The NAT Traversal Problem
NAT devices are commonly used to alleviate IPv4 address exhaustion by allowing the use of private IP addresses on home and corporate networks behind routers with a single public IP address facing the public Internet. The internal network devices communicate with hosts on the external network by changing the source address of outgoing requests to that of the NAT device and relaying replies back to the originating device. This leaves the internal network ill-suited to host servers, as the NAT device has no automatic method of determining the internal host for which incoming packets are destined. This is not a problem for home users behind NAT devices doing general web access and e-mail. However, applications such as peer-to-peer file sharing, VoIP services and the online services of current generation video game consoles require clients to be servers as well, thereby posing a problem for users behind NAT devices, as incoming requests cannot be easily correlated to the proper internal host. Furthermore many of these types of services carry IP address and port number information in the application data, potentially requiring substitution or special traversal techniques for NAT traversal.
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