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Prestige 2002 Series User’s Guide
42 Chapter 5 Introduction to VoIP
Figure 14 SIP Redirect Server
5.2.3.4 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.
5.2.4 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.
5.3 STUN
STUN (Simple Traversal of User Datagram Protocol (UDP) through Network Address
Translators) allows the Prestige to find the presence and types of NAT routers and/or firewalls
between it and the public Internet. STUN also allows the Prestige to find the public IP address
that NAT assigned, so the Prestige can embed it in the SIP data stream. See RFC 3489 for
details on STUN.
5.3.1 NAT
NAT routers are commonly used to translate private (or internal) IP addresses in packet
headers to public (or external) IP addresses and vice versa. NAT A NAT router maps a private
IP address and port pair to a public IP address and port, and whenever the NAT router receives
a packet with that public IP address and port, it knows how to reroute the packet back to the
private IP address and port. The figures in this section use lower-case letters (like a.b.c.d for
example) to represent public IP addresses. In the following figure, NAT maps the same IP
address to both A and B, but uses separate ports.