The Research Of SIP Based VoIP Applications Traversing NAT Or Firewall

In the past decade, great attention has been paid to a new technology named VoIP(Voice over IP), which challenges the traditional PSTN telephony with its powerful functions, cheap price and obvious facility. Not only voice is supported by VoIP., more services such as video, data and fax etc. is also supported. VoIP becomes one of the most representative and prospective technologies of the new network era.One of the key technologies VoIP depends on is signaling. H.323 and SIP are the two presently most widely used signaling protocols in VoIP. H.323 occupies the mainstream of VoIP signaling currently. Howerver, interest in SIP increases progressively, because SIP is in accordance with the trend of VoIP and the future networks, with its excellence in brief structure, distributed control pattern and close connection with Internet.With the arrival of its maturation, VoIP has been more rapidly developed and spread. However some difficulties and problems also arise unavoidably and the problem, how VoIP applications traverse NAT or Firewall, is one of them. The problem, along with some main ideas of its solution is introduced here. This article focuses on the resolution of this problem for SIP. Detailed introduction on SIP is made firstly, then, based on the knowledge of it, a solution named "SIP Application Level Gateway (SIP ALG)" is brought forward. The resolution tries to resolve the problem by modifying SIP messages in application level when they traverse NAT/FW, and controlling some behaviors of the NAT/FW. Principle and realization detail of the resolution, as well as test result is rendered in this article.