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About the H.323-ALG

If you use Voice-over-IP (VoIP) in your organization, you can add an H.323 or SIP (Session Initiation Protocol) ALG (Application Layer Gateway) to open the ports necessary to enable VoIP through your Firebox. An ALG is created in the same way as a proxy policy and offers similar configuration options. These ALGs have been created to work in a NAT environment to maintain security for privately addressed conferencing equipment protected by your Firebox.

H.323 is commonly used on videoconferencing equipment. SIP is commonly used with IP phones. You can use both H.323 and SIP ALGs at the same time, if necessary. To determine which ALG to add, consult the documentation for your VoIP devices or applications.

VoIP Components

It is important to understand that you usually implement VoIP by using either:

Peer-to-peer connections

In a peer-to-peer connection, each of the two devices knows the IP address of the other device and connects to the other directly, without the use of a proxy server to route their calls.

Host-based connections

Connections managed by a call management system (PBX). The call management system can be self-hosted, or hosted by a third-party service provider.

With H.323, the key component of call management is known as a gatekeeper. A gatekeeper manages VoIP calls for a group of users, and can be located on a network protected by your Firebox or at an external location. For example, some VoIP providers host a gatekeeper on their network that you must connect to before you can place a VoIP call. Other solutions require you to set up and maintain a gatekeeper on your network.

Coordination of the many components of a VoIP installation can be a difficult task. We recommend you make sure that VoIP connections work successfully before you add an H.323 or SIP ALG. This can help you to troubleshoot any problems.

ALG Functions

When you use an H.323-ALG, your Firebox:

  • Routes traffic for VoIP applications
  • Opens the ports necessary to make and receive calls, and to exchange audio and video media
  • Makes sure that VoIP connections use standard H.323 protocols
  • Generates log messages for auditing purposes

Many VoIP devices and servers use NAT (Network Address Translation) to open and close ports automatically. The H.323 and SIP ALGs also perform this function. You must disable NAT on your VoIP devices if you configure an H.323 or SIP ALG.

For more information on how to add a proxy to your configuration, see Add a Proxy Policy to Your Configuration.

Configure the H.323-ALG

See Also

About Proxy Policies and ALGs

About the SIP-ALG

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