Definition
Acknowledgement Number (TCP)
The acknowledgement number is a 32-bit field in the TCP header that specifies the next sequence number the sender of the segment expects to receive.
It acknowledges all data bytes with sequence numbers less than this value, meaning the receiver has successfully received every byte up to
acknowledgement number − 1.The ACK flag must be set for the acknowledgement field to be valid. Once a connection is established, every segment (except the initial SYN) typically carries an ACK.