Re: [yang-doctors] Yangdoctors last call review of draft-ietf-ospf-yang-09

[Martin Bjorklund <[e-mail]>]

Hi,

I think we can agree that the model in this I-D should use
derived-from-or-self() instead of string comparison, and conclude this
discussion here.  I suggest that if we need to further discuss the
representation of identityrefs, then we start a new thred on the
NETMOD ML.


/martin



Ladislav Lhotka <[e-mail]> wrote:
> On Tue, 2018-01-09 at 16:23 +0100, Martin Bjorklund wrote:
> > Ladislav Lhotka <[e-mail]> wrote:
> > > On Tue, 2018-01-09 at 09:06 +0100, Martin Bjorklund wrote:
> > 
> > > > Hi,
> > 
> > > > 
> > 
> > > > Ladislav Lhotka <[e-mail]> wrote:
> > 
> > > > > Hi Acee,
> > 
> > > > 
> > 
> > > > > 
> > 
> > > > 
> > 
> > > > > please see inline.
> > 
> > > > 
> > 
> > > > > 
> > 
> > > > 
> > 
> > > > > On Mon, 2018-01-08 at 19:28 +0000, Acee Lindem (acee) wrote:
> > 
> > > > 
> > 
> > > > > > Hi Lada,
> > 
> > > > 
> > 
> > > > > > 
> > 
> > > > 
> > 
> > > > > > Apologies for the delay. We somewhat got hung up on 4 and 6. See
> > inline.
> > 
> > > > 
> > 
> > > > > > 
> > 
> > > > 
> > 
> > > > > > On 12/6/17, 6:26 AM, "Ladislav Lhotka" <[e-mail]> wrote:
> > 
> > > > 
> > 
> > > > > > 
> > 
> > > > 
> > 
> > > > > > > Reviewer: Ladislav Lhotka
> > 
> > > > 
> > 
> > > > > > > Review result: Ready with Issues
> > 
> > > > 
> > 
> > > > > 
> > 
> > > > 
> > 
> > > > > ...
> > 
> > > > 
> > 
> > > > > 
> > 
> > > > 
> > 
> > > > > > > 
> > 
> > > > 
> > 
> > > > > > > 3. Maybe the draft could mention that implementations should supply
> > a
> > 
> > > > 
> > 
> > > > > > >   default routing domain as a system-controlled resource.
> > 
> > > > 
> > 
> > > > > > 
> > 
> > > > 
> > 
> > > > > > Isn’t this more of an RFC8022BIS statement? I guess we could state
> > this as
> > 
> > > > 
> > 
> > > > > > an assumption.
> > 
> > > > 
> > 
> > > > > 
> > 
> > > > 
> > 
> > > > > Probably, but it is not a YANG issue, so I'd leave it to you routing
> > folks
> > 
> > > > to
> > 
> > > > 
> > 
> > > > > decide.
> > 
> > > > 
> > 
> > > > > 
> > 
> > > > 
> > 
> > > > > >  
> > 
> > > > 
> > 
> > > > > > 
> > 
> > > > 
> > 
> > > > > > > 4. In "when" expressions, the module uses literal strings for
> > 
> > > > 
> > 
> > > > > > >   identities. This is known to be problematic, the XPath functions
> > 
> > > > 
> > 
> > > > > > >   derived-from() or derived-from-or-self() should be used instead.
> > 
> > > > 
> > 
> > > > > > 
> > 
> > > > 
> > 
> > > > > > Why is this problematic? Is it because the types can be extended?
> > 
> > > > 
> > 
> > > > > 
> > 
> > > > 
> > 
> > > > > That's one reason: derived identities should often also satisfy the
> > 
> > > > constraint.
> > 
> > > > 
> > 
> > > > > 
> > 
> > > > 
> > 
> > > > > But the more serious problem is that things like
> > 
> > > > 
> > 
> > > > > 
> > 
> > > > 
> > 
> > > > >     when "../../../../../../../rt:type = 'ospf:ospfv3'"
> > 
> > > > 
> > 
> > > > > 
> > 
> > > > 
> > 
> > > > > rely on plain string comparison that depends od the actual prefix used
> > for
> > 
> > > > the
> > 
> > > > 
> > 
> > > > > "rt:type" value. For one, according to RFC 7951 the JSON encoding of
> > this
> > 
> > > > value
> > 
> > > > 
> > 
> > > > > would be "ietf-ospf:ospfv3" so the above expression is always false. 
> > 
> > > > 
> > 
> > > > 
> > 
> > > > This is not correct; the when expression is not evaluated on the JSON
> > 
> > > > encoding.  See the last paragraph of section 9.10.3 in RFC 7950:
> > 
> > > > 
> > 
> > > >    The string value of a node of type "identityref" in a "must" or
> > 
> > > >    "when" XPath expression is the referred identity's qualified name
> > 
> > > >    with the prefix present.  If the referred identity is defined in an
> > 
> > > >    imported module, the prefix in the string value is the prefix defined
> > 
> > > >    in the corresponding "import" statement.  Otherwise, the prefix in
> > 
> > > >    the string value is the prefix for the current module.
> > 
> > > 
> > 
> > > This is weird, to say the least. The leafref instance may have an identity
> > value
> > 
> > > that is defined in a module that (has to be implemented by the server but)
> > 
> > > needn't be imported in the module that contains the XPath expression. So I
> > don't
> > 
> > > know what 'corresponding "import" statement' this paragraph is talking
> > about.
> > 
> > 
> > It has to import the module in order to give a prefix, which then can
> > be used in the XPath expression.
> 
> In the XPath expression above, do you mean the "rt" prefix of "rt:type"? If so,
> it is irrelevant for the string comparison, what's important is the *value* of
> the "rt:type" instance, which can be an identity defined in a module that
> needn't be imported by ietf-routing, ietf-ospf or whatever. Sec. 9.10.2:
> 
>    On a particular server, the valid values are further restricted to the set of
>      identities defined in the modules implemented by the server.
> 
> > 
> > > Also, potentially there can be a collision in prefixes and then this also
> > breaks
> > 
> > > down.
> > 
> > 
> > No, two modules cannot be imported with the same prefix.
> 
> I have to disagree. An identity derived from the "ietf-routing:control-protocol-
> type" base identity can be defined in a module that is not imported anywhere. If
> a server declares such a module as implemented, then "rt:type" may have this
> value per sec. 9.10.2.
> 
> And, consequently, there may be two different modules with conflicting prefixes
> defining identities that are derived from "ietf-routing:control-protocol-type". 
> 
> > 
> > > A moral of the namespace/prefix story in XML was that relying of namespace
> > 
> > > prefixes having a particular value is a really bad idea. I know that the
> > cited
> > 
> > > paragraph was intended to make such XPath string comparisons more
> > deterministic,
> > 
> > > but it is also problematic and should be avoided if possible.
> > 
> > 
> > Note that this prefix is under the control of the module designer
> > writing the XPath expression.  The same identityref value might use a
> 
> No, it is not. The prefixes appear in instance data.
> 
> Lada
> 
> > different prefix in some other module.
> > 
> > 
> > /martin
> > 
> > 
> > 
> > > 
> > 
> > > Lada
> > 
> > > 
> > 
> > > > 
> > 
> > > > So the equality test of the identityref is correct.
> > 
> > > > 
> > 
> > > > However, I agree that in most cases 'derived-from-or-self' should be
> > 
> > > > used, in order to handle derived identities.
> > 
> > > > 
> > 
> > > > 
> > 
> > > > /martin
> > 
> > > -- 
> > 
> > > Ladislav Lhotka
> > 
> > > Head, CZ.NIC Labs
> > 
> > > PGP Key ID: 0xB8F92B08A9F76C67
> > 
> > > 
> > 
> -- 
> Ladislav Lhotka
> Head, CZ.NIC Labs
> PGP Key ID: 0xB8F92B08A9F76C67
>