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

[Ladislav Lhotka <[e-mail]>]

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
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> > > 
> 
> > > > 
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> > > 
> 
> > > >     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