rfc4918
Network Working Group L. Dusseault, Ed.
Request for Comments: 4918 CommerceNet
Obsoletes: 2518 June 2007
Category: Standards Track
HTTP Extensions for Web Distributed Authoring and Versioning (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 IETF Trust (2007).
Abstract
Web Distributed Authoring and Versioning (WebDAV) consists of 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, URL namespace manipulation, and resource
locking (collision avoidance).
RFC 2518 was published in February 1999, and this specification
obsoletes RFC 2518 with minor revisions mostly due to
interoperability experience.
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Table of Contents
1. Introduction ....................................................7
2. Notational Conventions ..........................................8
3. Terminology .....................................................8
4. Data Model for Resource Properties .............................10
4.1. The Resource Property Model ...............................10
4.2. Properties and HTTP Headers ...............................10
4.3. Property Values ...........................................10
4.3.1. Example - Property with Mixed Content ..............12
4.4. Property Names ............................................14
4.5. Source Resources and Output Resources .....................14
5. Collections of Web Resources ...................................14
5.1. HTTP URL Namespace Model ..................................15
5.2. Collection Resources ......................................15
6. Locking ........................................................17
6.1. Lock Model ................................................18
6.2. Exclusive vs. Shared Locks ................................19
6.3. Required Support ..........................................20
6.4. Lock Creator and Privileges ...............................20
6.5. Lock Tokens ...............................................21
6.6. Lock Timeout ..............................................21
6.7. Lock Capability Discovery .................................22
6.8. Active Lock Discovery .....................................22
7. Write Lock .....................................................23
7.1. Write Locks and Properties ................................24
7.2. Avoiding Lost Updates .....................................24
7.3. Write Locks and Unmapped URLs .............................25
7.4. Write Locks and Collections ...............................26
7.5. Write Locks and the If Request Header .....................28
7.5.1. Example - Write Lock and COPY ......................28
7.5.2. Example - Deleting a Member of a Locked
Collection .........................................29
7.6. Write Locks and COPY/MOVE .................................30
7.7. Refreshing Write Locks ....................................30
8. General Request and Response Handling ..........................31
8.1. Precedence in Error Handling ..............................31
8.2. Use of XML ................................................31
8.3. URL Handling ..............................................32
8.3.1. Example - Correct URL Handling .....................32
8.4. Required Bodies in Requests ...............................33
8.5. HTTP Headers for Use in WebDAV ............................33
8.6. ETag ......................................................33
8.7. Including Error Response Bodies ...........................34
8.8. Impact of Namespace Operations on Cache Validators ........34
9. HTTP Methods for Distributed Authoring .........................35
9.1. PROPFIND Method ...........................................35
9.1.1. PROPFIND Status Codes ..............................37
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9.1.2. Status Codes for Use in 'propstat' Element .........37
9.1.3. Example - Retrieving Named Properties ..............38
9.1.4. Example - Using 'propname' to Retrieve All
Property Names .....................................39
9.1.5. Example - Using So-called 'allprop' ................41
9.1.6. Example - Using 'allprop' with 'include' ...........43
9.2. PROPPATCH Method ..........................................44
9.2.1. Status Codes for Use in 'propstat' Element .........44
9.2.2. Example - PROPPATCH ................................45
9.3. MKCOL Method ..............................................46
9.3.1. MKCOL Status Codes .................................47
9.3.2. Example - MKCOL ....................................47
9.4. GET, HEAD for Collections .................................48
9.5. POST for Collections ......................................48
9.6. DELETE Requirements .......................................48
9.6.1. DELETE for Collections .............................49
9.6.2. Example - DELETE ...................................49
9.7. PUT Requirements ..........................................50
9.7.1. PUT for Non-Collection Resources ...................50
9.7.2. PUT for Collections ................................51
9.8. COPY Method ...............................................51
9.8.1. COPY for Non-collection Resources ..................51
9.8.2. COPY for Properties ................................52
9.8.3. COPY for Collections ...............................52
9.8.4. COPY and Overwriting Destination Resources .........53
9.8.5. Status Codes .......................................54
9.8.6. Example - COPY with Overwrite ......................55
9.8.7. Example - COPY with No Overwrite ...................55
9.8.8. Example - COPY of a Collection .....................56
9.9. MOVE Method ...............................................56
9.9.1. MOVE for Properties ................................57
9.9.2. MOVE for Collections ...............................57
9.9.3. MOVE and the Overwrite Header ......................58
9.9.4. Status Codes .......................................59
9.9.5. Example - MOVE of a Non-Collection .................60
9.9.6. Example - MOVE of a Collection .....................60
9.10. LOCK Method ..............................................61
9.10.1. Creating a Lock on an Existing Resource ...........61
9.10.2. Refreshing Locks ..................................62
9.10.3. Depth and Locking .................................62
9.10.4. Locking Unmapped URLs .............................63
9.10.5. Lock Compatibility Table ..........................63
9.10.6. LOCK Responses ....................................63
9.10.7. Example - Simple Lock Request .....................64
9.10.8. Example - Refreshing a Write Lock .................65
9.10.9. Example - Multi-Resource Lock Request .............66
9.11. UNLOCK Method ............................................68
9.11.1. Status Codes ......................................68
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9.11.2. Example - UNLOCK ..................................69
10. HTTP Headers for Distributed Authoring ........................69
10.1. DAV Header ...............................................69
10.2. Depth Header .............................................70
10.3. Destination Header .......................................71
10.4. If Header ................................................72
10.4.1. Purpose ...........................................72
10.4.2. Syntax ............................................72
10.4.3. List Evaluation ...................................73
10.4.4. Matching State Tokens and ETags ...................74
10.4.5. If Header and Non-DAV-Aware Proxies ...............74
10.4.6. Example - No-tag Production .......................75
10.4.7. Example - Using "Not" with No-tag Production ......75
10.4.8. Example - Causing a Condition to Always
Evaluate to True ..................................75
10.4.9. Example - Tagged List If Header in COPY ...........76
10.4.10. Example - Matching Lock Tokens with
Collection Locks .................................76
10.4.11. Example - Matching ETags on Unmapped URLs ........76
10.5. Lock-Token Header ........................................77
10.6. Overwrite Header .........................................77
10.7. Timeout Request Header ...................................78
11. Status Code Extensions to HTTP/1.1 ............................78
11.1. 207 Multi-Status .........................................78
11.2. 422 Unprocessable Entity .................................78
11.3. 423 Locked ...............................................78
11.4. 424 Failed Dependency ....................................79
11.5. 507 Insufficient Storage .................................79
12. Use of HTTP Status Codes ......................................79
12.1. 412 Precondition Failed ..................................79
12.2. 414 Request-URI Too Long .................................79
13. Multi-Status Response .........................................80
13.1. Response Headers .........................................80
13.2. Handling Redirected Child Resources ......................81
13.3. Internal Status Codes ....................................81
14. XML Element Definitions .......................................81
14.1. activelock XML Element ...................................81
14.2. allprop XML Element ......................................82
14.3. collection XML Element ...................................82
14.4. depth XML Element ........................................82
14.5. error XML Element ........................................82
14.6. exclusive XML Element ....................................83
14.7. href XML Element .........................................83
14.8. include XML Element ......................................83
14.9. location XML Element .....................................83
14.10. lockentry XML Element ...................................84
14.11. lockinfo XML Element ....................................84
14.12. lockroot XML Element ....................................84
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14.13. lockscope XML Element ...................................84
14.14. locktoken XML Element ...................................85
14.15. locktype XML Element ....................................85
14.16. multistatus XML Element .................................85
14.17. owner XML Element .......................................85
14.18. prop XML Element ........................................86
14.19. propertyupdate XML Element ..............................86
14.20. propfind XML Element ....................................86
14.21. propname XML Element ....................................87
14.22. propstat XML Element ....................................87
14.23. remove XML Element ......................................87
14.24. response XML Element ....................................88
14.25. responsedescription XML Element .........................88
14.26. set XML Element .........................................88
14.27. shared XML Element ......................................89
14.28. status XML Element ......................................89
14.29. timeout XML Element .....................................89
14.30. write XML Element .......................................89
15. DAV Properties ................................................90
16. Precondition/Postcondition XML Elements .......................98
17. XML Extensibility in DAV .....................................101
18. DAV Compliance Classes .......................................103
18.1. Class 1 .................................................103
18.2. Class 2 .................................................103
18.3. Class 3 .................................................103
19. Internationalization Considerations ..........................104
20. Security Considerations ......................................105
20.1. Authentication of Clients ...............................105
20.2. Denial of Service .......................................106
20.3. Security through Obscurity ..............................106
20.4. Privacy Issues Connected to Locks .......................106
20.5. Privacy Issues Connected to Properties ..................107
20.6. Implications of XML Entities ............................107
20.7. Risks Connected with Lock Tokens ........................108
20.8. Hosting Malicious Content ...............................108
21. IANA Considerations ..........................................109
21.1. New URI Schemes .........................................109
21.2. XML Namespaces ..........................................109
21.3. Message Header Fields ...................................109
21.3.1. DAV ..............................................109
21.3.2. Depth ............................................110
21.3.3. Destination ......................................110
21.3.4. If ...............................................110
21.3.5. Lock-Token .......................................110
21.3.6. Overwrite ........................................111
21.3.7. Timeout ..........................................111
21.4. HTTP Status Codes .......................................111
22. Acknowledgements .............................................112
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23. Contributors to This Specification ...........................113
24. Authors of RFC 2518 ..........................................113
25. References ...................................................114
25.1. Normative References.....................................114
25.2. Informative References ..................................115
Appendix A. Notes on Processing XML Elements ....................117
A.1. Notes on Empty XML Elements ..............................117
A.2. Notes on Illegal XML Processing ..........................117
A.3. Example - XML Syntax Error ...............................117
A.4. Example - Unexpected XML Element .........................118
Appendix B. Notes on HTTP Client Compatibility ...................119
Appendix C. The 'opaquelocktoken' Scheme and URIs ................120
Appendix D. Lock-null Resources ..................................120
D.1. Guidance for Clients Using LOCK to Create Resources ......121
Appendix E. Guidance for Clients Desiring to Authenticate ........121
Appendix F. Summary of Changes from RFC 2518 .....................123
F.1. Changes for Both Client and Server Implementations .......123
F.2. Changes for Server Implementations .......................125
F.3. Other Changes ............................................126
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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.
Collections: The ability to create sets of documents and to retrieve
a hierarchical membership listing (like a directory listing in a file
system).
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, operations that change the mapping from URLs to
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].
This document does not specify the versioning operations suggested by
[RFC2291]. That work was done in a separate document, "Versioning
Extensions to WebDAV" [RFC3253].
The sections below provide a detailed introduction to various WebDAV
abstractions: resource properties (Section 4), collections of
resources (Section 5), locks (Section 6) in general, and write locks
(Section 7) specifically.
These abstractions are manipulated by the WebDAV-specific HTTP
methods (Section 9) and the extra HTTP headers (Section 10) used with
WebDAV methods. General considerations for handling HTTP requests
and responses in WebDAV are found in Section 8.
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.
This specification defines extra status codes developed for WebDAV
methods (Section 11) and describes existing HTTP status codes
(Section 12) as used in WebDAV. Since some WebDAV methods may
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operate over many resources, the Multi-Status response (Section 13)
has been introduced to return status information for multiple
resources. Finally, this version of WebDAV introduces precondition
and postcondition (Section 16) XML elements in error response bodies.
WebDAV uses XML ([REC-XML]) for property names and some values, and
also uses XML to marshal complicated requests and responses. This
specification contains DTD and text definitions of all properties
(Section 15) and all other XML elements (Section 14) used in
marshalling. WebDAV includes a few special rules on extending WebDAV
XML marshalling in backwards-compatible ways (Section 17).
Finishing off the specification are sections on what it means for a
resource to be compliant with this specification (Section 18), on
internationalization support (Section 19), and on security
(Section 20).
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 [RFC2616],
including the rules about implied linear whitespace. Since this
augmented BNF uses the basic production rules provided in Section 2.2
of [RFC2616], these rules apply to this document as well. Note this
is not the standard BNF syntax used in other RFCs.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Note that in natural language, a property like the "creationdate"
property in the "DAV:" XML namespace is sometimes referred to as
"DAV:creationdate" for brevity.
3. Terminology
URI/URL - A Uniform Resource Identifier and Uniform Resource Locator,
respectively. These terms (and the distinction between them) are
defined in [RFC3986].
URI/URL Mapping - A relation between an absolute URI and a resource.
Since a resource can represent items that are not network
retrievable, as well as those that are, it is possible for a resource
to have zero, one, or many URI mappings. Mapping a resource to an
"http" scheme URI makes it possible to submit HTTP protocol requests
to the resource using the URI.
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Path Segment - Informally, the characters found between slashes ("/")
in a URI. Formally, as defined in Section 3.3 of [RFC3986].
Collection - Informally, a resource that also acts as a container of
references to child resources. Formally, a resource that contains a
set of mappings between path segments and resources and meets the
requirements defined in Section 5.
Internal Member (of a Collection) - Informally, a child resource of a
collection. Formally, a resource referenced by a path segment
mapping contained in the collection.
Internal Member URL (of a Collection) - A URL of an internal member,
consisting of the URL of the collection (including trailing slash)
plus the path segment identifying the internal member.
Member (of a Collection) - Informally, a "descendant" of a
collection. Formally, an internal member of the collection, or,
recursively, a member of an internal member.
Member URL (of a Collection) - A URL that is either an internal
member URL of the collection itself, or is an internal member URL of
a member of that collection.
Property - A name/value pair that contains descriptive information
about a resource.
Live Property - A property whose semantics and syntax are enforced by
the server. For example, the live property DAV:getcontentlength 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.
Principal - A distinct human or computational actor that initiates
access to network resources.
State Token - A URI that represents a state of a resource. Lock
tokens are the only state tokens defined in this specification.
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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
protected and 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. 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 that allows a
principal to identify a set of properties in which the principal is
interested and to set or retrieve just those properties.
4.3. Property Values
The value of a property is always a (well-formed) XML fragment.
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 elements. Clients will not break when they encounter
extensions because they will still have the data specified in the
original schema and MUST ignore elements they do not understand.
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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, using the "xml:
lang" attribute, handles cases where the same character set is
employed by multiple human languages. Note that xml:lang scope is
recursive, so an xml:lang attribute on any element containing a
property name element applies to the property value unless it has
been overridden by a more locally scoped attribute. Note that a
property only has one value, in one language (or language MAY be left
undefined); a property does not have multiple values in different
languages or a single value in multiple languages.
A property is always represented with an XML element consisting of
the property name, called the "property name element". The simplest
example is an empty property, which is different from a property that
does not exist:
<R:title xmlns:R="http://www.example.com/ns/"></R:title>
The value of the property appears inside the property name element.
The value may be any kind of well-formed XML content, including both
text-only and mixed content. Servers MUST preserve the following XML
Information Items (using the terminology from [REC-XML-INFOSET]) in
storage and transmission of dead properties:
For the property name Element Information Item itself:
[namespace name]
[local name]
[attributes] named "xml:lang" or any such attribute in scope
[children] of type element or character
On all Element Information Items in the property value:
[namespace name]
[local name]
[attributes]
[children] of type element or character
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On Attribute Information Items in the property value:
[namespace name]
[local name]
[normalized value]
On Character Information Items in the property value:
[character code]
Since prefixes are used in some XML vocabularies (XPath and XML
Schema, for example), servers SHOULD preserve, for any Information
Item in the value:
[prefix]
XML Infoset attributes not listed above MAY be preserved by the
server, but clients MUST NOT rely on them being preserved. The above
rules would also apply by default to live properties, unless defined
otherwise.
Servers MUST ignore the XML attribute xml:space if present and never
use it to change whitespace handling. Whitespace in property values
is significant.
4.3.1. Example - Property with Mixed Content
Consider a dead property 'author' created by the client as follows:
<D:prop xml:lang="en" xmlns:D="DAV:">
<x:author xmlns:x='http://example.com/ns'>
<x:name>Jane Doe</x:name>
<!-- Jane's contact info -->
<x:uri type='email'
added='2005-11-26'>mailto:jane.doe@example.com</x:uri>
<x:uri type='web'
added='2005-11-27'>http://www.example.com</x:uri>
<x:notes xmlns:h='http://www.w3.org/1999/xhtml'>
Jane has been working way <h:em>too</h:em> long on the
long-awaited revision of <![CDATA[<RFC2518>]]>.
</x:notes>
</x:author>
</D:prop>
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When this property is requested, a server might return:
<D:prop xmlns:D='DAV:'><author
xml:lang='en'
xmlns:x='http://example.com/ns'
xmlns='http://example.com/ns'
xmlns:h='http://www.w3.org/1999/xhtml'>
<x:name>Jane Doe</x:name>
<x:uri added="2005-11-26" type="email"
>mailto:jane.doe@example.com</x:uri>
<x:uri added="2005-11-27" type="web"
>http://www.example.com</x:uri>
<x:notes>
Jane has been working way <h:em>too</h:em> long on the
long-awaited revision of <RFC2518>.
</x:notes>
</author>
</D:prop>
Note in this example:
o The [prefix] for the property name itself was not preserved, being
non-significant, whereas all other [prefix] values have been
preserved,
o attribute values have been rewritten with double quotes instead of
single quotes (quoting style is not significant), and attribute
order has not been preserved,
o the xml:lang attribute has been returned on the property name
element itself (it was in scope when the property was set, but the
exact position in the response is not considered significant as
long as it is in scope),
o whitespace between tags has been preserved everywhere (whitespace
between attributes not so),
o CDATA encapsulation was replaced with character escaping (the
reverse would also be legal),
o the comment item was stripped (as would have been a processing
instruction item).
Implementation note: there are cases such as editing scenarios where
clients may require that XML content is preserved character by
character (such as attribute ordering or quoting style). In this
case, clients should consider using a text-only property value by
escaping all characters that have a special meaning in XML parsing.
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4.4. 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 ([RFC3986]), 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 that 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.
4.5. Source Resources and Output Resources
Some HTTP resources are dynamically generated by the server. For
these resources, there presumably exists source code somewhere
governing how that resource is generated. The relationship of source
files to output HTTP resources may be one to one, one to many, many
to one, or many to many. There is no mechanism in HTTP to determine
whether a resource is even dynamic, let alone where its source files
exist or how to author them. Although this problem would usefully be
solved, interoperable WebDAV implementations have been widely
deployed without actually solving this problem, by dealing only with
static resources. Thus, the source vs. output problem is not solved
in this specification and has been deferred to a separate document.
5. Collections of Web Resources
This section provides a description of a type of Web resource, the
collection, and discusses its interactions with the HTTP URL
namespace and with HTTP methods. The purpose of a collection
resource is to model collection-like objects (e.g., file system
directories) within a server's namespace.
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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 URL.
The root, or top-level collection of the namespace under
consideration, is exempt from the previous rule. The top-level
collection of the namespace under consideration is not necessarily
the collection identified by the absolute path '/' -- it may be
identified by one or more path segments (e.g., /servlets/webdav/...)
Neither HTTP/1.1 nor WebDAV requires that the entire HTTP URL
namespace be consistent -- a WebDAV-compatible resource may not have
a parent collection. However, certain WebDAV methods are prohibited
from producing results that cause namespace inconsistencies.
As is implicit in [RFC2616] and [RFC3986], 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
Collection resources differ from other resources in that they also
act as containers. Some HTTP methods apply only to a collection, but
some apply to some or all of the resources inside the container
defined by the collection. When the scope of a method is not clear,
the client can specify what depth to apply. Depth can be either zero
levels (only the collection), one level (the collection and directly
contained resources), or infinite levels (the collection and all
contained resources recursively).
A collection's state consists of at least a set of mappings between
path segments and resources, and a set of properties on the
collection itself. In this document, a resource B will be said to be
contained in the collection resource A if there is a path segment
mapping that maps to B and that is contained in A. A collection MUST
contain at most one mapping for a given path segment, i.e., it is
illegal to have the same path segment mapped to more than one
resource.
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Properties defined on collections behave exactly as do properties on
non-collection resources. A collection MAY have additional state
such as entity bodies returned by GET.
For all WebDAV-compliant resources A and B, identified by URLs "U"
and "V", respectively, such that "V" is equal to "U/SEGMENT", A MUST
be a collection that contains a mapping from "SEGMENT" to B. So, if
resource B with URL "http://example.com/bar/blah" is WebDAV compliant
and if resource A with URL "http://example.com/bar/" is WebDAV
compliant, then resource A must be a collection and must contain
exactly one mapping from "blah" to B.
Although commonly a mapping consists of a single segment and a
resource, in general, a mapping consists of a set of segments and a
resource. This allows a server to treat a set of segments as
equivalent (i.e., either all of the segments are mapped to the same
resource, or none of the segments are mapped to a resource). For
example, a server that performs case-folding on segments will treat
the segments "ab", "Ab", "aB", and "AB" as equivalent. A client can
then use any of these segments to identify the resource. Note that a
PROPFIND result will select one of these equivalent segments to
identify the mapping, so there will be one PROPFIND response element
per mapping, not one per segment in the mapping.
Collection resources MAY have mappings to non-WebDAV-compliant
resources in the HTTP URL namespace hierarchy but are not required to
do so. For example, if resource X with URL
"http://example.com/bar/blah" is not WebDAV compliant and resource A
with "URL http://example.com/bar/" identifies a WebDAV collection,
then A may or may not have a mapping from "blah" to X.
If a WebDAV-compliant resource has no WebDAV-compliant internal
members 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 server MAY handle the
request as if the trailing slash were present. In this case, it
SHOULD return a Content-Location header in the response, pointing to
the URL ending with the "/". For example, if a client invokes a
method on http://example.com/blah (no trailing slash), the server may
respond as if the operation were invoked on http://example.com/blah/
(trailing slash), and should return a Content-Location header with
the value http://example.com/blah/. Wherever a server produces a URL
referring to a collection, the server SHOULD include the trailing
slash. In general, clients SHOULD use the trailing slash form of
collection names. If clients do not use the trailing slash form the
client needs to be prepared to see a redirect response. Clients will
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find the DAV:resourcetype property more reliable than the URL to find
out if a resource is a collection.
Clients MUST be able to support the case where WebDAV resources are
contained inside non-WebDAV resources. For example, if an OPTIONS
response from "http://example.com/servlet/dav/collection" indicates
WebDAV support, the client cannot assume that
"http://example.com/servlet/dav/" or its parent necessarily are
WebDAV collections.
A typical scenario in which mapped URLs do not appear as members of
their parent collection is the case where a server allows links or
redirects to non-WebDAV resources. For instance, "/col/link" might
not appear as a member of "/col/", although the server would respond
with a 302 status to a GET request to "/col/link"; thus, the URL
"/col/link" would indeed be mapped. Similarly, a dynamically-
generated page might have a URL mapping from "/col/index.html", thus
this resource might respond with a 200 OK to a GET request yet not
appear as a member of "/col/".
Some mappings to even WebDAV-compliant resources might not appear in
the parent collection. An example for this case are servers that
support multiple alias URLs for each WebDAV-compliant resource. A
server may implement case-insensitive URLs, thus "/col/a" and
"/col/A" identify the same resource, yet only either "a" or "A" is
reported upon listing the members of "/col". In cases where a server
treats a set of segments as equivalent, the server MUST expose only
one preferred segment per mapping, consistently chosen, in PROPFIND
responses.
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.
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6.1. Lock Model
This section provides a concise model for how locking behaves. Later
sections will provide more detail on some of the concepts and refer
back to these model statements. Normative statements related to LOCK
and UNLOCK method handling can be found in the sections on those
methods, whereas normative statements that cover any method are
gathered here.
1. A lock either directly or indirectly locks a resource.
2. A resource becomes directly locked when a LOCK request to a URL
of that resource creates a new lock. The "lock-root" of the new
lock is that URL. If at the time of the request, the URL is not
mapped to a resource, a new empty resource is created and
directly locked.
3. An exclusive lock (Section 6.2) conflicts with any other kind of
lock on the same resource, whether either lock is direct or
indirect. A server MUST NOT create conflicting locks on a
resource.
4. For a collection that is locked with a depth-infinity lock L, all
member resources are indirectly locked. Changes in membership of
such a collection affect the set of indirectly locked resources:
* If a member resource is added to the collection, the new
member resource MUST NOT already have a conflicting lock,
because the new resource MUST become indirectly locked by L.
* If a member resource stops being a member of the collection,
then the resource MUST no longer be indirectly locked by L.
5. Each lock is identified by a single globally unique lock token
(Section 6.5).
6. An UNLOCK request deletes the lock with the specified lock token.
After a lock is deleted, no resource is locked by that lock.
7. A lock token is "submitted" in a request when it appears in an
"If" header (Section 7, "Write Lock", discusses when token
submission is required for write locks).
8. If a request causes the lock-root of any lock to become an
unmapped URL, then the lock MUST also be deleted by that request.
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6.2. Exclusive vs. Shared Locks
The most basic form of lock is an exclusive lock. Exclusive locks
avoid having to deal with content change conflicts, without requiring
any coordination other than the methods described in this
specification.
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 that has both access privileges and a valid lock can use
the locked resource.
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 creator leaves without
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unlocking a resource. While both timeouts (Section 6.6) 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.
A successful request for a new shared lock MUST result in the
generation of a unique lock associated with the requesting principal.
Thus, if five principals have taken out shared write locks on the
same resource, there will be five locks and five lock tokens, one for
each principal.
6.3. Required Support
A WebDAV-compliant resource is not required to support locking in any
form. If the resource 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.
6.4. Lock Creator and Privileges
The creator of a lock has special privileges to use the lock to
modify the resource. When a locked resource is modified, a server
MUST check that the authenticated principal matches the lock creator
(in addition to checking for valid lock token submission).
The server MAY allow privileged users other than the lock creator to
destroy a lock (for example, the resource owner or an administrator).
The 'unlock' privilege in [RFC3744] was defined to provide that
permission.
There is no requirement for servers to accept LOCK requests from all
users or from anonymous users.
Note that having a lock does not confer full privilege to modify the
locked resource. Write access and other privileges MUST be enforced
through normal privilege or authentication mechanisms, not based on
the possible obscurity of lock token values.
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6.5. Lock Tokens
A lock token is a type of state token that identifies a particular
lock. Each lock has exactly one unique lock token generated by the
server. Clients MUST NOT attempt to interpret lock tokens in any
way.
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. Since lock tokens
are unique, a client MAY submit a lock token in an If header on a
resource other than the one that returned it.
When a LOCK operation creates a new lock, the new lock token is
returned in the Lock-Token response header defined in Section 10.5,
and also in the body of the response.
Servers MAY make lock tokens publicly readable (e.g., in the DAV:
lockdiscovery property). One use case for making lock tokens
readable is so that a long-lived lock can be removed by the resource
owner (the client that obtained the lock might have crashed or
disconnected before cleaning up the lock). Except for the case of
using UNLOCK under user guidance, a client SHOULD NOT use a lock
token created by another client instance.
This specification encourages servers to create Universally Unique
Identifiers (UUIDs) for lock tokens, and to use the URI form defined
by "A Universally Unique Identifier (UUID) URN Namespace"
([RFC4122]). However, servers are free to use any URI (e.g., from
another scheme) so long as it meets the uniqueness requirements. For
example, a valid lock token might be constructed using the
"opaquelocktoken" scheme defined in Appendix C.
Example: "urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6"
6.6. Lock Timeout
A lock MAY have a limited lifetime. The lifetime is suggested by the
client when creating or refreshing the lock, but the server
ultimately chooses the timeout value. Timeout is measured in seconds
remaining until lock expiration.
The timeout counter MUST be restarted if a refresh lock request is
successful (see Section 9.10.2). The timeout counter SHOULD NOT be
restarted at any other time.
If the timeout expires, then the lock SHOULD be removed. In this
case the server SHOULD act as if an UNLOCK method was executed by the
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server on the resource using the lock token of the timed-out lock,
performed with its override authority.
Servers are advised to pay close attention to the values submitted by
clients, as they will be indicative of the type of activity the
client intends to perform. For example, an applet running in a
browser may need to lock a resource, but because of the instability
of the environment within which the applet is running, the applet may
be turned off without warning. As a result, the applet is likely to
ask for a relatively small timeout value so that if the applet dies,
the lock can be quickly harvested. However, a document management
system is likely to ask for an extremely long timeout because its
user may be planning on going offline.
A client MUST NOT assume that just because the timeout has expired,
the lock has immediately been removed.
Likewise, a client MUST NOT assume that just because the timeout has
not expired, the lock still exists. Clients MUST assume that locks
can arbitrarily disappear at any time, regardless of the value given
in the Timeout header. The Timeout header only indicates the
behavior of the server if extraordinary circumstances do not occur.
For example, a sufficiently privileged user may remove a lock at any
time, or the system may crash in such a way that it loses the record
of the lock's existence.
6.7. 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. A
client can determine what lock types the server supports by
retrieving the DAV:supportedlock property.
Any DAV-compliant resource that supports the LOCK method MUST support
the DAV:supportedlock property.
6.8. 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 DAV:lockdiscovery
property is provided. This property lists all outstanding locks,
describes their type, and MAY even provide the lock tokens.
Any DAV-compliant resource that supports the LOCK method MUST support
the DAV:lockdiscovery property.
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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.
An exclusive write lock protects a resource: it prevents changes by
any principal other than the lock creator and in any case where the
lock token is not submitted (e.g., by a client process other than the
one holding the lock).
Clients MUST submit a lock-token they are authorized to use in any
request that modifies a write-locked resource. The list of
modifications covered by a write-lock include:
1. A change to any of the following aspects of any write-locked
resource:
* any variant,
* any dead property,
* any live property that is lockable (a live property is
lockable unless otherwise defined.)
2. For collections, any modification of an internal member URI. An
internal member URI of a collection is considered to be modified
if it is added, removed, or identifies a different resource.
More discussion on write locks and collections is found in
Section 7.4.
3. A modification of the mapping of the root of the write lock,
either to another resource or to no resource (e.g., DELETE).
Of the methods defined in HTTP and WebDAV, PUT, POST, PROPPATCH,
LOCK, UNLOCK, MOVE, COPY (for the destination resource), DELETE, and
MKCOL are affected by write locks. All other HTTP/WebDAV methods
defined so far -- GET in particular -- function independently of a
write lock.
The next few sections describe in more specific terms how write locks
interact with various operations.
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7.1. 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.
Only dead properties and live properties defined as lockable are
guaranteed not to change while write locked.
7.2. Avoiding Lost Updates
Although the write locks 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.
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.
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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.
7.3. Write Locks and Unmapped URLs
WebDAV provides the ability to send a LOCK request to an unmapped URL
in order to reserve the name for use. This is a simple way to avoid
the lost-update problem on the creation of a new resource (another
way is to use If-None-Match header specified in Section 14.26 of
[RFC2616]). It has the side benefit of locking the new resource
immediately for use of the creator.
Note that the lost-update problem is not an issue for collections
because MKCOL can only be used to create a collection, not to
overwrite an existing collection. When trying to lock a collection
upon creation, clients can attempt to increase the likelihood of
getting the lock by pipelining the MKCOL and LOCK requests together
(but because this doesn't convert two separate operations into one
atomic operation, there's no guarantee this will work).
A successful lock request to an unmapped URL MUST result in the
creation of a locked (non-collection) resource with empty content.
Subsequently, a successful PUT request (with the correct lock token)
provides the content for the resource. Note that the LOCK request
has no mechanism for the client to provide Content-Type or Content-
Language, thus the server will use defaults or empty values and rely
on the subsequent PUT request for correct values.
A resource created with a LOCK is empty but otherwise behaves in
every way as a normal resource. It behaves the same way as a
resource created by a PUT request with an empty body (and where a
Content-Type and Content-Language was not specified), followed by a
LOCK request to the same resource. Following from this model, a
locked empty resource:
o Can be read, deleted, moved, and copied, and in all ways behaves
as a regular non-collection resource.
o Appears as a member of its parent collection.
o SHOULD NOT disappear when its lock goes away (clients must
therefore be responsible for cleaning up their own mess, as with
any other operation or any non-empty resource).
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o MAY NOT have values for properties like DAV:getcontentlanguage
that haven't been specified yet by the client.
o Can be updated (have content added) with a PUT request.
o MUST NOT be converted into a collection. The server MUST fail a
MKCOL request (as it would with a MKCOL request to any existing
non-collection resource).
o MUST have defined values for DAV:lockdiscovery and DAV:
supportedlock properties.
o The response MUST indicate that a resource was created, by use of
the "201 Created" response code (a LOCK request to an existing
resource instead will result in 200 OK). The body must still
include the DAV:lockdiscovery property, as with a LOCK request to
an existing resource.
The client is expected to update the locked empty resource shortly
after locking it, using PUT and possibly PROPPATCH.
Alternatively and for backwards compatibility to [RFC2518], servers
MAY implement Lock-Null Resources (LNRs) instead (see definition in
Appendix D). Clients can easily interoperate both with servers that
support the old model LNRs and the recommended model of "locked empty
resources" by only attempting PUT after a LOCK to an unmapped URL,
not MKCOL or GET, and by not relying on specific properties of LNRs.
7.4. Write Locks and Collections
There are two kinds of collection write locks. A depth-0 write lock
on a collection protects the collection properties plus the internal
member URLs of that one collection, while not protecting the content
or properties of member resources (if the collection itself has any
entity bodies, those are also protected). A depth-infinity write
lock on a collection provides the same protection on that collection
and also provides write lock protection on every member resource.
Expressed otherwise, a write lock of either kind protects any request
that would create a new resource in a write locked collection, any
request that would remove an internal member URL of a write locked
collection, and any request that would change the segment name of any
internal member.
Thus, a collection write lock protects all the following actions:
o DELETE a collection's direct internal member,
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o MOVE an internal member out of the collection,
o MOVE an internal member into the collection,
o MOVE to rename an internal member within a collection,
o COPY an internal member into a collection, and
o PUT or MKCOL request that would create a new internal member.
The collection's lock token is required in addition to the lock token
on the internal member itself, if it is locked separately.
In addition, a depth-infinity lock affects all write operations to
all members of the locked collection. With a depth-infinity lock,
the resource identified by the root of the lock is directly locked,
and all its members are indirectly locked.
o Any new resource added as a descendant of a depth-infinity locked
collection becomes indirectly locked.
o Any indirectly locked resource moved out of the locked collection
into an unlocked collection is thereafter unlocked.
o Any indirectly locked resource moved out of a locked source
collection into a depth-infinity locked target collection remains
indirectly locked but is now protected by the lock on the target
collection (the target collection's lock token will thereafter be
required to make further changes).
If a depth-infinity write LOCK request is issued to a collection
containing member URLs identifying resources that are currently
locked in a manner that conflicts with the new lock (see Section 6.1,
point 3), the request MUST fail with a 423 (Locked) status code, and
the response SHOULD contain the 'no-conflicting-lock' precondition.
If a lock request causes the URL of a resource to be added as an
internal member URL of a depth-infinity locked collection, then the
new resource MUST be automatically protected by the lock. 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.
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7.5. Write Locks and the If Request Header
A user agent has to demonstrate knowledge of a lock when requesting
an operation on a locked resource. Otherwise, the following scenario
might occur. In the scenario, 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 for all locked resources that a method may
change or the method MUST fail. A lock token is submitted when it
appears in an If header. For example, if a resource is to be moved
and both the source and destination are locked, then two lock tokens
must be submitted in the If header, one for the source and the other
for the destination.
7.5.1. Example - Write Lock and COPY
>>Request
COPY /~fielding/index.html HTTP/1.1
Host: www.example.com
Destination: http://www.example.com/users/f/fielding/index.html
If: <http://www.example.com/users/f/fielding/index.html>
(<urn:uuid: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 (the one 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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7.5.2. Example - Deleting a Member of a Locked Collection
Consider a collection "/locked" with an exclusive, depth-infinity
write lock, and an attempt to delete an internal member "/locked/
member":
>>Request
DELETE /locked/member HTTP/1.1
Host: example.com
>>Response
HTTP/1.1 423 Locked
Content-Type: application/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:error xmlns:D="DAV:">
<D:lock-token-submitted>
<D:href>/locked/</D:href>
</D:lock-token-submitted>
</D:error>
Thus, the client would need to submit the lock token with the request
to make it succeed. To do that, various forms of the If header (see
Section 10.4) could be used.
"No-Tag-List" format:
If: (<urn:uuid:150852e2-3847-42d5-8cbe-0f4f296f26cf>)
"Tagged-List" format, for "http://example.com/locked/":
If: <http://example.com/locked/>
(<urn:uuid:150852e2-3847-42d5-8cbe-0f4f296f26cf>)
"Tagged-List" format, for "http://example.com/locked/member":
If: <http://example.com/locked/member>
(<urn:uuid:150852e2-3847-42d5-8cbe-0f4f296f26cf>)
Note that, for the purpose of submitting the lock token, the actual
form doesn't matter; what's relevant is that the lock token appears
in the If header, and that the If header itself evaluates to true.
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7.6. 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 a depth-infinity lock,
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, if there is an existing
lock at the destination, the server MUST add the moved resource to
the destination lock scope. For example, if the MOVE makes the
resource a child of a collection that has a depth-infinity lock, then
the resource will be added to that collection's lock. Additionally,
if a resource with a depth-infinity lock is moved to a destination
that is within the scope of the same lock (e.g., within the URL
namespace tree covered by the lock), the moved resource will again be
added to the lock. In both these examples, as specified in
Section 7.5, an If header must be submitted containing a lock token
for both the source and destination.
7.7. 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 request with an If header but
without a body. A server receiving a LOCK request with no body MUST
NOT create a new lock -- this form of the LOCK request is only to be
used to "refresh" an existing lock (meaning, at minimum, that any
timers associated with the lock MUST be reset).
Clients may submit Timeout headers of arbitrary value with their lock
refresh requests. Servers, as always, may ignore Timeout headers
submitted by the client, and a server MAY refresh a lock with a
timeout period that is different than the previous timeout period
used for the lock, provided it advertises the new value in the LOCK
refresh response.
If an error is received in response to a refresh LOCK request, the
client MUST NOT assume that the lock was refreshed.
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8. General Request and Response Handling
8.1. Precedence in Error Handling
Servers MUST return authorization errors in preference to other
errors. This avoids leaking information about protected resources
(e.g., a client that finds that a hidden resource exists by seeing a
423 Locked response to an anonymous request to the resource).
8.2. Use of XML
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 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.
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.
When XML is used for a request or response body, the Content-Type
type SHOULD be application/xml. Implementations MUST accept both
text/xml and application/xml in request and response bodies. Use of
text/xml is deprecated.
All DAV-compliant clients and resources MUST use XML parsers that are
compliant with [REC-XML] and [REC-XML-NAMES]. All XML used in either
requests or responses MUST be, at minimum, well formed and use
namespaces correctly. If a server receives XML that is not well-
formed, then the server MUST reject the entire request with a 400
(Bad Request). If a client receives XML that is not well-formed in a
response, then the client MUST NOT assume anything about the outcome
of the executed method and SHOULD treat the server as malfunctioning.
Note that processing XML submitted by an untrusted source may cause
risks connected to privacy, security, and service quality (see
Section 20). Servers MAY reject questionable requests (even though
they consist of well-formed XML), for instance, with a 400 (Bad
Request) status code and an optional response body explaining the
problem.
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8.3. URL Handling
URLs appear in many places in requests and responses.
Interoperability experience with [RFC2518] showed that many clients
parsing Multi-Status responses did not fully implement the full
Reference Resolution defined in Section 5 of [RFC3986]. Thus,
servers in particular need to be careful in handling URLs in
responses, to ensure that clients have enough context to be able to
interpret all the URLs. The rules in this section apply not only to
resource URLs in the 'href' element in Multi-Status responses, but
also to the Destination and If header resource URLs.
The sender has a choice between two approaches: using a relative
reference, which is resolved against the Request-URI, or a full URI.
A server MUST ensure that every 'href' value within a Multi-Status
response uses the same format.
WebDAV only uses one form of relative reference in its extensions,
the absolute path.
Simple-ref = absolute-URI | ( path-absolute [ "?" query ] )
The absolute-URI, path-absolute and query productions are defined in
Sections 4.3, 3.3, and 3.4 of [RFC3986].
Within Simple-ref productions, senders MUST NOT:
o use dot-segments ("." or ".."), or
o have prefixes that do not match the Request-URI (using the
comparison rules defined in Section 3.2.3 of [RFC2616]).
Identifiers for collections SHOULD end in a '/' character.
8.3.1. Example - Correct URL Handling
Consider the collection http://example.com/sample/ with the internal
member URL http://example.com/sample/a%20test and the PROPFIND
request below:
>>Request:
PROPFIND /sample/ HTTP/1.1
Host: example.com
Depth: 1
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In this case, the server should return two 'href' elements containing
either
o 'http://example.com/sample/' and
'http://example.com/sample/a%20test', or
o '/sample/' and '/sample/a%20test'
Note that even though the server may be storing the member resource
internally as 'a test', it has to be percent-encoded when used inside
a URI reference (see Section 2.1 of [RFC3986]). Also note that a
legal URI may still contain characters that need to be escaped within
XML character data, such as the ampersand character.
8.4. Required Bodies in Requests
Some of these new methods do not define bodies. Servers MUST examine
all requests for a body, even when a body was not expected. In cases
where a request body is present but would be ignored by a server, the
server MUST reject the request with 415 (Unsupported Media Type).
This informs the client (which may have been attempting to use an
extension) that the body could not be processed as the client
intended.
8.5. HTTP Headers for Use in WebDAV
HTTP defines many headers that can be used in WebDAV requests and
responses. Not all of these are appropriate in all situations and
some interactions may be undefined. Note that HTTP 1.1 requires the
Date header in all responses if possible (see Section 14.18,
[RFC2616]).
The server MUST do authorization checks before checking any HTTP
conditional header.
8.6. ETag
HTTP 1.1 recommends the use of ETags rather than modification dates,
for cache control, and there are even stronger reasons to prefer
ETags for authoring. Correct use of ETags is even more important in
a distributed authoring environment, because ETags are necessary
along with locks to avoid the lost-update problem. A client might
fail to renew a lock, for example, when the lock times out and the
client is accidentally offline or in the middle of a long upload.
When a client fails to renew the lock, it's quite possible the
resource can still be relocked and the user can go on editing, as
long as no changes were made in the meantime. ETags are required for
the client to be able to distinguish this case. Otherwise, the
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client is forced to ask the user whether to overwrite the resource on
the server without even being able to tell the user if it has
changed. Timestamps do not solve this problem nearly as well as
ETags.
Strong ETags are much more useful for authoring use cases than weak
ETags (see Section 13.3.3 of [RFC2616]). Semantic equivalence can be
a useful concept but that depends on the document type and the
application type, and interoperability might require some agreement
or standard outside the scope of this specification and HTTP. Note
also that weak ETags have certain restrictions in HTTP, e.g., these
cannot be used in If-Match headers.
Note that the meaning of an ETag in a PUT response is not clearly
defined either in this document or in RFC 2616 (i.e., whether the
ETag means that the resource is octet-for-octet equivalent to the
body of the PUT request, or whether the server could have made minor
changes in the formatting or content of the document upon storage).
This is an HTTP issue, not purely a WebDAV issue.
Because clients may be forced to prompt users or throw away changed
content if the ETag changes, a WebDAV server SHOULD NOT change the
ETag (or the Last-Modified time) for a resource that has an unchanged
body and location. The ETag represents the state of the body or
contents of the resource. There is no similar way to tell if
properties have changed.
8.7. Including Error Response Bodies
HTTP and WebDAV did not use the bodies of most error responses for
machine-parsable information until the specification for Versioning
Extensions to WebDAV introduced a mechanism to include more specific
information in the body of an error response (Section 1.6 of
[RFC3253]). The error body mechanism is appropriate to use with any
error response that may take a body but does not already have a body
defined. The mechanism is particularly appropriate when a status
code can mean many things (for example, 400 Bad Request can mean
required headers are missing, headers are incorrectly formatted, or
much more). This error body mechanism is covered in Section 16.
8.8. Impact of Namespace Operations on Cache Validators
Note that the HTTP response headers "Etag" and "Last-Modified" (see
[RFC2616], Sections 14.19 and 14.29) are defined per URL (not per
resource), and are used by clients for caching. Therefore servers
must ensure that executing any operation that affects the URL
namespace (such as COPY, MOVE, DELETE, PUT, or MKCOL) does preserve
their semantics, in particular:
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RFC 4918 WebDAV June 2007
o For any given URL, the "Last-Modified" value MUST increment every
time the representation returned upon GET changes (within the
limits of timestamp resolution).
o For any given URL, an "ETag" value MUST NOT be reused for
different representations returned by GET.
In practice this means that servers
o might have to increment "Last-Modified" timestamps for every
resource inside the destination namespace of a namespace operation
unless it can do so more selectively, and
o similarly, might have to re-assign "ETag" values for these
resources (unless the server allocates entity tags in a way so
that they are unique across the whole URL namespace managed by the
server).
Note that these considerations also apply to specific use cases, such
as using PUT to create a new resource at a URL that has been mapped
before, but has been deleted since then.
Finally, WebDAV properties (such as DAV:getetag and DAV:
getlastmodified) that inherit their semantics from HTTP headers must
behave accordingly.
9. HTTP Methods for Distributed Authoring
9.1. PROPFIND Method
The PROPFIND method retrieves properties defined on the resource
identified by the Request-URI, if the resource does not have any
internal members, or on the resource identified by the Request-URI
and potentially its member resources, if the resource is a collection
that has internal member URLs. All DAV-compliant resources MUST
support the PROPFIND method and the propfind XML element
(Section 14.20) along with all XML elements defined for use with that
element.
A client MUST submit a Depth header with a value of "0", "1", or
"infinity" with a PROPFIND request. Servers MUST support "0" and "1"
depth requests on WebDAV-compliant resources and SHOULD support
"infinity" requests. In practice, support for infinite-depth
requests MAY be disabled, due to the performance and security
concerns associated with this behavior. Servers SHOULD treat a
request without a Depth header as if a "Depth: infinity" header was
included.
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RFC 4918 WebDAV June 2007
A client may submit a 'propfind' XML element in the body of the
request method describing what information is being requested. It is
possible to:
o Request particular property values, by naming the properties
desired within the 'prop' element (the ordering of properties in
here MAY be ignored by the server),
o Request property values for those properties defined in this
specification (at a minimum) plus dead properties, by using the
'allprop' element (the 'include' element can be used with
'allprop' to instruct the server to also include additional live
properties that may not have been returned otherwise),
o Request a list of names of all the properties defined on the
resource, by using the 'propname' element.
A client may choose not to submit a request body. An empty PROPFIND
request body MUST be treated as if it were an 'allprop' request.
Note that 'allprop' does not return values for all live properties.
WebDAV servers increasingly have expensively-calculated or lengthy
properties (see [RFC3253] and [RFC3744]) and do not return all
properties already. Instead, WebDAV clients can use propname
requests to discover what live properties exist, and request named
properties when retrieving values. For a live property defined
elsewhere, that definition can specify whether or not that live
property would be returned in 'allprop' requests.
All servers MUST support returning a response of content type text/
xml or application/xml that contains a multistatus XML element that
describes the results of the attempts to retrieve the various
properties.
If there is an error retrieving a property, then a proper error
result MUST be included in the response. A request to retrieve the
value of a property that does not exist is an error and MUST be noted
with a 'response' XML element that contains a 404 (Not Found) status
value.
Consequently, the 'multistatus' XML element for a collection resource
MUST include a 'response' XML element for each member URL of the
collection, to whatever depth was requested. It SHOULD NOT include
any 'response' elements for resources that are not WebDAV-compliant.
Each 'response' element MUST contain an 'href' element that contains
the URL of the resource on which the properties in the prop XML
element are defined. Results for a PROPFIND on a collection resource
are returned as a flat list whose order of entries is not
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significant. Note that a resource may have only one value for a
property of a given name, so the property may only show up once in
PROPFIND responses.
Properties may be subject to access control. In the case of
'allprop' and 'propname' requests, if a principal does not have the
right to know whether a particular property exists, then the property
MAY be silently excluded from the response.
Some PROPFIND results MAY be cached, with care, as there is no cache
validation mechanism for most properties. This method is both safe
and idempotent (see Section 9.1 of [RFC2616]).
9.1.1. PROPFIND Status Codes
This section, as with similar sections for other methods, provides
some guidance on error codes and preconditions or postconditions
(defined in Section 16) that might be particularly useful with
PROPFIND.
403 Forbidden - A server MAY reject PROPFIND requests on collections
with depth header of "Infinity", in which case it SHOULD use this
error with the precondition code 'propfind-finite-depth' inside the
error body.
9.1.2. Status Codes for Use in 'propstat' Element
In PROPFIND responses, information about individual properties is
returned inside 'propstat' elements (see Section 14.22), each
containing an individual 'status' element containing information
about the properties appearing in it. The list below summarizes the
most common status codes used inside 'propstat'; however, clients
should be prepared to handle other 2/3/4/5xx series status codes as
well.
200 OK - A property exists and/or its value is successfully returned.
401 Unauthorized - The property cannot be viewed without appropriate
authorization.
403 Forbidden - The property cannot be viewed regardless of
authentication.
404 Not Found - The property does not exist.
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RFC 4918 WebDAV June 2007
9.1.3. Example - Retrieving Named Properties
>>Request
PROPFIND /file HTTP/1.1
Host: www.example.com
Content-type: application/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:propfind xmlns:D="DAV:">
<D:prop xmlns:R="http://ns.example.com/boxschema/">
<R:bigbox/>
<R:author/>
<R:DingALing/>
<R:Random/>
</D:prop>
</D:propfind>
>>Response
HTTP/1.1 207 Multi-Status
Content-Type: application/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:multistatus xmlns:D="DAV:">
<D:response xmlns:R="http://ns.example.com/boxschema/">
<D:href>http://www.example.com/file</D:href>
<D:propstat>
<D:prop>
<R:bigbox>
<R:BoxType>Box type A</R:BoxType>
</R:bigbox>
<R:author>
<R:Name>J.J. Johnson</R:Name>
</R:author>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
<D:propstat>
<D:prop><R:DingALing/><R:Random/></D:prop>
<D:status>HTTP/1.1 403 Forbidden</D:status>
<D:responsedescription> The user does not have access to the
DingALing property.
</D:responsedescription>
</D:propstat>
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</D:response>
<D:responsedescription> There has been an access violation error.
</D:responsedescription>
</D:multistatus>
In this example, PROPFIND is executed on a non-collection resource
http://www.example.com/file. The propfind XML element specifies the
name of four properties whose values are being requested. In this
case, only two properties were returned, since the principal issuing
the request did not have sufficient access rights to see the third
and fourth properties.
9.1.4. Example - Using 'propname' to Retrieve All Property Names
>>Request
PROPFIND /container/ HTTP/1.1
Host: www.example.com
Content-Type: application/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<propfind xmlns="DAV:">
<propname/>
</propfind>
>>Response
HTTP/1.1 207 Multi-Status
Content-Type: application/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<multistatus xmlns="DAV:">
<response>
<href>http://www.example.com/container/</href>
<propstat>
<prop xmlns:R="http://ns.example.com/boxschema/">
<R:bigbox/>
<R:author/>
<creationdate/>
<displayname/>
<resourcetype/>
<supportedlock/>
</prop>
<status>HTTP/1.1 200 OK</status>
Dusseault Standards Track [Page 39]
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</propstat>
</response>
<response>
<href>http://www.example.com/container/front.html</href>
<propstat>
<prop xmlns:R="http://ns.example.com/boxschema/">
<R:bigbox/>
<creationdate/>
<displayname/>
<getcontentlength/>
<getcontenttype/>
<getetag/>
<getlastmodified/>
<resourcetype/>
<supportedlock/>
</prop>
<status>HTTP/1.1 200 OK</status>
</propstat>
</response>
</multistatus>
In this example, PROPFIND is invoked on the collection resource
http://www.example.com/container/, with a propfind XML element
containing the propname XML element, meaning the name of all
properties should be returned. Since no Depth header is present, it
assumes its default value of "infinity", meaning the name of the
properties on the collection and all its descendants should be
returned.
Consistent with the previous example, resource
http://www.example.com/container/ has six properties defined on it:
bigbox and author in the "http://ns.example.com/boxschema/"
namespace, and creationdate, displayname, resourcetype, and
supportedlock in the "DAV:" namespace.
The resource http://www.example.com/container/index.html, a member of
the "container" collection, has nine properties defined on it, bigbox
in the "http://ns.example.com/boxschema/" namespace and creationdate,
displayname, getcontentlength, getcontenttype, getetag,
getlastmodified, resourcetype, and supportedlock in the "DAV:"
namespace.
This example also demonstrates the use of XML namespace scoping and
the default namespace. Since the "xmlns" attribute does not contain
a prefix, the namespace applies by default to all enclosed elements.
Hence, all elements that do not explicitly state the namespace to
which they belong are members of the "DAV:" namespace.
Dusseault Standards Track [Page 40]
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9.1.5. Example - Using So-called 'allprop'
Note that 'allprop', despite its name, which remains for backward-
compatibility, does not return every property, but only dead
properties and the live properties defined in this specification.
>>Request
PROPFIND /container/ HTTP/1.1
Host: www.example.com
Depth: 1
Content-Type: application/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:propfind xmlns:D="DAV:">
<D:allprop/>
</D:propfind>
>>Response
HTTP/1.1 207 Multi-Status
Content-Type: application/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:multistatus xmlns:D="DAV:">
<D:response>
<D:href>/container/</D:href>
<D:propstat>
<D:prop xmlns:R="http://ns.example.com/boxschema/">
<R:bigbox><R:BoxType>Box type A</R:BoxType></R:bigbox>
<R:author><R:Name>Hadrian</R:Name></R:author>
<D:creationdate>1997-12-01T17:42:21-08:00</D:creationdate>
<D:displayname>Example collection</D:displayname>
<D:resourcetype><D:collection/></D:resourcetype>
<D:supportedlock>
<D:lockentry>
<D:lockscope><D:exclusive/></D:lockscope>
<D:locktype><D:write/></D:locktype>
</D:lockentry>
<D:lockentry>
<D:lockscope><D:shared/></D:lockscope>
<D:locktype><D:write/></D:locktype>
</D:lockentry>
</D:supportedlock>
</D:prop>
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<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href>/container/front.html</D:href>
<D:propstat>
<D:prop xmlns:R="http://ns.example.com/boxschema/">
<R:bigbox><R:BoxType>Box type B</R:BoxType>
</R:bigbox>
<D:creationdate>1997-12-01T18:27:21-08:00</D:creationdate>
<D:displayname>Example HTML resource</D:displayname>
<D:getcontentlength>4525</D:getcontentlength>
<D:getcontenttype>text/html</D:getcontenttype>
<D:getetag>"zzyzx"</D:getetag>
<D:getlastmodified
>Mon, 12 Jan 1998 09:25:56 GMT</D:getlastmodified>
<D:resourcetype/>
<D:supportedlock>
<D:lockentry>
<D:lockscope><D:exclusive/></D:lockscope>
<D:locktype><D:write/></D:locktype>
</D:lockentry>
<D:lockentry>
<D:lockscope><D:shared/></D:lockscope>
<D:locktype><D:write/></D:locktype>
</D:lockentry>
</D:supportedlock>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:response>
</D:multistatus>
In this example, PROPFIND was invoked on the resource
http://www.example.com/container/ with a Depth header of 1, meaning
the request applies to the resource and its children, and a propfind
XML element containing the allprop XML element, meaning the request
should return the name and value of all the dead properties defined
on the resources, plus the name and value of all the properties
defined in this specification. This example illustrates the use of
relative references in the 'href' elements of the response.
The resource http://www.example.com/container/ has six properties
defined on it: 'bigbox' and 'author in the
"http://ns.example.com/boxschema/" namespace, DAV:creationdate, DAV:
displayname, DAV:resourcetype, and DAV:supportedlock.
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RFC 4918 WebDAV June 2007
The last four properties are WebDAV-specific, defined in Section 15.
Since GET is not supported on this resource, the get* properties
(e.g., DAV:getcontentlength) are not defined on this resource. The
WebDAV-specific properties assert that "container" was created on
December 1, 1997, at 5:42:21PM, in a time zone 8 hours west of GMT
(DAV:creationdate), has a name of "Example collection" (DAV:
displayname), a collection resource type (DAV:resourcetype), and
supports exclusive write and shared write locks (DAV:supportedlock).
The resource http://www.example.com/container/front.html has nine
properties defined on it:
'bigbox' in the "http://ns.example.com/boxschema/" namespace (another
instance of the "bigbox" property type), DAV:creationdate, DAV:
displayname, DAV:getcontentlength, DAV:getcontenttype, DAV:getetag,
DAV:getlastmodified, DAV:resourcetype, and DAV:supportedlock.
The DAV-specific properties assert that "front.html" was created on
December 1, 1997, at 6:27:21PM, in a time zone 8 hours west of GMT
(DAV:creationdate), has a name of "Example HTML resource" (DAV:
displayname), a content length of 4525 bytes (DAV:getcontentlength),
a MIME type of "text/html" (DAV:getcontenttype), an entity tag of
"zzyzx" (DAV:getetag), was last modified on Monday, January 12, 1998,
at 09:25:56 GMT (DAV:getlastmodified), has an empty resource type,
meaning that it is not a collection (DAV:resourcetype), and supports
both exclusive write and shared write locks (DAV:supportedlock).
9.1.6. Example - Using 'allprop' with 'include'
>>Request
PROPFIND /mycol/ HTTP/1.1
Host: www.example.com
Depth: 1
Content-Type: application/xml; charset="utf-8"
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:propfind xmlns:D="DAV:">
<D:allprop/>
<D:include>
<D:supported-live-property-set/>
<D:supported-report-set/>
</D:include>
</D:propfind>
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RFC 4918 WebDAV June 2007
In this example, PROPFIND is executed on the resource
http://www.example.com/mycol/ and its internal member resources. The
client requests the values of all live properties defined in this
specification, plus all dead properties, plus two more live
properties defined in [RFC3253]. The response is not shown.
9.2. PROPPATCH Method
The PROPPATCH method processes instructions specified in the request
body to set and/or remove properties defined on the resource
identified by the Request-URI.
All DAV-compliant resources MUST support the PROPPATCH method and
MUST process instructions that are specified using the
propertyupdate, set, and remove XML elements. Execution of the
directives in this method is, of course, subject to access control
constraints. DAV-compliant resources SHOULD support the setting of
arbitrary dead properties.
The request message body of a PROPPATCH method MUST contain the
propertyupdate XML element.
Servers MUST process PROPPATCH instructions in document order (an
exception to the normal rule that ordering is irrelevant).
Instructions MUST either all be executed or none executed. Thus, if
any error occurs during processing, all executed instructions MUST be
undone and a proper error result returned. Instruction processing
details can be found in the definition of the set and remove
instructions in Sections 14.23 and 14.26.
If a server attempts to make any of the property changes in a
PROPPATCH request (i.e., the request is not rejected for high-level
errors before processing the body), the response MUST be a Multi-
Status response as described in Section 9.2.1.
This method is idempotent, but not safe (see Section 9.1 of
[RFC2616]). Responses to this method MUST NOT be cached.
9.2.1. Status Codes for Use in 'propstat' Element
In PROPPATCH responses, information about individual properties is
returned inside 'propstat' elements (see Section 14.22), each
containing an individual 'status' element containing information
about the properties appearing in it. The list below summarizes the
most common status codes used inside 'propstat'; however, clients
should be prepared to handle other 2/3/4/5xx series status codes as
well.
Dusseault Standards Track [Page 44]
RFC 4918 WebDAV June 2007
200 (OK) - The property set or change succeeded. Note that if this
appears for one property, it appears for every property in the
response, due to the atomicity of PROPPATCH.
403 (Forbidden) - The client, for reasons the server chooses not to
specify, cannot alter one of the properti
