338 Appendix L SIPNN47923-501SIP ALGSome NAT routers can include a SIP Application Layer Gateway (ALG). A SIPALG allows VoIP calls to pass through NAT by examining and translating IPaddresses embedded in the data stream. When a VoIP device (SIP client) behindthe SIP ALG registers with the SIP register server, the SIP ALG translates thedevice’s private IP address inside the SIP data stream to a public IP address. Youdo not need to use STUN if your VoIP device is behind the SIP ALG.STUNUsing STUN (Simple Traversal of User Datagram Protocol (UDP) throughNetwork Address Translators), the VoIP device can the presence and types ofNAT routers, firewalls, or both between it and the public Internet. With STUN,the VoIP device can also find the public IP address that NAT assigned, so theVoIP device can embed it in the SIP data stream. See “STUN - Simple Traversalof User Datagram Protocol (UDP) Through Network Address Translators(NATs)” (RFC 3489) for details on STUN.Business Secure Router SIP ALG• SIP clients must be connected to the LAN. A SIP server must be on the WAN.• You can make and receive calls between the LAN and the WAN. You cannotmake a call between the LAN and the LAN.• The SIP ALG forwards UDP packets with a port 5060 destination to passthrough.• The Business Secure Router forwards SIP audio connections.
