SIMPLE WG A. Niemi
Internet-Draft Nokia Research Center
Expires: September 7, 2006 M. Lonnfors
E. Leppanen
Nokia
March 6, 2006
Publication of Partial Presence Information
draft-ietf-simple-partial-publish-04
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
The 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
actually changed since the previous update. As a consequence,
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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 . . . . . . . . . . . . . . . . . . . . . . 4
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 . . . . . . . . . . . . 6
4.3. Processing of Partial Publications . . . . . . . . . . . . 7
4.3.1. Processing <pidf-full> . . . . . . . . . . . . . . . . 8
4.3.2. Processing <pidf-diff> . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative references . . . . . . . . . . . . . . . . . . . 13
8.2. Informative references . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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1. Introduction
Session Initiation Protocol (SIP) Extension for Event State
Publication [RFC3903] 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) [RFC3863] and is by default
used in presence publication. The PIDF uses Extensible Markup
Language (XML) [W3C.REC-xml], and groups data into elements called
tuples. In addition, [I-D.ietf-simple-presence-data-model],
[I-D.ietf-simple-rpid], [I-D.ietf-simple-cipid], [I-D.ietf-simple-
future] and [I-D.ietf-simple-prescaps-ext] 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 after an
initial full state publication able to publish only those parts of
the presence document that have changed since the previous update.
This is accomplished using the partial PIDF [I-D.ietf-simple-partial-
pidf-format] document format to communicate a set of presence
document changes to the PA, who then applies the changes in sequence
to its version of the presence document.
This memo is structured in the following way: Section 3 gives an
overview of the partial publication mechanism; Section 4 includes the
detailed specification; Section 5 includes discussion of security
considerations; and Section 6 includes examples of partial
publication.
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, BCP 14
[RFC2119], and indicate requirement levels for compliant
implementations.
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This document makes use of the vocabulary defined in the Model for
Presence and Instant Messaging [RFC2778], the Event State Publication
Extension to SIP [RFC3903], and the PIDF Extension for Partial
Presence [I-D.ietf-simple-partial-pidf-format].
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.
3.1. Presence Publication
Event state publication is specified in [RFC3903].
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, successive modifying publications to
this initial presence state can communicate state deltas, i.e., one
or more changes to the presence information since the previous
update. Versioning of these partial publications is necessary to
guarantee that the changes are applied in the correct order. The
PUBLISH method [RFC3903] already accomplishes this using entity-tags
and conditional requests, which guarantee correct ordering of
publication updates.
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Note that the partial PIDF format [I-D.ietf-simple-partial-pidf-
format] contains the 'version' attribute that could be used for
versioning as well. However, we chose not to introduce an
additional versioning mechanism to partial publish, since that
would only add ambiquity and a potential, undefined error case
were the two versioning mechanisms to somehow contradict.
To initialize its publication of presence information, the PUA first
publishes a full state initial publication. The consequent modifying
publications can carry either state deltas or full state. Both
initial and modifying partial presence publications are accomplished
using the 'application/pidf-diff+xml' content type [I-D.ietf-simple-
partial-pidf-format], with the former using the <pidf-full> root
element, and the latter using the <pidf-diff> or <pidf-full> root
elements, respectively.
While the <pidf-full> encapsulates a regular PIDF document, the
<pidf-diff> can contain one or more operations for adding new
elements or attributes (<add> elements), replacing elements or
attributes whose content has changed (<replace> elements), or
indications of removal of certain elements or attributes (<remove>
elements). The PUA is free to decide the granularity by which
changes in presence information are communicated to the composer. It
may very well happen that there are enough changes to be communicated
that it is more efficient to send a full state publication instead of
a set of state deltas.
When the presence compositor receives a partial publication it
applies the included patch operations in sequence. The resulting
changed (or patched) presence document is then submitted to the
composition logic in the same manner as with a full state presence
publication. Similarly, any changes to the publication expiration
apply to the full, patched presence publication. In other words,
there is no possibility to roll back to an earlier version, except by
submitting a full state publication.
4. Client and Server Operation
Unless otherwise specified in this document, the presence user agent
and presence agent behavior are as defined in [RFC3903].
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 the partial PIDF format [I-D.ietf-simple-partial-
pidf-format], in addition to the baseline 'application/pidf+xml'
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content type defined in [RFC3863].
4.2. Generation of Partial Publications
Whenever a PUA decides to begin publication of partial presence
information, it first needs to make an initial publication. This
intial publication always carries full state. After the initial
publication, presence information can be updated using modifying
publications; the modifications can carry state deltas as well as
full state. Finally, the publication can be terminated by explicit
removal, or by expiration.
Both the initial and modifying publications make use of the partial
presence document format [I-D.ietf-simple-partial-pidf-format], and
all follow the normal rules for creating publications, as defined in
RFC 3903 [RFC3903], Section 4.
In addition, to construct an initial publication, the PUA uses the
following process:
o The Content-Type header field in the PUBLISH request MUST be set
to the value 'application/pidf-diff+xml'.
o The document in the body of the request is populated with a <pidf-
full> root element that includes the 'entity' attribute set to
identify the presentity.
o Under the <pidf-full> root element exists all of the children of a
PIDF [RFC3863] <presence> element. This document contains the
full state of which the PUA is aware, and MAY include elements
from any extension namespace.
If the initial PUBLISH request returns a 415 (Unsupported Media
Type), it means that the compositor did not understand the partial
publication format. In this case, the PUA MUST follow normal
precedures for handling a 400-class response, as specified in Section
8.1.3.5 of [RFC3261]. In addition, to find out a priori whether a
specific presence compositor supports partial presence publication,
the PUA MAY use the OPTIONS method, as described in [RFC3261].
To construct a partial modifying publication the following process is
followed:
o The Content-Type header field in the PUBLISH request MUST be set
to the value 'application/pidf-diff+xml'
o The document in the body of the request is populated with a <pidf-
diff> root element that includes the 'entity' attribute
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identifying the presentity.
o Under the <pidf-diff> root element exists a set of patch
operations that communicate the changes to the presentity's
presence information. These operations MUST be constructed in
sequence, and as defined in the partial PIDF format [I-D.ietf-
simple-partial-pidf-format].
The PUA is free to decide the granularity by which changes in the
presentity's presence information are communicated to the presence
compositor. In order to reduce unnecessary network traffic, the PUA
SHOULD batch several patch operations in a single PUBLISH request.
A reasonable granularity might be to batch state changes resulting
from related UI events together in a single PUBLISH request. For
example, when the user sets their status to "Away", several things
including freetext notes, service availability, and activities
might change as a result.
If the size of the delta state becomes more than the size of the full
state, the PUA SHOULD instead send a modifying publication carrying
full state, unless this size comparison is not possible.
To an implementation that generates state deltas directly out of
its internal events, it may not be trivial to determine the size
of the corresponding full state.
To construct a full state modifying publication, the following
process is followed:
o The Content-Type header field in the PUBLISH request MUST be set
to the value 'application/pidf-diff+xml'.
o The document in the body of the request is populated with a <pidf-
full> root element that includes the 'entity' attribute set to
identify the presentity.
o Under the <pidf-full> root element exists all of the children of a
PIDF [RFC3863] <presence> element. This document contains the
full state of which the PUA is aware, and MAY include elements
from any extension namespace.
4.3. Processing of Partial Publications
For each resource, the compositor maintains a record for each of the
publications. These are indexed using the entity-tag of the
publications.
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Processing of publications generally follows the guidelines set in
[RFC3903]. In addition, processing PUBLISH requests that contain
'application/pidf-diff+xml' require some extra processing that is
dependant on whether the request contains full or partial state.
4.3.1. Processing <pidf-full>
If the value of the Content-Type header field is 'application/
pidf-diff+xml', and the document therein contains a <pidf-full> root
element, the publication contains full presence information, and the
next step applies:
o The compositor MUST take the received presence document under the
<pidf-full> as the local presence document, replacing any previous
publications.
If any errors are encountered before the entire publication is
completely processed, the compositor MUST reject the request with a
500 (Server Internal Error) response, and revert back to its
original, locally stored presence information.
4.3.2. Processing <pidf-diff>
If the value of the Content-Type header field is 'application/
pidf-diff+xml', and the document in the body contains a <pidf-diff>
root element, the publication contains partial presence information
(state delta), and the next steps apply:
o If the publication containing the <pidf-diff> root element is a
modifying publication (i.e., contains an If-Match header field
with a valid entity-tag), the compositor MUST apply the included
patch operations in sequence against its locally stored presence
document.
o Else, the publication is an initial publication, for which only
<pidf-full> is allowed. Therefore the publication MUST be
rejected with an appropriate error response, such as a 400
(Invalid Partial Publication).
If a publication carrying partial presence information expires
without the PUA refreshing it, the compositor MUST clear the entire,
full state publication.
This means that the compositor does not keep a record of the
applied patches, and consequently (unlike some versioning
systems), the compositor does not roll back to an earlier version
if a particular partial publication were to expire.
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If any errors are encountered before the entire partial publication
is completely processed, including all of the patch operations in the
'application/pidf-diff+xml' body, the compositor MUST reject the
request with a 500 (Server Internal Error) response, and revert back
to its original, locally stored presence information.
5. Security Considerations
This specification relies on protocol behavior defined in [RFC3903].
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. Examples
The following message flow (Figure 1) shows an example of a presence
system that applies the partial publication mechanism.
First, the PUA sends an initial publication that contains full state.
In return, it receives a 200 OK response containing an entity-tag.
This entity-tag serves as a reference with which the initial full
state can be updated using partial publications containing state
deltas.
Then at some point the resource state changes, and the PUA assembles
these changes into a set of patch operations. It then sends a
modifying publication containing the patch operations, using the
entity-tag as a reference to the publication against which the
patches are to be applied. The compositor applies the received patch
operations to its local presence document in sequence, and returns a
200 OK which includes a new entity-tag.
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Presence Agent /
PUA Compositor
| (M1) PUBLISH |
|---------------------------->|
| (M2) 200 OK |
|<----------------------------|
| |
| |
| |
| (M3) PUBLISH |
|---------------------------->|
| (M4) 200 OK |
|<----------------------------|
| |
_|_ _|_
Figure 1: Partial Publication Message Flow
Message details:
(M1): PUA -> Compositor
PUBLISH sip:resource@example.com SIP/2.0
...
Event: presence
Expires: 3600
Content-Type: application/pidf-diff+xml
Content-Length: [Replace with real content length]
<?xml version="1.0" encoding="UTF-8"?>
<p:pidf-full xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:p="urn:ietf:params:xml:ns:pidf-diff"
xmlns:r="urn:ietf:params:xml:ns:pidf:rpid"
xmlns:c="urn:ietf:params:xml:ns:pidf:caps"
entity="pres:someone@example.com">
<tuple id="sg89ae">
<status>
<basic>open</basic>
<r:relationship>assistant</r:relationship>
</status>
<c:servcaps>
<c:audio>true</c:audio>
<c:video>false</c:video>
<c:message>true</c:message>
</c:servcaps>
<contact priority="0.8">tel:09012345678</contact>
</tuple>
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<tuple id="cg231jcr">
<status>
<basic>open</basic>
</status>
<contact priority="1.0">im:pep@example.com</contact>
</tuple>
<tuple id="r1230d">
<status>
<basic>closed</basic>
<r:activity>meeting</r:activity>
</status>
<r:homepage>http://example.com/~pep/</r:homepage>
<r:icon>http://example.com/~pep/icon.gif</r:icon>
<r:card>http://example.com/~pep/card.vcd</r:card>
<contact priority="0.9">sip:pep@example.com</contact>
</tuple>
<note xml:lang="en">Full state presence document</note>
<r:person>
<r:status>
<r:activities>
<r:on-the-phone/>
<r:busy/>
</r:activities>
</r:status>
</r:person>
<r:device id="urn:esn:600b40c7">
<r:status>
<c:devcaps>
<c:mobility>
<c:supported>
<c:mobile/>
</c:supported>
</c:mobility>
</c:devcaps>
</r:status>
</r:device>
</p:pidf-full>
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(M2): Compositor -> PUA
SIP/2.0 200 OK
...
SIP-ETag: 61763862389729
Expires: 3600
Content-Length: 0
(M3): PUA -> Compositor
PUBLISH sip:resource@example.com SIP/2.0
...
Event: presence
SIP-If-Match: 61763862389729
Expires: 3600
Content-Type: application/pidf-diff+xml
Content-Length: [Replace with real content length]
<?xml version="1.0" encoding="UTF-8"?>
<p:pidf-diff xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:p="urn:ietf:params:xml:ns:pidf-diff"
xmlns:r="urn:ietf:params:xml:ns:pidf:rpid"
entity="pres:someone@example.com">
<p:add sel="presence/note" pos="before"><tuple id="ert4773">
<status>
<basic>open</basic>
</status>
<contact priority="0.4">mailto:pep@example.com</contact>
<note xml:lang="en">This is a new tuple inserted
between the last tuple and note element</note>
</tuple>
</p:add>
<p:replace sel="*/tuple[@id='r1230d']/status/basic/text()"
>open</p:replace>
<p:remove sel="*/r:person/r:status/r:activities/r:busy"/>
<p:replace sel="*/tuple[@id='cg231jcr']/contact/@priority"
>0.7</p:replace>
</p:pidf-diff>
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(M4): Compositor -> PUA
SIP/2.0 200 OK
...
SIP-ETag: 18764920981476
Expires: 3600
Content-Length: 0
7. Acknowledgements
The authors would like to thank Atle Monrad, Christian Schmidt,
George Foti, Fridy Sharon-Fridman and Avshalom Houri for review
comments.
8. References
8.1. Normative references
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3903] Niemi, A., "Session Initiation Protocol (SIP) Extension
for Event State Publication", RFC 3903, October 2004.
[RFC3863] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr,
W., and J. Peterson, "Presence Information Data Format
(PIDF)", RFC 3863, August 2004.
[RFC3261] 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.
[I-D.ietf-simple-partial-pidf-format]
Lonnfors, M., "Presence Information Data format (PIDF)
Extension for Partial Presence",
draft-ietf-simple-partial-pidf-format-05 (work in
progress), October 2005.
8.2. Informative references
[RFC2778] Day, M., Rosenberg, J., and H. Sugano, "A Model for
Presence and Instant Messaging", RFC 2778, February 2000.
[I-D.ietf-simple-publish-reqs]
Campbell, B., "SIMPLE Presence Publication Requirements",
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draft-ietf-simple-publish-reqs-00 (work in progress),
February 2003.
[I-D.ietf-simple-presence-data-model]
Rosenberg, J., "A Data Model for Presence",
draft-ietf-simple-presence-data-model-07 (work in
progress), January 2006.
[I-D.ietf-simple-rpid]
Schulzrinne, H., "RPID: Rich Presence Extensions to the
Presence Information Data Format (PIDF)",
draft-ietf-simple-rpid-10 (work in progress),
December 2005.
[I-D.ietf-simple-future]
Schulzrinne, H., "Timed Presence Extensions to the
Presence Information Data Format (PIDF) to Indicate
Status Information for Past and Future Time Intervals",
draft-ietf-simple-future-05 (work in progress),
December 2005.
[I-D.ietf-simple-cipid]
Schulzrinne, H., "CIPID: Contact Information in Presence
Information Data Format", draft-ietf-simple-cipid-07 (work
in progress), December 2005.
[I-D.ietf-simple-prescaps-ext]
Lonnfors, M. and K. Kiss, "Session Initiation Protocol
(SIP) User Agent Capability Extension to Presence
Information Data Format (PIDF)",
draft-ietf-simple-prescaps-ext-06 (work in progress),
January 2006.
[W3C.REC-xml]
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>.
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Authors' Addresses
Aki Niemi
Nokia Research Center
P.O. Box 407
NOKIA GROUP, FIN 00045
Finland
Phone: +358 50 389 1644
Email: aki.niemi@nokia.com
Mikko Lonnfors
Nokia
Itamerenkatu 11-13
Helsinki
Finland
Phone: +358 71 8008000
Email: mikko.lonnfors@nokia.com
Eva Leppanen
Nokia
P.O BOX 785
Tampere
Finland
Email: eva-maria.leppanen@nokia.com
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