Understanding the H.323 ALG

The H.323 standard is a legacy VoIP protocol defined by the International Telecommunication Union (ITU-T). H.323 consists of a suite of protocols (such as H.225.0 and H.245) that are used for call signaling and call control for VoIP.

H.323 uses the ASN.1 coding format. It sets up the dynamic links for data, video, and audio streams, following the protocols Q.931 (with port number 1720) and H.245. There are three major processes in H.323:

• Gatekeeper Discovery—An endpoint finds its gatekeeper through the gatekeeper discovery process, through broadcast or unicast (to a known IP and the well-known UDP port 1719). (JUNOS software supports unicast only.)
• Endpoint Registration, Admission, and Status—An endpoint registers to a gatekeeper and asks for its management. Before making a call, an endpoint asks its gatekeeper for permission to place the call. In both registration and admission phases, the remote access server (RAS) channel is used. The Transport Service Access Point (TSAP) may be either the well-known UDP port (1719), or a dynamically assigned port from the discovery or registration phase.
• Call Control and Call Setup—Calls can be established within a zone or across two zones, or even across multiple zones (multipoint conference). The call setup and teardown is performed through the call signaling channel whose TSAP is the well-known TCP port (1720). The call control, including opening/closing media channels between two endpoints, is performed through the call control channel whose TSAP is dynamically assigned from the previous call signaling process. H.245 messages are used in the call control channel, and are encoded using ASN.1.

  Note: Detailed information on H.323 can be found in ITU-T RECOMMENDATION H.323.

The H.323 ALG lets you secure VoIP communication between terminal hosts, such as IP phones and multimedia devices. In such a telephony system, gatekeeper J-series device manage call registration, admission, and call status for VoIP calls. Gatekeepers can reside in the two different zones or in the same zone. (See Figure 93.)

Figure 93: H.323 Protocol

Note: The illustrations use IP phones for illustrative purposes, although it is possible to make configurations for other hosts that use VoIP, such as Microsoft NetMeeting multimedia devices.

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