tcp Congestion Analysis VI (HSTCP)

This paper analyzes the hstcp congestion algorithm of linux-4.19.12

The hstcp relative reno algorithm changes the window Growth Logic in congestion avoidance phase and the setting of ssthresh after packet loss through a fixed array of hstcp ﹣ AIMD ﹣ Vals related to sending window, that is, the sending window of AIMD (additional increase multiple decision) is changed

According to the current sending window snd ﹣ CWnd, find the array position I to meet the requirements of HSTCP ﹣ AIMD ﹣ Vals [I-1]. CWnd < snd ﹣ CWnd < = = HSTCP ﹣ AIMD ﹣ Vals [i]. CWnd

/* From AIMD tables from RFC 3649 appendix B,
 * with fixed-point MD scaled <<8.
 */
static const struct hstcp_aimd_val {
	unsigned int cwnd;
	unsigned int md;
} hstcp_aimd_vals[] = {
	{     38,  128, /*  0.50 */ },
	{    118,  112, /*  0.44 */ },
	{    221,  104, /*  0.41 */ },
	{    347,   98, /*  0.38 */ },
	{    495,   93, /*  0.37 */ },
	{    663,   89, /*  0.35 */ },
	{    851,   86, /*  0.34 */ },
	{   1058,   83, /*  0.33 */ },
	{   1284,   81, /*  0.32 */ },
	{   1529,   78, /*  0.31 */ },
	{   1793,   76, /*  0.30 */ },
	{   2076,   74, /*  0.29 */ },
	{   2378,   72, /*  0.28 */ },
	{   2699,   71, /*  0.28 */ },
	{   3039,   69, /*  0.27 */ },
	{   3399,   68, /*  0.27 */ },
	{   3778,   66, /*  0.26 */ },
	{   4177,   65, /*  0.26 */ },
	{   4596,   64, /*  0.25 */ },
	{   5036,   62, /*  0.25 */ },
	{   5497,   61, /*  0.24 */ },
	{   5979,   60, /*  0.24 */ },
	{   6483,   59, /*  0.23 */ },
	{   7009,   58, /*  0.23 */ },
	{   7558,   57, /*  0.22 */ },
	{   8130,   56, /*  0.22 */ },
	{   8726,   55, /*  0.22 */ },
	{   9346,   54, /*  0.21 */ },
	{   9991,   53, /*  0.21 */ },
	{  10661,   52, /*  0.21 */ },
	{  11358,   52, /*  0.20 */ },
	{  12082,   51, /*  0.20 */ },
	{  12834,   50, /*  0.20 */ },
	{  13614,   49, /*  0.19 */ },
	{  14424,   48, /*  0.19 */ },
	{  15265,   48, /*  0.19 */ },
	{  16137,   47, /*  0.19 */ },
	{  17042,   46, /*  0.18 */ },
	{  17981,   45, /*  0.18 */ },
	{  18955,   45, /*  0.18 */ },
	{  19965,   44, /*  0.17 */ },
	{  21013,   43, /*  0.17 */ },
	{  22101,   43, /*  0.17 */ },
	{  23230,   42, /*  0.17 */ },
	{  24402,   41, /*  0.16 */ },
	{  25618,   41, /*  0.16 */ },
	{  26881,   40, /*  0.16 */ },
	{  28193,   39, /*  0.16 */ },
	{  29557,   39, /*  0.15 */ },
	{  30975,   38, /*  0.15 */ },
	{  32450,   38, /*  0.15 */ },
	{  33986,   37, /*  0.15 */ },
	{  35586,   36, /*  0.14 */ },
	{  37253,   36, /*  0.14 */ },
	{  38992,   35, /*  0.14 */ },
	{  40808,   35, /*  0.14 */ },
	{  42707,   34, /*  0.13 */ },
	{  44694,   33, /*  0.13 */ },
	{  46776,   33, /*  0.13 */ },
	{  48961,   32, /*  0.13 */ },
	{  51258,   32, /*  0.13 */ },
	{  53677,   31, /*  0.12 */ },
	{  56230,   30, /*  0.12 */ },
	{  58932,   30, /*  0.12 */ },
	{  61799,   29, /*  0.12 */ },
	{  64851,   28, /*  0.11 */ },
	{  68113,   28, /*  0.11 */ },
	{  71617,   27, /*  0.11 */ },
	{  75401,   26, /*  0.10 */ },
	{  79517,   26, /*  0.10 */ },
	{  84035,   25, /*  0.10 */ },
	{  89053,   24, /*  0.10 */ },
};

The speed of AI phase (congestion avoidance phase) is I / snd ﹐ CWnd, not ack ed / snd ﹐ CWnd in reno

In MD phase (timeout packet loss), the reduction of s-sthresh is snd ﹣ cwnd-hstcp ﹣ AIMD ﹣ Vals [i]. MD * snd ﹣ CWnd. (note that in order to deal with decimals, the actual operation of sshresh and MD is shifted 8 bits to the right). Compared with reno fixed reduction of half (that is, md=0.5), for CWnd > 38, the bandwidth will be occupied

static void hstcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct hstcp *ca = inet_csk_ca(sk);

	if (!tcp_is_cwnd_limited(sk))
		return;

	if (tcp_in_slow_start(tp))
		tcp_slow_start(tp, acked);
	else {
		/* Update AIMD parameters.
		 *
		 * We want to guarantee that:
		 *     hstcp_aimd_vals[ca->ai-1].cwnd <
		 *     snd_cwnd <=
		 *     hstcp_aimd_vals[ca->ai].cwnd
		 */
		if (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd) {
			while (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd &&
			       ca->ai < HSTCP_AIMD_MAX - 1)
				ca->ai++;
		} else if (ca->ai && tp->snd_cwnd <= hstcp_aimd_vals[ca->ai-1].cwnd) {
			while (ca->ai && tp->snd_cwnd <= hstcp_aimd_vals[ca->ai-1].cwnd)
				ca->ai--;
		}

		/* Do additive increase */
		if (tp->snd_cwnd < tp->snd_cwnd_clamp) {
			/* cwnd = cwnd + a(w) / cwnd */
			tp->snd_cwnd_cnt += ca->ai + 1;
			if (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
				tp->snd_cwnd_cnt -= tp->snd_cwnd;
				tp->snd_cwnd++;
			}
		}
	}
}

static u32 hstcp_ssthresh(struct sock *sk)
{
	const struct tcp_sock *tp = tcp_sk(sk);
	struct hstcp *ca = inet_csk_ca(sk);

	/* Do multiplicative decrease */
	return max(tp->snd_cwnd - ((tp->snd_cwnd * hstcp_aimd_vals[ca->ai].md) >> 8), 2U);
}

 

Tags: Linux

Posted on Tue, 03 Dec 2019 16:15:07 -0500 by Bea