TLP_calc_PTO():
    If SRTT is available:
        PTO = 2 * SRTT
        If FlightSize is one segment:
           PTO += TLP.max_ack_delay
    Else:
        PTO = 1 sec

    If Now() + PTO > TCP_RTO_expiration():
        PTO = TCP_RTO_expiration() - Now()