PIM Reverse Path Forwarding (RPF) Vetor Conflict Resolution

[RFC7891] Section 7 defines the handling principles for PIM Join attributes with same type of RFF Vector and Explicit RFP Vector, but with different contents are received. However, it does not address scenarios where one downstream router includes a RFF Vector in its message while another does not. This leaves the handling of such conflicts unspecified. This document supplements the existing specifications by defining the processing rules for this specific conflict scenario.