What is IMS-ALG (IMS application level gateway)?

In the 3GPP IP Multimedia Subsystem architecture, the IM-ALG is a Deep Packet Inspection device which guarantees that addressing to be held inside SIP signalling is eliminated whenever needed to assist NAT traversal. Moreover, the IM-ALG will likewise screen SIP signalling to distinguish malicious attacks. The IM-ALG is a useful element of the Interconnection Border Control Function.