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XML Validity vs Extensibility

From: Lisa Dusseault <lisa@xythos.com>
Date: Fri, 1 Dec 2000 08:37:04 -0800
To: "James J. Hunt \(by way of \"Ralph R. Swick\" <swick@w3.org>\)" <jjh@ira.uka.de>, <ietf-dav-versioning@w3.org>, <w3c-dist-auth@w3.org>
Changing the syntax of the PROPFIND and PROPPATCH in the manner you suggest
would break current implementations.  This would change the bits on the wire
for all PROP* messages, even for ones which include no extensions.

While it's arguable that it would be nice to have required valid XML when
RFC 2818 was written, changing the bits on the wire for Proposed Standard
protocols isn't done except for worse problems (or much smaller changes)
than this.

As far as validating XML to check interoperability, feel free:  RFC2518
includes a DTD, and implementors are welcome to check their implementation
against the DTD by validating XML bodies.

The cat's already out of the bag and can't be put back in.


> -----Original Message-----
> From: "James J. Hunt" <jjh@ira.uka.de>
> To: ietf-dav-versioning@w3.org
> CC: w3c-dist-auth@w3.org, reuterj@ira.uka.de
> Message-ID: <"i41fs3.ira.0039901:001201.012827"@ira.uka.de>
> Dear Colleges,
> >From the experience we have gathered trying to implement the DeltaV
> protocol, we have some basic comments and suggestions to make before the
> end of the last call period for the DeltaV protocol.  We regard these
> issues to be important for the future development of DeltaV and WebDAV.
> This message is cross posted to the WebDAV mailing list because the main
> points pertain as much to WebDAV as to DeltaV.
> Valid XML should be used in WebDAV instead of simply well formed XML
> ====================================================================
> Using simply well formed XML misses half the benefit of XML: the
> automatic detection of syntactic errors.  This is the best way to insure
> that a protocol stays open and correct.  Any client or server can be
> easily tested for at least syntactic conformance.  This does not means
> that every transaction must be validated, but ensuring that it can is
> important.
> On the WebDAV list, there has been some discussion about validating XML
> for WebDAV, for example, see
> http://lists.w3.org/Archives/Public/w3c-dist-auth/1999OctDec/0242.html.
> There seemed to be some agreement that the current WebDAV specification
> uses a syntax that is well-formed, but is not suitable for XML
> validation, and that it would not be too hard to make WebDAV to conform
> to XML validation rules.  However, some participants did not agree on
> the advantages of validation.
> Interoperability is one of the key requirement on Internet protocols for
> supporting a heterogeneous network.  This holds true for the WWW and its
> protocols including HTTP, WebDAV, and DeltaV.  As WebDAV matures several
> extension protocols are emerging, among them the bindings and
> redirect-reference protocols, DeltaV, the access control lists
> specification, and the advanced collections protocol.  All these
> extension protocols contribute to the XML syntax of WebDAV.  Moreover,
> it is quite likely that vendors will introduce proprietary extensions to
> the XML syntax.  For an idea of the extent to which this can occur, one
> just need look at the source code of the DeltaV specification at
> http://www.webdav.org/deltav/protocol/draft-ietf-deltav-versioning
> -10.5.htm
> and notice all the tags inserted by Word that are not part of the HTML
> specification.
> All in all, the XML syntax of WebDAV can no longer be viewed as being
> simple.  Validation can help us to identify bugs in WebDAV applications.
> Actually, this has already help us to find some bugs in the WebDAV
> specification itself (e.g. WebDAV issues DTD_BOOBOO, MISSING_NS_SPEC,
> cp. http://www.ics.uci.edu/pub/ietf/webdav/protocol/issues.html).
> Validation may also force implementors to handle XML more carefully and
> thus improve reliability.
> Note that we do not want to force a WebDAV implementation to validate
> XML.  We only suggest letting the implementor decide if and when to use
> validation.  Currently, this is not possible without modifying an
> existing XML parser or writing a non-conforming XML parser.  Therefore,
> we strongly suggest to fix the WebDAV specification and its extension
> protocols like DeltaV to conform to the XML specification--including
> XML validation.  Then an implementor may choose when to apply
> validation.
> Another problem is to track version information about the protocols.
> The DAV header as returned from an OPTIONS request (e.g.: "DAV: 1, 2,
> version-selector-checkout") does not help at all.  However, when using
> a DTD, the client and the server could agree on a protocol revision
> (or otherwise stop communication with an appropriate error message or
> log) by using the DTD's header information.
> Making WebDAV properties suitable for XML validation
> ====================================================
> WebDAV currently expresses resource properties through a variety of
> tags, for example:
> <D:displayname xmlns:D="DAV:">Hello, world!</D:displayname>
> In this example, the name of the property is "DAV:displayname", and its
> value is "Hello, world!".  This approach has some disadvantages.
> First of all, if you want to list property names only, you currently
> have to do something like:
> <D:displayname xmlns:D="DAV:"/>
> But, in an XML DTD, an element can not be defined to have empty
> content and non-empty content at the same time.  Hence, the way WebDAV
> currently handles properties is not suitable for validating XML.
> This problem has already been discussed as anomaly in the DAV:prop
> element usage (see
> http://lists.w3.org/Archives/Public/w3c-dist-auth/2000JanMar/0029.html).
> The simple solution here is to model the property name as an element
> attribute value or as PCDATA of a generic tag for all properties:
> Solution using XML attributes:
> - ------------------------------
> <!ELEMENT property (property-name | property-value)* >
> <!ELEMENT property-name EMPTY>
> <!ELEMENT property-value ANY>
> <!ATTLIST property-name
>           name  CDATA   #REQUIRED>
> <!ATTLIST property-value
>           name  CDATA   #REQUIRED>
> Example:
> <D:property xmlns:D="DAV:">
>   <D:property-name name="displayname"/> <!--specifying property
> name only-->
>   <D:property-value name="source">      <!--specifying property name &
> value-->
>     <D:href>http://www.foo.bar</D:href>
>   </D:property-value>
> </D:property>
> Alternative solution without use of XML attributes:
> - ---------------------------------------------------
> <!ELEMENT property (property-name, property-value?)* >
> <!ELEMENT property-name (#PCDATA)>
> <!ELEMENT property-value ANY>
> Example:
> <D:property xmlns:D="DAV:">
>   <D:property-name>displayname</D:property-name> <!--property name only-->
>   <D:property-name>source</D:property-name>      <!--property
> name & value-->
>   <D:property-value>
>     <D:href>http://www.foo.bar</D:href>
>   </D:property-value>
> </D:property>
> DTD for DeltaV
> ==============
> Having all of the above said, DeltaV should, similarly to WebDAV, of
> course specify an adequate DTD for all elements introduced in DeltaV.
> The DTD need not be sent with every message.  A reference to a URL at
> www.webdav.org would be sufficient.
> Labels in DeltaV
> ================
> One of the recent changes in DeltaV was to remove labels from the core
> versioning part and put them into the options part of the protocol.
> We strongly suggest to undo this change.  Even if there exist two or
> three revision control systems that do not use labels, labels are
> essential for identifying sets of associated files in a repository of
> versions.  And, actually, they are really easy to implement
> (especially on top of a WebDAV implementation that requires built-in
> mechanisms for property storage).
> Sincerely,
> James J. Hunt
> Jürgen Reuter
> P.S. I will be at the IETF meeting in San Diego from the evening of the
>      10th to the early morning of the 15th to discuss these issues.
>      --- JJHunt
> P.P.S I will send a second mail pertaining to smaller points we have
>       discussed here that concern only DeltaV.
>       --- Jürgen Reuter
Received on Friday, 1 December 2000 12:05:31 UTC

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