thanks for your suggestion Aldo
finally i've put this in a script called pppoesrvrestart :
killall -15 mpd && sleep 2 && /usr/local/sbin/mpd -b -d /var/etc/mpd-vpn -p /var/run/mpd-vpn.pid pppoeand then added that in config.xml :
<minute>0</minute> <hour>0</hour> <mday>*</mday> <month>*</month> <wday>*</wday> <who>root</who> <command></command>/etc/pppoesrvrestartand pppoe server is restarting everyday at midnight
thanks all for your help