Figure 396 Sip Redirect Server - ZyXEL Communications ZyWall 35 User Manual

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ZyWALL 35 User's Guide

Figure 396 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.

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.

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

stream to a public IP address. You do not need to use STUN if your VoIP device is behind the

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.

Appendix J SIP Passthrough