Netgraph/mpd over USB modem problems

[Vicknesan AYADURAI]

Hi all,

I've been battling this unsuccessfully for a while now, and hope
someone can shed some light on how to fix this.

I'm trying to achieve a PPP dialup connection via a USB modem using
netgraph mpd. However there appears to be a problem between netgraph's
ng_tty node and /dev/ucom0, which I can't figure out/fix.

The initial mpd chat script works fine, and AT commands successfully
establish a call between the local (USB) and remote modem. When DCD
comes up, the netgraph nodes kick in, and the netgraph chain (ng_ppp
-> ng_tty -> etc) is automatically (and correctly) formed.

LCP packets are then sucessfully transmitted to the remote PPP host
via the USB port and modem. However, the subsequent responses arriving
at the local USB port, are not forwarded up the stack i.e., are not
received by the netgraph node(s).

It's possible (although not completely confirmed) that this is somehow
related to the changing line-disciplines from TTYDISC (during the chat
phase) to NETGRAPHDISC (during LCP/PPP phase) when the netgraph nodes
are formed.

I've verified that with the USB & serial modems (same modem actually):

a. user-space PPP works on both /dev/cuaa0 and /dev/ucom0
b. mpd works on /dev/cuaa0 but NOT /dev/ucom0

indicating that the individual components seem to work, but the
netgraph/USB combination has problems. The only difference between the
relevant serial and USB config files is: /dev/cuaa0 is changed to
/dev/ucom0 when switching from serial to USB.

I've tested this on GENERIC FreeBSD versions 4.9R, 5.2.1R and
yesterday, on a new installation of 5.3-RC2 -- all without success.
All releases display seemingly identical behaviour, i.e., establishing
the call successfully, but not receiving incoming LCP frames.  :-(

Anyone have any experience running netgraph nodes with USB modems?
Even more generally, anyone know if netgraph is even _meant_ to work
with USB...?

Desperately,
vick