Defined above is the books VPN, popular speakers, VPN is an encrypted tunnel.

上文是书本上定义的VPN，通俗的讲，VPN是一条加密的隧道。

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Connection protocol: This protocol multiplexes the encrypted tunnel to numerous logical channels, running over the User Authentication protocol.

连接协议：该协议多路传输加密隧道到多个逻辑通道，通过用户认证协议运行。

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Once a connection to the server has been established (including a connection via a proxy or over a TLS-encrypted tunnel), the client MUST send an opening handshake to the server.

假如开启了安全选项，客户端在打开连接后发送数据前，必须进行TLS握手。

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