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Network Working Group                                         J. Reschke
Internet-Draft                                                greenbytes
Intended status: Standards Track                      September 22, 2008
Expires: March 26, 2009


 Using POST to add to Web Distributed Authoring and Versioning (WebDAV)
                              Collections
                      draft-reschke-webdav-post-00

Status of this Memo

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Abstract

   The Hypertext Transfer Protocol (HTTP) Extensions for the Web
   Distributed Authoring and Versioning (WebDAV) do not define the
   behavior for the "POST" method when applied to collections, as the
   base specification (HTTP) leaves implementers lots of freedom for the
   semantics of "POST".

   This has lead to a situation where many WebDAV servers do not
   implement POST for collections at all, although it is well suited to
   be used for the purpose of adding new members to a collection, where
   the server remains in control of the newly assigned URL.  As a matter
   of fact, the Atom Publishing Protocol (AtomPub) uses POST exactly for



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   that purpose.  On the other hand, WebDAV-based protocols such as the
   Calendar Extensions to WebDAV (CalDAV) frequently require clients to
   pick a unique URL, although the server could easily perform that
   task.

   This specification defines a discovery mechanism through which
   servers can advertise support for POST requests with the
   aforementioned "add collection member" semantics.

Editorial Note (To be removed by RFC Editor before publication)

   Please send comments to the Distributed Authoring and Versioning
   (WebDAV) working group at <mailto:w3c-dist-auth@w3.org>, which may be
   joined by sending a message with subject "subscribe" to
   <mailto:w3c-dist-auth-request@w3.org>.  Discussions of the WEBDAV
   working group are archived at
   <http://lists.w3.org/Archives/Public/w3c-dist-auth/>.

   Note that although discussion takes place on the WebDAV working
   group's mailing list, this is not a working group document.

   XML versions, latest edits and the issues list for this document are
   available from
   <http://greenbytes.de/tech/webdav/#draft-reschke-webdav-post>.



























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Protocol Extension . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Definition of 'Add-Member' URI . . . . . . . . . . . . . .  6
     3.2.  Discovery  . . . . . . . . . . . . . . . . . . . . . . . .  6
       3.2.1.  p:add-member Property  . . . . . . . . . . . . . . . .  6
       3.2.2.  Example  . . . . . . . . . . . . . . . . . . . . . . .  6
       3.2.3.  add-member Link Relation . . . . . . . . . . . . . . .  7
     3.3.  Relation to AtomPub's 'Slug' Header  . . . . . . . . . . .  8
     3.4.  Example Operation  . . . . . . . . . . . . . . . . . . . .  8
   4.  Internationalization Considerations  . . . . . . . . . . . . .  8
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     7.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11






























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1.  Introduction

   The Hypertext Transfer Protocol (HTTP) Extensions for the Web
   Distributed Authoring and Versioning (WebDAV) ([RFC4918], Section
   9.5) do not define the behavior for the "POST" method when applied to
   collections, as the base specification (HTTP) leaves implementers
   lots of freedom for the semantics of "POST":

      9.5 POST for Collections

      Since by definition the actual function performed by POST is
      determined by the server and often depends on the particular
      resource, the behavior of POST when applied to collections cannot
      be meaningfully modified because it is largely undefined.  Thus,
      the semantics of POST are unmodified when applied to a collection.

   This has lead to a situation where many WebDAV servers do not
   implement POST for collections at all, although it is well suited to
   be used for the purpose of adding new members to a collection, where
   the server remains in control of the newly assigned URL.  As a matter
   of fact, the Atom Publishing Protocol (AtomPub) uses POST exactly for
   that purpose ([RFC5023], Section 9.2):

      9.2 Creating Resources with POST

      To add members to a Collection, clients send POST requests to the
      URI of the Collection.

   On the other hand, WebDAV-based protocols such as Calendaring
   Extensions to WebDAV (CalDAV) frequently require clients to pick a
   unique URL, although the server could easily perform that task
   ([RFC4791], Section 5.3.2):

      5.3.2 Creating Calendar Object Resources

      ...

      When servers create new resources, it's not hard for the server to
      choose an unmapped URI.  It's slightly tougher for clients,
      because a client might not want to examine all resources in the
      collection and might not want to lock the entire collection to
      ensure that a new resource isn't created with a name collision.
      (...)

      Note: the vCard Extensions to WebDAV (CardDAV)
      ([draft-ietf-vcarddav-carddav]) suffer from the same issue, and
      may be able to take advantage of this specification.




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   This specification defines a discovery mechanism through which
   servers can advertise support for POST requests with the
   aforementioned "add collection member" semantics.

      Note: the author previously proposed a new HTTP method for exactly
      this purpose ([draft-reschke-http-addmember]), but quite a few
      reviewers pointed out that this would duplicate the original
      semantics of POST.  Thus this proposal that avoids adding a new
      HTTP method.


2.  Terminology

   The terminology used here follows that in the WebDAV specification
   ([RFC4918]).

   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 [RFC2119].

   This document uses XML DTD fragments ([XML]) as a purely notational
   convention.  In particular:

   o  Element names use the namespace
      "http://purl.org/NET/webdav/post#".  When an XML element type in
      this namespace is referenced in this document outside of the
      context of an XML fragment, the string "p:" will be prefixed to
      the element name. [[ns: With sufficient support from the WebDAV
      WG's former mailing list and/or the IESG Applications Area the
      property defined later on could potentially be moved into the DAV:
      namespace.]]

   o  Element ordering is irrelevant.

   o  Extension elements/attributes (elements/attributes not already
      defined as valid child elements) may be added anywhere, except
      when explicitly stated otherwise.


3.  Protocol Extension

   Due to the reasons stated in Section 1, clients can not rely on a
   specific server behavior when POST is applied to a collection.  This
   problem is addressed by this specification by allowing servers to
   advertise a URI that has the desired "add member" semantics.

   Note that ervers that already use POST for a different purpose can
   just expose a different URI for that purpose.  Other servers can just



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   advertise the collection's own URI, thus avoiding to mint another URI
   for a limited purpose.

3.1.  Definition of 'Add-Member' URI

   The "Add-Member" URI of a WebDAV collection is a URI that will accept
   HTTP POST requests, and will interpret these as requests to store the
   enclosed entity as a new internal member of the collection.  The URI
   of the newly created resource is returned in the HTTP Location
   response header ([RFC2616], Section 14.30).

3.2.  Discovery

3.2.1.  p:add-member Property

   The p:add-member property is defined on WebDAV collections, and
   contains the "Add-Member" URI for that collection (embedded inside a
   DAV:href element).

     <!ELEMENT p:add-member (href)>
     <!-- href: defined in [RFC4918], Section 14.7 -->

   A PROPFIND/allprop request SHOULD NOT return this property (see
   [RFC4918], Section 9.1).

3.2.2.  Example

   >>Request

   PROPFIND /collection/ HTTP/1.1
   Host: example.com
   Content-Type: application/xml; charset="utf-8"
   Content-Lenght: 163

   <?xml version="1.0" encoding="utf-8" ?>
   <propfind xmlns="DAV:" xmlns:p="http://purl.org/NET/webdav/post#">
     <prop>
       <p:add-member/>
     </prop>
   </propfind>











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   >>Response

   HTTP/1.1 207 Multi-Status
   Content-Type: application/xml; charset="utf-8"
   Content-Length: 385

   <?xml version="1.0" encoding="utf-8" ?>
   <multistatus xmlns="DAV:" xmlns:p="http://purl.org/NET/webdav/post#">
     <response>
       <href>/collection/</href>
       <propstat>
         <prop>
           <p:add-member>
             <href>/collection/;add-member</href>
           </p:add-member>
         </prop>
         <status>HTTP/1.1 200 OK</status>
       </propstat>
     </response>
   </multistatus>
   Note that in this case, the server has minted a separate URI for the
   purpose of adding new content.

3.2.3.  add-member Link Relation

   There may be cases in which it is more efficient to expose the Add-
   Member URI in HTTP headers or in (X)HTML content.  For this use case,
   this specification defines a new link relation with the name:

     http://purl.org/NET/webdav/post#add-member

   It can be uses both in HTTP Link headers (see
   [draft-nottingham-http-link-header]):

   HEAD /collection/ HTTP/1.1
   Host: example.com


   HTTP/1.1 200 OK
   Link: </collection/;add-member>;
         rel="http://purl.org/NET/webdav/post#add-member"

   ...and in (X)HTML content:








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   <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
     <head>
       <title>Contents of /collection/</title>
       <link rel="http://purl.org/NET/webdav/post#add-member"
             href="/collection/;add-member" />
     </head
     <body>...</body>
   </html>

3.3.  Relation to AtomPub's 'Slug' Header

   In the AtomPub protocol, clients can use the entity header "Slug" to
   suggest parts of the URI to be created (see [RFC5023], Section 9.7).
   Note that servers are free to ignore this suggestion, or to use
   whatever algorithm that makes sense to generate the new URI.

   The same applies to the extension defined here: clients can use the
   "Slug" header as by its definition of a generic HTTP header.  Servers
   should process it exactly the way defined by AtomPub.

3.4.  Example Operation

   >>Request

   POST /collection/;add-member HTTP/1.1
   Host: example.com
   Content-Type: text/plain
   Slug: Sample Text
   Content-Lenght: 12

   Sample text.

   >>Response

   HTTP/1.1 201 Created
   Location: http://example.com/collection/sample%20text


4.  Internationalization Considerations

   This document does not introduce any new internationalization
   considerations beyond those discussed in Section 20 of [RFC4918].


5.  IANA Considerations

   This specification does not require any actions from IANA.




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6.  Security Considerations

   All security considerations connected to HTTP/WebDAV and XML apply
   for this specification as well, namely, [RFC4918] (Section 20) and
   [RFC3470] (Section 7).


7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918, June 2007.

   [XML]      Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fourth
              Edition)", W3C REC-xml-20060816, August 2006,
              <http://www.w3.org/TR/2006/REC-xml-20060816>.

   [draft-nottingham-http-link-header]
              Nottingham, M., "HTTP Header Linking",
              draft-nottingham-http-link-header-02 (work in progress),
              July 2008.

7.2.  Informative References

   [RFC3470]  Hollenbeck, S., Rose, M., and L. Masinter, "Guidelines for
              the Use of Extensible Markup Language (XML) within IETF
              Protocols", RFC 3470, BCP 70, January 2003.

   [RFC4791]  Daboo, C., Desruisseaux, B., and L. Dusseault,
              "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
              March 2007.

   [RFC5023]  Gregorio, J. and B. de hOra, "The Atom Publishing
              Protocol", RFC 5023, October 2007.

   [draft-ietf-vcarddav-carddav]
              Daboo, C., "vCard Extensions to WebDAV (CardDAV)",
              draft-ietf-vcarddav-carddav-01 (work in progress),
              July 2008.



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   [draft-reschke-http-addmember]
              Reschke, J., "The HTTP ADDMEMBER Method",
              draft-reschke-http-addmember-00 (work in progress),
              February 2005.


Index

   A
      Add-Member URI  6


Author's Address

   Julian F. Reschke
   greenbytes GmbH
   Hafenweg 16
   Muenster, NW  48155
   Germany

   Phone: +49 251 2807760
   Email: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/




























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