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Tuesday, November 20, 2007

MPDF: A User Agent Profile Data Set for Media Policy

Members of the IETF Session Initiation Proposal Investigation Working
Group have published an updated version of the "User Agent Profile
Data Set for Media Policy" Internet Draft. This draft specification
defines a document format for the media properties of Session Initiation
Protocol (SIP) sessions. Examples for media properties are the codecs
or media types used in a session. This document format is based on XML
and extends the Schema for SIP User Agent Profile Data Sets. It can be
used to describe the properties of a specific SIP session or to define
policies that are then applied to different SIP sessions. Section 8
supplies the RELAX NG Definition. The Framework for Session Initiation
Protocol (SIP) User Agent Profile Delivery and the Framework for SIP
Session Policies define mechanisms to convey session policies and
configuration information from a network server to a user agent. An
important piece of the information conveyed to the user agent relates
to the media properties of the SIP sessions set up by the user agent.
Examples for these media properties are the codecs and media types used,
the media-intermediaries to be traversed or the maximum bandwidth
available for media streams. The Media Policy Dataset Format (MPDF)
specification is defined in this document for SIP session media
properties. This format can be used in two ways: first, it can be used
to describe the properties of a given SIP session (e.g., the media types
and codecs used). These MPDF documents are called session info documents
and they are usually created based on the session description of a
session. Second, the MPDF format can be used to define policies for SIP
sessions in a session policy document. A session policy document defines
properties (e.g., the media types) that can or can not be used in a
session, independent of a specific session description. The two types
of MPDF documents, session information and session policy documents,
share the same set of XML elements to describe session properties. A
user agent can receive multiple session policy documents from different
sources. These documents need to be merged into a single document the
user agent can work with. This document specifies rules for merging each
of the XML elements defined. It should be noted that these merging rules
are part of the semantics of the XML element. User agents implement the
merging rules as part of implementing the element semantics. As a
consequence, it is not possible to build an entity that can mechanically
merge two session policy documents without understanding the semantics
of all elements in the input documents. More Information

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