SIMPLE J. Rosenberg
Internet-Draft dynamicsoft
Expires: December 22, 2003 June 23, 2003
The Extensible Markup Language (XML) Configuration Access Protocol
(XCAP)
draft-ietf-simple-xcap-00
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
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 is not a new protocol. XCAP maps XML document
sub-trees and element attributes to HTTP URIs, so that these
components can be directly accessed by HTTP.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview of Operation . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Application Usages . . . . . . . . . . . . . . . . . . . . . 6
5. URI Construction . . . . . . . . . . . . . . . . . . . . . . 8
5.1 Identifying the XML Document . . . . . . . . . . . . . . . . 8
5.2 Identifying the XML Nodes . . . . . . . . . . . . . . . . . 9
6. Client Operations . . . . . . . . . . . . . . . . . . . . . 10
6.1 Creating a New Document . . . . . . . . . . . . . . . . . . 10
6.2 Replace an Existing Document . . . . . . . . . . . . . . . . 10
6.3 Deleting a Document . . . . . . . . . . . . . . . . . . . . 10
6.4 Fetching a Document . . . . . . . . . . . . . . . . . . . . 10
6.5 Creating a New Element . . . . . . . . . . . . . . . . . . . 10
6.6 Replacing an Element in the Document . . . . . . . . . . . . 11
6.7 Delete an Element . . . . . . . . . . . . . . . . . . . . . 12
6.8 Fetch an Element . . . . . . . . . . . . . . . . . . . . . . 12
6.9 Create an Attribute . . . . . . . . . . . . . . . . . . . . 12
6.10 Replacing Attributes . . . . . . . . . . . . . . . . . . . . 13
6.11 Deleting Attributes . . . . . . . . . . . . . . . . . . . . 13
6.12 Fetching Attributes . . . . . . . . . . . . . . . . . . . . 13
6.13 Fetching Metadata . . . . . . . . . . . . . . . . . . . . . 13
6.14 Read/Modify/Write Transactions . . . . . . . . . . . . . . . 14
7. Server Behavior . . . . . . . . . . . . . . . . . . . . . . 15
7.1 POST Handling . . . . . . . . . . . . . . . . . . . . . . . 16
7.2 PUT Handling . . . . . . . . . . . . . . . . . . . . . . . . 17
7.3 GET Handling . . . . . . . . . . . . . . . . . . . . . . . . 18
7.4 DELETE Handling . . . . . . . . . . . . . . . . . . . . . . 18
7.5 Managing Modification Times . . . . . . . . . . . . . . . . 19
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9. Security Considerations . . . . . . . . . . . . . . . . . . 23
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . 24
Normative References . . . . . . . . . . . . . . . . . . . . 25
Informative References . . . . . . . . . . . . . . . . . . . 26
Author's Address . . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . 28
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1. Introduction
The Session Initiation Protocol for Instant Messaging and Presence
Leveraging Extensions (SIMPLE) working group has been developing
specifications for subscribing to, and receiving notifications of,
user presence [10]. An important aspect of user presence is
authorization policy. Indeed, the presence specification requires a
Presence Agent (PA) to both authenticate and authorize all
subscriptions before accepting them. However, it does not define how
the server determines the authorization status of a subscriber. Users
can set their authorization policy through web pages or voice
response systems. However, there is currently no protocol specified
for setting this policy. A protocol for this purpose is called an
authorization manipulation protocol.
Mechanisms have also been defined to support reactive authorization
[11][12]. Reactive authorization allows the user to be informed when
someone has attempted to subscribe to their presence when the server
is unable to determine an authorization policy. The user can then go
and set an authorization policy for the subscriber, using the same
unspecified mechanism for setting the policy.
Another important aspect of presence systems is the buddy list, also
known as the presence list. This is a list of users that a watcher
wishes to learn presence state for. This list can be stored in the
client, or it can be stored in a centralized server. In the latter
case, the client would subscribe to the list as a whole [13]. The
presence list can be set by using a web page or voice response
application. However, there is no protocol mechanism currently
specified to manage the presence list. Such a protocol is called a
presence list manipulation protocol.
The SIMPLE group has defined requirements for an authorization
manipulation protocol and a presence list manipulation protocol.
These protocols have similar requirements, and are captured in [14].
This document proposes a candidate for the authorization and presence
manipulation protocol, called the Extensible Markup Language (XML)
Configuration Access Protocol (XCAP). XCAP is not actually a new
protocol. XCAP is a set of conventions for using HTTP to read, write
and modify XML configuration data. XCAP is based heavily on ideas
borrowed from the Application Configuration Access Protocol (ACAP)
[15], 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.
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2. Overview of Operation
XCAP supports the needs of any application that needs access to data
defined by clients of the application. Each application that makes
use of XCAP specifies an application usage (Section 4). This
application usage defines the XML schema [1] 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 [2]. 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 subtree
of the document, or any attribute for any element within the
document. Thus, the HTTP URIs used by XCAP point to pieces of
information that are finer grained than the XML document itself.
With a standardized naming convention for components of XML
documents, the basic operations for accessing the data are simple.
Reading one of the components is just a standard HTTP GET operation.
Writing, creating or modifying one of the components is a standard
HTTP POST or PUT operation. Deleting a component is just a standard
DELETE operation. For example, to add a friend to a presence list, a
client would construct an XML document fragment which contains the
information on that friend. The client would then construct a URI
that refers to the location in the presence list document where this
new fragment is to be added. The client then performs a POST
operation against the URI, placing the document fragment into the
body of the POST request. To provide atomic read/modify/write
operations, the HTTP If-Unmodified-Since header field is used. The
HTTP POST operation used by the client would contain the date
obtained in the Last-Modified header field from the GET used to read
the data.
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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 [3] and
indicate requirement levels for compliant implementations.
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4. Application Usages
A central concept in XCAP is that of an application usage. An
application usage defines the way in which a specific application
makes use of XCAP. This definition is composed of several pieces of
information, such as an XML schema and constraints on values of one
element given values in another.
Application usages are documented in specifications which convey this
information. In particular, an application usage specification MUST
provide the following information:
Application Usage ID (AUID): Each application usage is associated
with a name, called an AUID. This name uniquely identifies the
application usage, and is different from all other AUIDs. 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 [16] based on the guidelines
in Section 10. The second namespace is the vendor-proprietary
namespace. Each AUID in that namespace is prefixed with the token
"vnd", followed by a period ("."), followed by a valid DNS name,
followed by another period, followed by any vendor defined token.
A vendor creating such an AUID MUST only create one using domain
names for which it is an administrator. As an example, the
example.com domain can create an AUID with the value
"vnd.example.com.foo" but cannot create one with the value
"vnd.example.org.bar". 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.
MIME Type: Each application usage MUST register a MIME type for
its XML documents. This is done based on the procedures of RFC
3023 [4].
XML Schema: Each application will have a unique schema which
defines the data needed by the application. In XCAP, this schema
is represented using XML schema. As an example, presence list data
is composed of a list of URIs, each of which represends a member
of presence list. [17] defines the XML schema for this data.
Additional Constraints: XML schemas can represent a variety of
constraints about data, such as ranges and types. However, schemas
cannot cover all types of data constraints, including constraints
introduced by data interdependencies. For example, one XML element
may contain an integer which defines the maximum number of
instances of another element. The application usage defines these
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additional constraints.
Data Semantics: The application usage needs to define detailed
semantics for each piece of data in the schema.
Naming Conventions: The data defined by the XML schema will be
used by any number of entities participating in the application.
In the case of presence list, the data is used by the Resource
List Server (RLS), which reads the data, and by the clients, which
write it. During the execution of the application (i.e., the
processing of the list subscription), specific documents will need
to be read or written. In order for the application to function
properly, there needs to be agreement on exactly which documents
are read or written by the application. This is an issue of naming
conventions; agreeing on how an application constructs the URI
representing the document that is to be read or written. The
application usage spells out this information.
Computed Data: Frequently, some of the data defined in the schema
is not independent; that is, its value depends on the values of
other elements in the document. As a result, when a client uses
XCAP to modify the independent pieces of the document, the server
needs to compute the dependent ones in order to fully populate the
document. The application usage needs to define which data
components are dependent, and how they are computed. As an
example, when the URI for a presence list is not specified by a
client, a URI is chosen by the server and filled in. This needs to
be specified by the application usage.
Authorization Policies: By default, an XCAP server will only allow
a user to access (read, write, delete or modify) their own
documents. The application usage can specify differing default
authorization policies. Of course, the default can always be
overriden by operator or user-specified policies.
Application usages are similar to dataset classes in ACAP.
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5. URI Construction
In order to manipulate a piece of configuration data, the data must
be represented by an HTTP URI. XCAP defines a specific naming
convention for constructing these URIs. This convention is very
similar to the naming conventions used for dataset classes in ACAP,
and makes use the XPath [5] specification for identifying nodes of an
XML document.
The HTTP URI consists of two parts:
XCAP-URI = Document-URI ["?" Node-Selector]
Document-URI = http_URL ;from RFC2616
Node-Selector = *uric ;Escape coded LocationPath from XPath
The first part, the Document-URI, selects a specific XML document. It
is a valid HTTP URL, subject to the constraints described here. The
constraints for constructing this URI are discussed below in Section
5.1. Once a document is selected, the remainder of the URI (the
Node-Selector) identifies which components of the document are being
addressed. The Node-Selector is an XPath [5] LocationPath expression,
subject to constraints described below.
5.1 Identifying the XML Document
XCAP mandates that a server organizes documents according to a
defined hierarchy. The root of this hierarchy is an HTTP URI called
the XCAP services root URI. This URI identifies the root of the tree
within the domain where all XCAP documents are stored. It can be any
valid HTTP URL, but MUST NOT contain a query string. As an example,
http://xcap.example.com/services might be used as the XCAP services
root URI within the example.com domain. Typically, the XCAP services
root URI is provisioned into client devices for bootstrapping
purposes.
Beneath the XCAP services root URI is a tree structure for organizing
documents. The first level of this tree consists of the XCAP AUID.
So, continuing the example above, all of the documents used by the
presence list application would be under http://xcap.example.com/
services/presence-lists.
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, within the directory structure for each application usage,
there are two sub-trees. One, called "users", holds the documents
that are applicable to only specific users, and the other, called
"global", holds documents applicable to all users.
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Within the "users" tree are zero or more sub-trees, each of which
identifies a documents that apply to a specific user. XCAP does not
itself define what it means for documents to "apply" to a user,
beyond specification of a baseline authorization policy.
Specifically, the default authorization policy is that only a user
who authenticates themself as user X can read, write, or otherwise
access in any way the documents within sub-tree X. Each application
usage can specify additional authorization policies which depend on
data used by the application itself.
The remainder of the URI (the path following "global" or the specific
user) is not constrained by this specification. The application usage
MAY introduce constraints, or may allow any structure to be used.
5.2 Identifying the XML Nodes
The second component of the XCAP URI specifies specific nodes of the
XML document which are to be accessed. Nodes, in this context, refers
to the definition provided in the XPath specification, and therefore
includes XML elements, attributes, text, namespaces, processing
instructions, comments, and roots. These nodes are identified by a
LocationPath expression, as defined in XPath. Either the abbreviated
or unabbreviated form MAY be used.
Contraints are imposed on the XPath expression based on the operation
being performed. These do not constrain the functions or axes that
can be used in the XPath expression, but rather constrain the
resulting node set. See Section 6 for details.
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6. Client Operations
An XCAP client is an HTTP 1.1 compliant client. An XCAP client
performs a set of primitive operations by invoking specific methods
against the server, using specific URIs, where the requests contain
bodies and headers subject to specific constraints. The set of
primitive operations, the methods used to accomplish them, and the
header and body constraints are described here.
6.1 Creating a New Document
To create a new document, the client constructs a URI that references
the location where the document is to be placed. This URI MUST NOT
contain a NodeSelector component, and MUST meet the constraints
described in Section 5.1. The client then invokes a PUT method on
that URI.
The content in the request MUST be an XML document compliant to the
schema associated with the application usage defined by the URI. For
example, if the client performs a PUT operation to http://
xcap.example.com/services/presence-lists/users/joe/mybuddies,
presence-lists is the application unique ID, and the schema defined
by it would dictate the body of the request.
6.2 Replace an Existing Document
To replace an existing document with a new one, the procedures of
Section 6.1 are followed; the Request-URI merely refers to an
existing document which is to be replaced with the content of the
request.
6.3 Deleting a Document
To delete a document, the client constructs a URI that references the
document to be deleted. By definition this URI will not contain a
NodeSelector component. The client then invokes a DELETE operation on
the URI to delete the document.
6.4 Fetching 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 to be fetched. It is useful for clients to perform
conditional GETs using the If-Modified-Since header field, in order
to check if a locally cached copy of the document is still valid.
6.5 Creating a New Element
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To create a new XML element within an existing document, the client
constructs a URI whose Document-URI points to the document to be
modified. The Node-Selector MUST be present, containing an expression
identifying the point in the document where the new element is to be
added. The node-set selected by the expression MUST contain only a
single XML element.
The client then invokes the HTTP POST method. The content in the
request MUST be an XML document. That XML document MUST be conformant
to the schema associated with the application usage defined by the
URI. The server will insert the document such that the first element
of the document becomes the next sibling immediately following the
element specified by the Request-URI. The client SHOULD be certain,
before making the request, that the resulting modified document will
also be conformant to the schema.
6.6 Replacing an Element in the Document
Replacing an element of the document constitutes storage of a
supplied entity under the specified URI, and is therefore
accomplished with the PUT method (as opposed to POST, which will
insert). The client constructs a URI whose Document-URI points to the
document to be modified. The Node-Selector MUST be present,
containing an expression identifying the element whose value is to be
replaced. The node-set selected by the expression MUST contain only a
single XML element.
The client then invokes the PUT method. The entity of the request
MUST be of type text/plain. The server will take the value of the
element specified by the request URI, and replace it with the content
of the PUT request. Here, value refers to the binary contents of an
XML document, starting with the beginning tag of the element, and
ending with the end tag. This differs from the "string value" defined
in XPath, which refers only to the values of the text element
descendants of an element. The client SHOULD be certain, before
making the request, that the resulting modified document will be
conformant to the schema.
The body of the request here is of type text/plain because the value
of an element need not be a valid XML document; frequently, it will
be text or CDATA. Of course, the value of an XML element may be other
XML elements, in which case the body of the request will be an XML
document fragment, and by itself not compliant to any schema.
Note that this operation only modifies the value of an element. It
cannot modify the attributes of the element. To do that, the replace
attribute operation is performed.
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6.7 Delete an Element
To delete elements from a document, the client constructs a URI whose
Document-URI points to the document containing the elements to be
deleted. The Node-Selector MUST be present, containing an expression
identifying the elements to be deleted. Unlike most of the other
operations, the node-set selected by the expression MAY contain
multiple elements.
The client then invokes the HTTP DELETE method. All of the elements
specified by the node set will be deleted by the server. The body of
the request SHOULD be empty. The client SHOULD be certain, before
making the request, that the resulting modified document will also be
conformant to the schema.
6.8 Fetch an Element
To fetch an element of a document, the client constructs a URI whose
Document-URI points to the document containing the element to be
fetched. The Node-Selector MUST be present, containing an expression
identifying the element whose value is to be fetched. The node-set
selected by the expression MUST contain only a single XML element.
The client then invokes the GET method. The response will contain an
XML document with the specified element as the one and only child of
the document root.
OPEN ISSUE: This only allows you to get one element at a time. We
could allow the XPath expression to specify multiple elements, and
then the response contains a document with each of those elements
as a child of the document root. However, that document might not
be compliant to the schema, and worse, the document doesnt
actually reflect any specific sub-tree of the actual document.
6.9 Create an Attribute
To create an attribute in an existing element of a document, the
client constructs a URI whose Document-URI points to the document to
be modified. The Node-Selector MUST be present, containing an
expression identifying an attribute that is to created. Specifically,
the last location step of the expression MUST specify an attribute
axis, and the context MUST specify a single element that exists
within the document.
The client then invokes the HTTP POST method. The content defined by
the request MUST be of type text/plain. A new attribute is added to
the element defined by the context, with the name specified by the
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node test in the last location step, with a value specified by the
body of the request. If an attribute of this name already exists, it
is replaced. The client SHOULD be certain, before making the request,
that the resulting modified document will also be conformant to the
schema.
6.10 Replacing Attributes
To replace an attribute in an existing element of a document, the
client constructs a URI whose Document-URI points to the document to
be modified. The Node-Selector MUST be present, containing an
expression identifying an attribute that is to be replaced.
The client then invokes the HTTP PUT method. The content defined by
the request MUST be of type text/plain. The value of the attribute
defined by the Node-Selector is replaced by the body of the request.
The client SHOULD be certain, before making the request, that the
resulting modified document will also be conformant to the schema.
6.11 Deleting Attributes
To delete attributes from the document, the client constructs a URI
whose Document-URI points to the document containing the attributes
to be deleted. The Node-Selector MUST be present, containing an
expression identifying the attributes to be deleted. Unlike most of
the other operations, the node-set selected by the expression MAY
contain multiple attributes.
The client then invokes the HTTP DELETE method. All of the attributes
specified by the node set will be deleted by the server. The body of
the request SHOULD be empty. The client SHOULD be certain, before
making the request, that the resulting modified document will also be
conformant to the schema.
6.12 Fetching Attributes
To fetch an attribute of a document, the client constructs a URI
whose Document-URI points to the document containing the attribute to
be fetched. The Node-Selector MUST be present, containing an
expression identifying the attribute whose value is to be fetched.
The node-set selected by the expression MUST contain only a single
XML attribute.
The client then invokes the GET method. The response will contain an
text/plain document with the value of the specified attribute.
6.13 Fetching Metadata
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Elements and attributes in an XML document have various meta-data
associated with them. For example, and XML element has a certain
number of child elements. That number is a piece of meta-data that
describes the element. Currently, there is no way to fetch meta-data
for XML elements or attributes.
OPEN ISSUE: Is this restriction OK? XPath does specify functions
for computing meta-data about node sets. We can't use them since
XCAP mandates that the request URI be a location set, which does
not include these other functions. We could relax the constraint
if this is deemed important.
6.14 Read/Modify/Write Transactions
It is anticipated that a common operation will be to read the current
version of a document or element, modify it on the client, and then
write the change back to the server. In order for the results to be
consistent with the client's expectations, the operation must be
atomic.
To accomplish this, the client stores the value of the Last-Modified
header field from the response to its GET operation used to read the
element, attribute, or document that is to be modified. To guarantee
atomicity, the PUT or POST operation used to write the changes back
to the server MUST contain an If-Unmodified-Since header field, whose
value is equal to the value from the prior GET response. If the
request fails with a 412 response, the client knows that another
update of the data has occurred before it was able to write the
results back. The client can then fetch the most recent version, and
attempt its modification again.
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7. Server Behavior
TODO: Specify an XML body type for the responses that contains
error conditions or success results.
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.
OPEN ISSUE: It may be possible to remove this restriction by
allowing an application usage to define operation in a server that
doesnt understand the usage. That may require some capabilities
discovery to be introduced, this constraint didnt seem that
problematic.
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 that is not recognized by the
server, it MUST reject the request with a 404 (Not Found).
Next, the server authenticates the request. All XCAP servers MUST
support HTTP Digest [6]. Furthermore, servers MUST support HTTP over
TLS, RFC 2818 [7]. It is RECOMMENDED that administrators use an HTTPS
URI as the XCAP root services 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 default
authorization policy is that only client X can access (create, read,
write, modify or delete) resources under the "users/X" directory. An
application usage can specify an alternate default authorization
policy specific to that usage. Of course, an administrator or
privileged user can override the default authorization policy,
although this specification provides no means for doing that.
Generally, if users need to be able to control authorization for
access to XCAP data, an XCAP application usage should be specified
which allows the user to set the policies as needed.
OPEN ISSUE: This is different from ACAP, where authorization
policies are built into the protocol. I think the default
generally will suffice, so I would rather not burden the baseline
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protocol with it.
Once authorized, the specific behavior depends on the method and what
the URI refers to.
7.1 POST Handling
If the URI contains only a Document-URI, the server examines the
entity body of the request. If there is no entity in the body, the
server MUST reject the request with a 409 response. If there is an
entity, but it is not well-formed, the server MUST reject the request
with a 409 response. If it is well-formed, but not compliant to the
schema associated with the application usage, the server MUST reject
the request with a 409 response. If it is compliant to the schema,
the server MUST store the document at the requested URI. If there is
not already a document stored at that URI, a 201 (Created) response
code MUST be sent, and it MUST include a Location header field
containing the value of the URI for the document (which will be the
same as the one in the Request-URI). Otherwise, a 200 OK response is
returned, and the document in the body replaces the existing one at
that URI. For either a 200 or 201 response, the new document is
returned in the body of the response, with a Content-Type equal to
the MIME type defined by the application usage.
If the Request URI contains a Node-Selector, the server MUST verify
that the document defined by the Document-URI exists. If no such
document exists on the server, the server MUST reject the request
with a 409 response code. If the document does exist, the server
evaluates the Node-Selector as an XPath RelativeLocationPath,
relative to the root of the document. If the Node-Selector does not
comply to the grammar for RelativeLocationPath, the server MUST
reject the request with a 400 response code. If the Node-Selector
does comply, and it evaluates to anything other than the empty set, a
single attribute node or single element node, the server MUST reject
the request with a 409 response code.
If the Node-Selector evaluates to the empty set, and the last
location step is on the attribute axis, and the expression without
the last location step evaluates to a single element node, the server
adds an attribute to that element. Its name is the name given in the
node test of the last location step, and its value is taken from the
body of the request. The server then generates a 200 OK response,
whose body contains the value of the attribute, with a Content-Type
of text/plain.
If the Node-Selector evaluates to a single element node, the server
takes the content of the request, and inserts it into the document
specified by the URI such that the selected element is the immediate
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sibling of the nodes defined by the content of the request. The
server then generates a 200 OK response, whose body contains the
parent element of the new elements just inserted. The parent element
is represented by extracting the contents of the XML document,
starting with, and including, the begin tag of the element, up to,
and including, the end tag for the element. The Content-Type of the
response is set to application/xml.
OPEN ISSUE: We can't use the MIME type for the application usage,
since the schema may not allow for the document to start with any
element defined by the schema. Is that OK? I think so.
If the Node-Selector evaluates to a single attribute node, the server
takes the content of the request, and sets it as the value of the
attribute specified by the body of the request. The server then
generates a 200 OK response, whose body contains the value of the
attribute, with a Content-Type of text/plain.
If the result of the POST is a document which does not comply with
the XML schema for the application usage, the server MUST NOT perform
the POST, and MUST reject the request with a 409 (Conflict).
7.2 PUT Handling
When the Request URI contains only the Document-URI, the semantics of
PUT are as defined in HTTP 1.1 Section 9.6 - the content of the
request is placed at the specified location.
If the Request URI contains a Node-Selector, the server MUST verify
that the document defined by the Document-URI exists. If no such
document exists on the server, the server MUST reject the request
with a 409 response code. If the document does exist, the server
evaluates the Node-Selector as an XPath RelativeLocationPath,
relative to the root of the document. If the Node-Selector does not
comply to the grammar for RelativeLocationPath, the server MUST
reject the request with a 400 response code. If the Node-Selector
does comply, and it evaluates to anything other than the a single
element node or attribute node, the server MUST reject the request
with a 409 response code.
If the Node-Selector evaluates to a single element node, the server
takes the content of the request, and replaces the value of that
element (where value is defined as all of the content - elements,
text, or CDATA - between the begin and end tags of the element) with
that content. The server then returns a 200 OK response.
OPEN ISSUE: PUT is not quite right here, since a subsequent GET on
the same URI will not return exactly the same thing - the begin
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and end tags will be present. This may need to be POST, but then,
how to differentiate a replace with an append operation?
If the Node-Selector evaluates to a single attribute node, the server
takes the content of the request, and sets it as the value of the
attribute. It then returns a 200 OK response.
If the result of the PUT is a document which does not comply with the
XML schema for the application usage, the server MUST NOT perform the
PUT, and MUST reject the request with a 409 (Conflict).
7.3 GET Handling
If the request URI contains only a Document-URI, the server returns
the document specified by the URI if it exists, else returns a 404
response.
If the request URI specifies a Node-Selector, the server verifies
that the document specified by the Document-URI exists. If it does
not exist, the server returns a 404 (Not Found) response. If the
document does exist, the server evaluates the Node-Selector as an
XPath RelativeLocationPath, relative to the root of the document. If
the Node-Selector does not comply to the grammar for
RelativeLocationPath, the server MUST reject the request with a 400
response code. If the Node-Selector does comply, and it evaluates to
anything other than the a single element node or attribute node, the
server MUST reject the request with a 409 response code.
If the Node-Selector evaluates to a single element node, the server
takes the document text, starting with, and including, the begin tag
of the element, up to, and including, the end tag for the element,
and places it into the body of a 200 OK response, setting the
Content-Type to application/xml.
If the Node-Selector evaluates to a single attribute node, the server
takes the value of the attribute and returns it as the content of the
200 OK response, setting the Content-Type to text/plain.
OPEN ISSUE: Do we need to say anything about HEAD? We havent said
anything about meta-data so far; most of that is just regular HTTP
usage, I think.
7.4 DELETE Handling
If the request URI contains only a Document-URI, the server deletes
the document specified by the URI if it exists and returns a 200 OK
response, else returns a 404 response.
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If the request URI specifies a Node-Selector, the server verifies
that the document specified by the Document-URI exists. If it does
not exist, the server returns a 404 (Not Found) response. If the
document does exist, the server evaluates the Node-Selector as an
XPath RelativeLocationPath, relative to the root of the document. If
the Node-Selector does not comply to the grammar for
RelativeLocationPath, the server MUST reject the request with a 400
response code. If the Node-Selector does comply, and it evaluates to
the empty set, the server MUST reject the request with a 404 (Not
Found).
Otherwise, the server removes all of the data defined by the
node-set. Specifically, any elements in the node set are removed from
the document, and any attributes in the node set are removed from the
document. It then returns a 200 OK response.
If the result of the deletion is a document which does not comply
with the XML schema for the application usage, the server MUST NOT
perform the deletion, and MUST reject the request with a 409
(Conflict).
7.5 Managing Modification Times
An XCAP server MUST maintain modification times for all resources
that can be referenced by a URI. Specifically, this means that each
document, and within the document, each element and attribute, MUST
be associated with a modification time maintained by the server.
These modification times are needed to support condition GET, POST
and PUT requests.
When a PUT or POST request is made that creates or replaces a
document, the modification time of that document and all elements and
attributes within is set to the current time. When a PUT request is
made to a URI referencing an XML element, the modification time of
that element and all of its enclosed children and their attributes is
set to the current time. Furthermore, the modification time of all
elements which are ancestors of that element have their modification
time set to the current time. However, the modification times of
attributes belong to elements that are ancestors of the modified
element do not have their modification times changed.
When a POST request is made to a URI referencing an XML element, the
modification time of all of the elements and their attributes within
the document in the body of the request is set to the current time.
Furthermore, the modification time of the element which is the new
parent of the elements in the request, and all of its ancestors, have
their modification time set to the current time. However, the
modification times of their attributes are unchanged.
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When a POST request is made to a URI referencing an XML attribute,
the modification time of that attribute, its element, and all
elements that are ancestors of that element is set to the current
time.
When a DELETE request is made to a URI referencing an element, the
modification time of all ancestors of that element is set to the
current time. When a DELETE request is made to a URI referencing an
attribute, the modification time of its element, and all ancestors of
that element, is set to the current time.
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8. Examples
This section goes through several examples, making use of the
presence-lists [17] XCAP application usage.
First, a user Bill creates a new presence-list, initially with no
users in it:
PUT
http://xcap.example.com/services/presence-lists/users/bill/fr.xml HTTP/1.1
Content-Type:application/presence-lists+xml
<?xml version="1.0" encoding="UTF-8"?>
<presence-lists xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<list name="friends" uri="sip:friends@example.com" subscribable="true">
</list>
</presence-lists>
Next, Bill adds an entry to the list:
PUT
http://xcap.example.com/services/presence-lists/users/bill/fr.xml?
presence-lists/list[@name="friends"] HTTP/1.1
Content-Type:text/plain
<entry name="Bill" uri="sip:bill@example.com">
<display-name>Bill Doe</display-name>
</entry>
Note how the URI in the PUT request selects the list element whose
name attribute is "friends". The body of that request replaces the
existing value of that element, which was empty.
Next, Bill adds another entry to the list, which is another list that
has three entries:
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POST
http://xcap.example.com/services/presence-lists/users/bill/fr.xml?
presence-lists/list[@name="friends"]/entry[@name="Bill"] HTTP/1.1
Content-Type:text/plain
<list name="close-friends" uri="sip:close-friends@example.com"
subscribable="true">
<entry name="Joe" uri="sip:joe@example.com">
<display-name>Joe Smith</display-name>
</entry>
<entry name="Nancy" uri="sip:nancy@example.com">
<display-name>Nancy Gross</display-name>
</entry>
<entry name="Petri" uri="sip:petri@example.com">
<display-name>Petri Aukia</display-name>
</entry>
</list>
Then, Bill decides he doesnt want Petri on the list, so he deletes
the entry:
DELETE
http://xcap.example.com/services/presence-lists/users/bill/fr.xml?
presence-lists/list/list/entry[@name="Petri"] HTTP/1.1
Bill decides to check on the URI for Nancy:
GET
http://xcap.example.com/services/presence-lists/users/bill/fr.xml?
presence-lists/list/list/entry[@name="Nancy"]/@uri HTTP/1.1
and the server responds:
HTTP/1.1 200 OK
Content-Type:text/plain
sip:nancy@example.com
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9. Security Considerations
Frequently, the data manipulated by XCAP contains sensitive
information. To avoid eavesdroppers from seeing this information,
XCAP RECOMMENDS that an admistrator hand out an https URI as the XCAP
root services URI. This will result in TLS-encrypted communications
between the client and server, preventing any eavesdropping.
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 [8] 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 [6] to establish
its identity, and this SHOULD be done over a TLS connection.
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10. IANA Considerations
This specification instructs IANA to create a new registry for XCAP
application usage IDs (AUIDs).
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 twenty characters long. The name MUST consist of
alphanum [9] characters only.
Descriptive text that describes the application usage.
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Normative References
[1] Thompson, H., Beech, D., Maloney, M. and N. Mendelsohn, "XML
Schema Part 1: Structures", W3C REC REC-xmlschema-1-20010502,
May 2001.
[2] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Murata, M., St. Laurent, S. and D. Kohn, "XML Media Types", RFC
3023, January 2001.
[5] Clark, J. and S. DeRose, "XML Path Language (XPath) Version
1.0", W3C REC REC-xpath-19991116, November 1999.
[6] 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.
[7] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[8] Khare, R. and S. Lawrence, "Upgrading to TLS Within HTTP/1.1",
RFC 2817, May 2000.
[9] 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.
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Informative References
[10] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", draft-ietf-simple-presence-10 (work
in progress), January 2003.
[11] Rosenberg, J., "A Watcher Information Event Template-Package
for the Session Initiation Protocol (SIP)",
draft-ietf-simple-winfo-package-05 (work in progress), January
2003.
[12] Rosenberg, J., "An Extensible Markup Language (XML) Based
Format for Watcher Information",
draft-ietf-simple-winfo-format-04 (work in progress), January
2003.
[13] Rosenberg, J., Roach, A. and B. Campbell, "A Session Initiation
Protocol (SIP) Event Notification Extension for Resource
Lists", draft-ietf-simple-event-list-04 (work in progress),
June 2003.
[14] Rosenberg, J. and M. Isomaki, "Requirements for Manipulation of
Data Elements in Session Initiation Protocol (SIP) for Instant
Messaging and Presence Leveraging Extensions (SIMPLE) Systems",
draft-ietf-simple-data-req-02 (work in progress), April 2003.
[15] Newman, C. and J. Myers, "ACAP -- Application Configuration
Access Protocol", RFC 2244, November 1997.
[16] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[17] Rosenberg, J., "An Extensible Markup Language (XML)
Configuration Access Protocol (XCAP) Usage for Presence Lists",
draft-rosenberg-simple-xcap-list-usage-00 (work in progress),
May 2003.
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Author's Address
Jonathan Rosenberg
dynamicsoft
600 Lanidex Plaza
Parsippany, NJ 07054
US
Phone: +1 973 952-5000
EMail: jdrosen@dynamicsoft.com
URI: http://www.jdrosen.net
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Rosenberg Expires December 22, 2003 [Page 29]
