RUT230 has "VoIP passthrough support H.232 and SIP-alg protocol NAT helpers, allowing proper routing of VoIP packets"
The NAT Helpers section provides you the option to add firewall exceptions for some VoIP protocols, namely SIP and H.323. In other words, these functions provide a pass-through for VoIP communications between the router's LAN and WAN.
FTP, SIP and H.323 protocols are harder to filter by firewalls since they violate layering by introducing OSI layer 3/4 parameters in the OSI layer 7. NAT helpers are modules that are able to assist the firewall in tracking these protocols. These helpers create the so-called expectations that can be used to open necessary ports for RELATED connections. For example: FTP, GRE and PPTP helpers are enabled by default.
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