TCP Congestion Control Brief Description by Pseudo Code

class TCPCongestionControl
  # rwnd receiver's advertised window
  def initialize(segment_size)
    # segment size announced by the other end, or the default,
    # typically 536 or 512
    @segment_size = segment_size
    @cwnd = segment_size
    @ssthresh =  65535 # bytes
    @in_fast_recovery = false
    # other logic ....
  end

  def receive
    # indicate congestion
    if current_algorithm == :slow_start && new_ack?
      # exponential growth
      @cwnd += @segment_size
      # may go_to congestion_avoidance state
      # Slow start continues until TCP is halfway to where it was when congestion
      # occurred (since it recorded half of the window size that caused
      # the problem in step 2), and then congestion avoidance takes over.
    elsif current_algorithm == :congestion_avoidance && new_ack?
      # linear growth
      @cwnd += segsize * segsize / @cwnd
    elsif three_or_more_duplicate_ack?
      # TCP does not know whether a duplicate ACK is caused by a lost segment or just a reordering of segments
      # it waits for a small number and assume it is just a a reordering of segments
      # 3 or more duplicate ACKs are received in a row,
      # it is a strong indication that a segment has been lost
      # go_to fast recovery
      @in_fast_recovery = true
      @ssthresh = current_window_size / 2
      retransmit_missing # retransmit directly without waiting timer
      @cwnd = @ssthresh + 3 * @segment_size
    # This inflates the congestion window by the number of segments that have left the network and which the other end has cached (3).
    elsif current_algorithm == :fast_recovery && new_ack?
      @cwnd = @ssthresh # go_to congestion_avoidance
      @in_fast_recovery = false
    elsif (current_algorithm == :slow_start || current_algorithm == :congestion_avoidance) && duplicate_ack?
      @ssthresh = current_window_size / 2 # go_to congestion avoidance
    elsif current_algorithm == :fast_recovery && duplicate_ack?
      @cwnd += @segment_size
      # This inflates the congestion window for the additional segment that has left the network.  Transmit a packet, if allowed by the new value of cwnd.
    elsif timeout?
      # slow_star and congestion_avoidance should come into here
      # not sure when timout occurs when fast_recevery should come into here too...
      @ssthresh = current_window_size / 2
      @cwnd = segment_size # go_to slow start
    end
    
    maybe_transmit_packet
  end

  private
  def current_algorithm
    return :fast_recovery if @in_fast_recovery

    @cwnd <= @ssthresh ? :slow_start : :congestion_avoidance
  end

  def current_window_size
    min(@cwnd, @rwnd)
  end
end