I use PPP with an active-filter for on-demand dialup to my ISP.
I want it to wake up on any TCP connection that's not smtp, or on
udp/kerberos.

This filter does most of what I want:
	active-filter 'udp port kerberos or udp port kerberos4 or (tcp and not tcp port smtp)'

but it apparently (from experience) somehow does not wake up for certain
packets.

For example, I connect via telnet to some remote host, and leave the
connection idle.  After the idle timeout, pppd hangs up the modem.  Some
time later, I type in the telnet window, yet pppd does not wake up
again.

I suspect it's related to not recognizing VJ compression packets, but
I'm not sure.  When I leave the connection idle, it shuts down with this
(according to tcpdump):

20:20:11.115996 [|ip]

When I try to type again at the telnet connection, tcpdump sees this:
20:21:05.230184 204.215.140.63.2785 > 18.71.0.20.23: P 63:65(2) ack 3690 win 16384 [tos 0x10]

but pppd doesn't reopen the connection.  If I open another TCP
connection from scratch, pppd will wake up and reconnect the PPP link.

So, can somebody suggest either (a) a filter expression that does what I
want here, or (b) what's broken in pppd's filter interpretation?

==John