1 files changed, 231 insertions, 0 deletions
diff --git a/src/core/or/congestion_control_westwood.c b/src/core/or/congestion_control_westwood.c
new file mode 100644
index 0000000000..4b24234212
--- /dev/null
+++ b/src/core/or/congestion_control_westwood.c
@@ -0,0 +1,231 @@
+/* Copyright (c) 2019-2021, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * \file congestion_control_westwood.c
+ * \brief Code that implements the TOR_WESTWOOD congestion control algorithm
+ *        from Proposal #324.
+ */
+
+#define TOR_CONGESTION_CONTROL_WESTWOOD_PRIVATE
+
+#include "core/or/or.h"
+
+#include "core/or/crypt_path.h"
+#include "core/or/or_circuit_st.h"
+#include "core/or/sendme.h"
+#include "core/or/congestion_control_st.h"
+#include "core/or/congestion_control_common.h"
+#include "core/or/congestion_control_westwood.h"
+#include "core/or/circuitlist.h"
+#include "core/or/circuituse.h"
+#include "core/or/origin_circuit_st.h"
+#include "core/or/channel.h"
+#include "feature/nodelist/networkstatus.h"
+
+#define USEC_ONE_MS (1000)
+
+#define WESTWOOD_CWND_BACKOFF_M 75
+#define WESTWOOD_RTT_BACKOFF_M 100
+#define WESTWOOD_RTT_THRESH 33
+#define WESTWOOD_MIN_BACKOFF 0
+
+/**
+ * Cache westwood consensus parameters.
+ */
+void
+congestion_control_westwood_set_params(congestion_control_t *cc)
+{
+  tor_assert(cc->cc_alg == CC_ALG_WESTWOOD);
+
+  cc->westwood_params.cwnd_backoff_m =
+   networkstatus_get_param(NULL, "cc_westwood_cwnd_m",
+      WESTWOOD_CWND_BACKOFF_M,
+      0,
+      100);
+
+  cc->westwood_params.rtt_backoff_m =
+   networkstatus_get_param(NULL, "cc_westwood_rtt_m",
+      WESTWOOD_RTT_BACKOFF_M,
+      50,
+      100);
+
+  cc->westwood_params.rtt_thresh =
+   networkstatus_get_param(NULL, "cc_westwood_rtt_thresh",
+      WESTWOOD_RTT_THRESH,
+      0,
+      100);
+
+  cc->westwood_params.min_backoff =
+   networkstatus_get_param(NULL, "cc_westwood_min_backoff",
+      WESTWOOD_MIN_BACKOFF,
+      0,
+      1);
+}
+
+/**
+ * Return the RTT threshhold that signals congestion.
+ *
+ * Computed from the threshold parameter that specifies a
+ * percent between the min and max RTT obseved so far.
+ */
+static inline uint64_t
+westwood_rtt_signal(const congestion_control_t *cc)
+{
+  return ((100 - cc->westwood_params.rtt_thresh)*cc->min_rtt_usec +
+          cc->westwood_params.rtt_thresh*(cc)->max_rtt_usec)/100;
+}
+
+/**
+ * Compute a backoff to reduce the max RTT.
+ *
+ * This may be necessary to ensure that westwood does not have
+ * a runaway condition where congestion inflates the max RTT, which
+ * inflates the congestion threshold. That cannot happen with one
+ * Westwood instance, but it may happen in aggregate. Hence, this is
+ * a safety parameter, in case we need it.
+ */
+static inline uint64_t
+westwood_rtt_max_backoff(const congestion_control_t *cc)
+{
+  return cc->min_rtt_usec +
+          (cc->westwood_params.rtt_backoff_m *
+          (cc->max_rtt_usec - cc->min_rtt_usec))/100;
+}
+
+/**
+ * Returns true if the circuit is experiencing congestion, as per
+ * TOR_WESTWOOD rules.
+ */
+static inline bool
+westwood_is_congested(const congestion_control_t *cc)
+{
+  /* If the local channel is blocked, that is always congestion */
+  if (cc->blocked_chan)
+    return true;
+
+  /* If the min RTT is within 1ms of the signal, then there is not enough
+   * range in RTTs to signify congestion. Treat that as not congested. */
+  if (westwood_rtt_signal(cc) < cc->min_rtt_usec ||
+      westwood_rtt_signal(cc) - cc->min_rtt_usec < USEC_ONE_MS)
+    return false;
+
+  /* If the EWMA-smoothed RTT exceeds the westwood RTT threshhold,
+   * then it is congestion. */
+  if (cc->ewma_rtt_usec > westwood_rtt_signal(cc))
+    return true;
+
+  return false;
+}
+
+/**
+ * Process a SENDME and update the congestion window according to the
+ * rules specified in TOR_WESTWOOD of Proposal #324.
+ *
+ * Essentially, this algorithm uses a threshhold of 'rtt_thresh', which
+ * is a midpoint between the min and max RTT. If the RTT exceeds this
+ * threshhold, then queue delay due to congestion is assumed to be present,
+ * and the algirithm reduces the congestion window. If the RTT is below the
+ * threshhold, the circuit is not congested (ie: queue delay is low), and we
+ * increase the congestion window.
+ *
+ * The congestion window is updated only once every congestion window worth of
+ * packets, even if the signal persists. It is also updated whenever the
+ * upstream orcon blocks, or unblocks. This minimizes local client queues.
+ */
+int
+congestion_control_westwood_process_sendme(congestion_control_t *cc,
+                                           const circuit_t *circ,
+                                           const crypt_path_t *layer_hint)
+{
+  tor_assert(cc && cc->cc_alg == CC_ALG_WESTWOOD);
+  tor_assert(circ);
+
+  /* Update ack counter until next congestion signal event is allowed */
+  if (cc->next_cc_event)
+    cc->next_cc_event--;
+
+  /* If we were unable to update our circuit estimates, Westwood must
+   * *not* update its cwnd, otherwise it could run to infinity, or to 0.
+   * Just update inflight from the sendme and return. */
+  if (!congestion_control_update_circuit_estimates(cc, circ, layer_hint)) {
+    cc->inflight = cc->inflight - cc->sendme_inc;
+    return 0;
+  }
+
+  /* We only update anything once per window */
+  if (cc->next_cc_event == 0) {
+    if (!westwood_is_congested(cc)) {
+      if (cc->in_slow_start) {
+        cc->cwnd = MAX(cc->cwnd + CWND_INC_SS(cc),
+                       cc->bdp[cc->bdp_alg]);
+      } else {
+        cc->cwnd = cc->cwnd + CWND_INC(cc);
+      }
+    } else {
+      if (cc->westwood_params.min_backoff)
+        cc->cwnd = MIN(cc->cwnd*cc->westwood_params.cwnd_backoff_m/100,
+                       cc->bdp[cc->bdp_alg]);
+      else
+        cc->cwnd = MAX(cc->cwnd*cc->westwood_params.cwnd_backoff_m/100,
+                       cc->bdp[cc->bdp_alg]);
+
+      cc->in_slow_start = 0;
+
+      // Because Westwood's congestion can runaway and boost max rtt,
+      // which increases its congestion signal, we backoff the max rtt
+      // too.
+      cc->max_rtt_usec = westwood_rtt_max_backoff(cc);
+
+      log_info(LD_CIRC, "CC: TOR_WESTWOOD congestion. New max RTT: %"PRIu64,
+                 cc->max_rtt_usec/1000);
+    }
+
+    /* cwnd can never fall below 1 increment */
+    cc->cwnd = MAX(cc->cwnd, cc->cwnd_min);
+
+    /* Schedule next update */
+    cc->next_cc_event = CWND_UPDATE_RATE(cc);
+
+    if (CIRCUIT_IS_ORIGIN(circ)) {
+      log_info(LD_CIRC,
+                 "CC: TOR_WESTWOOD Circuit %d "
+                 "CWND: %"PRIu64", "
+                 "INFL: %"PRIu64", "
+                 "NCCE: %"PRIu64", "
+                 "WRTT: %"PRIu64", "
+                 "WSIG: %"PRIu64", "
+                 "SS: %d",
+               CONST_TO_ORIGIN_CIRCUIT(circ)->global_identifier,
+               cc->cwnd,
+               cc->inflight,
+               cc->next_cc_event,
+               cc->ewma_rtt_usec/1000,
+               westwood_rtt_signal(cc)/1000,
+               cc->in_slow_start
+               );
+    } else {
+      log_info(LD_CIRC,
+                 "CC: TOR_WESTWOOD Circuit %"PRIu64":%d "
+                 "CWND: %"PRIu64", "
+                 "INFL: %"PRIu64", "
+                 "NCCE: %"PRIu64", "
+                 "WRTT: %"PRIu64", "
+                 "WSIG: %"PRIu64", "
+                 "SS: %d",
+               circ->n_chan->global_identifier, circ->n_circ_id,
+               cc->cwnd,
+               cc->inflight,
+               cc->next_cc_event,
+               cc->ewma_rtt_usec/1000,
+               westwood_rtt_signal(cc)/1000,
+               cc->in_slow_start
+               );
+    }
+  }
+
+  /* Update inflight with ack */
+  cc->inflight = cc->inflight - cc->sendme_inc;
+
+  return 0;
+}