H323 is a suite of ITU protocols for audio-video conferencing and collaboration applications. H323 consists of h.225 call signaling protocols and h.245, the control protocol for media communication. During h.225 negotiation, endpoints create a call by exchanging call signaling messages on the control channel and negotiate a new control channel for h.245. A new control connection is created for h.245 messages. Messages are exchanged on the h.245 control channel to open media channels.
The JUNOS stateful firewall service monitors the h.225 control channel to open the h.245 control channel. Once the h.245 channel is created, the stateful firewall service also monitors this channel for media channel information and allows the media traffic through the firewall. The H323 ALG supports static destination, static source, and dynamic source NAT by rewriting the appropriate addresses and ports in the h.225 and h.245 messages.