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ZyWALL 110/310/1100 Series User’s Guide 233
CHAPTER 15
ALG
15.1 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 135 SIP ALG Example
The ALG feature is only needed for traffic that goes through the ZyWALL’s NAT.
15.1.1 What You Can Do in this Chapter
Use the ALG screen (Section 15.2 on page 236) to set up SIP, H.323, and FTP ALG settings.
15.1.2 What You Need to Know
Application Layer Gateway (ALG), NAT and Firewall
The ZyWALL can function as an Application Layer Gateway (ALG) to allow certain NAT un-friendly
applications (such as SIP) to operate properly through the ZyWALL’s NAT and firewall. The ZyWALL
dynamically creates an implicit NAT session and firewall session for the application’s traffic from the
WAN to the LAN. The ALG on the ZyWALL supports all of the ZyWALL’s NAT mapping types.