NAT traversal is a cover all term used to describe the various methods employed to set up and maintain a TCP/IP network or UDP connections that cross NAT gateways. These techniques are needed in most peer to peer and VoIP applications. While there are many different ways of taking care of these issues there is no one set way that works for everyone as there is no real standardization of NAT protocols. Some techniques must have access to publicly routed IP address server while others use only the particular server in question.
The problem with NAT traversal is that since the system is designed to use many different computers on the public network and most people now make use of at least one firewall it makes it more difficult for the process to function. What you need to understand is that a NAT router is a simple device that is designed to allow several computers to share access to the Internet via a single IP address. This may not seem like much of a problem as long as you are sending communications out and setting permission levels that allow others to communicate with your network.
However if you are into peer to peer file sharing or online gaming then NAT traversal can become more of a challenge. There are different techniques and applications that can help overcome these problems, however as previously stated the large variety of different NAT routers and protocols makes it very difficult to create one application that can be used on all routers to make the traversal much easier. However new standards are being released that call for more standardization under RFC 4787 so that those who play online with the different gaming systems from Sony, Nintendo and Microsoft will all work smoothly.
The concept behind NAT traversal is that you need to have access to a router on a public IP to act as a relay for the information that is to be passed back and forth. It works like this computer A wants to communicate with computer B both of which are using their own unique NAT routers, to do this A sends a request to an intermediary or public server that can then relay it to B in a form that it can read. While this is an oversimplified version the public server sends out the information that can traverse the firewall in the NAT router of both sender and recipient allowing for the exchange of information freely.
From:nattraversal.net
This page is wiki editable click here to edit this page.在计算机科学中，NAT穿越（NAT traversal）涉及TCP/IP网络中的一个常见问题，即在处于使用了NAT设备的私有TCP/IP网络中的主机之间建立连接的问题。
会遇到这个问题的通常是那些客户端网络交互应用程序的开发人员，尤其是在对等网络和VoIP领域中。IPsec VPN客户普遍使用NAT-T来达到使ESP包通过NAT的目的。
尽管有许多穿越NAT的技术，但没有一项是完美的，这是因为NAT的行为是非标准化的。这些技术中的大多数都要求有一个公共服务器，而且这个服务器 使用的是一个众所周知的、从全球任何地方都能访问得到的IP地址。一些方法仅在建立连接时需要使用这个服务器，而其它的方法则通过这个服务器中继所有的数据——这就引入了带宽开销的问题。
两种常用的NAT穿越技术是：UDP 打洞和STUN。 除此之外，还有TURN， ICE，ALG，以及SBC。
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