Figure 26 Stun - ZyXEL Communications P-2024 User Manual

Some NAT routers may include a SIP Application Layer Gateway (ALG). An Application
Layer Gateway (ALG) manages a specific protocol (such as SIP, H.323 or FTP) at the
application layer.
A SIP ALG allows SIP calls to pass through NAT by examining and translating IP addresses
embedded in the data stream.
When the P-2024 registers with the SIP register server, the SIP ALG translates the P-2024's
private IP address inside the SIP data stream to a public IP address. You do not need to use
STUN or an outbound proxy (see
ALG.
6.1.7.2 Use NAT
If you know the NAT router's public IP address and SIP port number, you can use the Use
NAT feature to manually configure the P-2024 to use a them in the SIP messages. This
eliminates the need for STUN or a SIP ALG.
You must also configure the NAT router to forward traffic with this port number to the P-2024.
6.1.7.3 STUN
STUN (Simple Traversal of User Datagram Protocol (UDP) through Network Address
Translators) allows the P-2024 to find the presence and types of NAT routers and/or firewalls
between it and the public Internet. STUN also allows the P-2024 to find the public IP address
that NAT assigned, so the P-2024 can embed it in the SIP data stream. STUN does not work
with symmetric NAT routers or firewalls. See RFC 3489 for details on STUN.
The following figure shows how STUN works.
1 The P-2024 (A) sends SIP packets to the STUN server (B).
2 The STUN server (B) finds the public IP address and port number that the NAT router
used on the P-2024's SIP packets and sends them to the P-2024.
3 The P-2024 uses the public IP address and port number in the SIP packets that it sends to
the SIP server (C).

Figure 26 STUN

P-2024 User's Guide
Section 6.1.7.4 on page 70) if your P-2024 is behind a SIP
Chapter 6 SIP
69