This section describes the Session Description Protocol (SDP) media-level attributes defined in Request for Comments (RFCs) 4574, 4796, and 5576. RFC 4574 defines a new SDP media-level attribute: “label.” The “label” attribute carries a pointer to a media stream in the context of an arbitrary network application that uses SDP. The sender of the SDP document can attach the “label” attribute to a particular media stream or streams. The application can then use the provided pointer to refer to each particular media stream in its context. RFC 4796 defines another new SDP media-level attribute: “content.” The “content” attribute defines the content of the media stream to a more detailed level than the media description line. The sender of an SDP session description can attach the “content” attribute to one or more media streams. The receiving application can then treat each media stream differently (for example, show it on a big or small screen) based on its content. The SDP also provides mechanisms to describe attributes of multimedia sessions and of individual media streams, for example, Real-time Transport Protocol sessions, within a multimedia session, but does not provide any mechanism to describe individual media sources within a media stream. RFC 5576 defines a mechanism, known as a source-specific media attribute, to describe Real-Time Transport Protocol media sources. Source-specific media attributes are identified by their synchronization source identifiers, in SDP, to associate attributes with these sources, and to express relationships among sources. It also defines several source-level attributes that can be used to describe properties of media sources.