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RFC 2518

HTTP Extensions for Distributed Authoring -- WEBDAV

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Network Working Group                                        Y. Goland
Request for Comments: 2518                                   Microsoft
Category: Standards Track                                 E. Whitehead
                                                             UC Irvine
                                                              A. Faizi
                                                              Netscape
                                                             S. Carter
                                                                Novell
                                                             D. Jensen
                                                                Novell
                                                         February 1999


          HTTP Extensions for Distributed Authoring -- WEBDAV

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

This document specifies a set of methods, headers, and content-types ancillary to HTTP/1.1 for the management of resource properties, creation and management of resource collections, namespace manipulation, and resource locking (collision avoidance).

Table of Contents

ABSTRACT............................................................1 1 INTRODUCTION .....................................................5 2 NOTATIONAL CONVENTIONS ...........................................7 3 TERMINOLOGY ......................................................7 4 DATA MODEL FOR RESOURCE PROPERTIES ...............................8 4.1 The Resource Property Model ...................................8 4.2 Existing Metadata Proposals ...................................8 4.3 Properties and HTTP Headers ...................................9 4.4 Property Values ...............................................9 4.5 Property Names ...............................................10 4.6 Media Independent Links ......................................10 5 COLLECTIONS OF WEB RESOURCES ....................................11
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   5.1  HTTP URL Namespace Model .....................................11
   5.2  Collection Resources .........................................11
   5.3  Creation and Retrieval of Collection Resources ...............12
   5.4  Source Resources and Output Resources ........................13
   6 LOCKING .........................................................14
   6.1  Exclusive Vs. Shared Locks ...................................14
   6.2  Required Support .............................................16
   6.3  Lock Tokens ..................................................16
   6.4  opaquelocktoken Lock Token URI Scheme ........................16
    6.4.1  Node Field Generation Without the IEEE 802 Address ........17
   6.5  Lock Capability Discovery ....................................19
   6.6  Active Lock Discovery ........................................19
   6.7  Usage Considerations .........................................19
   7 WRITE LOCK ......................................................20
   7.1  Methods Restricted by Write Locks ............................20
   7.2  Write Locks and Lock Tokens ..................................20
   7.3  Write Locks and Properties ...................................20
   7.4  Write Locks and Null Resources ...............................21
   7.5  Write Locks and Collections ..................................21
   7.6  Write Locks and the If Request Header ........................22
    7.6.1  Example - Write Lock ......................................22
   7.7  Write Locks and COPY/MOVE ....................................23
   7.8  Refreshing Write Locks .......................................23
   8 HTTP METHODS FOR DISTRIBUTED AUTHORING ..........................23
   8.1  PROPFIND .....................................................24
    8.1.1  Example - Retrieving Named Properties .....................25
    8.1.2  Example - Using allprop to Retrieve All Properties ........26
    8.1.3  Example - Using propname to Retrieve all Property Names ...29
   8.2  PROPPATCH ....................................................31
    8.2.1  Status Codes for use with 207 (Multi-Status) ..............31
    8.2.2  Example - PROPPATCH .......................................32
   8.3  MKCOL Method .................................................33
    8.3.1  Request ...................................................33
    8.3.2  Status Codes ..............................................33
    8.3.3  Example - MKCOL ...........................................34
   8.4  GET, HEAD for Collections ....................................34
   8.5  POST for Collections .........................................35
   8.6  DELETE .......................................................35
    8.6.1  DELETE for Non-Collection Resources .......................35
    8.6.2  DELETE for Collections ....................................36
   8.7  PUT ..........................................................36
    8.7.1  PUT for Non-Collection Resources ..........................36
    8.7.2  PUT for Collections .......................................37
   8.8  COPY Method ..................................................37
    8.8.1  COPY for HTTP/1.1 resources ...............................37
    8.8.2  COPY for Properties .......................................38
    8.8.3  COPY for Collections ......................................38
    8.8.4  COPY and the Overwrite Header .............................39
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    8.8.5  Status Codes ..............................................39
    8.8.6  Example - COPY with Overwrite .............................40
    8.8.7  Example - COPY with No Overwrite ..........................40
    8.8.8  Example - COPY of a Collection ............................41
   8.9  MOVE Method ..................................................42
    8.9.1  MOVE for Properties .......................................42
    8.9.2  MOVE for Collections ......................................42
    8.9.3  MOVE and the Overwrite Header .............................43
    8.9.4  Status Codes ..............................................43
    8.9.5  Example - MOVE of a Non-Collection ........................44
    8.9.6  Example - MOVE of a Collection ............................44
   8.10 LOCK Method ..................................................45
    8.10.1 Operation .................................................46
    8.10.2 The Effect of Locks on Properties and Collections .........46
    8.10.3 Locking Replicated Resources ..............................46
    8.10.4 Depth and Locking .........................................46
    8.10.5 Interaction with other Methods ............................47
    8.10.6 Lock Compatibility Table ..................................47
    8.10.7 Status Codes ..............................................48
    8.10.8 Example - Simple Lock Request .............................48
    8.10.9 Example - Refreshing a Write Lock .........................49
    8.10.10 Example - Multi-Resource Lock Request ....................50
   8.11 UNLOCK Method ................................................51
    8.11.1 Example - UNLOCK ..........................................52
   9 HTTP HEADERS FOR DISTRIBUTED AUTHORING ..........................52
   9.1  DAV Header ...................................................52
   9.2  Depth Header .................................................52
   9.3  Destination Header ...........................................54
   9.4  If Header ....................................................54
    9.4.1  No-tag-list Production ....................................55
    9.4.2  Tagged-list Production ....................................55
    9.4.3  not Production ............................................56
    9.4.4  Matching Function .........................................56
    9.4.5  If Header and Non-DAV Compliant Proxies ...................57
   9.5  Lock-Token Header ............................................57
   9.6  Overwrite Header .............................................57
   9.7  Status-URI Response Header ...................................57
   9.8  Timeout Request Header .......................................58
   10  STATUS CODE EXTENSIONS TO HTTP/1.1 ............................59
   10.1 102 Processing ...............................................59
   10.2 207 Multi-Status .............................................59
   10.3 422 Unprocessable Entity .....................................60
   10.4 423 Locked ...................................................60
   10.5 424 Failed Dependency ........................................60
   10.6 507 Insufficient Storage .....................................60
   11  MULTI-STATUS RESPONSE .........................................60
   12  XML ELEMENT DEFINITIONS .......................................61
   12.1 activelock XML Element .......................................61
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    12.1.1 depth XML Element .........................................61
    12.1.2 locktoken XML Element .....................................61
    12.1.3 timeout XML Element .......................................61
   12.2 collection XML Element .......................................62
   12.3 href XML Element .............................................62
   12.4 link XML Element .............................................62
    12.4.1 dst XML Element ...........................................62
    12.4.2 src XML Element ...........................................62
   12.5 lockentry XML Element ........................................63
   12.6 lockinfo XML Element .........................................63
   12.7 lockscope XML Element ........................................63
    12.7.1 exclusive XML Element .....................................63
    12.7.2 shared XML Element ........................................63
   12.8 locktype XML Element .........................................64
    12.8.1 write XML Element .........................................64
   12.9 multistatus XML Element ......................................64
    12.9.1 response XML Element ......................................64
    12.9.2 responsedescription XML Element ...........................65
   12.10 owner XML Element ...........................................65
   12.11 prop XML element ............................................66
   12.12 propertybehavior XML element ................................66
    12.12.1 keepalive XML element ....................................66
    12.12.2 omit XML element .........................................67
   12.13 propertyupdate XML element ..................................67
    12.13.1 remove XML element .......................................67
    12.13.2 set XML element ..........................................67
   12.14 propfind XML Element ........................................68
    12.14.1 allprop XML Element ......................................68
    12.14.2 propname XML Element .....................................68
   13  DAV PROPERTIES ................................................68
   13.1 creationdate Property ........................................69
   13.2 displayname Property .........................................69
   13.3 getcontentlanguage Property ..................................69
   13.4 getcontentlength Property ....................................69
   13.5 getcontenttype Property ......................................70
   13.6 getetag Property .............................................70
   13.7 getlastmodified Property .....................................70
   13.8 lockdiscovery Property .......................................71
    13.8.1 Example - Retrieving the lockdiscovery Property ...........71
   13.9 resourcetype Property ........................................72
   13.10 source Property .............................................72
    13.10.1 Example - A source Property ..............................72
   13.11 supportedlock Property ......................................73
    13.11.1 Example - Retrieving the supportedlock Property ..........73
   14  INSTRUCTIONS FOR PROCESSING XML IN DAV ........................74
   15  DAV COMPLIANCE CLASSES ........................................75
   15.1 Class 1 ......................................................75
   15.2 Class 2 ......................................................75
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   16  INTERNATIONALIZATION CONSIDERATIONS ...........................76
   17  SECURITY CONSIDERATIONS .......................................77
   17.1 Authentication of Clients ....................................77
   17.2 Denial of Service ............................................78
   17.3 Security through Obscurity ...................................78
   17.4 Privacy Issues Connected to Locks ............................78
   17.5 Privacy Issues Connected to Properties .......................79
   17.6 Reduction of Security due to Source Link .....................79
   17.7 Implications of XML External Entities ........................79
   17.8 Risks Connected with Lock Tokens .............................80
   18  IANA CONSIDERATIONS ...........................................80
   19  INTELLECTUAL PROPERTY .........................................81
   20  ACKNOWLEDGEMENTS ..............................................82
   21  REFERENCES ....................................................82
   21.1 Normative References .........................................82
   21.2 Informational References .....................................83
   22  AUTHORS' ADDRESSES ............................................84
   23  APPENDICES ....................................................86
   23.1 Appendix 1 - WebDAV Document Type Definition .................86
   23.2 Appendix 2 - ISO 8601 Date and Time Profile ..................88
   23.3 Appendix 3 - Notes on Processing XML Elements ................89
    23.3.1 Notes on Empty XML Elements ...............................89
    23.3.2 Notes on Illegal XML Processing ...........................89
   23.4 Appendix 4 -- XML Namespaces for WebDAV ......................92
    23.4.1 Introduction ..............................................92
    23.4.2 Meaning of Qualified Names ................................92
   24  FULL COPYRIGHT STATEMENT ......................................94



1 Introduction

This document describes an extension to the HTTP/1.1 protocol that allows clients to perform remote web content authoring operations. This extension provides a coherent set of methods, headers, request entity body formats, and response entity body formats that provide operations for: Properties: The ability to create, remove, and query information about Web pages, such as their authors, creation dates, etc. Also, the ability to link pages of any media type to related pages. Collections: The ability to create sets of documents and to retrieve a hierarchical membership listing (like a directory listing in a file system).
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   Locking: The ability to keep more than one person from working on a
   document at the same time. This prevents the "lost update problem,"
   in which modifications are lost as first one author then another
   writes changes without merging the other author's changes.

   Namespace Operations: The ability to instruct the server to copy and
   move Web resources.

   Requirements and rationale for these operations are described in a
   companion document, "Requirements for a Distributed Authoring and
   Versioning Protocol for the World Wide Web" [RFC2291].

   The sections below provide a detailed introduction to resource
   properties (section 4), collections of resources (section 5), and
   locking operations (section 6).  These sections introduce the
   abstractions manipulated by the WebDAV-specific HTTP methods
   described in section 8, "HTTP Methods for Distributed Authoring".

   In HTTP/1.1, method parameter information was exclusively encoded in
   HTTP headers. Unlike HTTP/1.1, WebDAV encodes method parameter
   information either in an Extensible Markup Language (XML) [REC-XML]
   request entity body, or in an HTTP header.  The use of XML to encode
   method parameters was motivated by the ability to add extra XML
   elements to existing structures, providing extensibility; and by
   XML's ability to encode information in ISO 10646 character sets,
   providing internationalization support. As a rule of thumb,
   parameters are encoded in XML entity bodies when they have unbounded
   length, or when they may be shown to a human user and hence require
   encoding in an ISO 10646 character set.  Otherwise, parameters are
   encoded within HTTP headers.  Section 9 describes the new HTTP
   headers used with WebDAV methods.

   In addition to encoding method parameters, XML is used in WebDAV to
   encode the responses from methods, providing the extensibility and
   internationalization advantages of XML for method output, as well as
   input.

   XML elements used in this specification are defined in section 12.

   The XML namespace extension (Appendix 4) is also used in this
   specification in order to allow for new XML elements to be added
   without fear of colliding with other element names.

   While the status codes provided by HTTP/1.1 are sufficient to
   describe most error conditions encountered by WebDAV methods, there
   are some errors that do not fall neatly into the existing categories.
   New status codes developed for the WebDAV methods are defined in
   section 10.  Since some WebDAV methods may operate over many
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   resources, the Multi-Status response has been introduced to return
   status information for multiple resources.  The Multi-Status response
   is described in section 11.

   WebDAV employs the property mechanism to store information about the
   current state of the resource.  For example, when a lock is taken out
   on a resource, a lock information property describes the current
   state of the lock. Section 13 defines the properties used within the
   WebDAV specification.

   Finishing off the specification are sections on what it means to be
   compliant with this specification (section 15), on
   internationalization support (section 16), and on security (section
   17).

2 Notational Conventions

Since this document describes a set of extensions to the HTTP/1.1 protocol, the augmented BNF used herein to describe protocol elements is exactly the same as described in section 2.1 of [RFC2068]. Since this augmented BNF uses the basic production rules provided in section 2.2 of [RFC2068], these rules apply to this document as well. 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 [RFC2119].

3 Terminology

URI/URL - A Uniform Resource Identifier and Uniform Resource Locator, respectively. These terms (and the distinction between them) are defined in [RFC2396]. Collection - A resource that contains a set of URIs, termed member URIs, which identify member resources and meets the requirements in section 5 of this specification. Member URI - A URI which is a member of the set of URIs contained by a collection. Internal Member URI - A Member URI that is immediately relative to the URI of the collection (the definition of immediately relative is given in section 5.2). Property - A name/value pair that contains descriptive information about a resource.
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   Live Property - A property whose semantics and syntax are enforced by
   the server.  For example, the live "getcontentlength" property has
   its value, the length of the entity returned by a GET request,
   automatically calculated by the server.

   Dead Property - A property whose semantics and syntax are not
   enforced by the server.  The server only records the value of a dead
   property; the client is responsible for maintaining the consistency
   of the syntax and semantics of a dead property.

   Null Resource - A resource which responds with a 404 (Not Found) to
   any HTTP/1.1 or DAV method except for PUT, MKCOL, OPTIONS and LOCK.
   A NULL resource MUST NOT appear as a member of its parent collection.

4 Data Model for Resource Properties

4.1 The Resource Property Model

Properties are pieces of data that describe the state of a resource. Properties are data about data. Properties are used in distributed authoring environments to provide for efficient discovery and management of resources. For example, a 'subject' property might allow for the indexing of all resources by their subject, and an 'author' property might allow for the discovery of what authors have written which documents. The DAV property model consists of name/value pairs. The name of a property identifies the property's syntax and semantics, and provides an address by which to refer to its syntax and semantics. There are two categories of properties: "live" and "dead". A live property has its syntax and semantics enforced by the server. Live properties include cases where a) the value of a property is read- only, maintained by the server, and b) the value of the property is maintained by the client, but the server performs syntax checking on submitted values. All instances of a given live property MUST comply with the definition associated with that property name. A dead property has its syntax and semantics enforced by the client; the server merely records the value of the property verbatim.

4.2 Existing Metadata Proposals

Properties have long played an essential role in the maintenance of large document repositories, and many current proposals contain some notion of a property, or discuss web metadata more generally. These include PICS [REC-PICS], PICS-NG, XML, Web Collections, and several proposals on representing relationships within HTML. Work on PICS-NG
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   and Web Collections has been subsumed by the Resource Description
   Framework (RDF) metadata activity of the World Wide Web Consortium.
   RDF consists of a network-based data model and an XML representation
   of that model.

   Some proposals come from a digital library perspective.  These
   include the Dublin Core [RFC2413] metadata set and the Warwick
   Framework [WF], a container architecture for different metadata
   schemas.  The literature includes many examples of metadata,
   including MARC [USMARC], a bibliographic metadata format, and a
   technical report bibliographic format employed by the Dienst system
   [RFC1807]. Additionally, the proceedings from the first IEEE Metadata
   conference describe many community-specific metadata sets.

   Participants of the 1996 Metadata II Workshop in Warwick, UK [WF],
   noted that "new metadata sets will develop as the networked
   infrastructure matures" and "different communities will propose,
   design, and be responsible for different types of metadata." These
   observations can be corroborated by noting that many community-
   specific sets of metadata already exist, and there is significant
   motivation for the development of new forms of metadata as many
   communities increasingly make their data available in digital form,
   requiring a metadata format to assist data location and cataloging.

4.3 Properties and HTTP Headers

Properties already exist, in a limited sense, in HTTP message headers. However, in distributed authoring environments a relatively large number of properties are needed to describe the state of a resource, and setting/returning them all through HTTP headers is inefficient. Thus a mechanism is needed which allows a principal to identify a set of properties in which the principal is interested and to set or retrieve just those properties.

4.4 Property Values

The value of a property when expressed in XML MUST be well formed. XML has been chosen because it is a flexible, self-describing, structured data format that supports rich schema definitions, and because of its support for multiple character sets. XML's self- describing nature allows any property's value to be extended by adding new elements. Older clients will not break when they encounter extensions because they will still have the data specified in the original schema and will ignore elements they do not understand. XML's support for multiple character sets allows any human-readable property to be encoded and read in a character set familiar to the user. XML's support for multiple human languages,
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   using the "xml:lang" attribute, handles cases where the same
   character set is employed by multiple human languages.

4.5 Property Names

A property name is a universally unique identifier that is associated with a schema that provides information about the syntax and semantics of the property. Because a property's name is universally unique, clients can depend upon consistent behavior for a particular property across multiple resources, on the same and across different servers, so long as that property is "live" on the resources in question, and the implementation of the live property is faithful to its definition. The XML namespace mechanism, which is based on URIs [RFC2396], is used to name properties because it prevents namespace collisions and provides for varying degrees of administrative control. The property namespace is flat; that is, no hierarchy of properties is explicitly recognized. Thus, if a property A and a property A/B exist on a resource, there is no recognition of any relationship between the two properties. It is expected that a separate specification will eventually be produced which will address issues relating to hierarchical properties. Finally, it is not possible to define the same property twice on a single resource, as this would cause a collision in the resource's property namespace.

4.6 Media Independent Links

Although HTML resources support links to other resources, the Web needs more general support for links between resources of any media type (media types are also known as MIME types, or content types). WebDAV provides such links. A WebDAV link is a special type of property value, formally defined in section 12.4, that allows typed connections to be established between resources of any media type. The property value consists of source and destination Uniform Resource Identifiers (URIs); the property name identifies the link type.
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5 Collections of Web Resources

This section provides a description of a new type of Web resource, the collection, and discusses its interactions with the HTTP URL namespace. The purpose of a collection resource is to model collection-like objects (e.g., file system directories) within a server's namespace. All DAV compliant resources MUST support the HTTP URL namespace model specified herein.

5.1 HTTP URL Namespace Model

The HTTP URL namespace is a hierarchical namespace where the hierarchy is delimited with the "/" character. An HTTP URL namespace is said to be consistent if it meets the following conditions: for every URL in the HTTP hierarchy there exists a collection that contains that URL as an internal member. The root, or top-level collection of the namespace under consideration is exempt from the previous rule. Neither HTTP/1.1 nor WebDAV require that the entire HTTP URL namespace be consistent. However, certain WebDAV methods are prohibited from producing results that cause namespace inconsistencies. Although implicit in [RFC2068] and [RFC2396], any resource, including collection resources, MAY be identified by more than one URI. For example, a resource could be identified by multiple HTTP URLs.

5.2 Collection Resources

A collection is a resource whose state consists of at least a list of internal member URIs and a set of properties, but which may have additional state such as entity bodies returned by GET. An internal member URI MUST be immediately relative to a base URI of the collection. That is, the internal member URI is equal to a containing collection's URI plus an additional segment for non- collection resources, or additional segment plus trailing slash "/" for collection resources, where segment is defined in section 3.3 of [RFC2396]. Any given internal member URI MUST only belong to the collection once, i.e., it is illegal to have multiple instances of the same URI in a collection. Properties defined on collections behave exactly as do properties on non-collection resources.
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   For all WebDAV compliant resources A and B, identified by URIs U and
   V, for which U is immediately relative to V, B MUST be a collection
   that has U as an internal member URI. So, if the resource with URL
   http://foo.com/bar/blah is WebDAV compliant and if the resource with
   URL http://foo.com/bar/ is WebDAV compliant then the resource with
   URL http://foo.com/bar/ must be a collection and must contain URL
   http://foo.com/bar/blah as an internal member.

   Collection resources MAY list the URLs of non-WebDAV compliant
   children in the HTTP URL namespace hierarchy as internal members but
   are not required to do so. For example, if the resource with URL
   http://foo.com/bar/blah is not WebDAV compliant and the URL
   http://foo.com/bar/ identifies a collection then URL
   http://foo.com/bar/blah may or may not be an internal member of the
   collection with URL http://foo.com/bar/.

   If a WebDAV compliant resource has no WebDAV compliant children in
   the HTTP URL namespace hierarchy then the WebDAV compliant resource
   is not required to be a collection.

   There is a standing convention that when a collection is referred to
   by its name without a trailing slash, the trailing slash is
   automatically appended.  Due to this, a resource may accept a URI
   without a trailing "/" to point to a collection. In this case it
   SHOULD return a content-location header in the response pointing to
   the URI ending with the "/".  For example, if a client invokes a
   method on http://foo.bar/blah (no trailing slash), the resource
   http://foo.bar/blah/ (trailing slash) may respond as if the operation
   were invoked on it, and should return a content-location header with
   http://foo.bar/blah/ in it.  In general clients SHOULD use the "/"
   form of collection names.

   A resource MAY be a collection but not be WebDAV compliant.  That is,
   the resource may comply with all the rules set out in this
   specification regarding how a collection is to behave without
   necessarily supporting all methods that a WebDAV compliant resource
   is required to support.  In such a case the resource may return the
   DAV:resourcetype property with the value DAV:collection but MUST NOT
   return a DAV header containing the value "1" on an OPTIONS response.

5.3 Creation and Retrieval of Collection Resources

This document specifies the MKCOL method to create new collection resources, rather than using the existing HTTP/1.1 PUT or POST method, for the following reasons:
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   In HTTP/1.1, the PUT method is defined to store the request body at
   the location specified by the Request-URI.  While a description
   format for a collection can readily be constructed for use with PUT,
   the implications of sending such a description to the server are
   undesirable.  For example, if a description of a collection that
   omitted some existing resources were PUT to a server, this might be
   interpreted as a command to remove those members.  This would extend
   PUT to perform DELETE functionality, which is undesirable since it
   changes the semantics of PUT, and makes it difficult to control
   DELETE functionality with an access control scheme based on methods.

   While the POST method is sufficiently open-ended that a "create a
   collection" POST command could be constructed, this is undesirable
   because it would be difficult to separate access control for
   collection creation from other uses of POST.

   The exact definition of the behavior of GET and PUT on collections is
   defined later in this document.

5.4 Source Resources and Output Resources

For many resources, the entity returned by a GET method exactly matches the persistent state of the resource, for example, a GIF file stored on a disk. For this simple case, the URI at which a resource is accessed is identical to the URI at which the source (the persistent state) of the resource is accessed. This is also the case for HTML source files that are not processed by the server prior to transmission. However, the server can sometimes process HTML resources before they are transmitted as a return entity body. For example, a server- side-include directive within an HTML file might instruct a server to replace the directive with another value, such as the current date. In this case, what is returned by GET (HTML plus date) differs from the persistent state of the resource (HTML plus directive). Typically there is no way to access the HTML resource containing the unprocessed directive. Sometimes the entity returned by GET is the output of a data- producing process that is described by one or more source resources (that may not even have a location in the URI namespace). A single data-producing process may dynamically generate the state of a potentially large number of output resources. An example of this is a CGI script that describes a "finger" gateway process that maps part of the namespace of a server into finger requests, such as http://www.foo.bar.org/finger_gateway/user@host.
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   In the absence of distributed authoring capabilities, it is
   acceptable to have no mapping of source resource(s) to the URI
   namespace. In fact, preventing access to the source resource(s) has
   desirable security benefits.  However, if remote editing of the
   source resource(s) is desired, the source resource(s) should be given
   a location in the URI namespace.  This source location should not be
   one of the locations at which the generated output is retrievable,
   since in general it is impossible for the server to differentiate
   requests for source resources from requests for process output
   resources.  There is often a many-to-many relationship between source
   resources and output resources.

   On WebDAV compliant servers the URI of the source resource(s) may be
   stored in a link on the output resource with type DAV:source (see
   section 13.10 for a description of the source link property).
   Storing the source URIs in links on the output resources places the
   burden of discovering the source on the authoring client.  Note that
   the value of a source link is not guaranteed to point to the correct
   source.  Source links may break or incorrect values may be entered.
   Also note that not all servers will allow the client to set the
   source link value.  For example a server which generates source links
   on the fly for its CGI files will most likely not allow a client to
   set the source link value.

6 Locking

The ability to lock a resource provides a mechanism for serializing access to that resource. Using a lock, an authoring client can provide a reasonable guarantee that another principal will not modify a resource while it is being edited. In this way, a client can prevent the "lost update" problem. This specification allows locks to vary over two client-specified parameters, the number of principals involved (exclusive vs. shared) and the type of access to be granted. This document defines locking for only one access type, write. However, the syntax is extensible, and permits the eventual specification of locking for other access types.

6.1 Exclusive Vs. Shared Locks

The most basic form of lock is an exclusive lock. This is a lock where the access right in question is only granted to a single principal. The need for this arbitration results from a desire to avoid having to merge results.
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   However, there are times when the goal of a lock is not to exclude
   others from exercising an access right but rather to provide a
   mechanism for principals to indicate that they intend to exercise
   their access rights.  Shared locks are provided for this case.  A
   shared lock allows multiple principals to receive a lock.  Hence any
   principal with appropriate access can get the lock.

   With shared locks there are two trust sets that affect a resource.
   The first trust set is created by access permissions.  Principals who
   are trusted, for example, may have permission to write to the
   resource.  Among those who have access permission to write to the
   resource, the set of principals who have taken out a shared lock also
   must trust each other, creating a (typically) smaller trust set
   within the access permission write set.

   Starting with every possible principal on the Internet, in most
   situations the vast majority of these principals will not have write
   access to a given resource.  Of the small number who do have write
   access, some principals may decide to guarantee their edits are free
   from overwrite conflicts by using exclusive write locks.  Others may
   decide they trust their collaborators will not overwrite their work
   (the potential set of collaborators being the set of principals who
   have write permission) and use a shared lock, which informs their
   collaborators that a principal may be working on the resource.

   The WebDAV extensions to HTTP do not need to provide all of the
   communications paths necessary for principals to coordinate their
   activities.  When using shared locks, principals may use any out of
   band communication channel to coordinate their work (e.g., face-to-
   face interaction, written notes, post-it notes on the screen,
   telephone conversation, Email, etc.)  The intent of a shared lock is
   to let collaborators know who else may be working on a resource.

   Shared locks are included because experience from web distributed
   authoring systems has indicated that exclusive locks are often too
   rigid.  An exclusive lock is used to enforce a particular editing
   process: take out an exclusive lock, read the resource, perform
   edits, write the resource, release the lock.  This editing process
   has the problem that locks are not always properly released, for
   example when a program crashes, or when a lock owner leaves without
   unlocking a resource.  While both timeouts and administrative action
   can be used to remove an offending lock, neither mechanism may be
   available when needed; the timeout may be long or the administrator
   may not be available.
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6.2 Required Support

A WebDAV compliant server is not required to support locking in any form. If the server does support locking it may choose to support any combination of exclusive and shared locks for any access types. The reason for this flexibility is that locking policy strikes to the very heart of the resource management and versioning systems employed by various storage repositories. These repositories require control over what sort of locking will be made available. For example, some repositories only support shared write locks while others only provide support for exclusive write locks while yet others use no locking at all. As each system is sufficiently different to merit exclusion of certain locking features, this specification leaves locking as the sole axis of negotiation within WebDAV.

6.3 Lock Tokens

A lock token is a type of state token, represented as a URI, which identifies a particular lock. A lock token is returned by every successful LOCK operation in the lockdiscovery property in the response body, and can also be found through lock discovery on a resource. Lock token URIs MUST be unique across all resources for all time. This uniqueness constraint allows lock tokens to be submitted across resources and servers without fear of confusion. This specification provides a lock token URI scheme called opaquelocktoken that meets the uniqueness requirements. However resources are free to return any URI scheme so long as it meets the uniqueness requirements. Having a lock token provides no special access rights. Anyone can find out anyone else's lock token by performing lock discovery. Locks MUST be enforced based upon whatever authentication mechanism is used by the server, not based on the secrecy of the token values.

6.4 opaquelocktoken Lock Token URI Scheme

The opaquelocktoken URI scheme is designed to be unique across all resources for all time. Due to this uniqueness quality, a client may submit an opaque lock token in an If header on a resource other than the one that returned it. All resources MUST recognize the opaquelocktoken scheme and, at minimum, recognize that the lock token does not refer to an outstanding lock on the resource.
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   In order to guarantee uniqueness across all resources for all time
   the opaquelocktoken requires the use of the Universal Unique
   Identifier (UUID) mechanism, as described in [ISO-11578].

   Opaquelocktoken generators, however, have a choice of how they create
   these tokens.  They can either generate a new UUID for every lock
   token they create or they can create a single UUID  and then add
   extension characters.  If the second method is selected then the
   program generating the extensions MUST guarantee that the same
   extension will never be used twice with the associated UUID.

   OpaqueLockToken-URI = "opaquelocktoken:" UUID [Extension]  ; The UUID
   production is the string representation of a UUID, as defined in
   [ISO-11578]. Note that white space (LWS) is not allowed between
   elements of this production.

   Extension = path  ; path is defined in section 3.2.1 of RFC 2068
   [RFC2068]

6.4.1 Node Field Generation Without the IEEE 802 Address

UUIDs, as defined in [ISO-11578], contain a "node" field that contains one of the IEEE 802 addresses for the server machine. As noted in section 17.8, there are several security risks associated with exposing a machine's IEEE 802 address. This section provides an alternate mechanism for generating the "node" field of a UUID which does not employ an IEEE 802 address. WebDAV servers MAY use this algorithm for creating the node field when generating UUIDs. The text in this section is originally from an Internet-Draft by Paul Leach and Rich Salz, who are noted here to properly attribute their work. The ideal solution is to obtain a 47 bit cryptographic quality random number, and use it as the low 47 bits of the node ID, with the most significant bit of the first octet of the node ID set to 1. This bit is the unicast/multicast bit, which will never be set in IEEE 802 addresses obtained from network cards; hence, there can never be a conflict between UUIDs generated by machines with and without network cards. If a system does not have a primitive to generate cryptographic quality random numbers, then in most systems there are usually a fairly large number of sources of randomness available from which one can be generated. Such sources are system specific, but often include:
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     - the percent of memory in use
     - the size of main memory in bytes
     - the amount of free main memory in bytes
     - the size of the paging or swap file in bytes
     - free bytes of paging or swap file
     - the total size of user virtual address space in bytes
     - the total available user address space bytes
     - the size of boot disk drive in bytes
     - the free disk space on boot drive in bytes
     - the current time
     - the amount of time since the system booted
     - the individual sizes of files in various system directories
     - the creation, last read, and modification times of files in
       various system directories
     - the utilization factors of various system resources (heap, etc.)
     - current mouse cursor position
     - current caret position
     - current number of running processes, threads
     - handles or IDs of the desktop window and the active window
     - the value of stack pointer of the caller
     - the process and thread ID of caller
     - various processor architecture specific performance counters
       (instructions executed, cache misses, TLB misses)

   (Note that it is precisely the above kinds of sources of randomness
   that are used to seed cryptographic quality random number generators
   on systems without special hardware for their construction.)

   In addition, items such as the computer's name and the name of the
   operating system, while not strictly speaking random, will help
   differentiate the results from those obtained by other systems.

   The exact algorithm to generate a node ID using these data is system
   specific, because both the data available and the functions to obtain
   them are often very system specific. However, assuming that one can
   concatenate all the values from the randomness sources into a buffer,
   and that a cryptographic hash function such as MD5 is available, then
   any 6 bytes of the MD5 hash of the buffer, with the multicast bit
   (the high bit of the first byte) set will be an appropriately random
   node ID.

   Other hash functions, such as SHA-1, can also be used. The only
   requirement is that the result be suitably random _ in the sense that
   the outputs from a set uniformly distributed inputs are themselves
   uniformly distributed, and that a single bit change in the input can
   be expected to cause half of the output bits to change.
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6.5 Lock Capability Discovery

Since server lock support is optional, a client trying to lock a resource on a server can either try the lock and hope for the best, or perform some form of discovery to determine what lock capabilities the server supports. This is known as lock capability discovery. Lock capability discovery differs from discovery of supported access control types, since there may be access control types without corresponding lock types. A client can determine what lock types the server supports by retrieving the supportedlock property. Any DAV compliant resource that supports the LOCK method MUST support the supportedlock property.

6.6 Active Lock Discovery

If another principal locks a resource that a principal wishes to access, it is useful for the second principal to be able to find out who the first principal is. For this purpose the lockdiscovery property is provided. This property lists all outstanding locks, describes their type, and where available, provides their lock token. Any DAV compliant resource that supports the LOCK method MUST support the lockdiscovery property.

6.7 Usage Considerations

Although the locking mechanisms specified here provide some help in preventing lost updates, they cannot guarantee that updates will never be lost. Consider the following scenario: Two clients A and B are interested in editing the resource ' index.html'. Client A is an HTTP client rather than a WebDAV client, and so does not know how to perform locking. Client A doesn't lock the document, but does a GET and begins editing. Client B does LOCK, performs a GET and begins editing. Client B finishes editing, performs a PUT, then an UNLOCK. Client A performs a PUT, overwriting and losing all of B's changes. There are several reasons why the WebDAV protocol itself cannot prevent this situation. First, it cannot force all clients to use locking because it must be compatible with HTTP clients that do not comprehend locking. Second, it cannot require servers to support locking because of the variety of repository implementations, some of which rely on reservations and merging rather than on locking. Finally, being stateless, it cannot enforce a sequence of operations like LOCK / GET / PUT / UNLOCK.
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   WebDAV servers that support locking can reduce the likelihood that
   clients will accidentally overwrite each other's changes by requiring
   clients to lock resources before modifying them.  Such servers would
   effectively prevent HTTP 1.0 and HTTP 1.1 clients from modifying
   resources.

   WebDAV clients can be good citizens by using a lock / retrieve /
   write /unlock sequence of operations (at least by default) whenever
   they interact with a WebDAV server that supports locking.

   HTTP 1.1 clients can be good citizens, avoiding overwriting other
   clients' changes, by using entity tags in If-Match headers with any
   requests that would modify resources.

   Information managers may attempt to prevent overwrites by
   implementing client-side procedures requiring locking before
   modifying WebDAV resources.

7 Write Lock

This section describes the semantics specific to the write lock type. The write lock is a specific instance of a lock type, and is the only lock type described in this specification.

7.1 Methods Restricted by Write Locks

A write lock MUST prevent a principal without the lock from successfully executing a PUT, POST, PROPPATCH, LOCK, UNLOCK, MOVE, DELETE, or MKCOL on the locked resource. All other current methods, GET in particular, function independently of the lock. Note, however, that as new methods are created it will be necessary to specify how they interact with a write lock.

7.2 Write Locks and Lock Tokens

A successful request for an exclusive or shared write lock MUST result in the generation of a unique lock token associated with the requesting principal. Thus if five principals have a shared write lock on the same resource there will be five lock tokens, one for each principal.

7.3 Write Locks and Properties

While those without a write lock may not alter a property on a resource it is still possible for the values of live properties to change, even while locked, due to the requirements of their schemas.
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   Only dead properties and live properties defined to respect locks are
   guaranteed not to change while write locked.

7.4 Write Locks and Null Resources

It is possible to assert a write lock on a null resource in order to lock the name. A write locked null resource, referred to as a lock-null resource, MUST respond with a 404 (Not Found) or 405 (Method Not Allowed) to any HTTP/1.1 or DAV methods except for PUT, MKCOL, OPTIONS, PROPFIND, LOCK, and UNLOCK. A lock-null resource MUST appear as a member of its parent collection. Additionally the lock-null resource MUST have defined on it all mandatory DAV properties. Most of these properties, such as all the get* properties, will have no value as a lock-null resource does not support the GET method. Lock-Null resources MUST have defined values for lockdiscovery and supportedlock properties. Until a method such as PUT or MKCOL is successfully executed on the lock-null resource the resource MUST stay in the lock-null state. However, once a PUT or MKCOL is successfully executed on a lock-null resource the resource ceases to be in the lock-null state. If the resource is unlocked, for any reason, without a PUT, MKCOL, or similar method having been successfully executed upon it then the resource MUST return to the null state.

7.5 Write Locks and Collections

A write lock on a collection, whether created by a "Depth: 0" or "Depth: infinity" lock request, prevents the addition or removal of member URIs of the collection by non-lock owners. As a consequence, when a principal issues a PUT or POST request to create a new resource under a URI which needs to be an internal member of a write locked collection to maintain HTTP namespace consistency, or issues a DELETE to remove a resource which has a URI which is an existing internal member URI of a write locked collection, this request MUST fail if the principal does not have a write lock on the collection. However, if a write lock request is issued to a collection containing member URIs identifying resources that are currently locked in a manner which conflicts with the write lock, the request MUST fail with a 423 (Locked) status code. If a lock owner causes the URI of a resource to be added as an internal member URI of a locked collection then the new resource MUST be automatically added to the lock. This is the only mechanism that
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   allows a resource to be added to a write lock.  Thus, for example, if
   the collection /a/b/ is write locked and the resource /c is moved to
   /a/b/c then resource /a/b/c will be added to the write lock.

7.6 Write Locks and the If Request Header

If a user agent is not required to have knowledge about a lock when requesting an operation on a locked resource, the following scenario might occur. Program A, run by User A, takes out a write lock on a resource. Program B, also run by User A, has no knowledge of the lock taken out by Program A, yet performs a PUT to the locked resource. In this scenario, the PUT succeeds because locks are associated with a principal, not a program, and thus program B, because it is acting with principal A's credential, is allowed to perform the PUT. However, had program B known about the lock, it would not have overwritten the resource, preferring instead to present a dialog box describing the conflict to the user. Due to this scenario, a mechanism is needed to prevent different programs from accidentally ignoring locks taken out by other programs with the same authorization. In order to prevent these collisions a lock token MUST be submitted by an authorized principal in the If header for all locked resources that a method may interact with or the method MUST fail. For example, if a resource is to be moved and both the source and destination are locked then two lock tokens must be submitted, one for the source and the other for the destination.

7.6.1 Example - Write Lock

>>Request COPY /~fielding/index.html HTTP/1.1 Host: www.ics.uci.edu Destination: http://www.ics.uci.edu/users/f/fielding/index.html If: <http://www.ics.uci.edu/users/f/fielding/index.html> (<opaquelocktoken:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>) >>Response HTTP/1.1 204 No Content In this example, even though both the source and destination are locked, only one lock token must be submitted, for the lock on the destination. This is because the source resource is not modified by a COPY, and hence unaffected by the write lock. In this example, user agent authentication has previously occurred via a mechanism outside the scope of the HTTP protocol, in the underlying transport layer.
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7.7 Write Locks and COPY/MOVE

A COPY method invocation MUST NOT duplicate any write locks active on the source. However, as previously noted, if the COPY copies the resource into a collection that is locked with "Depth: infinity", then the resource will be added to the lock. A successful MOVE request on a write locked resource MUST NOT move the write lock with the resource. However, the resource is subject to being added to an existing lock at the destination, as specified in section 7.5. For example, if the MOVE makes the resource a child of a collection that is locked with "Depth: infinity", then the resource will be added to that collection's lock. Additionally, if a resource locked with "Depth: infinity" is moved to a destination that is within the scope of the same lock (e.g., within the namespace tree covered by the lock), the moved resource will again be a added to the lock. In both these examples, as specified in section 7.6, an If header must be submitted containing a lock token for both the source and destination.

7.8 Refreshing Write Locks

A client MUST NOT submit the same write lock request twice. Note that a client is always aware it is resubmitting the same lock request because it must include the lock token in the If header in order to make the request for a resource that is already locked. However, a client may submit a LOCK method with an If header but without a body. This form of LOCK MUST only be used to "refresh" a lock. Meaning, at minimum, that any timers associated with the lock MUST be re-set. A server may return a Timeout header with a lock refresh that is different than the Timeout header returned when the lock was originally requested. Additionally clients may submit Timeout headers of arbitrary value with their lock refresh requests. Servers, as always, may ignore Timeout headers submitted by the client. If an error is received in response to a refresh LOCK request the client SHOULD assume that the lock was not refreshed.


(page 23 continued on part 2)

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