TCP Sender Response to Loss

• So far we’ve only considered loss-less cwnd growth

• • Loss changes the adaptive sliding window behavior

• TCP assumes that all loss occurs because of congestion

• • Bit errors are rare
  • So, TCP will “back off” if it thinks the network is congested

• A TCP sender can detect loss in two ways

• • No ACK (timeout) - almost definitely loss
  • Duplicate ACKs - might be loss, not sure

• When a TCP sender times out

• • cwnd = MSS
  • ssthresh = max(fs/2, 2*MSS)
  • • where fs is the flight size (# of unACKed bytes in the network)
  • Retransmit first unACKed packet
  • Go into Slow Start