WRAP supports the EXPRESS [3] single-source model of multicast. Multi-source multicast applications can be supported with EXPRESS by relaying the multicast through a node that is a source of an existing relay multicast channel, similar to the rendezvous point in PIM-SM, but performed at the WRAP or the application layer.
A subscriber joins a multicast channel by specifying the associated RA, reverse token, and destination multicast address. It receives this information by looking up the multicast channel by name in the TRIAD directory service.
A multicast WRAP header contains the same multicast address G repeated r times, where r represents the maximum number of relay hops in the multicast tree; this allows multicast WRAP relaying to performed identically to unicast WRAP relaying. As multicast packets are relayed, group addresses may be translated so that the (S,G) pair upon which IPv4 routers do multicast delivery is unique. However, a single intra-realm channel can be reused within a realm to deliver multiple inter-realm channels.