Sip Alg - Nortel 222 User Manual

342 Appendix M SIP

SIP ALG

Some NAT routers can include a SIP Application Layer Gateway (ALG). A SIP
ALG allows VoIP calls to pass through NAT by examining and translating IP
addresses embedded in the data stream. When a VoIP device (SIP client) behind
the SIP ALG registers with the SIP register server, the SIP ALG translates the
device's private IP address inside the SIP data stream to a public IP address. You
do not need to use STUN if your VoIP device is behind the SIP ALG.
STUN
Using STUN (Simple Traversal of User Datagram Protocol (UDP) through
Network Address Translators), the VoIP device can the presence and types of
NAT 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 the
VoIP device can embed it in the SIP data stream. See "STUN - Simple Traversal
of User Datagram Protocol (UDP) Through Network Address Translators
(NATs)" (RFC 3489) for details on STUN.
Business Secure Router SIP ALG
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NN47922-501
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 cannot
make a call between the LAN and the LAN.
The SIP ALG forwards UDP packets with a port 5060 destination to pass
through.
The Business Secure Router forwards SIP audio connections.