I wrote:
>    Of course, if LQM (Line Quality Monitoring) is working properly, pppd
>    should notice this condition itself and cycle the line. Have you tried
>    watching tcpdump on your ppp interface to see if LQM messages are
>    being sent/received?

Brad Spencer <brad@anduin.eldar.org> writes:

> I was not aware that PPP had such a thing.  What criteria does it use to
> determine "quality"??

See RFC 1989. Every "n" seconds, the sides send each other a link
quality report (LQR) packet, and each side determines whether or not
to drop the line depending on the statistics it receives in this
report (and on how many reports have been missed).

To tell you the truth, I've only used LQM on Morning Star PPP, not on
NetBSD PPP, so I don't even know if it supports it (I looked through
the pppd manpage and didn't find mention of it). What NetBSD pppd
*does* support is a simple "send LCP Echo Request packets, and drop
the line if you don't receive Echo Replies" option, which should have
approximately the same effect - check out the "lcp-echo-failure" and
"lcp-echo-interval" options.