Re: [netmod] Use XML namespaces in YANG document examples

[Tim Bray <[e-mail]>]

Hi everyone, I'm the person on the XML directorate who raised this issue in
conversation with Ian Farrer in a review of draft-ietf-dhc-dhcpv6-yang.
After a lot of back and forth, I think we came to agreement on the
necessary language to address this issue.  I believe that language has.
been included in draft-ietf-i2nsf-nsf-monitoring-data-model, see
https://datatracker.ietf.org/doc/html/draft-ietf-i2nsf-nsf-monitoring-data-model-13#section-10

I've excerpted an email exchange with Ian Farrer that I think makes the
potential problem concrete:

Hi Ian, I don't think we've met.  I'm the grumpy person on the "XML
Directorate" who's been whining about the namespace prefixes in YANG
internet-drafts. One quick issue: I'm a little surprised, is anyone still
using XML in this kind of thing any more in 2021?

Anyhow, below I've excerpted the issue that's still troubling me. Here's
the XML:

 <interfaces xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces"
     xmlns:ianaift="urn:ietf:params:xml:ns:yang:iana-if-type">
     <interface>
       <name>eth0</name>
       <type>ianaift:ethernetCsmacd</type>
       <description>DHCPv6 Relay Interface</description>
       <enabled>true</enabled>
     </interface>
   </interfaces>

So my question is, I see the XML namespace prefix and the prefix for the
<type> element content are identical. Is this a coincidence?  For example,
would the following work, changing the namespace prefix to "foo"?


 <interfaces xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces"
     xmlns:foo="urn:ietf:params:xml:ns:yang:iana-if-type">
     <interface>
       <name>eth0</name>
       <type>ianaift:ethernetCsmacd</type>
       <description>DHCPv6 Relay Interface</description>
       <enabled>true</enabled>
     </interface>
   </interfaces>

[if - This example would not work and fails validation with yanglint:

$ yanglint --strict --verbose -t config -p $IETFYANG
$IETFYANG/iana-if-type.yang $IETFYANG/ietf-interfaces.yang test1.xml
err : Invalid value "ianaift:ethernetCsmacd" in "type" element.
(/ietf-interfaces:interfaces/interface[name='eth0']/type)
]


Follow-up, would the following work, foo for both namespace and content
prefix?

<interfaces xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces"
     xmlns:foo="urn:ietf:params:xml:ns:yang:iana-if-type">
     <interface>
       <name>eth0</name>
       <type>foo:ethernetCsmacd</type>
       <description>DHCPv6 Relay Interface</description>
       <enabled>true</enabled>
     </interface>
   </interfaces>

Thanks in advance!


[if - This does validate with yanglint, however the convention in the IETF
examples I’ve seen seems to be to use the prefix that is defined in the
original YANG module for imports for consistency, e.g. (from
iana-if-type.yang):


On Thu, Feb 3, 2022 at 8:03 AM Andy Bierman <[e-mail]> wrote:

> Hi,
>
> I think the text from sec 4 refers to the usage within an application.
> The XML instance document is the on-the-wire representation and
> the I-D example looks correct.
>
> https://www.w3.org/TR/xml-names/#ns-qualnames
>
>
> Andy
>
>
> On Thu, Feb 3, 2022 at 3:53 AM tom petch <[e-mail]> wrote:
>
>> From: netmod <[e-mail]> on behalf of [e-mail] <
>> [e-mail]>
>> Sent: 03 February 2022 09:37
>>
>> Hi,
>>
>> A draft I have been working on (
>> https://datatracker.ietf.org/doc/draft-ietf-dhc-dhcpv6-yang/) contains a
>> number of XML configuration examples. During the XML expert review, a
>> question has been raised about the use of XML namespaces in these examples.
>> I’m raising it here as I don’t have the XML knowledge to answer.
>>
>> <tp>
>>
>> Ian
>>
>> This looks like the issue I raised on this list 14jan2022 with a subject
>> line of
>> XML and prefix
>> although I have not checked that the usage is exactly the same; the 'XML
>> Expert' comment would appear to be.
>>
>> Tom Petch
>>
>> In my example:
>>
>> <interfaces xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces"
>>
>>      xmlns:ianaift="urn:ietf:params:xml:ns:yang:iana-if-type">
>>      <interface>
>>        <name>eth0</name>
>>        <type>ianaift:ethernetCsmacd</type>
>>        <description>DHCPv6 Relay Interface</description>
>>        <enabled>true</enabled>
>>      </interface>
>>    </interfaces>
>>
>> The question is related to the use of the ‘ianaift:’ prefix. This is
>> quite commonly use in XML examples in YANG documents (e.g. RFC8344) so I
>> think the question is generally applicable.
>>
>> The specific comments from the expert review are:
>>
>> -
>> For the correct processing of these documents requires that whatever XML
>> software is being used makes available to application code the namespace
>> prefixes.
>>
>> Whilst the recommended tools (e.g. yanglint) provides this function, it
>> is not an XML best practice. Quoting from the Namespaces in XML, section 4:
>> "Note that the prefix functions only as a placeholder for a namespace name.
>> Applications SHOULD use the namespace name, not the prefix, in constructing
>> names whose scope extends beyond the containing document.”
>>
>> I think that violating a SHOULD assertion in a W3C standard is a problem.
>>
>> There is no requirement for XML processors to provide this prefix
>> information, and software that (quite legally) doesn't, will not work
>> correctly with YANG documents constructed as specified in this I-D.
>>
>> 1, YANG specifications should note this fact and specify that software
>> which is used to process YANG documents MUST provide an interface such that
>> applications can retrieve the prefix-namespace mappings.
>> 2, For constructs such as <type>ianaift:ethernetCsmacd</type> the
>> Internet-Draft should specify that the prefix ("ianaift" in this case) MUST
>> be identical to the xmlns namespace prefix representing the namespace name
>> urn:ietf:params:xml:ns:yang:iana-if-type
>> 3, Alternately, the draft could specify that for the namespace
>> urn:ietf:params:xml:ns:yang:iana-if-type, the XML namespace prefix ianaift
>> MUST be used. Another XML bad practice because software that generates XML
>> programmatically should feel free to generate synthetic prefixes without
>> breaking the content, but at least this would solve the problem.
>> -
>>
>> BCP216 (RFC8407 - Guidelines for Authors and Reviewers of Documents
>> Containing YANG modules) doesn’t make any mention of how XML namespaces
>> should be used, only that example XML/ JSON should be included and that
>> these examples need to be validated (pyang and yanglint are mentioned for
>> this).
>>
>> Does this guidance need to be updated to reflect expert review comments
>> above?
>>
>> Thanks,
>> Ian
>>
>>
>>
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>