Transport Layer 3-73 TCP fast retransmit  time-out period often relatively long:  long delay before resending lost packet  detect lost segments via duplicate ACKs  sender often sends many segments back- to-back  if segment is lost, there will likely be many duplicate ACKs if sender receives 3 ACKs for same data ( triple duplicate ACKs ), resend unacked segment with smallest seq #  likely that unacked segment lost, so don't wait for timeout TCP fast retransmit
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