http2session.alpnProtocol

• 类型：<string> | <undefined>

如果 Http2Session 尚未连接到套接字，值将为 undefined；如果 Http2Session 未连接到 TLSSocket，值将为 h2c；或者将返回已连接 TLSSocket 自身的 alpnProtocol 属性的值。

【Value will be undefined if the Http2Session is not yet connected to a socket, h2c if the Http2Session is not connected to a TLSSocket, or will return the value of the connected TLSSocket's own alpnProtocol property.】