Aethra Maia XC, Use And Installation Manual

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association between a private IP address and a public IP address reserved for H.323 videoconferences)
for every terminal that must be accessible from an external connection.
The NAT device substitutes the static IP address in the payload and header setup packet sent from the
internal terminal to the external terminal. The destination terminal uses that address for addressing the
reply packets, which are routed through the NAT device to the internal terminal.
Firewall ALG
Application Level Gateways (ALGs) are firewalls programmed to recognize specific IP protocols like H.323.
Instead of looking only at the information contained in packet headers to determine whether to transmit or
block packets, ALGs analyze in detail the data contained in the payload packet. The H.323 protocol inserts
important control information such as audio and video port identification in the payload packets. The
terminal expects to receive audio and video connections from the remote calling terminal on these ports.
By analyzing which port the terminal expects to use, the ALG dynamically opens only those ports, leaving
the others closed to preserve network security. An example of a firewall ALG follows.
The Aethra Application Level Gateway is present in the Aethra Stargate xDSL Router and allows any
videoconferencing terminal, independent of its manufacturer, resolve the NAT/firewall problem. The
Stargate router is capable of checking every incoming and outgoing H.323 call and dynamically opening
only the ports being used for the H.323 videoconference.
The Stargate router also supports NAT functionality and is therefore capable of substituting the public NAT
address for the private IP address automatically inserted in the H.323 payload packets by the internal
terminal. When the Aethra ALG functionality is used with an Aethra videoconferencing system, the “Aethra
NAT” function of the videoconferencing system must be disabled because the network equipment is H.323
compatible.