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RE: RFC2518 (WebDAV) / RFC2396 (URI) inconsistency

From: Stefan Eissing <stefan.eissing@greenbytes.de>
Date: Wed, 21 Nov 2001 10:46:25 +0100
To: "Jim Whitehead" <ejw@cse.ucsc.edu>, <w3c-dist-auth@w3.org>
Message-ID: <NDBBKJABLJNMLJELONBKAENJDBAA.stefan.eissing@greenbytes.de>

> From: w3c-dist-auth-request@w3.org
> [mailto:w3c-dist-auth-request@w3.org]On Behalf Of Jim Whitehead
> Let me reconstruct how we got here.
> * WebDAV uses the "dav:" URI scheme for the name of XML elements used in
> protocol messages, and for the name of properties.

I try to summarize my knowledge of XML element names, so either
Julian can correct me, or everyone knows what we are talking about:

Unfortunately, the name of XML elements in XML 1.0 ist just the name
of the element as written in the document. So, the name of the elements
  <D:creationdate xmlns:D="DAV:"/>
  <x:creationdate xmlns:x="DAV:"/>
is "D:creationdate" and "x:creationdate". These two _names_ are not
equivalent for XML parsers!

In namespace-aware parsers, the name does not change. However such
parsers subdivide a name in two parts: namespace-prefix + localname.
This gives in our example:
    nsprefix        localname
    D               creationdate
    x               creationdate
NS-aware parsers require that each namespace-prefix be defined in
 a namespace declaration (as it is the case in the example). Note
that the names of the elements are still unchanged and they are still
not equivalent!

Now, XML-NS does a trick and defines equivalence for a new thing
called an expanded element type, consisting of localname and the
belonging namespace-URI. This gives in our example:
    localname       namespace-URI
    creationdate    DAV:          (which is no URI, unfortunately)
    creationdate    DAV:
which indeed are equivalent.

Coming back to WebDAV:

It is misleading to say that the name of the property
creationdate in WebDAV is DAV:creationdate. Because when you
serialize it to <D:creationdate xmlns:D="DAV:"/> the name
of the XML element is "D:creationdate".

Another misleading thing is to assume that the localname of
an XML element is appended to the namespace URI. As Julian pointed
out, this is neither defined, nor always possible, since allowed
character sets for XML element names and URIs differ. Example:
<D:prop-A xmlns:D="DAV:"/> would give DAV:prop-A which is not
a valid URI either.

And finally as for Jim's recommendation to XML-NS: requiring XML-NS
to accept DAV: in namespace URIs, means to give up the notion that
the namespace is an URI. Instead the namespace-thingie would be
an unstructured sequence of characters.


> * While the BNF for "dav:" URIs has never been explicitly defined
> (something
> we should do when we revise RFC 2518), if we had, its definition
> would be as
> follows:
> davuri = davscheme ":" davvalue
> davscheme = "dav"
> davvalue = opaque_part
> From RFC 2396 we know that the opaque_part is defined as:
> opaque_part   = uric_no_slash *uric
> That is, the "dav:" URI scheme can be considered a member of the class of
> non-hierarchical URIs described on page 12 of RFC 2396. In particular, it
> was never the intent that just the string "dav:" would be
> considered a full
> URI. The string "dav:" is a URI scheme name, not a URI. The string "dav:"
> plus a string matching the production "opaque_part" is a URI.
> * WebDAV marshals "dav:" URIs that are the name of XML elements as a
> {namespace} + {opaque_part} pair.  So, for example, "dav:creationdate" is
> <D:creationdate xmlns:D="dav:">.
> * The XML Namespace recommendation requires that the namespace
> identifier be
> a URI.
> * Since "dav:" scheme URIs are members of the class of non-hierarchical
> URIs, the only constant part is the URI scheme name itself,
> "dav:". From the
> definition of non-hierarchical URIs given in RFC 2396, ALL
> non-hierarchical
> URIs will share this quality. Since "dav:" is the only constant
> part, it is
> the only part of a "dav:" scheme URI suitable for use as the namespace
> identifier.
> To summarize:
> * The "dav:" URI scheme is perfectly legal according to RFC 2396.
> Therefore,
> no change is needed to RFC 2396.
> * It is only the use of the "dav:" URI scheme name as an namespace
> identifier that is violating any specification. Either RFC 2518 or the XML
> Namespaces specification could be changed to rectify this.
> In my opinion, it is natural to want to use the URI scheme name as the XML
> namespace identifier.  That is:
> <D:getcontentlength xmlns:D="DAV:">
> is more natural than:
> <D:getcontentlength xmlns:D="http://www.webdav.org/">
> Not to mention that it uses fewer bytes on the wire (not a huge
> consideration these days, but those bytes add up over millions of daily
> users).
> As a result, I recommend that the XML namespace recommendation be modified
> to allow the use of just the URI scheme name as a namespace identifier,
> perhaps limited to just members of the set of non-hierarchical URIs. It
> seems clear to me that the XML namespace recommendation was written with
> only the class of hierarchical URIs in mind, and as a result it's not too
> surprising that a glitch arose in the first use with
> non-hierarchical URIs.
> Based on Julian's experience, and our experience with multiple WebDAV
> implementations, accepting a URI scheme name as a namespace
> identifier would
> codify existing, interoperable, practice.
> - Jim
Received on Wednesday, 21 November 2001 04:44:51 UTC

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