acknowledgment number (TCP)
The data field Acknowledgement number is a field of the TCP header. The confirmation number, for which four bytes are available, is used to confirm TCP data packets received without errors.
This is a flow control mechanism based on sequential numbering of the data bytes.
By sending the acknowledgement number back to the sender, the receiver confirms to the sender that all data has been received and at the same time indicates which sequence number it expects to receive next. Before sending back, the incremented sequence number is entered into this field.