text: /*
   * Copyright (c) 2018-2019 Cisco and/or its affiliates.
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at:
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  #include <vnet/tcp/tcp.h>
  #include <math.h>
  
  #define beta_cubic 	0.7
  #define cubic_c		0.4
  #define west_const 	(3 * (1 - beta_cubic) / (1 + beta_cubic))
  
  typedef struct cubic_cfg_
  {
    u8 fast_convergence;
    u32 ssthresh;
  } cubic_cfg_t;
  
  static cubic_cfg_t cubic_cfg = {
    .fast_convergence = 1,
    .ssthresh = 0x7FFFFFFFU,
  };
  
  typedef struct cubic_data_
  {
    /** time period (in seconds) needed to increase the current window
     *  size to W_max if there are no further congestion events */
    f64 K;
  
    /** time (in sec) since the start of current congestion avoidance */
    f64 t_start;
  
    /** Inflection point of the cubic function (in snd_mss segments) */
    u32 w_max;
  
  } __clib_packed cubic_data_t;
  
  STATIC_ASSERT (sizeof (cubic_data_t) <= TCP_CC_DATA_SZ, "cubic data len");
  
  static inline f64
  cubic_time (u32 thread_index)
  {
    return transport_time_now (thread_index);
  }
  
  /**
   * RFC 8312 Eq. 1
   *
   * CUBIC window increase function. Time and K need to be provided in seconds.
   */
  static inline u64
  W_cubic (cubic_data_t * cd, f64 t)
  {
    f64 diff = t - cd->K;
  
    /* W_cubic(t) = C*(t-K)^3 + W_max */
    return cubic_c * diff * diff * diff + cd->w_max;
  }
  
  /**
   * RFC 8312 Eq. 2
   */
  static inline f64
  K_cubic (cubic_data_t * cd, u32 wnd)
  {
    /* K = cubic_root(W_max*(1-beta_cubic)/C)
     * Because the current window may be less than W_max * beta_cubic because
     * of fast convergence, we pass it as parameter */
    return pow ((f64) (cd->w_max - wnd) / cubic_c, 1 / 3.0);
  }
  
  /**
   * RFC 8312 Eq. 4
   *
   * Estimates the window size of AIMD(alpha_aimd, beta_aimd) for
   * alpha_aimd=3*(1-beta_cubic)/(1+beta_cubic) and beta_aimd=beta_cubic.
   * Time (t) and rtt should be provided in seconds
   */
  static inline u32
  W_est (cubic_data_t * cd, f64 t, f64 rtt)
  {
    /* W_est(t) = W_max*beta_cubic+[3*(1-beta_cubic)/(1+beta_cubic)]*(t/RTT) */
    return cd->w_max * beta_cubic + west_const * (t / rtt);
  }
  
  static void
  cubic_congestion (tcp_connection_t * tc)
  {
    cubic_data_t *cd = (cubic_data_t *) tcp_cc_data (tc);
    u32 w_max;
  
    w_max = tc->cwnd / tc->snd_mss;
    if (cubic_cfg.fast_convergence && w_max < cd->w_max)
      w_max = w_max * ((1.0 + beta_cubic) / 2.0);
  
    cd->w_max = w_max;
    tc->ssthresh = clib_max (tc->cwnd * beta_cubic, 2 * tc->snd_mss);
    tc->cwnd = tc->ssthresh;
  }
  
  static void
  cubic_loss (tcp_connection_t * tc)
  {
    cubic_data_t *cd = (cubic_data_t *) tcp_cc_data (tc);
  
    tc->cwnd = tcp_loss_wnd (tc);
    cd->t_start = cubic_time (tc->c_thread_index);
    cd->K = 0;
    cd->w_max = tc->cwnd / tc->snd_mss;
  }
  
  static void
  cubic_recovered (tcp_connection_t * tc)
  {
    cubic_data_t *cd = (cubic_data_t *) tcp_cc_data (tc);
    cd->t_start = cubic_time (tc->c_thread_index);
    tc->cwnd = tc->ssthresh;
    cd->K = K_cubic (cd, tc->cwnd / tc->snd_mss);
  }
  
  static void
  cubic_cwnd_accumulate (tcp_connection_t * tc, u32 thresh, u32 bytes_acked)
  {
    /* We just updated the threshold and don't know how large the previous
     * one was. Still, optimistically increase cwnd by one segment and
     * clear the accumulated bytes. */
    if (tc->cwnd_acc_bytes > thresh)
      {
        tc->cwnd += tc->snd_mss;
        tc->cwnd_acc_bytes = 0;
      }
  
    tcp_cwnd_accumulate (tc, thresh, tc->bytes_acked);
  }
  
  static void
  cubic_rcv_ack (tcp_connection_t * tc, tcp_rate_sample_t * rs)
  {
    cubic_data_t *cd = (cubic_data_t *) tcp_cc_data (tc);
    u64 w_cubic, w_aimd;
    f64 t, rtt_sec;
    u32 thresh;
  
    /* Constrained by tx fifo, can't grow further */
    if (tc->cwnd >= tc->tx_fifo_size)
      return;
  
    if (tcp_in_slowstart (tc))
      {
        tc->cwnd += tc->bytes_acked;
        return;
      }
  
    t = cubic_time (tc->c_thread_index) - cd->t_start;
    rtt_sec = clib_min (tc->mrtt_us, (f64) tc->srtt * TCP_TICK);
  
    w_cubic = W_cubic (cd, t + rtt_sec) * tc->snd_mss;
    w_aimd = (u64) W_est (cd, t, rtt_sec) * tc->snd_mss;
    if (w_cubic < w_aimd)
      {
        cubic_cwnd_accumulate (tc, tc->cwnd, tc->bytes_acked);
      }
    else
      {
        if (w_cubic > tc->cwnd)
  	{
  	  /* For NewReno and slow start, we increment cwnd based on the
  	   * number of bytes acked, not the number of acks received. In
  	   * particular, for NewReno we increment the cwnd by 1 snd_mss
  	   * only after we accumulate 1 cwnd of acked bytes (RFC 3465).
  	   *
  	   * For Cubic, as per RFC 8312 we should increment cwnd by
  	   * (w_cubic - cwnd)/cwnd for each ack. Instead of using that,
  	   * we compute the number of packets that need to be acked
  	   * before adding snd_mss to cwnd and compute the threshold
  	   */
  	  thresh = (tc->snd_mss * tc->cwnd) / (w_cubic - tc->cwnd);
  
  	  /* Make sure we don't increase cwnd more often than every segment */
  	  thresh = clib_max (thresh, tc->snd_mss);
  	}
        else
  	{
  	  /* Practically we can't increment so just inflate threshold */
  	  thresh = 50 * tc->cwnd;
  	}
        cubic_cwnd_accumulate (tc, thresh, tc->bytes_acked);
      }
  }
  
  static void
  cubic_conn_init (tcp_connection_t * tc)
  {
    cubic_data_t *cd = (cubic_data_t *) tcp_cc_data (tc);
    tc->ssthresh = cubic_cfg.ssthresh;
    tc->cwnd = tcp_initial_cwnd (tc);
    cd->w_max = 0;
    cd->K = 0;
    cd->t_start = cubic_time (tc->c_thread_index);
  }
  
  static uword
  cubic_unformat_config (unformat_input_t * input)
  {
    u32 ssthresh = 0x7FFFFFFFU;
  
    if (!input)
      return 0;
  
    unformat_skip_white_space (input);
  
    while (unformat_check_input (input) != UNFORMAT_END_OF_INPUT)
      {
        if (unformat (input, "no-fast-convergence"))
  	cubic_cfg.fast_convergence = 0;
        else if (unformat (input, "ssthresh %u", &ssthresh))
  	cubic_cfg.ssthresh = ssthresh;
        else
  	return 0;
      }
    return 1;
  }
  
  const static tcp_cc_algorithm_t tcp_cubic = {
    .name = "cubic",
    .unformat_cfg = cubic_unformat_config,
    .congestion = cubic_congestion,
    .loss = cubic_loss,
    .recovered = cubic_recovered,
    .rcv_ack = cubic_rcv_ack,
    .rcv_cong_ack = newreno_rcv_cong_ack,
    .init = cubic_conn_init,
  };
  
  clib_error_t *
  cubic_init (vlib_main_t * vm)
  {
    clib_error_t *error = 0;
  
    tcp_cc_algo_register (TCP_CC_CUBIC, &tcp_cubic);
  
    return error;
  }
  
  VLIB_INIT_FUNCTION (cubic_init);
  
  /*
   * fd.io coding-style-patch-verification: ON
   *
   * Local Variables:
   * eval: (c-set-style "gnu")
   * End:
   */
  