3.1.5.2.1 Session Variable Relationships for the Sender
The DATA packet MUST NOT be sent if the value of the SeqNumForSend variable of the Session object is equivalent to the value of the HighWaterForSend variable of the Session object.
Otherwise, the value of the SEQNUM field of the DATA packet is set to the value of the SeqNumForSend variable plus 1, the DATA packet is sent, and the value of the SeqNumForSend variable is incremented by 1.
Upon receiving a packet, the value of the HighWaterForSend variable isset to the value of the WNDW field of the received packet.
Note The value of the send window size equals the value of the HighWaterForSend variable minus the value of the SeqNumForSend variable. The send window is considered closed when the value of the send window size is 0. The maximum send window size for the implementation described in this document is 4.