On Tue, 2018-01-09 at 16:23 +0100, Martin Bjorklund wrote:
> Ladislav Lhotka <lhotka@xxxxxx> wrote:
> > On Tue, 2018-01-09 at 09:06 +0100, Martin Bjorklund wrote:
>
> > > Hi,
>
> > >
>
> > > Ladislav Lhotka <lhotka@xxxxxx> 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" <lhotka@xxxxxx> 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