1         2          3          4      
              ----------|----------|----------|---------- 
                     SND.UNA    SND.NXT    SND.UNA        
                                          +SND.WND        

        1 - old sequence numbers that have been acknowledged  
        2 - sequence numbers of unacknowledged data            
        3 - sequence numbers allowed for new data transmission 
        4 - future sequence numbers that are not yet allowed