How can we solve the problem of traversing NAT in both directions for SIP traffic?
The NAT problem for SIP signaling can be solved by simply implementing a smart registrar which does not save the contact address as presented by the device in the SIP Register message but rather based on the real IP:port combination the message originates from. Once registered with the SIP Registrar, either the phone or the SIP Registrar must maintain the communication channel open by sending keep-alive packets to the SIP device before the binding expires in the NAT device. The packets could be either SIP packets send by the device or IP packets sent by the Registrar. It is possible depending on the type of NAT that sending keep-alive packets from outside is not enough, in this case the device inside NAT should take care of this by lowering the Registration interval bellow the NAT binding expiration time (usually a value of 85 seconds will suffice). Now, having a permanent communication path open between the SIP Registrar and the SIP device, it is always possible to ring the device beh
