Unreliability: 不可靠性(不靠谱), 是指client向server发出一个请求后, 非得等到server的回复才罢休, 如果server不回复或者回复的packet丢失时, client将呆滞在那里永远等待, 这是不可靠性.
A call that waits for a network operation to complete is said to block the caller. The term blocking is used to describe a call like recv() that makes the client wait until new data arrives.
