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); }