Hi all,   
   This may be simple but I just begin learning TCP deeply.   
   I am reading TCP FACK paper MM96(http://conferences.sigcomm.org/   
   igcomm/1996/papers/mathis.pdf)   
   and I don't  quite understand below statement:   
      
   if the sender receives an ACK which advances snd.fack beyond the value of   
   snd.nxt at the time a segment was retransmitted (and that retransmitted   
   segment is otherwise unaccounted for), the sender knows that segment which was   
   retransmitted has been lost.   
      
   could someone give more explanation or examples, why the sender can determine   
   the retransmitted segment has been lost?   
      
   thanks,   
   Jan   
      
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