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.

 

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