Through Firewall / Nat And Sip-based Voip

With the flourishing development of IP network broadband services in recent years,the text_based multimedia communication system standard SIP has got more and moreextensive application in the video conference and VoIP services. But the application ofVoIP services brings a question of meritting attention: In order to solve the networksecurity problem and resist the deficient problem of IP address, a large number ofenterprises and CPN(Customer Premise Network) have adopted NAT and firewall. But theprotocols (such as SIP, H.323 etc.) that VoIP adopts is unable to traverse NAT or firewallsmoothly. As a result,the Video Conference products based on VoIP unable to use onBroadband Metropolitan Area network and Intranet.This paper firstly expatiates the technic background and develop specifications ofVolP, analyzes and studies the communication technologies of the SIP_based VolP systemand SIP protocols in detail,especially it's calling process and the security technologies ofthe SIP-based VoIP System; and also studies the basic firewall and NAT technologies.Based on those analysis and studies,this paper summarizes the problems that may occurwhen the SIP_based VoIP system communications across the sub-networks that using NATand firewall. Then this paper list and compare several solutions such as SIPALG, MIDCOM solution, STUN solution, Full Proxy solution and tunnel traversing solution,and emphatically expatiates the basic theories and implement frame of the tunneltraversing solution.Finally, this paper evaluates the rough performance of the tunnel traversing solutionthrough a enterprise-level VoIP applications instance, and brings forth the proposition thatcontinue study.