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ZyWALL USG 300 User’s Guide
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CHAPTER 21
ALG
21.1 ALG Overview
Application Layer Gateway (ALG) allows the following applications to operate
properly through the ZyWALL’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 323 SIP ALG Example
The ALG feature is only needed for traffic that goes through the ZyWALL’s NAT.
21.1.1 What You Can Do in this Chapter
Use the ALG screen (Section 21.2 on page 431) to set up SIP, H.323, and FTP ALG
settings.