/* * CAIA Delay-Gradient (CDG) congestion control * * This implementation is based on the paper: * D.A. Hayes and G. Armitage. "Revisiting TCP congestion control using * delay gradients." In IFIP Networking, pages 328-341. Springer, 2011. * * Scavenger traffic (Less-than-Best-Effort) should disable coexistence * heuristics using parameters use_shadow=0 and use_ineff=0. * * Parameters window, backoff_beta, and backoff_factor are crucial for * throughput and delay. Future work is needed to determine better defaults, * and to provide guidelines for use in different environments/contexts. * * Except for window, knobs are configured via /sys/module/tcp_cdg/parameters/. * Parameter window is only configurable when loading tcp_cdg as a module. * * Notable differences from paper/FreeBSD: * o Using Hybrid Slow start and Proportional Rate Reduction. * o Add toggle for shadow window mechanism. Suggested by David Hayes. * o Add toggle for non-congestion loss tolerance. * o Scaling parameter G is changed to a backoff factor; * conversion is given by: backoff_factor = 1000/(G * window). * o Limit shadow window to 2 * cwnd, or to cwnd when application limited. * o More accurate e^-x. */ #include <linux/kernel.h> #include <linux/random.h> #include <linux/module.h> #include <net/tcp.h> #define HYSTART_ACK_TRAIN 1 #define HYSTART_DELAY 2 static int window __read_mostly = 8; static unsigned int backoff_beta __read_mostly = 0.7071 * 1024; /* sqrt 0.5 */ static unsigned int backoff_factor __read_mostly = 42; static unsigned int hystart_detect __read_mostly = 3; static unsigned int use_ineff __read_mostly = 5; static bool use_shadow __read_mostly = true; static bool use_tolerance __read_mostly; module_param(window, int, 0444); MODULE_PARM_DESC(window, "gradient window size (power of two <= 256)"); module_param(backoff_beta, uint, 0644); MODULE_PARM_DESC(backoff_beta, "backoff beta (0-1024)"); module_param(backoff_factor, uint, 0644); MODULE_PARM_DESC(backoff_factor, "backoff probability scale factor"); module_param(hystart_detect, uint, 0644); MODULE_PARM_DESC(hystart_detect, "use Hybrid Slow start " "(0: disabled, 1: ACK train, 2: delay threshold, 3: both)"); module_param(use_ineff, uint, 0644); MODULE_PARM_DESC(use_ineff, "use ineffectual backoff detection (threshold)"); module_param(use_shadow, bool, 0644); MODULE_PARM_DESC(use_shadow, "use shadow window heuristic"); module_param(use_tolerance, bool, 0644); MODULE_PARM_DESC(use_tolerance, "use loss tolerance heuristic"); struct cdg_minmax { union { struct { s32 min; s32 max; }; u64 v64; }; }; enum cdg_state { CDG_UNKNOWN = 0, CDG_NONFULL = 1, CDG_FULL = 2, CDG_BACKOFF = 3, }; struct cdg { struct cdg_minmax rtt; struct cdg_minmax rtt_prev; struct cdg_minmax *gradients; struct cdg_minmax gsum; bool gfilled; u8 tail; u8 state; u8 delack; u32 rtt_seq; u32 undo_cwnd; u32 shadow_wnd; u16 backoff_cnt; u16 sample_cnt; s32 delay_min; u32 last_ack; u32 round_start; }; /** * nexp_u32 - negative base-e exponential * @ux: x in units of micro * * Returns exp(ux * -1e-6) * U32_MAX. */ static u32 __pure nexp_u32(u32 ux) { static const u16 v[] = { /* exp(-x)*65536-1 for x = 0, 0.000256, 0.000512, ... */ 65535, 65518, 65501, 65468, 65401, 65267, 65001, 64470, 63422, 61378, 57484, 50423, 38795, 22965, 8047, 987, 14, }; u32 msb = ux >> 8; u32 res; int i; /* Cut off when ux >= 2^24 (actual result is <= 222/U32_MAX). */ if (msb > U16_MAX) return 0; /* Scale first eight bits linearly: */ res = U32_MAX - (ux & 0xff) * (U32_MAX / 1000000); /* Obtain e^(x + y + ...) by computing e^x * e^y * ...: */ for (i = 1; msb; i++, msb >>= 1) { u32 y = v[i & -(msb & 1)] + U32_C(1); res = ((u64)res * y) >> 16; } return res; } /* Based on the HyStart algorithm (by Ha et al.) that is implemented in * tcp_cubic. Differences/experimental changes: * o Using Hayes' delayed ACK filter. * o Using a usec clock for the ACK train. * o Reset ACK train when application limited. * o Invoked at any cwnd (i.e. also when cwnd < 16). * o Invoked only when cwnd < ssthresh (i.e. not when cwnd == ssthresh). */ static void tcp_cdg_hystart_update(struct sock *sk) { struct cdg *ca = inet_csk_ca(sk); struct tcp_sock *tp = tcp_sk(sk); ca->delay_min = min_not_zero(ca->delay_min, ca->rtt.min); if (ca->delay_min == 0) return; if (hystart_detect & HYSTART_ACK_TRAIN) { u32 now_us = div_u64(local_clock(), NSEC_PER_USEC); if (ca->last_ack == 0 || !tcp_is_cwnd_limited(sk)) { ca->last_ack = now_us; ca->round_start = now_us; } else if (before(now_us, ca->last_ack + 3000)) { u32 base_owd = max(ca->delay_min / 2U, 125U); ca->last_ack = now_us; if (after(now_us, ca->round_start + base_owd)) { NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPHYSTARTTRAINDETECT); NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPHYSTARTTRAINCWND, tp->snd_cwnd); tp->snd_ssthresh = tp->snd_cwnd; return; } } } if (hystart_detect & HYSTART_DELAY) { if (ca->sample_cnt < 8) { ca->sample_cnt++; } else { s32 thresh = max(ca->delay_min + ca->delay_min / 8U, 125U); if (ca->rtt.min > thresh) { NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPHYSTARTDELAYDETECT); NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPHYSTARTDELAYCWND, tp->snd_cwnd); tp->snd_ssthresh = tp->snd_cwnd; } } } } static s32 tcp_cdg_grad(struct cdg *ca) { s32 gmin = ca->rtt.min - ca->rtt_prev.min; s32 gmax = ca->rtt.max - ca->rtt_prev.max; s32 grad; if (ca->gradients) { ca->gsum.min += gmin - ca->gradients[ca->tail].min; ca->gsum.max += gmax - ca->gradients[ca->tail].max; ca->gradients[ca->tail].min = gmin; ca->gradients[ca->tail].max = gmax; ca->tail = (ca->tail + 1) & (window - 1); gmin = ca->gsum.min; gmax = ca->gsum.max; } /* We keep sums to ignore gradients during cwnd reductions; * the paper's smoothed gradients otherwise simplify to: * (rtt_latest - rtt_oldest) / window. * * We also drop division by window here. */ grad = gmin > 0 ? gmin : gmax; /* Extrapolate missing values in gradient window: */ if (!ca->gfilled) { if (!ca->gradients && window > 1) grad *= window; /* Memory allocation failed. */ else if (ca->tail == 0) ca->gfilled = true; else grad = (grad * window) / (int)ca->tail; } /* Backoff was effectual: */ if (gmin <= -32 || gmax <= -32) ca->backoff_cnt = 0; if (use_tolerance) { /* Reduce small variations to zero: */ gmin = DIV_ROUND_CLOSEST(gmin, 64); gmax = DIV_ROUND_CLOSEST(gmax, 64); if (gmin > 0 && gmax <= 0) ca->state = CDG_FULL; else if ((gmin > 0 && gmax > 0) || gmax < 0) ca->state = CDG_NONFULL; } return grad; } static bool tcp_cdg_backoff(struct sock *sk, u32 grad) { struct cdg *ca = inet_csk_ca(sk); struct tcp_sock *tp = tcp_sk(sk); if (prandom_u32() <= nexp_u32(grad * backoff_factor)) return false; if (use_ineff) { ca->backoff_cnt++; if (ca->backoff_cnt > use_ineff) return false; } ca->shadow_wnd = max(ca->shadow_wnd, tp->snd_cwnd); ca->state = CDG_BACKOFF; tcp_enter_cwr(sk); return true; } /* Not called in CWR or Recovery state. */ static void tcp_cdg_cong_avoid(struct sock *sk, u32 ack, u32 acked) { struct cdg *ca = inet_csk_ca(sk); struct tcp_sock *tp = tcp_sk(sk); u32 prior_snd_cwnd; u32 incr; if (tcp_in_slow_start(tp) && hystart_detect) tcp_cdg_hystart_update(sk); if (after(ack, ca->rtt_seq) && ca->rtt.v64) { s32 grad = 0; if (ca->rtt_prev.v64) grad = tcp_cdg_grad(ca); ca->rtt_seq = tp->snd_nxt; ca->rtt_prev = ca->rtt; ca->rtt.v64 = 0; ca->last_ack = 0; ca->sample_cnt = 0; if (grad > 0 && tcp_cdg_backoff(sk, grad)) return; } if (!tcp_is_cwnd_limited(sk)) { ca->shadow_wnd = min(ca->shadow_wnd, tp->snd_cwnd); return; } prior_snd_cwnd = tp->snd_cwnd; tcp_reno_cong_avoid(sk, ack, acked); incr = tp->snd_cwnd - prior_snd_cwnd; ca->shadow_wnd = max(ca->shadow_wnd, ca->shadow_wnd + incr); } static void tcp_cdg_acked(struct sock *sk, const struct ack_sample *sample) { struct cdg *ca = inet_csk_ca(sk); struct tcp_sock *tp = tcp_sk(sk); if (sample->rtt_us <= 0) return; /* A heuristic for filtering delayed ACKs, adapted from: * D.A. Hayes. "Timing enhancements to the FreeBSD kernel to support * delay and rate based TCP mechanisms." TR 100219A. CAIA, 2010. */ if (tp->sacked_out == 0) { if (sample->pkts_acked == 1 && ca->delack) { /* A delayed ACK is only used for the minimum if it is * provenly lower than an existing non-zero minimum. */ ca->rtt.min = min(ca->rtt.min, sample->rtt_us); ca->delack--; return; } else if (sample->pkts_acked > 1 && ca->delack < 5) { ca->delack++; } } ca->rtt.min = min_not_zero(ca->rtt.min, sample->rtt_us); ca->rtt.max = max(ca->rtt.max, sample->rtt_us); } static u32 tcp_cdg_ssthresh(struct sock *sk) { struct cdg *ca = inet_csk_ca(sk); struct tcp_sock *tp = tcp_sk(sk); ca->undo_cwnd = tp->snd_cwnd; if (ca->state == CDG_BACKOFF) return max(2U, (tp->snd_cwnd * min(1024U, backoff_beta)) >> 10); if (ca->state == CDG_NONFULL && use_tolerance) return tp->snd_cwnd; ca->shadow_wnd = min(ca->shadow_wnd >> 1, tp->snd_cwnd); if (use_shadow) return max3(2U, ca->shadow_wnd, tp->snd_cwnd >> 1); return max(2U, tp->snd_cwnd >> 1); } static u32 tcp_cdg_undo_cwnd(struct sock *sk) { struct cdg *ca = inet_csk_ca(sk); return max(tcp_sk(sk)->snd_cwnd, ca->undo_cwnd); } static void tcp_cdg_cwnd_event(struct sock *sk, const enum tcp_ca_event ev) { struct cdg *ca = inet_csk_ca(sk); struct tcp_sock *tp = tcp_sk(sk); struct cdg_minmax *gradients; switch (ev) { case CA_EVENT_CWND_RESTART: gradients = ca->gradients; if (gradients) memset(gradients, 0, window * sizeof(gradients[0])); memset(ca, 0, sizeof(*ca)); ca->gradients = gradients; ca->rtt_seq = tp->snd_nxt; ca->shadow_wnd = tp->snd_cwnd; break; case CA_EVENT_COMPLETE_CWR: ca->state = CDG_UNKNOWN; ca->rtt_seq = tp->snd_nxt; ca->rtt_prev = ca->rtt; ca->rtt.v64 = 0; break; default: break; } } static void tcp_cdg_init(struct sock *sk) { struct cdg *ca = inet_csk_ca(sk); struct tcp_sock *tp = tcp_sk(sk); /* We silently fall back to window = 1 if allocation fails. */ if (window > 1) ca->gradients = kcalloc(window, sizeof(ca->gradients[0]), GFP_NOWAIT | __GFP_NOWARN); ca->rtt_seq = tp->snd_nxt; ca->shadow_wnd = tp->snd_cwnd; } static void tcp_cdg_release(struct sock *sk) { struct cdg *ca = inet_csk_ca(sk); kfree(ca->gradients); } struct tcp_congestion_ops tcp_cdg __read_mostly = { .cong_avoid = tcp_cdg_cong_avoid, .cwnd_event = tcp_cdg_cwnd_event, .pkts_acked = tcp_cdg_acked, .undo_cwnd = tcp_cdg_undo_cwnd, .ssthresh = tcp_cdg_ssthresh, .release = tcp_cdg_release, .init = tcp_cdg_init, .owner = THIS_MODULE, .name = "cdg", }; static int __init tcp_cdg_register(void) { if (backoff_beta > 1024 || window < 1 || window > 256) return -ERANGE; if (!is_power_of_2(window)) return -EINVAL; BUILD_BUG_ON(sizeof(struct cdg) > ICSK_CA_PRIV_SIZE); tcp_register_congestion_control(&tcp_cdg); return 0; } static void __exit tcp_cdg_unregister(void) { tcp_unregister_congestion_control(&tcp_cdg); } module_init(tcp_cdg_register); module_exit(tcp_cdg_unregister); MODULE_AUTHOR("Kenneth Klette Jonassen"); MODULE_LICENSE("GPL"); MODULE_DESCRIPTION("TCP CDG");