This document discusses Bidirectional PIM (BIDIR-PIM), a variant of

PIM Sparse-Mode that builds bidirectional shared trees connecting

multicast sources and receivers. Bidirectional trees are built using

a fail-safe Designated Forwarder (DF) election mechanism operating on

each link of a multicast topology. With the assistance of the DF,

multicast data is natively forwarded from sources to the Rendezvous-

Point (RP) and hence along the shared tree to receivers without

requiring source-specific state. The DF election takes place at RP

discovery time and provides the route to the RP, thus eliminating the

requirement for data-driven protocol events.