tcp-bic算法

Cong Avoid

low_window = 14
BICTCP_B = 4
max_increment = 16
smooth_part = 20

if (cwnd < ca->last_max_cwnd) {
    __u32 dist = (ca->last_max_cwnd - cwnd) / BICTCP_B;
    if (dist > max_increment)
        ca->cnt = cwnd / max_increment;
    else if (dist <= 1U)
        ca->cnt = (cwnd * smooth_part) / BICTCP_B;
    else
        ca->cnt = cwnd / dist;
} else {
    if (cwnd < ca->last_max_cwnd + BICTCP_B)
        ca->cnt = (cwnd * smooth_part) / BICTCP_B;
    else if (cwnd < ca->last_max_cwnd + max_increment*(BICTCP_B-1))
        ca->cnt = (cwnd * (BICTCP_B-1)) / (cwnd - ca->last_max_cwnd);
    else
        ca->cnt = cwnd / max_increment;

    /* delayed_ack = (15/16) * delayed_ack + (1/16)cnt */
    ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;
}

增长曲线分为六段，离last_max_cwnd越近增长越慢

• cwnd < last_max_cwnd
  • dist > max_increment
  • 1U < dist <= max_increment
  • dist <= 1U
• cwnd >= last_max_cwnd
  • cwnd < last_max_cwnd + BICTCP_B
  • cwnd < last_max_cwnd + max_increment*(BICTCP_B-1)
  • cwnd >= last_max_cwnd + max_increment*(BICTCP_B-1)

Recalc Ssthresh

    if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
        /* last_max_cwnd = cwnd *(1024 + 819) / 2048 = 0.8999 * cwnd */
        ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta)) / (2 * BICTCP_BETA_SCALE);
    else
        ca->last_max_cwnd = tp->snd_cwnd;

    ca->loss_cwnd = tp->snd_cwnd;

    if (tp->snd_cwnd <= low_window)
        return max(tp->snd_cwnd >> 1U, 2U);
    else
        /* ssthresh = cwnd * (819/1024) = 0.7998 * cwnd */
        return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);

snd_ssthresh略小于last_max_cwnd，方便前期使用慢启动快速逼近阈值。