Incoming calls not recieved

General
6

Hi @danielfox32bit,

Yeah, the NAT table entry in firewalls (local or ISP), with shorter delays, certainly plays in the equation with UDP protocols. TCP not only helps in this regard, but it goes even more complex when you compare both protocols.

To put it simply:

  • In UDP there are no “connections” between two sockets.
  • Data packets are just “sent in the air” to a desired destination without any notion of having been received, nor the order they will be received.
  • So there is no guarantee on the server side it has been received, and no guarantee on the client side it had to receive something.
  • This is the best solution when speed and high traffic is required, like call centres, for example.

The other thing is that SIP/VoIP uses a set of communication ports that are usually blocked on firewalls and also, they need to be sent to a specific device. On top of that add NAT transversal issues for which UDP is way more susceptible. So these things make it technically more challenging to work properly. (Firewall settings, port forwarding, etc.)

The simple solution for small businesses and home users is TCP.

  • It establishes a connection between the client and the server.
  • Since the client established the connection, there are usually no special firewall settings required (in rare cases depending of each setup).
  • All packet exchanges are guaranteed to arrive, and in order, except if the connection drops.
  • Potentially gives better call quality, can introduce delays in real-time communications, and can simplify firewall and router issues.
  • Concerning router settings, one must usually disable any “ALG” software option in the firewall because it can cause problems with the SIP/TCP connection.

The best solution is TCP/TLS.

  • It will provide additional security so people cannot “listen” to your conversations.
  • Concerning “ALG” software on routers, when using TLS it can be fine since the protocol is encrypted, but on old implementations of ALG it can still raise connection issues. If uncertain, just disable it too.

Kind regards,
Alexandre

1 Like