|Network Working Group||Y. Goland|
|Request for Comments: 2518||Microsoft|
|Category: Standards Track||E. Whitehead|
HTTP Extensions for Distributed Authoring -- WEBDAV
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 © The Internet Society (1999). All Rights Reserved.
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).
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).
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 23.4) 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 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.
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].
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.
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.
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.
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 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.
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.
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, using the "xml:lang" attribute, handles cases where the same character set is employed by multiple human languages.
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.
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.
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.
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.
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.
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.
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:
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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:
(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.
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.
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.
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.
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.
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.
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.
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.
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 to respect locks are guaranteed not to change while write locked.
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.
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 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.
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.
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>)
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.
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.
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.
The following status codes are added to those defined in HTTP/1.1 [RFC2068].
The 102 (Processing) status code is an interim response used to inform the client that the server has accepted the complete request, but has not yet completed it. This status code SHOULD only be sent when the server has a reasonable expectation that the request will take significant time to complete. As guidance, if a method is taking longer than 20 seconds (a reasonable, but arbitrary value) to process the server SHOULD return a 102 (Processing) response. The server MUST send a final response after the request has been completed.
Methods can potentially take a long period of time to process, especially methods that support the Depth header. In such cases the client may time-out the connection while waiting for a response. To prevent this the server may return a 102 (Processing) status code to indicate to the client that the server is still processing the method.
The 207 (Multi-Status) status code provides status for multiple independent operations (see Section 11 for more information).
The 422 (Unprocessable Entity) status code means the server understands the content type of the request entity (hence a 415(Unsupported Media Type) status code is inappropriate), and the syntax of the request entity is correct (thus a 400 (Bad Request) status code is inappropriate) but was unable to process the contained instructions. For example, this error condition may occur if an XML request body contains well-formed (i.e., syntactically correct), but semantically erroneous XML instructions.
The 423 (Locked) status code means the source or destination resource of a method is locked.
The 424 (Failed Dependency) status code means that the method could not be performed on the resource because the requested action depended on another action and that action failed. For example, if a command in a PROPPATCH method fails then, at minimum, the rest of the commands will also fail with 424 (Failed Dependency).
The 507 (Insufficient Storage) status code means the method could not be performed on the resource because the server is unable to store the representation needed to successfully complete the request. This condition is considered to be temporary. If the request which received this status code was the result of a user action, the request MUST NOT be repeated until it is requested by a separate user action.
The default 207 (Multi-Status) response body is a text/xml or application/xml HTTP entity that contains a single XML element called multistatus, which contains a set of XML elements called response which contain 200, 300, 400, and 500 series status codes generated during the method invocation. 100 series status codes SHOULD NOT be recorded in a response XML element.
In the section below, the final line of each section gives the element type declaration using the format defined in [REC-XML]. The "Value" field, where present, specifies further restrictions on the allowable contents of the XML element using BNF (i.e., to further restrict the values of a PCDATA element).
<!ELEMENT depth (#PCDATA) >
<!ELEMENT collection EMPTY >
<!ELEMENT href (#PCDATA)>
<!ELEMENT multistatus (response+, responsedescription?) >
<!ELEMENT propstat (prop, status, responsedescription?) >
<!ELEMENT responsedescription (#PCDATA) >
<!ELEMENT owner ANY>
<!ELEMENT prop ANY>
<!ELEMENT propertybehavior (omit | keepalive) >
<!ELEMENT omit EMPTY >
<!ELEMENT propertyupdate (remove | set)+ >
<!ELEMENT allprop EMPTY >
For DAV properties, the name of the property is also the same as the name of the XML element that contains its value. In the section below, the final line of each section gives the element type declaration using the format defined in [REC-XML]. The "Value" field, where present, specifies further restrictions on the allowable contents of the XML element using BNF (i.e., to further restrict the values of a PCDATA element).
<!ELEMENT creationdate (#PCDATA) >
<!ELEMENT displayname (#PCDATA) >
<!ELEMENT getcontentlanguage (#PCDATA) >
<!ELEMENT getcontentlength (#PCDATA) >
<!ELEMENT getcontenttype (#PCDATA) >
<!ELEMENT getetag (#PCDATA) >
<!ELEMENT getlastmodified (#PCDATA) >
<!ELEMENT lockdiscovery (activelock)* >
PROPFIND /container/ HTTP/1.1 Host: www.foo.bar Content-Length: xxxx Content-Type: text/xml; charset="utf-8" <?xml version="1.0" encoding="utf-8" ?> <D:propfind xmlns:D='DAV:'> <D:prop><D:lockdiscovery/></D:prop> </D:propfind>
HTTP/1.1 207 Multi-Status Content-Type: text/xml; charset="utf-8" Content-Length: xxxx <?xml version="1.0" encoding="utf-8" ?> <D:multistatus xmlns:D='DAV:'> <D:response> <D:href>http://www.foo.bar/container/</D:href> <D:propstat> <D:prop> <D:lockdiscovery> <D:activelock> <D:locktype><D:write/></D:locktype> <D:lockscope><D:exclusive/></D:lockscope> <D:depth>0</D:depth> <D:owner>Jane Smith</D:owner> <D:timeout>Infinite</D:timeout> <D:locktoken> <D:href> opaquelocktoken:f81de2ad-7f3d-a1b2-4f3c-00a0c91a9d76 </D:href> </D:locktoken> </D:activelock> </D:lockdiscovery> </D:prop> <D:status>HTTP/1.1 200 OK</D:status> </D:propstat> </D:response> </D:multistatus>
This resource has a single exclusive write lock on it, with an infinite timeout.
<!ELEMENT resourcetype ANY >
<?xml version="1.0" encoding="utf-8" ?> <D:prop xmlns:D="DAV:" xmlns:F="http://www.foocorp.com/Project/"> <D:source> <D:link> <F:projfiles>Source</F:projfiles> <D:src>http://foo.bar/program</D:src> <D:dst>http://foo.bar/src/main.c</D:dst> </D:link> <D:link> <F:projfiles>Library</F:projfiles> <D:src>http://foo.bar/program</D:src> <D:dst>http://foo.bar/src/main.lib</D:dst> </D:link> <D:link> <F:projfiles>Makefile</F:projfiles> <D:src>http://foo.bar/program</D:src> <D:dst>http://foo.bar/src/makefile</D:dst> </D:link> </D:source> </D:prop>
In this example the resource http://foo.bar/program has a source property that contains three links. Each link contains three elements, two of which, src and dst, are part of the DAV schema defined in this document, and one which is defined by the schema http://www.foocorp.com/project/ (Source, Library, and Makefile). A client which only implements the elements in the DAV spec will not understand the foocorp elements and will ignore them, thus seeing the expected source and destination links. An enhanced client may know about the foocorp elements and be able to present the user with additional information about the links. This example demonstrates the power of XML markup, allowing element values to be enhanced without breaking older clients.
<!ELEMENT supportedlock (lockentry)* >
PROPFIND /container/ HTTP/1.1 Host: www.foo.bar Content-Length: xxxx Content-Type: text/xml; charset="utf-8" <?xml version="1.0" encoding="utf-8" ?> <D:propfind xmlns:D="DAV:"> <D:prop><D:supportedlock/></D:prop> </D:propfind>
HTTP/1.1 207 Multi-Status Content-Type: text/xml; charset="utf-8" Content-Length: xxxx <?xml version="1.0" encoding="utf-8" ?> <D:multistatus xmlns:D="DAV:"> <D:response> <D:href>http://www.foo.bar/container/</D:href> <D:propstat> <D:prop> <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>
All DAV compliant resources MUST ignore any unknown XML element and all its children encountered while processing a DAV method that uses XML as its command language.
This restriction also applies to the processing, by clients, of DAV property values where unknown XML elements SHOULD be ignored unless the property's schema declares otherwise.
This restriction does not apply to setting dead DAV properties on the server where the server MUST record unknown XML elements.
Additionally, this restriction does not apply to the use of XML where XML happens to be the content type of the entity body, for example, when used as the body of a PUT.
Since XML can be transported as text/xml or application/xml, a DAV server MUST accept DAV method requests with XML parameters transported as either text/xml or application/xml, and DAV client MUST accept XML responses using either text/xml or application/xml.
A DAV compliant resource can choose from two classes of compliance. A client can discover the compliance classes of a resource by executing OPTIONS on the resource, and examining the "DAV" header which is returned.
Since this document describes extensions to the HTTP/1.1 protocol, minimally all DAV compliant resources, clients, and proxies MUST be compliant with [RFC2068].
Compliance classes are not necessarily sequential. A resource that is class 2 compliant must also be class 1 compliant; but if additional compliance classes are defined later, a resource that is class 1, 2, and 4 compliant might not be class 3 compliant. Also note that identifiers other than numbers may be used as compliance class identifiers.
A class 1 compliant resource MUST meet all "MUST" requirements in all sections of this document.
Class 1 compliant resources MUST return, at minimum, the value "1" in the DAV header on all responses to the OPTIONS method.
A class 2 compliant resource MUST meet all class 1 requirements and support the LOCK method, the supportedlock property, the lockdiscovery property, the Time-Out response header and the Lock-Token request header. A class "2" compliant resource SHOULD also support the Time-Out request header and the owner XML element.
Class 2 compliant resources MUST return, at minimum, the values "1" and "2" in the DAV header on all responses to the OPTIONS method.
In the realm of internationalization, this specification complies with the IETF Character Set Policy [RFC2277]. In this specification, human-readable fields can be found either in the value of a property, or in an error message returned in a response entity body. In both cases, the human-readable content is encoded using XML, which has explicit provisions for character set tagging and encoding, and requires that XML processors read XML elements encoded, at minimum, using the UTF-8 [UTF-8] encoding of the ISO 10646 multilingual plane. XML examples in this specification demonstrate use of the charset parameter of the Content-Type header, as defined in [RFC2376], as well as the XML "encoding" attribute, which together provide charset identification information for MIME and XML processors.
XML also provides a language tagging capability for specifying the language of the contents of a particular XML element. XML uses either IANA registered language tags (see [RFC1766]) or ISO 639 language tags [ISO-639] in the "xml:lang" attribute of an XML element to identify the language of its content and attributes.
WebDAV applications MUST support the character set tagging, character set encoding, and the language tagging functionality of the XML specification. Implementors of WebDAV applications are strongly encouraged to read "XML Media Types" [RFC2376] for instruction on which MIME media type to use for XML transport, and on use of the charset parameter of the Content-Type header.
Names used within this specification fall into three categories: names of protocol elements such as methods and headers, names of XML elements, and names of properties. Naming of protocol elements follows the precedent of HTTP, using English names encoded in USASCII for methods and headers. Since these protocol elements are not visible to users, and are in fact simply long token identifiers, they do not need to support encoding in multiple character sets. Similarly, though the names of XML elements used in this specification are English names encoded in UTF-8, these names are not visible to the user, and hence do not need to support multiple character set encodings.
The name of a property defined on a resource is a URI. Although some applications (e.g., a generic property viewer) will display property URIs directly to their users, it is expected that the typical application will use a fixed set of properties, and will provide a mapping from the property name URI to a human-readable field when displaying the property name to a user. It is only in the case where the set of properties is not known ahead of time that an application need display a property name URI to a user. We recommend that applications provide human-readable property names wherever feasible.
For error reporting, we follow the convention of HTTP/1.1 status codes, including with each status code a short, English description of the code (e.g., 423 (Locked)). While the possibility exists that a poorly crafted user agent would display this message to a user, internationalized applications will ignore this message, and display an appropriate message in the user's language and character set.
Since interoperation of clients and servers does not require locale information, this specification does not specify any mechanism for transmission of this information.
This section is provided to detail issues concerning security implications of which WebDAV applications need to be aware.
All of the security considerations of HTTP/1.1 (discussed in [RFC2068]) and XML (discussed in [RFC2376]) also apply to WebDAV. In addition, the security risks inherent in remote authoring require stronger authentication technology, introduce several new privacy concerns, and may increase the hazards from poor server design. These issues are detailed below.
Due to their emphasis on authoring, WebDAV servers need to use authentication technology to protect not just access to a network resource, but the integrity of the resource as well. Furthermore, the introduction of locking functionality requires support for authentication.
A password sent in the clear over an insecure channel is an inadequate means for protecting the accessibility and integrity of a resource as the password may be intercepted. Since Basic authentication for HTTP/1.1 performs essentially clear text transmission of a password, Basic authentication MUST NOT be used to authenticate a WebDAV client to a server unless the connection is secure. Furthermore, a WebDAV server MUST NOT send Basic authentication credentials in a WWW-Authenticate header unless the connection is secure. Examples of secure connections include a Transport Layer Security (TLS) connection employing a strong cipher suite with mutual authentication of client and server, or a connection over a network which is physically secure, for example, an isolated network in a building with restricted access.
WebDAV applications MUST support the Digest authentication scheme [RFC2069]. Since Digest authentication verifies that both parties to a communication know a shared secret, a password, without having to send that secret in the clear, Digest authentication avoids the security problems inherent in Basic authentication while providing a level of authentication which is useful in a wide range of scenarios.
Denial of service attacks are of special concern to WebDAV servers. WebDAV plus HTTP enables denial of service attacks on every part of a system's resources.
The underlying storage can be attacked by PUTting extremely large files.
Asking for recursive operations on large collections can attack processing time.
Making multiple pipelined requests on multiple connections can attack network connections.
WebDAV servers need to be aware of the possibility of a denial of service attack at all levels.
WebDAV provides, through the PROPFIND method, a mechanism for listing the member resources of a collection. This greatly diminishes the effectiveness of security or privacy techniques that rely only on the difficulty of discovering the names of network resources. Users of WebDAV servers are encouraged to use access control techniques to prevent unwanted access to resources, rather than depending on the relative obscurity of their resource names.
When submitting a lock request a user agent may also submit an owner XML field giving contact information for the person taking out the lock (for those cases where a person, rather than a robot, is taking out the lock). This contact information is stored in a lockdiscovery property on the resource, and can be used by other collaborators to begin negotiation over access to the resource. However, in many cases this contact information can be very private, and should not be widely disseminated. Servers SHOULD limit read access to the lockdiscovery property as appropriate. Furthermore, user agents SHOULD provide control over whether contact information is sent at all, and if contact information is sent, control over exactly what information is sent.
Since property values are typically used to hold information such as the author of a document, there is the possibility that privacy concerns could arise stemming from widespread access to a resource's property data. To reduce the risk of inadvertent release of private information via properties, servers are encouraged to develop access control mechanisms that separate read access to the resource body and read access to the resource's properties. This allows a user to control the dissemination of their property data without overly restricting access to the resource's contents.
HTTP/1.1 warns against providing read access to script code because it may contain sensitive information. Yet WebDAV, via its source link facility, can potentially provide a URI for script resources so they may be authored. For HTTP/1.1, a server could reasonably prevent access to source resources due to the predominance of read-only access. WebDAV, with its emphasis on authoring, encourages read and write access to source resources, and provides the source link facility to identify the source. This reduces the security benefits of eliminating access to source resources. Users and administrators of WebDAV servers should be very cautious when allowing remote authoring of scripts, limiting read and write access to the source resources to authorized principals.
XML supports a facility known as "external entities", defined in section 4.2.2 of [REC-XML], which instruct an XML processor to retrieve and perform an inline include of XML located at a particular URI. An external XML entity can be used to append or modify the document type declaration (DTD) associated with an XML document. An external XML entity can also be used to include XML within the content of an XML document. For non-validating XML, such as the XML used in this specification, including an external XML entity is not required by [REC-XML]. However, [REC-XML] does state that an XML processor may, at its discretion, include the external XML entity.
External XML entities have no inherent trustworthiness and are subject to all the attacks that are endemic to any HTTP GET request. Furthermore, it is possible for an external XML entity to modify the DTD, and hence affect the final form of an XML document, in the worst case significantly modifying its semantics, or exposing the XML processor to the security risks discussed in [RFC2376]. Therefore, implementers must be aware that external XML entities should be treated as untrustworthy.
There is also the scalability risk that would accompany a widely deployed application which made use of external XML entities. In this situation, it is possible that there would be significant numbers of requests for one external XML entity, potentially overloading any server which fields requests for the resource containing the external XML entity.
This specification, in Section 6.4, requires the use of Universal Unique Identifiers (UUIDs) for lock tokens, in order to guarantee their uniqueness across space and time. UUIDs, as defined in [ISO-11578], contain a "node" field which "consists of the IEEE address, usually the host address. For systems with multiple IEEE 802 nodes, any available node address can be used." Since a WebDAV server will issue many locks over its lifetime, the implication is that it will also be publicly exposing its IEEE 802 address.
There are several risks associated with exposure of IEEE 802 addresses. Using the IEEE 802 address:
Section 6.4.1 of this specification details an alternate mechanism for generating the "node" field of a UUID without using an IEEE 802 address, which alleviates the risks associated with exposure of IEEE 802 addresses by using an alternate source of uniqueness.
This document defines two namespaces, the namespace of property names, and the namespace of WebDAV-specific XML elements used within property values. URIs are used for both names, for several reasons. Assignment of a URI does not require a request to a central naming authority, and hence allow WebDAV property names and XML elements to be quickly defined by any WebDAV user or application. URIs also provide a unique address space, ensuring that the distributed users of WebDAV will not have collisions among the property names and XML elements they create.
This specification defines a distinguished set of property names and XML elements that are understood by all WebDAV applications. The property names and XML elements in this specification are all derived from the base URI DAV: by adding a suffix to this URI, for example, DAV:creationdate for the "creationdate" property.
This specification also defines a URI scheme for the encoding of lock tokens, the opaquelocktoken URI scheme described in Section 6.4.
To ensure correct interoperation based on this specification, IANA must reserve the URI namespaces starting with "DAV:" and with "opaquelocktoken:" for use by this specification, its revisions, and related WebDAV specifications.
The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use other technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director.
A specification such as this thrives on piercing critical review and withers from apathetic neglect. The authors gratefully acknowledge the contributions of the following people, whose insights were so valuable at every stage of our work.
Terry Allen, Harald Alvestrand, Jim Amsden, Becky Anderson, Alan Babich, Sanford Barr, Dylan Barrell, Bernard Chester, Tim Berners-Lee, Dan Connolly, Jim Cunningham, Ron Daniel, Jr., Jim Davis, Keith Dawson, Mark Day, Brian Deen, Martin Duerst, David Durand, Lee Farrell, Chuck Fay, Wesley Felter, Roy Fielding, Mark Fisher, Alan Freier, George Florentine, Jim Gettys, Phill Hallam-Baker, Dennis Hamilton, Steve Henning, Mead Himelstein, Alex Hopmann, Andre van der Hoek, Ben Laurie, Paul Leach, Ora Lassila, Karen MacArthur, Steven Martin, Larry Masinter, Michael Mealling, Keith Moore, Thomas Narten, Henrik Nielsen, Kenji Ota, Bob Parker, Glenn Peterson, Jon Radoff, Saveen Reddy, Henry Sanders, Christopher Seiwald, Judith Slein, Mike Spreitzer, Einar Stefferud, Greg Stein, Ralph Swick, Kenji Takahashi, Richard N. Taylor, Robert Thau, John Turner, Sankar Virdhagriswaran, Fabio Vitali, Gregory Woodhouse, and Lauren Wood.
Two from this list deserve special mention. The contributions by Larry Masinter have been invaluable, both in helping the formation of the working group and in patiently coaching the authors along the way. In so many ways he has set high standards we have toiled to meet. The contributions of Judith Slein in clarifying the requirements, and in patiently reviewing draft after draft, both improved this specification and expanded our minds on document management.
We would also like to thank John Turner for developing the XML DTD.
|[RFC1766]||Alvestrand, H., “Tags for the Identification of Languages”, RFC 1766, March 1995.|
|[RFC2277]||Alvestrand, H., “IETF Policy on Character Sets and Languages”, BCP 18, RFC 2277, January 1998.|
|[RFC2119]||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.|
|[RFC2396]||Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifiers (URI): Generic Syntax”, RFC 2396, August 1998.|
|[REC-XML]||Bray, T., Paoli, J., and C. Sperberg-McQueen, “Extensible Markup Language (XML) 1.0”, W3C REC-xml-19980210, February 1998, <http://www.w3.org/TR/1998/REC-xml-19980210>.|
|[REC-XML-NAMES]||Bray, T., Hollander, D., and A. Layman, “Namespaces in XML”, W3C REC-xml-names-19990114, January 1999, <http://www.w3.org/TR/1999/REC-xml-names-19990114>.|
|[RFC2069]||Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P., Luotonen, A., Sink, E., and L. Stewart, “An Extension to HTTP : Digest Access Authentication”, RFC 2069, January 1997.|
|[RFC2068]||Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2068, January 1997.|
|[ISO-639]||International Organization for Standardization, “ISO 639:1988. Code for the representation of names of languages.”, 1988.|
|[ISO-8601]||International Organization for Standardization, “ISO 8601, Data elements and interchange formats-Information interchange--Representation of dates and times”, June 1988.|
|[ISO-11578]||International Organization for Standardization, “ISO/IEC 11578:1996. Information technology - Open Systems Interconnection - Remote Procedure Call (RPC)”, 1996.|
|[RFC2141]||Moats, R., “URN Syntax”, RFC 2141, May 1997.|
|[UTF-8]||Yergeau, F., “UTF-8, a transformation format of ISO 10646”, RFC 2279, January 1998.|
|[RFC2026]||Bradner, S., “The Internet Standards Process -- Revision 3”, BCP 9, RFC 2026, October 1996.|
|[RFC1807]||Lasher, R. and D. Cohen, “A Format for Bibliographic Records”, RFC 1807, June 1995.|
|[WF]||Lagoze, C., “The Warwick Framework: A Container Architecture for Diverse Sets of Metadata”, July 1996, <http://www.dlib.org/dlib/july96/lagoze/07lagoze.html>.|
D-Lib Magazine, July/August 1996.
|[USMARC]||Network Development and MARC Standards, Office, Washington DC: Cataloging Distribution Service, Library of Congress., “USMARC Format for Bibliographic Data”, 1994.|
|[REC-PICS]||Miller, J., Krauskopf, T., Resnick, P., and W. Treese, “PICS Label Distribution Label Syntax and Communication Protocols, Version 1.1”, W3C REC-PICS-labels-961031, October 1996, <http://www.w3.org/pub/WWW/TR/REC-PICS-labels-961031.html>.|
|[RFC2291]||Slein, J., Vitali, F., Whitehead, E., and D. Durand, “Requirements for a Distributed Authoring and Versioning Protocol for the World Wide Web”, RFC 2291, February 1998.|
|[RFC2413]||Weibel, S., Kunze, J., Lagoze, C., and M. Wolf, “Dublin Core Metadata for Resource Discovery”, RFC 2413, September 1998.|
|[RFC2376]||Whitehead, E. and M. Makoto, “XML Media Types”, RFC 2376, July 1998.|
This section provides a document type definition, following the rules in [REC-XML], for the XML elements used in the protocol stream and in the values of properties. It collects the element definitions given in sections 12 and 13.
<!DOCTYPE webdav-1.0 [ <!--============ XML Elements from Section 12 ==================--> <!ELEMENT activelock (lockscope, locktype, depth, owner?, timeout?, locktoken?) > <!ELEMENT lockentry (lockscope, locktype) > <!ELEMENT lockinfo (lockscope, locktype, owner?) > <!ELEMENT locktype (write) > <!ELEMENT write EMPTY > <!ELEMENT lockscope (exclusive | shared) > <!ELEMENT exclusive EMPTY > <!ELEMENT shared EMPTY > <!ELEMENT depth (#PCDATA) > <!ELEMENT owner ANY > <!ELEMENT timeout (#PCDATA) > <!ELEMENT locktoken (href+) > <!ELEMENT href (#PCDATA) > <!ELEMENT link (src+, dst+) > <!ELEMENT dst (#PCDATA) > <!ELEMENT src (#PCDATA) > <!ELEMENT multistatus (response+, responsedescription?) > <!ELEMENT response (href, ((href*, status)|(propstat+)), responsedescription?) > <!ELEMENT status (#PCDATA) > <!ELEMENT propstat (prop, status, responsedescription?) > <!ELEMENT responsedescription (#PCDATA) > <!ELEMENT prop ANY > <!ELEMENT propertybehavior (omit | keepalive) > <!ELEMENT omit EMPTY > <!ELEMENT keepalive (#PCDATA | href+) > <!ELEMENT propertyupdate (remove | set)+ > <!ELEMENT remove (prop) > <!ELEMENT set (prop) > <!ELEMENT propfind (allprop | propname | prop) > <!ELEMENT allprop EMPTY > <!ELEMENT propname EMPTY > <!ELEMENT collection EMPTY > <!--=========== Property Elements from Section 13 ===============--> <!ELEMENT creationdate (#PCDATA) > <!ELEMENT displayname (#PCDATA) > <!ELEMENT getcontentlanguage (#PCDATA) > <!ELEMENT getcontentlength (#PCDATA) > <!ELEMENT getcontenttype (#PCDATA) > <!ELEMENT getetag (#PCDATA) > <!ELEMENT getlastmodified (#PCDATA) > <!ELEMENT lockdiscovery (activelock)* > <!ELEMENT resourcetype ANY > <!ELEMENT source (link)* > <!ELEMENT supportedlock (lockentry)* > ]>
The creationdate property specifies the use of the ISO 8601 date format [ISO-8601]. This section defines a profile of the ISO 8601 date format for use with this specification. This profile is quoted from an Internet-Draft by Chris Newman, and is mentioned here to properly attribute his work.
date-time = full-date "T" full-time full-date = date-fullyear "-" date-month "-" date-mday full-time = partial-time time-offset date-fullyear = 4DIGIT date-month = 2DIGIT ; 01-12 date-mday = 2DIGIT ; 01-28, 01-29, 01-30, 01-31 based on month/year time-hour = 2DIGIT ; 00-23 time-minute = 2DIGIT ; 00-59 time-second = 2DIGIT ; 00-59, 00-60 based on leap second rules time-secfrac = "." 1*DIGIT time-numoffset = ("+" / "-") time-hour ":" time-minute time-offset = "Z" / time-numoffset partial-time = time-hour ":" time-minute ":" time-second [time-secfrac]
Numeric offsets are calculated as local time minus UTC (Coordinated Universal Time). So the equivalent time in UTC can be determined by subtracting the offset from the local time. For example, 18:50:00-04:00 is the same time as 22:58:00Z.
If the time in UTC is known, but the offset to local time is unknown, this can be represented with an offset of "-00:00". This differs from an offset of "Z" which implies that UTC is the preferred reference point for the specified time.
XML supports two mechanisms for indicating that an XML element does not have any content. The first is to declare an XML element of the form <A></A>. The second is to declare an XML element of the form <A/>. The two XML elements are semantically identical.
It is a violation of the XML specification to use the <A></A> form if the associated DTD declares the element to be EMPTY (e.g., <!ELEMENT A EMPTY>). If such a statement is included, then the empty element format, <A/> must be used. If the element is not declared to be EMPTY, then either form <A></A> or <A/> may be used for empty elements.
XML is a flexible data format that makes it easy to submit data that appears legal but in fact is not. The philosophy of "Be flexible in what you accept and strict in what you send" still applies, but it must not be applied inappropriately. XML is extremely flexible in dealing with issues of white space, element ordering, inserting new elements, etc. This flexibility does not require extension, especially not in the area of the meaning of elements.
There is no kindness in accepting illegal combinations of XML elements. At best it will cause an unwanted result and at worst it can cause real damage.
The following request body for a PROPFIND method is illegal.
<?xml version="1.0" encoding="utf-8" ?> <D:propfind xmlns:D="DAV:"> <D:allprop/> <D:propname/> </D:propfind>
The definition of the propfind element only allows for the allprop or the propname element, not both. Thus the above is an error and must be responded to with a 400 (Bad Request).
Imagine, however, that a server wanted to be "kind" and decided to pick the allprop element as the true element and respond to it. A client running over a bandwidth limited line who intended to execute a propname would be in for a big surprise if the server treated the command as an allprop.
Additionally, if a server were lenient and decided to reply to this request, the results would vary randomly from server to server, with some servers executing the allprop directive, and others executing the propname directive. This reduces interoperability rather than increasing it.
The previous example was illegal because it contained two elements that were explicitly banned from appearing together in the propfind element. However, XML is an extensible language, so one can imagine new elements being defined for use with propfind. Below is the request body of a PROPFIND and, like the previous example, must be rejected with a 400 (Bad Request) by a server that does not understand the expired-props element.
<?xml version="1.0" encoding="utf-8" ?> <D:propfind xmlns:D="DAV:" xmlns:E="http://www.foo.bar/standards/props/"> <E:expired-props/> </D:propfind>
To understand why a 400 (Bad Request) is returned let us look at the request body as the server unfamiliar with expired-props sees it.
<?xml version="1.0" encoding="utf-8" ?> <D:propfind xmlns:D="DAV:" xmlns:E="http://www.foo.bar/standards/props/"> </D:propfind>
As the server does not understand the expired-props element, according to the WebDAV-specific XML processing rules specified in Section 14, it must ignore it. Thus the server sees an empty propfind, which by the definition of the propfind element is illegal.
Please note that had the extension been additive it would not necessarily have resulted in a 400 (Bad Request). For example, imagine the following request body for a PROPFIND:
<?xml version="1.0" encoding="utf-8" ?> <D:propfind xmlns:D="DAV:" xmlns:E="http://www.foo.bar/standards/props/"> <D:propname/> <E:leave-out>*boss*</E:leave-out> </D:propfind>
The previous example contains the fictitious element leave-out. Its purpose is to prevent the return of any property whose name matches the submitted pattern. If the previous example were submitted to a server unfamiliar with leave-out, the only result would be that the leave-out element would be ignored and a propname would be executed.
All DAV compliant systems MUST support the XML namespace extensions as specified in [REC-XML-NAMES].
[Note to the reader: This section does not appear in [REC-XML-NAMES], but is necessary to avoid ambiguity for WebDAV XML processors.]
WebDAV compliant XML processors MUST interpret a qualified name as a URI constructed by appending the LocalPart to the namespace name URI.
<del:glider xmlns:del="http://www.del.jensen.org/"> <del:glidername> Johnny Updraft </del:glidername> <del:glideraccidents/> </del:glider>
In this example, the qualified element name "del:glider" is interpreted as the URL "http://www.del.jensen.org/glider".
<bar:glider xmlns:del="http://www.del.jensen.org/"> <bar:glidername> Johnny Updraft </bar:glidername> <bar:glideraccidents/> </bar:glider>
Even though this example is syntactically different from the previous example, it is semantically identical. Each instance of the namespace name "bar" is replaced with "http://www.del.jensen.org/" and then appended to the local name for each element tag. The resulting tag names in this example are exactly the same as for the previous example.
<foo:r xmlns:foo="http://www.del.jensen.org/glide"> <foo:rname> Johnny Updraft </foo:rname> <foo:raccidents/> </foo:r>
This example is semantically identical to the two previous ones. Each instance of the namespace name "foo" is replaced with "http://www.del.jensen.org/glide" which is then appended to the local name for each element tag, the resulting tag names are identical to those in the previous examples.
Y. Y. Goland
One Microsoft Way
Redmond, WA 98052-6399
E. J. Whitehead, Jr.
Dept. Of Information and Computer Science, University of California, Irvine
Irvine, CA 92697-3425
685 East Middlefield Road
Mountain View, CA 94043
S. R. Carter
1555 N. Technology Way
M/S ORM F111
Orem, UT 84097-2399
1555 N. Technology Way
M/S ORM F111
Orem, UT 84097-2399
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