MEDIUM: quic: Cubic congestion control algorithm implementation
Cubic is the congestion control algorithm used by default by the Linux kernel
since 2.6.15 version. This algorithm is supposed to achieve good scalability and
fairness between flows using the same network path, it should also be used by QUIC
by default. This patch implements this algorithm and select it as default algorithm
for the congestion control.
Must be backported to 2.6.
diff --git a/src/quic_cc_cubic.c b/src/quic_cc_cubic.c
new file mode 100644
index 0000000..be48b6f
--- /dev/null
+++ b/src/quic_cc_cubic.c
@@ -0,0 +1,284 @@
+#include <haproxy/trace.h>
+#include <haproxy/quic_cc.h>
+
+/* This source file is highly inspired from Linux kernel source file
+ * implementation for TCP Cubic. In fact, we have no choice if we do
+ * not want to use any floating point operations to be fast!
+ * (See net/ipv4/tcp_cubic.c)
+ */
+#define TRACE_SOURCE &trace_quic
+
+#define CUBIC_BETA_SCALE 1024
+#define CUBIC_BETA_SCALE_SHIFT 10
+/* beta = 0.7 ; C = 0.4 */
+#define CUBIC_BETA 717 /* CUBIC_BETA / CUBIC_BETA_SCALE = 0.7 */
+#define CUBIC_C 410 /* CUBIC_C / CUBIC_BETA_SCALE = 0.4 */
+
+#define CUBIC_BETA_SCALE_FACTOR_SHIFT (3 * CUBIC_BETA_SCALE_SHIFT)
+#define TIME_SCALE_FACTOR_SHIFT 10
+
+/* The maximum value which may be cubed an multiplied by CUBIC_BETA */
+#define CUBIC_DIFF_TIME_LIMIT 355535ULL /* ms */
+
+/* K cube factor: (1 - beta) / c */
+struct cubic {
+ uint32_t ssthresh;
+ uint32_t remaining_inc;
+ uint32_t remaining_tcp_inc;
+ uint32_t epoch_start;
+ uint32_t origin_point;
+ uint32_t K;
+ uint32_t last_w_max;
+ uint32_t tcp_wnd;
+ uint32_t recovery_start_time;
+};
+
+static void quic_cc_cubic_reset(struct quic_cc *cc)
+{
+ struct cubic *c = quic_cc_priv(cc);
+
+ cc->algo->state = QUIC_CC_ST_SS;
+
+ c->ssthresh = QUIC_CC_INFINITE_SSTHESH;
+ c->remaining_inc = 0;
+ c->remaining_tcp_inc = 0;
+ c->epoch_start = 0;
+ c->origin_point = 0;
+ c->K = 0;
+ c->last_w_max = 0;
+ c->tcp_wnd = 0;
+ c->recovery_start_time = 0;
+}
+
+static int quic_cc_cubic_init(struct quic_cc *cc)
+{
+ quic_cc_cubic_reset(cc);
+ return 1;
+}
+
+/* Cubic root.
+ * Highly inspired from Linux kernel sources.
+ * See net/ipv4/tcp_cubic.c
+ */
+static uint32_t cubic_root(uint64_t val)
+{
+ uint32_t x, b, shift;
+
+ static const uint8_t v[] = {
+ 0, 54, 54, 54, 118, 118, 118, 118,
+ 123, 129, 134, 138, 143, 147, 151, 156,
+ 157, 161, 164, 168, 170, 173, 176, 179,
+ 181, 185, 187, 190, 192, 194, 197, 199,
+ 200, 202, 204, 206, 209, 211, 213, 215,
+ 217, 219, 221, 222, 224, 225, 227, 229,
+ 231, 232, 234, 236, 237, 239, 240, 242,
+ 244, 245, 246, 248, 250, 251, 252, 254,
+ };
+
+ b = my_flsl(val);
+ if (b < 7) {
+ /* val in [0..63] */
+ return ((uint32_t)v[(uint32_t)val] + 35) >> 6;
+ }
+
+ b = ((b * 84) >> 8) - 1;
+ shift = (val >> (b * 3));
+
+ x = ((uint32_t)(((uint32_t)v[shift] + 10) << b)) >> 6;
+
+ x = 2 * x + (uint32_t)(val / ((uint64_t)x * (uint64_t)(x - 1)));
+ x = ((x * 341) >> 10);
+
+ return x;
+}
+
+static inline void quic_cubic_update(struct quic_cc *cc, uint32_t acked)
+{
+ struct cubic *c = quic_cc_priv(cc);
+ struct quic_path *path = container_of(cc, struct quic_path, cc);
+ /* Current cwnd as number of packets */
+ uint32_t t, target, inc, inc_diff;
+ uint64_t delta, diff;
+
+ if (!c->epoch_start) {
+ c->epoch_start = now_ms;
+ if (c->last_w_max <= path->cwnd) {
+ c->K = 0;
+ c->origin_point = path->cwnd;
+ }
+ else {
+ /* K = cubic_root((1 - beta) * W_max / C) */
+ c->K = cubic_root((c->last_w_max - path->cwnd) *
+ (CUBIC_BETA_SCALE - CUBIC_BETA) / CUBIC_C / path->mtu) << TIME_SCALE_FACTOR_SHIFT;
+ c->origin_point = c->last_w_max;
+ }
+
+ c->tcp_wnd = path->cwnd;
+ c->remaining_inc = 0;
+ c->remaining_tcp_inc = 0;
+ }
+
+ t = now_ms + path->loss.rtt_min - c->epoch_start;
+ if (t < c->K) {
+ diff = c->K - t;
+ }
+ else {
+ diff = t - c->K;
+ }
+
+ if (diff > CUBIC_DIFF_TIME_LIMIT) {
+ /* TODO : should not happen if we handle the case
+ * of very late acks receipt. This must be handled as a congestion
+ * control event: a very late ack should trigger a congestion
+ * control algorithm reset.
+ */
+ quic_cc_cubic_reset(cc);
+ return;
+ }
+
+ delta = path->mtu * ((CUBIC_C * diff * diff * diff) >> (10 + 3 * TIME_SCALE_FACTOR_SHIFT));
+ if (t < c->K)
+ target = c->origin_point - delta;
+ else
+ target = c->origin_point + delta;
+
+ if (target > path->cwnd) {
+ inc_diff = c->remaining_inc + path->mtu * (target - path->cwnd);
+ c->remaining_inc = inc_diff % path->cwnd;
+ inc = inc_diff / path->cwnd;
+ }
+ else {
+ /* small increment */
+ inc_diff = c->remaining_inc + path->mtu;
+ c->remaining_inc = inc_diff % (100 * path->cwnd);
+ inc = inc_diff / (100 * path->cwnd);
+ }
+
+ inc_diff = c->remaining_tcp_inc + path->mtu * acked;
+ c->tcp_wnd += inc_diff / path->cwnd;
+ c->remaining_tcp_inc = inc_diff % path->cwnd;
+ /* TCP friendliness */
+ if (c->tcp_wnd > path->cwnd) {
+ uint32_t tcp_inc = path->mtu * (c->tcp_wnd - path->cwnd) / path->cwnd;
+ if (tcp_inc > inc)
+ inc = tcp_inc;
+ }
+
+ path->cwnd += inc;
+}
+
+static void quic_cc_cubic_slow_start(struct quic_cc *cc)
+{
+ quic_cc_cubic_reset(cc);
+}
+
+static void quic_enter_recovery(struct quic_cc *cc)
+{
+ struct quic_path *path = container_of(cc, struct quic_path, cc);
+ struct cubic *c = quic_cc_priv(cc);
+ /* Current cwnd as number of packets */
+
+ c->epoch_start = 0;
+ c->recovery_start_time = now_ms;
+ /* Fast convergence */
+ if (path->cwnd < c->last_w_max) {
+ /* (1 + beta) * path->cwnd / 2 */
+ c->last_w_max = (path->cwnd * (CUBIC_BETA_SCALE + CUBIC_BETA) / 2) >> CUBIC_BETA_SCALE_SHIFT;
+ }
+ else {
+ c->last_w_max = path->cwnd;
+ }
+ path->cwnd = (CUBIC_BETA * path->cwnd) >> CUBIC_BETA_SCALE_SHIFT;
+ c->ssthresh = QUIC_MAX(path->cwnd, path->min_cwnd);
+}
+
+/* Congestion slow-start callback. */
+static void quic_cc_cubic_ss_cb(struct quic_cc *cc, struct quic_cc_event *ev)
+{
+ struct quic_path *path = container_of(cc, struct quic_path, cc);
+ struct cubic *c = quic_cc_priv(cc);
+
+ TRACE_ENTER(QUIC_EV_CONN_CC, cc->qc, ev);
+ switch (ev->type) {
+ case QUIC_CC_EVT_ACK:
+ /* Do not increase the congestion window in recovery period. */
+ if (ev->ack.time_sent <= c->recovery_start_time)
+ goto out;
+
+ path->cwnd += ev->ack.acked;
+ /* Exit to congestion avoidance if slow start threshold is reached. */
+ if (path->cwnd >= c->ssthresh)
+ cc->algo->state = QUIC_CC_ST_CA;
+ break;
+
+ case QUIC_CC_EVT_LOSS:
+ /* Do not decrease the congestion window when already in recovery period. */
+ if (ev->loss.time_sent <= c->recovery_start_time)
+ goto out;
+
+ quic_enter_recovery(cc);
+ /* Exit to congestion avoidance. */
+ cc->algo->state = QUIC_CC_ST_CA;
+ break;
+
+ case QUIC_CC_EVT_ECN_CE:
+ /* TODO */
+ break;
+ }
+
+ out:
+ TRACE_LEAVE(QUIC_EV_CONN_CC, cc->qc, NULL, cc);
+}
+
+/* Congestion avoidance callback. */
+static void quic_cc_cubic_ca_cb(struct quic_cc *cc, struct quic_cc_event *ev)
+{
+ struct cubic *c = quic_cc_priv(cc);
+
+ TRACE_ENTER(QUIC_EV_CONN_CC, cc->qc, ev);
+ switch (ev->type) {
+ case QUIC_CC_EVT_ACK:
+ /* Do not increase the congestion window when already in recovery period. */
+ if (ev->ack.time_sent <= c->recovery_start_time)
+ goto out;
+
+ quic_cubic_update(cc, ev->ack.acked);
+ break;
+ case QUIC_CC_EVT_LOSS:
+ /* Do not decrease the congestion window when already in recovery period. */
+ if (ev->loss.time_sent <= c->recovery_start_time)
+ goto out;
+
+ quic_enter_recovery(cc);
+ break;
+ case QUIC_CC_EVT_ECN_CE:
+ /* TODO */
+ break;
+ }
+
+ out:
+ TRACE_LEAVE(QUIC_EV_CONN_CC, cc->qc, NULL, cc);
+}
+
+static void (*quic_cc_cubic_state_cbs[])(struct quic_cc *cc,
+ struct quic_cc_event *ev) = {
+ [QUIC_CC_ST_SS] = quic_cc_cubic_ss_cb,
+ [QUIC_CC_ST_CA] = quic_cc_cubic_ca_cb,
+};
+
+static void quic_cc_cubic_event(struct quic_cc *cc, struct quic_cc_event *ev)
+{
+ return quic_cc_cubic_state_cbs[cc->algo->state](cc, ev);
+}
+
+static void quic_cc_cubic_state_trace(struct buffer *buf, const struct quic_cc *cc)
+{
+}
+
+struct quic_cc_algo quic_cc_algo_cubic = {
+ .type = QUIC_CC_ALGO_TP_CUBIC,
+ .init = quic_cc_cubic_init,
+ .event = quic_cc_cubic_event,
+ .slow_start = quic_cc_cubic_slow_start,
+ .state_trace = quic_cc_cubic_state_trace,
+};