ZyWALL USG Series User’s Guide
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CHAPTER 15
ALG
15.1 ALG Overview
Application Layer Gateway (ALG) allows the following applications to operate properly through the
Zyxel Device’s NAT.
• SIP - Session Initiation Protocol (SIP) - An application-layer protocol that can be used to create voice
and multimedia sessions over Internet.
• H.323 - A teleconferencing protocol suite that provides audio, data and video conferencing.
• FTP - File Transfer Protocol - an Internet file transfer service.
The following example shows SIP signaling (1) and audio (2) sessions between SIP clients A and B and the
SIP server.
Figure 314 SIP ALG Example
The ALG feature is only needed for traffic that goes through the Zyxel Device’s NAT.
15.1.1 What You Need to Know
Application Layer Gateway (ALG), NAT and Security Policy
The Zyxel Device can function as an Application Layer Gateway (ALG) to allow certain NAT un-friendly
applications (such as SIP) to operate properly through the Zyxel Device’s NAT and security policy. The
Zyxel Device dynamically creates an implicit NAT session and security policy session for the application’s
traffic from the WAN to the LAN. The ALG on the Zyxel Device supports all of the Zyxel Device’s NAT
mapping types.
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