This was discussed on the list before, but AFAIK, the problem hasn't been
fixed:

When some site is sending high-priority packets through a PPP link, all
other packet types are completely blocked.  Normally, the high-priority
packets are used by interactive connections, but I just ran into another
ftp site that sends out all the packets with the interactive flag set.

This blocks all other connections AND the PPP link LCP packets.  I have
configured PPP to drop the connection and restart it if the LCP packets
aren't getting through - so, during this 4 MB data transfer, my PPP
connection has been dropped about 15 times.

Has this been fixed?  If not, it _should_ be fixed.  This "feature" makes
the PPP link drop all the time when using ftp sites like this.  Or, if
someone decides to flood a PPP link with high-priority packets, the link
will be totally unusable.

Sigh.

  -jm