TCP Congestion Avoidance works alongside Slow Start to make sure a TCP connection is not being overrun with too much data. After a connection is established the Slow Start mechanism begins and the sender window continues to increase exponentially. Each acknowledgement the sender receives warrants the sending window size to double.

  • SENDER –> 1 Segment
  • SENDER –> 2 Segments
  • RECEIVER –> 2 ACKs
  • SENDER –> 4 Segments
  • RECEIVER –> 4 ACKs
  • SENDER –> 8 Segments
  • RECEIVER –> 8 ACKs

A value created when establishing a TCP connection is the Slow Start Threshold (ssthresh). The ssthresh is used by the sender to know whether the TCP connection is in the Slow Start phase or the Congestion Avoidance phase. If the ssthresh is greater than the Congestion Window (CWND) then the TCP connection is in the Slow Start phase. The Slow Start phase will continue exponential growth (double amount of segments per ACK) unless there is data loss on the connection. If the ssthresh is larger than the CWND, then the TCP connection is in the Congestion Avoidance phase. In the Congestion Avoidance phase the CWND is increased by 1 segment only if ALL segments in the window have been acknowledged by the receiver. The idea is to slow down the increasing rate of transmitting data to stabilize the link or utilize the proper amount of physical bandwidth available.

The sender would discover loss/congestion over the connection through either receiving 3 of the same ACKs or a retransmission timeout (no ACK received after timer). If a sender deems there is segment or data loss over a connection then CWND is decreased to 1 MSS and the slow start process begins again.


  • There are different versions of TCP that are more beneficial for LFNs, Long Flat Networks.
  • ssthresh is calculated differently sometimes. Sometimes the value is set to infinite, meaning the CWND will grow infinitely until loss occurs.

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