I have noticed (using tcpdump and simple TCP client-server application)
that TCP frame are always sent with ACK flag even if the sender doesn't
receive new frames (i.e. it send the same Acknowledgment Number).
RFC 2581 (section 4.1) states that:
"A TCP receiver MUST NOT generate more than one ACK for every
segment, other than to update the offered window as the receiving
application consumes new data"
According to the above statement it seems that LINUX TCP / IP violates