ACK (ACKnowledgment) 的意思是： TCP首部中的确认标志，对已接受到的TCP报文进行确认。
============================================= ACK - ACK signal In some digital communication protocols, ACK is the name of a signal that data has been received successfully (for example, with an acceptable number of errors). The ACK signal is sent by the receiving station (destination) back to the sending station (source) after the receipt of a recognizable block of data of specific size. In order to be recognizable, the data block must conform to the protocol in use. When the source receives the ACK signal from the destination, it transmits the next block of data. If the source fails to receive the ACK signal, it either repeats the block of data or else ceases transmission, depending on the protocol. The ACK signal is usually an ASCII character that is reserved for that purpose. In some protocols, there are various ACK signals that indicate the successful reception and recognition of specific commands, such as power-down or standby. 什么是ACK信号？ 在数字通信协议中，如果接受方成功的接收到数据，那么会回复一个ACK信号。
通常ACK信号有自己固定的格式，长度大小，由接受方回复给发送方。
其格式取决于采取的网络协议。
当发送方接收到ACK信号时，就可以发送下一个数据。
如果发送方没有收到信号，那么发送方可能会重发当前数据包，也可能停止传送数据。
具体情况取决于所采用的网络协议。
ACK信号通常是一个ASCII字符，不同的协议中ACK信号都不一样。
Acknowledge Character 就是确认字符
