3.1.5.1.2 Receiving an ACK Packet
When an ACK packet is received, the following applies:
If the value of the WNDW field of the ACK packet is greater than the value of the HighWaterForSend variable of the Session object, the receiver of the ACK packet MUST do the following:
If an ACK packet is received in the FIN RECEIVED state, an error SHOULD be raised to the higher layer and the underlying transport connection SHOULD be closed.
If the value of the SEQNUM field of the ACK packet is not equal to the value of the SeqNumForRecv variable of the Session object, an error is raised to the higher layer and the underlying transport connection is closed.