keep-alive

基本解释

In order to maintain presence, the chat application can send keep-alive messages on the websocket to prevent it being closed due to an idle timeout.
为了保持存在的状态，聊天应用要发送“连接保持（Keep Alive）”的消息给WebSocket，来避免该连接因为闲置超时而关闭。
Since clients do not wish to be forcefully disconnected, they will automatically send a keep-alive message if the connection is inactive for the number of seconds specified by this parameter.
由于客户机不希望强制断开连接，因此，当连接闲置的时间为此参数指定的秒数时，客户机将自动发送 keep-alive 消息。
Persistent connections specify that an outgoing HTTP response should use a persistent (keep-alive) connection instead of a connection that closes after one request or response exchange occurs.
持久连接指定传出的 HTTP 响应应该使用持久（保持活动状态）连接，而不是在请求或响应交换后关闭的连接。

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