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

[Ladislav Lhotka <[e-mail]>]

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.

Also, potentially there can be a collision in prefixes and then this also breaks
down.

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. 

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