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HAP T ER
This chapter covers how to use the NXC's ALG feature to allow certain applications to pass
through the NXC.
18.1 ALG Introduction
The NXC can function as an Application Layer Gateway (ALG) to allow certain NAT un-
friendly applications (such as SIP) to operate properly through the NXC's NAT.
Some applications cannot operate through NAT (are NAT un-friendly) because they embed IP
addresses and port numbers in their packets' data payload. The NXC examines and uses IP
address and port number information embedded in the VoIP traffic's data stream. When a
device behind the NXC uses an application for which the NXC has VoIP pass through enabled,
the NXC translates the device's private IP address inside the data stream to a public IP address.
It also records session port numbers and allows the related sessions to go through the firewall
so the application's traffic can come in from the WAN to the LAN.
The NXC only needs to use the ALG feature for traffic that goes through the NXC's NAT. The
firewall allows related sessions for VoIP applications that register with a server. The firewall
allows or blocks peer to peer VoIP traffic based on the firewall rules.
You do not need to use a TURN (Traversal Using Relay NAT) server for VoIP devices behind
the NXC when you enable the SIP ALG.
NXC CLI Reference Guide
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ALG
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