Section: 17.5.2 Session start procedure
Content: The BM-SC initiates the MBMS session start procedure when it is ready to send data. This informs the GGSN of the imminent start of the transmission and MBMS session attributes are provided to the GGSNs included in the list of downstream nodes in BM-SC. For a multicast MBMS service these are the GGSNs that have previously registered for the corresponding MBMS bearer service. The bearer plane is allocated. BM-SC and GGSN shall at least support IP unicast encapsulation of IP multicast datagrams, which shall be default mode of sending user plane data. BM-SC may support sending user plane IP multicast datagrams to GGSN, and GGSN also may support this mode of operation. Figure 27: MBMS Session Start procedure 1. The BM-SC sends a RAR message to indicate the impending start of the transmission and to provide the session attributes to the GGSNs listed in the "list of downstream nodes" parameter of the corresponding MBMS Bearer Context. BM-SC may indicate to GGSN that BM-SC supports sending the user plane IP multicast data without IP unicast encapsulation. In such case BM-SC shall send multicast source address as specified by IETF RFC 4604 [73] and IETF RFC 4607 [74]. The BM-SC sets the state attribute of its MBMS Bearer Context to ‘Active’. By sending "CN IP Multicast Distribution" parameter to GGSN, the BM-SC indicates if IP multicast mechanism should be used for user plane data distribution to UTRAN. "MBMS HC Indicator" parameter, if present, indicates that a header compression is used for MBMS user plane data. 2. For a broadcast MBMS bearer service the GGSN creates an MBMS Bearer Context. The GGSN stores the session attributes in the MBMS Bearer Context, sets the state attribute of its MBMS Bearer Context to ‘Active’ and sends a RAA message to the BM-SC. In case GGSN receives BM-SC multicast source address, which indicates BM-SC support for both modes of sending user plane data, GGSN decides in which mode GGSN shall receive the user plane data. In case GGSN decides to receive unicast encapsulated data, then GGSN shall send own IP address and UDP port for the encapsulating unicast IP and UDP layer of the user plane to BM-SC. In case GGSN decides to receive IP multicast packets, then GGSN shall join the multicast group as specified by IETF RFC 4604 [73] and IETF RFC 4607 [74], and indicate to BM-SC about the decision.