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Chapter 9 SIP
P-2304R-P1 Series User’s Guide
110
Figure 56 SIP Redirect Server
9.1.5.4 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.
9.1.6 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.
9.1.7 NAT and SIP
The ZyXEL Device must register its public IP address with a SIP register server. If there is a
NAT router between the ZyXEL Device and the SIP register server, the ZyXEL Device
probably has a private IP address. The ZyXEL Device lists its IP address in the SIP message
that it sends to the SIP register server. NAT does not translate this IP address in the SIP
message. The SIP register server gets the ZyXEL Device’s IP address from inside the SIP
message and maps it to your SIP identity. If the ZyXEL Device has a private IP address listed
in the SIP message, the SIP server cannot map it to your SIP identity. See Chapter 8 on page
97 for more information about NAT.
Use a SIP ALG (Application Layer Gateway), Use NAT, STUN, or outbound proxy to allow
the ZyXEL Device to list its public IP address in the SIP messages.
9.1.7.1 SIP ALG
See Section 8.1.3 on page 99.