Research And Implementation Of SIP End-To-End Application In The Non Peer-To-Peer Network

"Unbeknownst to those outside the field of telecommunications, a quiet revolution is taking place". The cost of traditional calls is high, especially those with long-distance. In recent years, with the development of network technology, the gradual spread of broadband business and the substantial increase of the network bandwidth, people are no longer satisfied with surfing web pages, sending and receiving e-mail and seeking information, but hopeful that communication business will be also involved into the network. As a result, VoIP technology has been widely developed.Nowadays, there are two main VoIP standards: H.323 and SIP. H.323 inherits the traditional advantage of the telecom industry, and has been fully developed. After being adopted by 3GPP as the core protocol of IMS (IP Multimedia Subsystem), SIP has been widely concerned about. With its access-independent feature, SIP can provide multimedia services, which conforms to the trend of network convergence.There are many problems in the development of SIP, of which NAT/Firewall traversal is the most representative one. IPv6 isn't popular at present, and many users access the Internet though NAT. NAT has temporarily solved the problem of lacking IP addresses, and can protect the internal network by hiding internal IP addresses. While SIP requires IP address information, which causes that SIP protocol can not be parsed in existing NAT equipments and makes NAT a big problem.Many experts have brought forward many NAT traversal options, including application level gateway, MIDCOM, STUN, TURN, and so on. But because of the diversity of NAT networks, these options are not widely applicable, such as STUN will not be able to travel the symmetric NAT. And some options need to upgrade the existing NAT equipments and/or firewalls, which restricts the extensive application of these technologies. An integrated solution has been expected for a long time.This paper presents an integrated NAT traversal solution, which can travel all types of NAT without any upgrade of the existing NAT equipments and firewalls. It takes the traits of SIP, makes use of existing technologies, and integrates them, rather than creates a new interactive protocol. It mainly uses two existing solutions: STUN and ALG STUN protocol is simple and easy to use; ALG can travel all types of NAT. And it also avoids their disadvantages: STUN can't travel symmetric NAT, and ALG needs to upgrade existing equipment. Different solutions are applicable to different problems. The solution presented in this paper adopts different ways based on different types of networks: the public SIP terminals connect the SIP server directly; STUN traversal method is used for non-symmetric NAT network and a way similar to ALG traversal is for symmetric NAT network. We call it a way similar to ALG because its traversal behaviors are much like the ALG's. But it isn't a module of NAT equipments, so it avoids upgrading the existing equipments, and solves the problems that in former solutions, all the flow goes through the NAT devices, which increases the burden.