获得一个成功的连接前，连接协商可能反复交换多个握手消息。
Connection negotiation may be iterative and several handshake messages may be exchanged before a successful connection is obtained.
服务端也可能要求连接重新协商，这要发送一个不成功的连接响应，并在握手消息中建议重连。
Servers may also request connection renegotiation by sending an unsuccessful connect response with advice to reconnect with a handshake message.
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