Hi Kai,
since 2.4.21 is coming soon i'm sending you an updated isdn ppp
filter patch. The only change since the last version fixes a memory
leak in an ioctl error path. I've also updated the ipppd man page
(cut-n-paste from pppd).

To build ipppd run autoconf in ipppd subdir after patching and
add "--enable-ippp-filter" to the configure options.

Please submit to Marcello if you are comfortable with it.

Best regards,
Patrick