The SIP-Based System Used in Connection with a Firewall

Different methods, such as ALG, STUN, TURN and FCP have been proposed for using SIP together with firewalls and NAT. The most suitable solution has to be determined in every situation. Some of these methods cause more network traffic and capacity requirements, while others require support from the clients being used in the network. In this paper a system which makes it possible to offer SIP service to customers behind NAT without making any changes to firewalls is presented. The system is based on an RTP proxy and a SIP server, which modifies SIP messages.