Design And Implementation Of TURN Based Solution To Symmetric NAT

In order to resolve the problem of the shortage of IPv4 addresses before IPv6 widely used, NAT technology has been proposed. Some standard application protocols work well with NAT. However, some end-to-end services rely on data packets, such as SIP in which the message body contains the IP and port information needed by the following RTP media stream. This will cause the network application programs using SIP as the signaling not to work properly. Therefore, the NAT traversing problem is the one which needs to be solved urgently.There exists several NAT traversing methods, but each has its limitation. For example, ALG requires that the existing NAT equipments must support the corresponding software update. The STUN method can not traverse symmetric NAT. For the traversing of symmetric NAT, there are neither standard protocols nor mature technologies. This makes the traversing of symmetric NAT to be a difficult today.The control/media channel carrying voice/video protocols based on IP faces the challenge in traversing symmetric NAT. This issue deals with this problem, and introduces a TURN based solution. The TURN client traverses through the NAT to communicate with external host by the forwarding of TURN server. With the combination of STUN, this scheme can not only traverse all types of NAT, but also reduce the load of TURN server. Besides, it dose not need any modification of the NATs. Compared with the traditional traversing methods, its greatest feature is that it can not only traverse the general types of NAT, but also traverse the symmetric NAT. This solution has been adopted successfully in the VoIP Gateway based on the embedded Linux platform.