is not reflected in Section

8.8 in the requests to IANA.



YL>> Correct. Will add Error-value =3 and its description in Section 8.8.



Section 8.5



Isn't this mentioned in Section 4.4 (not 4.3)?



Section 8.6



As above, isn't this mentioned in Section 4.4 (not 4.3)?



YL>> Yes, in both cases, it should’ve been Section 4.4.





----------------------------------------------------------------------

COMMENT:

----------------------------------------------------------------------



Section 3



   A Lambda Switch Capable (LSC) Label Switched Path (LSP) may span one

   or several transparent segments, which are delimited by 3R

   regenerators (Re-amplification, Re-shaping, Re-timing) typically

   with electronic regenerator and optional wavelength conversion. Each

   transparent segment or path in WSON is referred to as an optical

   path. An optical path may span multiple fiber links and the path

   should be assigned the same wavelength for each link. In such case,

   the optical path is said to satisfy the wavelength-continuity

   constraint. Figure 1 illustrates the relationship between a LSC LSP

   and transparent segments (optical paths).



Some nit-level remarks: using "optical path" for both transparent segments and paths is perhaps confusing; perhaps "an optical path in WSON refers to a transparent pathe that can comprise of one or more segments".  If the path "should be assigned the same wavelength for each link", what kind of constraint is that?  Is it just a nice "feel-good" thing for humans, is there some physical requirement for it, does it make planning simpler?  I understand that this is lowercase and thus not intended to be normative, but on first read it feels like the implication is that there is some aspect of the physics that drives this to be the case; I don't actually believe that to be true, though.  What entities would care if the "wavelength-continuity constraint" is satisfied?  (That is, is it really a "constraint" or more of a "property"?)



YL>> The constraint is called wavelength continuity without which no data will be transmitted. This is a MUST constraint physically speaking. LSP can comprise of multiple transparent segments (which are optical paths) via 3R conversion (OEO). That is why we chose to use LSP to differentiate it from optical path. Optical path is used to mean optical transparent path (which does not have any 3R in the path).



I'm a little confused by Figure 1, shich weems to imply by link labels that the middle nodes that are not makred "(3R)" are LSC nodes, but isn't it the case that if these nodes make use of their lamda switching capabilities that they will cease to be transparent and instead also be 3Rs?



YL>> LSC means to switch lambdas from the incoming one to the outgoing one. Transparent means the incoming and outgoing are the same lambas. When you do 3R, it becomes non-transpaent as the incoming lambda has to be converted via OEO (3R).



   Note that two optical paths within a WSON LSP do not need to operate

   on the same wavelength (due to the wavelength conversion

   capabilities). Two optical paths that share a common fiber link

   cannot be assigned the same wavelength; Otherwise, the two signals



Are these "two optical paths" part of the same or different LSPs?  My best reading of the two instances of the phrase are that the first one is the same LSP and the second usage is for different LSPs, which is pretty confusing to the reader (if true).



YL>> The same LSP. LSP is an end-to-end notion in the context of WSON. It may comprise multiple segments as shown in the figure.



Please expand PSC and TDM.



YL>> OK. Thanks for pointing out. PSC: Packet Switch Capable; TDM: Time Division Multiplexing



                            In order to improve the signal quality and

   limit some optical effects several advanced modulation processing

   capabilities are used. [...]



Used by this document specifically, or in general usage?



YL>> Specifically in this document. See Section 4.4.



   This document, however, does not address optical impairments as part

   of RWA path computation.



Do we need a link/reference for "optical impairments"?



YL>> I can add reference [RFC6566].



Section 4.1



   o  Reserved (16 bits): Reserved for future use and SHOULD be zeroed.



Do we also want to say "ignored on receipt"?



YL>> Certainly. Added.



Please expand TED and NMS (IGP is "well-known" per https://www.rfc-editor.org/materials/abbrev.expansion.txt).



YL>> Done. TED = Traffic Engineering Database; NMS = Network Management System



Section 4.3





   Note that "interfaces" are assumed to be bidirectional.



You haven't used the term "interface" (with or without scare quotes) yet, so this is a dangling reference.



YL>> I can change to “links”.



How are future "Action" values to be defined?



   o  Reserved (16 bits): Reserved for future use and SHOULD be zeroed.



And ignored on receipt?



YL>> “Ignored on receipt” is added.



Section 4.3.2



If you're going to copy the Label Set format here from RFC 7579, maybe you could say something like "repeated here for convenience, with the base label internal structure included" to spare the reader from having to go compare the two formats?  This also holds for the list of Action values and the other field descriptions not already incorporated from RFC 7579 by reference.



YL>> Agree and added.



RFC 6205 says that the "Identifier" is a per-node assigned and scoped value that may change on a per-hop basis.  I don't see where our base label gets scoped to a node (just that it's part of a PCReq message which does not seem scoped to a node), so this seems problematic.



YL>> See the comments in the DISCUSS section.



Section 4.4



   Path computation for WSON includes checking of signal processing

   capabilities at each interface against requested capability; this

   requirement MAY be implemented by the IGP.  [...]



How is the IGP supposed to check the processing capabilities of an interface against a given request?  I'd suggest rephrasing this text to parallel the text in Section 4.3 about how mechanisms to know the interface capabilities can include IGP or NMS.



YL>> OK. s/this requirement MAY be implemented by the IGP/ the PCE MUST have mechanisms to know the signal processing capabilities at each interface, e.g. by means of the Traffic Engineering Database (TED) either via IGP or Network Management System (NMS).





   The supported signal processing capabilities are those described in

   [RFC7446]:



"Supported by what?"  Perhaps rephrase as "The signal processing capabilities considered in the RWA Information Model [RFC7446] are:".



YL>> Agreed and changed as such.





Section 4.4.1



   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |X|  Type = X   |     Length    |   Reserved    | Attribute     |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



I strongly suggest using a different placeholder than 'X' in "Type = X"

since there is a separate 'X' bit.



YL>> Replaced with TBD.



   Reserved bits (8 bits) are for future use and SHOULD be zeroed.



And ignored on receipt?



YL>> Agree and added.



  The Attribute field (8 bits) indicates how the exclusion sub-object

   is to be interpreted. The Attribute can only be 0 (Interface) or 1

   (Node).



I would suggest phrasing this more like "[RFC5521] defines several Attribute values; the only permitted Attribute values for this sub-object are [...]".



YL>> Agreed and changed.



Section 4.4.2



   This is supported by adding the sub-object "WSON Processing Hop

   Attribute TLV" defined for ERO in Section 4.2 [RFC7689] to the PCEP

   IRO object [RFC5440].



The referenced structure is defined as an RSVP-TE LSP attribute.

I cannot find any evidence that its usage in PCEP is defined, nor any TLV or subobject type allocated for its usage with PCEP.  (Is there some generic equivalence or mapping between (G)MPLS EROs and IROs and the PCEP analogues that I haven't encountered yet?)  Don't we need to allocate an IRO Subobject value for this usage in a PCEP IRO object?  Also, the WSON Processing Hop Attribute field is encoded as a sequence of sub-TLVs; if we want to reuse the same sub-TLVs from the existing usage, don't we need to document the linkage from the existing registry to the new usage somewhere?

How does the error handling translate to PCEP usage?

This seems rather underspecified.



YL>> Please see DISCUSS section.



Section 5



   Option (b) allows distributed label allocation (performed during

   signaling) to complete wavelength allocation.



   The Wavelength Allocation TLV type is TBD4 (See Section 8.4). The

   TLV data is defined as follows:



Could you maybe give a bit more of transition/explanation, e.g., whether this TLV is used for both (a) and (b), that it's a hop attribute that appears in a TLV in the ERO's TLV list, etc.



YL>> Added “This TLV is used for both (a) and (b) before the figure. The latter is discussed in the last paragraph of Section 5.



   This TLV is encoded as an attributes TLV, per [RFC5420], which is

   carried in the ERO LSP Attribute Subobjects per [RFC7570].



RFC 7570 seems to call these "Hop Attribute Subobjects", if I'm finding the right place.  Using consistent naming would be a big help to the (confused) reader.



YL>> Agreed. Changed.

Section 6



Thank you for the Manageability Considerations section; it helps give a picture of how this slots into the broader ecosystem.



YL>> Thanks.