This uses two messages. echo request echo response As often as every 60 seconds, a GSN can send an echo request to every other GSN with which it has an active connection.
这种机制依靠以下两种消息实现： Echo Request Echo Response 每隔60秒，一个网元可以向每个有活跃连接的对端发送一个Echo Request，来确认连接有效。