[u-u] Cheap/free VOIP home phone

D. Hugh Redelmeier hugh at mimosa.com
Tue May 26 11:51:20 EDT 2020

For you, it may be better not to fight this technical war.  But this
is a technical group, so I'll post my thoughts about what might be
going on.

Your 15 minute timeout sure sounds to me to be a NAT issue.  Most 
implementations of NAT have time-outs on table entries.

Your phone will use the SIP protocol for negotiating the connection
but the actual VoIP data uses the RTP protocol.

Both are probably carried over UDP.  They could use TCP but that isn't 
really as natural.

(It would be better from a security standpoint if your communication were 
tunneled.  That would also probably fix your particular problem.  But
I don't imagine netTALK provides tunneling.)

UDP is a datagram protocol.  This really makes NAT unNATural.  If NAT is a 
hack, UDP NAT is doubly so.  It's easy for a protocol to use UDP for a
bi-directional channel, but the UDP packets have no way of indicating 
this.  And yet the NAT implementation must somehow make this inference.  
See, for example STUN.

The normal way of avoiding NAT time-outs is regular keep-alive messages of 
some sort, in both directions.  Usually there is a setting somewhere to 
enable and tune keep-alives going in VoIP systems.  Usually VoIP providers 
also provide STUN servers.

You tried to avoid NAT by using the router's DMZ feature.  And yet you
still had the 15-minute failure.  The problem MIGHT be that you
directed SIP ports to the VoIP device but not the ports used by RTP.
So your SIP traffic is probably still going through NAT.  It is RTP
that is failing you.

All this could be figured out via tcpdump.  Too bad you don't control
the router so you cannot insert tcpdump into the traffic.  That's
another reason I use my own router, not Rogers'.

On another topic, probably of only theoretical interest:

If you use a Rogers modem in bridge mode, supplying your own router,
your router gets one routable IP address.  Apparently, if your modem
is a CODA-4582, you can attache a second device to the Modem's
ethernet ports and get a second routable IP address!  No NAT involved.
So you could put your VoIP device beside your router, not behind it.

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