Implement RFC3742 Limited Slow Start

RFC3742 updates the cwnd every sendme during slow start, and backs off of the
exponential growth based on a cap parameter.

3 files changed, 191 insertions, 91 deletions

diff --git a/src/core/or/congestion_control_common.c b/src/core/or/congestion_control_common.c
index 42f816690f..8f09a80bae 100644
--- a/src/core/or/congestion_control_common.c
+++ b/src/core/or/congestion_control_common.c
@@ -42,7 +42,7 @@
 #define CC_ALG_DFLT_ALWAYS (CC_ALG_VEGAS)
 
 #define CWND_INC_DFLT (TLS_RECORD_MAX_CELLS)
-#define CWND_INC_PCT_SS_DFLT (50)
+#define CWND_INC_PCT_SS_DFLT (100)
 #define CWND_INC_RATE_DFLT (1)
 
 #define CWND_MIN_DFLT (SENDME_INC_DFLT)
diff --git a/src/core/or/congestion_control_st.h b/src/core/or/congestion_control_st.h
index bc547b59d9..f88e1f4a9a 100644
--- a/src/core/or/congestion_control_st.h
+++ b/src/core/or/congestion_control_st.h
@@ -97,6 +97,10 @@ struct westwood_params_t {
 
 /** Vegas algorithm parameters. */
 struct vegas_params_t {
+    /** The slow-start cwnd cap for RFC3742 */
+    uint32_t ss_cwnd_cap;
+    /** The maximum slow-start cwnd */
+    uint32_t ss_cwnd_max;
     /** The queue use allowed before we exit slow start */
     uint16_t gamma;
     /** The queue use below which we increment cwnd */
@@ -228,6 +232,16 @@ static inline uint64_t CWND_UPDATE_RATE(const struct congestion_control_t *cc)
 }
 
 /**
+ * Gives us the number of SENDMEs in a CWND, rounded.
+ */
+static inline uint64_t SENDME_PER_CWND(const struct congestion_control_t *cc)
+{
+  /* We add cwnd_inc_rate*sendme_inc/2 to round to nearest integer number
+   * of acks */
+  return ((cc->cwnd + cc->sendme_inc/2)/cc->sendme_inc);
+}
+
+/**
  * Returns the amount to increment the congestion window each update,
  * during slow start.
  *
diff --git a/src/core/or/congestion_control_vegas.c b/src/core/or/congestion_control_vegas.c
index 5451d7849c..9e10e1311e 100644
--- a/src/core/or/congestion_control_vegas.c
+++ b/src/core/or/congestion_control_vegas.c
@@ -26,29 +26,35 @@
 
 #define OUTBUF_CELLS (2*TLS_RECORD_MAX_CELLS)
 
+#define SS_CWND_MAX_DFLT (5000)
+
 /* sbws circs are two hops, so params are based on 2 outbufs of cells */
 #define VEGAS_ALPHA_SBWS_DFLT (2*OUTBUF_CELLS-TLS_RECORD_MAX_CELLS)
 #define VEGAS_BETA_SBWS_DFLT (2*OUTBUF_CELLS)
 #define VEGAS_GAMMA_SBWS_DFLT (2*OUTBUF_CELLS)
 #define VEGAS_DELTA_SBWS_DFLT (4*OUTBUF_CELLS)
+#define VEGAS_SSCAP_SBWS_DFLT (400)
 
 /* Exits are three hops, so params are based on 3 outbufs of cells */
 #define VEGAS_ALPHA_EXIT_DFLT (3*OUTBUF_CELLS-TLS_RECORD_MAX_CELLS)
 #define VEGAS_BETA_EXIT_DFLT (3*OUTBUF_CELLS)
 #define VEGAS_GAMMA_EXIT_DFLT (3*OUTBUF_CELLS)
 #define VEGAS_DELTA_EXIT_DFLT (5*OUTBUF_CELLS)
+#define VEGAS_SSCAP_EXIT_DFLT (500)
 
 /* Onion rends are six hops, so params are based on 6 outbufs of cells */
 #define VEGAS_ALPHA_ONION_DFLT (6*OUTBUF_CELLS-TLS_RECORD_MAX_CELLS)
 #define VEGAS_BETA_ONION_DFLT (6*OUTBUF_CELLS)
 #define VEGAS_GAMMA_ONION_DFLT (6*OUTBUF_CELLS)
 #define VEGAS_DELTA_ONION_DFLT (8*OUTBUF_CELLS)
+#define VEGAS_SSCAP_ONION_DFLT (600)
 
 /* Single Onions are three hops, so params are based on 3 outbufs of cells */
 #define VEGAS_ALPHA_SOS_DFLT (3*OUTBUF_CELLS-TLS_RECORD_MAX_CELLS)
 #define VEGAS_BETA_SOS_DFLT (3*OUTBUF_CELLS)
 #define VEGAS_GAMMA_SOS_DFLT (3*OUTBUF_CELLS)
 #define VEGAS_DELTA_SOS_DFLT (5*OUTBUF_CELLS)
+#define VEGAS_SSCAP_SOS_DFLT (500)
 
 /* Vanguard Onions are 7 hops (or 8 if both sides use vanguards, but that
  * should be rare), so params are based on 7 outbufs of cells */
@@ -56,26 +62,15 @@
 #define VEGAS_BETA_VG_DFLT (7*OUTBUF_CELLS)
 #define VEGAS_GAMMA_VG_DFLT (7*OUTBUF_CELLS)
 #define VEGAS_DELTA_VG_DFLT (9*OUTBUF_CELLS)
-
-#define VEGAS_BDP_MIX_PCT       100
+#define VEGAS_SSCAP_VG_DFLT (600)
 
 /**
- * The original TCP Vegas used only a congestion window BDP estimator. We
- * believe that the piecewise estimator is likely to perform better, but
- * for purposes of experimentation, we might as well have a way to blend
- * them. It also lets us set Vegas to its original estimator while other
- * algorithms on the same network use piecewise (by setting the
- * 'vegas_bdp_mix_pct' consensus parameter to 100, while leaving the
- * 'cc_bdp_alg' parameter set to piecewise).
- *
- * Returns a percentage weighted average between the CWND estimator and
- * the specified consensus BDP estimator.
+ * The original TCP Vegas congestion window BDP estimator.
  */
 static inline uint64_t
-vegas_bdp_mix(const congestion_control_t *cc)
+vegas_bdp(const congestion_control_t *cc)
 {
-  return cc->vegas_params.bdp_mix_pct*cc->bdp[BDP_ALG_CWND_RTT]/100 +
-         (100-cc->vegas_params.bdp_mix_pct)*cc->bdp[cc->bdp_alg]/100;
+  return cc->bdp[BDP_ALG_CWND_RTT];
 }
 
 /**
@@ -87,8 +82,8 @@ congestion_control_vegas_set_params(congestion_control_t *cc,
 {
   tor_assert(cc->cc_alg == CC_ALG_VEGAS);
   const char *alpha_str = NULL, *beta_str = NULL, *gamma_str = NULL;
-  const char *delta_str = NULL;
-  int alpha, beta, gamma, delta;
+  const char *delta_str = NULL, *sscap_str = NULL;
+  int alpha, beta, gamma, delta, ss_cwnd_cap;
 
   switch (path) {
     case CC_PATH_SBWS:
@@ -96,56 +91,78 @@ congestion_control_vegas_set_params(congestion_control_t *cc,
       beta_str = "cc_vegas_beta_sbws";
       gamma_str = "cc_vegas_gamma_sbws";
       delta_str = "cc_vegas_delta_sbws";
+      sscap_str = "cc_sscap_sbws";
       alpha = VEGAS_ALPHA_SBWS_DFLT;
       beta = VEGAS_BETA_SBWS_DFLT;
       gamma = VEGAS_GAMMA_SBWS_DFLT;
       delta = VEGAS_DELTA_SBWS_DFLT;
+      ss_cwnd_cap = VEGAS_SSCAP_SBWS_DFLT;
       break;
     case CC_PATH_EXIT:
       alpha_str = "cc_vegas_alpha_exit";
       beta_str = "cc_vegas_beta_exit";
       gamma_str = "cc_vegas_gamma_exit";
       delta_str = "cc_vegas_delta_exit";
+      sscap_str = "cc_sscap_exit";
       alpha = VEGAS_ALPHA_EXIT_DFLT;
       beta = VEGAS_BETA_EXIT_DFLT;
       gamma = VEGAS_GAMMA_EXIT_DFLT;
       delta = VEGAS_DELTA_EXIT_DFLT;
+      ss_cwnd_cap = VEGAS_SSCAP_EXIT_DFLT;
       break;
     case CC_PATH_ONION:
       alpha_str = "cc_vegas_alpha_onion";
       beta_str = "cc_vegas_beta_onion";
       gamma_str = "cc_vegas_gamma_onion";
       delta_str = "cc_vegas_delta_onion";
+      sscap_str = "cc_sscap_onion";
       alpha = VEGAS_ALPHA_ONION_DFLT;
       beta = VEGAS_BETA_ONION_DFLT;
       gamma = VEGAS_GAMMA_ONION_DFLT;
       delta = VEGAS_DELTA_ONION_DFLT;
+      ss_cwnd_cap = VEGAS_SSCAP_ONION_DFLT;
       break;
     case CC_PATH_ONION_SOS:
       alpha_str = "cc_vegas_alpha_sos";
       beta_str = "cc_vegas_beta_sos";
       gamma_str = "cc_vegas_gamma_sos";
       delta_str = "cc_vegas_delta_sos";
+      sscap_str = "cc_sscap_sos";
       alpha = VEGAS_ALPHA_SOS_DFLT;
       beta = VEGAS_BETA_SOS_DFLT;
       gamma = VEGAS_GAMMA_SOS_DFLT;
       delta = VEGAS_DELTA_SOS_DFLT;
+      ss_cwnd_cap = VEGAS_SSCAP_SOS_DFLT;
       break;
     case CC_PATH_ONION_VG:
       alpha_str = "cc_vegas_alpha_vg";
       beta_str = "cc_vegas_beta_vg";
       gamma_str = "cc_vegas_gamma_vg";
       delta_str = "cc_vegas_delta_vg";
+      sscap_str = "cc_sscap_vg";
       alpha = VEGAS_ALPHA_VG_DFLT;
       beta = VEGAS_BETA_VG_DFLT;
       gamma = VEGAS_GAMMA_VG_DFLT;
       delta = VEGAS_DELTA_VG_DFLT;
+      ss_cwnd_cap = VEGAS_SSCAP_VG_DFLT;
       break;
     default:
       tor_assert(0);
       break;
   }
 
+  cc->vegas_params.ss_cwnd_cap =
+   networkstatus_get_param(NULL, sscap_str,
+      ss_cwnd_cap,
+      100,
+      INT32_MAX);
+
+  cc->vegas_params.ss_cwnd_max =
+   networkstatus_get_param(NULL, "cc_ss_max",
+      SS_CWND_MAX_DFLT,
+      500,
+      INT32_MAX);
+
   cc->vegas_params.alpha =
    networkstatus_get_param(NULL, alpha_str,
       alpha,
@@ -169,12 +186,107 @@ congestion_control_vegas_set_params(congestion_control_t *cc,
       delta,
       0,
       INT32_MAX);
+}
 
-  cc->vegas_params.bdp_mix_pct =
-   networkstatus_get_param(NULL, "cc_vegas_bdp_mix",
-      VEGAS_BDP_MIX_PCT,
-      0,
-      100);
+/**
+ * Common log function for tracking all vegas state.
+ */
+static void
+congestion_control_vegas_log(const circuit_t *circ,
+                             const congestion_control_t *cc)
+{
+  uint64_t queue_use = cc->cwnd - vegas_bdp(cc);
+
+  if (CIRCUIT_IS_ORIGIN(circ) &&
+      circ->purpose == CIRCUIT_PURPOSE_S_REND_JOINED) {
+    log_info(LD_CIRC,
+               "CC: TOR_VEGAS Onion Circuit %d "
+               "RTT: %"PRIu64", %"PRIu64", %"PRIu64", "
+               "CWND: %"PRIu64", "
+               "INFL: %"PRIu64", "
+               "VBDP: %"PRIu64", "
+               "QUSE: %"PRIu64", "
+               "BWE: %"PRIu64", "
+               "SS: %d",
+             CONST_TO_ORIGIN_CIRCUIT(circ)->global_identifier,
+             cc->min_rtt_usec/1000,
+             cc->ewma_rtt_usec/1000,
+             cc->max_rtt_usec/1000,
+             cc->cwnd,
+             cc->inflight,
+             vegas_bdp(cc),
+             queue_use,
+             cc->cwnd*CELL_MAX_NETWORK_SIZE*1000/
+                MAX(cc->min_rtt_usec,cc->ewma_rtt_usec),
+             cc->in_slow_start
+             );
+  } else {
+    log_info(LD_CIRC,
+               "CC: TOR_VEGAS "
+               "RTT: %"PRIu64", %"PRIu64", %"PRIu64", "
+               "CWND: %"PRIu64", "
+               "INFL: %"PRIu64", "
+               "VBDP: %"PRIu64", "
+               "QUSE: %"PRIu64", "
+               "BWE: %"PRIu64", "
+               "SS: %d",
+             cc->min_rtt_usec/1000,
+             cc->ewma_rtt_usec/1000,
+             cc->max_rtt_usec/1000,
+             cc->cwnd,
+             cc->inflight,
+             vegas_bdp(cc),
+             queue_use,
+             cc->cwnd*CELL_MAX_NETWORK_SIZE*1000/
+                MAX(cc->min_rtt_usec,cc->ewma_rtt_usec),
+             cc->in_slow_start
+             );
+  }
+}
+
+/**
+ * Implements RFC3742: Limited Slow Start.
+ * https://datatracker.ietf.org/doc/html/rfc3742#section-2
+ */
+static inline uint64_t
+rfc3742_ss_inc(const congestion_control_t *cc)
+{
+  if (cc->cwnd <= cc->vegas_params.ss_cwnd_cap) {
+    /* If less than the cap, round and always grow by at least 1 sendme_inc. */
+    return ((uint64_t)cc->cwnd_inc_pct_ss*cc->sendme_inc + 50)/100;
+  } else {
+    // K = int(cwnd/(0.5 max_ssthresh));
+    //  => K = 2*cwnd/max_ssthresh
+    // cwnd += int(MSS/K);
+    //  => cwnd += MSS*max_ssthresh/(2*cwnd)
+    return ((uint64_t)cc->sendme_inc*cc->vegas_params.ss_cwnd_cap + cc->cwnd)/
+            (2*cc->cwnd);
+  }
+}
+
+/**
+ * Exit Vegas slow start.
+ *
+ * This function sets our slow-start state to 0, and emits logs
+ * and control port information signifying end of slow start.
+ * It also schedules the next CWND update for steady-state.
+ */
+static void
+congestion_control_vegas_exit_slow_start(const circuit_t *circ,
+                                         congestion_control_t *cc)
+{
+  congestion_control_vegas_log(circ, cc);
+  cc->in_slow_start = 0;
+  cc->next_cc_event = CWND_UPDATE_RATE(cc);
+  congestion_control_vegas_log(circ, cc);
+
+  /* We need to report that slow start has exited ASAP,
+   * for sbws bandwidth measurement. */
+  if (CIRCUIT_IS_ORIGIN(circ)) {
+    /* We must discard const here because the event modifies fields :/ */
+    control_event_circ_bandwidth_used_for_circ(
+            TO_ORIGIN_CIRCUIT((circuit_t*)circ));
+  }
 }
 
 /**
@@ -217,43 +329,45 @@ congestion_control_vegas_process_sendme(congestion_control_t *cc,
     return 0;
   }
 
-  /* We only update anything once per window */
-  if (cc->next_cc_event == 0) {
-    /* The queue use is the amount in which our cwnd is above BDP;
-     * if it is below, then 0 queue use. */
-    if (vegas_bdp_mix(cc) > cc->cwnd)
-      queue_use = 0;
-    else
-      queue_use = cc->cwnd - vegas_bdp_mix(cc);
-
-    if (cc->in_slow_start) {
-      if (queue_use < cc->vegas_params.gamma && !cc->blocked_chan) {
-        /* Grow to BDP immediately, then exponential growth until
-         * congestion signal. Increment by at least 2 sendme's worth. */
-        cc->cwnd = MAX(cc->cwnd + MAX(CWND_INC_SS(cc), 2*cc->sendme_inc),
-                       vegas_bdp_mix(cc));
+  /* The queue use is the amount in which our cwnd is above BDP;
+   * if it is below, then 0 queue use. */
+  if (vegas_bdp(cc) > cc->cwnd)
+    queue_use = 0; // This should not happen anymore..
+  else
+    queue_use = cc->cwnd - vegas_bdp(cc);
+
+  if (cc->in_slow_start) {
+    if (queue_use < cc->vegas_params.gamma && !cc->blocked_chan) {
+      /* Get the "Limited Slow Start" increment */
+      uint64_t inc = rfc3742_ss_inc(cc);
+
+      // Check if inc is less than what we would do in steady-state
+      // avoidance
+      if (inc*SENDME_PER_CWND(cc) <= CWND_INC(cc)) {
+        cc->cwnd += inc;
+        congestion_control_vegas_exit_slow_start(circ, cc);
       } else {
-        /* Congestion signal: Set cwnd to gamma threshhold */
-        cc->cwnd = vegas_bdp_mix(cc) + cc->vegas_params.gamma;
-        cc->in_slow_start = 0;
-        log_info(LD_CIRC, "CC: TOR_VEGAS exiting slow start");
-
-        /* We need to report that slow start has exited ASAP,
-         * for sbws bandwidth measurement. */
-        if (CIRCUIT_IS_ORIGIN(circ)) {
-          /* We must discard const here because the event modifies fields :/ */
-          control_event_circ_bandwidth_used_for_circ(
-                  TO_ORIGIN_CIRCUIT((circuit_t*)circ));
-        }
+        cc->cwnd += inc;
+        cc->next_cc_event = 1; // Technically irellevant, but for consistency
       }
     } else {
-      if (queue_use > cc->vegas_params.delta) {
-        cc->cwnd = vegas_bdp_mix(cc) + cc->vegas_params.delta - CWND_INC(cc);
-      } else if (queue_use > cc->vegas_params.beta || cc->blocked_chan) {
-        cc->cwnd -= CWND_INC(cc);
-      } else if (queue_use < cc->vegas_params.alpha) {
-        cc->cwnd += CWND_INC(cc);
-      }
+      /* Congestion signal: Set cwnd to gamma threshhold */
+      cc->cwnd = vegas_bdp(cc) + cc->vegas_params.gamma;
+      congestion_control_vegas_exit_slow_start(circ, cc);
+    }
+
+    if (cc->cwnd >= cc->vegas_params.ss_cwnd_max) {
+      cc->cwnd = cc->vegas_params.ss_cwnd_max;
+      congestion_control_vegas_exit_slow_start(circ, cc);
+    }
+  /* After slow start, We only update once per window */
+  } else if (cc->next_cc_event == 0) {
+    if (queue_use > cc->vegas_params.delta) {
+      cc->cwnd = vegas_bdp(cc) + cc->vegas_params.delta - CWND_INC(cc);
+    } else if (queue_use > cc->vegas_params.beta || cc->blocked_chan) {
+      cc->cwnd -= CWND_INC(cc);
+    } else if (queue_use < cc->vegas_params.alpha) {
+      cc->cwnd += CWND_INC(cc);
     }
 
     /* cwnd can never fall below 1 increment */
@@ -262,41 +376,13 @@ congestion_control_vegas_process_sendme(congestion_control_t *cc,
     /* Schedule next update */
     cc->next_cc_event = CWND_UPDATE_RATE(cc);
 
-    if (CIRCUIT_IS_ORIGIN(circ)) {
-      log_info(LD_CIRC,
-                 "CC: TOR_VEGAS Circuit %d "
-                 "CWND: %"PRIu64", "
-                 "INFL: %"PRIu64", "
-                 "VBDP: %"PRIu64", "
-                 "QUSE: %"PRIu64", "
-                 "NCCE: %"PRIu64", "
-                 "SS: %d",
-               CONST_TO_ORIGIN_CIRCUIT(circ)->global_identifier,
-               cc->cwnd,
-               cc->inflight,
-               vegas_bdp_mix(cc),
-               queue_use,
-               cc->next_cc_event,
-               cc->in_slow_start
-               );
-    } else {
+    congestion_control_vegas_log(circ, cc);
+
+    /* Log if we're above the ss_cap */
+    if (cc->cwnd >= cc->vegas_params.ss_cwnd_max) {
       log_info(LD_CIRC,
-                 "CC: TOR_VEGAS Circuit %"PRIu64":%d "
-                 "CWND: %"PRIu64", "
-                 "INFL: %"PRIu64", "
-                 "VBDP: %"PRIu64", "
-                 "QUSE: %"PRIu64", "
-                 "NCCE: %"PRIu64", "
-                 "SS: %d",
-               CONST_TO_OR_CIRCUIT(circ)->p_chan->global_identifier,
-               CONST_TO_OR_CIRCUIT(circ)->p_circ_id,
-               cc->cwnd,
-               cc->inflight,
-               vegas_bdp_mix(cc),
-               queue_use,
-               cc->next_cc_event,
-               cc->in_slow_start
-               );
+            "CC: TOR_VEGAS above ss_max in steady state for circ %d: %"PRIu64,
+            circ->purpose, cc->cwnd);
     }
   }