Network Working Group M. Isomaki
Request for Comments: 4827 E. Leppanen
Category: Standards Track Nokia
May 2007An Extensible Markup Language (XML) Configuration Access Protocol (XCAP)
Usage for Manipulating Presence Document Contents
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 (C) The IETF Trust (2007).
This document describes a usage of the Extensible Markup Language
(XML) Configuration Access Protocol (XCAP) for manipulating the
contents of Presence Information Data Format (PIDF) based presence
documents. It is intended to be used in Session Initiation Protocol
(SIP) based presence systems, where the Event State Compositor can
use the XCAP-manipulated presence document as one of the inputs on
which it builds the overall presence state for the presentity.
The Session Initiation Protocol (SIP) for Instant Messaging and
Presence (SIMPLE) specifications allow a user, called a watcher, to
subscribe to another user, called a presentity, in order to learn its
presence information . The presence data model has been specified
in . The data model makes a clean separation between person-,
service-, and device-related information.
A SIP-based mechanism, SIP PUBLISH method, has been defined for
publishing presence state . Using SIP PUBLISH, a Presence User
Agent (PUA) can publish its view of the presence state, independently
of and without the need to learn about the states set by other PUAs.
However, SIP PUBLISH has a limited scope and does not address all the
requirements for setting presence state. The main issue is that SIP
PUBLISH creates a soft state that expires after the negotiated
lifetime unless it is refreshed. This makes it unsuitable for cases
where the state should prevail without active devices capable of
refreshing the state.
There are three main use cases where setting of permanent presence
state that is independent of activeness of any particular device is
useful. The first case concerns setting person-related state. The
presentity would often like to set its presence state even for
periods when it has no active devices capable of publishing
available. Good examples are traveling, vacations, and so on. The
second case is about setting state for services that are open for
communication, even if the presentity does not have a device running
that service online. Examples of these kinds of services include
e-mail, Multimedia Messaging Service (MMS), and Short Message Service
(SMS). In these services, the presentity is provisioned with a
server that makes the service persistently available, at least in
certain forms, and it would be good to be able to advertise this to
the watchers. Since it is not realistic to assume that all e-mail,
MMS, or SMS servers can publish presence state on their own (and even
if this were possible, such state would almost never change), this
has to be done by some other device. And since the availability of
the service is not dependent on that device, it would be impractical
to require that device to be constantly active just to publish such
availability. The third case concerns setting the default state of
any person, service, or device in the absence of any device capable
of actively publishing such state. For instance, the presentity
might want to advertise that his or her voice service is currently
closed, just to let the watchers know that such service might be open
at some point. Again, this type of default state is independent of
any particular device and can be considered rather persistent.
Even though SIP PUBLISH remains the main way of publishing presence
state in SIMPLE-based presence systems and is especially well-suited
for publishing dynamic state (which presence mainly is), it needs to
be complemented by the mechanism described in this document to
address the use cases presented above.
XML Configuration Access Protocol (XCAP)  allows a client to read,
write, and modify application configuration data stored in XML format
on a server. The data has no expiration time, so it must be
explicitly inserted and deleted. The protocol allows multiple
clients to manipulate the data, provided that they are authorized to
do so. XCAP is already used in SIMPLE-based presence systems for
manipulation of presence lists and presence authorization policies.
This makes XCAP an ideal choice for doing device-independent presence
This document defines an XML Configuration Access Protocol (XCAP)
application usage for manipulating the contents of presence document.
Presence Information Document Format (PIDF)  is used as the
presence document format, since the event state compositor already
has to support it, as it is used in SIP PUBLISH.
Section 3 describes in detail how the presence document manipulated
with XCAP is related to soft state publishing done with SIP PUBLISH.
XCAP requires application usages to standardize several pieces of
information, including a unique application usage ID (AUID) and an
XML schema for the manipulated data. These are specified starting
from Section 4.
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  and
indicate requirement levels for compliant implementations.
Comprehensive terminology of presence and event state publishing is
provided in "Session Initiation Protocol (SIP) Extension for Event
State Publication" .
3. Relationship with Presence State Published Using SIP PUBLISH
The framework for publishing presence state is described in Figure 1.
A central part of the framework is the event state compositor
element, whose function is to compose presence information received
from several sources into a single coherent presence document.
The presence state manipulated with XCAP can be seen as one of the
information sources for the compositor to be combined with the soft
state information published using SIP PUBLISH. This is illustrated
in Figure 1. It is expected that, in the normal case, there can be
several PUAs publishing their separate views with SIP PUBLISH, but
only a single XCAP manipulated presence document. As shown in the
figure, multiple XCAP clients (for instance, in different physical
devices) can manipulate the same document on the XCAP server, but
this still creates only one input to the event state compositor. The
XCAP server stores the XCAP manipulated presence document under the
"users" tree in the XCAP document hierarchy. See Section 9 for
details and Section 11 for an example.
As individual inputs, the presence states set by XCAP and SIP PUBLISH
are completely separated, and it is not possible to directly
manipulate the state set by one mechanism with the other. How the
compositor treats XCAP-based inputs with respect to SIP PUBLISH-based
inputs is a matter of compositor policy, which is beyond the scope of
this specification. Since the SIP PUBLISH specification already
mandates the compositor to be able to construct the overall presence
state from multiple inputs, which may contain non-orthogonal (or in
some ways even conflicting) information, this XCAP usage does not
impose any new requirements on the compositor functionality.
| Event State | | Presence |<-- SIP SUBSCRIBE
| Compositor +---------+ Agent |--> SIP NOTIFY
| | | (PA) |
^ ^ ^
| | |
| | | +---------------+
+--------+ | +-------| XCAP server |
| | +-------+-------+
| | ^ ^
| SIP Publish | | XCAP |
| | | |
+--+--+ +--+--+ +-------+ +-------+
| PUA | | PUA | | XCAP | | XCAP |
| | | | | client| | client|
+-----+ +-----+ +-------+ +-------+
Figure 1: Framework for Presence Publishing and Event State
The protocol interface between XCAP server and the event state
compositor is not specified here.
4. Application Usage ID
XCAP requires application usages to define a unique application usage
ID (AUID) in either the IETF tree or a vendor tree. This
specification defines the 'pidf-manipulation' AUID within the IETF
tree, via the IANA registration in Section 13.
5. MIME Type
The MIME type for this application usage is 'application/pidf+xml'.
6. Structure of Manipulated Presence Information
The XML Schema of the presence information is defined in the Presence
Information Data Format (PIDF) . The PIDF also defines a
mechanism for extending presence information. See , , ,
and  for currently defined PIDF extensions and their XML Schemas.
The namespace URI for PIDF is 'urn:ietf:params:xml:ns:pidf' which is
also the XCAP default document namespace.
7. Additional Constraints
There are no constraints on the document beyond those described in
the XML schemas (PIDF and its extensions) and in the description of
8. Resource Interdependencies
There are no resource interdependencies beyond the possible
interdependencies defined in PIDF  and XCAP  that need to be
defined for this application usage.
9. Naming Conventions
The XCAP server MUST store only a single XCAP manipulated presence
document for each user. The presence document MUST be located under
the "users" tree, using filename "index". See an example in
10. Authorization Policies
This application usage does not modify the default XCAP authorization
policy, which allows only a user (owner) to read, write, or modify
their own documents. A server can allow privileged users to modify
documents that they do not own, but the establishment and indication
of such policies is outside the scope of this document.
The section provides an example of a presence document provided by an
XCAP Client to an XCAP Server. The presence document illustrates the
situation where a (human) presentity has left for vacation, and
before that, has set his presence information so that he is only
available via e-mail. In the absence of any published soft state
information, this would be the sole input to the compositor forming
the presence document. The example document contains PIDF extensions
specified in "RPID: Rich Presence Extensions to the Presence
Information Data Format (PIDF)"  and "CIPID: Contact Information
in Presence Information Data Format" .
It is assumed that the presentity is a SIP user with Address-of-
Record (AOR) sip:email@example.com. The XCAP root URI for
example.com is assumed to be http://xcap.example.com. The XCAP User
Identifier (XUI) is assumed to be identical to the SIP AOR, according
to XCAP recommendations. In this case, the presence document would
be located at http://xcap.example.com/pidf-manipulation/users/
The presence document is created with the following XCAP operation:
PUT /pidf-manipulation/users/sip:firstname.lastname@example.org/index HTTP/1.1
<?xml version="1.0" encoding="UTF-8"?>
<note>I'm available only by e-mail.</note>
<note>I'm reading mail a couple of times a week</note>
When the user wants to change the note related to e-mail service,
it is done with the following XCAP operation:
<note>I'm reading mails on Tuesdays and Fridays</note>
12. Security Considerations
A presence document may contain information that is highly sensitive.
Its delivery to watchers needs to happen strictly according to the
relevant authorization policies. It is also important that only
authorized clients are able to manipulate the presence information.
The XCAP base specification mandates that all XCAP servers MUST
implement HTTP Digest authentication specified in RFC 2617 .
Furthermore, XCAP servers MUST implement HTTP over TLS . It is
recommended that administrators of XCAP servers use an HTTPS URI as
the XCAP root services' URI, so that the digest client authentication
occurs over TLS. By using these means, XCAP client and server can
ensure the confidentiality and integrity of the XCAP presence
document manipulation operations, and that only authorized clients
are allowed to perform them.
13. IANA Considerations
There is an IANA consideration associated with this specification.
13.1. XCAP Application Usage ID
This section registers a new XCAP Application Usage ID (AUID)
according to the IANA procedures defined in .
Name of the AUID: pidf-manipulation
Description: Pidf-manipulation application usage defines how XCAP is
used to manipulate the contents of PIDF-based presence documents.
The authors would like to thank Jari Urpalainen, Jonathan Rosenberg,
Hisham Khartabil, Aki Niemi, Mikko Lonnfors, Oliver Biot, Alex Audu,
Krisztian Kiss, Jose Costa-Requena, George Foti, and Paul Kyzivat for
15.1. Normative References
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Rosenberg, J., "The Extensible Markup Language (XML)
Configuration Access Protocol (XCAP)", RFC 4825, May 2007.
 Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and
J. Peterson, "Presence Information Data Format (PIDF)",
RFC 3863, August 2004.
 Niemi, A., "Session Initiation Protocol (SIP) Extension for
Event State Publication", RFC 3903, October 2004.
 Franks, J., "HTTP Authentication: Basic and Digest Access
Authentication", RFC 2617, June 1999.
 Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
15.2. Informative References
 Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.
 Schulzrinne, H., Gurbani, V., Kyzivat, P., and J. Rosenberg,
"RPID: Rich Presence Extensions to the Presence Information
Data Format (PIDF)", RFC 4480, July 2006.
 Schulzrinne, H., "CIPID: Contact Information for the Presence
Information Data Format", RFC 4482, July 2006.
 Rosenberg, J., "A Data Model for Presence", RFC 4479,
 Lonnfors, M. and K. Kiss, "Session Initiation Protocol (SIP)
User Agent Capability Extension to Presence Information Data
Format (PIDF)", Work in Progress, July 2006.
 Schulzrinne, H., "Timed Presence Extensions to the Presence
Information Data Format (PIDF) to Indicate Status Information
for Past and Future Time Intervals", RFC 4481, July 2006.
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