Research Of TCP Traversal Technology Based On Multi-type NAT

Transmission Control Protocol/Internet Protocol(TCP/IP), as the basic Internet protocol over thirty years, but some of its rules can't apply to the present Internet. The most striking problem is the shortage of IP addresses. To solve this problem, the next generation Internet Protocol version 6(IPv6) expands the space of addresses to 128 bit, which greatly increasing the number of IP addresses, and guaranteeing more safety, convenience, speed than Protocol Version 4(IPV4). However, devices with IPV4 have been widely applied in practical life so it will take very long time for devices with IPV6 to replace them. In the past ten years, Network Address Translation(NAT) has become the critical technology to relive the problems of IP addresses shortage.The score of NAT is to substitute internal IP address for public IP address, making network devices share one public IP Address to save IP addresses in large. Thus, it can hide internal structure of local area network well and enhance its safety. However, NAT is contradictory to TCP/IP. Many end-to-end applications in TCP/IP are limited by NAT. For instance, basic Internet operations, such as the limitation of remote accessing to family or company network devices. Therefore, NAT traversal is becoming more and more important. Transmission Control Protocol(TCP) provides more credible connections than User Data-gram Protocol(UDP) while its three handshake procedure makes it more difficult to traversal NAT. Based on what I have mentioned above, the content of this paper is as follows.Firstly, minor symmetric NAT in the Internet, which chooses port at random, might fail to predict the right port mapping and finally fail to realize traversal, which due to its special way of choosing port mapping. After researching shortcomings of current way, we analyzed characteristics of port mapping and propos a new method to detect port. Moreover, comparing these two methods we had found that the new method is more likely to succeed.Secondly, some NAT refuses SYN packet even with mapping record, causing the failure of TCP handshake. STUNT is the most frequently used technology to filter the limitation of NAT. We analyzed its principles and process, pointed out its weakness, such as stability and connection approach, and gived a new method to filter NAT, which turns out to be better than STUNT by experiment.Lastly, combining all the researches, we invented an universal multiple NAT port detection scheme. The new detection scheme together with the new filtering scheme, which constituted a NAT traversal system can be used in multiple environments and its feasibility has been examined by test.