I want to develop a P2P Gaming application. What is the fastest and efficient way to implement NAT traversal and Peer discovery. I have read about STUN and TURN server. But what are the Open source parts available and how can I use them?. 1. Does those STUN server work on 3G networks? 2. Does UDP HOLE PUNCHING works on 3G networks?
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If you have a videogame, you probably want to have a central server that contains all the game data and allow the users to be clients. You know, like Puzzles and Dragons, where you have a username and password and sign in to a server.
Google "Stun Client" and "ICE protocol". There are plenty of open source implementations
STUN fails on mobile networks because mobile networks have symmetric, large scale NAT. Traversing that kind of nat doesn't work with STUN (see reason below)
Why Mobile Broadband NAT cannot be bypassed?
It boils down to statistics. In order for a connection to be established, you have to send a packet into the port that they are at and they have to send a packet into the port that you are at. If you send to the wrong port number or they send to the wrong port number, you miss and no connection is established. If both of you simultaneously bind to a port and send out a packet directed at the other's ip address, you have a 1 in 65535 (65535 being the number of port on an ip address) chance of sending a packet into their port and they have a roughly 1 in 65535 chance of sending a packet into your port. So the chance of you establishing a connection is (1/65535) * (1/65535), or (1 / 65535^2).
You can't know your port number for any subsequent connection because for every new outbound connection, the router randomly gives you a new port number on the interval between 1024 and 65535. So if you ask a server what you ip and port number is, it may tell you the right ip (your ip address doesn't change very often unless you turn off your phone or something like that), but the port number will change. If you try to guess the port number there is a ( (65535-1024-1) / (65535-1024) ), or 99.998% change of you guessing it wrong, assuming that the number of possible port numbers to choose from is (65535-1024).
So unless the port numbers are predictable (which in many 4G networks they are not), you're hosed - no chance.
Your best bet is to use stun and not support evil routers.
