path: root/doc/rfc/rfc4437.txt

Network Working Group                                       J. Whitehead
Request for Comments: 4437                               U.C. Santa Cruz
Category: Experimental                                          G. Clemm
                                                                     IBM
                                                         J. Reschke, Ed.
                                                              greenbytes
                                                              March 2006


           Web Distributed Authoring and Versioning (WebDAV)
                      Redirect Reference Resources


Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This specification defines an extension to Web Distributed Authoring
   and Versioning (WebDAV) to allow clients to author HTTP redirect
   reference resources whose default response is an HTTP/1.1 3xx
   (Redirection) status code.  A redirect reference makes it possible to
   access the target resourced indirectly through any URI mapped to the
   redirect reference resource.  This specification does not address
   remapping of trees of resources or regular expression based
   redirections.  There are no integrity guarantees associated with
   redirect reference resources.  Other mechanisms can also be used to
   achieve the same functionality as this specification.  This
   specification allows operators to experiment with this mechanism and
   develop experience on what is the best approach to the problem.

Table of Contents

   1. Introduction ....................................................3
   2. Notational Conventions ..........................................4
   3. Terminology .....................................................4
   4. Overview of Redirect Reference Resources ........................5
   5. Operations on Redirect Reference Resources ......................6
   6. MKREDIRECTREF Method ............................................7




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      6.1. Example: Creating a Redirect Reference Resource
           with MKREDIRECTREF .........................................8
   7. UPDATEREDIRECTREF Method ........................................9
      7.1. Example: Updating a Redirect Reference Resource with
           UPDATEREDIRECTREF .........................................10
   8. Operations on Collections That Contain Redirect
      Reference Resources ............................................11
      8.1. Example: PROPFIND on a Collection with Redirect
           Reference .................................................11
      8.2. Example: PROPFIND with Apply-To-Redirect-Ref on a
           Collection with Redirect Reference Resources ..............13
   9. Operations on Targets of Redirect Reference Resources ..........15
   10. Relative References in DAV:reftarget ..........................15
      10.1. Example: Resolving a Relative Reference in a
            Multi-Status Response.....................................16
   11. Redirect References to Collections ............................17
   12. Headers .......................................................18
      12.1. Redirect-Ref Response Header .............................18
      12.2. Apply-To-Redirect-Ref Request Header .....................19
   13. Redirect Reference Resource Properties ........................19
      13.1. DAV:redirect-lifetime (protected) ........................19
      13.2. DAV:reftarget (protected) ................................19
   14. XML Elements ..................................................19
      14.1. redirectref XML Element ..................................19
   15. Extensions to the DAV:response XML Element for Multi-Status
       Responses .....................................................20
   16. Capability Discovery ..........................................20
      16.1. Example: Discovery of Support for Redirect
            Reference Resources ......................................20
   17. Security Considerations .......................................21
      17.1. Privacy Concerns .........................................21
      17.2. Redirect Loops ...........................................21
      17.3. Redirect Reference Resources and Denial of Service .......21
      17.4. Revealing Private Locations ..............................22
   18. Internationalization Considerations ...........................22
   19. IANA Considerations ...........................................22
      19.1. HTTP headers .............................................22
           19.1.1. Redirect-Ref ......................................22
           19.1.2. Apply-To-Redirect-Ref .............................23
   20. Contributors ..................................................23
   21. Acknowledgements ..............................................23
   22. Normative References ..........................................23









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1.  Introduction

   This specification extends the Web Distributed Authoring Protocol
   (WebDAV) to enable clients to create new access paths to existing
   resources.  This capability is useful for several reasons.

   WebDAV makes it possible to organize HTTP resources into hierarchies,
   placing them into groupings, known as collections, that are more
   easily browsed and manipulated than a single flat collection.
   However, hierarchies require categorization decisions that locate
   resources at a single location in the hierarchy, a drawback when a
   resource has multiple valid categories.  For example, in a hierarchy
   of vehicle descriptions containing collections for cars and boats, a
   description of a combination car/boat vehicle could belong in either
   collection.  Ideally, the description should be accessible from both.
   Allowing clients to create new URIs that access the existing resource
   lets them put that resource into multiple collections.

   Hierarchies also make resource sharing more difficult, since
   resources that have utility across many collections are still forced
   into a single collection.  For example, the mathematics department at
   one university might create a collection of information on fractals
   that contains bindings to some local resources, but also provides
   access to some resources at other universities.  For many reasons, it
   may be undesirable to make physical copies of the shared resources:
   to conserve disk space, to respect copyright constraints, or to make
   any changes in the shared resources visible automatically.  Being
   able to create new access paths to existing resources in other
   collections or even on other unrelated systems is useful for this
   sort of case.

   The redirect reference resources defined here provide a mechanism for
   creating alternative access paths to existing resources.  A redirect
   reference resource is a resource in one collection whose purpose is
   to redirect requests to another resource (its target), possibly in a
   different collection.  In this way, it allows clients to submit
   requests to the target resource from another collection.  It
   redirects most requests to the target resource using an HTTP status
   code from the 3xx range (Redirection), thereby providing a form of
   mediated access to the target resource.











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   A redirect reference is a resource with properties but with no body
   of its own.  Properties of a redirect reference resource can contain
   information such as who created the reference, when, and why.  Since
   redirect reference resources are implemented using HTTP 3xx
   responses, it generally takes two round trips to submit a request to
   the intended resource.  Redirect references work equally well for
   local resources and for resources that reside on a different system
   from the reference.

   The remainder of this document is structured as follows: Section 3
   defines terms that will be used throughout the specification.
   Section 4 provides an overview of redirect reference resources.
   Section 5 defines the semantics of existing methods when applied to
   redirect reference resources.  Section 6 discusses how to create a
   redirect reference resource, and Section 7 discusses updating
   redirect references.  Section 8 discusses their semantics when
   applied to collections that contain redirect reference resources.
   Sections 9 through 11 discuss several other issues raised by the
   existence of redirect reference resources.  Sections 12 through 15
   define the new headers, properties, and XML elements required to
   support redirect reference resources.  Section 16 discusses
   capability discovery.  Sections 17 through 19 present the security,
   internationalization, and IANA concerns raised by this specification.
   The remaining sections provide a variety of supporting information.

2.  Notational Conventions

   Since this document describes a set of extensions to the WebDAV
   Distributed Authoring Protocol [RFC2518], itself an extension to the
   HTTP/1.1 protocol, the augmented BNF used here to describe protocol
   elements is exactly the same as described in Section 2.1 of
   [RFC2616].  Since this augmented BNF uses the basic production rules
   provided in Section 2.2 of [RFC2616], these rules apply to this
   document as well.

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in [RFC2119].

3.  Terminology

   The terminology used here follows and extends that in the WebDAV
   Distributed Authoring Protocol specification [RFC2518].  Definitions
   of the terms resource, Uniform Resource Identifier (URI), and Uniform
   Resource Locator (URL) are provided in [RFC3986].






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   Redirect Reference Resource

      A resource created to redirect all requests made to it, using an
      HTTP status code from the 3xx range, to a defined target resource.

   Non-Reference Resource

      A resource that is not a reference to another resource.

   Target Resource

      The resource to which requests are redirected by a redirect
      reference resource.  A target resource can be anything that can be
      identified by an absolute URI (see [RFC3986], "absolute-URI").

   This document uses the terms "precondition", "postcondition", and
   "protected property" as defined in [RFC3253].  Servers MUST report
   pre-/postcondition failures as described in Section 1.6 of this
   document.

4.  Overview of Redirect Reference Resources

   For all operations submitted to a redirect reference resource, the
   default response is a 302 (Found), accompanied by the Redirect-Ref
   header (defined in Section 12.1, below) and the Location header
   ([RFC2616], Section 14.30) set to the URI of the target resource.
   With this information, the client can resubmit the request to the URI
   of the target resource.

   A redirect reference resource never automatically forwards requests
   to its target resource.  Redirect resources bring the same benefits
   as links in HTML documents.  They can be created and maintained
   without the involvement or even knowledge of their target resource.
   This reduces the cost of linking between resources.

   If the client is aware that it is operating on a redirect reference
   resource, it can resolve the reference by retrieving the reference
   resource's DAV:reftarget property (defined in Section 13.2, below),
   whose value contains the URI of the target resource.  It can then
   submit requests to the target resource.

   A redirect reference resource is a new type of resource.  To
   distinguish redirect reference resources from non-reference
   resources, a new value of the DAV:resourcetype property (defined in
   [RFC2518]), DAV:redirectref, is defined in Section 14.1, below.

   Since a redirect reference resource is a resource, methods can be
   applied to the reference resource as well as to its target resource.



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   The Apply-To-Redirect-Ref request header (defined in Section 12.2,
   below) is provided so that referencing-aware clients can control
   whether an operation is applied to the redirect reference resource or
   standard HTTP/WebDAV behaviour (redirection with a 3xx status code)
   should occur.  The Apply-To-Redirect-Ref header can be used with most
   requests to redirect reference resources.  This header is
   particularly useful with PROPFIND, to retrieve the reference
   resource's own properties.

   Implementation Note: Operations on the target of a redirect reference
   usually do not affect the redirect reference itself.  However,
   clients should not rely on this behaviour (for instance, some servers
   may update redirect references as a result of namespace operations on
   the reference's target).

5.  Operations on Redirect Reference Resources

   Although non-referencing-aware clients cannot create reference
   resources, they should be able to submit requests through the
   reference resources created by reference-aware WebDAV clients.  They
   should be able to follow any references to their targets.  To make
   this possible, a server that receives any request made via a redirect
   reference resource MUST return a 3xx range (Redirection) status code,
   unless the request includes an Apply-To-Redirect-Ref header
   specifying "T".  The client and server MUST follow [RFC2616], Section
   10.3, but with these additional rules:

   o  The Location response header MUST contain a URI (see [RFC3986],
      Section 3) that identifies the target of the reference resource.

   o  The response MUST include the Redirect-Ref header.  This header
      allows reference-aware WebDAV clients to recognize the resource as
      a reference resource and to understand the reason for the
      redirection.

   A reference-aware WebDAV client can, like a non-referencing client,
   resubmit the request to the URI in the Location header in order to
   operate on the target resource.  Alternatively, it can resubmit the
   request to the URI of the redirect reference resource with the
   "Apply-To-Redirect-Ref: T" header in order to operate on the
   reference resource itself.  In this case, the request MUST be applied
   to the reference resource itself, and a 3xx response MUST NOT be
   returned.

   As redirect references do not have bodies, GET and PUT requests with
   "Apply-To-Redirect-Ref: T" MUST fail with status 403 (forbidden).





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6.  MKREDIRECTREF Method

   The MKREDIRECTREF method requests the creation of a redirect
   reference resource.

   If a MKREDIRECTREF request fails, the server state preceding the
   request MUST be restored.

   Responses from a MKREDIRECTREF request MUST NOT be cached, as
   MKREDIRECTREF has non-idempotent and non-safe semantics (see
   [RFC2616], Section 9.1).

   Marshalling

      The request body MUST be a DAV:mkredirectref XML element.

      <!ELEMENT mkredirectref (reftarget, redirect-lifetime?)>
      <!ELEMENT reftarget (href)>
      <!ELEMENT redirect-lifetime (permanent | temporary)>
      <!ELEMENT permanent EMPTY>
      <!ELEMENT temporary EMPTY>

      The DAV:href element is defined in [RFC2518] (Section 12.3) and
      MUST contain either a URI or a relative-ref (see [RFC3986],
      Sections 3 and 4.2).

      If no DAV:redirect-lifetime element is specified, the server MUST
      behave as if a value of DAV:temporary was specified.

      If the request succeeds, the server MUST return 201 (Created)
      status.

      If a response body for a successful request is included, it MUST
      be a DAV:mkredirectref-response XML element.  Note that this
      document does not define any elements for the MKREDIRECTREF
      response body, but the DAV:mkredirectref-response element is
      defined to ensure interoperability between future extensions that
      do define elements for the response body.

      <!ELEMENT mkredirectref-response ANY>

   Preconditions

      (DAV:resource-must-be-null): A resource MUST NOT exist at the
      Request-URI.

      (DAV:parent-resource-must-be-non-null): The Request-URI minus the
      last past segment MUST identify a collection.



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      (DAV:name-allowed): The last segment of the Request-URI is
      available for use as a resource name.

      (DAV:locked-update-allowed): If the collection identified by the
      Request-URI minus the last path segment is write-locked, then the
      appropriate token MUST be specified in an If request header.

      (DAV:redirect-lifetime-supported): If the request body contains a
      DAV:redirect-lifetime element, the server MUST support the
      specified lifetime.  Support for DAV:temporary is REQUIRED, while
      support for DAV:permanent is OPTIONAL.

      (DAV:legal-reftarget): The specified is a legal URI or relative-
      ref.

   Postconditions

      (DAV:new-redirectref): a new redirect reference resource is
      created whose DAV:reftarget property has the value specified in
      the request body.

6.1.  Example: Creating a Redirect Reference Resource with MKREDIRECTREF

   >> Request:

   MKREDIRECTREF /~whitehead/dav/spec08.ref HTTP/1.1
   Host: www.example.com
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx

   <?xml version="1.0" encoding="utf-8" ?>
   <D:mkredirectref xmlns:D="DAV:">
     <D:reftarget>
       <D:href>/i-d/draft-webdav-protocol-08.txt</D:href>
     </D:reftarget>
   </D:mkredirectref>

   >> Response:

   HTTP/1.1 201 Created











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   This request resulted in the creation of a new redirect reference
   resource at http://www.example.com/~whitehead/dav/spec08.ref, which
   points to the resource identified by the DAV:reftarget property.  In
   this example, the target resource is identified by the URI
   http://www.example.com/i-d/draft-webdav-protocol-08.txt.  The
   redirect reference resource's DAV:resourcetype property is set to
   DAV:redirectref, and its DAV:redirect-lifetime property has the value
   DAV:temporary.

7.  UPDATEREDIRECTREF Method

   The UPDATEREDIRECTREF method requests the update of a redirect
   reference resource.

   If a UPDATEREDIRECTREF request fails, the server state preceding the
   request MUST be restored.

   Responses from a UPDATEREDIRECTREF request MUST NOT be cached, as
   UPDATEREDIRECTREF has non-safe semantics (see [RFC2616], Section
   9.1).

   Marshalling

      The request body MUST be a DAV:updateredirectref XML element.

      <!ELEMENT updateredirectref (reftarget?, redirect-lifetime?)>

      See Section 6 for a definition of DAV:reftarget and DAV:redirect-
      lifetime.

      If no DAV:reftarget element is specified, the server MUST NOT
      change the target of the redirect reference.

      If no DAV:redirect-lifetime element is specified, the server MUST
      NOT change the lifetime of the redirect reference.

      If a response body for a successful request is included, it MUST
      be a DAV:updateredirectref-response XML element.  Note that this
      document does not define any elements for the UPDATEREDIRECTREF
      response body, but the DAV:updateredirectref-response element is
      defined to ensure interoperability between future extensions that
      do define elements for the response body.

      <!ELEMENT updateredirectref-response ANY>







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   Preconditions

      (DAV:locked-update-allowed): if the resource is write-locked, then
      the appropriate token MUST be specified in an If request header.

      (DAV:must-be-redirectref): the resource identified by the
      Request-URI must be a redirect reference resource as defined by
      this specification.

      (DAV:redirect-lifetime-supported): see Section 6.

      (DAV:redirect-lifetime-update-supported): servers MAY support
      changing the DAV:redirect-lifetime property; if they don't, this
      condition code can be used to signal failure.

      (DAV:legal-reftarget): see Section 6.

   Postconditions

      (DAV:redirectref-updated): the DAV:reftarget and DAV:redirect-
      lifetime properties of the redirect reference have been updated
      accordingly.

7.1.  Example: Updating a Redirect Reference Resource with
      UPDATEREDIRECTREF

   >> Request:

   UPDATEREDIRECTREF /~whitehead/dav/spec08.ref HTTP/1.1
   Host: www.example.com
   Apply-To-Redirect-Ref: T
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx

   <?xml version="1.0" encoding="utf-8" ?>
   <D:updateredirectref xmlns:D="DAV:">
     <D:reftarget>
       <D:href>/i-d/draft-webdav-protocol-08b.txt</D:href>
     </D:reftarget>
   </D:updateredirectref>

   >> Response:

   HTTP/1.1 200 OK

   This request has updated the redirect reference's DAV:reftarget
   property to "/i-d/draft-webdav-protocol-08b.txt" and has not changed
   the DAV:redirect-lifetime value.  Note that the "Apply-To-Redirect-



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   Ref" request header must be used; otherwise, the request would result
   in a redirect (3xx) response status.

8.  Operations on Collections That Contain Redirect Reference Resources

   According to [RFC2518], Section 9.2, methods that have defined
   interactions with the "Depth" request header should apply all other
   request headers to each resource in scope.  However, applying this
   principle to the "Apply-To-Redirect-Ref" header uniformly would make
   it impractical to implement this specification on top of existing
   servers and also would result in unexpected server behaviour for
   clients that do not take the existence of redirect references into
   account.  On the other hand, the definition of the "Depth" header
   allows alternate behaviours to be explicitly defined.

   For this reason, this specification defines the interaction between
   "Depth" and "Apply-To-Redirect-Ref" request headers on a case-by-case
   basis and also provides a default for methods not mentioned here that
   do not specify the behaviour themselves.

    +-------------+-----------------+------------------+-----------+
    | method name | defined in      | supported depths | behaviour |
    +-------------+-----------------+------------------+-----------+
    | COPY        | [RFC2518], 8.9  | 0, infinity      | "T"       |
    | DELETE      | [RFC2518], 8.7  | infinity         | "T"       |
    | LOCK        | [RFC2518], 8.11 | 0, infinity      | "T"       |
    | MOVE        | [RFC2518], 8.10 | 0, infinity      | "T"       |
    | PROPFIND    | [RFC2518], 8.2  | 0, 1, infinity   | inherit   |
    | REPORT      | [RFC3253], 3.6  | 0, 1, infinity   | inherit   |
    | default     |                 |                  | "T"       |
    +-------------+-----------------+------------------+-----------+

   When the behaviour is defined to be "inherit", the method should
   follow RFC2518's default behaviour for "Depth" operations, which
   means applying the value given for "Apply-To-Redirect-Ref" to each
   resource in scope.  On the other hand, when it is defined to be "T",
   the method should behave as if a "Apply-To-Redirect-Ref: T" header
   was specified for each operation on child resources.  The latter
   ensures that "Depth: infinity" operations will not fail unexpectedly
   just because there was a redirect reference resource in scope.

8.1.  Example: PROPFIND on a Collection with Redirect Reference
      Resources

   Suppose a PROPFIND request with Depth: infinity is submitted to the
   following collection, with the members shown here:





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   /MyCollection/
        (non-reference resource) diary.html
        (redirect reference resource) nunavut

   >> Request:

   PROPFIND /MyCollection/ HTTP/1.1
   Host: example.com
   Depth: infinity
   Apply-To-Redirect-Ref: F
   Content-Type: text/xml
   Content-Length: xxxx

   <?xml version="1.0" ?>
   <D:propfind xmlns:D="DAV: ">
     <D:prop xmlns:J="http://example.com/jsprops/">
       <D:resourcetype/>
       <J:keywords/>
     </D:prop>
   </D:propfind>

   >> Response:

   HTTP/1.1 207 Multi-Status
   Content-Type: text/xml
   Content-Length: xxxx

   <?xml version="1.0" ?>
   <D:multistatus xmlns:D="DAV:" xmlns:J="http://example.com/jsprops/">
     <D:response>
       <D:href>/MyCollection/</D:href>
       <D:propstat>
         <D:prop>
           <D:resourcetype><D:collection/></D:resourcetype>
           <J:keywords>diary, interests, hobbies</J:keywords>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
     </D:response>
     <D:response>
       <D:href>/MyCollection/diary.html</D:href>
       <D:propstat>
         <D:prop>
           <D:resourcetype/>
           <J:keywords>diary, travel, family, history</J:keywords>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>



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     </D:response>
     <D:response>
       <D:href>/MyCollection/nunavut</D:href>
       <D:status>HTTP/1.1 302 Found</D:status>
       <D:location>
         <D:href>http://example.ca/art/inuit/</D:href>
       </D:location>
     </D:response>
   </D:multistatus>

   In this example, the Depth header is set to infinity, and the Apply-
   To-Redirect-Ref header is set to "F".  The collection contains one
   URI that identifies a redirect reference resource.  The response
   element for the redirect reference resource has a status of 302
   (Found) and includes a DAV:location extension element to allow
   clients to retrieve the properties of its target resource.  (The
   response element for the redirect reference resource does not include
   the requested properties.  The client can submit another PROPFIND
   request to the URI in the DAV:location pseudo-property to retrieve
   those properties.)

8.2.  Example: PROPFIND with Apply-To-Redirect-Ref on a Collection with
      Redirect Reference Resources

   Suppose a PROPFIND request with "Apply-To-Redirect-Ref: T" and Depth:
   infinity is submitted to the following collection, with the members
   shown here:

   /MyCollection/
        (non-reference resource) diary.html
        (redirect reference resource) nunavut

   >> Request:

   PROPFIND /MyCollection/ HTTP/1.1
   Host: example.com
   Depth: infinity
   Apply-To-Redirect-Ref: T
   Content-Type: text/xml
   Content-Length: xxxx











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   <?xml version="1.0" ?>
   <D:propfind xmlns:D="DAV:">
     <D:prop>
       <D:resourcetype/>
       <D:reftarget/>
       <D:redirect-lifetime/>
     </D:prop>
   </D:propfind>

   >> Response:

   HTTP/1.1 207 Multi-Status
   Content-Type: text/xml
   Content-Length: xxxx

   <?xml version="1.0" ?>
   <D:multistatus xmlns:D="DAV:">
     <D:response>
       <D:href>/MyCollection/</D:href>
       <D:propstat>
         <D:prop>
           <D:resourcetype><D:collection/></D:resourcetype>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
       <D:propstat>
         <D:prop>
           <D:reftarget/>
           <D:redirect-lifetime/>
         </D:prop>
         <D:status>HTTP/1.1 404 Not Found</D:status>
       </D:propstat>
     </D:response>
     <D:response>
       <D:href>/MyCollection/diary.html</D:href>
       <D:propstat>
         <D:prop>
           <D:resourcetype/>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
       <D:propstat>
         <D:prop>
           <D:reftarget/>
           <D:redirect-lifetime/>
         </D:prop>
         <D:status>HTTP/1.1 404 Not Found</D:status>
       </D:propstat>



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     </D:response>
     <D:response>
       <D:href>/MyCollection/nunavut</D:href>
       <D:propstat>
         <D:prop>
           <D:resourcetype><D:redirectref/></D:resourcetype>
           <D:reftarget>
             <D:href>http://example.ca/art/inuit/</D:href>
           </D:reftarget>
           <D:redirect-lifetime><D:temporary/></D:redirect-lifetime>
         </D:prop>
       <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
     </D:response>
   </D:multistatus>

   Since the "Apply-To-Redirect-Ref: T" header is present, the response
   shows the properties of the redirect reference resource in the
   collection rather than reporting a 302 status.

9.  Operations on Targets of Redirect Reference Resources

   Operations on targets of redirect reference resources have no effect
   on the reference resource.

10.  Relative References in DAV:reftarget

   The URI in the href in a DAV:reftarget property MAY be a relative
   reference.  In this case, the base URI to be used for resolving it to
   absolute form is the URI used in the HTTP message to identify the
   redirect reference resource to which the DAV:reftarget property
   belongs.

   When DAV:reftarget appears in the context of a Multi-Status response,
   it is in a DAV:response element that contains a single DAV:href
   element.  The value of this DAV:href element serves as the base URI
   for resolving a relative reference in DAV:reftarget.  The value of
   DAV:href may itself be relative, in which case it must be resolved
   first in order to serve as the base URI for the relative reference in
   DAV:reftarget.  If the DAV:href element is relative, its base URI is
   constructed from the scheme component "http", the value of the Host
   header in the request, and the Request-URI.









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10.1.  Example: Resolving a Relative Reference in a Multi-Status
       Response

   >> Request:

   PROPFIND /geog/ HTTP/1.1
   Host: example.com
   Apply-To-Redirect-Ref: T
   Depth: 1
   Content-Type: text/xml
   Content-Length: nnn

   <?xml version="1.0" ?>
   <D:propfind xmlns:D="DAV:">
     <D:prop>
       <D:resourcetype/>
       <D:reftarget/>
     </D:prop>
   </D:propfind>

   >> Response:

   HTTP/1.1 207 Multi-Status
   Content-Type: text/xml
   Content-Length: nnn

   <?xml version="1/0" ?>
   <D:multistatus xmlns:D="DAV:">
     <D:response>
       <D:href>/geog/</D:href>
       <D:propstat>
         <D:prop>
           <D:resourcetype><D:collection/></D:resourcetype>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
       <D:propstat>
         <D:prop><D:reftarget/></D:prop>
         <D:status>HTTP/1.1 404 Not Found</D:status>
       </D:propstat>
     </D:response>
     <D:response>
       <D:href>/geog/stats.html</D:href>
       <D:propstat>
         <D:prop>
           <D:resourcetype><D:redirectref/></D:resourcetype>
           <D:reftarget>
             <D:href>statistics/population/1997.html</D:href>



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           </D:reftarget>
         </D:prop>
       <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
     </D:response>
   </D:multistatus>

   In this example, the relative reference
   "statistics/population/1997.html" is returned as the value of the
   DAV:reftarget property for the reference resource identified by href
   /geog/stats.html.  The href is itself a relative reference, which
   resolves to http://example.com/geog/stats.html.  This is the base URI
   for resolving the relative reference in reftarget.  The absolute URI
   of reftarget is
   http://example.com/geog/statistics/population/1997.html.

11.  Redirect References to Collections

   In a Request-URI /segment1/segment2/segment3, any of the three
   segments may identify a redirect reference resource.  (See [RFC3986],
   Section 3.3, for definitions of "path" and "segment".)  If any
   segment in a Request-URI identifies a redirect reference resource,
   the response SHOULD be a 3xx.  The value of the Location header in
   the response is as follows:

   The leftmost path segment of the Request-URI that identifies a
   redirect reference resource, together with all path segments and
   separators to the left of it, is replaced by the value of the
   redirect reference resource's DAV:reftarget property (resolved to an
   absolute URI).  The remainder of the Request-URI is concatenated to
   this path.

   Note: If the DAV:reftarget property ends with a "/" and the remainder
   of the Request-URI is non-empty (and therefore must begin with a
   "/"), the final "/" in the DAV:reftarget property is dropped before
   the remainder of the Request-URI is appended.

   Consider Request-URI /x/y/z.html.  Suppose that /x/ is a redirect
   reference resource, whose target resource is collection /a/, which
   contains redirect reference resource y whose target resource is
   collection /b/, which contains redirect reference resource z.html,
   whose target resource is /c/d.html.









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                        /x/y/z.html
                            |
                            | /x -> /a
                            |
                            v
                        /a/y/z.html
                            |
                            | /a/y -> /b
                            |
                            v
                        /b/z.html
                            |
                            | /b/z.html -> /c/d.html
                            |
                            v
                        /c/d.html

   In this case, the client must follow up three separate 3xx responses
   before finally reaching the target resource.  The server responds to
   the initial request with a 3xx with Location: /a/y/z.html, and the
   client resubmits the request to /a/y/z.html.  The server responds to
   this request with a 3xx with Location: /b/z.html, and the client
   resubmits the request to /b/z.html.  The server responds to this
   request with a 3xx with Location: /c/d.html, and the client resubmits
   the request to /c/d.html.  This final request succeeds.

      Note: The behaviour described above may have a very serious impact
      on the efficiency of mapping Request-URIs to resources in HTTP
      request processing.  Therefore, servers MAY respond with a 404
      status code if the cost of checking all leading path segments for
      redirect references seems prohibitive.

12.  Headers

12.1.  Redirect-Ref Response Header

   Redirect-Ref = "Redirect-Ref:" (URI | relative-ref)
   ; URI: see [RFC3986], Section 3
   ; relative-ref: see [RFC3986], Section 4.2

   The Redirect-Ref header is used in all 3xx responses from redirect
   reference resources.  The value is the link target as specified
   during redirect reference resource creation.








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12.2.  Apply-To-Redirect-Ref Request Header

   Apply-To-Redirect-Ref = "Apply-To-Redirect-Ref" ":" ("T" | "F")

   The optional Apply-To-Redirect-Ref header can be used on any request
   to a redirect reference resource.  When it is present and set to "T",
   the request MUST be applied to the reference resource itself, and a
   3xx response MUST NOT be returned.

   If the Apply-To-Redirect-Ref header is used on a request to any other
   sort of resource besides a redirect reference resource, the server
   MUST ignore it.

13.  Redirect Reference Resource Properties

   The properties defined below are REQUIRED on redirect reference
   resources.  A PROPFIND/allprop request SHOULD NOT return any of the
   properties defined in this document.

13.1.  DAV:redirect-lifetime (protected)

   This property provides information about the lifetime of a redirect.
   It can be either DAV:permanent (HTTP status 301) or DAV:temporary
   (HTTP status 302).  Future protocols may define additional values.

   <!ELEMENT redirect-lifetime (permanent | temporary)>
   <!ELEMENT permanent EMPTY>
   <!ELEMENT temporary EMPTY>

13.2.  DAV:reftarget (protected)

   This property provides an efficient way for clients to discover the
   URI of the target resource.  This is a read-only property after its
   initial creation.  Its value can only be set in a MKREDIRECTREF
   request.  The value is a DAV:href element containing the URI of the
   target resource.

   <!ELEMENT reftarget href >

14.  XML Elements

14.1.  redirectref XML Element

   Name: redirectref

   Namespace: DAV:





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   Purpose: Used as the value of the DAV:resourcetype property to
      specify that the resource type is a redirect reference resource.

   <!ELEMENT redirectref EMPTY >

15.  Extensions to the DAV:response XML Element for Multi-Status
     Responses

   As described in Section 8, the DAV:location element may be returned
   in the DAV:response element of a 207 Multi-Status response, to allow
   clients to resubmit their requests to the target resource of a
   redirect reference resource.

   Consequently, the definition of the DAV:response XML element changes
   to the following:

   <!ELEMENT response (href, ((href*, status)|(propstat+)),
                       responsedescription?, location?) >
   <!ELEMENT location (href) >

16.  Capability Discovery

   Sections 9.1 and 15 of [RFC2518] describe the use of compliance
   classes with the DAV header in responses to OPTIONS, to indicate
   which parts of the WebDAV Distributed Authoring protocols the
   resource supports.  This specification defines an OPTIONAL extension
   to [RFC2518].  It defines a new compliance class, called
   redirectrefs, for use with the DAV header in responses to OPTIONS
   requests.  If a resource does support redirect references, its
   response to an OPTIONS request may indicate that it does, by listing
   the new redirectrefs compliance class in the DAV header and by
   listing the MKREDIRECTREF method as one it supports.

   When responding to an OPTIONS request, any type of resource can
   include redirectrefs in the value of the DAV header.  Doing so
   indicates that the server permits a redirect reference resource at
   the Request-URI.

16.1.  Example: Discovery of Support for Redirect Reference Resources

   >> Request:

   OPTIONS /somecollection/someresource HTTP/1.1
   Host: example.org







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   >> Response:

   HTTP/1.1 200 OK
   Allow: OPTIONS, GET, HEAD, POST, PUT, DELETE, TRACE, COPY, MOVE
   Allow: MKCOL, PROPFIND, PROPPATCH, LOCK, UNLOCK, MKREDIRECTREF
   DAV: 1, 2, redirectrefs

   The DAV header in the response indicates that the resource
   /somecollection/someresource is level 1 and level 2 compliant, as
   defined in [RFC2518].  In addition, /somecollection/someresource
   supports redirect reference resources.  The Allow header indicates
   that MKREDIRECTREF requests can be submitted to
   /somecollection/someresource.

17.  Security Considerations

   This section is provided to make applications that implement this
   protocol aware of the security implications of this protocol.

   All of the security considerations of HTTP/1.1 and the WebDAV
   Distributed Authoring Protocol specification also apply to this
   protocol specification.  In addition, redirect reference resources
   introduce several new security concerns and increase the risk of some
   existing threats.  These issues are detailed below.

17.1.  Privacy Concerns

   By creating redirect reference resources on a trusted server, it is
   possible for a hostile agent to induce users to send private
   information to a target on an unrelated system.  This risk is
   mitigated somewhat, since clients are required to notify the user of
   the redirection for any request other than GET or HEAD.  (See
   [RFC2616], Section 10.3.3, 302 Found.)

17.2.  Redirect Loops

   Although redirect loops were already possible in HTTP 1.1, the
   introduction of the MKREDIRECTREF method creates a new avenue for
   clients to create loops accidentally or maliciously.  If the
   reference resource and its target are on the same server, the server
   may be able to detect MKREDIRECTREF requests that would create loops.
   See also [RFC2616], Section 10.3, "Redirection 3xx."

17.3.  Redirect Reference Resources and Denial of Service

   Denial of service attacks were already possible by posting URLs that
   were intended for limited use at heavily used Web sites.  The
   introduction of MKREDIRECTREF creates a new avenue for similar denial



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   of service attacks.  Clients can now create redirect reference
   resources at heavily used sites to target locations that were not
   designed for heavy usage.

17.4.  Revealing Private Locations

   There are several ways that redirect reference resources may reveal
   information about collection structures.  First, the DAV:reftarget
   property of every redirect reference resource contains the URI of the
   target resource.  Anyone who has access to the reference resource can
   discover the collection path that leads to the target resource.  The
   owner of the target resource may have wanted to limit knowledge of
   this collection structure.

   Sufficiently powerful access control mechanisms can control this risk
   to some extent.  Property-level access control could prevent users
   from examining the DAV:reftarget property.  (The Location header
   returned in responses to requests on redirect reference resources
   reveals the same information, however.)

   This risk is no greater than the similar risk posed by HTML links.

18.  Internationalization Considerations

   All internationalization considerations mentioned in [RFC2518] also
   apply to this document.

19.  IANA Considerations

   All IANA considerations mentioned in [RFC2518] also apply to this
   document.

19.1.  HTTP headers

   This document specifies the two new HTTP headers listed below.

19.1.1.  Redirect-Ref

   Header field name: Redirect-Ref

   Applicable protocol: http

   Status: standard

   Author/Change controller: IETF

   Specification document: this specification (Section 12.1)




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19.1.2  Apply-To-Redirect-Ref

   Header field name: Apply-To-Redirect-Ref

   Applicable protocol: http

   Status: standard

   Author/Change controller: IETF

   Specification document: this specification (Section 12.2)

20.  Contributors

   Many thanks to Jason Crawford, Jim Davis, Chuck Fay, and Judith
   Slein, who can take credit for big parts of the original design of
   this specification.

21.  Acknowledgements

   This document has benefited from thoughtful discussion by Jim Amsden,
   Peter Carlson, Steve Carter, Tyson Chihaya, Ken Coar, Ellis Cohen,
   Bruce Cragun, Spencer Dawkins, Mark Day, Rajiv Dulepet, David Durand,
   Lisa Dusseault, Stefan Eissing, Roy Fielding, Yaron Goland, Fred
   Hitt, Alex Hopmann, James Hunt, Marcus Jager, Chris Kaler, Manoj
   Kasichainula, Rohit Khare, Daniel LaLiberte, Steve Martin, Larry
   Masinter, Jeff McAffer, Joe Orton, Surendra Koduru Reddy, Juergen
   Reuter, Max Rible, Sam Ruby, Bradley Sergeant, Nick Shelness, John
   Stracke, John Tigue, John Turner, Kevin Wiggen, and others.

22.  Normative References

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

   [RFC2518]  Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D.
              Jensen, "HTTP Extensions for Distributed Authoring --
              WEBDAV", RFC 2518, February 1999.

   [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.

   [RFC3253]  Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J.
              Whitehead, "Versioning Extensions to WebDAV (Web
              Distributed Authoring and Versioning)", RFC 3253, March
              2002.




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   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

Authors' Addresses

   Jim Whitehead
   UC Santa Cruz, Dept. of Computer Science
   1156 High Street
   Santa Cruz, CA  95064
   US

   EMail: ejw@cse.ucsc.edu


   Geoff Clemm
   IBM
   20 Maguire Road
   Lexington, MA  02421
   US

   EMail: geoffrey.clemm@us.ibm.com


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

   Phone: +49 251 2807760
   Fax:   +49 251 2807761
   EMail: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/

















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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
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   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).







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