Reset the min value if we hit cwnd_min.

This can avoid circuits getting stuck due to an abnormally low min value.

2 files changed, 55 insertions, 1 deletions

diff --git a/src/core/or/congestion_control_common.c b/src/core/or/congestion_control_common.c
index b90c78729a..25446621d1 100644
--- a/src/core/or/congestion_control_common.c
+++ b/src/core/or/congestion_control_common.c
@@ -54,6 +54,8 @@
 #define N_EWMA_MAX_DFLT (10)
 #define N_EWMA_SS_DFLT (2)
 
+#define RTT_RESET_PCT_DFLT (100)
+
 /* BDP algorithms for each congestion control algorithms use the piecewise
  * estimattor. See section 3.1.4 of proposal 324. */
 #define WESTWOOD_BDP_ALG BDP_ALG_PIECEWISE
@@ -119,6 +121,12 @@ static uint8_t n_ewma_ss;
 static uint8_t bwe_sendme_min;
 
 /**
+ * Percentage of the current RTT to use when reseting the minimum RTT
+ * for a circuit. (RTT is reset when the cwnd hits cwnd_min).
+ */
+static uint8_t rtt_reset_pct;
+
+/**
  * Update global congestion control related consensus parameter values,
  * every consensus update.
  */
@@ -163,6 +171,14 @@ congestion_control_new_consensus_params(const networkstatus_t *ns)
         CWND_MAX_MIN,
         CWND_MAX_MAX);
 
+#define RTT_RESET_PCT_MIN (0)
+#define RTT_RESET_PCT_MAX (100)
+  rtt_reset_pct =
+    networkstatus_get_param(NULL, "cc_rtt_reset_pct",
+        RTT_RESET_PCT_DFLT,
+        RTT_RESET_PCT_MIN,
+        RTT_RESET_PCT_MAX);
+
 #define SENDME_INC_MIN 1
 #define SENDME_INC_MAX (255)
   cc_sendme_inc =
@@ -857,7 +873,22 @@ congestion_control_update_circuit_rtt(congestion_control_t *cc,
     cc->max_rtt_usec = rtt;
   }
 
-  if (cc->min_rtt_usec == 0 || cc->ewma_rtt_usec < cc->min_rtt_usec) {
+  if (cc->min_rtt_usec == 0) {
+    // If we do not have a min_rtt yet, use current ewma
+    cc->min_rtt_usec = cc->ewma_rtt_usec;
+  } else if (cc->cwnd == cc->cwnd_min) {
+    // Raise min rtt if cwnd hit cwnd_min. This gets us out of a wedge state
+    // if we hit cwnd_min due to an abnormally low rtt.
+    uint64_t new_rtt = percent_max_mix(cc->ewma_rtt_usec, cc->min_rtt_usec,
+                                       rtt_reset_pct);
+
+    static ratelim_t rtt_notice_limit = RATELIM_INIT(300);
+    log_fn_ratelim(&rtt_notice_limit, LOG_NOTICE, LD_CIRC,
+            "Resetting circ RTT from %"PRIu64" to %"PRIu64" due to low cwnd",
+            cc->min_rtt_usec/1000, new_rtt/1000);
+
+    cc->min_rtt_usec = new_rtt;
+  } else if (cc->ewma_rtt_usec < cc->min_rtt_usec) {
     // Using the EWMA for min instead of current RTT helps average out
     // effects from other conns
     cc->min_rtt_usec = cc->ewma_rtt_usec;
diff --git a/src/core/or/congestion_control_common.h b/src/core/or/congestion_control_common.h
index 71e984f914..50af945d62 100644
--- a/src/core/or/congestion_control_common.h
+++ b/src/core/or/congestion_control_common.h
@@ -145,6 +145,29 @@ n_count_ewma(uint64_t curr, uint64_t prev, uint64_t N)
     return (2*curr + (N-1)*prev)/(N+1);
 }
 
+/**
+ * Helper function that gives us a percentile weighted-average between
+ * two values. The pct_max argument specifies the percentage weight of the
+ * maximum of a and b, when computing this weighted-average.
+ *
+ * This also allows this function to be used as either MIN() or a MAX()
+ * by this parameterization. It is MIN() when pct_max==0;
+ * it is MAX() when pct_max==100; it is avg() when pct_max==50; it is a
+ * weighted-average for values in between.
+ */
+static inline uint64_t
+percent_max_mix(uint64_t a, uint64_t b, uint8_t pct_max)
+{
+  uint64_t max = MAX(a, b);
+  uint64_t min = MIN(a, b);
+
+  if (BUG(pct_max > 100)) {
+    return max;
+  }
+
+  return pct_max*max/100 + (100-pct_max)*min/100;
+}
+
 /* Private section starts. */
 #ifdef TOR_CONGESTION_CONTROL_PRIVATE