NAT 穿透

/*********************************************************************************
 *                                  NAT 穿透
 * 说明：
 *     NAT网络数据穿透，了解一下。
 *
 *                                              2017-10-30 深圳 南山平山村 曾剑锋
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一、参考文档：
    1. 怎样用python实现NAT穿透
        https://groups.google.com/forum/#!topic/python-cn/qwEpn_nP5ZU
    2. Python实现STUN+TURN+P2P聊天
        https://laike9m.com/blog/pythonshi-xian-stunturnp2pliao-tian,29/
    3. PyPunchP2P
        https://github.com/laike9m/PyPunchP2P

二、NAT类型：
    1. Full Cone（完全穿透）: Any external host can send a packet to the internal host, by sending a packet to the mapped external address.（任何外部主机发送数据包到内部主机映射的外部地址，内部主机都能接收到数据包。）
    2. Restricted Cone（Address Restricted Cone 地址限制穿透）: An external host (with IP address X) can send a packet to the internal host only if the internal host had previously sent a packet to IP address X. Once an internal address (iAddr:port1) is mapped to an external address (eAddr:port2), any packets from iAddr:port1 will be sent through eAddr:port2. Any external host can send packets to iAddr:port1 by sending packets to eAddr:port2。（只有当一个内部主机已经提前发送了一个数据包给外部主机（IP地址X），外部主机才能发送数据包到内部主机。一旦一个内部主机地址（iAddr:port1）映射到外部主机地址（eAddr:port2），任何从iAddr:port1发送的数据包都会通过eAddr:port2发出去，任何外部主机可以发送数据包到eAddr:port2转发到iAddr:port1中。）
    3. Port Restricted Cone（端口限制穿透）: A port restricted cone NAT is like a restricted cone NAT, but the restriction includes port numbers.（端口限制穿透NAT很像地址限制穿透NAT，其中的限制条件还要加上端口号。）
    4. Symmetric NAT（对称穿透）: Each request from the same internal IP address and port to a specific destination IP address and port is mapped to a unique external source IP address and port. If the same internal host sends a packet even with the same source address and port but to a different destination, a different mapping is used. Only an external host that receives a packet from an internal host can send a packet back.（每个相同的内部IP、Port都会映射成唯一的外部IP、Prort，如果内部的同一台主机发送数据包给不同的目标主机，会采用不同的映射，只有收到内部主机发送的数据包的外部主机才能发送回数据。）