PPP Autodialling
================

To enable autodialling the PPP interface needs to remain ifconfig'd up even
when the line is really down.  The default route that PPP usually adds must
not be deleted otherwise Internet will not let any data through to PPP.

[In the sifdown routine the IFF_UP flag in ifr.ifr_flags is not altered.  The
behaviour of the DCI4 st_link_status flag in the Dib structure and the
moreflags flag in the ppp_softc structure are not changed].


When PPP receives a packet it checks whether the interface is really up or
down.  If it is down then it calls SWI Dialler_Dial, telling the dialler to
use the previous parameters.  PPP then returns ENETUNREACH and the packet is
discarded.

[This is done at the start of the pppoutput function.]


When it is time to bring the interface back up PPP checks to see if it has
already added a default route.  It it has then it changes the existing
default route (in case the gateway has changed); if not then it adds a
default route.

[The changes to the routing behaviour are implemented in the ipcp_down and
ipcp_up functions.  There is a new function in the sys-riscos file to change
a route].
