The Extensible Markup Language (XML)

Configuration Access Protocol (XCAP)

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 © The IETF Trust (2007).

Abstract

This specification defines the Extensible Markup Language (XML) Configuration Access Protocol (XCAP). XCAP allows a client to read, write, and modify application configuration data stored in XML format on a server. XCAP maps XML document sub-trees and element attributes to HTTP URIs, so that these components can be directly accessed by HTTP.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  5
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Application Usages . . . . . . . . . . . . . . . . . . . . . .  7
     5.1.  Application Unique ID (AUID) . . . . . . . . . . . . . . .  7
     5.2.  Default Document Namespace . . . . . . . . . . . . . . . .  8
     5.3.  Data Validation  . . . . . . . . . . . . . . . . . . . . .  9
     5.4.  Data Semantics . . . . . . . . . . . . . . . . . . . . . . 10
     5.5.  Naming Conventions . . . . . . . . . . . . . . . . . . . . 11
     5.6.  Resource Interdependencies . . . . . . . . . . . . . . . . 11
     5.7.  Authorization Policies . . . . . . . . . . . . . . . . . . 12
     5.8.  Data Extensibility . . . . . . . . . . . . . . . . . . . . 12
     5.9.  Documenting Application Usages . . . . . . . . . . . . . . 13
     5.10. Guidelines for Creating Application Usages . . . . . . . . 13
   6.  URI Construction . . . . . . . . . . . . . . . . . . . . . . . 15
     6.1.  XCAP Root  . . . . . . . . . . . . . . . . . . . . . . . . 15
     6.2.  Document Selector  . . . . . . . . . . . . . . . . . . . . 16
     6.3.  Node Selector  . . . . . . . . . . . . . . . . . . . . . . 18
     6.4.  Namespace Bindings for the Selector  . . . . . . . . . . . 23
   7.  Client Operations  . . . . . . . . . . . . . . . . . . . . . . 24
     7.1.  Create or Replace a Document . . . . . . . . . . . . . . . 26
     7.2.  Delete a Document  . . . . . . . . . . . . . . . . . . . . 26
     7.3.  Fetch a Document . . . . . . . . . . . . . . . . . . . . . 26
     7.4.  Create or Replace an Element . . . . . . . . . . . . . . . 26
     7.5.  Delete an Element  . . . . . . . . . . . . . . . . . . . . 29
     7.6.  Fetch an Element . . . . . . . . . . . . . . . . . . . . . 30
     7.7.  Create or Replace an Attribute . . . . . . . . . . . . . . 30
     7.8.  Delete an Attribute  . . . . . . . . . . . . . . . . . . . 31
     7.9.  Fetch an Attribute . . . . . . . . . . . . . . . . . . . . 31
     7.10. Fetch Namespace Bindings . . . . . . . . . . . . . . . . . 32
     7.11. Conditional Operations . . . . . . . . . . . . . . . . . . 32
   8.  Server Behavior  . . . . . . . . . . . . . . . . . . . . . . . 34
     8.1.  POST Handling  . . . . . . . . . . . . . . . . . . . . . . 35
     8.2.  PUT Handling . . . . . . . . . . . . . . . . . . . . . . . 35
       8.2.1.  Locating the Parent  . . . . . . . . . . . . . . . . . 35
       8.2.2.  Verifying Document Content . . . . . . . . . . . . . . 36
       8.2.3.  Creation . . . . . . . . . . . . . . . . . . . . . . . 37
       8.2.4.  Replacement  . . . . . . . . . . . . . . . . . . . . . 41
       8.2.5.  Validation . . . . . . . . . . . . . . . . . . . . . . 42
       8.2.6.  Conditional Processing . . . . . . . . . . . . . . . . 43
       8.2.7.  Resource Interdependencies . . . . . . . . . . . . . . 44
     8.3.  GET Handling . . . . . . . . . . . . . . . . . . . . . . . 44
     8.4.  DELETE Handling  . . . . . . . . . . . . . . . . . . . . . 45
     8.5.  Managing Etags . . . . . . . . . . . . . . . . . . . . . . 46
   9.  Cache Control  . . . . . . . . . . . . . . . . . . . . . . . . 47
   10. Namespace Binding Format . . . . . . . . . . . . . . . . . . . 47
   11. Detailed Conflict Reports  . . . . . . . . . . . . . . . . . . 47
     11.1. Document Structure . . . . . . . . . . . . . . . . . . . . 48
     11.2. XML Schema . . . . . . . . . . . . . . . . . . . . . . . . 50
   12. XCAP Server Capabilities . . . . . . . . . . . . . . . . . . . 53
     12.1. Application Unique ID (AUID) . . . . . . . . . . . . . . . 54
     12.2. XML Schema . . . . . . . . . . . . . . . . . . . . . . . . 54
     12.3. Default Document Namespace . . . . . . . . . . . . . . . . 56
     12.4. MIME Type  . . . . . . . . . . . . . . . . . . . . . . . . 56
     12.5. Validation Constraints . . . . . . . . . . . . . . . . . . 56
     12.6. Data Semantics . . . . . . . . . . . . . . . . . . . . . . 56
     12.7. Naming Conventions . . . . . . . . . . . . . . . . . . . . 56
     12.8. Resource Interdependencies . . . . . . . . . . . . . . . . 56
     12.9. Authorization Policies . . . . . . . . . . . . . . . . . . 56
   13. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 59
   15. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 60
     15.1. XCAP Application Unique IDs  . . . . . . . . . . . . . . . 60
     15.2. MIME Types . . . . . . . . . . . . . . . . . . . . . . . . 61
       15.2.1. application/xcap-el+xml MIME Type  . . . . . . . . . . 61
       15.2.2. application/xcap-att+xml MIME Type . . . . . . . . . . 62
       15.2.3. application/xcap-ns+xml MIME Type  . . . . . . . . . . 63
       15.2.4. application/xcap-error+xml MIME Type . . . . . . . . . 64
       15.2.5. application/xcap-caps+xml MIME Type  . . . . . . . . . 64
     15.3. URN Sub-Namespace Registrations  . . . . . . . . . . . . . 65
       15.3.1. urn:ietf:params:xml:ns:xcap-error  . . . . . . . . . . 65
       15.3.2. urn:ietf:params:xml:ns:xcap-caps . . . . . . . . . . . 66
     15.4. XML Schema Registrations . . . . . . . . . . . . . . . . . 67
       15.4.1. XCAP Error Schema Registration . . . . . . . . . . . . 67
       15.4.2. XCAP Capabilities Schema Registration  . . . . . . . . 67
   16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 67
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 67
     17.1. Normative References . . . . . . . . . . . . . . . . . . . 67
     17.2. Informative References . . . . . . . . . . . . . . . . . . 69

1. Introduction

In many communications applications, such as Voice over IP, instant messaging, and presence, it is necessary for network servers to access per-user information in the process of servicing a request. This per-user information resides within the network, but is managed by the end user themselves. Its management can be done through a multiplicity of access points, including the web, a wireless handset, or a PC application.

There are many examples of per-user information. One is presence [20] authorization policy, which defines rules about which watchers are allowed to subscribe to a presentity, and what information they are allowed to access. Another is presence lists, which are lists of users whose presence is desired by a watcher [26]. One way to obtain presence information for the list is to subscribe to a resource which represents that list [21]. In this case, the Resource List Server (RLS) requires access to this list in order to process a SIP [16] SUBSCRIBE [28] request for it. Another way to obtain presence for the users on the list is for a watcher to subscribe to each user individually. In that case, it is convenient to have a server store the list, and when the client boots, it fetches the list from the server. This would allow a user to access their resource lists from different clients.

This specification describes a protocol that can be used to manipulate this per-user data. It is called the Extensible Markup Language (XML) Configuration Access Protocol (XCAP). XCAP is a set of conventions for mapping XML documents and document components into HTTP URIs, rules for how the modification of one resource affects another, data validation constraints, and authorization policies associated with access to those resources. Because of this structure, normal HTTP primitives can be used to manipulate the data. XCAP is based heavily on ideas borrowed from the Application Configuration Access Protocol (ACAP) [25], but it is not an extension of it, nor does it have any dependencies on it. Like ACAP, XCAP is meant to support the configuration needs for a multiplicity of applications, rather than just a single one.

XCAP was not designed as a general purpose XML search protocol, XML database update protocol, nor a general purpose, XML-based configuration protocol for network elements.

2. Overview of Operation

Each application (where an application refers to a use case that implies a collection of data and associated semantics) that makes use of XCAP specifies an application usage (Section 5). This application usage defines the XML schema [2] for the data used by the application, along with other key pieces of information. The principal task of XCAP is to allow clients to read, write, modify, create, and delete pieces of that data. These operations are supported using HTTP/1.1 [6]. An XCAP server acts as a repository for collections of XML documents. There will be documents stored for each application. Within each application, there are documents stored for each user. Each user can have a multiplicity of documents for a particular application. To access some component of one of those documents, XCAP defines an algorithm for constructing a URI that can be used to reference that component. Components refer to any element or attribute within the document. Thus, the HTTP URIs used by XCAP point to a document, or to pieces of information that are finer grained than the XML document itself. An HTTP resource that follows the naming conventions and validation constraints defined here is called an XCAP resource.

Since XCAP resources are also HTTP resources, they can be accessed using HTTP methods. Reading an XCAP resource is accomplished with HTTP GET, creating or modifying one is done with HTTP PUT, and removing one of the resources is done with an HTTP DELETE. XCAP resources do not represent processing scripts; as a result, POST operations to HTTP URIs representing XCAP resources are not defined. Properties that HTTP associates with resources, such as entity tags, also apply to XCAP resources. Indeed, entity tags are particularly useful in XCAP, as they allow a number of conditional operations to be performed.

XML documents that are equivalent for the purposes of many applications may differ in their physical representation. With XCAP resources, the canonical form with comments [19] of an XML document determines the logical equivalence. In other words, the canonical specification determines how significant whitespace MUST be processed. It also implies that, for example, new inserted attributes may appear in any order within the physical representation.

3. Terminology

In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 [7] and indicate requirement levels for compliant implementations.

4. Definitions

The following terms are used throughout this document:

   XCAP Resource:  An HTTP resource representing an XML document, an
      element within an XML document, or an attribute of an element
      within an XML document that follows the naming and validation
      constraints of XCAP.
   
   XCAP Server:  An HTTP server that understands how to follow the
      naming and validation constraints defined in this specification.
   
   XCAP Client:  An HTTP client that understands how to follow the
      naming and validation constraints defined in this specification.
   
   Application:  A collection of software components within a network
      whose operation depends on data managed and stored on an XCAP
      server.
   
   Application Usage:  Detailed information on the interaction of an
      application with the XCAP server.
   
   Application Unique ID (AUID):  A unique identifier within the
      namespace of application unique IDs created by this specification
      that differentiates XCAP resources accessed by one application
      from XCAP resources accessed by another.
   
   Naming Conventions:  The part of an application usage that specifies
      well-known URIs used by an application, or more generally,
      specifies the URIs that are typically accessed by an application
      during its processing.
   
   XCAP User Identifier (XUI):  The XUI is a string, valid as a path
      element in an HTTP URI, that is associated with each user served
      by the XCAP server.
   
   XCAP Root:  A context that contains all the documents across all
      application usages and users that are managed by the server.
   
   Document Selector:  A sequence of path segments, with each segment
      being separated by a "/", that identify the XML document within an
      XCAP root that is being selected.
   
   Node Selector:  A sequence of path segments, with each segment being
      separated by a "/", that identify the XML node (element or
      attribute) being selected within a document.
   
   Node Selector Separator:  A single path segment equal to two tilde
      characters "~~" that is used to separate the document selector
      from the node selector within an HTTP URI.
   
   Document URI:  The HTTP URI containing the XCAP root and document
      selector, resulting in the selection of a specific document.  As a
      result, performing a GET against the document URI would retrieve
      the document.
   
   Node URI:  The HTTP URI containing the XCAP root, document selector,
      node selector separator, and node selector, resulting in the
      selection of a specific XML node.
   
   XCAP Root URI:  An HTTP URI that represents the XCAP root.  Although
      a syntactically valid URI, the XCAP Root URI does not correspond
      to an actual resource on an XCAP server.  Actual resources are
      created by appending additional path information to the XCAP Root
      URI.
   
   Global Tree:  A URI that represents the parent for all global
      documents for a particular application usage within a particular
      XCAP root.
   
   Home Directory:  A URI that represents the parent for all documents
      for a particular user for a particular application usage within a
      particular XCAP root.
   
   Positional Insertion:  A PUT operation that results in the insertion
      of a new element into a document such that its position, relative
      to other children of the same parent, is set by the client.

5. Application Usages

Each XCAP resource on a server is associated with an application. In order for an application to use those resources, application specific conventions must be specified. Those conventions include the XML schema that defines the structure and constraints of the data, well- known URIs to bootstrap access to the data, and so on. All of those application specific conventions are defined by the application usage.

5.1. Application Unique ID (AUID)

Each application usage is associated with a name, called an Application Unique ID (AUID). This name uniquely identifies the application usage within the namespace of application usages, and is different from AUIDs used by other applications. AUIDs exist in one of two namespaces. The first namespace is the IETF namespace. This namespace contains a set of tokens, each of which is registered with IANA. These registrations occur with the publication of standards track RFCs [27], based on the guidelines in Section 15. The second namespace is the vendor-proprietary namespace. Each AUID in that namespace is prefixed with the reverse domain name of the organization creating the AUID, followed by a period, followed by any vendor defined token. As an example, the example.com domain can create an AUID with the value "com.example.foo" but cannot create one with the value "org.example.foo". AUIDs within the vendor namespace do not need to be registered with IANA. The vendor namespace is also meant to be used in lab environments where no central registry is needed. The syntax for AUIDs, expressed in ABNF [12] (and using some of the BNF defined in RFC 3986 [13]), is:

   AUID             =  global-a-uid / vendor-a-uid
   global-a-uid     =  a-uid
   a-uid            =  1*a-uid-char
   vendor-a-uid     =  rev-hostname "." a-uid
   rev-hostname     =  toplabel *( "." domainlabel  )
   domainlabel      =  alphanum
                       / alphanum *( alphanum / "-" ) alphanum
   toplabel         =  ALPHA / ALPHA *( alphanum / "-" ) alphanum
   a-uid-char       =  a-uid-unreserved / pct-encoded / sub-delims
                       / ":" / "@"
                                  ;pct-encoded from RFC 3986
                                  ;sub-delims from RFC 3986
   alphanum         = ALPHA / DIGIT
                                  ;DIGIT from RFC 4234
                                  ;ALPHA from RFC 4234
   a-uid-unreserved = ALPHA / DIGIT / "-" / "_" / "~"

The allowed characters for the auid production is a subset of the pchar production defined in RFC 3986. In particular, it omits the ".", which allows for the auid to be separated from the reverse hostname.

5.2. Default Document Namespace

In order for the XCAP server to match a URI to an element or attribute of a document, any XML namespace prefixes used within the URI must be expanded [3]. This expansion requires a namespace binding context. That context maps namespace prefixes to namespace URIs. It also defines a default namespace that applies to elements in the URI without namespace prefixes. The namespace binding context comes from two sources. First, the mapping of namespace prefixes to namespace URIs is obtained from the URI itself (see Section 6.4). However, the default document namespace is defined by the application usage itself, and applies to all URIs referencing resources within that application usage. All application usages MUST define a namespace URI that represents the default document namespace to be used when evaluating URIs. The default document namespace does not apply to elements or attributes within the documents themselves -- it applies only to the evaluation of URIs within that application usage. Indeed, the term 'default document namespace' is distinct from the term 'default namespace'. The latter has the standard meaning within XML documents, and the former refers to the default used in evaluation of XCAP URIs. XCAP does not change in any way the mechanisms for determining the default namespace within XML documents. However, if a document contains a URI representing an XCAP resource, the default document namespace defined by the application usage applies to that URI as well.

5.3. Data Validation

One of the responsibilities of an XCAP server is to validate the content of each XCAP resource when an XCAP client tries to modify one. This is done using two mechanisms. Firstly, all application usages MUST describe their document contents using XML schema [2]. The application usage MUST also identify the MIME type for documents compliant to that schema.

Unfortunately, XML schemas cannot represent every form of data constraint. As an example, one XML element may contain an integer that defines the maximum number of instances of another element. This constraint cannot be represented with XML schema. However, such constraints may be important to the application usage. The application usage defines any additional constraints beyond those in the schema.

Of particular importance are uniqueness constraints. In many cases, an application will require that there be only one instance of some element or attribute within a particular scope. Each uniqueness constraint needs to be specified by identifying the field, or combinations of fields, that need to be unique, and then identifying the scope in which that uniqueness applies. One typical scope is the set of all elements of a certain name within the same parent. Another typical scope is the set of all URIs valid within a particular domain. In some cases, these constraints can be specified using XML schema, which provides the <unique> element for this purpose. Other uniqueness constraints, such as URI uniqueness across a domain, cannot be expressed by schema. Whether or not the schema is used to express some of the uniqueness requirements, the application usage MUST specify all uniqueness requirements when it defines its data validation needs.

For example, the resource lists application usage [22] requires that each <list> element have a unique value for the "name" attribute within a single parent. As another example, the RLS services application usage [22] requires that the value of the "uri" attribute of the <service> element be a URI that is unique within the domain of the URI.

URI constraints represent another form of constraints. These are constraints on the scheme or structure of the scheme-specific part of the URI. These kinds of constraints cannot be expressed in an XML schema. If these constraints are important to an application usage, they need to be explicitly called out.

Another important data constraint is referential integrity. Referential integrity is important when the name or value of an element or attribute is used as a key to select another element or attribute. An application usage MAY specify referential integrity constraints. However, XCAP servers are not a replacement for Relational Database Management Systems (RDBMS), and therefore clients MUST NOT depend on servers to maintain referential integrity. XCAP clients are responsible for making all the appropriate changes to documents in order to maintain referential integrity.

Another constraint is character encoding. XML allows documents to be encoded using several different character sets. However, this specification mandates that all documents used with XCAP MUST be encoded using UTF-8. This cannot be changed by an application usage.

The data validation information is consumed by both clients, which use them to make sure they construct requests that will be accepted by the server, and by servers, which validate the constraints when they receive a request (with the exception of referential integrity constraints, which are not validated by the server).

5.4. Data Semantics

For each application usage, the data present in the XML document has a well-defined semantic. The application usage defines that semantic, so that a client can properly construct a document in order to achieve the desired result. They are not used by the server, as it is purposefully unaware of the semantics of the data it is managing. The data semantics are expressed in English prose by the application usage.

One particularly important semantic is the base URI that is to be used for the resolution of any relative URI references pointed to XCAP resources. As discussed below, relative URI references pointing to XCAP resources cannot be resolved using the retrieval URI as the base URI. Therefore, it is up to the application usage to specify the base URI.

5.5. Naming Conventions

In addition to defining the meaning of the document in the context of a particular application, an application usage has to specify how the applications obtain the documents they need. In particular, it needs to define any well-known URIs used for bootstrapping purposes, and document any other conventions on the URIs used by an application. It should also document how documents reference each other. These conventions are called naming conventions.

For many application usages, users need only a single document. In such a case, it is RECOMMENDED that the application usage require that this document be called "index" and exist within the user's home directory.

As an example, the RLS services application usage allows an RLS to obtain the contents of a resource list when the RLS receives a SUBSCRIBE request for a SIP URI identifying an RLS service. The application usage specifies that the list of service definitions is present within a specific document with a specific name within the global tree. This allows the RLS to perform a single XCAP request to fetch the service definition for the service associated with the SIP URI in a SUBSCRIBE request.

Naming conventions are used by XCAP clients to construct their URIs. The XCAP server does not make use of them.

5.6. Resource Interdependencies

When a user modifies an XCAP resource, the content of many other resources is affected. For example, when a user deletes an XML element within a document, it does so by issuing a DELETE request against the URI for the element resource. However, deleting this element also deletes all child elements and their attributes, each of which is also an XCAP resource. As such, manipulation of one resource affects the state of other resources.

For the most part, these interdependencies are fully specified by the XML schema used by the application usage. However, in some application usages, there is a need for the server to relate resources together, and such a relationship cannot be specified through a schema. This occurs when changes in one document will affect another document. Typically, this is the case when an application usage is defining a document that acts as a collection of information defined in other documents.

As an example, when a user creates a new RLS service (that is, it creates a new <service> element within an RLS services document), the server adds that element to a read-only global list of services maintained by the server in the global tree. This read-only global list is accessed by the RLS when processing a SIP SUBSCRIBE request.

Resource interdependencies are used by both XCAP clients and servers.

5.7. Authorization Policies

By default, each user is able to access (read, modify, and delete) all the documents below their home directory, and any user is able to read documents within the global directory. However, only trusted users, explicitly provisioned into the server, can modify global documents.

The application usage can specify a different authorization policy that applies to all documents associated with that application usage. An application usage can also specify whether another application usage is used to define the authorization policies. An application usage for setting authorization policies can also be defined subsequent to the definition of the main application usage. In such a case, the main application usage needs only to specify that such a usage will be defined in the future.

If an application usage does not wish to change the default authorization policy, it can merely state that the default policy is used.

The authorization policies defined by the application usage are used by the XCAP server during its operation.

5.8. Data Extensibility

An XCAP server MUST understand an application usage in order to process an HTTP request made against a resource for that particular application usage. However, it is not required for the server to understand all of the contents of a document used by an application usage. A server is required to understand the baseline schema defined by the application usage. However, those schemas can define points of extensibility where new content can be added from other namespaces and corresponding schemas. Sometimes, the server will understand those namespaces and therefore have access to their schemas. Sometimes, it will not.

A server MUST allow for documents that contain elements from namespaces not known to the server. In such a case, the server cannot validate that such content is schema compliant; it will only verify that the XML is well-formed.

If a client wants to verify that a server supports a particular namespace before operating on a resource, it can query the server for its capabilities using the XCAP Capabilities application usage, discussed in Section 12.

5.9. Documenting Application Usages

Application usages are documented in specifications that convey the information described above. In particular, an application usage specification MUST provide the following information:

• Application Unique ID (AUID): If the application usage is meant for general use on the Internet, the application usage MUST register the AUID into the IETF tree using the IANA procedures defined in Section 15.

• XML Schema

• Default Document Namespace

• MIME Type

• Validation Constraints

• Data Semantics

• Naming Conventions

• Resource Interdependencies

• Authorization Policies

5.10. Guidelines for Creating Application Usages

The primary design task when creating a new application usage is to define the schema. Although XCAP can be used with any XML document, intelligent schema design will improve the efficiency and utility of the document when it is manipulated with XCAP.

XCAP provides three fundamental ways to select elements amongst a set of siblings: by the expanded name of the element, by its position, or by the value of a specific attribute. Positional selection always allows a client to get exactly what it wants. However, it requires a client to cache a copy of the document in order to construct the predicate. Furthermore, if a client performs a PUT, it requires the client to reconstruct the PUT processing that a server would follow in order to update its local cached copy. Otherwise, the client will be forced to re-GET the document after every PUT, which is inefficient. As such, it is a good idea to design schemas such that common operations can be performed without requiring the client to cache a copy of the document.

Without positional selection, a client can pick the element at each step by its expanded name or the value of an attribute. Many schemas include elements that can be repeated within a parent (often, minOccurs equals zero or one, and maxOccurs is unbounded). As such, all of the elements have the same name. This leaves the attribute value as the only way to select an element. Because of this, if an application usage expects the user to manipulate elements or attributes that are descendants of an element that can repeat, that element SHOULD include, in its schema, an attribute that can be suitably used as a unique index. Furthermore, the naming conventions defined by that application usage SHOULD specify this uniqueness constraint explicitly.

URIs often make a good choice for such a unique index. They have fundamental uniqueness properties, and are also usually of semantic significance in the application usage. However, care must be taken when using a URI as an attribute value. URI equality is usually complex. However, attribute equality is performed by the server using XML rules, which are based on case sensitive string comparison. Thus, XCAP will match URIs based on lexical equality, not functional equality. In such cases, an application usage SHOULD consider these implications carefully.

XCAP provides the ability of a client to operate on a single element, attribute, or document at a time. As a result, it may be possible that common operations the client might perform will require a sequence of multiple requests. This is inefficient, and introduces the possibility of failure conditions when another client modifies the document in the middle of a sequence. In such a case, the client will be forced to detect this case using entity tags (discussed below in Section 7.11), and undo its previous changes. This is very difficult.

As a result, the schemas SHOULD be defined so that common operations generally require a single request to perform. Consider an example. Let's say an application usage is defining permissions for users to perform certain operations. The schema can be designed in two ways. The top level of the tree can identify users, and within each user, there can be the permissions associated with the user. In an alternative design, the top level of the tree identifies each permission, and within that permission, the set of users who have it.

If, in this application usage, it is common to change the permission for a user from one value to another, the former schema design is better for xcap; it will require a single PUT to make such a change. In the latter case, either the entire document needs to be replaced (which is a single operation), or two PUT operations need to occur -- one to remove the user from the old permission, and one to add the user to the new permission.

Naming conventions form another key part of the design of an application usage. The application usage should be certain that XCAP clients know where to "start" to retrieve and modify documents of interest. Generally, this will involve the specification of a well- known document at a well-known URI. That document can contain references to other documents that the client needs to read or modify.

6. URI Construction

In order to manipulate an XCAP resource, the data must be represented by an HTTP URI. XCAP defines a specific naming convention for constructing these URIs. The URI is constructed by concatenating the XCAP root with the document selector with the node selector separator with a percent-encoded form of the node selector. This is followed by an optional query component that defines namespace bindings used in evaluating the URI. The XCAP root is the enclosing context in which all XCAP resources live. The document selector is a path that identifies a document within the XCAP root. The node selector separator is a path segment with a value of double tilde ("~~"), and SHOULD NOT be percent-encoded, as advised in Section 2.3 of RFC 3986 [13]. URIs containing %7E%7E should be normalized to ~~ for comparison; they are equivalent. The node selector separator is a piece of syntactic sugar that separates the document selector from the node selector. The node selector is an expression that identifies a component of the document, such as an element or attribute. It is possible that a "~~" appears as part of the node selector itself; in such a case, the first "~~" in the URI is the node selector separator.

The sections below describe these components in more detail.

6.1. XCAP Root

The root of the XCAP hierarchy is called the XCAP root. It defines the context in which all other resources exist. The XCAP root is represented with an HTTP URI, called the XCAP Root URI. This URI is a valid HTTP URI; however, it doesn't point to any resource that actually exists on the server. Its purpose is to identify the root of the tree within the domain where all XCAP documents are stored.

It can be any valid HTTP URI, but MUST NOT contain a query component (a complete XCAP URI may have a query component, but it is not part of the XCAP root URI). It is RECOMMENDED that it be equal to xcap.domain, where domain is the domain of the provider. As an example, "http://xcap.example.com" might be used as the XCAP root URI within the example.com domain. Typically, the XCAP root URI is provisioned into client devices. If not explicitly provisioned, clients SHOULD assume the form xcap.domain, where domain is the domain of their service provider (for SIP, this would be the domain part of their Address-of-Record (AOR)). A server or domain MAY support multiple XCAP root URIs. In such a case, it is effectively operating as if it were serving separate domains. There is never information carryover or interactions between resources in different XCAP root URIs.

When a client generates an HTTP request to a URI identifying an XCAP resource, RFC 2616 procedures for the construction of the Request-URI apply. In particular, the authority component of the URI may not be present in the Request-URI if the request is sent directly to the origin server.

The XCAP root URI can also be a relative HTTP URI. It is the responsibility of the application usage to specify the base URI for an HTTP URI representing an XCAP resource whenever such a URI appears within a document defined by that application usage. Generally speaking, it is unsafe to use the retrieval URI as the base URI. This is because any URI that points to an ancestor for a particular element or attribute can contain content including that element or attribute. If that element or attribute contained a relative URI reference, it would be resolved relative to whatever happened to be used to retrieve the content, and this will often not be the base URI defined by the application usage.

6.2. Document Selector

Each document within the XCAP root is identified by its document selector. The document selector is a sequence of path segments, separated by a slash ("/"). These path segments define a hierarchical structure for organizing documents within any XCAP root. The first path segment MUST be the XCAP AUID. So, continuing the example above, all of the documents used by the resource lists application would be under "http://xcap.example.com/resource-lists".

• Implementors making use of HTTP servlets should be aware that XCAP may require them to get authorization from the server administrator to place resources within this specific subset of the URI namespace.

It is assumed that each application will have data that is set by users, and/or it will have global data that applies to all users. As a result, beneath each AUID, there are two sub-trees. One, called "users", holds the documents that are applicable to specific users, and the other, called "global", holds documents applicable to all users. The sub-tree beneath "global" is called the global tree. The path segment after the AUID MUST either be "global" or "users".

Within the "users" tree are zero or more sub-trees, each of which identifies documents that apply to a specific user. Each user known to the server is associated with a username, called the XCAP User Identifier (XUI). Typically, an endpoint is provisioned with the value of the XUI. For systems that support SIP applications, it is RECOMMENDED that the XUI be equal to the Address-of-Record (AOR) for the user (i.e., sip:joe@example.com). Since SIP endpoints generally know their AOR, they will also know their XUI. As a consequence, if no XUI is explicitly provisioned, a SIP User Agent SHOULD assume it is equal to their AOR. This XUI MUST be used as the path segment beneath the "users" segment. Since the SIP URI allows for characters that are not permitted in HTTP URI path segments (such as the '?' and '/' characters, which are permitted in the user part of the SIP URI), any such characters MUST be percent encoded. The sub-tree beneath an XUI for a particular user is called their home directory. "User" in this context should be interpreted loosely; a user might correspond to a device, for example.

XCAP does not itself define what it means for documents to "apply" to a user, beyond specification of a baseline authorization policy, described below in Section 8. Each application usage can specify additional authorization policies that depend on data used by the application itself.

The remainder of the document selector (the path following "global" or the XUI) points to specific documents for that application usage. Subdirectories are permitted, but are NOT RECOMMENDED. XCAP provides no way to create sub-directories or to list their contents, thus limiting their utility. If subdirectories are used, there MUST NOT be a document in a directory with the same name as a sub-directory.

The final path segment in the document selector identifies the actual document in the hierarchy. This is equivalent to a filename, except that XCAP does not require that its document resources be stored as files in a file system. However, the term "filename" is used to describe the final path segment in the document selector. In traditional filesystems, the filename would have a filename extension, such as ".xml". There is nothing in this specification that requires or prevents such extensions from being used in the filename. In some cases, the application usage will specify a naming convention for documents, and those naming conventions may or may not specify a file extension. For example, in the RLS services application usage [22], documents in the user's home directory with the filename "index" will be used by the server to compute the global index, which is also a document with the filename "index". Barring specific guidelines in the application usage, if a user has a single document for a particular application usage, this SHOULD be called "index".

When the naming conventions in an application usage do not constrain the filename conventions (or, more generally, the document selector), an application will know the filename (or more generally, the document selector) because it is included as a reference in a document accessed by the client. As another example, within the index document defined by RLS services, the <service> element has a child element called <resource-list> whose content is a URI pointing to a resource list within the users home directory.

As a result, if the user creates a new document, and then references that document from a well-known document (such as the index document above), it doesn't matter whether or not the user includes an extension in the filename, as long as the user is consistent and maintains referential integrity.

As an example, the path segment "/resource-lists/users/sip:joe@example.com/index" is a document selector. Concatenating the XCAP root URI with the document selector produces the HTTP URI "http://xcap.example.com/resource-lists/users/ sip:joe@example.com/index". In this URI, the AUID is "resource- lists", and the document is in the user tree with the XUI "sip:joe@example.com" with filename "index".

6.3. Node Selector

The node selector specifies specific nodes of the XML document that are to be accessed. A node refers to an XML element, an attribute of an element, or a set of namespace bindings. The node selector is an expression that identifies an element, attribute, or set of namespace bindings. Its grammar is:

   node-selector          = element-selector ["/" terminal-selector]
   terminal-selector      = attribute-selector / namespace-selector /
                            extension-selector
   element-selector       = step *( "/" step)
   step                   = by-name / by-pos / by-attr / by-pos-attr /
                            extension-selector
   by-name                = NameorAny
   by-pos                 = NameorAny "[" position "]"
   
   position               = 1*DIGIT
   attr-test              = "@" att-name "=" att-value
   by-attr                = NameorAny "[" attr-test "]"
   by-pos-attr            = NameorAny "[" position "]" "[" attr-test "]"
   NameorAny              = QName / "*"   ; QName from XML Namespaces
   att-name               = QName
   att-value              = AttValue      ; from XML specification
   attribute-selector     = "@" att-name
   namespace-selector     = "namespace::*"
   extension-selector     = 1*( %x00-2e / %x30-ff )  ; anything but "/"

The QName grammar is defined in the XML namespaces [3] specification, and the AttValue grammar is defined in the XML specification XML 1.0 [1].

The extension-selector is included for purposes of extensibility. It can be composed of any character except the slash, which is the delimiter amongst steps. Any characters in an extension that cannot be represented in a URI MUST be percent-encoded before placement into a URI.

Note that the double quote, left square bracket and right square bracket characters, which are meaningful to XCAP, cannot be directly represented in the HTTP URI. As a result, they are percent-encoded when placed within the HTTP URI. In addition to these characters, an apostrophe (') character can be used as a delimiter within XPath expressions. Furthermore, since XML allows for non-ASCII characters, the names of elements and attributes may not be directly representable in a URI. Any such characters MUST be represented by converting them to an octet sequence corresponding to their representation in UTF-8, and then percent-encoding that sequence of octets.

Similarly, the XML specification defines the QName production for the grammar for element and attribute names, and the AttValue production for the attribute values. Unfortunately, the characters permitted by these productions include some that are not allowed for pchar, which is the production for the allowed set of characters in path segments in the URI. The AttValue production allows many such characters within the US-ASCII set, including the space. Those characters MUST be percent-encoded when placed in the URI. Furthermore, QName and AttValue allow many Unicode characters, outside of US-ASCII. When these characters need to be represented in the HTTP URI, they are percent-encoded. To do this, the data should be encoded first as octets according to the UTF-8 character encoding [18], and then only those octets that do not correspond to characters in the pchar set should be percent-encoded. For example, the character A would be represented as "A", the character LATIN CAPITAL LETTER A WITH GRAVE would be represented as "%C3%80", and the character KATAKANA LETTER A would be represented as "%E3%82%A2".

As a result, the grammar above represents the expressions processed by the XCAP server internally after it has decoded the URI. The on- the-wire format is dictated by RFC 3986 [13]. In the discussions and examples below, when the node selectors are not part of an HTTP URI, they are presented in their internal format prior to encoding. If an example includes a node selector within an HTTP URI, it is presented in its percent-encoded form.

The node selector is based on the concepts in XPath [10]. Indeed, the node selector expression, before it is percent-encoded for representation in the HTTP URI, happens to be a valid XPath expression. However, XPath provides a set of functionality far richer than is needed here, and its breadth would introduce much unneeded complexity into XCAP.

To determine the XML element, attribute, or namespace bindings selected by the node selector, processing begins at the root node of the XML document. The first step in the element selector is then taken. Each step chooses a single XML element within the current document context. The document context is the point within the XML document from which a specific step is evaluated. The document context begins at the root node of the document. When a step determines an element within that context, that element becomes the new context for evaluation of the next step. Each step can select an element by its name (expanded), by a combination of name and attribute value, by name and position, or by name, position and attribute. In all cases, the name can be wildcarded, so that all elements get selected.

The selection operation operates as follows. Within the current document context, the children of that context are enumerated in document order. If the context is the root node of the document, its child element is the root element of the document. If the context is an element, its children are all of the children of that element (naturally). Next, those elements whose name is not a match for NameorAny are discarded. An element name is a match if NameorAny is the wildcard, or if it is not a wildcard, the element name matches NameorAny. Matching is discussed below. The result is an ordered list of elements.

The elements in the list are further filtered by the predicates, which are the expressions in square brackets following NameorAny. Each predicate further prunes the elements from the current ordered list. These predicates are evaluated in order. If the content of the predicate is a position, the position-th element is selected

(that is, treat "position" as a variable, and take the element whose position equals that variable), and all others are discarded. If there are fewer elements in the list than the value of position, the result is a no-match.

If the content of the predicate is an attribute name and value, all elements possessing an attribute with that name and value are selected, and all others are discarded. Note that, although a document can have namespace declarations within elements, those elements cannot be selected using a namespace declaration as a predicate. That is, a step like "el-name[@xmlns='namespace']" will never match an element, even if there is an element in the list that specifies a default namespace of "namespace". In other words, a namespace node is NOT an attribute. If the namespaces in scope for an element are needed, they can be selected using the namespace- selector described below. If there are no elements with attributes having the given name and value, the result is a no-match.

After the predicates have been applied, the result will be a no-match, one element, or multiple elements. If the result is multiple elements, the node selector is invalid. Each step in a node selector MUST produce a single element to form the context for the next step. This is more restrictive than general XPath expressions, which allow a context to contain multiple nodes. If the result is a no-match, the node selector is invalid. The node selector is only valid if a single element was selected. This element becomes the context for the evaluation of the next step in the node selector expression.

The last location step is either the previously described element selector or a "terminal selector". If the terminal selector is an attribute selector, the server checks to see if there is an attribute with the same expanded name in the current element context. If there is not, the result is considered a no-match. Otherwise, that attribute is selected. If the terminal selector is a namespace selector, the result is equal to the set of namespace bindings in scope for the element, including the possible default namespace declaration. This specification defines a syntax for representing namespace bindings, so they can be returned to the client in an HTTP response.

As a result, once the entire node selector is evaluated against the document, the result will either be a no-match, invalid, a single element, a single attribute, or a set of namespace bindings.

Matching of element names is performed as follows. The element being compared in the step has its name expanded as described in XML namespaces [3]. The element name in the step is also expanded. This expansion requires that any namespace prefix is converted to its namespace URI. Doing that requires a set of bindings from prefixes to namespace URIs. This set of bindings is obtained from the query component of the URI (see Section 6.4). If the prefix of the QName of an element is empty, the corresponding URI is then the default document namespace URI defined by the application usage, or null if not defined. Comparisons are then performed as described in XML namespaces [3]. Note that the namespace prefix expansions described here are different than those specified in the XPath 1.0 specification, but are closer to those currently defined by the XPath 2.0 specification [24].

Matching of attribute names proceeds in a similar way. The attribute in the document has its name expanded as described in XML namespaces [3]. If the attribute name in the attribute selector has a namespace prefix, its name is expanded using the namespace bindings obtained from the query component of the URI. An unprefixed attribute QName is in no namespace.

Comments, text content (including whitespace), and processing instructions can be present in a document, but cannot be selected by the expressions defined here. Of course, if such information is present in a document, and a user selects an XML element enclosing that data, that information would be included in a resulting GET, for example. Furthermore, whitespace is respected by XCAP. If a client PUTs an element or document that contains whitespace, the server retains that whitespace, and will return the element or document back to the client with exactly the same whitespace. Similarly, when an element is inserted, no additional whitespace is added around the inserted element, and the element gets inserted in a very specific location relative to any whitespace, comments, or processing instructions around it. Section 8.2.3 describes where the insertion occurs.

As an example, consider the following XML document:

   <?xml version="1.0"?>
   <watcherinfo xmlns="urn:ietf:params:xml:ns:watcherinfo"
                version="0" state="full">
     <watcher-list resource="sip:professor@example.net"
                   package="presence">
       <watcher status="active"
                id="8ajksjda7s"
                duration-subscribed="509"
                event="approved">sip:userA@example.net</watcher>
       <watcher status="pending"
                id="hh8juja87s997-ass7"
                display-name="Mr. Subscriber"
                event="subscribe">sip:userB@example.org</watcher>
     </watcher-list>
   </watcherinfo>

Figure 3: Example XML Document

Assuming that the default document namespace for this application usage is "urn:ietf:params:xml:ns:watcherinfo", the node selector watcherinfo/watcher-list/watcher[@id="8ajksjda7s"] would select the following XML element:

<watcher status="active"

       id="8ajksjda7s"
       duration-subscribed="509"
       event="approved">sip:userA@example.net</watcher>

6.4. Namespace Bindings for the Selector

In order to expand the namespace prefixes used in the node selector, a set of bindings from those namespace prefixes to namespace URI must be used. Those bindings are contained in the query component of the URI. If no query component is present, it means that only the default document namespace (as identified by the application usage) is defined. The query component is formatted as a valid xpointer expression [5] after suitable URI encoding as defined in Section 4.1 of the Xpointer framework. This xpointer expression SHOULD only contain expressions from the xmlns() scheme [4]. A server compliant to this specification MUST ignore any xpointer expressions not from the xmlns() scheme. The xmlns() xpointer expressions define the set of namespace bindings in use for evaluating the URI.

Note that xpointer expressions were originally designed for usage within fragment identifiers of URIs. However, within XCAP, they are used within query components of URIs.

The following example shows a more complex matching operation, this time including the usage of namespace bindings. Consider the following document:

   <?xml version="1.0"?>
   <foo xmlns="urn:test:default-namespace">
     <ns1:bar xmlns:ns1="urn:test:namespace1-uri"
              xmlns="urn:test:namespace1-uri">
       <baz/>
       <ns2:baz xmlns:ns2="urn:test:namespace2-uri"/>
     </ns1:bar>
     <ns3:hi xmlns:ns3="urn:test:namespace3-uri">
       <there/>
     </ns3:hi>
   </foo>

Assume that this document has a document URI of "http://xcap.example.com/test/users/sip:joe@example.com/index", where "test" is the application usage. This application usage defines a default document namespace of "urn:test:default-namespace". The XCAP URI:

   http://xcap.example.com/test/users/sip:joe@example.com/index/
   ~~/foo/a:bar/b:baz?xmlns(a=urn:test:namespace1-uri)
   xmlns(b=urn:test:namespace1-uri)

will select the first <baz> child element of the <bar> element in the document. The XCAP URI:

   http://xcap.example.com/test/users/sip:joe@example.com/index/
   ~~/foo/a:bar/b:baz?xmlns(a=urn:test:namespace1-uri)
   xmlns(b=urn:test:namespace2-uri)

will select the second <baz> child element of the <bar> element in the document. The following XCAP URI will also select the second <baz> child element of the <bar> element in the document:

   http://xcap.example.com/test/users/sip:joe@example.com/index/
   ~~/d:foo/a:bar/b:baz?xmlns(a=urn:test:namespace1-uri)
   xmlns(b=urn:test:namespace2-uri)
   xmlns(d=urn:test:default-namespace)

7. Client Operations

An XCAP client is an HTTP/1.1 compliant client. Specific data manipulation tasks are accomplished by invoking the right set of HTTP methods with the right set of headers on the server. This section describes those in detail.

In all cases where the client modifies a document, by deleting or inserting a document, element or attribute resource, the client SHOULD verify that, if the operation were to succeed, the resulting document would meet the data constraints defined by the application usage, including schema validation. For example, if the client performs a PUT operation to "http://xcap.example.com/rls-services/ users/sip:joe@example.com/mybuddies", rls-services is the application unique ID, and the constraints defined by it SHOULD be followed.

The client will know what URI to use based on the naming conventions described by the application usage.

If the document, after modification, does not meet the data constraints, the server will reject it with a 409. The 409 response may contain an XML body, formatted according to the schema in Section 11.2, which provides further information on the nature of the error. The client MAY use this information to try and alter the request so that, this time, it might succeed. The client SHOULD NOT simply retry the request without changing some aspect of it.

In some cases, the application usage will dictate a uniqueness constraint that the client cannot guarantee on its own. One such example is that a URI has to be unique within a domain. Typically, the client is not the owner of the domain, and so it cannot be sure that a URI is unique. In such a case, the client can either generate a sufficiently random identifier, or it can pick a "vanity" identifier in the hopes that it is not taken. In either case, if the identifier is not unique, the server will reject the request with a 409 and suggest alternatives that the client can use to try again. If the server does not suggest alternatives, the client SHOULD attempt to use random identifiers with increasing amounts of randomness.

HTTP also specifies that PUT and DELETE requests are idempotent. This means that, if the client performs a PUT on a document and it succeeds, it can perform the same PUT, and the resulting document will look the same. Similarly, when a client performs a DELETE, if it succeeds, a subsequent DELETE to the same URI will generate a 404; the resource no longer exists on the server since it was deleted by the previous DELETE operation. To maintain this property, the client SHOULD construct its URIs such that, after the modification has taken place, the URI in the request will point to the resource just inserted for PUT (i.e., the body of the request), and will point to nothing for DELETE. If this property is maintained, it is the case that GET to the URI in the PUT will return the same content (i.e., GET(PUT(X)) == x). This property implies idempotency. Although a request can still be idempotent if it does not possess this property, XCAP does not permit such requests. If the client's request does not have this property, the server will reject the request with a 409 and indicate a cannot-insert error condition.

If the result of the PUT is a 200 or 201 response, the operation was successful. Other response codes to any request, such as a redirection, are processed as per RFC 2616 [6].

7.1. Create or Replace a Document

To create or replace a document, the client constructs a URI that references the location where the document is to be placed. This URI MUST be a document URI, and therefore contain the XCAP root and document selector. The client then invokes a PUT method on that URI.

The MIME content type MUST be the type defined by the application usage. For example, it would be "application/rls-services+xml" for an RLS services [22] document, and not "application/xml".

If the Request-URI identifies a document that already exists in the server, the PUT operation replaces that document with the content of the request. If the Request-URI does not identify an existing document, the document is created on the server at that specific URI.

7.2. Delete a Document

To delete a document, the client constructs a URI that references the document to be deleted. This URI MUST be a document URI. The client then invokes a DELETE operation on the URI to delete the document.

7.3. Fetch a Document

As one would expect, fetching a document is trivially accomplished by performing an HTTP GET request with the Request URI set to the document URI.

7.4. Create or Replace an Element

To create or replace an XML element within an existing document, the client constructs a URI whose document selector points to the document to be modified. The node selector MUST be present in the URI, delimited from the document selector with the node selector separator. The query component MUST be present if the node selector makes use of namespace prefixes, in which case, the xmlns() expressions in the query component MUST define those prefixes. To create this element within the document, the node selector is constructed such that it is a no-match against the current document, but if the element in the body of the request was added to the document as desired by the client, the node selector would select that element. To replace an element in the document, the node selector is constructed so that it is a match against the element in the current document to be replaced, as well as a match to the new element (present in the body of the PUT request) that is to replace it.

Oftentimes, the client will wish to insert an element into a document in a certain position relative to other children of the same parent. This is called a positional insertion. They often arise because the schema constrains where the element can occur, or because ordering of elements is significant within the schema. To accomplish this, the client can use a node selector of the following form:

     parent/*[position][unique-attribute-value]

Here, "parent" is an expression for the parent of the element to be inserted. "position" is the position amongst the existing child elements of this parent where the new element is to be inserted. "unique-attribute-value" is an attribute name and value for the element to be inserted, which is different from the current element in "position". The second predicate is needed so that the overall expression is a no-match when evaluated against the current children. Otherwise, the PUT would replace the existing element in that position. Note that in addition to wildcard "*" a QName can also be used as a node test. The insert logic is described in more detail in Section 8.2.3.

   Consider the example document in Figure 3.  The client would like to
   insert a new <watcher> element as the second element underneath
   <watcher-list>.  However, it cannot just PUT to a URI with the
   watcherinfo/watcher-list/*[2] node selector; this node selector would
   select the existing second child element of <watcher-list> and
   replace it.  Thus, the PUT has to be made to a URI with watcherinfo/
   watcher-list/*[2][@id="hhggff"] as the node selector, where "hhggff"
   is the value of the "id" attribute of the new element to be inserted.
   This node-selector is a no-match against the current document, and
   would be a match against the new element if it was inserted as the
   second child element of <watcher-list>.

The "*" indicates that all element children of <watcher-info> are to be considered when computing the position for insertion. If, instead of a wildcard *, an element name (QName) was present, the expression above would insert the new element as the position-th element amongst those with the same expanded name (see Section 8.2.3 for a discussion on insertion rules).

Once the client constructs the URI, it invokes the HTTP PUT method. The content in the request MUST be an XML element. Specifically, it contains the element, starting with the opening bracket for the begin tag for that element, including the attributes and content of that element (whether it be text or other child elements), and ending with the closing bracket for the end tag for that element. The MIME type in the request MUST be "application/xcap-el+xml", defined in Section 15.2.1. If the node selector, when evaluated against the current document, results in a no-match, the server performs a creation operation. If the node selector, when evaluated against the current document, is a match for an element in the current document, the server replaces it with the content of the PUT request. This replacement is complete; that is, the old element (including its attributes, namespace declarations and content: text, element, comment and processing instruction nodes) are removed, and the new one, including its attributes, namespace declarations and content, is put in its place.

To be certain that element insertions have the GET(PUT(x))==x property, the client can check that the attribute predicates in the final path segment of the URI match the attributes of the element in the body of the request. As an example of a request that would not have this property, and therefore would not be idempotent, consider the following PUT request (URIs are line-folded for readability):

   PUT
   /rls-services/users/sip:bill@example.com/index/~~/rls-services/
   service%5b@uri=%22sip:good-friends@example.com%22%5d
    HTTP/1.1
   Content-Type:application/xcap-el+xml
   Host: xcap.example.com
   
   <service uri="sip:mybuddies@example.com">
     <resource-list>http://xcap.example.com/resource-lists/users
   /sip:joe@example.com/index/~~/resource-lists/list%5b@name=%22l1%22%5d
   </resource-list>
     <packages>
      <package>presence</package>
     </packages>
   </service>

This request will fail with a 409. The Request URI contains a final path segment with a predicate based on attributes: @uri="sip:good-friends@example.com". However, this will not match the value of the "uri" attribute in the element in the body (sip:mybuddies@example.com).

The GET(PUT(x))==x property introduces some limitations on the types of operations possible. It will not be possible to replace an element with one that has a new value for an attribute that is the sole unique element identifier, if the URI contained a node selector that was using the previous value of that attribute for purposes of selecting the element. This is exactly the use case in the example above. To get around this limitation, the selection can be done by position instead of attribute value, or the parent of the element to be replaced can be selected, and then the body of the PUT operation would contain the parent, the child to be replaced, and all other siblings.

7.5. Delete an Element

To delete an element from a document, the client constructs a URI whose document selector points to the document containing the element to be deleted. The node selector MUST identify a single element. The node selector MUST be present following the node selector separator, and identify the specific element to be deleted. Furthermore, the node selector MUST match no element after the deletion of the target element. This is required to maintain the idempotency property of HTTP deletions. The query component MUST be present if the node selector makes use of namespace prefixes, in which case the xmlns() expressions in the query component MUST define those prefixes.

If the client wishes to delete an element in a specific position, this is referred to as a positional deletion. Like a positional insertion, the node selector has the following form:

     parent/*[position][unique-attribute-value]

Where "parent" is an expression for the parent of the element to be deleted, "position" is the position of the element to be deleted amongst the existing child elements of this parent, and "unique- attribute-value" is an attribute name and value for the element to be deleted, where this attribute name and value are different than any of the siblings of the element.

   Positional deletions without using a unique attribute name and value
   are possible, but only in limited cases where idempotency is
   guaranteed.  In particular, if a DELETE operation refers to an
   element by name and position alone (parent/elname[n]), this is
   permitted only when the element to be deleted is the last element
   amongst all its siblings with that name.  Similarly, if a DELETE
   operation refers to an element by position alone (parent/*[n]), this
   is permitted only when the element to be deleted is the last amongst
   all sibling elements, regardless of name.

The client then invokes the HTTP DELETE method. The server will remove the element from the document (including its attributes, namespace declarations, and its descendant nodes, such as any children).

7.6. Fetch an Element

To fetch an element of a document, the client constructs a URI whose document selector points to the document containing the element to be fetched. The node selector MUST be present following the node selector separator, and must identify the element to be fetched. The query component MUST be present if the node selector makes use of namespace prefixes, in which case the xmlns() expressions in the query component MUST define those prefixes.

The client then invokes the GET method. The 200 OK response will contain that XML element. Specifically, it contains the content of the XML document, starting with the opening bracket for the begin tag for that element, and ending with the closing bracket for the end tag for that element. This will, as a result, include all attributes, namespace declarations and descendant nodes: elements, comments, text, and processing instructions of that element.

7.7. Create or Replace an Attribute

To create or replace an attribute in an existing element of a document, the client constructs a URI whose document selector points to the document to be modified. The node selector, following the node selector separator, MUST be present. The node selector MUST be constructed such that, if the attribute was created or replaced as desired, the node selector would select that attribute. If the node selector, when evaluated against the current document, results in a no-match, it is a creation operation. If it matches an existing attribute, it is a replacement operation. The query component MUST be present if the node selector makes use of namespace prefixes, in which case the xmlns() expressions in the query component MUST define those prefixes.

The client then invokes the HTTP PUT method. The content defined by the request MUST be the value of the attribute, compliant to the grammar for AttValue as defined in XML 1.0 [1]. Note that, unlike when AttValue is present in the URI, there is no percent-encoding of the body. This request MUST be sent with the Content-Type of "application/xcap-att+xml" as defined in Section 15.2.2. The server will add the attribute such that, if the node selector is evaluated on the resulting document, it will return the attribute present in the request.

To be certain that attribute insertions have the GET(PUT(x))==x property, the client can check that any attribute predicate in the path segment that selects the element into which the attribute is inserted, matches a different attribute than the one being inserted by the request. As an example of a request that would not have this property, and therefore would not be idempotent, consider the following PUT request (URIs are line-folded for readability):

   PUT
   /rls-services/users/sip:bill@example.com/index/~~/rls-services
   /service%5b@uri=%22sip:good-friends@example.com%22%5d/@uri
    HTTP/1.1
   Content-Type:application/xcap-att+xml
   Host: xcap.example.com
   
   "sip:bad-friends@example.com"

This request will fail with a 409.

As with element insertions and replacements, the GET(PUT(x))==x property introduces limitations on attribute replacements. It will not be possible to replace the attribute value of an attribute, when that attribute is the sole unique element identifier, and the URI contains a node selector that uses the previous value of the attribute to select the affected element. This is the use case in the example above. Instead, the element can be selected positionally, or its entire parent replaced.

7.8. Delete an Attribute

To delete an attribute from the document, the client constructs a URI whose document selector points to the document containing the attribute to be deleted. The node selector MUST be present following the node selector separator, and evaluate to an attribute in the document to be deleted. The query component MUST be present if the node selector makes use of namespace prefixes, in which case the xmlns() expressions in the query component MUST define those prefixes.

The client then invokes the HTTP DELETE method. The server will remove the attribute from the document.

7.9. Fetch an Attribute

To fetch an attribute of a document, the client constructs a URI whose document selector points to the document containing the attribute to be fetched. The node selector MUST be present following the node selector separator, containing an expression identifying the attribute whose value is to be fetched. The query component MUST be present if the node selector makes use of namespace prefixes, in which case the xmlns() expressions in the query component MUST define those prefixes.

The client then invokes the GET method. The 200 OK response will contain an "application/xcap-att+xml" document with the specified attribute, formatted according to the grammar of AttValue as defined in the XML 1.0 specifications.

7.10. Fetch Namespace Bindings

If a client wishes to insert an element or attribute into a document, and that element or attribute is part of a namespace declared elsewhere in the document, the client will need to know the namespace bindings in order to construct the XML content in the request. If the client has a cached copy of the document, it will know the bindings. However, if it doesn't have the whole document cached, it can be useful to fetch just the bindings that are in scope for an element, in order to construct a subsequent PUT request.

To get those bindings, the client constructs a URI whose document selector points to the document containing the element whose namespace bindings are to be fetched. The node selector MUST be present following the node selector separator, containing an expression identifying the desired namespace bindings. The query component MUST be present if the node selector makes use of namespace prefixes, in which case the xmlns() expressions in the query component MUST define those prefixes.

The client then invokes the GET method. The 200 OK response will contain an "application/xcap-ns+xml" document with the namespace definitions. The format for this document is defined in Section 10.

A client cannot set the namespace prefixes in scope for an element. As such, a node selector that identifies namespace prefixes MUST NOT appear in a PUT or DELETE request.

7.11. Conditional Operations

The HTTP specification defines several header fields that can be used by a client to make the processing of the request conditional. In particular, the If-None-Match and If-Match header fields allow a client to make them conditional on the current value of the entity tag for the resource. These conditional operations are particularly useful for XCAP resources.

For example, it is anticipated that clients will frequently wish to cache the current version of a document. So, when the client starts up, it will fetch the current document from the server and store it.

When it does so, the GET response will contain the entity tag for the document resource. Each resource within a document maintained by the server will share the same value of the entity tag. As a result, the entity tag returned by the server for the document resource is applicable to element and attribute resources within the document.

If the client wishes to insert or modify an element or attribute within the document, but it wants to be certain that the document hasn't been modified since the client last operated on it, it can include an If-Match header field in the request, containing the value of the entity tag known to the client for all resources within the document. If the document has changed, the server will reject this request with a 412 response. In that case, the client will need to flush its cached version, fetch the entire document, and store the new entity tag returned by the server in the 200 OK to the GET request. It can then retry the request, placing the new entity tag in the If-Match header field. If this succeeds, the Etag header field in the response to PUT contains the entity tag for the resource that was just inserted or modified. Because all resources in a document share the same value for their entity tag, this entity tag value can be applied to the modification of other resources.

A client can also conditionally delete elements or attributes by including an If-Match header field in DELETE requests. Note that the 200 OK responses to a DELETE will contain an Etag header field, containing the entity tag for all of the other resources in the document, even though the resource identified by the DELETE request no longer exists.

When a client uses conditional PUT and DELETE operations, it can apply those changes to its local cached copy, and update the value of the entity tag for the locally cached copy based on the Etag header field returned in the response. As long as no other clients try to modify the document, the client will be able to perform conditional operations on the document without ever having to perform separate GET operations to synchronize the document and its entity tags with the server. If another client tries to modify the document, this will be detected by the conditional mechanisms, and the client will need to perform a GET to resynchronize its copy unless it has some other means to learn about the change.

If a client does not perform a conditional operation, but did have a cached copy of the document, that cached copy will become invalid once the operation is performed (indeed, it may have become invalid even beforehand). Unconditional operations should only be performed by clients when knowledge of the entire document is not important for the operation to succeed.

As another example, a when a client fetches a document, and there is an older version cached, it is useful for clients to use a conditional GET in order to reduce network usage if the cached copy is still valid. This is done by including, in the GET request, the If-None-Match header field with a value equal to the current etag held by the client for the document. The server will only generate a 200 OK response if the etag held by the server differs than that held by the client. If it doesn't differ, the server will respond with a 304 response.

8. Server Behavior

An XCAP server is an HTTP/1.1 compliant origin server. The behaviors mandated by this specification relate to the way in which the HTTP URI is interpreted and the content is constructed.

An XCAP server MUST be explicitly aware of the application usage against which requests are being made. That is, the server must be explicitly configured to handle URIs for each specific application usage, and must be aware of the constraints imposed by that application usage.

When the server receives a request, the treatment depends on the URI. If the URI refers to an application usage not understood by the server, the server MUST reject the request with a 404 (Not Found) response. If the URI refers to a user (identified by an XUI) that is not recognized by the
server, it MUST reject the request with a 404 (Not Found). If the URI includes extension-selectors that the server doesn't understand, it MUST reject the request with a 404 (Not Found).

Next, the server authenticates the request. All XCAP servers MUST implement HTTP Digest [11]. Furthermore, servers MUST implement HTTP over TLS, RFC 2818 [14]. It is RECOMMENDED that administrators use an HTTPS URI as the XCAP root URI, so that the digest client authentication occurs over TLS.

Next, the server determines if the client has authorization to perform the requested operation on the resource. The application usage defines the authorization policies. An application usage may specify that the default is used. This default is described in Section 5.7.

Next, the server makes sure that it can properly evaluate the request URI. The server MUST separate the document selector from the node selector, by splitting the URI at the first instance of the node selector separator ("~~"). The server MUST check the node selector in the request URI, if present. If any qualified names are present that use a namespace prefix, and that prefix is not defined in an xmlns() expression in the query component of the request URI, the server MUST reject the request with a 400 response.

After checking the namespace prefix definitions, the specific behavior depends on the method and what the URI refers to.

8.1. POST Handling

XCAP resources do not represent processing scripts. As a result, POST operations to HTTP URIs representing XCAP resources are not defined. A server receiving such a request for an XCAP resource SHOULD return a 405.

8.2. PUT Handling

The behavior of a server in receipt of a PUT request is as specified in HTTP/1.1, Section 9.6 -- the content of the request is placed at the specified location. This section serves to define the notion of "placement" and "specified location" within the context of XCAP resources.

If the request URI contained a namespace-selector, the server MUST reject the request with a 405 (Method Not Allowed) and MUST include an Allow header field including the GET method.

8.2.1. Locating the Parent

The first step the server performs is to locate the parent, whether it is a directory or element, in which the resource is to be placed. To do that, the server removes the last path segment from the URI. The rest of the URI refers to the parent. This parent can be a document, element, or prefix of a document selector (called a directory, even though this specification does not mandate that documents are actually stored in a filesystem). This URI is called the parent URI. The path segment that was removed is called the target selector, and the node (element, document, or attribute) it describes is called the target node.

If the parent URI has no node selector separator, it is referring to the directory into which the document should be inserted. In normal XCAP operations, this will be either the user's home directory or the global directory, which will always exist on the server. However, if an application usage is making use of subdirectories (despite the fact that this is not recommended), it is possible that the directory into which the document should be inserted does not exist. In this case, the server MUST return a 409 response, and SHOULD include a detailed conflict report including the <no-parent> element. Detailed conflict reports are discussed in Section 11. If the directory does exist, the server checks to see if there is a document with the same filename as the target node. If there is, the operation is the replacement operation, discussed in Section 8.2.4. If it does not exist, it is the creation operation discussed in Section 8.2.3.

If the parent URI has a node selector separator, the document selector is extracted, and that document is retrieved. If the document does not exist, the server MUST return a 409 response, and SHOULD include a detailed conflict report including the <no-parent> element. If it does exist, the node selector is extracted and decoded (recall that the node selector is percent-encoded). The node selector is applied to the document based on the matching operations discussed in Section 6.3. If the result is a no-match or invalid, the server MUST return a 409 response, and SHOULD include a detailed conflict report including the <no-parent> element.

If the node-selector is valid, the server examines the target selector, and evaluates it within the context of the parent node. If the target node exists within the parent, the operation is a replacement, as described in Section 8.2.4. If it does not exist, it is the creation operation, discussed in Section 8.2.3.

Before performing the replacement or creation, as determined based on the logic above, the server validates the content of the request as described in Section 8.2.2.

8.2.2. Verifying Document Content

If the PUT request is for a document (the request URI had no node selector separator), the content of the request body has to be a well-formed XML document. If it is not, the server MUST reject the request with a 409 response code. That response SHOULD include a detailed conflict report including the <not-well-formed> element. If the document is well-formed but not UTF-8 encoded, the server MUST reject the request with a 409 response code. That response SHOULD include a detailed conflict report including the <not-utf-8> element. If the MIME type in the Content-Type header field of the request is not equal to the MIME type defined for the application usage, the server MUST reject the request with a 415.

If the PUT request is for an element, the content of the request body has to be a well-balanced region of an XML document, also known as an XML fragment body in The XML Fragment Interchange [23] specification, including only a single element. If it is not, the server MUST reject the request with a 409 response code. That response SHOULD include a detailed conflict report including the <not-xml-frag> element. If the fragment body is well-balanced but contains characters outside of the UTF-8 character set, the server MUST reject the request with a 409 response code. That response SHOULD include a detailed conflict report including the <not-utf-8> element. If the MIME type in the Content-Type header field of the request is not equal to "application/xcap-el+xml", the server MUST reject the request with a 415.

If the PUT request is for an attribute, the content of the request body has to be a sequence of characters that comply with the grammar for AttValue as defined above. If it is not, the server MUST reject the request with a 409 response code. That response SHOULD include a detailed conflict report including the <not-xml-att-value> element. If the attribute value is valid but contains characters outside of the UTF-8 character set, the server MUST reject the request with a 409 response code. That response SHOULD include a detailed conflict report including the <not-utf-8> element.If the MIME type in the Content-Type header field of the request is not equal to "application/xcap-att+xml", the server MUST reject the request with a 415.

8.2.3. Creation

The steps in this sub-section are followed if the PUT request will result in the creation of a new document, element, or attribute.

If the PUT request is for a document, the content of the request body is placed into the directory, and its filename is associated with the target node, which is a document.

If the PUT request is for an element, the server inserts the content of the request body as a new child element of the parent element selected in Section 8.2.1. The insertion is done such that the request URI, when evaluated, would now point to the element that was inserted. There exist three possible ways in which new elements are positioned.

First, if there were no other sibling elements with the same expanded name, and the insertion is not positionally constrained, the new element is inserted such that it is the last element amongst all element siblings. Furthermore, if there were comment, text, or processing instruction nodes after the former last element, they MUST occur prior to the insertion of the new element. This case occurs when one of the following are true:

• The element name in the target selector is not wildcarded. There could be an attribute selector (in which case, it would have to match an attribute of the element being inserted), and the position in the target selector will either be absent or have a value of 1 (a value greater than 1 would always result in rejection of the request, since this is the first element with the given name underneath the parent).

• The element name in the target selector is wildcarded, but there are no other elements underneath the same parent. There could be an attribute selector (in which case, it would have to match an attribute of the element being inserted), and the position in the target selector will either be absent or have a value of 1 (a value greater than 1 would always result in rejection of the request, since this is the first element underneath the parent).

• The element name in the target selector is wildcarded, and there are other elements underneath the same parent. However, there is an attribute selector that matches none of the attributes in the other sibling elements underneath the parent, but does match an attribute of the element to be inserted. The position in the target selector is absent.

Secondly, if there were sibling elements with the same name already in the document, but the insertion is positionally unconstrained, the server MUST insert the element such that it is in the "earliest last" position. "Earliest last" means that the new element MUST be inserted so that there are no elements after it with the same expanded name, and for all insertion positions where this is true, it is inserted such that as many sibling nodes (element, comment, text, or processing instruction) appear after it as possible. This case occurs when the target selector is defined by a by-name or by-attr production, and there is no position indicated.

Lastly, if the element is positionally constrained, the server MUST insert the element so that it is in the "earliest nth" position. When n>1 and NameofAny is not a wildcard, the element MUST be inserted so that there are n-1 sibling elements before it with the same expanded name. If there are not n-1 sibling elements with the same expanded name, the request will fail. When n>1 and NameorAny is a wildcard, the element MUST be inserted so that there are n-1 sibling elements before it, each of which can have any expanded name. If there are not n-1 sibling elements in the document, the request will fail. In both of these cases, the new element is inserted such that as many sibling nodes appear after it as possible. When n=1 and

NameorAny is not a wildcard, the insertion is positionally constrained when an element with the same expanded name already appears as a child of the same parent. In this case, the new element MUST appear just before the existing first element with this same expanded name. When n=1 and NameorAny is wildcarded, the insertion is positionally constrained when there is also an attribute selector that didn't match the first sibling of the parent (if it did match, or was absent, this wouldn't have been an insertion). In this case, the new element MUST appear just before all existing elements, regardless of their expanded name.

In practice, this insertion logic keeps elements with the same expanded names closely together. This simplifies the application logic when the content model is described by XML schema with <sequence> rules and maxOccurs="unbounded" cardinalities, like:

   <xs:element name="foobar">
     <xs:complexType>
       <xs:sequence>
         <xs:element ref="foo" maxOccurs="unbounded" />
         <xs:element ref="bar" maxOccurs="unbounded" />
       </xs:sequence>
     </xs:complexType>
   </xs:element>

Based on this schema, the document contains some number of <foo> elements followed by some number of <bar> elements. Either <bar> or <foo> elements may easily be added without wildcards and positional constraints. Note that if "minOccurs" cardinality of <foo> element were zero and <foo> elements do not yet exist, a positional predicate with the * wildcard must be used.

The whole insert logic is best described by complete examples. Consider the following document:

   <?xml version="1.0"?>
   <root>
    <el1 att="first"/>
    <el1 att="second"/>
    <!-- comment -->
    <el2 att="first"/>
   </root>

A PUT request whose content is <el1 att="third"/> and whose node selector is root/el1[@att="third"] would result in the following document:

   <?xml version="1.0"?>
   <root>
    <el1 att="first"/>
    <el1 att="second"/><el1 att="third"/>
    <!-- comment -->
    <el2 att="first"/>
   </root>
   Notice how it has been inserted as the third <el1> element in the
   document, and just before the comment and whitespace nodes.  It would
   have been inserted in exactly the same place if the node selector had
   been root/el1[3][@att="third"] or root/*[3][@att="third"].

If the content of the request had been <el3 att="first"/> and the node selector was root/el3, it would result in the following document:

   <?xml version="1.0"?>
   <root>
    <el1 att="first"/>
    <el1 att="second"/>
    <!-- comment -->
    <el2 att="first"/>
   <el3 att="first"/></root>

A PUT request whose content is <el2 att="2"/> and whose node selector is root/el2[@att="2"] would result in the following document:

   <?xml version="1.0"?>
   <root>
    <el1 att="first"/>
    <el1 att="second"/>
    <!-- comment -->
    <el2 att="first"/><el2 att="2"/>
   </root>
   
   It would have been inserted in exactly the same place if the node
   selector had been root/el2[2][@att="2"].  However, a selector root/
   *[2][@att="2"] would result in the following document:
   
   <?xml version="1.0"?>
   <root>
    <el1 att="first"/><el2 att="2"/>
    <el1 att="second"/>
    <!-- comment -->
    <el2 att="first"/>
   </root>
   Lastly, if the node selector had been root/el2[1][@att="2"] the
   result would be:
   
   <?xml version="1.0"?>
   <root>
    <el1 att="first"/>
    <el1 att="second"/>
    <!-- comment -->
    <el2 att="2"/><el2 att="first"/>
   </root>

It is possible that the element cannot be inserted such that the request URI, when evaluated, returns the content provided in the request. Such a request is not allowed for PUT. This happens when the element in the body is not described by the expression in the target selector. An example of this case is described in Section 7.4. If this happens, the server MUST NOT perform the insertion, and MUST reject the request with a 409 response. The body of the response SHOULD contain a detailed conflict report containing the <cannot-insert> element. It is important to note that schema compliance does not play a role while performing the insertion. That is, the decision of where the element gets inserted is dictated entirely by the structure of the request-URI, the current document, and the rules in this specification.

If the element being inserted (or any of its children) contain namespace declarations, those declarations are retained when the element is inserted, even if those same declarations exist in a parent element after insertion. The XCAP server MUST NOT remove redundant namespace declarations or otherwise change the namespace declarations that were present in the element being inserted.

If the PUT request is for an attribute, the server inserts the content of the request body as the value of the attribute. The name of the attribute is equal to the att-name from the attribute-selector in the target selector.

Assuming that the insertion can be accomplished, the server verifies that the insertion results in a document that meets the constraints of the application usage. This is discussed in Section 8.2.5.

8.2.4. Replacement

The steps in this sub-section are followed if the PUT request will result in the replacement of a document, element, or attribute with the contents of the request.

If the PUT request is for a document, the content of the request body is placed into the directory, replacing the document with the same filename.

If the PUT request is for an element, the server replaces the target node with the content of the request body. As in the creation case, it is possible that, after replacement, the request URI does not select the element that was just inserted. If this happens, the server MUST NOT perform the replacement, and MUST reject the request with a 409 response. The body of the response SHOULD contain a detailed conflict report containing the <cannot-insert> element.

As with creation, replacement of an element does not result in the changing or elimination of namespace declarations within the newly modified element.

If the PUT request is for an attribute, the server sets the value of the selected attribute to the content of the request body. It is possible in the replacement case (but not in the creation case), that, after replacement of the attribute, the request URI no longer selects the attribute that was just replaced. The scenario in which this can happen is discussed in Section 7.7. If this is the case, the server MUST NOT perform the replacement, and MUST reject the request with a 409 response. The body of the response SHOULD contain a detailed conflict report containing the <cannot-insert> element.

8.2.5. Validation

Once the document, element, or attribute has been tentatively inserted, the server needs to verify that the resulting document meets the data constraints outlined by the application usage.

First, the server checks that the final document is compliant with the schema. If it is not, the server MUST NOT perform the insertion. It MUST reject the request with a 409 response. That response SHOULD contain a detailed conflict report containing the <schema-validation- error> element. If a schema allows for elements or attributes from other namespaces, and the new document contains elements or attributes from an unknown namespace, the server MUST allow the change. In other words, it is not necessary for an XCAP server to understand the namespaces and corresponding schemas for elements and attributes within a document, as long as the schema itself allows for such elements or attributes to be included. Of course, such unknown namespaces would not be advertised by the server in its XCAP capabilities document, discussed in Section 12.

If the final document contains elements or attributes from a namespace that the server does understand (and has consequently advertised in its XCAP capabilities document), but the server does not have the schema for that particular element or attribute, the server MUST reject the request with a 409 response. That response SHOULD contain a detailed conflict report containing the <schema- validation-error> element.

Next, the server checks for any uniqueness constraints identified by the application usage. If the application usage required that a particular element or attribute had a unique value within a specific scope, the server would check that this uniqueness property still exists. If the application usage required that a URI within the document was unique within the domain, the server checks whether it is the case. If any of these uniqueness constraints are not met, the server MUST NOT perform the insertion. It MUST reject the request with a 409 response. That response SHOULD contain a detailed conflict report containing the <uniqueness-failure> element. That element can contain suggested values that the client can use to retry. These SHOULD be values that, at the time the server generates the 409, would meet the uniqueness constraints.

The server also checks for URI constraints and other non-schema data constraints. If the document fails one of these constraints, the server MUST NOT perform the insertion. It MUST reject the request with a 409 response. That response SHOULD contain a detailed conflict report containing the <constraint-failure> element. That element indicates that the document failed non-schema data constraints explicitly called out by the application usage.

Element or attribute removals have similar constraints. The server checks the document for schema validity and compliance to constraints defined by the application usage, and rejects the request as described above, if either check fails.

8.2.6. Conditional Processing

A PUT request for an XCAP resource, like any other HTTP resource, can be made conditional through usage of the If-Match and If-None-Match header fields. For a replacement, these are processed as defined in [6]. For an insertion of an element or attribute, conditional operations are permitted. The entity tag that is used for the procedures in [6] is the one for all of the resources within the same document as the parent of the element or attribute being inserted. One way to think of this is that, logically speaking, upon receipt of the PUT request, the XCAP server instantiates the etag for the resource referenced by the request, and then applies the processing of the request. Because of this behavior, it is not possible to perform a conditional insert on an attribute or element that is conditioned on the operation being an insertion and not a replacement. In other words, a conditional PUT of an element or attribute with an If-None-Match: * will always fail.

8.2.7. Resource Interdependencies

Because XCAP resources include elements, attributes, and documents, each of which has its own HTTP URI, the creation or modification of one resource affects the state of many others. For example, insertion of a document creates resources on the server for all of the elements and attributes within that document. After the server has performed the insertion associated with the PUT, the server MUST create and/or modify those resources affected by that PUT. The structure of the document completely defines the inter-relationship between those resources.

However, the application usage can specify other resource inter- dependencies. The server MUST create or modify the resources specified by the application usage.

If the creation or replacement was successful, and the resource interdependencies are resolved, the server returns a 201 Created or 200 OK, respectively. Note that a 201 Created is generated for creation of new documents, elements, or attributes. A 200 OK response to PUT MUST not contain any content. Per the recommendations of RFC 2616, the 201 can contain a Location header field and entity that identify the resource that was created. An entity tag MUST be included in all successful responses to a PUT.

8.3. GET Handling

The semantics of GET are as specified in RFC 2616. This section clarifies the specific content to be returned for a particular URI that represents an XCAP resource.

If the request URI contains only a document selector, the server returns the document specified by the URI if it exists, else returns a 404 response. The MIME type of the body of the 200 OK response MUST be the MIME type defined by that application usage (i.e., "application/resource-lists+xml").

If the request URI contains a node selector, the server obtains the document specified by the document selector, and if it is found, evaluates the node-selector within that document. If no document is found, or if the node-selector is a no-match or invalid, the server returns a 404 response. Otherwise, the server returns a 200 OK response. If the node selector identifies an XML element, that element is returned in the 200 OK response as an XML fragment body containing the selected element. The server MUST NOT add namespace bindings representing namespaces used by the element or its children, but declared in ancestor elements; the client will either know these bindings already (since it has a cached copy of the whole document), or it can learn them by explicitly querying for the bindings. The MIME type of the response MUST be "application/xcap-el+xml". If the node selector identifies an XML attribute, the value of that attribute is returned in the body of the response. The MIME type of the response MUST be "application/xcap-att+xml". If the node selector identifies a set of namespace bindings, the server computes the set of namespace bindings in scope for the element (including the default) and encodes it using the "application/xcap-ns+xml" format defined in Section 10. That document is then returned in the body of the response.

GET operations can be conditional, and follow the procedures defined in [6].

Note that the GET of a resource that was just PUT might not be octet- for-octet equivalent to what was PUT, due to XML normalization and equivalency rules.

A successful response to a GET MUST include an entity tag.

8.4. DELETE Handling

The semantics of DELETE are as specified in RFC 2616. This section clarifies the specific content to be deleted for a particular URI that represents an XCAP resource.

If the request URI contained a namespace-selector, the server MUST reject the request with a 405 (Method Not Allowed) and MUST include an Allow header field including the GET method.

If the request URI contains only a document selector, the server deletes the document specified by the URI if it exists and returns a 200 OK, else returns a 404 response.

If the request URI contains a node selector, the server obtains the document specified by the document selector, and if it is found, evaluates the node-selector within that document. If no document is found, or if the node-selector is a no-match or invalid (note that it will be invalid if multiple elements or attributes are selected), the server returns a 404 response. Otherwise, the server removes the specified element or attribute from the document and performs the validation checks defined in Section 8.2.5. Note that this deletion does not include any white space around the element that was deleted; the XCAP server MUST preserve surrounding whitespace. It is possible that, after deletion, the request URI selects another element in the document. If this happens, the server MUST NOT perform the deletion, and MUST reject the request with a 409 response. The body of the response SHOULD contain a detailed conflict report containing the <cannot-delete> element. If the deletion will cause a failure of one of the constraints, the deletion MUST NOT take place. The server follows the procedures in Section 8.2.5 for computing the 409 response. If the deletion results in a document that is still valid, the server MUST perform the deletion, process the resource interdependencies defined by the application usage, and return a 200 OK response.

DELETE operations can be conditional, and follow the procedures defined in [6].

Before the server returns the 200 OK response to a DELETE, it MUST process the resource interdependencies as defined in Section 8.2.7. As long as the document still exists after the delete operation, any successful response to DELETE MUST include the entity tag of the document.

8.5. Managing Etags

An XCAP server MUST maintain entity tags for all resources that it maintains. This specification introduces the additional constraint that when one resource within a document (including the document itself) changes, that resource is assigned a new etag, and all other resources within that document MUST be assigned the same etag value. Effectively, there is a single etag for the entire document. An XCAP server MUST include the Etag header field in all 200 or 201 responses to PUT, GET, and DELETE, assuming the document itself still exists after the operation. In the case of a DELETE, the entity tag refers to the value of the entity tag for the document after the deletion of the element or attribute.

XCAP resources do not introduce new requirements on the strength of the entity tags.

As a result of this constraint, when a client makes a change to an element or attribute within a document, the response to that operation will convey the entity tag of the resource that was just affected. Since the client knows that this entity tag value is shared by all of the other resources in the document, the client can make conditional requests against other resources using that entity tag.

9. Cache Control

An XCAP resource is a valid HTTP resource, and therefore, it can be cached by clients and network caches. Network caches, however, will not be aware of the interdependencies between XCAP resources. As such, a change to an element in a document by a client will invalidate other XCAP resources affected by the change. For application usages containing data that is likely to be dynamic or written by clients, servers SHOULD indicate a no-cache directive.

10. Namespace Binding Format

A node-selector can identify a set of namespace bindings that are in scope for a particular element. In order to convey these bindings in a GET response, a way is needed to encode them.

Encoding is trivially done by including a single XML element in an XML fragment body. This element has the same local-name as the element whose namespace bindings are desired, and also the same namespace-prefix. The element has an xmlns attribute identifying the default namespace in scope, and an xmlns:prefix declaration for each prefix that is in scope.

For example, consider the XML document in Section 6.4. The node- selector df:foo/df2:bar/df2:baz/namespace::* will select the namespaces in scope for the <baz> element in the document, assuming the request is accompanied by a query component that contains xmlns(df=urn:test:default-namespace) and xmlns(df2=urn:test:namespace1-uri). A GET containing this node selector and namespace bindings will produce the following result:

<baz xmlns="urn:test:namespace1-uri"

        xmlns:ns1="urn:tes:namespace1-uri"/>

It is important to note that the client does not need to know the actual namespace bindings in order to construct the URI. It does need to know the namespace URI for each element in the node-selector. The namespace bindings present in the query component are defined by the client, mapping those URIs to a set of prefixes. The bindings returned by the server are the actual bindings used in the document.

11. Detailed Conflict Reports

In cases where the server returns a 409 error response, that response will usually include a document in the body of the response which provides further details on the nature of the error. This document is an XML document, formatted according to the schema of

Section 11.2. Its MIME type, registered by this specification, is "application/xcap-error+xml".

11.1. Document Structure

The document structure is simple. It contains the root element <xcap-error>. The content of this element is a specific error condition. Each error condition is represented by a different element. This allows for different error conditions to provide different data about the nature of the error. All error elements support a "phrase" attribute, which can contain text meant for rendering to a human user.

The following error elements are defined by this specification:

   <not-well-formed>:  This indicates that the body of the request was
      not a well-formed XML document.
   
   <not-xml-frag>:  This indicates that the request was supposed to
      contain a valid XML fragment body, but did not.  Most likely this
      is because the XML in the body was malformed or not balanced.
   
   <no-parent>:  This indicates that an attempt to insert a document,
      element, or attribute failed because the directory, document, or
      element into which the insertion was supposed to occur does not
      exist.  This error element can contain an optional <ancestor>
      element, which provides an HTTP URI that represents the closest
      parent that would be a valid point of insertion.  This HTTP URI
      MAY be a relative URI, relative to the document itself.  Because
      this is a valid HTTP URI, its node selector component MUST be
      percent-encoded.
   
   <schema-validation-error>:  This element indicates that the document
      was not compliant to the schema after the requested operation was
      performed.
   
   <not-xml-att-value>:  This indicates that the request was supposed to
      contain a valid XML attribute value, but did not.
   
   <cannot-insert>:  This indicates that the requested PUT operation
      could not be performed because a GET of that resource after the
      PUT would not yield the content of the PUT request.
   
   <cannot-delete>:  This indicates that the requested DELETE operation
      could not be performed because it would not be idempotent.
   
   <uniqueness-failure>:  This indicates that the requested operation
      would result in a document that did not meet a uniqueness
      constraint defined by the application usage.  For each URI,
      element, or attribute specified by the client that is not unique,
      an <exists> element is present as the content of the error
      element.  Each <exists> element has a "field" attribute that
      contains a relative URI identifying the XML element or attribute
      whose value needs to be unique, but wasn't.  The relative URI is
      relative to the document itself, and will therefore start with the
      root element.  The query component of the URI MUST be present if
      the node selector portion of the URI contains namespace prefixes.
      Since the "field" node selector is a valid HTTP URI, it MUST be
      percent-encoded.  The <exists> element can optionally contain a
      list of <alt-value> elements.  Each one is a suggested alternate
      value that does not currently exist on the server.
   
   <constraint-failure>:  This indicates that the requested operation
      would result in a document that failed a data constraint defined
      by the application usage, but not enforced by the schema or a
      uniqueness constraint.
   
   <extension>:  This indicates an error condition that is defined by an
      extension to XCAP.  Clients that do not understand the content of
      the extension element MUST discard the xcap-error document and
      treat the error as an unqualified 409.
   
   <not-utf-8>:  This indicates that the request could not be completed
      because it would have produced a document not encoded in UTF-8.

As an example, the following document indicates that the user attempted to create an RLS service using the URI sip:friends@example.com, but that URI already exists:

   <?xml version="1.0" encoding="UTF-8"?>
   <xcap-error xmlns="urn:ietf:params:xml:ns:xcap-error">
    <uniqueness-failure>
     <exists field="rls-services/service/@uri">
       <alt-value>sip:mybuddies@example.com</alt-value>
     </exists>
    </uniqueness-failure>
   </xcap-error>

11.2. XML Schema

<?xml version="1.0" encoding="UTF-8"?> <xs:schema targetNamespace="urn:ietf:params:xml:ns:xcap-error"

xmlns="urn:ietf:params:xml:ns:xcap-error" xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">

    <xs:element name="error-element" abstract="true"/>
    <xs:element name="xcap-error">
     <xs:annotation>
      <xs:documentation>Indicates the reason for the error.
     </xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:sequence>
       <xs:element ref="error-element"/>
      </xs:sequence>
     </xs:complexType>
    </xs:element>
    
    <xs:element name="extension" substitutionGroup="error-element">
     <xs:complexType>
      <xs:sequence>
       <xs:any namespace="##any" processContents="lax"
               minOccurs="0" maxOccurs="unbounded"/>
      </xs:sequence>
     </xs:complexType>
    </xs:element>
   
    <xs:element name="schema-validation-error"
     substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This element indicates
   that the document was not compliant to the schema after the requested
   operation was performed.</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>
   
    <xs:element name="not-xml-frag" substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that the request was supposed to
   contain a valid XML fragment body, but did not.</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>
   
    <xs:element name="no-parent" substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that an attempt to insert
   an element, attribute, or document failed because the document or
   element into which the insertion was
   supposed to occur does not exist.</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:sequence>
       <xs:element name="ancestor" type="xs:anyURI" minOccurs="0">
        <xs:annotation>
         <xs:documentation>Contains an HTTP URI that points to the
   element that is the closest ancestor that does exist.
         </xs:documentation>
        </xs:annotation>
       </xs:element>
      </xs:sequence>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>
   
    <xs:element name="cannot-insert" substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that the requested
   PUT operation could not be performed because a GET of that resource
   after the PUT would not yield the content of the PUT request.
      </xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>
   
    <xs:element name="not-xml-att-value"
     substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that the
   request was supposed to contain a valid XML attribute value, but did
   not.</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>
   
    <xs:element name="uniqueness-failure"
     substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that the
   requested operation would result in a document that did not meet a
   uniqueness constraint defined by the application usage.
      </xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:sequence>
       <xs:element name="exists" maxOccurs="unbounded">
        <xs:annotation>
         <xs:documentation>For each URI,
   element, or attribute specified by the client that is not unique,
   one of these is present.</xs:documentation>
        </xs:annotation>
        <xs:complexType>
         <xs:sequence minOccurs="0">
          <xs:element name="alt-value" type="xs:string"
           maxOccurs="unbounded">
           <xs:annotation>
            <xs:documentation>An optional set of alternate values can be
   provided.</xs:documentation>
           </xs:annotation>
          </xs:element>
         </xs:sequence>
         <xs:attribute name="field" type="xs:string" use="required"/>
        </xs:complexType>
       </xs:element>
      </xs:sequence>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>
   
    <xs:element name="not-well-formed"
     substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that the body of the request was
   not a well-formed document.</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>
    <xs:element name="constraint-failure"
     substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that the
   requested operation would result in a document that failed a data
   constraint defined by the application usage, but not enforced by the
   schema or a uniqueness constraint.</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>
   
    <xs:element name="cannot-delete" substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that the requested DELETE
   operation could not be performed because it would not be
   idempotent.</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>
   
    <xs:element name="not-utf-8" substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that the request could not be
         completed because it would have produced a document not
         encoded in UTF-8.</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>
   </xs:schema>

12. XCAP Server Capabilities

XCAP can be extended through the addition of new application usages and extensions to the core protocol. Application usages may define MIME types with XML schemas that allow new extension nodes from new namespaces. It will often be necessary for a client to determine what extensions, application usages, or namespaces a server supports before making a request. To enable that, this specification defines an application usage with the AUID "xcap-caps". All XCAP servers MUST support this application usage. This usage defines a single document within the global tree that lists the capabilities of the server. Clients can read this well-known document, and therefore learn the capabilities of the server.

The structure of the document is simple. The root element is <xcap- caps>. Its children are <auids>, <extensions>, and <namespaces>. Each of these contain a list of AUIDs, extensions, and namespaces supported by the server. Extensions are named by tokens defined by the extension, and typically define new selectors. Namespaces are identified by their namespace URI. To 'support' a namespace, the server must have the schemas for all elements within that namespace, and be able to validate them if they appear within documents. Since all XCAP servers support the "xcap-caps" AUID, it MUST be listed in the <auids> element, and the "urn:ietf:params:xml:ns:xcap-caps" namespace MUST be listed in the <namespaces> element.

The following sections provide the information needed to define this application usage.

12.1. Application Unique ID (AUID)

This specification defines the "xcap-caps" AUID within the IETF tree, via the IANA registration in Section 15.

12.2. XML Schema

<?xml version="1.0" encoding="UTF-8"?> <xs:schema targetNamespace="urn:ietf:params:xml:ns:xcap-caps"

xmlns="urn:ietf:params:xml:ns:xcap-caps" xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified"> <xs:element name="xcap-caps">

     <xs:annotation>
      <xs:documentation>Root element for xcap-caps</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:sequence>
       <xs:element name="auids">
        <xs:annotation>
         <xs:documentation>List of supported AUID.</xs:documentation>
        </xs:annotation>
        <xs:complexType>
         <xs:sequence minOccurs="0" maxOccurs="unbounded">
          <xs:element name="auid" type="auidType"/>
         </xs:sequence>
        </xs:complexType>
       </xs:element>
       <xs:element name="extensions" minOccurs="0">
        <xs:annotation>
         <xs:documentation>List of supported extensions.
         </xs:documentation>
        </xs:annotation>
        <xs:complexType>
         <xs:sequence minOccurs="0" maxOccurs="unbounded">
          <xs:element name="extension" type="extensionType"/>
         </xs:sequence>
        </xs:complexType>
       </xs:element>
       <xs:element name="namespaces">
        <xs:annotation>
         <xs:documentation>List of supported namespaces.
         </xs:documentation>
        </xs:annotation>
        <xs:complexType>
         <xs:sequence minOccurs="0" maxOccurs="unbounded">
          <xs:element name="namespace" type="namespaceType"/>
         </xs:sequence>
        </xs:complexType>
       </xs:element>
       <xs:any namespace="##other" processContents="lax"
        minOccurs="0" maxOccurs="unbounded"/>
      </xs:sequence>
     </xs:complexType>
    </xs:element>
    <xs:simpleType name="auidType">
     <xs:annotation>
      <xs:documentation>AUID Type</xs:documentation>
     </xs:annotation>
     <xs:restriction base="xs:string"/>
    </xs:simpleType>
    <xs:simpleType name="extensionType">
     <xs:annotation>
      <xs:documentation>Extension Type</xs:documentation>
     </xs:annotation>
     <xs:restriction base="xs:string"/>
    </xs:simpleType>
    <xs:simpleType name="namespaceType">
     <xs:annotation>
      <xs:documentation>Namespace type</xs:documentation>
     </xs:annotation>
     <xs:restriction base="xs:anyURI"/>
    </xs:simpleType>
   </xs:schema>

12.3. Default Document Namespace

   The default document namespace used in evaluating a URI is
   urn:ietf:params:xml:ns:xcap-caps.

12.4. MIME Type

Documents conformant to this schema are known by the MIME type "application/xcap-caps+xml", registered in Section 15.2.5.

12.5. Validation Constraints

There are no additional validation constraints associated with this application usage.

12.6. Data Semantics

Data semantics are defined above.

12.7. Naming Conventions

A server MUST maintain a single instance of the document in the global tree, using the filename "index". There MUST NOT be an instance of this document in the user's tree.

12.8. Resource Interdependencies

There are no resource interdependencies in this application usage beyond those defined by the schema.

12.9. Authorization Policies

This application usage does not change the default authorization policy defined by XCAP.

13. Examples

This section goes through several examples, making use of the resource-lists and rls-services [22] XCAP application usages.

First, a user Bill creates a new document (see Section 7.1). This document is a new resource-list, initially with a single list, called friends, with no users in it:

   PUT
   /resource-lists/users/sip:bill@example.com/index HTTP/1.1
   Content-Type:application/resource-lists+xml
   Host: xcap.example.com
   <?xml version="1.0" encoding="UTF-8"?>
   <resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists">
     <list name="friends">
     </list>
   </resource-lists>

Figure 24: New Document

Next, Bill creates an RLS services document defining a single RLS service referencing this list. This service has a URI of sip:myfriends@example.com (URIs are line-folded for readability):

   PUT
   /rls-services/users/sip:bill@example.com/index HTTP/1.1
   Content-Type:application/rls-services+xml
   Host: xcap.example.com
   
   <?xml version="1.0" encoding="UTF-8"?>
   <rls-services xmlns="urn:ietf:params:xml:ns:rls-services">
   <service uri="sip:myfriends@example.com">
     <resource-list>http://xcap.example.com/resource-lists/users/
   sip:bill@example.com/index/~~/resource-lists/
   list%5b@name=%22friends%22%5d
   </resource-list>
     <packages>
      <package>presence</package>
     </packages>
    </service>
   </rls-services>

Figure 25: RLS Services Example

Next, Bill creates an element in the resource-lists document (Section 7.4). In particular, he adds an entry to the list:

   PUT
   /resource-lists/users/sip:bill@example.com/index
   /~~/resource-lists/list%5b@name=%22friends%22%5d/entry HTTP/1.1
   Content-Type:application/xcap-el+xml
   Host: xcap.example.com

<entry uri="sip:bob@example.com">

       <display-name>Bob Jones</display-name>
     </entry>

Figure 26: Resource Lists Document

Next, Bill fetches the document (Section 7.3):

   GET
   /resource-lists/users/sip:bill@example.com/index HTTP/1.1

Figure 27: Fetch Operation

And the result is (note how white space text nodes appear in the document):

   HTTP/1.1 200 OK
   Etag: "wwhha"
   Content-Type: application/resource-lists+xml
   
   <?xml version="1.0" encoding="UTF-8"?>
   <resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists">
     <list name="friends">
     <entry uri="sip:bob@example.com">
       <display-name>Bob Jones</display-name>
     </entry></list>
   </resource-lists>

Figure 28: Results of Fetch

Next, Bill adds another entry to the list, which is another list that has three entries. This is another element creation (Section 7.4):

   PUT
   /resource-lists/users/sip:bill@example.com/index/~~/
   resource-lists/list%5b@name=%22friends%22%5d/
   list%5b@name=%22close-friends%22%5d HTTP/1.1
   Content-Type: application/xcap-el+xml
   Host: xcap.example.com
   
   <list name="close-friends">
      <entry uri="sip:joe@example.com">
        <display-name>Joe Smith</display-name>
      </entry>
      <entry uri="sip:nancy@example.com">
        <display-name>Nancy Gross</display-name>
      </entry>
      <entry uri="sip:petri@example.com">
        <display-name>Petri Aukia</display-name>
      </entry>
   </list>

Figure 29: Adding an Entry

Then, Bill decides he doesn't want Petri on the list, so he deletes the entry (Section 7.5):

   DELETE
   /resource-lists/users/sip:bill@example.com/index/
   ~~/resource-lists/list/list/
   entry%5b@uri=%22sip:petri@example.com%22%5d HTTP/1.1
   Host: xcap.example.com

Figure 30: Deleting an Entry

Bill decides to check on the URI for Nancy, so he fetches a particular attribute (Section 7.6):

   GET
   /resource-lists/users/sip:bill@example.com/index/
   ~~/resource-lists/list/list/entry%5b2%5d/@uri HTTP/1.1
   Host: xcap.example.com

Figure 31: Fetching an Attribute

and the server responds:

   HTTP/1.1 200 OK
   Etag: "ad88"
   Content-Type:application/xcap-att+xml
   
   "sip:nancy@example.com"

Figure 32: Results of Fetch

14. Security Considerations

Frequently, the data manipulated by XCAP contains sensitive information. To avoid eavesdroppers from seeing this information, it is RECOMMENDED that an administrator hand out an HTTPS URI as the XCAP root URI. This will result in TLS-encrypted communications between the client and server, preventing any eavesdropping. Clients MUST implement TLS, assuring that such URIs will be usable by the client.

Client and server authentication are also important. A client needs to be sure it is talking to the server it believes it is contacting. Otherwise, it may be given false information, which can lead to denial-of-service attacks against a client. To prevent this, a client SHOULD attempt to upgrade [15] any connections to TLS. Similarly, authorization of read and write operations against the data is important, and this requires client authentication. As a result, a server SHOULD challenge a client using HTTP Digest [11] to establish its identity, and this SHOULD be done over a TLS connection. Clients MUST implement digest authentication, assuring interoperability with servers that challenge the client. Servers MUST NOT perform basic authentication without a TLS connection to the client.

Because XCAP is a usage of HTTP and not a separate protocol, it runs on the same port numbers as HTTP traffic normally does. This makes it difficult to apply port-based filtering rules in firewalls to separate the treatment of XCAP traffic from other HTTP traffic.

However, this problem exists broadly today because HTTP is used to access a wide breadth of content, all on the same port, and XCAP views application configuration documents as just another type of HTTP content. As such, separate treatment of XCAP traffic from other HTTP traffic requires firewalls to examine the URL itself. There is no foolproof way to identify a URL as pointing to an XCAP resource. However, the presence of the double tilde (~~) is a strong hint that the URL points to an XML element or attribute. As always, care must be taken in looking for the double-tilde due to the breadth of ways in which a URI can be encoded on-the-wire [29] [13].

15. IANA Considerations

There are several IANA considerations associated with this specification.

15.1. XCAP Application Unique IDs

Per this specification's instructions, IANA created a new registry for XCAP application unique IDs (AUIDs). This registry is defined as a table that contains three columns:

   AUID:  This will be a string provided in the IANA registrations into
      the registry.
   
   Description:  This is text that is supplied by the IANA registration
      into the registry.
   
   Reference:  This is a reference to the RFC containing the
      registration.

Per this specification's instructions, IANA created this table with an initial entry. The resulting table looks like:

   Application Unique
       ID (AUID)          Description                      Reference
   --------------------   -------------------------------  ---------
   xcap-caps              Capabilities of an XCAP server   RFC 4825

XCAP AUIDs are registered by the IANA when they are published in standards track RFCs. The IANA Considerations section of the RFC must include the following information, which appears in the IANA registry along with the RFC number of the publication.

• Name of the AUID. The name MAY be of any length, but SHOULD be no more than 20 characters long. The name MUST consist of alphanum and mark [16] characters only.

• Descriptive text that describes the application usage.

15.2. MIME Types

This specification requests the registration of several new MIME types according to the procedures of RFC 4288 [8] and guidelines in RFC 3023 [9].

15.2.1. application/xcap-el+xml MIME Type

   MIME media type name:  application
   
   MIME subtype name:  xcap-el+xml
   
   Mandatory parameters:  none
   
   Optional parameters:  Same as charset parameter application/xml as
      specified in RFC 3023 [9].
   
   Encoding considerations:  Same as encoding considerations of
      application/xml as specified in RFC 3023 [9].
   
   Security considerations:  See Section 10 of RFC 3023 [9].
   
   Interoperability considerations:  none
   
   Published specification:  RFC 4825
   Applications that use this media type:  This document type has been
      used to support transport of XML fragment bodies in RFC 4825, the
      XML Configuration Access Protocol (XCAP).

Additional Information:

         Magic Number: none
         
         File Extension: .xel

Macintosh file type code: "TEXT"

Personal and email address for further information:

      Jonathan Rosenberg, jdrosen@jdrosen.net
   
   Intended usage:  COMMON
   
   Author/Change controller:  The IETF.

15.2.2. application/xcap-att+xml MIME Type

   MIME media type name:  application
   
   MIME subtype name:  xcap-att+xml
   
   Mandatory parameters:  none
   
   Optional parameters:  Same as charset parameter application/xml as
      specified in RFC 3023 [9].
   
   Encoding considerations:  Same as encoding considerations of
      application/xml as specified in RFC 3023 [9].
   
   Security considerations:  See Section 10 of RFC 3023 [9].
   
   Interoperability considerations:  none
   
   Published specification:  RFC 4825
   
   Applications that use this media type:  This document type has been
      used to support transport of XML attribute values in RFC 4825, the
      XML Configuration Access Protocol (XCAP).

Additional Information:

         Magic Number: none
         
         File Extension: .xav

Macintosh file type code: "TEXT"

Personal and email address for further information:

      Jonathan Rosenberg, jdrosen@jdrosen.net
   
   Intended usage:  COMMON
   
   Author/Change controller:  The IETF.

15.2.3. application/xcap-ns+xml MIME Type

   MIME media type name:  application
   
   MIME subtype name:  xcap-ns+xml
   
   Mandatory parameters:  none
   
   Optional parameters:  Same as charset parameter application/xml as
      specified in RFC 3023 [9].
   
   Encoding considerations:  Same as encoding considerations of
      application/xml as specified in RFC 3023 [9].
   
   Security considerations:  See Section 10 of RFC 3023 [9].
   
   Interoperability considerations:  none
   
   Published specification:  RFC 4825
   
   Applications that use this media type:  This document type has been
      used to support transport of XML fragment bodies in RFC 4825, the
      XML Configuration Access Protocol (XCAP).

Additional Information:

      Magic Number:  none
      
      File Extension:  .xns
      
      Macintosh file type code:  "TEXT"

Personal and email address for further information:

      Jonathan Rosenberg, jdrosen@jdrosen.net
   
   Intended usage:  COMMON
   
   Author/Change controller:  The IETF.

15.2.4. application/xcap-error+xml MIME Type

   MIME media type name:  application
   
   MIME subtype name:  xcap-error+xml
   
   Mandatory parameters:  none
   
   Optional parameters:  Same as charset parameter application/xml as
      specified in RFC 3023 [9].
   
   Encoding considerations:  Same as encoding considerations of
      application/xml as specified in RFC 3023 [9].
   
   Security considerations:  See Section 10 of RFC 3023 [9].
   
   Interoperability considerations:  none
   
   Published specification:  RFC 4825
   
   Applications that use this media type:  This document type conveys
      error conditions defined in RFC 4825

Additional Information:

      Magic Number:  none
      
      File Extension:  .xer
      
      Macintosh file type code:  "TEXT"

Personal and email address for further information:

      Jonathan Rosenberg, jdrosen@jdrosen.net
   
   Intended usage:  COMMON
   
   Author/Change controller:  The IETF.

15.2.5. application/xcap-caps+xml MIME Type

   MIME media type name:  application
   
   MIME subtype name:  xcap-caps+xml
   
   Mandatory parameters:  none
   
   Optional parameters:  Same as charset parameter application/xml as
      specified in RFC 3023 [9].
   
   Encoding considerations:  Same as encoding considerations of
      application/xml as specified in RFC 3023 [9].
   
   Security considerations:  See Section 10 of RFC 3023 [9].
   
   Interoperability considerations:  none
   
   Published specification:  RFC 4825
   
   Applications that use this media type:  This document type conveys
      capabilities of an XML Configuration Access Protocol (XCAP)
      server, as defined in RFC 4825.

Additional Information:

      Magic Number:  none
      
      File Extension:  .xca
      
      Macintosh file type code:  "TEXT"

Personal and email address for further information:

      Jonathan Rosenberg, jdrosen@jdrosen.net
   
   Intended usage:  COMMON
   
   Author/Change controller:  The IETF.

15.3. URN Sub-Namespace Registrations

This specification registers several new XML namespaces, as per the guidelines in RFC 3688 [17].

15.3.1. urn:ietf:params:xml:ns:xcap-error

   URI:  The URI for this namespace is urn:ietf:params:xml:ns:xcap-error
   
   Registrant Contact:  IETF, SIMPLE working group, (simple@ietf.org),
      Jonathan Rosenberg (jdrosen@jdrosen.net).

XML:

           BEGIN
           <?xml version="1.0"?>
           <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
              "http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
           <html xmlns="http://www.w3.org/1999/xhtml">
           <head>
             <meta http-equiv="content-type"
                content="text/html;charset=iso-8859-1"/>
             <title>XCAP Error Namespace</title>
           </head>
           <body>
             <h1>Namespace for XCAP Error Documents</h1>
             <h2>urn:ietf:params:xml:ns:xcap-error</h2>
             <p>See <a href="http://www.rfc-editor.org/rfc/rfc4825.txt">
                RFC4825</a></p>
           </body>
           </html>
           END

15.3.2. urn:ietf:params:xml:ns:xcap-caps

   URI:  The URI for this namespace is urn:ietf:params:xml:ns:xcap-caps
   
   Registrant Contact:  IETF, SIMPLE working group, (simple@ietf.org),
      Jonathan Rosenberg (jdrosen@jdrosen.net).

XML:

           BEGIN
           <?xml version="1.0"?>
           <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
              "http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
           <html xmlns="http://www.w3.org/1999/xhtml">
           <head>
             <meta http-equiv="content-type"
                content="text/html;charset=iso-8859-1"/>
             <title>XCAP Capabilities Namespace</title>
           </head>
           <body>
             <h1>Namespace for XCAP Capability Documents</h1>
             <h2>urn:ietf:params:xml:ns:xcap-caps</h2>
             <p>See <a href="http://www.rfc-editor.org/rfc/rfc4825.txt">
                RFC4825</a></p>
           </body>
           </html>
           END

15.4. XML Schema Registrations

This section registers two XML schemas per the procedures in [17].

15.4.1. XCAP Error Schema Registration

   URI:  urn:ietf:params:xml:schema:xcap-error
   
   Registrant Contact:  IETF, SIMPLE working group, (simple@ietf.org),
      Jonathan Rosenberg (jdrosen@jdrosen.net).
   
   XML Schema:  The XML for this schema can be found as the sole content
      of Section 11.2.

15.4.2. XCAP Capabilities Schema Registration

   URI:  urn:ietf:params:xml:schema:xcap-caps
   
   Registrant Contact:  IETF, SIMPLE working group, (simple@ietf.org),
      Jonathan Rosenberg (jdrosen@jdrosen.net).
   
   XML Schema:  The XML for this schema can be found as the sole content
      of Section 12.2.

16. Acknowledgements

The author would like to thank Jari Urpalainen, who has contributed many important comments and has assisted with edit passes in the document. The author would also like to thank Ben Campbell, Eva-Maria Leppanen, Hisham Khartabil, Chris Newman, Joel Halpern, Lisa Dusseault, Tim Bray, Pete Cordell, Jeroen van Bemmel, Christian Schmidt, and Spencer Dawkins for their input and comments. A special thanks to Ted Hardie for his input and support.

17. References

17.1. Normative References

   [1]   Maler, E., Yergeau, F., Paoli, J., Bray, T., and C. Sperberg-
         McQueen, "Extensible Markup Language (XML) 1.0 (Third
         Edition)", World Wide Web Consortium FirstEdition REC-xml-
         20040204, February 2004,
         <http://www.w3.org/TR/2004/REC-xml-20040204>.
   
   [2]   Thompson, H., Maloney, M., Mendelsohn, N., and D. Beech, "XML
         Schema Part 1: Structures Second Edition", World Wide Web
         Consortium Recommendation REC-xmlschema-1-20041028,
         October 2004,
         <http://www.w3.org/TR/2004/REC-xmlschema-1-20041028>.
   
   [3]   Layman, A., Hollander, D., and T. Bray, "Namespaces in XML",
         World Wide Web Consortium FirstEdition REC-xml-names-19990114,
         January 1999,
         <http://www.w3.org/TR/1999/REC-xml-names-19990114>.
   
   [4]   Daniel, R., DeRose, S., Maler, E., and J. Marsh, "XPointer
         xmlns() Scheme", World Wide Web Consortium Recommendation REC-
         xptr-xmlns-20030325, March 2003,
         <http://www.w3.org/TR/2003/REC-xptr-xmlns-20030325>.
   
   [5]   Grosso, P., Marsh, J., Maler, E., and N. Walsh, "XPointer
         Framework", World Wide Web Consortium Recommendation REC-xptr-
         framework-20030325, March 2003,
         <http://www.w3.org/TR/2003/REC-xptr-framework-20030325>.
   
   [6]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
         Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
         HTTP/1.1", RFC 2616, June 1999.
   
   [7]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.
   
   [8]   Freed, N. and J. Klensin, "Media Type Specifications and
         Registration Procedures", BCP 13, RFC 4288, December 2005.
   
   [9]   Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types",
         RFC 3023, January 2001.
   
   [10]  Clark, J. and S. DeRose, "XML Path Language (XPath) Version
         1.0", World Wide Web Consortium Recommendation REC-xpath-
         19991116, November 1999,
         <http://www.w3.org/TR/1999/REC-xpath-19991116>.
   
   [11]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
         Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication:
         Basic and Digest Access Authentication", RFC 2617, June 1999.
   
   [12]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 4234, October 2005.
   
   [13]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
         Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
         January 2005.
   
   [14]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
   
   [15]  Khare, R. and S. Lawrence, "Upgrading to TLS Within HTTP/1.1",
         RFC 2817, May 2000.
   
   [16]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.
   
   [17]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
         January 2004.
   
   [18]  Yergeau, F., "UTF-8, a transformation format of ISO 10646",
         STD 63, RFC 3629, November 2003.
   
   [19]  Boyer, J., "Canonical XML Version 1.0", World Wide Web
         Consortium Recommendation REC-xml-c14n-20010315, March 2001,
         <http://www.w3.org/TR/2001/REC-xml-c14n-20010315>.

17.2. Informative References

   [20]  Rosenberg, J., "A Presence Event Package for the Session
         Initiation Protocol (SIP)", RFC 3856, August 2004.
   
   [21]  Roach, A., Campbell, B., and J. Rosenberg, "A Session
         Initiation Protocol (SIP) Event Notification Extension for
         Resource Lists", RFC 4662, August 2006.
   
   [22]  Rosenberg, J., "Extensible Markup Language (XML) Formats for
         Representing Resource Lists", RFC 4826, May 2007.
   
   [23]  Grosso, P. and D. Veillard, "XML Fragment Interchange", World
         Wide Web Consortium CR CR-xml-fragment-20010212, February 2001,
         <http://www.w3.org/TR/2001/CR-xml-fragment-20010212>.
   
   [24]  Berglund, A., Boag, S., Chamberlin, D., Fernandez, M., Kay, M.,
         Robie, J., and J. Simeon, "XML Path Language (XPath) 2.0",
         World Wide Web Consortium
         CR http://www.w3.org/TR/2005/CR-xpath20-20051103,
         November 2005.
   
   [25]  Newman, C. and J. Myers, "ACAP -- Application Configuration
         Access Protocol", RFC 2244, November 1997.
   
   [26]  Day, M., Rosenberg, J., and H. Sugano, "A Model for Presence
         and Instant Messaging", RFC 2778, February 2000.
   
   [27]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434,
         October 1998.
   
   [28]  Roach, A., "Session Initiation Protocol (SIP)-Specific Event
         Notification", RFC 3265, June 2002.
   
   [29]  Duerst, M. and M. Suignard, "Internationalized Resource
         Identifiers (IRIs)", RFC 3987, January 2005.

Author's Address

Jonathan Rosenberg
Cisco
Edison, NJ
US

   EMail: jdrosen@cisco.com
   URI:   http://www.jdrosen.net

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