Alg; Alg Overview; What You Need To Know; Chapter 14 Alg - ZyXEL Communications ZyWall USG20-VPN User Manual

14.1 ALG Overview

Application Layer Gateway (ALG) allows the following applications to operate properly through the
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 176 SIP ALG Example
The ALG feature is only needed for traffic that goes through the USG's NAT.

14.1.1 What You Need to Know

Application Layer Gateway (ALG), NAT and Security Policy
The USG can function as an Application Layer Gateway (ALG) to allow certain NAT un-friendly
applications (such as SIP) to operate properly through the USG's NAT and security policy. The 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 USG supports all of the USG's NAT mapping types.
FTP ALG
The FTP ALG allows TCP packets with a specified port destination to pass through. If the FTP server
is located on the LAN, you must also configure NAT (port forwarding) and security policies if you
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