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

[Martin Bjorklund <[e-mail]>]

Hi,

The second argument must be a string, so you should add quotes:

  must "derived-from( "
     + "/rt:routing/rt:control-plane-protocols/"
     + "rt:control-plane-protocol[rt:name=current()]/"
     + "rt:type, 'ospf:ospf-protocol')";
    -------------^------------------^   


/martin


"Acee Lindem (acee)" <[e-mail]> wrote:
> Martin, Lada,
> 
> I believe this got lost with all the discussion of schema mount/yang
> library during roughly the same time frame.  You have both advocated
> using derived-from()/dervived-from-or-self() for ietf-ospf.yang
> checking. However, either I have misinterpreted the RFC 7950
> description of these YANG functions or the yangvalidator validation is
> broken. Please see below.
> 
> Thanks,
> Acee 
> 
> On 1/22/18, 1:42 PM, "Acee Lindem (acee)" <[e-mail]> wrote:
> 
>     Hi Martin, Lada, 
>     
>     In experimenting with this, I get YANGLINT validation errors. I’m not
>     sure what I’m missing but the first argument to
>     derived-from()/derive-from-or-self() is the schema node and the second
>     is the identity – correct?
>     For example, the following YANG leaf:
>     
>     identity ospf-protocol {
>         base "rt:routing-protocol";
>         description "Any version the OSPF protocol";
>       }
>     
>       identity ospfv2 {
>         base "ospf-protocol";
>         description "OSPFv2";
>       }
>     
>       identity ospfv3 {
>         base "ospf-protocol";
>         description "OSPFv3";
>       }
>     
>     leaf routing-protocol-name {
>           type leafref {
>             path "/rt:routing/rt:control-plane-protocols/"
>                + "rt:control-plane-protocol/rt:name";
>           }
>           must "derived-from( "
>     	+ "/rt:routing/rt:control-plane-protocols/"
>             + "rt:control-plane-protocol[rt:name=current()]/"
>             + "rt:type, ospf:ospf-protocol)";
>           description
>            "OSPF routing protocol instance name.";
>         }
>     
>     Gives me: 
>     
>     warn: Schema node "ietf-ospf:ospf-protocol" not found (derived-from(
>     /ietf-routing:routing/ietf-routing:control-plane-protocols/ietf-routing:control-plane-protocol[ietf-routing:name=current()]/ietf-routing:type,
>     ietf-ospf:ospf-protocol) with context node
>     "/ietf-ospf:if-state-change/routing-protocol-name".
>     
>     Thanks
>     Acee 
>     
>     On 1/10/18, 4:36 AM, "Martin Bjorklund" <[e-mail]> wrote:
>     
>         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
>         > 
>         
>     
>     
> 
>