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
ZyWALL/USG’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 278 SIP ALG Example
The ALG feature is only needed for traffic that goes through the ZyWALL/USG’s NAT.
15.1.1 What You Need to Know
Application Layer Gateway (ALG), NAT and Security Policy
The ZyWALL/USG can function as an Application Layer Gateway (ALG) to allow certain NAT un-
friendly applications (such as SIP) to operate properly through the ZyWALL/USG’s NAT and security
policy. The ZyWALL/USG 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 ZyWALL/USG supports all of the
ZyWALL/USG’s NAT mapping types.
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