rfc 4601 PIM query

HI,
I have one doubt about PIM register messages.

rfc 4601 sec. 4.4 PIM register messsages :

    "The Designated Router (DR) on a LAN or point-to-point link
    encapsulates multicast packets from local sources to the RP for the
    relevant group unless it recently received a Register-Stop message
    for that (S,G) or (*,G) from the RP.  When the DR receives a
    Register-Stop message from the RP, it starts a Register-Stop Timer to
    maintain this state.  Just before the Register-Stop Timer expires,
    the DR sends a Null-Register Message to the RP to allow the RP to
    refresh the Register-Stop information at the DR.  If the Register-
    Stop Timer actually expires, the DR will resume encapsulating packets
    from the source to the RP."

I m confused after reading " Just before the Register-Stop Timer 
expires". Does this mean we need one more timer here?

Thanks
Ajay