tcp: add CDG congestion control

CAIA Delay-Gradient (CDG) is a TCP congestion control that modifies
the TCP sender in order to [1]:

  o Use the delay gradient as a congestion signal.
  o Back off with an average probability that is independent of the RTT.
  o Coexist with flows that use loss-based congestion control, i.e.,
    flows that are unresponsive to the delay signal.
  o Tolerate packet loss unrelated to congestion. (Disabled by default.)

Its FreeBSD implementation was presented for the ICCRG in July 2012;
slides are available at http://www.ietf.org/proceedings/84/iccrg.html

Running the experiment scenarios in [1] suggests that our implementation
achieves more goodput compared with FreeBSD 10.0 senders, although it also
causes more queueing delay for a given backoff factor.

The loss tolerance heuristic is disabled by default due to safety concerns
for its use in the Internet [2, p. 45-46].

We use a variant of the Hybrid Slow start algorithm in tcp_cubic to reduce
the probability of slow start overshoot.

[1] D.A. Hayes and G. Armitage. "Revisiting TCP congestion control using
    delay gradients." In Networking 2011, pages 328-341. Springer, 2011.
[2] K.K. Jonassen. "Implementing CAIA Delay-Gradient in Linux."
    MSc thesis. Department of Informatics, University of Oslo, 2015.

Cc: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: David Hayes <davihay@ifi.uio.no>
Cc: Andreas Petlund <apetlund@simula.no>
Cc: Dave Taht <dave.taht@bufferbloat.net>
Cc: Nicolas Kuhn <nicolas.kuhn@telecom-bretagne.eu>
Signed-off-by: Kenneth Klette Jonassen <kennetkl@ifi.uio.no>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 433 insertions, 0 deletions

diff --git a/net/ipv4/tcp_cdg.c b/net/ipv4/tcp_cdg.c
new file mode 100644
index 000000000000..a52ce2db53f2
--- /dev/null
+++ b/net/ipv4/tcp_cdg.c
@@ -0,0 +1,433 @@
+/*
+ * 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 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 minmax rtt;
+        struct minmax rtt_prev;
+        struct minmax *gradients;
+        struct 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 = 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_BH(sock_net(sk),
+                                                 LINUX_MIB_TCPHYSTARTTRAINDETECT);
+                                NET_ADD_STATS_BH(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_BH(sock_net(sk),
+                                                 LINUX_MIB_TCPHYSTARTDELAYDETECT);
+                                NET_ADD_STATS_BH(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 (tp->snd_cwnd < tp->snd_ssthresh && 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, u32 num_acked, s32 rtt_us)
+{
+        struct cdg *ca = inet_csk_ca(sk);
+        struct tcp_sock *tp = tcp_sk(sk);
+
+        if (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 (num_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, rtt_us);
+                        ca->delack--;
+                        return;
+                } else if (num_acked > 1 && ca->delack < 5) {
+                        ca->delack++;
+                }
+        }
+
+        ca->rtt.min = min_not_zero(ca->rtt.min, rtt_us);
+        ca->rtt.max = max(ca->rtt.max, 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 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");