The forthcoming Next Generation Network (NGN) is an all IP network. It should guarantee multimedia communications to traverse NAT and firewall. The existing traversal methods are not suitable for service provider to set up a real system in NGN. Multi-agent System for Multimedia Communication Traversing NAT and Firewall (MASMCT) is given to resolve the problem. MASMCT is secure and easy to set up for service provider. It has a two-layer multi-agent architecture and can be classified as client agents and server agents. Each of them in MASMCT is transparent for endpoint users and softswitch. They coact and relay call signaling message and media data between endpoint and softswitch. One of its key aspects is the introduction to the mechanism and function model of MASMCT. The Protocol of Multiplexed Connection for Bundled Session (MCBS) defined in this paper is used by agents to communicate with each other, which can be the standard protocol in the future NGN.