ZyXEL Communications 200 Series 用户手册

ZyWALL USG 100/200 Series User’s Guide

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. 

Use the ALG screen (

) to set up SIP, H.323, and FTP ALG settings.