多项选择题
A.GFC-4 bits of generic flow control B.Forward-explicit congestion notification（FECN） C.VCI-16 bits of virtual channel identifier D.CLP-1 bit of congestion loss E.HEC-8 bit of header error control F.ATM Discard Eligibility（DE）bit
A.lowest cost B.highest bandwidth C.lowest hop-count D.path attributes E.lowest delay
A.It prevents message looping，Session Advertisement（SA）messages must be RPF checked B.RPF check should be done against the route to the Rendez-vous Point of the corresponding PIM-SM domain，which originated the SA C.RPF checking Session Advertisement（SA）messages will cause message looping D.RPF check should be done against the route to the source S of the corresponding PIM-SM domain