keep-alive 机制

• 发送 ping/pong 管理包

• 发出 ping 后，一直没有 pong 包响应，就认为对端死掉，相当于 socket error/end 事件

• At handshake, client send keep-alive-interval to noradle-dispatcher

• noradle-dispatcher call socket.setTimeout(num, cb) to monitor timeout event

• when timeout detected, set state to probe, and send a ping frame, and wait for 3s with another setTimeout

• if no pong is received, treat the connection as dead, call socket.end, release resources

• if pong is received within time limit, it's ok

graceful quit 机制

发送 quit 管理包，通知对端不要再发送新包，当前请求的还允许继续

连接中断事件检测

发现异常中断事件，需要看是否有在途请求，并作响应处理