<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">Hi,<br class=""><br class="">After looking into this a lot more, I’m still not closer to a solution or a root cause. For a while, I thought enabling debug logging for pppd (via the NM_PPP_DEBUG environment variable for NetworkManager) resolved the issue. However, it didn’t reliably. For the debug and non-debug case, I captured straces. In this particular instance, the debug-case didn’t show any problems re-establishing a connection after the connection and pppd was terminated while the non-debug case did. I have attached both straces. When looking at the straces, a couple of things seemed odd to me:<br class=""><br class="">1. After the modem hangs up, the TTY is in a state where ioctls to the TTY return I/O errors. How is pppd supposed restore the termios settings when it’s hung up?<br class="">2. NetworkManager doesn’t wait to signal pppd with SIGTERM until pppd is in PHASE_DEAD, even though this should be happening. Ref <a href="https://github.com/paulusmack/ppp/issues/6#issuecomment-51176255" class="">https://github.com/paulusmack/ppp/issues/6#issuecomment-51176255</a><br class="">3. The termios flags present when pppd starts up differ for the debug/non-debug cases: c_iflags=0x5 vs 0x4 and c_lflags=0 vs 0x8a21. Why would that be the case? Doesn’t ModemManager set this up deterministically? Does it matter?<br class=""><br class="">Besides that, my current working theory is that the problem is somehow caused by the periodic connection status check. If that detects a lost connection (+CGACT: 1,0) before pppd has terminated, it will emit the MM_PORT_CONNECTED notification which calls back to port_connected(). There, we try to re-acquire exclusive access to the TTY which will fail because pppd is still running. Afterwards, data_watch_enable starts to watch the TTY which immediately triggers with the modem hangup.<div class=""><br class=""></div><div class="">Sven</div><div class=""><br class=""></div><div class=""></div></body></html>