Editing Protocol-Independent Multicast (section)

===Multicast clients===
A router receives explicit Join/Prune messages from those neighboring routers that have downstream group members.

* In order to join a multicast group, G, a host conveys its membership information through the [[Internet Group Management Protocol]] (IGMP).
* The router then forwards data packets addressed to a multicast group G to only those interfaces on which explicit joins have been received.
* A Designated Router (DR) sends periodic Join/Prune messages toward a group-specific Rendezvous Point (RP) for each group for which it has active members.
** Note that one router will be automatically or statically designated as the rendezvous point (RP), and all routers must explicitly join through the RP.
* Each router along the path toward the RP builds a wild card (any-source) state for the group and sends Join/Prune messages on toward the RP.
** The term route entry is used to refer to the state maintained in a router to represent the distribution tree.
** A route entry may include such fields as:
*** source address
*** the group address
*** the incoming interface from which packets are accepted
*** the list of outgoing interfaces to which packets are sent
*** timers, flag bits, etc.
** The wild card route entry's incoming interface points toward the RP
** The outgoing interfaces point to the neighboring downstream routers that have sent Join/Prune messages toward the RP as well as the directly connected hosts which have requested membership to group G.
* This state creates a shared, RP-centered, distribution tree that reaches all group members.