Overview

SIP NAT Traversal Controller API is an ECOM plug-in API that provides the connection between the SIP stack and an external UNSAF protocol module. The Network Address Translator (NAT) devices are found between the SIP User Agent (SIP UA) and SIP proxy, and have the UA as an outbound proxy. This creates the problems to maintain SIP connectivity between the UA and the remaining of the SIP network that are described as follows:

• The SIP UA registers its contact to SIP registrar using the private IP address & UDP/TCP port pair within the contact URI. These addresses are not accessible from the proxy. The NAT maps the private addresses to some other addresses which can be public or private, in another address range. The proxy can access the UA when you send the SIP requests and responses to addresses that the NAT allocates for the UA. You must have the Contact header of the REGISTER request to contain addresses allocated by the NAT for the UA, instead of the private addresses of the UA.

• The NAT allocates IP addresses to the UA when the UA sends IP packets to a destination through the NAT. The NAT type determines if the address-port pairs are either applicable or dependent on the IP address and UDP or TCP port. The UA sends the IP packets to the UDP or TCP port.

  Depending on the NAT type, the addresses within the contact URI must be those that any device outside the NAT can see and use or those that only the specific destination (SIP proxy in this case) can see. To detect addresses the UA uses Unilateral Self-Address Fixing (UNSAF) protocols defined within IETF. These protocols include the Simple Traversal of UDP through NATs (STUN) or Traversal Using Relay NAT (TURN).

  The allocation created by NAT is not permanent. It expires if the UDP flow or TCP connection between the end-points remains idle. To prevent expiration the UA send keepalive messages within the flow to refresh the mapping. Different types of keepalive messages are used like Carriage-Return/Line-Feed (CRLF) sequences, OPTIONS requests or STUN binding requests

To enable all these processes the SIP stack contains an ECOM plug-in API, SIP NAT Traversal Controller API. The stack uses these plug-ins to access and use a separate UNSAF protocol implementation. This implementation supports protocols like STUN and TURN. These protocols enable the SIP stack to detect the server reflexive and relayed addresses for the UA The protocols also implements the keepalive messages for the flow toward the outbound proxy.