Re: [Idr] Benjamin Kaduk's No Objection on draft-ietf-idr-bgp-ls-segment-routing-msd-17: (with COMMENT)

[Jeff Tantsura <[e-mail]>]

Hi Ben,

Many thanks for your comments!
Please see inline

Cheers,
Jeff
On May 4, 2020, 2:53 PM -0700, Benjamin Kaduk via Datatracker <[e-mail]>, wrote:
> Benjamin Kaduk has entered the following ballot position for
> draft-ietf-idr-bgp-ls-segment-routing-msd-17: No Objection
>
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>
>
>
> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
>
> Not a whole lot of interesting comments on this one; it seems like a
> pretty straightforward codepoint allocation with relevant (security and
> other) considerations covered in other documents. I do appreciate the
> discussion in the security considerations section; it looks good.
> [jeff] thank you!
>
> This document talks about the MSD as being the depth of a stack that a
> given node can "impose". Is there a similar consideration (or protocol
> element) for what a given node can read/process? (I think I remember
> such a document going by, but don't have enough keywords to search for
> it efficiently.)
>
> [jeff] The drafts talking about Readable Label Stack Deepth Capability (RLSDC) are
> draft-ietf-isis-mpls-elc/draft-ietf-ospf-mpls-elc (subcase in RFC8662)
>
> Is there a specific error-handling behavior to use when a Node MSD TLV
> is received with a value larger than the value of a Link MSD TLV
> associated with that node (for the same MSD-type)? Is that one of the
> things "left to the consumer of the BGP-LS information" by Sectoin 7?
>
> [jeff] the rule is that most specific (Link MSD) always wins,
> e.g if there’s a Node MSD of 5 and Link MSD of 3 for interface-3 and a router with 3 interfaces -
> interface-1 and interface-2 will use the value signaled through Node MSD (5), while interface-3 would use the value from Link MSD
> In general - the logic is indeed left to the consumer of this information. Advertising different values in Node and Link MSD is not an error.
>
>
> Section 1
>
> In the future, it is expected that new MSD-Types will be defined to
> signal additional capabilities, e.g., ELs, SIDs that can be imposed
> through recirculation, or SIDs associated with another data plane
> such as IPv6. MSD advertisements may be useful even if SR itself is
> not enabled. For example, in a non-SR MPLS network, MSD defines the
> maximum label depth.
>
> It's slightly interesting that we still call it "MSD" even though there
> are potential uses in a broader scope. Perhaps just a historical legacy
> and we need to remain consistent with the name in common use...
> Though, perhaps the terminology section in this document does not need
> to be quite so constrained by the past.
>
> [jeff] we have adopted all new use cases to refer to MSD, so far works OK.
> Indeed, there’s quite some history
>
> Section 3
>
> identified by the corresponding Router-ID. Node MSD is the smallest
> MSD supported by the node on the set of interfaces configured for
> use. MSD values may be learned via a hardware API or may be
>
> It is a shame that the semantics are already locked in (as was noted by
> a previous review that I can't find right now?) and the efficient
> encoding of "large per-node value with smaller per-link values for
> exceptions" is not admissible.
>
> [jeff] please see my response to Martin - https://mailarchive.ietf.org/arch/msg/idr/ngvWcLl8OZqdrsWDtFmHA7cbiGs/
>
> Section 5
>
> MSD-type is not supported. The correct interpretation MUST be
> specified when an MSD-type is defined in [RFC8491].
>
> Is this "is defined in the registry defined in [RFC8491]" or "is defined
> according to the procedures in [RFC8491]" or something else? The
> current text doesn't scan properly, as it implies that a future new
> MSD-type will be defined in RFC 8491, an immutable document.
>
> [jeff] This doesn’t read correct, thanks!
> RFC8491 states: "The correct interpretation MUST be specified when an MSD-Type is defined"
> The correct interpretation MUST be specified when an MSD-type is defined, as described in [RFC8491] seems like a better text, would that be OK?
>
>
> Section 7
>
> Specifically, the malformed attribute tests for syntactic checks in
> the Fault Management section of [RFC7752] now encompass the new BGP-
> LS Attribute TLVs defined in this document. [...]
>
> Interestingly, the referenced section of 7752 does not seem to
> explicitly say that other invariant properties of TLV lengths (e.g.,
> that they must be a multiple of 2 as for the ones defined by this
> document) should be checked.
>
> [jeff] RFC7752 is being updated as we speak (draft-ietf-idr-rfc7752bis), would a good addition.
> IDR WG is CCed in this email.
>
> [RFC7752]. The MSD information introduced in BGP-LS by this
> specification, may be used by BGP-LS consumer applications like a SR
> path computation engine (PCE) to learn the SR SID stack handling
>
> https://www.rfc-editor.org/materials/abbrev.expansion.txt lists "PCE" as
> having a well-known expansion to "Path Computation Element" (not
> "engine") that can be used directly in abbreviated form without any
> expansion given.
>
> [jeff]ack, this is a typo, thanks and changed
>
> Section 8
>
> issues when propagating the TLVs into BGP-LS. The advertisement of
> the node and link attribute information defined in this document
> presents no additional risk beyond that associated with the existing
> node and link attribute information already supported in [RFC7752].
>
> I'd suggest hedging to "no significant additional risk", though I do not
> have an example of additional marginal risk at hand.
> [jeff] ack, added
>
>
>