Between Intranet and the Internet, a firm firewall is usually established and communication is greatly restricted. This is a measure to make enterprise network secure and we have to accept some degree of loss of convenience. Communication protocol which can pass through firewall is often determined by the enterprise security policy, and generally only web-site access from the inside of the company and dispatch and receipt of e-mails are allowed. The enterprise security policy is just like the constitution of the Information Management Division in the company, and once established, it is quite difficult to change it.
Therefore, if only specific application can pass through firewall securely without modifying the security policy, the merit will be large. For instance, the use of IP phone is expected to grow in the future, replacing cell phone. The merit of IP phone is that it does not cost anything for communication. If more access points are established and wireless LAN area is widespread, IP phone will certainly replace cell phone rapidly. If IP phone communication calls pass through firewall securely, business communication will also be made by IP phone, enabling a great deal of merit therefrom (Fig.1).
Thus, we propose an IP phone system which goes through firewall without causing any problems. We expect to use a standard type IP phone, namely an IP phone system where SIP (Session Initiation Protocol) is used for dialing, and VoIP based on UDP is used for voice communication.