ZyXEL Communications ZyXEL ZyWALL 5 - Manual (Page 593)

ZyWALL 5 User’s Guide
592 Appendix H SIP Passthrough
Figure 36 SIP Redirect Server
SIP Register Server
A SIP register server maintains a database of SIP identity-to-IP address (or domain name)
mapping. The register server checks your user name and password when you register.
RTP
When you make a VoIP call using SIP, the RTP (Real time Transport Protocol) is used to
handle voice data transfer. See RFC 1889 for details on RTP.
SIP ALG
Some NAT routers may 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
STUN (Simple Traversal of User Datagram Protocol (UDP) through Network Address
Translators) allows the VoIP device to find the presence and types of NAT routers and/or
firewalls between it and the public Internet. STUN also allows the VoIP device to find the
public IP address that NAT assigned, so the VoIP device can embed it in the SIP data stream.
See RFC 3489 for details on STUN.