Detecting the type of NAT firewall using messages

A computer program product for determining a type of a Network Address Translation (NAT) firewall at a system outside the NAT firewall, the computer program product comprising a non-transitory computer readable storage medium and computer program instructions, recorded on the computer readable medium and executable by a processor of the system, for performing the steps of:

• receiving at the system outside the NAT firewall a Session Initiation Protocol (SIP) message from a device inside the NAT firewall, the SIP message addressed to a first IP address and port of the system outside the NAT firewall;
• transmitting a plurality of SIP messages to the device inside the NAT firewall, at least one of the plurality of SIP messages addressed from the first IP address and port of the system outside the NAT firewall and at least one of the SIP messages addressed from a second IP address and port of the system outside the NAT firewall, wherein each SIP message is formed so that when the device inside the NAT firewall responds to the SIP message, the response is sent to a different IP address and port of the system outside the NAT firewall than the first IP address and port;
• receiving responses at the system outside the NAT firewall to at least some of the plurality of SIP messages transmitted to the device inside the NAT firewall;
• and
• determining at the system outside the NAT firewall the type of the NAT firewall based on the received responses, which responses are received to SIP messages addressed from the second IP address and port of the system outside the NAT firewall, and which responses are received addressed to the IP address and port of the system outside the NAT firewall that is different than the first IP address and port.

A system for determining a type of a Network Address Translation (NAT) firewall, the system outside the NAT firewall, comprising a processor operable to execute computer program instructions, a memory operable to store computer program instructions executable by the processor, and computer program instructions stored in the memory and executable to perform the steps of:

• receiving at the system outside the NAT firewall a Session Initiation Protocol (SIP) message from a device inside the NAT firewall, the SIP message addressed to a first IP address and port of the system outside the NAT firewall;
• transmitting a plurality of SIP messages to the device inside the NAT firewall, at least one of the plurality of SIP messages addressed from the first IP address and port of the system outside the NAT firewall and at least one of the SIP messages addressed from a second IP address and port of the system outside the NAT firewall, wherein each SIP message is formed so that when the device inside the NAT firewall responds to the SIP message, the response is sent to a different IP address and port of the system outside the NAT firewall than the first IP address and port;
• receiving responses at the system outside the NAT firewall to at least some of the plurality of SIP messages transmitted to the device inside the NAT firewall;
• and
• determining at the system outside the NAT firewall the type of the NAT firewall based on the received responses, which responses are received to SIP messages addressed from the second IP address and port of the system outside the NAT firewall, and which responses are received addressed to the IP address and port of the system outside the NAT firewall that is different than the first IP address and port.