UE激活MBMS服务的组播地址的设置

nomad2

3gpp 23.246, section 8.2 MBMS multicast service activation

第2条消息，是IGMP Join消息，规范上这样写，

The UE sends an IGMP (IPv4)or MLD (IPv6) Join message over the default PDP context to signal its interestin receiving a particular multicast MBMS bearer service identified by an IPmulticast address.

不明白的是，这里的“IPmulticast address”是怎么得到的呢？UE是怎么知道想要激活的服务的组播地址的？谢谢。