Fast Retransmit

• When a TCP sender receives a 3rd duplicate ACK for the same packet, it assumes the packet has been lost

• • Less than 3 duplicate ACKs could be re-ordering

• But since some packets are getting through, the loss was probably transient

• • So the back-off is less severe

• Upon receiving the 3rd duplicate ACK

• • Retransmit missing packet
  • ssthresh = max(fs/2, 2*MSS)
  • cwnd = ssthresh + 3*MSS
  • • Three packets must have gotten through, so we should reflect that
  • Enter Fast Recovery mode