what is the reason to send an ack message by client before server window is over?

the 开发者_开发技巧 situation is this :

the client starting connection establishment and requesting a file . the server start to send the file but in the middle of the server sending window the client send an ack message . is it because the server sending rate is higher ? is there more reason or condition for that to happen?

There is no requirement to wait for the end of the window; quoting RFC 2018

The receiver SHOULD send an ACK for every valid segment that arrives containing new data

Short answer: This behaviour reduces latency.