Re: [netmod] Benjamin Kaduk's Discuss on draft-ietf-netmod-geo-location-08: (with DISCUSS and COMMENT)

[Christian Hopps <[e-mail]>]

Benjamin Kaduk <[e-mail]> writes:

> Hi Christian,
>
> Sorry for the very delayed reply (and thanks to Rob for the nudge).
>
> On Wed, May 26, 2021 at 06:04:58PM -0400, Christian Hopps wrote:
>>
>> Benjamin Kaduk via Datatracker <[e-mail]> writes:
>>
>> > Benjamin Kaduk has entered the following ballot position for
>> > draft-ietf-netmod-geo-location-08: 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 DISCUSS and COMMENT positions.
>> >
>> >
>> > The document, along with other ballot positions, can be found here:
>> > https://datatracker.ietf.org/doc/draft-ietf-netmod-geo-location/
>> >
>> >
>> >
>> > ----------------------------------------------------------------------
>> > DISCUSS:
>> > ----------------------------------------------------------------------
>> >
>> > I think we lack sufficient precision (forgive the pun) in how we talk
>> > about "accuracy" and "precision".  Are the leafs that claim to specify
>> > "accuracy" specifying a precision?  If so, the precision of a specific
>> > measurement, the precision of the measurements that led to the creation
>> > of the coordinate frame, or something else?  Are they doing so in
>> > relative terms (e.g., percentage) or absolute terms (e.g., degrees and
>> > meters)?  (There are "units" directives only for "height-accuracy" and
>> > not the others.)  How can we we say that we'll have 16 fraction-digits of
>> > precision for lat/long when the maximum accuracy we can say that a
>> > geodetic-system has only gives us 6 fraction-digits for coord-accuracy?
>> > When we say that the "precision of this measurement is indicated by the
>> > reference-frame" is that the same thing as the relevant "-accuracy"
>> > nodes, or something else?
>>
>> Yes, the geodesic-datum is what defines the values and their accuracy. For the
>> precision in the value we choose the fractional digits based on what might be
>> needed, but not to prescribe anything. For decimal degrees e.g., we only need
>> 100s values the rest can be left to the fractional portion.
>
> Unfortunately, even your description here still doesn't help me understand
> what the intended semantics of these values are.
>
> To help illustrate my confusion, here are a few possible things that could
> be what is intended to be conveyed:
>
> - the geodetic-datum description of the object has been measured to be
>   within a known delta of the actual object being described, at all points
>   on the object that the coordinate system can describe
>
> - the geodetic-datum description of the object is capable of determining
>   relative differences between points on the object to within a particular
>   delta of precision, but those individual coordinate values may be farther
>   than that delta from the actual point on the object that was referred to
>
> - the values that are reported in this YANG module reflect measurements
>   that were made and are known to be within some delta of the coordinate
>   system's value that they are reported as
>
> - the values that are reported in this YANG module reflect measurements
>   thare are known to be distinguishable from other measurements to within
>   some delta of other measurements relative to that coordinate system, even
>   though the actual position being indicated may diverge from the reported
>   value by more than that delta
>
> - the values that are reported in this YANG module reflect measurements
>   that were made and are known to be within some delta of the actual point
>   on the object that the coordinates refer to
>
> - the values that are reported in this YANG module reflect measurements
>   that were and are known to be distinguishable from other measurements of
>   points on that object within some delta, but the actual distance from the
>   measured point to the point on the object indicated by the reported
>   coordinates may be larger than that delta
>
> In short, there are at least three classes of things at play here: the
> actual object itself, the coordinate system used to model the object, and
> values reported in the YANG module (which are assumed to ultimately derive
> from some form of measurement).  To talk about accuracy or precision
> implies a relationship between elements of two of those classes, and I
> don't even know which of those classes you're trying to talk about.

Let's start with a simple baseline, if you want to dig any deeper than the well understood Lat+Long; do you know what a geodetic datum is? This is required knowledge if you want to get into anything more than the obvious Lat+Long use of this grouping. It defines the coordinates and also the accuracy of measurements.

It is out way out of scope for this YANG grouping to try and explain the huge field of geographic locations and geodetic datum and systems.

Thanks,
Chris.

>
>> > ----------------------------------------------------------------------
>> > COMMENT:
>> > ----------------------------------------------------------------------
>> >
>> > (I support Roman's Discuss.)
>> >
>> > Why do we only define velocity in terms of north/east/up, when we could
>> > be in x/y/z coordinates where there is no clear "north" or "east"?
>> >
>> > It would have been helpful for the shepherd review to point to the
>> > thread at
>> > https://mailarchive.ietf.org/arch/msg/netmod/dA9olZfEVa3clGdfvNYEFXUEMJw/
>> > that attempted to discuss the feedback from the yangdoctor review -- the
>> > mail with the review itself got no direct replies.
>>
>> This is a very edge case of this grouping meant really to handle something
>> like continental drift with long stored values. One can keep drilling down on
>> this particular velocity value seemingly forever, but then we aren't getting
>> our work done. I think it's enough to say that if the usable values don't work
>> for a use case at this point, then they don't work.
>
> Sure, that's most of why this is in the non-blocking COMMENT section.
>
>
> The rest of this (including the follow-up) all looks fine; thanks for the
> replies and updates!
>
> -Ben
>
>> > Section 2.1
>> >
>> >    In addition to the "geodetic-datum" value, we allow refining the
>> >    coordinate and height accuracy using "coord-accuracy" and "height-
>> >
>> > My understanding is that "refine" is a YANG keyword, and the current
>> > module/tree structure does not seem consistent with this description
>> > referring to use of the YANG keyword (since we can just set new values
>> > directly without needing to "refine" the YANG structure itself).  A
>> > different word here might be appropriate.
>>
>> Ok changed to "overriding".
>>
>>
>> >    Finally, we define an optional feature which allows for changing the
>> >    system for which the above values are defined.  This optional feature
>> >    adds an "alternate-system" value to the reference frame.  This value
>> >    is normally not present which implies the natural universe is the
>> >    system.  The use of this value is intended to allow for creating
>> >    virtual realities or perhaps alternate coordinate systems.  The
>> >    definition of alternate systems is outside the scope of this
>> >    document.
>> >
>> > This paragraph doesn't really convince me that we need to include the
>> > "alternate-system" capability in the proposed-standard version of this
>> > YANG module at this time.
>>
>> It doesn't hurt anything to include it and it was asked for by the person who
>> came up with the shape of this grouping (Peter L.). Unless there's a strong
>> objection I'd prefer to leave it in deference to the person who asked for it.
>>
>> > Section 2.3
>> >
>> >    meters per second.  The values "v-north" and "v-east" are relative to
>> >    true north as defined by the reference frame for the astronomical
>> >    body, "v-up" is perpendicular to the plane defined by "v-north" and
>> >    "v-east", and is pointed away from the center of mass.
>> >
>> > When I read this I wondered if the "plane defined by v-north and v-east"
>> > was taken at the initial snapshot position, or continuously updated with
>> > the effect of v-north and v-east drift.  Given the stated application,
>> > it's unlikely that it actually would matter, though, so it's not clear
>> > that we should change the text to cover it.
>> >
>> > Section 3
>> >
>> >                   and 91..126). The IANA registry further restricts the
>> >                   value by converting all spaces (' ') to dashes ('-')";
>> >
>> > Is there a reason why we shouldn't disallow spaces via the regex (and
>> > obviate the need for special processing at IANA)?
>>
>> The thinking is to allow users to do more than what we IANA is limited to.
>>
>> > Section 5.1.2
>> >
>> > The following subsection suggests that there is a "heading" field in the
>> > W3C structure/API, but I don't see one listed in Figure 1.
>> >
>> > Section 6.1
>> >
>> > What are suitable references for the "me" and "mola-vik-1" geoedtic
>> > systems?  I do not see how just the listed descriptions provide a "clear
>> > definition" even for the two coordinate values latitude/longitude.
>> >
>> > Section 7
>> >
>> > Thanks for using the template for security considerations for YANG
>> > models!  I think that since some of the portions of the template do not
>> > apply, they can safely be removed.  In particular, the "these are the
>> > subtrees and data nodes and their sensitivity/vulnerability" lines can
>> > go, and the clause about "can have a negative effect on network
>> > operations" may be worth tweaking (network operations may not be the
>> > most likely thing to be impacted).  I think it's also okay to drop the
>> > paragraph/sentence about RPCs.
>> >
>> > Section 8
>> >
>> > The [WGS84] and [EGM08] links don't work for me.  ([EGM96] does.)
>>
>> I've removed the links as they are not stable. Just going with standard title, author pub date now.
>>
>> > Section 9
>> >
>> > It seems like RFC 7950 is more properly classified as normative, since
>> > you can't really make sense of YANG without ... knowing YANG.  I think
>> > 8340 is sometimes listed as normative as well, but the case is not quite
>> > as clear, here.
>> >
>> > NITS
>> >
>> > Abstract
>> >
>> >    This document defines a generic geographical location object YANG
>> >    grouping.  [...]
>> >
>> > I'm having a hard time seeing what role the word "object" is playing
>> > here, especially since in the next sentence we just refer to the
>> > "geographical location grouping".
>>
>> Removed "object"
>>
>> > Section 3
>> >
>> >          description
>> >            "A location on an astronomical body (e.g., 'earth')
>> >             somewhere in a universe.";
>> >
>> > I guess in some alternate-systems the "astronomical body" bit may not
>> > really be accurate.  (And possibly in some cartesian coordinate frames,
>> > too, but that's less clear.)
>> >
>> >              type string {
>> >                pattern '[ -@\[-\^_-~]*';
>> >
>> > If I'm reading my table correctly, '^' and '_' are adjacent, so this
>> > rather-reader-unfriendly regex formulation can't even be justified as
>> > the minimal encoding.
>>
>> This has to do with working with limitations in the tools. The minimal encoding does not work unfortunately.
>>
>> >                 '67p/churyumov-gerasimenko (a comet). The value should
>> >                 be comprised of all lower case ASCII characters not
>> >                 including control characters (i.e., values 32..64, and
>> >                 91..126).  [...]
>> >
>> > "all lower case ASCII characters" inherently excludes control
>> > characters, so "all lower case ASCII characters not including control
>> > characters" is redundant.
>> > Also, that doesn't match up the listed range of values (or the regex).
>> > (Also^2, that doesn't match the given comet name, which has numbers and
>> > punctuation.)
>>
>> Changed to:
>>
>> "The ASCII value SHOULD have upper case converted to lower case characters and not include control characters (i.e., values 32..64, and 91..126). Any preceding 'the' in the name SHOULD NOT be included.";
>>
>> >                   for Cartesian coordinates. When coord-accuracy is
>> >                   specified, it overrides the geodetic-datum implied
>> >                   accuracy.";
>> >                   [...]
>> >                  "The accuracy of height value for ellipsoidal
>> >                   coordinates, this value is not used with Cartesian
>> >                   coordinates. When specified, it overrides the
>> >                   geodetic-datum implied default.";
>> >
>> > I suggest using parallel language for "when specified, overrides implied
>> > default".  (That is, "coord-accuracy" is currently explicitly named but
>> > "height-accuracy" is not.)
>>
>> Done.
>>
>> >
>> >            leaf v-up {
>> >              type decimal64 {
>> >                fraction-digits 12;
>> >              }
>> >              units "meters per second";
>> >              description
>> >                "v-up is the rate of change (i.e., speed) away from the
>> >                 center of mass.";
>> >
>> > "center of mass" may not be universally applicable, e.g., to cartesian
>> > coordinates, binary systems, extremely massive objects that are not the
>> > astronomical-body of the reference-frame.
>>
>> In this case the value is not relevant. Again, this isn't a big part of this
>> specification, it's meant to track things like continental drift. If it's not
>> useful as defined then it's simply not useful for that use.
>>
>> > Section 4
>> >
>> > We probably should expand CRS at/before first usage.
>>
>> Done, just under the table.
>>
>> > Section 6.1
>> >
>> >    Each entry should be sufficient to define the 3 coordinate values (2
>> >    if height is not required).  So for example the "wgs-84" is defined
>> >
>> > I'd suggest flipping the order, for "should be sufficient to define the
>> > 2 coordinate values, and to define height if height is required".
>>
>> Done.