Internet Engineering Task Force (IETF) G. Camarillo Request for Comments: 6502 Ericsson Category: Standards Track S. Srinivasan ISSN: 2070-1721 R. Even Gesher Erove Ltd J. Urpalainen Nokia March 2012
Conference Event Package Data Format Extension for Centralized Conferencing (XCON)
This document specifies the notification mechanism for XCON (centralized conferencing). This mechanism reuses the SIP (Session Initiation Protocol) event package for conference state. Additionally, the notification mechanism includes support for the XCON data model and for partial notifications.
Status of This Memo
This is an Internet Standards Track document.
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The XCON (Centralized Conferencing) framework [RFC5239] defines a notification service that provides updates about a conference instance's state to authorized parties using a notification protocol, as shown in Figure 1. This document specifies how to use the SIP (Session Initiation Protocol [RFC3261]) event package for conference state defined in [RFC4575] as a notification protocol between a client and a conference's notification server.
Figure 1: Notification service and protocol in the XCON architecture
In addition to specifying the SIP event package for conference state, [RFC4575] specifies a data format to be used with the event package. The XCON data model [RFC6501] extends that format with new elements and attributes so that the extended format supports more functionality (e.g., floor control). The notification protocol specified in this document supports all the data defined in the XCON data model (i.e., the data originally defined in [RFC4575] plus all the extensions defined in [RFC6501]) plus a partial notification mechanism based on XML patch operations [RFC5261].
In order to obtain notifications from a conference server's notification service, a client subscribes to the 'conference' event package at the server as specified in [RFC4575]. Per [RFC4575], NOTIFY requests within this event package can carry an XML document in the "application/conference-info+xml" format. Additionally, per this specification, NOTIFY requests can also carry XML documents in the "application/xcon-conference-info+xml" and the "application/ xcon-conference-info-diff+xml" formats.
A document in the "application/xcon-conference-info+xml" format provides the user agent with the whole state of a conference instance. A document in the "application/ xcon-conference-info-diff+xml" format provides the user agent with the changes the state of the conference instance has experienced since the last notification sent to the user agent.
Subscribers signal support for full notifications by including the "application/xcon-conference-info+xml" format in the Accept header field of the SUBSCRIBE requests they generate. If a client subscribing to the 'conference' event package generates an Accept header field that includes the MIME type "application/ xcon-conference-info+xml", the server has the option of returning documents that follow the XML format specified in [RFC6501] and are carried in "application/xcon-conference-info+xml" message bodies.
Conference servers that implement the SIP event package for conference state and support the "application/ xcon-conference-info+xml" MIME type MUST also support the "application/conference-info+xml" MIME type. This way, legacy clients, which only support "application/conference-info+xml", are able to receive notifications in a format they understand.
Clients that implement the SIP event package for conference state and support the "application/xcon-conference-info+xml" MIME type SHOULD also support the "application/conference-info+xml" MIME type. This way, these clients are able to receive notifications from legacy servers, which only support "application/conference-info+xml", in a format they understand.
The conference state reported by this event package may contain many elements. When the "xcon-conference-info+xml" format is used and there is a change in the state of an element, the server generates a notification with the whole conference state. Generating large notifications to report small changes does not meet the efficiency requirements of some bandwidth-constrained environments. The partial notifications mechanism specified in this section is a more efficient way to report changes in the conference state.
The SIP event package for conference state defined a partial notification mechanism based on <state> elements. Servers compliant with this specification MUST NOT use that partial notification mechanism. Instead, they MUST use the mechanism specified in this section.
Subscribers signal support for partial notifications by including the "application/xcon-conference-info-diff+xml" format in the Accept header field of the SUBSCRIBE requests they generate. If a client subscribing to the 'conference' event package generates an Accept header field that includes the MIME type "application/ xcon-conference-info-diff+xml", the server has the option of returning documents that follow the XML format specified in Section 5.4 and are carried in "application/ xcon-conference-diff-info+xml" message bodies.
Once a subscription is accepted and installed, the server MUST deliver full state in its first notification. To report full state, the server MUST set the Content-Type header field to the value "application/xcon-conference-info+xml".
In order to deliver a partial notification, the server MUST set the Content-Type header field to the value "application/ xcon-conference-info-diff+xml". When the server generates a partial notification, the server SHOULD only include the information that has changed compared to the previous notification. It is up to the server's local policy to determine what is considered as a change to the previous state.
The server MUST construct partial notifications according to the following logic: all the information that has been added to the document is listed inside <add> elements. All information that has been removed from the document is listed inside <remove> elements, and all information that has been changed is listed under <replace> elements.
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The server MUST NOT send a new NOTIFY request with a partial notification until it has received a final response from the subscriber for the previous one or the previous NOTIFY request has timed out.
When the server receives a SUBSCRIBE request (refresh or termination) within the associated subscription, it SHOULD send a NOTIFY request containing the full document using the 'application/ xcon-conference-info+xml' content type.
If the server has used a content type other than 'application/ xcon-conference-info+xml' in notifications within the existing subscription and changes to deliver partial notifications, the server MUST deliver full state using the 'application/ xcon-conference-info+xml' content type before generating its first partial notification.
When a subscriber receives the first notification containing full state in a 'application/xcon-conference-info+xml' MIME body, the subscriber MUST store the received full document as its local copy.
When the subscriber receives a subsequent notification, the subscriber MUST modify its locally stored information according to the following logic:
o If the notification carries an 'application/ xcon-conference-info+xml' document, the subscriber MUST replace its local copy of the document with the document received in the notification.
o If the notification carries an 'application/ xcon-conference-info-diff+xml' document, the subscriber MUST apply the changes indicated in the received 'application/ xcon-conference-info-diff+xml' document to its local copy of the full document.
If the subscriber encounters a processing error while processing an 'application/xcon-conference-info-diff+xml' encoded document, the subscriber SHOULD renew its subscription. A subscriber can fall back to normal operations by not including the "application/ xcon-conference-info-diff+xml" format in a new SUBSCRIBE request.
If the server changes the content type used in notifications within the existing subscription, the subscriber MUST discard all the previously received information and process the new content as specified for that content type.
An xcon-conference-info-diff document is an XML [W3C.REC-xml-20081126] document that MUST be well-formed and SHOULD be valid. The namespace URI for the <conference-info-diff> root document element is defined in [RFC6501]:
The root document element <conference-info-diff> has a single mandatory attribute, "entity". The value of this attribute is the conference object identifier (XCON-URI) that identifies the conference being described in the document.
The content of the <conference-info-diff> element is an unordered sequence of <add>, <replace>, and <remove> elements followed by elements from other namespaces for the purposes of extensibility. Any such unknown elements MUST be ignored by the client. The <add>, <replace>, and <remove> elements can contain other extension attributes than what are defined in the corresponding base types of [RFC5261].
This document specifies how to deliver notifications using the SIP event package for conference state in two new formats. The fact that notifications are encoded in a different format does not have
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security implications. Section 8 of [RFC4575] contains security considerations related to the use of the event package. Implementers of the event package need to follow those considerations regardless of the format used to encode their notifications.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types", RFC 3023, January 2001.
[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.
[RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, "A Session Initiation Protocol (SIP) Event Package for Conference State", RFC 4575, August 2006.
[RFC5261] Urpalainen, J., "An Extensible Markup Language (XML) Patch Operations Framework Utilizing XML Path Language (XPath) Selectors", RFC 5261, September 2008.
[RFC6501] Novo, O., Camarillo, G., Morgan, D., and J. Urpalainen, "Conference Information Data Model for Centralized Conferencing (XCON)", RFC 6501, March 2012.
[W3C.REC-xml-20081126] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth Edition)", World Wide Web Consortium Recommendation REC- xml-20081126, November 2008, <http://www.w3.org/TR/2008/REC-xml-20081126>.