An open discussion list for topics related to the eduGAIN interfederation service.

[Janusz Ulanowski <janusz.ulanowski AT heanet.ie>]

On 26/02/16 13:05, Olav Morken wrote:
> On 2016-02-26 11:50, Ian Young wrote:
> > > On 26 Feb 2016, at 10:09, Peter Schober <peter.schober AT univie.ac.at>
> > > wrote:
> > >
> > > Namespaces don't need to be defined in the root element, they can be
> > > defined on the element using the element or attribute itself, or even
> > > in appropriate parent elements, IIRC.
> >
> > Part of the confusion in this area is because the terminology is
> > confusing. Although we often say that this is a "namespace
> > declaration"...
> >
> >      xmlns:foo="bar"
> >
> > ... in fact what we mean is that it is a declaration of the namespace
> > prefix foo to refer to the namespace name bar. The namespace name
> > itself is what counts for XML processors, not the prefix. Indeed, you
> > can declare multiple prefixes at different levels of the document to
> > represent the *same* namespace name. Similarly, you can use a
> > namespace name without any prefix at all:
> >
> >     <element xmlns="bar"/>
> >
> > None of this should matter for validation or any other processing,
> > just the <namespace name, local name> assigned to each element and
> > attribute.
>
> Actually, there is a place where this breaks: Where namespace prefixes
> are used in attribute values. Consider the following example:
>
>    <md:RoleDescriptor xmlns:foo="urn:foo" xsi:type="foo:SomeType" [...]>
>
> If "foo" was changed to something else, this would obviously break.
> There are other places where the prefixes are used as well, such as the
> PrefixList attribute used in XML signatures. (Though that is admittedly
> less interesting to preserve.)
>
> The problem is that without knowing the schema of the element that you
> are processing, you cannot know if any prefixes are significant for that
> element.
>
> In the XML metadata processing code of SimpleSAMLphp, I ended up solving
> this by grabbing every active prefix whenever we find an unknown XML
> element (<md:RoleDescriptor>, an unknown extension, etc.). It will then
> serialize out the element with those prefixes added.
>
> For example:
>
>    <ns:EntitiesDescriptor xmlns:ns="urn:oasis:names:tc:SAML:2.0:metadata">
>      <ns:Extensions>
>        <foo:Foo xmlns:foo="urn:foo" />
>      </ns:Extensions>
>    </ns:EntitiesDescriptor>
>
> Would become:
>
>    <md:EntitiesDescriptor xmlns:md="urn:oasis:names:tc:SAML:2.0:metadata">
>      <md:Extensions>
>        <foo:Foo xmlns:ns="urn:oasis:names:tc:SAML:2.0:metadata"
> xmlns:foo="urn:foo" />
>      </md:Extensions>
>    </md:EntitiesDescriptor>
>
> It is ugly, but has a high chance of success. Extra namespace prefixes
> should never hurt, but missing namespace prefixes may create problems.
>
> Best regards,
> Olav Morken
> UNINETT / Feide

Thanks very much to all for your responses

--
Janusz