ZyWALL User's Guide

ZyWALL USG 100/200 Series User’s Guide
325
CHAPTER 18
ALG
18.1 ALG Overview
Application Layer Gateway (ALG) allows the following applications to operate properly
through the ZyWALL’s NAT.
• FTP - File Transfer Protocol (FTP) is an Internet file transfer service.
• SIP - Session Initiation Protocol (SIP) is an application-layer protocol that can be used to
create voice and multimedia sessions over Internet.
• H.323 - This is a teleconferencing protocol suite that provides audio, data and video
conferencing.
The following example shows SIP signaling (1) and audio (2) sessions between SIP clients A
and B and the SIP server.
Figure 232 SIP ALG Example
The ZyWALL only needs to use the ALG feature for traffic that goes through the ZyWALL’s
NAT.
18.1.1 What You Can Do in the ALG Screen
Use the ALG screen (Section 18.2 on page 328) to set up SIP, H.323, and FTP ALG settings.