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SIMPLE WG M. Lonnfors
Internet-Draft A. Niemi
Expires: April 24, 2005 Nokia Research Center
E. Leppanen
Nokia
October 24, 2004
Partial Publication of Presence Information
draft-ietf-simple-partial-publish-01
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
Session Initiation Protocol (SIP) Extension for Event State
Publication describes a mechanism with which a presence user agent is
able to publish presence information to a presence agent. Using the
Presence Information Data Format (PIDF), each presence publication
contains full state, regardless of how much of that information has
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actually changed since the previous update. As a consequence,
updating a sizeable presence document with small changes bear a
considerable overhead and is therefore inefficient. Especially with
low bandwidth and high latency links, this can constitue a
considerable burden to the system. This memo defines a solution that
aids in reducing the impact of those constraints and increases
transport efficiency by introducing a mechanism that allows for
publication of partial presence information.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions and Document Conventions . . . . . . . . . . . . . 3
3. Overall Operation . . . . . . . . . . . . . . . . . . . . . . 3
3.1 Presence Publication . . . . . . . . . . . . . . . . . . . 4
3.2 Partial Presence Publication . . . . . . . . . . . . . . . 4
4. Client and Server Operation . . . . . . . . . . . . . . . . . 5
4.1 Content-type for Partial Publications . . . . . . . . . . 5
4.2 Generation of Partial Publications . . . . . . . . . . . . 5
4.3 Processing of Partial Publications . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1 Normative references . . . . . . . . . . . . . . . . . . . . 7
7.2 Informative references . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . 10
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1. Introduction
Session Initiation Protocol (SIP) Extension for Event State
Publication [3] allows Presence User Agents ('PUA') to publish
presence information of a user ('presentity'). The Presence Agent
('PA') collects publications from one or several presence user
agents, and generates the composite event state of the presentity.
The baseline format for presence information is defined in the
Presence Information Data Format (PIDF) [4] and is by default used in
presence publication. The PIDF uses Extensible Markup Language (XML)
[12], and groups data into elements called tuples. In addition, [8],
[9], [10], and [11] define extension elements that provide various
additional features to PIDF.
Presence publication by default uses the PIDF document format, and
each publication contains full state regardless of how much of the
presence information has actually changed since the previous update.
As a consequence, updating a sizeable presence document especially
with small changes bears a considerable overhead and is therefore
inefficient. Publication of information over low bandwidth and high
latency links further exacerbates this inefficiency.
This memo specifies a mechanism with which the PUA is able to publish
only those parts of the presence document that have changed since the
previous update. This is accomplished using the partial PIDF format
[2].
2. Definitions and Document Conventions
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 [1]and
indicate requirement levels for compliant implementations.
This document makes use of the vocabulary defined in RFC 2778 [6],
the State Publication Extension to SIP [3], and PIDF Extension to
Partial Presence [2].
3. Overall Operation
This section introduces the baseline functionality for presence
publication, and gives an overview of the partial publication
mechanism. This section is informational in nature. It does not
contain any normative statements.
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3.1 Presence Publication
Event state publication is specified in [3].
The publication of presence information consists of a presence user
agent sending a PUBLISH request targeted to the address-of-record of
the presentity, and serviced by a presence agent or compositor. The
body of the PUBLISH request carries full event state in the form of a
presence document.
The compositor processes the PUBLISH request and stores the presence
information. It also assigns an entity-tag that is used to identify
the publication. This entity-tag is returned to the PUA in the
response to the PUBLISH request.
The PUA uses the entity-tag in the following PUBLISH request for
identifying the publication that the request is meant to refresh,
modify or remove. Presence information is stored in an initial
publication, and maintained using the refreshing and modifying
publications. Presence information disappears either by expilicitly
removing it or when it meets its expiration time.
3.2 Partial Presence Publication
The partial publication mechanism enables the PUA to update only
parts of its presence information, namely those sections of the
presence document that have changed. The initial publication always
carries full state. However, modifying publications that update the
initial presence state only carry partial state. Versioning of the
partial publications guarantees that the changes are applied in the
correct order. Even though the PUBLISH mechanism in itself already
accomplishes this using entity-tags, versioning is important in case
the updates traverse a gateway into a system without such guarantees.
To initialize its publications, the PUA first publishes a full state
initial publication using the PIDF document format. The consequent
updates result in the publication of partial presence state, using
the 'application/pidf-partial+xml' content type [2]. The partial
state may contain operations for adding new elements or attributes
(<add> elements), replacing elements or attributes whose content has
changed (<replace> elements) , and it may also indicate removal of
certain elements or attributes (<remove> elements). The PUA is free
to decide the granularity in which changes in presence information
are reported to the composer.
When the presence composer receives a partial publication it performs
the included operations in sequence. The resulting changed presence
document is then submitted to the composition logic in the same
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manner as with a full state presence publication.
4. Client and Server Operation
Unless otherwise specified in this document, the presence user agent
and presence agent behavior are as defined in the Event State
Publication Extension to SIP [3].
4.1 Content-type for Partial Publications
The entities supporting the partial publication extension described
in this document MUST support the 'application/pidf-diff+xml' content
type defined in [2] in addition to the baseline
'application/pidf+xml' content type defined in [4].
4.2 Generation of Partial Publications
Whenever a PUA decides to begin publication of presence information,
it first needs to make an initial publication. After the initial
publication, presence information can be updated using modifying
publications, using the partial presence document format of [2].
Finally, the publication can be terminated by explicit removal, or by
expiration.
To construct an initial publication, the PUA uses the following
logic:
o The Content-Type header field in the PUBLISH request MUST be set
to the value 'application/pidf+xml'
o The body of the request is populated with a PIDF document
containing the full state of which the PUA is aware.
o The version number local to the PUA MUST be initialized, i.e., set
to zero.
OPEN ISSUE: Should the initial publication also contain pidf-diff
instead of plain pidf? The major difference is in whether the PUA
falls back to normal behavior before or after the initial
publication. If a full publication using the pidf-diff is rejected
with 415, the PUA ends up sending the same data over again. Not a
major issue, but all other things being equal, seems like a reason to
use pidf in initial publications.
To construct a modifying PUBLISH request the following logic is
followed:
o The Content-Type header field in the PUBLISH request MUST be set
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to the value 'application/pidf-diff+xml'
o The value of the "version" attribute MUST be set to the local
version value incremented by one.
o The "entity" attribute MUST be set to the same value as in the
initial publication.
o The events that have changed the presentity's presence information
MUST be constructed into operations in sequence, as defined in
[2].
The PUA is free to decide the granularity by which changes in the
presentity's presence information are reported to the presence
compositor. In order to reduce unnecessary network traffic, the PUA
SHOULD be able to batch several partial publications together. For
example, a reasonable granularity would be to batch events related to
a single UI event together in a single PUBLISH request.
If a modifying publication carrying partial presence information is
not understood by the presence composer, it will reject the request
with a 415 (Unsupported Media Type). If the PUA receives a 415
reponse, it MUST fall back to full state presence updates. To find
out whether a specific presence compositor supports partial presence
publication, the PUA MAY use the OPTIONS method, as described in [5].
4.3 Processing of Partial Publications
Processing of publications generally follows the guidelines set in
[3]. In addition, processing modifying PUBLISH requests, the
following logic is followed:
o If the value of the Content-Type header field is
'application/pidf-diff+xml', the publication is partial, and the
next steps apply.
o The compositor MUST apply the partial publication operations in
sequence against its locally stored presence information.
o If any errors are encountered before the entire partial
publication is completely processed, the compositor MUST reject
the request with a 500 (Server Internal Error), and revert back to
its locally stored presence information.
OPEN ISSUE: Anything else needed here? This is pretty straight
forward, since the PUBLISH mechanism prohibits overlapping
publications and guarantees in-order delivery using entity-tags.
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5. Security Considerations
This specification relies on protocol behavior defined in [3].
General event state publication related security considerations are
extensively discussed in that specification and all the identified
security considerations apply to this document in entirety. In
addition, this specification adds no new security considerations.
6. Acknowledgements
The authors would like to thank Atle Monrad, Christian Schmidt and
George Foti for review comments.
7. References
7.1 Normative references
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Lonnfors, M., Leppanen, E. and H. Khartabil, "Presence
Information Data format (PIDF) Extension for Partial Presence",
draft-ietf-simple-partial-pidf-format-01 (work in progress),
April 2004.
[3] Niemi, A., "An Event State Publication Extension to the Session
Initiation Protocol (SIP)", draft-ietf-sip-publish-04 (work in
progress), May 2004.
[4] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W. and
J. Peterson, "Presence Information Data Format (PIDF)", RFC
3863, August 2004.
[5] 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.
7.2 Informative references
[6] Day, M., Rosenberg, J. and H. Sugano, "A Model for Presence and
Instant Messaging", RFC 2778, February 2000.
[7] Campbell, B., "SIMPLE Presence Publication Requirements",
draft-ietf-simple-publish-reqs-00 (work in progress), February
2003.
[8] Rosenberg, J., "A Data Model for Presence",
draft-ietf-simple-presence-data-model-00 (work in progress),
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September 2004.
[9] Schulzrinne, H., Gurbani, V., Kyzivat, P. and J. Rosenberg,
"RPID: Rich Presence: Extensions to the Presence Information
Data Format (PIDF)", draft-ietf-simple-rpid-03 (work in
progress), March 2004.
[10] Schulzrinne, H., "CIPID: Contact Information in Presence
Information Data Format", draft-ietf-simple-cipid-03 (work in
progress), July 2004.
[11] Lonnfors, M. and K. Kiss, "User agent capability presence
status extension", draft-ietf-simple-prescaps-ext-01 (work in
progress), May 2004.
[12] Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler,
"Extensible Markup Language (XML) 1.0 (2nd ed)", W3C REC-xml,
October 2000, <http://www.w3.org/TR/REC-xml>.
Authors' Addresses
Mikko Lonnfors
Nokia Research Center
Itamerenkatu 11-13
Helsinki
Finland
Phone: +358 71 8008000
EMail: mikko.lonnfors@nokia.com
Aki Niemi
Nokia Research Center
P.O. Box 407
NOKIA GROUP, FIN 00045
Finland
Phone: +358 50 389 1644
EMail: aki.niemi@nokia.com
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Eva Leppanen
Nokia
P.O BOX 785
Tampere
Finland
EMail: eva-maria.leppanen@nokia.com
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