ZyXEL Communications ZyWALL 300 User Manual

 Chapter 18 ALG

ZyWALL USG 300 User’s Guide

Figure 171   H.323 ALG Example 

The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol that 
handles the setting up, altering and tearing down of voice and multimedia sessions over the 
Internet. SIP is used in VoIP (Voice over IP), the sending of voice signals over the Internet 
Protocol.

SIP signaling is separate from the media for which it handles sessions. The media that is 
exchanged during the session can use a different path from that of the signaling. SIP handles 
telephone calls and can interface with traditional circuit-switched telephone networks.

• SIP clients can be connected to the LAN or DMZ. A SIP server must be on the WAN. 

• Using the SIP ALG allows you to use bandwidth management on SIP traffic.

• The SIP ALG handles SIP calls that go through NAT or that the ZyWALL routes. You can 

also make other SIP calls that do not go through NAT or routing. Examples would be calls 
between LAN IP addresses that are on the same subnet.

• The SIP ALG supports peer-to-peer SIP calls. The firewall (by default) allows peer to peer 

calls from the LAN zone to go to the WAN zone and blocks peer to peer calls from the 
WAN zone to the LAN zone.

• The SIP ALG allows UDP packets with a port 5060 destination to pass through.

• The ZyWALL allows SIP audio connections.

The following example shows SIP signaling (1) and audio (2) sessions between SIP clients A 
and B and the SIP server. 

Figure 172   SIP ALG Example