Network Working Group M. Lonnfors

Network Working Group M. Lonnfors
Request for Comments: 5263 J. Costa-Requena
Category: Standards Track E. Leppanen
Nokia
H. Khartabil
Ericsson
September 2008

Session Initiation Protocol (SIP) Extension for
Partial Notification of Presence Information

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.

Abstract

By default, presence delivered using the presence event package for
the Session Initiation Protocol (SIP) is represented in the Presence
Information Data Format (PIDF). A PIDF document contains a set of
elements, each representing a different aspect of the presence being
reported. When any subset of the elements change, even just a single
element, a new document containing the full set of elements is
delivered. This memo defines an extension allowing delivery of only
the presence data that has actually changed.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Introduction to the Partial Notification Mechanism . . . . . . 3
3.1. Basic Presence Agent Operation . . . . . . . . . . . . . . 3
3.2. Operation with Partial Notification . . . . . . . . . . . 3
4. Client and Server Operations . . . . . . . . . . . . . . . . . 4
4.1. Content-Type for Partial Notifications . . . . . . . . . . 4
4.2. Watcher Generation of SUBSCRIBE Requests . . . . . . . . . 4
4.3. Presence Agent Processing of SUBSCRIBE Requests . . . . . 4
4.4. Presence Agent Generation of Partial Notifications . . . . 5
4.5. Watcher Processing of NOTIFY Requests . . . . . . . . . . 6
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . . 14

1. Introduction

A presence event package for Session Initiation Protocol (SIP) [3]
allows users ('watchers') to subscribe to other users'
('presentities') presence information. The presence information is
composed of multiple pieces of data that are delivered to the
watcher. The size of the presence information document can be large
(i.e., the presence document can contain an arbitrary number of
elements called presence tuples that convey data). As specified in
RFC 2778 [9] and the presence event package for SIP [3], a Presence
Agent (PA) always delivers in presence notifications all the presence
data that has been authorized for a certain watcher. This is done
regardless of what presence data has changed compared to last
notification. It may not be reasonable to send the complete presence
information over low bandwidth and high latency links when only part
of the presence information has actually changed. This may end up
degrading the presence service and causing bad perception at the
watcher side.

This document defines a partial notification approach where the
presence server delivers to the watchers only those parts of the
presence information that have changed compared to the presence
information sent in the previous notifications. This reduces the
amount of data that is transported over the network.

This mechanism utilizes the presence event package for SIP [3] and a
new MIME type for carrying partial Presence Information Data Format
documents [2].

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2. 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 [9],
RFC 3265 [6], the presence event package for SIP [3], and the PIDF
extension for Partial Presence [2].

3. Introduction to the Partial Notification Mechanism

This chapter briefly introduces the regular functionality of the
presence service, and gives an overview of the partial notification
solution. This section is informational in nature. It does not
contain any normative statements.

3.1. Basic Presence Agent Operation

The presence service normally operates so that a watcher sends a SIP
SUBSCRIBE request targeted to the presentity. The request is routed
to the presence agent where the presentity's presence information is
stored. The SUBSCRIBE request can include an Accept header field
that indicates the supported content types.

The presence agent receives the SUBSCRIBE request and if there is no
Accept header indicating the supported content types or the Accept
header contains the default PIDF content type, the PA will generate
presence notifications using the default PIDF format [5]. The PIDF
document can contain one or multiple XML elements. The PIDF document
includes a set of elements defined in RFC 2778 [9], and its
extensions for representing the presence information. This PIDF
document will be carried in the body of a NOTIFY request constructed
as per RFC 3265 [6]. During basic operation, the presence document
always contains the full state corresponding to the presence status
of the presentity, as determined by the PA local policy and
authorization rules.

3.2. Operation with Partial Notification

The partial notification mechanism allows a watcher to request that,
whenever possible, notifications contain only presence information
that has actually changed. A watcher that wants to receive partial
notifications according to this document, creates a SIP SUBSCRIBE
request similar to that of a regular presence subscription. However,
the SIP SUBSCRIBE request contains an Accept header field whose value

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contains the "application/pidf-diff+xml" tag as well as the
"application/pidf+xml" tag.

When the presence agent receives the subscription, it examines the
Accept header field value and if the "application/pidf-diff+xml"
value is present, it can decide to use the partial notifications
mechanism specified in this memo. The presence agent builds NOTIFY
requests that contain the Content-Type header field set to
"application/pidf-diff+xml". The first NOTIFY request that contains
presence information will contain a full presence document.
Subsequent NOTIFY requests can contain partial presence documents.
This behavior is described in detail in Section 4.

4. Client and Server Operations

Unless otherwise specified in this document, the regular watcher and
presence agent behavior is applied as defined in the SIP presence
event package [3].

4.1. Content-Type for Partial Notifications

Entities supporting the partial notification extension described in
this document MUST support the 'application/pidf-diff+xml' content
type specified in the PIDF extension for partial presence [2].

4.2. Watcher Generation of SUBSCRIBE Requests

A SUBSCRIBE request can be used to negotiate the preferred content
type to be used in the notifications. The Accept header field is
used for this purpose as specified in RFC 3261 [4]. When a watcher
wants to allow the presence agent to send partial notifications the
watcher MUST include an Accept header field in its SUBSCRIBE request.
The value of the Accept header field MUST contain 'application/
pidf-diff+xml' (in addition to 'application/pidf+xml' required by the
SIP presence event package [3]). The watcher MAY include a "q"
parameter with each Accept value to indicate the relative preference
of that value.

4.3. Presence Agent Processing of SUBSCRIBE Requests

The presence agent receives subscriptions from watchers and generates
notifications according to the SIP presence event package [3]. If
the watcher has indicated the supported content types in the Accept
header field of the SUBSCRIBE request, the presence agent compares
the values included in the Accept header field with the supported
ones, and decides which one to use. If the watcher has indicated
preferred accept values by means of "q" parameters, the presence

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agent SHOULD base the decision on those preferences, unless otherwise
indicated by the presence agent local policy.

4.4. Presence Agent Generation of Partial Notifications

Once a subscription is accepted and installed, the PA MUST deliver
the full state of the presence information in the first partial
notification that contains a presence document having <pidf-full>
root element. If the presence agent decides to send notifications
that include a presence document according to this specification, the
presence agent MUST build a presence document according to the PIDF
extension for Partial Presence [2] and MUST set the Content-Type
header field to the value 'application/pidf-diff+xml'.

When using the 'application/pidf-diff+xml' MIME type, the PA MUST
include a "version" attribute; for the first partial notification
(within a given subscription), the PA MUST initialize version to
value one (1). This version counter is scoped to the subscription,
and is incremented by one within each partial notification. The
version value is only reset when the given subscription is
terminated. It is not reset when the subscription is refreshed.

When the PA generates a partial presence document, the PA SHOULD
include only that presence information that has changed compared to
the previous notifications. It is up to the PA's local policy to
determine what is considered as a change to the previous state.

The PA MUST construct the partial presence document according to the
following logic:

o The PA MUST construct the presence information according to the
PIDF extension for Partial Presence [2]. All the information that
has been added to the presence document is listed inside <add>
elements. All information that has been removed from the presence
document is listed inside <remove> elements, and all information
that has been changed is listed under <replace> elements.

o The PA MUST include a "version" attribute in the presence
document. The PA MUST increment the version number by one
compared to the earlier successfully sent presence document in the
PIDF extension for Partial Presence [2] format to the watcher
associated with a certain subscription.

The PA MUST NOT send a new NOTIFY request that contains a partial
notification for the same Request-URI until it has received a final
response from the watcher for the previous one or the previous NOTIFY
request has timed out.

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When the PA receives a SUBSCRIBE request (refresh or termination)
within the associated subscription, it SHOULD send a NOTIFY request
containing the full presence document.

If the PA has used other than the 'application/pidf-diff+xml' content
type in notifications within the existing subscription, and changes
to deliver partial notifications, the PA MUST deliver the full state
of the presence information containing a presence document having
<pidf-full> root element as the first partial notification.

4.5. Watcher Processing of NOTIFY Requests

Watcher processes all NOTIFY requests that contain 'application/
pidf+xml' content type as specified in RFC 3856 [3].

When the watcher receives the first notification containing the
'application/pidf-diff+xml' MIME body the watcher MUST initialize an
internal version counter, related to this subscription, to the value
of the "version" included in the presence document. This version
counter is scoped to the subscription. The watcher MUST also store
the received full presence document as its local copy.

When the watcher receives a subsequent 'application/pidf-diff+xml'
encoded presence document the watcher MUST compare the received
"version" attribute with the local version counter. If the watcher
receives a presence document with the "version" attribute value equal
or lower than the locally stored version number, it is considered a
PA failure, and the watcher SHOULD discard the document without
further processing. Otherwise, the watcher MUST modify its locally
stored information according to the following logic:

o If the root element of the presence document is <pidf-full>, the
watcher must replace its local copy of the presence document with
the presence document received in the 'application/pidf-diff+xml'
body and set the internal version value to the value of the
"version" attribute included in the presence document.

o If the root element of the presence document is <pidf-diff> and
the received version number is incremented by one compared with
the local version counter, the watcher MUST apply the changes to
its local copy of the full presence document indicated in the
received 'application/pidf-diff+xml' document as specified in PIDF
extension for Partial Presence [2]. The watcher MUST increment
the local version counter by one.

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o If the root element of the presence document is <pidf-diff> and
the received "version" value is higher by more than one compared
with the locally stored value, the watcher assumes that one or
more NOTIFYs were lost. The watcher SHOULD either refresh the
subscription in order to receive a full presence document or
terminate the subscription.

If the watcher encounters a processing error while processing the
received 'application/pidf-diff+xml' encoded presence document, look
at Section 5.1 of [8]. In this case, the watcher SHOULD renew the
subscription. The watcher MAY also fall back to normal presence
operations by not inserting 'application/pidf-diff+xml' in a new
SUBSCRIBE request. It is hardly reasonable to signal this error to
the notifier even if the error exists in the notifier process.

If the PA changes the content type used in notifications within the
existing subscription, the watcher MUST discard all the previously
received presence information (except local version counter) from
that particular presentity and process the new content as specified
for that content type. The local version counter MUST NOT be
discarded because if the PA changes back to 'application/
pidf-diff+xml', the MIME type version counter will continue to
increase from the last version value.

5. Examples

The following message flow shows an example applying the partial
notifications mechanism.

A watcher sends a SUBSCRIBE request declaring support for the default
presence format ('application/pidf+xml) and for the partial
notification format ('application/pidf-diff+xml') in the Accept
header field value. The watcher uses the "q" parameter to set the
preference for receiving partial notifications. The PA accepts the
subscription and, based on the "q" parameter value, selects to send
partial notifications in NOTIFY requests. The first NOTIFY request
includes the full state of presence information. The following
notifications only include information about delta of the presence
information from the previous NOTIFY requests.

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Watcher Presence Agent PUA
| F1 SUBSCRIBE | |
|-------------------------->| |
| F2 200 OK | |
|<--------------------------| |
| F3 NOTIFY | |
|<--------------------------| |
| F4 200 OK | |
|-------------------------->| |
| | |
| | Update presence |
| |<----------------------- |
| | |
| F5 NOTIFY | |
|<--------------------------| |
| F6 200 OK | |
|-------------------------->| |

Message Details

F1 SUBSCRIBE watcher->example.com server

SUBSCRIBE sip:[email protected] SIP/2.0
Via: SIP/2.0/TCP watcherhost.example.com;
branch=z9hG4bKnashds7
To: <sip:[email protected]>
From: <sip:[email protected]> ;tag=xfg9
Call-ID: [email protected]
CSeq: 17766 SUBSCRIBE
Max-Forwards: 70
Event: presence
Accept: application/pidf+xml;q=0.3,
application/pidf-diff+xml;q=1
Contact: <sip:[email protected]>
Expires: 3600
Content-Length: 0

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The PA accepts the subscription and generates a 200 OK response
to the SUBSCRIBE request

F2 200 OK example.com server ->watcher

SIP/2.0 200 OK
Via: SIP/2.0/TCP watcherhost.example.com;
branch=z9hG4bKnashds7
;received=192.0.2.1
To: <sip:[email protected]>;tag=ffd2
From: <sip:[email protected]>;tag=xfg9
Call-ID: [email protected]
CSeq: 17766 SUBSCRIBE
Event: presence
Expires: 3600
Contact: <sip:server.example.com>
Content-Length: 0

The PA, based on the "q" parameter value in the Accept header
of the SUBSCRIBE request (F1), decides to use partial
notifications. The PA creates the first NOTIFY request that
includes the full presence document.

F3 NOTIFY example.com server -> watcher

NOTIFY sip:[email protected] SIP/2.0
Via: SIP/2.0/TCP server.example.com;
branch=z9hG4bKna998sk
To: <sip:[email protected]>;tag=xfg9
From: <sip:[email protected]>;tag=ffd2
Call-ID: [email protected]
Event: presence
Subscription-State: active;expires=3599
Max-Forwards: 70
CSeq: 8775 NOTIFY
Contact: <sip:server.example.com>
Content-Type: application/pidf-diff+xml
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"
xmlns:cp="urn:ietf:params:xml:ns:pidf:cipid"
xmlns:dm="urn:ietf:params:xml:ns:pidf:data-model"
entity="sip:[email protected]"
version="1">

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<tuple id="sg89ae">
<status>
<basic>open</basic>
</status>
<c:servcaps>
<c:audio>true</c:audio>
<c:video>false</c:video>
<c:message>true</c:message>
</c:servcaps>
<r:relationship><r:assistant/></r:relationship>
<contact priority="0.8">tel:09012345678</contact>
</tuple>

<tuple id="cg231jcr">
<status>
<basic>open</basic>
</status>
<contact priority="1.0">im:[email protected]</contact>
</tuple>

<tuple id="r1230d">
<status>
<basic>closed</basic>
</status>
<cp:homepage>http://example.com/~res/</cp:homepage>
<cp:icon>http://example.com/~res/icon.gif</cp:icon>
<cp:card>http://example.com/~res/card.vcd</cp:card>
<contact priority="0.9">sip:[email protected]</contact>
</tuple>

<note xml:lang="en">Full state presence document</note>

<dm:person id="fdkfj">
<r:activities>
<r:on-the-phone/>
<r:busy/>
</r:activities>
</dm:person>

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<dm:device id="u00b40c7">
<c:devcaps>
<c:mobility>
<c:supported>
<c:mobile/>
</c:supported>
</c:mobility>
</c:devcaps>
<dm:deviceID>mac:xxx</dm:deviceID>
</dm:device>

</p:pidf-full>

F4 200 OK watcher -> example.com server

SIP/2.0 200 OK
Via: SIP/2.0/TCP server.example.com;
branch=z9hG4bKna998sk
;received=192.0.2.2
To: <sip:[email protected]>;tag=xfg9
From: <sip:[email protected]>;tag=ffd2
Call-ID: [email protected]
CSeq: 8775 NOTIFY
Content-Length: 0

At a later time, the presentity's presence information
changes. The PA generates a NOTIFY request
that includes information about the changes.

F5 NOTIFY example.com server -> watcher

NOTIFY sip:[email protected] SIP/2.0
Via: SIP/2.0/TCP server.example.com;
branch=z9hG4bKna998sl
To: <sip:[email protected]>;tag=xfg9
From: <sip:[email protected]>;tag=ffd2
Call-ID: [email protected]
CSeq: 8776 NOTIFY
Event: presence
Subscription-State: active;expires=3543
Max-Forwards: 70
Contact: <sip:server.example.com>
Content-Type: application/pidf-diff+xml
Content-Length: ...

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<?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"
xmlns:dm="urn:ietf:params:xml:ns:pidf:data-model"
entity="sip:[email protected]"
version="2">

<p:add sel="presence/note" pos="before"><tuple id="ert4773">
<status>
<basic>open</basic>
</status>
<contact priority="0.4">mailto:[email protected]</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="*/dm:person/r:activities/r:busy"/>

<p:replace sel="*/tuple[@id='cg231jcr']/contact/@priority"
>0.7</p:replace>

</p:pidf-diff>

F6 200 OK watcher-> example.com server

SIP/2.0 200 OK
Via: SIP/2.0/TCP server.example.com;
branch=z9hG4bKna998sl
;received=192.0.2.2
To: <sip:[email protected]>;tag=xfg9
From: <sip:[email protected]>;tag=ffd2
Call-ID: [email protected]
CSeq: 8776 NOTIFY
Content-Length: 0

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6. Security Considerations

This specification relies on the presence event package for SIP [3].
Partial notifications can reveal information about what has changed
compared to the previous notification. This can make it easier for
an eavesdropper to know what kind of changes are happening in the
presentity's presence information. However, the same information can
be found if the presence event package is used with baseline PIDF
[5].

A third party can inject a NOTIFY request with partial state that
will cause the watcher to think it has missed a partial notification
and to request a new full presence document. This is no worse than
what we have without this extension since a party that could perform
such action could also send a NOTIFY with Subscription-State:
terminated and achieve the same effect without knowing about the
extension. Partial Notification does not make the situation any
worse, and the protection mechanisms from the existing system apply
to preventing this attack against the partial notification mechanism.

Presence-related security considerations are extensively discussed in
the presence event package for SIP [3] and all those identified
security considerations apply to this document as well. Issues
described in the presence event package for SIP [3], including
confidentiality, message integrity and authenticity, outbound
authentication, replay prevention, Denial-of-Service (DoS) attacks
against thirst parties and DoS attacks against servers all apply here
without any change.

It is RECOMMENDED that TLS [7] be used between elements to provide
hop-by-hop confidentially protection. Furthermore, S/MIME MAY be
used for integrity and authenticity of SUBSCRIBE and NOTIFY requests.
This is described in Section 23 of RFC 3261.

7. Acknowledgments

The authors would like to thank Jari Urpalainen, Jyrki Aarnos,
Jonathan Rosenberg, Dean Willis, Kriztian Kiss, Juha Kalliokulju,
Miguel Garcia, Anders Kristensen, Yannis Pavlidis, Ben Cambell,
Robert Sparks, and Tim Moran for their valuable comments.

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8. References

8.1. Normative References

[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[2] Lonnfors, M., Khartabil, H., Leppanen, E., and J. Urpalainen,
"Presence Information Data Format (PIDF) Extension for Partial
Presence", RFC 5262, August 008.

[3] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.

[4] 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.

[5] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and
J. Peterson, "Presence Information Data Format (PIDF)",
RFC 3863, August 2004.

[6] Roach, A., "SIP-Specific Event Notification", RFC 3265,
June 2002.

[7] Dierks, T. and E. Rescorla, "The TLS Protocol Version 1.1",
RFC 4346, April 2006.

[8] Urpalainen, J., "An Extensible Markup Language (XML) Patch
Operations Framework Utilizing XML Path Language (XPath)
Selectors", RFC 5261, September 2008.

8.2. Informative References

[9] Day, M., Rosenberg, J., and H. Sugano, "A Model for Presence and
Instant Messaging", RFC 2778, February 2000.

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Authors' Addresses

Mikko Lonnfors
Nokia
P.O. Box 321
Helsinki
Finland

Phone: +358 71 8008000
EMail: [email protected]

Jose Costa-Requena
Nokia
P.O. Box 321
Helsinki
Finland

Phone: +358 71 8008000
EMail: [email protected]

Eva Leppanen
Nokia
Lempaala
Finland

EMail: [email protected]

Hisham Khartabil
Ericsson
Melbourne
Australia

Phone: +61 416 108 890
EMail: [email protected]

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Full Copyright Statement

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