One document matched: draft-ietf-webdav-properties-extension-00.txt
WebDAV J. Amsden
Internet Draft IBM
Document: draft-ietf-webdav-properties-extension-00.txt September 1999
Category: Informational
Expires: March, 2000
Proposed Extensions to WebDAV Properties
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1].
Internet-Drafts are working documents of the Internet Engineering
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progress."
The list of current Internet-Drafts can be accessed at
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1. Abstract
The WebDAV protocol defines facilities for adding properties to Web
resources. These properties consist of name/value pairs interchanged
using XML. The protocol includes the PROPFIND method for accessing
properties, and the PROPPATCH method for updating and removing
properties. Experience building both WebDAV clients and a WebDAV
server indicates the need for three minor extensions to the current
properties protocol: 1) allow PROPPATCH to create and initialize the
properties of a resource that did not exist, 2) distinguish between
adding a new property, and setting the value of an existing property
of a resource, and 3) give client applications more control in
specifying how PROPPATCH errors should be handled.
2. Conventions used in this document
Since this document describes a set of extensions to the HTTP/1.1
protocol, the augmented BNF used here to describe protocol an element
is exactly the same as described in Section 2.1 of [HTTP]. Since
this augmented BNF uses the basic production rules provided in
Section 2.2 of [HTTP], these rules apply to this document as well.
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In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC-2119 [2].
3. Terminology
The terminology used here extends that defined in the WebDAV
Distributed Authoring Protocol specification [WebDAV]. Definitions of
the terms resource, Uniform Resource Identifier (URI), and Uniform
Resource Locator (URL) are provided in [URI].
Null Property - A property that responds with 404 (Not Found) to a
PROPFIND method. A null property MUST not appear in the list of
properties of a resource the PROPFIND is specified with DAV:allprop.
4. Introduction
This Internet Draft specification introduces three simple extensions
or modifications to the handling of WebDAV properties. All are based
on experience gained through building a WebDAV class 2 server, and
sample client applications. The extensions are all compatible with
existing WebDAV property semantics, and all involve property updates
through the PROPPATCH method.
4.1 PROPPATCH on a Null Resource
HTTP and WebDAV do not specify methods for creating resources; they
are created as the side effect of other methods. For example, PUT on
a null resource (a resource that does not exist) creates the resource
and establishes its initial contents. LOCK on a null resource creates
a lock-null resource, which has properties, but no content. COPY and
MOVE can create resources at the destination URI. It is often
necessary to create resources and initialize their state before any
other access is allowed in order to ensure the integrity of the
operations. WebDAV currently has no protocol for creating and
initializing the properties of a resource in a single method. This
Internet Draft proposes that PROPPATCH on a null resource creates the
resource with empty contents, and initializes its properties based on
the propertyupdate elements in the request entity body.
The WebDAV spec is currently somewhat silent about PROPPATCH on a
null resource. It does say that PROPPATCH on a lock null resource (a
resource created by the LOCK method) will fail, but this is
inconsistent with PUT which is allowed, and changes the state of the
resource from lock null to resource. [HTTP] uses status 404, Not
Found, to indicate the server has not found anything matching the
Request-URI. It does not say that the server cannot create a resource
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at the Request-URI. The HTTP and WebDAV PUT method on a null resource
is defined to create the resource and set its initial contents.
Allowing PROPPATCH on a null resource to create the resource and set
its initial properties is consistent with the PUT semantics, and
other resource state changing methods that create resources as a side
effect.
4.2 Adding New Properties
Typical database management systems distinguish three kinds of
updates; add, remove, and change. It is often necessary to
distinguish between add and change in order to ensure a property is
added only once, and that its value is not inadvertently changed
after it has been initialized.
The WebDAV PROPPATCH method updates a resource's properties based on
the instructions contained in the DAV:propertyupdate element of the
request entity body. The propertyupdate element currently supports
setting and removing properties. DAV:set creates a new property if it
does not exist, and sets its value. If the property already exists,
the value is changed.
This Internet Draft proposes the addition of a DAV:add propertyupdate
that adds a new property and sets its value, but fails if the
property already exists. In addition, this draft proposes that
DAV:set on a null property (a property that does not exist) fails
instead of creating a new property. This may help prevent erroneous
client property updates resulting from setting the values of
properties where the property name is incorrectly specified.
4.3 Handling PROPPATCH Errors
Client applications currently use PROPPATCH to update the properties
of a resource. PROPPATCH supports the update of a number of
properties in a single method request in order to reduce
communication overhead and maintain resource state integrity with
atomic updates.
Currently, the WebDAV spec dictates that the result of a PROPPATCH is
an atomic transaction. That is, either all the updates succeed, or
none of them succeed and the updates are rolled back. This can result
in excessive client burden. A client application may attempt to make
a number of updates with PROPPATCH, and doesn't care if some of them
don't succeed. This is often the case when some of the properties are
live on the server, but the client has no way to determine which are
live without attempting to update their value and getting an error.
This situation arises when different servers support different live
properties. Updating the desired properties, ignoring the ones that
are live requires the client to attempt the update, examine the
DAV:multistatus response that is returned, construct a new PROPPATCH
entity request body removing the properties that failed, and re-
submitting the PROPPATCH request.
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This Internet Draft proposes that a mechanism be added to the
DAV:propertyupdate element that gives client applications more
control over updating resource properties by allowing the protocol to
specify how to handle errors on each of the updates in the
DAV:propertyupdate of a PROPPATCH entity request body. This is
similar to the DAV:propertybehavior element of the COPY and MOVE
entity request body that is used to specify how properties are copied
to the destination resource.
By making this change to the PROPPATCH entity request body, it is now
possible to completely specify the semantics of COPY and MOVE in
terms of the more primitive GET, PUT, PROPFIND, and PROPPATCH
methods. This is not currently possible because there is no way to
process the DAV:propertybehavior element of the COPY or MOVE entity
request body with a PROPPATCH method.
5. PROPPATCH Protocol Changes
This section specifies changes to section 8.2 PROPPATCH of [WebDAV]
and section 12 to support the semantics described above.
Update section 8.2 PROPPATCH to indicate that PROPPATCH on a null
resource creates the resource at the given Request-URI, and
initializes its properties as specified in the DAV:propertyupdate
element of the PROPPATCH entity request body.
Change section 8.2 PROPPATCH to include simple error processing of
PROPPATCH method requests. Update the DAV:set and DAV:remove elements
to include information describing how the client wishes to handle
errors. Change the semantics of DAV:set to fail on a null property.
Change section 12.13.2 set XML element to:
12.13.2 set XML element
Name: set
Namespace: DAV:
Purpose: List the DAV property values to be set for a
resource.
Description: The set XML element MUST contain only a prop XML
element. The elements contained by the prop XML element inside the
set XML element MUST specify the name and value of properties that
are set on the resource identified by the Request-URI. If the
property does not exist then a 404 (Not Found) status is returned,
and the property update fails. The DAV:updatebehavior element
specifies how update failures are to be handled. Language tagging
information in the property's value (in the "xml:lang" attribute, if
present) MUST be persistently stored along with the property, and
MUST be subsequently retrievable using PROPFIND.
<!ELEMENT set (prop, updatebehavior?) >
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Change section 12.13.1 remove XML element to:
12.13.1 remove XML element
Name: remove
Namespace: DAV:
Purpose: List the DAV property values to be removed from a
resource.
Description: Remove instructs that the properties specified in the
prop should be removed. Specifying the removal of a null property is
not an error. All the XML elements in a prop XML element inside of a
remove XML element MUST be empty, as only the names of properties to
be removed are required. The DAV:updatebehavior element specifies how
update failures are to be handled.
<!ELEMENT remove (prop, updatebehavior?) >
Extend the DAV:propertyupdate element of the PROPPATCH entity request
body to include element DAV:add. The DAV:add element has the same
contents as DAV:set.
Adding a property fails with 403 (Forbidden) if the property already
exists. Setting a null property results in a 404 (Not Found) status.
Change to section 12.13 propertyupdate XML element to:
12.13 propertyupdate XML element
Name: propertyupdate
Namespace: DAV:
Purpose: Contains a request to alter the properties on a
resource
Description: This XML element is a container for the information
required to modify the properties on the resource. This XML element
is multi-valued to support adding, changing, and/or removing
properties.
<!ELEMENT propertyupdate (add | set | remove) >
Add section 12.13.3, add XML element as follows:
12.13.3 add XML element
Name: add
Namespace: DAV:
Purpose: List the DAV property values to be added to a
resource.
Description: The add XML element MUST contain only a prop XML
element. The elements contained by the prop XML element inside the
add XML element MUST specify the name and value of properties that
are to be added to the resource identified by the Request-URI. If a
property already exists then a 403 (Forbidden) status is returned,
and the property is not added. The DAV:updatebehavior element
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specifies how update failures are to be handled. Language tagging
information in the property's value (in the "xml:lang" attribute, if
present) MUST be persistently stored along with the property, and
MUST be subsequently retrievable using PROPFIND.
<!ELEMENT add (prop, propertyupdate?) >
Add sections to section 12.13 to specify error processing on property
updates.
12.13.4 updatebehavior XML element
Name: updatebehavior
Namespace: DAV:
Purpose: Specify how errors on property updates are to be
handled.
Description: An updatebehavior specifies how errors are to be handled
on the corresponding DAV:add, DAV:set, or DAV:remove propertyupdate.
The client can specify that the error should be ignored, or that it
must succeed. If DAV:mustsucceed is specified, then the PROPPATCH
method will fail if any of the updates fail. This is the default
behavior.
<!ELEMENT updatebehavior (ignore | mustsucceed) >
12.13.5 ignore XML element
Name: ignore
Namespace: DAV:
Purpose: Causes corresponding property update errors to be
ignored
Description: The default behavior for a PROPPATCH method is all
property updates must succeed, or none of them succeed. If an
updatebehavior is not included, it is equivalent to the default
behavior or specifying <mustsucceed>*</mustsucceed> meaning that all
the updates must be successful or none of them are performed. The
DAV:ignore element specifies that the server should make best-effort
property updates. Any error caused by the associated propertyupdate
is ignored. The error is reported in the resulting DAV:multistatus,
but the rest of the updates specified in the PROPPATCH entity request
body are processed as if this propertyupdate was not specified.
<!ELEMENT ignore EMPTY >
12.13.6 mustsucceed XML element
Name: mustsucceed
Namespace: DAV:
Purpose: Specifies the corresponding property update must
succeed.
Description: The default behavior for a PROPPATCH method is all
property updates must succeed, or none of them succeed. The
DAV:mustsucceed element allows the client to specify a list of named
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properties whose property update must succeed. Any error caused by
the associated propertyupdate causes the PROPPATCH method to fail,
and all property updates are rolled back. If a value of "*" is given
for the mustsucceed XML element, this designates that all property
updates must succeed. "*" is the only PCDATA value that can be
specified.
<!ELEMENT mustsucceed (#PCDATA | href+)>
Note: element DAV:mustsucceed may not be necessary because the
default behavior is for the PROPPATCH to fail if any propertyupdate
fails. It is really only necessary to specify which errors are to be
ignored. I have included it here for completeness, and consistency
with the DAV:keepalive XML element.
6. Formal Syntax
< Commonly used grammar is BNF grammar defined in RFC-2234.
Suggested wording.>
This section summarizes the changes and additions to the WebDAV DTD
given in section 24.1 of [WebDAV].
<!ELEMENT propertyupdate (add | set | remove)+>
<!ELEMENT add (prop, updatebehavior?) >
<!ELEMENT set (prop, updatebehavior?) >
<!ELEMENT remove (prop, updatebehavior?) >
<!ELEMENT updatebehavior (ignore | mustsucceed)>
<!ELEMENT ignore EMPTY>
<!ELEMENT mustsucceed (#PCDATA | href+)>
7. Security Considerations
This section is provided to detail issues concerning security
implications of which WebDAV applications need to be aware.
All of the security considerations of HTTP/1.1 and the WebDAV
Distributed Authoring Protocol specification also apply to WebDAV
collections.
These changes may introduce a denial of service security violation
through the possibility of a client application exhausting server
resources with PROPPATCH methods. However, the same possibility
exists with PUT operations, so no new security issues are introduced.
8. IANA Considerations
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This document uses the namespaces defined by [WebDAV] for properties
and
XML elements. All other IANA considerations mentioned in [WebDAV] also
apply to this document.
9. Copyright
To be supplied by the RFC Editor.
10. Intellectual Property
To be supplied by the RFC Editor.
11. References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
2 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement
Levels." RFC 2119, BCP 14. Harvard University. March, 1997.
[XML] T. Bray, J. Paoli, C.M. Sperberg-McQueen, "Extensible Markup
Language (XML)." World Wide Web Consortium Recommendation REC-xml-
19980210. http://www.w3.org/TR/1998/REC-xml-19980210.
[HTTP] R. Fielding, J. Gettys, J. Mogul, H. Frystyk, T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1." RFC 2068. UC Irvine, DEC,
MIT/LCS. January, 1997.
[WebDAV] Y. Y. Goland, E. J. Whitehead, Jr., A. Faizi, S. R. Carter,
D.
Jensen, "HTTP Extensions for Distributed Authoring - WebDAV." RFC
2518.
Microsoft, U.C. Irvine, Netscape, Novell. February, 1999.
11. Acknowledgments
This draft has benefited from thoughtful discussion by Jim Whitehead,
Geoff Clemm, and others.
12. Author's Addresses
James Amsden
IBM
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3039 Cornwallis Road
RPT, NC
Phone: 919-461-3919
Email: jamsden@us.ibm.com
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