Research And Design Of VoIP Over NAT Based On ALG

Nowadays, with the rapid development of Internet, many new applications of business have been supported. Along with the development, more and more people have paid attention to the problem of the depletion address resource. The long-term solution was expected to be a proposal for a new Internet protocol with larger addresses and we now know that the next generation of IP will be IPv6. But the Internet Network from IPv4 to IPv6 will have many works to do in future. People want to find an easy way to sovle the problem. NAT [RFC1631] is widely thought of as a short-term solution until the time of IPv6 network coming.The edging router with NAT can make a translation to the address in the IP packet outbound and inbound according to the NAT rule. Through a proper Intranet stratery with NAT, not only the communication with Internet can be provided to Enterprise, but also the sercurity of Intranet can be guaranteed by some custom-built rules.Multimedia technology is one of the focus realms in recent computer network science. Enterprise can reduce their spending on communication by using VoIP and obtain higher qulity of service. H.323 standard and SIP are two kinds of most important and widely-using VoIP protocol.And the solution of VoIP through NAT is also paid much more attention. By far, some solutions have been advised for the topic. Application level gateway, called ALG, is the most popular way. And many network equipment producers configure ALG in their their NAT module.Base on ALG, the paper gives a comprehensive solution for SIP through NAT gateway. Further more, the network environment inside and outsid the NAT gateway and the SIP packet context are sufficiently considerd. At the same time, H.323 through NAT gateway is also mentioned in the paper. For the comparison between H.323 and SIP through NAT gateway, the advantage of SIP in VoIP communication is been proved.