用Android的话来说，不能够及时响应的应用经常使系统弹出令人恐惧的“应用没有响应”(ANR)的消息。
In Android terms, applications that are insufficiently responsive will frequently cause the system to pop up the dreaded "application Not Responding" (ANR) message.
如果存在到主机的有效路由，而且它能够响应ping，但是应用服务没有响应，那么可以使用telnet检查到相关端口的连接。
If a valid route exists to the host, and it's responding to pings but the application service is failing to respond, then you can check connectivity to the relevant ports using Telnet.
如果主机是已知的，但是用户抱怨这个主机本身或它上面运行的应用程序没有响应，那么使用ping并查看数据包丢失率是否为0%(见清单6)。
If a host is known but you find that users are complaining that the host itself or an application running on it isn't responding, use ping and look for 0% packet loss (see Listing 6).
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