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[tor-commits] [tor/master] Circuit padding tests.

To: tor-commits@xxxxxxxxxxxxxxxxxxxx
Subject: [tor-commits] [tor/master] Circuit padding tests.
From: nickm@xxxxxxxxxxxxxx
Date: Mon, 14 Jan 2019 19:50:35 +0000 (UTC)
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Patch-author: George Kadianakis <desnacked@xxxxxxxxxx>
Sender: "tor-commits" <tor-commits-bounces@xxxxxxxxxxxxxxxxxxxx>
commit a336d816a68e5eaddd9d80f7179699274b367a1d
Author: George Kadianakis <desnacked@xxxxxxxxxx>
Date:   Wed Jan 2 15:19:12 2019 +0200

    Circuit padding tests.
    
    Co-authored-by: George Kadianakis <desnacked@xxxxxxxxxx>
---
 src/core/or/circuitpadding.c               |   55 +-
 src/core/or/circuitpadding.h               |    5 +
 src/lib/smartlist_core/smartlist_foreach.h |    5 +
 src/lib/time/compat_time.c                 |    4 +-
 src/lib/time/compat_time.h                 |    2 +-
 src/test/Makefile.nmake                    |    1 +
 src/test/include.am                        |    1 +
 src/test/test.c                            |    1 +
 src/test/test.h                            |    1 +
 src/test/test_circuitpadding.c             | 2358 ++++++++++++++++++++++++++++
 src/test/test_containers.c                 |   25 +
 11 files changed, 2438 insertions(+), 20 deletions(-)

diff --git a/src/core/or/circuitpadding.c b/src/core/or/circuitpadding.c
index 9d65e2cf2..6cfbf4ba5 100644
--- a/src/core/or/circuitpadding.c
+++ b/src/core/or/circuitpadding.c
@@ -157,7 +157,7 @@ circpad_circuit_machineinfo_new(circuit_t *on_circ, int machine_index)
  * invalid state.
  */
 STATIC const circpad_state_t *
-circpad_machine_current_state(circpad_machineinfo_t *mi)
+circpad_machine_current_state(const circpad_machineinfo_t *mi)
 {
   const circpad_machine_t *machine = CIRCPAD_GET_MACHINE(mi);
 
@@ -189,7 +189,7 @@ circpad_machine_current_state(circpad_machineinfo_t *mi)
  * It has a usec value of CIRCPAD_DELAY_INFINITE (UINT32_MAX).
  */
 STATIC circpad_delay_t
-circpad_histogram_bin_to_usec(circpad_machineinfo_t *mi,
+circpad_histogram_bin_to_usec(const circpad_machineinfo_t *mi,
                               circpad_hist_index_t bin)
 {
   const circpad_state_t *state = circpad_machine_current_state(mi);
@@ -224,6 +224,18 @@ circpad_histogram_bin_to_usec(circpad_machineinfo_t *mi,
                               CIRCPAD_DELAY_INFINITE);
 }
 
+/** Return the midpoint of the histogram bin <b>bin_index</b>. */
+static circpad_delay_t
+circpad_get_histogram_bin_midpoint(const circpad_machineinfo_t *mi,
+                           int bin_index)
+{
+  circpad_delay_t left_bound = circpad_histogram_bin_to_usec(mi, bin_index);
+  circpad_delay_t right_bound =
+    circpad_histogram_bin_to_usec(mi, bin_index+1)-1;
+
+  return left_bound + (right_bound - left_bound)/2;
+}
+
 /**
  * Return the bin that contains the usec argument.
  * "Contains" is defined as us in [lower, upper).
@@ -446,6 +458,8 @@ circpad_machine_sample_delay(circpad_machineinfo_t *mi)
   tor_assert(curr_bin < CIRCPAD_INFINITY_BIN(state));
 
   bin_start = circpad_histogram_bin_to_usec(mi, curr_bin);
+  /* We don't need to reduct 1 from the upper bound because the random range
+   * function below samples from [bin_start, bin_end) */
   bin_end = circpad_histogram_bin_to_usec(mi, curr_bin+1);
 
   /* Truncate the high bin in case it's the infinity bin:
@@ -532,7 +546,7 @@ circpad_distribution_sample(circpad_distribution_t dist)
  * greater than the target, and that has tokens remaining.
  */
 static circpad_hist_index_t
-circpad_machine_first_higher_index(circpad_machineinfo_t *mi,
+circpad_machine_first_higher_index(const circpad_machineinfo_t *mi,
                                    circpad_delay_t target_bin_usec)
 {
   circpad_hist_index_t bin = circpad_histogram_usec_to_bin(mi,
@@ -554,7 +568,7 @@ circpad_machine_first_higher_index(circpad_machineinfo_t *mi,
  * <b>target_bin_usec</b>, and that still has tokens remaining.
  */
 static circpad_hist_index_t
-circpad_machine_first_lower_index(circpad_machineinfo_t *mi,
+circpad_machine_first_lower_index(const circpad_machineinfo_t *mi,
                                   circpad_delay_t target_bin_usec)
 {
   circpad_hist_index_t bin = circpad_histogram_usec_to_bin(mi,
@@ -619,7 +633,9 @@ circpad_machine_remove_lower_token(circpad_machineinfo_t *mi,
 /**
  * Remove a token from the closest non-empty bin to the target.
  *
- * If use_usec is true, measure "closest" in terms of bin start usec.
+ * If use_usec is true, measure "closest" in terms of the next closest bin
+ * midpoint.
+ *
  * If it is false, use bin index distance only.
  */
 STATIC void
@@ -639,8 +655,9 @@ circpad_machine_remove_closest_token(circpad_machineinfo_t *mi,
     return;
   }
 
+  /* Take care of edge cases first */
   if (higher == mi->histogram_len && lower == -1) {
-    // Bins are empty
+    /* All bins are empty */
     return;
   } else if (higher == mi->histogram_len) {
     /* All higher bins are empty */
@@ -654,6 +671,7 @@ circpad_machine_remove_closest_token(circpad_machineinfo_t *mi,
     return;
   }
 
+  /* Now handle the intermediate cases */
   if (use_usec) {
     /* Find the closest bin midpoint to the target */
     circpad_delay_t lower_usec = circpad_get_histogram_bin_midpoint(mi, lower);
@@ -677,6 +695,9 @@ circpad_machine_remove_closest_token(circpad_machineinfo_t *mi,
       ENSURE_BIN_CAPACITY(lower);
       bin_to_remove = lower;
     }
+    mi->histogram[bin_to_remove]--;
+    log_debug(LD_GENERAL, "Removing token from bin %d", bin_to_remove);
+    return;
   } else {
     if (current - lower > higher - current) {
       // Higher bin is closer
@@ -756,7 +777,7 @@ check_machine_token_supply(circpad_machineinfo_t *mi)
  *
  * Returns 1 if we transition states, 0 otherwise.
  */
-circpad_decision_t
+STATIC circpad_decision_t
 circpad_machine_remove_token(circpad_machineinfo_t *mi)
 {
   const circpad_state_t *state = NULL;
@@ -841,10 +862,10 @@ circpad_machine_remove_token(circpad_machineinfo_t *mi)
  *
  * Returns negative on error, 0 on success.
  */
-static signed_error_t
-circpad_send_command_to_hop(origin_circuit_t *circ, uint8_t hopnum,
-                            uint8_t relay_command, const uint8_t *payload,
-                            ssize_t payload_len)
+MOCK_IMPL(STATIC signed_error_t,
+circpad_send_command_to_hop,(origin_circuit_t *circ, uint8_t hopnum,
+                             uint8_t relay_command, const uint8_t *payload,
+                             ssize_t payload_len))
 {
   crypt_path_t *target_hop = circuit_get_cpath_hop(circ, hopnum);
   signed_error_t ret;
@@ -1017,7 +1038,7 @@ circpad_new_consensus_params(const networkstatus_t *ns)
  *
  * Returns 1 if limits are set and we've hit them. Otherwise returns 0.
  */
-static bool
+STATIC bool
 circpad_machine_reached_padding_limit(circpad_machineinfo_t *mi)
 {
   const circpad_machine_t *machine = CIRCPAD_GET_MACHINE(mi);
@@ -1025,7 +1046,7 @@ circpad_machine_reached_padding_limit(circpad_machineinfo_t *mi)
   /* If machine_padding_pct is non-zero, and we've sent more
    * than the allowed count of padding cells, then check our
    * percent limits for this machine. */
-  if (machine->max_padding_percent &&
+   if (machine->max_padding_percent &&
       mi->padding_sent >= machine->allowed_padding_count) {
     uint32_t total_cells = mi->padding_sent + mi->nonpadding_sent;
 
@@ -1046,7 +1067,7 @@ circpad_machine_reached_padding_limit(circpad_machineinfo_t *mi)
 
     /* Check the percent */
     if ((100*circpad_global_padding_sent) / total_cells >
-         circpad_global_max_padding_percent) {
+        circpad_global_max_padding_percent) {
       return 1; // global limit reached. Stop.
     }
   }
@@ -1205,9 +1226,9 @@ circpad_machine_transitioned_to_end(circpad_machineinfo_t *mi)
  *
  * Returns 1 if we transition states, 0 otherwise.
  */
-circpad_decision_t
-circpad_machine_transition(circpad_machineinfo_t *mi,
-                           circpad_event_t event)
+MOCK_IMPL(circpad_decision_t,
+circpad_machine_transition,(circpad_machineinfo_t *mi,
+                            circpad_event_t event))
 {
   const circpad_state_t *state =
       circpad_machine_current_state(mi);
diff --git a/src/core/or/circuitpadding.h b/src/core/or/circuitpadding.h
index 24034a454..4680c6be4 100644
--- a/src/core/or/circuitpadding.h
+++ b/src/core/or/circuitpadding.h
@@ -675,6 +675,11 @@ STATIC void circpad_machine_remove_closest_token(circpad_machineinfo_t *mi,
                                          bool use_usec);
 STATIC void circpad_machine_setup_tokens(circpad_machineinfo_t *mi);
 
+MOCK_DECL(STATIC signed_error_t,
+circpad_send_command_to_hop,(origin_circuit_t *circ, uint8_t hopnum,
+                             uint8_t relay_command, const uint8_t *payload,
+                             ssize_t payload_len));
+
 #ifdef TOR_UNIT_TESTS
 extern smartlist_t *origin_padding_machines;
 extern smartlist_t *relay_padding_machines;
diff --git a/src/lib/smartlist_core/smartlist_foreach.h b/src/lib/smartlist_core/smartlist_foreach.h
index 14f2930c9..c9afebd6a 100644
--- a/src/lib/smartlist_core/smartlist_foreach.h
+++ b/src/lib/smartlist_core/smartlist_foreach.h
@@ -83,6 +83,11 @@
          ++var ## _sl_idx) {                                    \
       var = (sl)->list[var ## _sl_idx];
 
+/** Iterates over the items in smartlist <b>sl</b> in reverse order, similar to
+ *  SMARTLIST_FOREACH_BEGIN
+ *
+ * NOTE: This macro is incompatible with SMARTLIST_DEL_CURRENT.
+ */
 #define SMARTLIST_FOREACH_REVERSE_BEGIN(sl, type, var)  \
   STMT_BEGIN                                                       \
     int var ## _sl_idx, var ## _sl_len=(sl)->num_used;             \
diff --git a/src/lib/time/compat_time.c b/src/lib/time/compat_time.c
index f1ddb4fdc..387b0fad2 100644
--- a/src/lib/time/compat_time.c
+++ b/src/lib/time/compat_time.c
@@ -787,8 +787,8 @@ monotime_absolute_nsec(void)
   return monotime_diff_nsec(&initialized_at, &now);
 }
 
-uint64_t
-monotime_absolute_usec(void)
+MOCK_IMPL(uint64_t,
+monotime_absolute_usec,(void))
 {
   return monotime_absolute_nsec() / 1000;
 }
diff --git a/src/lib/time/compat_time.h b/src/lib/time/compat_time.h
index 44fab62de..bf1bd2880 100644
--- a/src/lib/time/compat_time.h
+++ b/src/lib/time/compat_time.h
@@ -103,7 +103,7 @@ uint64_t monotime_absolute_nsec(void);
 /**
  * Return the number of microseconds since the timer system was initialized.
  */
-uint64_t monotime_absolute_usec(void);
+MOCK_DECL(uint64_t, monotime_absolute_usec,(void));
 /**
  * Return the number of milliseconds since the timer system was initialized.
  */
diff --git a/src/test/Makefile.nmake b/src/test/Makefile.nmake
index aa16a22b5..ca6a84cf8 100644
--- a/src/test/Makefile.nmake
+++ b/src/test/Makefile.nmake
@@ -19,6 +19,7 @@ TEST_OBJECTS = test.obj test_addr.obj test_channel.obj test_channeltls.obj \
 	test_cell_formats.obj test_relay.obj test_replay.obj \
 	test_channelpadding.obj \
 	test_circuitstats.obj \
+	test_circuitpadding.obj \
 	test_scheduler.obj test_introduce.obj test_hs.obj tinytest.obj
 
 tinytest.obj: ..\ext\tinytest.c
diff --git a/src/test/include.am b/src/test/include.am
index 4725e8cba..4da0b8439 100644
--- a/src/test/include.am
+++ b/src/test/include.am
@@ -101,6 +101,7 @@ src_test_test_SOURCES += \
 	src/test/test_cell_queue.c \
 	src/test/test_channel.c \
 	src/test/test_channelpadding.c \
+	src/test/test_circuitpadding.c \
 	src/test/test_channeltls.c \
 	src/test/test_checkdir.c \
 	src/test/test_circuitlist.c \
diff --git a/src/test/test.c b/src/test/test.c
index 13e8c7170..a0a138b03 100644
--- a/src/test/test.c
+++ b/src/test/test.c
@@ -845,6 +845,7 @@ struct testgroup_t testgroups[] = {
   { "channeltls/", channeltls_tests },
   { "checkdir/", checkdir_tests },
   { "circuitbuild/", circuitbuild_tests },
+  { "circuitpadding/", circuitpadding_tests },
   { "circuitlist/", circuitlist_tests },
   { "circuitmux/", circuitmux_tests },
   { "circuitstats/", circuitstats_tests },
diff --git a/src/test/test.h b/src/test/test.h
index 9f754469c..9f6eb0a7e 100644
--- a/src/test/test.h
+++ b/src/test/test.h
@@ -187,6 +187,7 @@ extern struct testcase_t cell_format_tests[];
 extern struct testcase_t cell_queue_tests[];
 extern struct testcase_t channel_tests[];
 extern struct testcase_t channelpadding_tests[];
+extern struct testcase_t circuitpadding_tests[];
 extern struct testcase_t channeltls_tests[];
 extern struct testcase_t checkdir_tests[];
 extern struct testcase_t circuitbuild_tests[];
diff --git a/src/test/test_circuitpadding.c b/src/test/test_circuitpadding.c
new file mode 100644
index 000000000..78f93f7b2
--- /dev/null
+++ b/src/test/test_circuitpadding.c
@@ -0,0 +1,2358 @@
+#define TOR_CHANNEL_INTERNAL_
+#define TOR_TIMERS_PRIVATE
+#define CIRCUITPADDING_PRIVATE
+#define NETWORKSTATUS_PRIVATE
+
+#include "core/or/or.h"
+#include "test.h"
+#include "lib/testsupport/testsupport.h"
+#include "core/or/connection_or.h"
+#include "core/or/channel.h"
+#include "core/or/channeltls.h"
+#include <event.h>
+#include "lib/evloop/compat_libevent.h"
+#include "lib/time/compat_time.h"
+#include "core/or/relay.h"
+#include "core/or/circuitlist.h"
+#include "core/or/circuitbuild.h"
+#include "core/or/circuitpadding.h"
+#include "core/crypto/relay_crypto.h"
+#include "core/or/protover.h"
+#include "feature/nodelist/nodelist.h"
+#include "lib/evloop/compat_libevent.h"
+#include "app/config/config.h"
+
+#include "feature/nodelist/routerstatus_st.h"
+#include "feature/nodelist/networkstatus_st.h"
+#include "feature/nodelist/node_st.h"
+#include "core/or/cell_st.h"
+#include "core/or/crypt_path_st.h"
+#include "core/or/or_circuit_st.h"
+#include "core/or/origin_circuit_st.h"
+
+extern smartlist_t *connection_array;
+
+#define USEC_PER_SEC (1000000)
+#define NSEC_PER_USEC (1000)
+#define NSEC_PER_MSEC (1000*1000)
+
+circid_t get_unique_circ_id_by_chan(channel_t *chan);
+void helper_create_basic_machine(void);
+static void helper_create_conditional_machines(void);
+
+static or_circuit_t * new_fake_orcirc(channel_t *nchan, channel_t *pchan);
+channel_t *new_fake_channel(void);
+void test_circuitpadding_negotiation(void *arg);
+void test_circuitpadding_wronghop(void *arg);
+void test_circuitpadding_conditions(void *arg);
+
+void test_circuitpadding_serialize(void *arg);
+void test_circuitpadding_rtt(void *arg);
+void test_circuitpadding_tokens(void *arg);
+void test_circuitpadding_circuitsetup_machine(void *arg);
+
+static void
+simulate_single_hop_extend(circuit_t *client, circuit_t *mid_relay,
+                           int padding);
+void free_fake_orcirc(circuit_t *circ);
+void free_fake_origin_circuit(origin_circuit_t *circ);
+
+static int deliver_negotiated = 1;
+static int64_t curr_mocked_time;
+
+static node_t padding_node;
+static node_t non_padding_node;
+
+static channel_t dummy_channel;
+static circpad_machine_t circ_client_machine;
+
+static void
+timers_advance_and_run(int64_t msec_update)
+{
+  curr_mocked_time += msec_update*NSEC_PER_MSEC;
+  monotime_coarse_set_mock_time_nsec(curr_mocked_time);
+  monotime_set_mock_time_nsec(curr_mocked_time);
+  timers_run_pending();
+}
+
+static void
+nodes_init(void)
+{
+  padding_node.rs = tor_malloc_zero(sizeof(routerstatus_t));
+  padding_node.rs->pv.supports_padding = 1;
+
+  non_padding_node.rs = tor_malloc_zero(sizeof(routerstatus_t));
+  non_padding_node.rs->pv.supports_padding = 0;
+}
+
+static void
+nodes_free(void)
+{
+  tor_free(padding_node.rs);
+
+  tor_free(non_padding_node.rs);
+}
+
+static const node_t *
+node_get_by_id_mock(const char *identity_digest)
+{
+  if (identity_digest[0] == 1) {
+    return &padding_node;
+  } else if (identity_digest[0] == 0) {
+    return &non_padding_node;
+  }
+
+  return NULL;
+}
+
+static or_circuit_t *
+new_fake_orcirc(channel_t *nchan, channel_t *pchan)
+{
+  or_circuit_t *orcirc = NULL;
+  circuit_t *circ = NULL;
+  crypt_path_t tmp_cpath;
+  char whatevs_key[CPATH_KEY_MATERIAL_LEN];
+
+  orcirc = tor_malloc_zero(sizeof(*orcirc));
+  circ = &(orcirc->base_);
+  circ->magic = OR_CIRCUIT_MAGIC;
+
+  //circ->n_chan = nchan;
+  circ->n_circ_id = get_unique_circ_id_by_chan(nchan);
+  circ->n_mux = NULL; /* ?? */
+  cell_queue_init(&(circ->n_chan_cells));
+  circ->n_hop = NULL;
+  circ->streams_blocked_on_n_chan = 0;
+  circ->streams_blocked_on_p_chan = 0;
+  circ->n_delete_pending = 0;
+  circ->p_delete_pending = 0;
+  circ->received_destroy = 0;
+  circ->state = CIRCUIT_STATE_OPEN;
+  circ->purpose = CIRCUIT_PURPOSE_OR;
+  circ->package_window = CIRCWINDOW_START_MAX;
+  circ->deliver_window = CIRCWINDOW_START_MAX;
+  circ->n_chan_create_cell = NULL;
+
+  //orcirc->p_chan = pchan;
+  orcirc->p_circ_id = get_unique_circ_id_by_chan(pchan);
+  cell_queue_init(&(orcirc->p_chan_cells));
+
+  circuit_set_p_circid_chan(orcirc, orcirc->p_circ_id, pchan);
+  circuit_set_n_circid_chan(circ, circ->n_circ_id, nchan);
+
+  memset(&tmp_cpath, 0, sizeof(tmp_cpath));
+  if (circuit_init_cpath_crypto(&tmp_cpath, whatevs_key,
+                                sizeof(whatevs_key), 0, 0)<0) {
+    log_warn(LD_BUG,"Circuit initialization failed");
+    return NULL;
+  }
+  orcirc->crypto = tmp_cpath.crypto;
+
+  return orcirc;
+}
+
+void
+free_fake_orcirc(circuit_t *circ)
+{
+  or_circuit_t *orcirc = TO_OR_CIRCUIT(circ);
+
+  relay_crypto_clear(&orcirc->crypto);
+
+  circpad_circuit_free_all_machineinfos(circ);
+  tor_free(circ);
+}
+
+void
+free_fake_origin_circuit(origin_circuit_t *circ)
+{
+  circpad_circuit_free_all_machineinfos(TO_CIRCUIT(circ));
+  circuit_clear_cpath(circ);
+  tor_free(circ);
+}
+
+void dummy_nop_timer(void);
+
+//static int dont_stop_libevent = 0;
+
+static circuit_t *client_side;
+static circuit_t *relay_side;
+
+static int n_client_cells = 0;
+static int n_relay_cells = 0;
+
+static int
+circuit_package_relay_cell_mock(cell_t *cell, circuit_t *circ,
+                           cell_direction_t cell_direction,
+                           crypt_path_t *layer_hint, streamid_t on_stream,
+                           const char *filename, int lineno);
+
+static void
+circuitmux_attach_circuit_mock(circuitmux_t *cmux, circuit_t *circ,
+                               cell_direction_t direction);
+
+static void
+circuitmux_attach_circuit_mock(circuitmux_t *cmux, circuit_t *circ,
+                               cell_direction_t direction)
+{
+  (void)cmux;
+  (void)circ;
+  (void)direction;
+
+  return;
+}
+
+static int
+circuit_package_relay_cell_mock(cell_t *cell, circuit_t *circ,
+                           cell_direction_t cell_direction,
+                           crypt_path_t *layer_hint, streamid_t on_stream,
+                           const char *filename, int lineno)
+{
+  (void)cell; (void)on_stream; (void)filename; (void)lineno;
+
+  if (circ == client_side) {
+    if (cell->payload[0] == RELAY_COMMAND_PADDING_NEGOTIATE) {
+      // Deliver to relay
+      circpad_handle_padding_negotiate(relay_side, cell);
+    } else {
+
+      int is_target_hop = circpad_padding_is_from_expected_hop(circ,
+                                                             layer_hint);
+      tt_int_op(cell_direction, OP_EQ, CELL_DIRECTION_OUT);
+      tt_int_op(is_target_hop, OP_EQ, 1);
+
+      // No need to pretend a padding cell was sent: This event is
+      // now emitted internally when the circuitpadding code sends them.
+      //circpad_cell_event_padding_sent(client_side);
+
+      // Receive padding cell at middle
+      circpad_deliver_recognized_relay_cell_events(relay_side,
+              cell->payload[0], NULL);
+    }
+    n_client_cells++;
+  } else if (circ == relay_side) {
+    tt_int_op(cell_direction, OP_EQ, CELL_DIRECTION_IN);
+
+    if (cell->payload[0] == RELAY_COMMAND_PADDING_NEGOTIATED) {
+      // XXX: blah need right layer_hint..
+      if (deliver_negotiated)
+        circpad_handle_padding_negotiated(client_side, cell,
+                                          TO_ORIGIN_CIRCUIT(client_side)
+                                             ->cpath->next);
+    } else if (cell->payload[0] == RELAY_COMMAND_PADDING_NEGOTIATE) {
+      circpad_handle_padding_negotiate(client_side, cell);
+    } else {
+      // No need to pretend a padding cell was sent: This event is
+      // now emitted internally when the circuitpadding code sends them.
+      //circpad_cell_event_padding_sent(relay_side);
+
+      // Receive padding cell at client
+      circpad_deliver_recognized_relay_cell_events(client_side,
+              cell->payload[0],
+              TO_ORIGIN_CIRCUIT(client_side)->cpath->next);
+    }
+
+    n_relay_cells++;
+  }
+
+ done:
+  timers_advance_and_run(1);
+  return 0;
+}
+
+// Test reading and writing padding to strings (or options_t + consensus)
+void
+test_circuitpadding_serialize(void *arg)
+{
+  (void)arg;
+}
+
+static signed_error_t
+circpad_send_command_to_hop_mock(origin_circuit_t *circ, uint8_t hopnum,
+                                 uint8_t relay_command, const uint8_t *payload,
+                                 ssize_t payload_len)
+{
+  (void) circ;
+  (void) hopnum;
+  (void) relay_command;
+  (void) payload;
+  (void) payload_len;
+  return 0;
+}
+
+void
+test_circuitpadding_rtt(void *arg)
+{
+  /* Test Plan:
+   *
+   * 1. Test RTT measurement server side
+   *    a. test usage of measured RTT
+   * 2. Test termination of RTT measurement
+   *    a. test non-update of RTT
+   * 3. Test client side circuit and non-application of RTT..
+   */
+  circpad_delay_t rtt_estimate;
+  (void)arg;
+
+  MOCK(circuitmux_attach_circuit, circuitmux_attach_circuit_mock);
+  MOCK(circpad_send_command_to_hop, circpad_send_command_to_hop_mock);
+
+  dummy_channel.cmux = circuitmux_alloc();
+  relay_side = TO_CIRCUIT(new_fake_orcirc(&dummy_channel, &dummy_channel));
+  client_side = TO_CIRCUIT(origin_circuit_new());
+  relay_side->purpose = CIRCUIT_PURPOSE_OR;
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+
+  monotime_init();
+  monotime_enable_test_mocking();
+  monotime_set_mock_time_nsec(1*NSEC_PER_USEC);
+  monotime_coarse_set_mock_time_nsec(1*NSEC_PER_USEC);
+  curr_mocked_time = 1*NSEC_PER_USEC;
+
+  timers_initialize();
+  circpad_machines_init();
+  helper_create_basic_machine();
+
+  MOCK(circuit_package_relay_cell,
+       circuit_package_relay_cell_mock);
+
+  client_side->padding_machine[0] = &circ_client_machine;
+  client_side->padding_info[0] = circpad_circuit_machineinfo_new(client_side,
+                                                                 0);
+
+  relay_side->padding_machine[0] = &circ_client_machine;
+  relay_side->padding_info[0] = circpad_circuit_machineinfo_new(client_side,0);
+
+  /* Test 1: Test measuring RTT */
+  circpad_cell_event_nonpadding_received((circuit_t*)relay_side);
+  tt_int_op(relay_side->padding_info[0]->last_received_time_usec, OP_NE, 0);
+
+  timers_advance_and_run(20);
+
+  circpad_cell_event_nonpadding_sent((circuit_t*)relay_side);
+  tt_int_op(relay_side->padding_info[0]->last_received_time_usec, OP_EQ, 0);
+
+  tt_int_op(relay_side->padding_info[0]->rtt_estimate_usec, OP_GE, 19000);
+  tt_int_op(relay_side->padding_info[0]->rtt_estimate_usec, OP_LE, 30000);
+  tt_int_op(circpad_histogram_bin_to_usec(relay_side->padding_info[0], 0),
+            OP_EQ,
+            relay_side->padding_info[0]->rtt_estimate_usec+
+            circpad_machine_current_state(
+             relay_side->padding_info[0])->start_usec);
+
+  circpad_cell_event_nonpadding_received((circuit_t*)relay_side);
+  circpad_cell_event_nonpadding_received((circuit_t*)relay_side);
+  tt_int_op(relay_side->padding_info[0]->last_received_time_usec, OP_NE, 0);
+  timers_advance_and_run(20);
+  circpad_cell_event_nonpadding_sent((circuit_t*)relay_side);
+  circpad_cell_event_nonpadding_sent((circuit_t*)relay_side);
+  tt_int_op(relay_side->padding_info[0]->last_received_time_usec, OP_EQ, 0);
+
+  tt_int_op(relay_side->padding_info[0]->rtt_estimate_usec, OP_GE, 20000);
+  tt_int_op(relay_side->padding_info[0]->rtt_estimate_usec, OP_LE, 21000);
+  tt_int_op(circpad_histogram_bin_to_usec(relay_side->padding_info[0], 0),
+            OP_EQ,
+            relay_side->padding_info[0]->rtt_estimate_usec+
+            circpad_machine_current_state(
+             relay_side->padding_info[0])->start_usec);
+
+  /* Test 2: Termination of RTT measurement (from the previous test) */
+  tt_int_op(relay_side->padding_info[0]->stop_rtt_update, OP_EQ, 1);
+  rtt_estimate = relay_side->padding_info[0]->rtt_estimate_usec;
+
+  circpad_cell_event_nonpadding_received((circuit_t*)relay_side);
+  timers_advance_and_run(4);
+  circpad_cell_event_nonpadding_sent((circuit_t*)relay_side);
+
+  tt_int_op(relay_side->padding_info[0]->rtt_estimate_usec, OP_EQ,
+            rtt_estimate);
+  tt_int_op(relay_side->padding_info[0]->last_received_time_usec, OP_EQ, 0);
+  tt_int_op(relay_side->padding_info[0]->stop_rtt_update, OP_EQ, 1);
+  tt_int_op(circpad_histogram_bin_to_usec(relay_side->padding_info[0], 0),
+            OP_EQ,
+            relay_side->padding_info[0]->rtt_estimate_usec+
+            circpad_machine_current_state(
+             relay_side->padding_info[0])->start_usec);
+
+  /* Test 3: Make sure client side machine properly ignores RTT */
+  circpad_cell_event_nonpadding_received((circuit_t*)client_side);
+  tt_int_op(client_side->padding_info[0]->last_received_time_usec, OP_EQ, 0);
+
+  timers_advance_and_run(20);
+  circpad_cell_event_nonpadding_sent((circuit_t*)client_side);
+  tt_int_op(client_side->padding_info[0]->last_received_time_usec, OP_EQ, 0);
+
+  tt_int_op(client_side->padding_info[0]->rtt_estimate_usec, OP_EQ, 0);
+  tt_int_op(circpad_histogram_bin_to_usec(client_side->padding_info[0], 0),
+            OP_NE, client_side->padding_info[0]->rtt_estimate_usec);
+  tt_int_op(circpad_histogram_bin_to_usec(client_side->padding_info[0], 0),
+            OP_EQ,
+            circpad_machine_current_state(
+                client_side->padding_info[0])->start_usec);
+ done:
+  free_fake_orcirc(relay_side);
+  circuitmux_detach_all_circuits(dummy_channel.cmux, NULL);
+  circuitmux_free(dummy_channel.cmux);
+  timers_shutdown();
+  monotime_disable_test_mocking();
+  UNMOCK(circuit_package_relay_cell);
+  UNMOCK(circuitmux_attach_circuit);
+  tor_free(circ_client_machine.states);
+
+  return;
+}
+
+void
+helper_create_basic_machine(void)
+{
+  /* Start, burst */
+  circpad_machine_states_init(&circ_client_machine, 2);
+
+  circ_client_machine.states[CIRCPAD_STATE_START].
+      next_state[CIRCPAD_EVENT_NONPADDING_RECV] = CIRCPAD_STATE_BURST;
+
+  circ_client_machine.states[CIRCPAD_STATE_BURST].
+      next_state[CIRCPAD_EVENT_PADDING_RECV] = CIRCPAD_STATE_BURST;
+  circ_client_machine.states[CIRCPAD_STATE_BURST].
+      next_state[CIRCPAD_EVENT_NONPADDING_RECV] = CIRCPAD_STATE_BURST;
+
+  circ_client_machine.states[CIRCPAD_STATE_BURST].
+      next_state[CIRCPAD_EVENT_NONPADDING_SENT] = CIRCPAD_STATE_CANCEL;
+
+  // FIXME: Is this what we want?
+  circ_client_machine.states[CIRCPAD_STATE_BURST].token_removal =
+      CIRCPAD_TOKEN_REMOVAL_HIGHER;
+
+  // FIXME: Tune this histogram
+  circ_client_machine.states[CIRCPAD_STATE_BURST].histogram_len = 5;
+  circ_client_machine.states[CIRCPAD_STATE_BURST].start_usec = 500;
+  circ_client_machine.states[CIRCPAD_STATE_BURST].range_usec = 1000000;
+  circ_client_machine.states[CIRCPAD_STATE_BURST].histogram[0] = 1;
+  circ_client_machine.states[CIRCPAD_STATE_BURST].histogram[1] = 0;
+  circ_client_machine.states[CIRCPAD_STATE_BURST].histogram[2] = 2;
+  circ_client_machine.states[CIRCPAD_STATE_BURST].histogram[3] = 2;
+  circ_client_machine.states[CIRCPAD_STATE_BURST].histogram[4] = 2;
+  circ_client_machine.states[CIRCPAD_STATE_BURST].histogram_total_tokens = 7;
+  circ_client_machine.states[CIRCPAD_STATE_BURST].use_rtt_estimate = 1;
+
+  return;
+}
+
+#define BIG_HISTOGRAM_LEN 10
+
+/** Setup a machine with a big histogram */
+static void
+helper_create_machine_with_big_histogram(circpad_removal_t removal_strategy)
+{
+  const int tokens_per_bin = 2;
+
+  /* Start, burst */
+  circpad_machine_states_init(&circ_client_machine, 2);
+
+  circpad_state_t *burst_state =
+    &circ_client_machine.states[CIRCPAD_STATE_BURST];
+
+  circ_client_machine.states[CIRCPAD_STATE_START].
+    next_state[CIRCPAD_EVENT_NONPADDING_RECV] = CIRCPAD_STATE_BURST;
+
+  burst_state->next_state[CIRCPAD_EVENT_PADDING_RECV] = CIRCPAD_STATE_BURST;
+  burst_state->next_state[CIRCPAD_EVENT_NONPADDING_RECV] =CIRCPAD_STATE_BURST;
+
+  burst_state->next_state[CIRCPAD_EVENT_NONPADDING_SENT] =CIRCPAD_STATE_CANCEL;
+
+  burst_state->token_removal = CIRCPAD_TOKEN_REMOVAL_HIGHER;
+
+  burst_state->histogram_len = BIG_HISTOGRAM_LEN;
+  burst_state->start_usec = 0;
+  burst_state->range_usec = 1000;
+
+  int n_tokens = 0;
+  for (int i = 0; i < BIG_HISTOGRAM_LEN ; i++) {
+    burst_state->histogram[i] = tokens_per_bin;
+    n_tokens += tokens_per_bin;
+  }
+
+  burst_state->histogram_total_tokens = n_tokens;
+  burst_state->length_dist.type = CIRCPAD_DIST_UNIFORM;
+  burst_state->length_dist.param1 = n_tokens;
+  burst_state->length_dist.param2 = n_tokens;
+  burst_state->max_length = n_tokens;
+  burst_state->length_includes_nonpadding = 1;
+  burst_state->use_rtt_estimate = 0;
+  burst_state->token_removal = removal_strategy;
+}
+
+static circpad_decision_t
+circpad_machine_schedule_padding_mock(circpad_machineinfo_t *mi)
+{
+  (void)mi;
+  return 0;
+}
+
+static uint64_t
+mock_monotime_absolute_usec(void)
+{
+  return 100;
+}
+
+/** Test higher token removal strategy by bin  */
+static void
+test_circuitpadding_token_removal_higher(void *arg)
+{
+  circpad_machineinfo_t *mi;
+  (void)arg;
+
+  /* Mock it up */
+  MOCK(monotime_absolute_usec, mock_monotime_absolute_usec);
+  MOCK(circpad_machine_schedule_padding,circpad_machine_schedule_padding_mock);
+
+  /* Setup test environment (time etc.) */
+  client_side = (circuit_t *)origin_circuit_new();
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+  monotime_enable_test_mocking();
+
+  /* Create test machine */
+  helper_create_machine_with_big_histogram(CIRCPAD_TOKEN_REMOVAL_HIGHER);
+  client_side->padding_machine[0] = &circ_client_machine;
+  client_side->padding_info[0] =
+    circpad_circuit_machineinfo_new(client_side, 0);
+
+  /* move the machine to the right state */
+  circpad_cell_event_nonpadding_received((circuit_t*)client_side);
+  tt_int_op(client_side->padding_info[0]->current_state, OP_EQ,
+            CIRCPAD_STATE_BURST);
+
+  /* Get the machine and setup tokens */
+  mi = client_side->padding_info[0];
+  tt_assert(mi);
+
+  /*************************************************************************/
+
+  uint64_t current_time = monotime_absolute_usec();
+
+  /* Test left boundaries of each histogram bin: */
+  const circpad_delay_t bin_left_bounds[] =
+    {0, 1, 7, 15, 31, 62, 125, 250, 500, CIRCPAD_DELAY_INFINITE};
+  for (int i = 0; i < BIG_HISTOGRAM_LEN ; i++) {
+    tt_uint_op(bin_left_bounds[i], OP_EQ,
+               circpad_histogram_bin_to_usec(mi, i));
+  }
+
+  /* Check that all bins have two tokens right now */
+  for (int i = 0; i < BIG_HISTOGRAM_LEN ; i++) {
+    tt_int_op(mi->histogram[i], OP_EQ, 2);
+  }
+
+  /* This is the right order to remove tokens from this histogram. That is, we
+   * first remove tokens from the 4th bin since 57 usec is nearest to the 4th
+   * bin midpoint (31 + (62-31)/2 == 46). Then we remove from the 3rd bin for
+   * the same reason, then from the 5th, etc. */
+  const int bin_removal_order[] = {4, 5, 6, 7, 8};
+  unsigned i;
+
+  /* Remove all tokens from all bins apart from the infinity bin */
+  for (i = 0; i < sizeof(bin_removal_order)/sizeof(int) ; i++) {
+    int bin_to_remove = bin_removal_order[i];
+    log_debug(LD_GENERAL, "Testing that %d attempt removes %d bin",
+              i, bin_to_remove);
+
+    tt_int_op(mi->histogram[bin_to_remove], OP_EQ, 2);
+
+    mi->padding_scheduled_at_usec = current_time - 57;
+    circpad_machine_remove_token(mi);
+
+    tt_int_op(mi->histogram[bin_to_remove], OP_EQ, 1);
+
+    mi->padding_scheduled_at_usec = current_time - 57;
+    circpad_machine_remove_token(mi);
+
+    /* Test that we cleaned out this bin. Don't do this in the case of the last
+       bin since the tokens will get refilled */
+    if (i != BIG_HISTOGRAM_LEN - 2) {
+      tt_int_op(mi->histogram[bin_to_remove], OP_EQ, 0);
+    }
+  }
+
+  /* Check that all lowe bins are not touched */
+  for (i=0; i < 4 ; i++) {
+    tt_int_op(mi->histogram[i], OP_EQ, 2);
+  }
+
+  /* Test below the lowest bin, for coverage */
+  tt_int_op(client_side->padding_info[0]->current_state, OP_EQ,
+            CIRCPAD_STATE_BURST);
+  circ_client_machine.states[CIRCPAD_STATE_BURST].start_usec = 100;
+  mi->padding_scheduled_at_usec = current_time - 1;
+  circpad_machine_remove_token(mi);
+  tt_int_op(mi->histogram[0], OP_EQ, 1);
+
+ done:
+  free_fake_origin_circuit(TO_ORIGIN_CIRCUIT(client_side));
+  monotime_disable_test_mocking();
+  tor_free(circ_client_machine.states);
+}
+
+/** Test lower token removal strategy by bin  */
+static void
+test_circuitpadding_token_removal_lower(void *arg)
+{
+  circpad_machineinfo_t *mi;
+  (void)arg;
+
+  /* Mock it up */
+  MOCK(monotime_absolute_usec, mock_monotime_absolute_usec);
+  MOCK(circpad_machine_schedule_padding,circpad_machine_schedule_padding_mock);
+
+  /* Setup test environment (time etc.) */
+  client_side = (circuit_t *)origin_circuit_new();
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+  monotime_enable_test_mocking();
+
+  /* Create test machine */
+  helper_create_machine_with_big_histogram(CIRCPAD_TOKEN_REMOVAL_LOWER);
+  client_side->padding_machine[0] = &circ_client_machine;
+  client_side->padding_info[0] =
+    circpad_circuit_machineinfo_new(client_side, 0);
+
+  /* move the machine to the right state */
+  circpad_cell_event_nonpadding_received((circuit_t*)client_side);
+  tt_int_op(client_side->padding_info[0]->current_state, OP_EQ,
+            CIRCPAD_STATE_BURST);
+
+  /* Get the machine and setup tokens */
+  mi = client_side->padding_info[0];
+  tt_assert(mi);
+
+  /*************************************************************************/
+
+  uint64_t current_time = monotime_absolute_usec();
+
+  /* Test left boundaries of each histogram bin: */
+  const circpad_delay_t bin_left_bounds[] =
+    {0, 1, 7, 15, 31, 62, 125, 250, 500, CIRCPAD_DELAY_INFINITE};
+  for (int i = 0; i < BIG_HISTOGRAM_LEN ; i++) {
+    tt_uint_op(bin_left_bounds[i], OP_EQ,
+               circpad_histogram_bin_to_usec(mi, i));
+  }
+
+  /* Check that all bins have two tokens right now */
+  for (int i = 0; i < BIG_HISTOGRAM_LEN ; i++) {
+    tt_int_op(mi->histogram[i], OP_EQ, 2);
+  }
+
+  /* This is the right order to remove tokens from this histogram. That is, we
+   * first remove tokens from the 4th bin since 57 usec is nearest to the 4th
+   * bin midpoint (31 + (62-31)/2 == 46). Then we remove from the 3rd bin for
+   * the same reason, then from the 5th, etc. */
+  const int bin_removal_order[] = {4, 3, 2, 1, 0};
+  unsigned i;
+
+  /* Remove all tokens from all bins apart from the infinity bin */
+  for (i = 0; i < sizeof(bin_removal_order)/sizeof(int) ; i++) {
+    int bin_to_remove = bin_removal_order[i];
+    log_debug(LD_GENERAL, "Testing that %d attempt removes %d bin",
+              i, bin_to_remove);
+
+    tt_int_op(mi->histogram[bin_to_remove], OP_EQ, 2);
+
+    mi->padding_scheduled_at_usec = current_time - 57;
+    circpad_machine_remove_token(mi);
+
+    tt_int_op(mi->histogram[bin_to_remove], OP_EQ, 1);
+
+    mi->padding_scheduled_at_usec = current_time - 57;
+    circpad_machine_remove_token(mi);
+
+    /* Test that we cleaned out this bin. Don't do this in the case of the last
+       bin since the tokens will get refilled */
+    if (i != BIG_HISTOGRAM_LEN - 2) {
+      tt_int_op(mi->histogram[bin_to_remove], OP_EQ, 0);
+    }
+  }
+
+  /* Check that all higher bins are untouched */
+  for (i = 5; i < BIG_HISTOGRAM_LEN ; i++) {
+    tt_int_op(mi->histogram[i], OP_EQ, 2);
+  }
+
+  /* Test above the highest bin, for coverage */
+  tt_int_op(client_side->padding_info[0]->current_state, OP_EQ,
+            CIRCPAD_STATE_BURST);
+  circ_client_machine.states[CIRCPAD_STATE_BURST].start_usec = 100;
+  mi->padding_scheduled_at_usec = current_time - 29202;
+  circpad_machine_remove_token(mi);
+  tt_int_op(mi->histogram[BIG_HISTOGRAM_LEN-2], OP_EQ, 1);
+
+ done:
+  free_fake_origin_circuit(TO_ORIGIN_CIRCUIT(client_side));
+  monotime_disable_test_mocking();
+  tor_free(circ_client_machine.states);
+}
+
+/** Test closest token removal strategy by bin  */
+static void
+test_circuitpadding_closest_token_removal(void *arg)
+{
+  circpad_machineinfo_t *mi;
+  (void)arg;
+
+  /* Mock it up */
+  MOCK(monotime_absolute_usec, mock_monotime_absolute_usec);
+  MOCK(circpad_machine_schedule_padding,circpad_machine_schedule_padding_mock);
+
+  /* Setup test environment (time etc.) */
+  client_side = (circuit_t *)origin_circuit_new();
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+  monotime_enable_test_mocking();
+
+  /* Create test machine */
+  helper_create_machine_with_big_histogram(CIRCPAD_TOKEN_REMOVAL_CLOSEST);
+  client_side->padding_machine[0] = &circ_client_machine;
+  client_side->padding_info[0] =
+    circpad_circuit_machineinfo_new(client_side, 0);
+
+  /* move the machine to the right state */
+  circpad_cell_event_nonpadding_received((circuit_t*)client_side);
+  tt_int_op(client_side->padding_info[0]->current_state, OP_EQ,
+            CIRCPAD_STATE_BURST);
+
+  /* Get the machine and setup tokens */
+  mi = client_side->padding_info[0];
+  tt_assert(mi);
+
+  /*************************************************************************/
+
+  uint64_t current_time = monotime_absolute_usec();
+
+  /* Test left boundaries of each histogram bin: */
+  const circpad_delay_t bin_left_bounds[] =
+    {0, 1, 7, 15, 31, 62, 125, 250, 500, CIRCPAD_DELAY_INFINITE};
+  for (int i = 0; i < BIG_HISTOGRAM_LEN ; i++) {
+    tt_uint_op(bin_left_bounds[i], OP_EQ,
+               circpad_histogram_bin_to_usec(mi, i));
+  }
+
+  /* Check that all bins have two tokens right now */
+  for (int i = 0; i < BIG_HISTOGRAM_LEN ; i++) {
+    tt_int_op(mi->histogram[i], OP_EQ, 2);
+  }
+
+  /* This is the right order to remove tokens from this histogram. That is, we
+   * first remove tokens from the 4th bin since 57 usec is nearest to the 4th
+   * bin midpoint (31 + (62-31)/2 == 46). Then we remove from the 3rd bin for
+   * the same reason, then from the 5th, etc. */
+  const int bin_removal_order[] = {4, 3, 5, 2, 6, 1, 7, 0, 8, 9};
+
+  /* Remove all tokens from all bins apart from the infinity bin */
+  for (int i = 0; i < BIG_HISTOGRAM_LEN-1 ; i++) {
+    int bin_to_remove = bin_removal_order[i];
+    log_debug(LD_GENERAL, "Testing that %d attempt removes %d bin",
+              i, bin_to_remove);
+
+    tt_int_op(mi->histogram[bin_to_remove], OP_EQ, 2);
+
+    mi->padding_scheduled_at_usec = current_time - 57;
+    circpad_machine_remove_token(mi);
+
+    tt_int_op(mi->histogram[bin_to_remove], OP_EQ, 1);
+
+    mi->padding_scheduled_at_usec = current_time - 57;
+    circpad_machine_remove_token(mi);
+
+    /* Test that we cleaned out this bin. Don't do this in the case of the last
+       bin since the tokens will get refilled */
+    if (i != BIG_HISTOGRAM_LEN - 2) {
+      tt_int_op(mi->histogram[bin_to_remove], OP_EQ, 0);
+    }
+  }
+
+  /* Check that all bins have been refilled */
+  for (int i = 0; i < BIG_HISTOGRAM_LEN ; i++) {
+    tt_int_op(mi->histogram[i], OP_EQ, 2);
+  }
+
+  /* Test below the lowest bin, for coverage */
+  tt_int_op(client_side->padding_info[0]->current_state, OP_EQ,
+            CIRCPAD_STATE_BURST);
+  circ_client_machine.states[CIRCPAD_STATE_BURST].start_usec = 100;
+  mi->padding_scheduled_at_usec = current_time - 102;
+  mi->histogram[0] = 0;
+  circpad_machine_remove_token(mi);
+  tt_int_op(mi->histogram[1], OP_EQ, 1);
+
+  /* Test above the highest bin, for coverage */
+  tt_int_op(client_side->padding_info[0]->current_state, OP_EQ,
+            CIRCPAD_STATE_BURST);
+  circ_client_machine.states[CIRCPAD_STATE_BURST].start_usec = 100;
+  mi->padding_scheduled_at_usec = current_time - 29202;
+  circpad_machine_remove_token(mi);
+  tt_int_op(mi->histogram[BIG_HISTOGRAM_LEN-2], OP_EQ, 1);
+
+ done:
+  free_fake_origin_circuit(TO_ORIGIN_CIRCUIT(client_side));
+  monotime_disable_test_mocking();
+  tor_free(circ_client_machine.states);
+}
+
+/** Test closest token removal strategy with usec  */
+static void
+test_circuitpadding_closest_token_removal_usec(void *arg)
+{
+  circpad_machineinfo_t *mi;
+  (void)arg;
+
+  /* Mock it up */
+  MOCK(monotime_absolute_usec, mock_monotime_absolute_usec);
+  MOCK(circpad_machine_schedule_padding,circpad_machine_schedule_padding_mock);
+
+  /* Setup test environment (time etc.) */
+  client_side = (circuit_t *)origin_circuit_new();
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+  monotime_enable_test_mocking();
+
+  /* Create test machine */
+  helper_create_machine_with_big_histogram(CIRCPAD_TOKEN_REMOVAL_CLOSEST_USEC);
+  client_side->padding_machine[0] = &circ_client_machine;
+  client_side->padding_info[0] =
+    circpad_circuit_machineinfo_new(client_side, 0);
+
+  /* move the machine to the right state */
+  circpad_cell_event_nonpadding_received((circuit_t*)client_side);
+  tt_int_op(client_side->padding_info[0]->current_state, OP_EQ,
+            CIRCPAD_STATE_BURST);
+
+  /* Get the machine and setup tokens */
+  mi = client_side->padding_info[0];
+  tt_assert(mi);
+
+  /*************************************************************************/
+
+  uint64_t current_time = monotime_absolute_usec();
+
+  /* Test left boundaries of each histogram bin: */
+  const circpad_delay_t bin_left_bounds[] =
+    {0, 1, 7, 15, 31, 62, 125, 250, 500, CIRCPAD_DELAY_INFINITE};
+  for (int i = 0; i < BIG_HISTOGRAM_LEN ; i++) {
+    tt_uint_op(bin_left_bounds[i], OP_EQ,
+               circpad_histogram_bin_to_usec(mi, i));
+  }
+
+  /* XXX we want to test remove_token_exact and
+     circpad_machine_remove_closest_token() with usec */
+
+  /* Check that all bins have two tokens right now */
+  for (int i = 0; i < BIG_HISTOGRAM_LEN ; i++) {
+    tt_int_op(mi->histogram[i], OP_EQ, 2);
+  }
+
+  /* This is the right order to remove tokens from this histogram. That is, we
+   * first remove tokens from the 4th bin since 57 usec is nearest to the 4th
+   * bin midpoint (31 + (62-31)/2 == 46). Then we remove from the 3rd bin for
+   * the same reason, then from the 5th, etc. */
+  const int bin_removal_order[] = {4, 3, 5, 2, 1, 0, 6, 7, 8, 9};
+
+  /* Remove all tokens from all bins apart from the infinity bin */
+  for (int i = 0; i < BIG_HISTOGRAM_LEN-1 ; i++) {
+    int bin_to_remove = bin_removal_order[i];
+    log_debug(LD_GENERAL, "Testing that %d attempt removes %d bin",
+              i, bin_to_remove);
+
+    tt_int_op(mi->histogram[bin_to_remove], OP_EQ, 2);
+
+    mi->padding_scheduled_at_usec = current_time - 57;
+    circpad_machine_remove_token(mi);
+
+    tt_int_op(mi->histogram[bin_to_remove], OP_EQ, 1);
+
+    mi->padding_scheduled_at_usec = current_time - 57;
+    circpad_machine_remove_token(mi);
+
+    /* Test that we cleaned out this bin. Don't do this in the case of the last
+       bin since the tokens will get refilled */
+    if (i != BIG_HISTOGRAM_LEN - 2) {
+      tt_int_op(mi->histogram[bin_to_remove], OP_EQ, 0);
+    }
+  }
+
+  /* Check that all bins have been refilled */
+  for (int i = 0; i < BIG_HISTOGRAM_LEN ; i++) {
+    tt_int_op(mi->histogram[i], OP_EQ, 2);
+  }
+
+  /* Test below the lowest bin, for coverage */
+  tt_int_op(client_side->padding_info[0]->current_state, OP_EQ,
+            CIRCPAD_STATE_BURST);
+  circ_client_machine.states[CIRCPAD_STATE_BURST].start_usec = 100;
+  mi->padding_scheduled_at_usec = current_time - 102;
+  mi->histogram[0] = 0;
+  circpad_machine_remove_token(mi);
+  tt_int_op(mi->histogram[1], OP_EQ, 1);
+
+  /* Test above the highest bin, for coverage */
+  tt_int_op(client_side->padding_info[0]->current_state, OP_EQ,
+            CIRCPAD_STATE_BURST);
+  circ_client_machine.states[CIRCPAD_STATE_BURST].start_usec = 100;
+  mi->padding_scheduled_at_usec = current_time - 29202;
+  circpad_machine_remove_token(mi);
+  tt_int_op(mi->histogram[BIG_HISTOGRAM_LEN-2], OP_EQ, 1);
+
+ done:
+  free_fake_origin_circuit(TO_ORIGIN_CIRCUIT(client_side));
+  monotime_disable_test_mocking();
+  tor_free(circ_client_machine.states);
+}
+
+/** Test closest token removal strategy with usec  */
+static void
+test_circuitpadding_token_removal_exact(void *arg)
+{
+  circpad_machineinfo_t *mi;
+  (void)arg;
+
+  /* Mock it up */
+  MOCK(monotime_absolute_usec, mock_monotime_absolute_usec);
+  MOCK(circpad_machine_schedule_padding,circpad_machine_schedule_padding_mock);
+
+  /* Setup test environment (time etc.) */
+  client_side = (circuit_t *)origin_circuit_new();
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+  monotime_enable_test_mocking();
+
+  /* Create test machine */
+  helper_create_machine_with_big_histogram(CIRCPAD_TOKEN_REMOVAL_EXACT);
+  client_side->padding_machine[0] = &circ_client_machine;
+  client_side->padding_info[0] =
+    circpad_circuit_machineinfo_new(client_side, 0);
+
+  /* move the machine to the right state */
+  circpad_cell_event_nonpadding_received((circuit_t*)client_side);
+  tt_int_op(client_side->padding_info[0]->current_state, OP_EQ,
+            CIRCPAD_STATE_BURST);
+
+  /* Get the machine and setup tokens */
+  mi = client_side->padding_info[0];
+  tt_assert(mi);
+
+  /**********************************************************************/
+  uint64_t current_time = monotime_absolute_usec();
+
+  /* Ensure that we will clear out bin #4 with this usec */
+  mi->padding_scheduled_at_usec = current_time - 57;
+  tt_int_op(mi->histogram[4], OP_EQ, 2);
+  circpad_machine_remove_token(mi);
+  mi->padding_scheduled_at_usec = current_time - 57;
+  tt_int_op(mi->histogram[4], OP_EQ, 1);
+  circpad_machine_remove_token(mi);
+  tt_int_op(mi->histogram[4], OP_EQ, 0);
+
+  /* Ensure that we will not remove any other tokens even tho we try to, since
+   * this is what the exact strategy dictates */
+  mi->padding_scheduled_at_usec = current_time - 57;
+  circpad_machine_remove_token(mi);
+  for (int i = 0; i < BIG_HISTOGRAM_LEN ; i++) {
+    if (i != 4) {
+      tt_int_op(mi->histogram[i], OP_EQ, 2);
+    }
+  }
+
+ done:
+  free_fake_origin_circuit(TO_ORIGIN_CIRCUIT(client_side));
+  monotime_disable_test_mocking();
+  tor_free(circ_client_machine.states);
+}
+
+#undef BIG_HISTOGRAM_LEN
+
+void
+test_circuitpadding_tokens(void *arg)
+{
+  const circpad_state_t *state;
+  circpad_machineinfo_t *mi;
+  (void)arg;
+
+  /** Test plan:
+   *
+   * 1. Test symmetry between bin_to_usec and usec_to_bin
+   *    a. Test conversion
+   *    b. Test edge transitions (lower, upper)
+   * 2. Test remove higher on an empty bin
+   *    a. Normal bin
+   *    b. Infinity bin
+   *    c. Bin 0
+   *    d. No higher
+   * 3. Test remove lower
+   *    a. Normal bin
+   *    b. Bin 0
+   *    c. No lower
+   * 4. Test remove closest
+   *    a. Closest lower
+   *    b. Closest higher
+   *    c. Closest 0
+   *    d. Closest Infinity
+   */
+  client_side = TO_CIRCUIT(origin_circuit_new());
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+
+  monotime_init();
+  monotime_enable_test_mocking();
+  monotime_set_mock_time_nsec(1*NSEC_PER_USEC);
+  monotime_coarse_set_mock_time_nsec(1*NSEC_PER_USEC);
+  curr_mocked_time = 1*NSEC_PER_USEC;
+
+  timers_initialize();
+
+  helper_create_basic_machine();
+  client_side->padding_machine[0] = &circ_client_machine;
+  client_side->padding_info[0] = circpad_circuit_machineinfo_new(client_side,
+                                                                 0);
+
+  mi = client_side->padding_info[0];
+
+  // Pretend a non-padding cell was sent
+  // XXX: This messes us up.. Padding gets scheduled..
+  circpad_cell_event_nonpadding_sent((circuit_t*)client_side);
+  circpad_cell_event_nonpadding_received((circuit_t*)client_side);
+  /* We have to save the infinity bin because one inf delay
+   * could have been chosen when we transition to burst */
+  circpad_hist_token_t inf_bin = mi->histogram[4];
+
+  tt_int_op(client_side->padding_info[0]->current_state, OP_EQ,
+            CIRCPAD_STATE_BURST);
+
+  state = circpad_machine_current_state(client_side->padding_info[0]);
+
+  // Test 0: convert bin->usec->bin
+  // Bin 0+1 have different semantics
+  for (int bin = 0; bin < 2; bin++) {
+    circpad_delay_t usec =
+        circpad_histogram_bin_to_usec(client_side->padding_info[0], bin);
+    int bin2 = circpad_histogram_usec_to_bin(client_side->padding_info[0],
+                                             usec);
+    tt_int_op(bin, OP_EQ, bin2);
+  }
+  for (int bin = 2; bin < state->histogram_len-1; bin++) {
+    circpad_delay_t usec =
+        circpad_histogram_bin_to_usec(client_side->padding_info[0], bin);
+    int bin2 = circpad_histogram_usec_to_bin(client_side->padding_info[0],
+                                             usec);
+    tt_int_op(bin, OP_EQ, bin2);
+    /* Verify we round down */
+    bin2 = circpad_histogram_usec_to_bin(client_side->padding_info[0],
+                                             usec+3);
+    tt_int_op(bin, OP_EQ, bin2);
+
+    bin2 = circpad_histogram_usec_to_bin(client_side->padding_info[0],
+                                             usec-1);
+    tt_int_op(bin, OP_EQ, bin2+1);
+  }
+
+  // Test 1: converting usec->bin->usec->bin
+  // Bin 0+1 have different semantics.
+  for (circpad_delay_t i = 0; i <= state->start_usec+1; i++) {
+    int bin = circpad_histogram_usec_to_bin(client_side->padding_info[0],
+                                            i);
+    circpad_delay_t usec =
+        circpad_histogram_bin_to_usec(client_side->padding_info[0], bin);
+    int bin2 = circpad_histogram_usec_to_bin(client_side->padding_info[0],
+                                             usec);
+    tt_int_op(bin, OP_EQ, bin2);
+    tt_int_op(i, OP_LE, usec);
+  }
+  for (circpad_delay_t i = state->start_usec+1;
+           i <= state->start_usec + state->range_usec; i++) {
+    int bin = circpad_histogram_usec_to_bin(client_side->padding_info[0],
+                                            i);
+    circpad_delay_t usec =
+        circpad_histogram_bin_to_usec(client_side->padding_info[0], bin);
+    int bin2 = circpad_histogram_usec_to_bin(client_side->padding_info[0],
+                                             usec);
+    tt_int_op(bin, OP_EQ, bin2);
+    tt_int_op(i, OP_GE, usec);
+  }
+
+  /* 2.a. Normal higher bin */
+  {
+    tt_int_op(mi->histogram[2], OP_EQ, 2);
+    tt_int_op(mi->histogram[3], OP_EQ, 2);
+    circpad_machine_remove_higher_token(mi,
+         circpad_histogram_bin_to_usec(mi, 2)+1);
+    tt_int_op(mi->histogram[3], OP_EQ, 2);
+    tt_int_op(mi->histogram[2], OP_EQ, 1);
+
+    circpad_machine_remove_higher_token(mi,
+         circpad_histogram_bin_to_usec(mi, 2)+1);
+    tt_int_op(mi->histogram[2], OP_EQ, 0);
+
+    tt_int_op(mi->histogram[3], OP_EQ, 2);
+    circpad_machine_remove_higher_token(mi,
+         circpad_histogram_bin_to_usec(mi, 2)+1);
+    circpad_machine_remove_higher_token(mi,
+         circpad_histogram_bin_to_usec(mi, 2)+1);
+    tt_int_op(mi->histogram[3], OP_EQ, 0);
+    circpad_machine_remove_higher_token(mi,
+         circpad_histogram_bin_to_usec(mi, 2)+1);
+    tt_int_op(mi->histogram[3], OP_EQ, 0);
+  }
+
+  /* 2.b. Higher Infinity bin */
+  {
+    tt_int_op(mi->histogram[4], OP_EQ, inf_bin);
+    circpad_machine_remove_higher_token(mi,
+         circpad_histogram_bin_to_usec(mi, 2)+1);
+    tt_int_op(mi->histogram[4], OP_EQ, inf_bin);
+
+    /* Test past the infinity bin */
+    circpad_machine_remove_higher_token(mi,
+         circpad_histogram_bin_to_usec(mi, 5)+1000000);
+
+    tt_int_op(mi->histogram[4], OP_EQ, inf_bin);
+  }
+
+  /* 2.c. Bin 0 */
+  {
+    tt_int_op(mi->histogram[0], OP_EQ, 1);
+    circpad_machine_remove_higher_token(mi,
+         state->start_usec/2);
+    tt_int_op(mi->histogram[0], OP_EQ, 0);
+  }
+
+  /* Drain the infinity bin and cause a refill */
+  while (inf_bin != 0) {
+    tt_int_op(mi->histogram[4], OP_EQ, inf_bin);
+    circpad_cell_event_nonpadding_received((circuit_t*)client_side);
+    inf_bin--;
+  }
+
+  circpad_cell_event_nonpadding_sent((circuit_t*)client_side);
+
+  // We should have refilled here.
+  tt_int_op(mi->histogram[4], OP_EQ, 2);
+
+  /* 3.a. Bin 0 */
+  {
+    tt_int_op(mi->histogram[0], OP_EQ, 1);
+    circpad_machine_remove_higher_token(mi,
+         state->start_usec/2);
+    tt_int_op(mi->histogram[0], OP_EQ, 0);
+  }
+
+  /* 3.b. Test remove lower normal bin */
+  {
+    tt_int_op(mi->histogram[3], OP_EQ, 2);
+    circpad_machine_remove_lower_token(mi,
+         circpad_histogram_bin_to_usec(mi, 3)+1);
+    circpad_machine_remove_lower_token(mi,
+         circpad_histogram_bin_to_usec(mi, 3)+1);
+    tt_int_op(mi->histogram[3], OP_EQ, 0);
+    tt_int_op(mi->histogram[2], OP_EQ, 2);
+    circpad_machine_remove_lower_token(mi,
+         circpad_histogram_bin_to_usec(mi, 3)+1);
+    circpad_machine_remove_lower_token(mi,
+         circpad_histogram_bin_to_usec(mi, 3)+1);
+    /* 3.c. No lower */
+    circpad_machine_remove_lower_token(mi,
+         circpad_histogram_bin_to_usec(mi, 3)+1);
+    tt_int_op(mi->histogram[2], OP_EQ, 0);
+  }
+
+  /* 4. Test remove closest
+   *    a. Closest lower
+   *    b. Closest higher
+   *    c. Closest 0
+   *    d. Closest Infinity
+   */
+  circpad_machine_setup_tokens(mi);
+  tt_int_op(mi->histogram[2], OP_EQ, 2);
+  circpad_machine_remove_closest_token(mi,
+         circpad_histogram_bin_to_usec(mi, 2)+1, 0);
+  circpad_machine_remove_closest_token(mi,
+         circpad_histogram_bin_to_usec(mi, 2)+1, 0);
+  tt_int_op(mi->histogram[2], OP_EQ, 0);
+  tt_int_op(mi->histogram[3], OP_EQ, 2);
+  circpad_machine_remove_closest_token(mi,
+         circpad_histogram_bin_to_usec(mi, 2)+1, 0);
+  circpad_machine_remove_closest_token(mi,
+         circpad_histogram_bin_to_usec(mi, 2)+1, 0);
+  tt_int_op(mi->histogram[3], OP_EQ, 0);
+  tt_int_op(mi->histogram[0], OP_EQ, 1);
+  circpad_machine_remove_closest_token(mi,
+         circpad_histogram_bin_to_usec(mi, 2)+1, 0);
+  tt_int_op(mi->histogram[0], OP_EQ, 0);
+  tt_int_op(mi->histogram[4], OP_EQ, 2);
+  circpad_machine_remove_closest_token(mi,
+         circpad_histogram_bin_to_usec(mi, 2)+1, 0);
+  tt_int_op(mi->histogram[4], OP_EQ, 2);
+
+  /* 5. Test remove closest usec
+   *    a. Closest 0
+   *    b. Closest lower (below midpoint)
+   *    c. Closest higher (above midpoint)
+   *    d. Closest Infinity
+   */
+  circpad_machine_setup_tokens(mi);
+
+  tt_int_op(mi->histogram[0], OP_EQ, 1);
+  circpad_machine_remove_closest_token(mi,
+         circpad_histogram_bin_to_usec(mi, 0)/3, 1);
+  tt_int_op(mi->histogram[0], OP_EQ, 0);
+  tt_int_op(mi->histogram[2], OP_EQ, 2);
+  circpad_machine_remove_closest_token(mi,
+         circpad_histogram_bin_to_usec(mi, 0)/3, 1);
+  circpad_machine_remove_closest_token(mi,
+         circpad_histogram_bin_to_usec(mi, 0)/3, 1);
+  tt_int_op(mi->histogram[2], OP_EQ, 0);
+  tt_int_op(mi->histogram[3], OP_EQ, 2);
+  circpad_machine_remove_closest_token(mi,
+         circpad_histogram_bin_to_usec(mi, 4), 1);
+  circpad_machine_remove_closest_token(mi,
+         circpad_histogram_bin_to_usec(mi, 4), 1);
+  tt_int_op(mi->histogram[3], OP_EQ, 0);
+  tt_int_op(mi->histogram[4], OP_EQ, 2);
+  circpad_machine_remove_closest_token(mi,
+         circpad_histogram_bin_to_usec(mi, 4), 1);
+  circpad_machine_remove_closest_token(mi,
+         circpad_histogram_bin_to_usec(mi, 4), 1);
+  tt_int_op(mi->histogram[4], OP_EQ, 2);
+
+  // XXX: Need more coverage of the actual usec branches
+
+ done:
+  free_fake_origin_circuit(TO_ORIGIN_CIRCUIT(client_side));
+  monotime_disable_test_mocking();
+  tor_free(circ_client_machine.states);
+}
+
+void
+test_circuitpadding_wronghop(void *arg)
+{
+  /**
+   * Test plan:
+   * 1. Padding sent from hop 1 and 3 to client
+   * 2. Send negotiated from hop 1 and 3 to client
+   * 3. Garbled negotiated cell
+   * 4. Padding negotiate sent to client
+   * 5. Send negotiate stop command for unknown machine
+   * 6. Send negotiated to relay
+   * 7. Garbled padding negotiate cell
+   */
+  (void)arg;
+  uint32_t read_bw = 0, overhead_bw = 0;
+  cell_t cell;
+  signed_error_t ret;
+  origin_circuit_t *orig_client;
+
+  MOCK(circuitmux_attach_circuit, circuitmux_attach_circuit_mock);
+
+  client_side = (circuit_t *)origin_circuit_new();
+  dummy_channel.cmux = circuitmux_alloc();
+  relay_side = (circuit_t *)new_fake_orcirc(&dummy_channel,
+                                            &dummy_channel);
+  orig_client = TO_ORIGIN_CIRCUIT(client_side);
+
+  relay_side->purpose = CIRCUIT_PURPOSE_OR;
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+  nodes_init();
+
+  monotime_init();
+  monotime_enable_test_mocking();
+  monotime_set_mock_time_nsec(1*TOR_NSEC_PER_USEC);
+  monotime_coarse_set_mock_time_nsec(1*TOR_NSEC_PER_USEC);
+  curr_mocked_time = 1*TOR_NSEC_PER_USEC;
+
+  timers_initialize();
+  circpad_machines_init();
+
+  MOCK(node_get_by_id,
+       node_get_by_id_mock);
+
+  MOCK(circuit_package_relay_cell,
+       circuit_package_relay_cell_mock);
+
+  /* Build three hops */
+  simulate_single_hop_extend(client_side, relay_side, 1);
+  simulate_single_hop_extend(client_side, relay_side, 1);
+  simulate_single_hop_extend(client_side, relay_side, 1);
+
+  /* verify padding was negotiated */
+  tt_ptr_op(relay_side->padding_machine[0], OP_NE, NULL);
+  tt_ptr_op(relay_side->padding_info[0], OP_NE, NULL);
+
+  /* verify echo was sent */
+  tt_int_op(n_relay_cells, OP_EQ, 1);
+  tt_int_op(n_client_cells, OP_EQ, 1);
+
+  read_bw = orig_client->n_delivered_read_circ_bw;
+  overhead_bw = orig_client->n_overhead_read_circ_bw;
+
+  /* 1. Test padding from first and third hop */
+  circpad_deliver_recognized_relay_cell_events(client_side,
+              RELAY_COMMAND_DROP,
+              TO_ORIGIN_CIRCUIT(client_side)->cpath);
+  tt_int_op(read_bw, OP_EQ,
+            orig_client->n_delivered_read_circ_bw);
+  tt_int_op(overhead_bw, OP_EQ,
+            orig_client->n_overhead_read_circ_bw);
+
+  circpad_deliver_recognized_relay_cell_events(client_side,
+              RELAY_COMMAND_DROP,
+              TO_ORIGIN_CIRCUIT(client_side)->cpath->next->next);
+  tt_int_op(read_bw, OP_EQ,
+            orig_client->n_delivered_read_circ_bw);
+  tt_int_op(overhead_bw, OP_EQ,
+            orig_client->n_overhead_read_circ_bw);
+
+  circpad_deliver_recognized_relay_cell_events(client_side,
+              RELAY_COMMAND_DROP,
+              TO_ORIGIN_CIRCUIT(client_side)->cpath->next);
+  tt_int_op(read_bw, OP_EQ,
+            orig_client->n_delivered_read_circ_bw);
+  tt_int_op(overhead_bw, OP_LT,
+            orig_client->n_overhead_read_circ_bw);
+
+  /* 2. Test padding negotiated not handled from hops 1,3 */
+  ret = circpad_handle_padding_negotiated(client_side, &cell,
+          TO_ORIGIN_CIRCUIT(client_side)->cpath);
+  tt_int_op(ret, OP_EQ, -1);
+
+  ret = circpad_handle_padding_negotiated(client_side, &cell,
+          TO_ORIGIN_CIRCUIT(client_side)->cpath->next->next);
+  tt_int_op(ret, OP_EQ, -1);
+
+  /* 3. Garbled negotiated cell */
+  memset(&cell, 255, sizeof(cell));
+  ret = circpad_handle_padding_negotiated(client_side, &cell,
+          TO_ORIGIN_CIRCUIT(client_side)->cpath->next);
+  tt_int_op(ret, OP_EQ, -1);
+
+  /* 4. Test that negotiate is dropped at origin */
+  read_bw = orig_client->n_delivered_read_circ_bw;
+  overhead_bw = orig_client->n_overhead_read_circ_bw;
+  relay_send_command_from_edge(0, relay_side,
+                               RELAY_COMMAND_PADDING_NEGOTIATE,
+                               (void*)cell.payload,
+                               (size_t)3, NULL);
+  tt_int_op(read_bw, OP_EQ,
+            orig_client->n_delivered_read_circ_bw);
+  tt_int_op(overhead_bw, OP_EQ,
+            orig_client->n_overhead_read_circ_bw);
+
+  tt_int_op(n_relay_cells, OP_EQ, 2);
+  tt_int_op(n_client_cells, OP_EQ, 1);
+
+  /* 5. Test that asking to stop the wrong machine does nothing */
+  circpad_negotiate_padding(TO_ORIGIN_CIRCUIT(client_side),
+                            255, 2, CIRCPAD_COMMAND_STOP);
+  tt_ptr_op(client_side->padding_machine[0], OP_NE, NULL);
+  tt_ptr_op(client_side->padding_info[0], OP_NE, NULL);
+  tt_ptr_op(relay_side->padding_machine[0], OP_NE, NULL);
+  tt_ptr_op(relay_side->padding_info[0], OP_NE, NULL);
+  tt_int_op(n_relay_cells, OP_EQ, 3);
+  tt_int_op(n_client_cells, OP_EQ, 2);
+
+  /* 6. Sending negotiated command to relay does nothing */
+  ret = circpad_handle_padding_negotiated(relay_side, &cell, NULL);
+  tt_int_op(ret, OP_EQ, -1);
+
+  /* 7. Test garbled negotated cell (bad command 255) */
+  memset(&cell, 0, sizeof(cell));
+  ret = circpad_handle_padding_negotiate(relay_side, &cell);
+  tt_int_op(ret, OP_EQ, -1);
+  tt_int_op(n_client_cells, OP_EQ, 2);
+
+  /* Test 2: Test no padding */
+  free_fake_origin_circuit(TO_ORIGIN_CIRCUIT(client_side));
+  free_fake_orcirc(relay_side);
+
+  client_side = (circuit_t *)origin_circuit_new();
+  relay_side = (circuit_t *)new_fake_orcirc(&dummy_channel,
+                                            &dummy_channel);
+  relay_side->purpose = CIRCUIT_PURPOSE_OR;
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+
+  simulate_single_hop_extend(client_side, relay_side, 1);
+  simulate_single_hop_extend(client_side, relay_side, 0);
+
+  /* verify no padding was negotiated */
+  tt_ptr_op(relay_side->padding_machine[0], OP_EQ, NULL);
+  tt_ptr_op(client_side->padding_machine[0], OP_EQ, NULL);
+  tt_int_op(n_relay_cells, OP_EQ, 3);
+  tt_int_op(n_client_cells, OP_EQ, 2);
+
+  /* verify no echo was sent */
+  tt_int_op(n_relay_cells, OP_EQ, 3);
+  tt_int_op(n_client_cells, OP_EQ, 2);
+
+  /* Finish circuit */
+  simulate_single_hop_extend(client_side, relay_side, 1);
+
+  /* Spoof padding negotiated on circuit with no padding */
+  circpad_padding_negotiated(relay_side,
+                             CIRCPAD_MACHINE_CIRC_SETUP,
+                             CIRCPAD_COMMAND_START,
+                             CIRCPAD_RESPONSE_OK);
+
+  /* verify no padding was negotiated */
+  tt_ptr_op(relay_side->padding_machine[0], OP_EQ, NULL);
+  tt_ptr_op(client_side->padding_machine[0], OP_EQ, NULL);
+
+  circpad_padding_negotiated(relay_side,
+                             CIRCPAD_MACHINE_CIRC_SETUP,
+                             CIRCPAD_COMMAND_START,
+                             CIRCPAD_RESPONSE_ERR);
+
+  /* verify no padding was negotiated */
+  tt_ptr_op(relay_side->padding_machine[0], OP_EQ, NULL);
+  tt_ptr_op(client_side->padding_machine[0], OP_EQ, NULL);
+
+ done:
+  free_fake_origin_circuit(TO_ORIGIN_CIRCUIT(client_side));
+  free_fake_orcirc(relay_side);
+  circuitmux_detach_all_circuits(dummy_channel.cmux, NULL);
+  circuitmux_free(dummy_channel.cmux);
+  monotime_disable_test_mocking();
+  UNMOCK(node_get_by_id);
+  UNMOCK(circuit_package_relay_cell);
+  UNMOCK(circuitmux_attach_circuit);
+  nodes_free();
+}
+
+void
+test_circuitpadding_negotiation(void *arg)
+{
+  /**
+   * Test plan:
+   * 1. Test circuit where padding is supported by middle
+   *    a. Make sure padding negotiation is sent
+   *    b. Test padding negotiation delivery and parsing
+   * 2. Test circuit where padding is unsupported by middle
+   *    a. Make sure padding negotiation is not sent
+   * 3. Test failure to negotiate a machine due to desync.
+   */
+  (void)arg;
+
+  MOCK(circuitmux_attach_circuit, circuitmux_attach_circuit_mock);
+
+  client_side = TO_CIRCUIT(origin_circuit_new());
+  dummy_channel.cmux = circuitmux_alloc();
+  relay_side = TO_CIRCUIT(new_fake_orcirc(&dummy_channel, &dummy_channel));
+
+  relay_side->purpose = CIRCUIT_PURPOSE_OR;
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+  nodes_init();
+
+  monotime_init();
+  monotime_enable_test_mocking();
+  monotime_set_mock_time_nsec(1*NSEC_PER_USEC);
+  monotime_coarse_set_mock_time_nsec(1*NSEC_PER_USEC);
+  curr_mocked_time = 1*NSEC_PER_USEC;
+
+  timers_initialize();
+  circpad_machines_init();
+
+  MOCK(node_get_by_id,
+       node_get_by_id_mock);
+
+  MOCK(circuit_package_relay_cell,
+       circuit_package_relay_cell_mock);
+
+  /* Build two hops */
+  simulate_single_hop_extend(client_side, relay_side, 1);
+  simulate_single_hop_extend(client_side, relay_side, 1);
+
+  /* verify padding was negotiated */
+  tt_ptr_op(relay_side->padding_machine[0], OP_NE, NULL);
+  tt_ptr_op(relay_side->padding_info[0], OP_NE, NULL);
+
+  /* verify echo was sent */
+  tt_int_op(n_relay_cells, OP_EQ, 1);
+  tt_int_op(n_client_cells, OP_EQ, 1);
+
+  /* Finish circuit */
+  simulate_single_hop_extend(client_side, relay_side, 1);
+
+  /* Test 2: Test no padding */
+  free_fake_origin_circuit(TO_ORIGIN_CIRCUIT(client_side));
+  free_fake_orcirc(relay_side);
+
+  client_side = TO_CIRCUIT(origin_circuit_new());
+  relay_side = TO_CIRCUIT(new_fake_orcirc(&dummy_channel, &dummy_channel));
+  relay_side->purpose = CIRCUIT_PURPOSE_OR;
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+
+  simulate_single_hop_extend(client_side, relay_side, 1);
+  simulate_single_hop_extend(client_side, relay_side, 0);
+
+  /* verify no padding was negotiated */
+  tt_ptr_op(client_side->padding_machine[0], OP_EQ, NULL);
+  tt_ptr_op(relay_side->padding_machine[0], OP_EQ, NULL);
+  tt_int_op(n_relay_cells, OP_EQ, 1);
+  tt_int_op(n_client_cells, OP_EQ, 1);
+
+  /* verify no echo was sent */
+  tt_int_op(n_relay_cells, OP_EQ, 1);
+  tt_int_op(n_client_cells, OP_EQ, 1);
+
+  /* Finish circuit */
+  simulate_single_hop_extend(client_side, relay_side, 1);
+
+  /* Force negotiate padding. */
+  circpad_negotiate_padding(TO_ORIGIN_CIRCUIT(client_side),
+                            CIRCPAD_MACHINE_CIRC_SETUP,
+                            2, CIRCPAD_COMMAND_START);
+
+  /* verify no padding was negotiated */
+  tt_ptr_op(relay_side->padding_machine[0], OP_EQ, NULL);
+  tt_ptr_op(client_side->padding_machine[0], OP_EQ, NULL);
+
+  /* verify no echo was sent */
+  tt_int_op(n_relay_cells, OP_EQ, 1);
+  tt_int_op(n_client_cells, OP_EQ, 1);
+
+  /* 3. Test failure to negotiate a machine due to desync */
+  free_fake_origin_circuit(TO_ORIGIN_CIRCUIT(client_side));
+  free_fake_orcirc(relay_side);
+
+  client_side = TO_CIRCUIT(origin_circuit_new());
+  relay_side = TO_CIRCUIT(new_fake_orcirc(&dummy_channel, &dummy_channel));
+  relay_side->purpose = CIRCUIT_PURPOSE_OR;
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+
+  SMARTLIST_FOREACH(relay_padding_machines,
+          circpad_machine_t *,
+          m, tor_free(m->states); tor_free(m));
+  smartlist_free(relay_padding_machines);
+  relay_padding_machines = smartlist_new();
+
+  simulate_single_hop_extend(client_side, relay_side, 1);
+  simulate_single_hop_extend(client_side, relay_side, 1);
+
+  /* verify echo was sent */
+  tt_int_op(n_client_cells, OP_EQ, 2);
+  tt_int_op(n_relay_cells, OP_EQ, 2);
+
+  /* verify no padding was negotiated */
+  tt_ptr_op(client_side->padding_info[0], OP_EQ, NULL);
+  tt_ptr_op(client_side->padding_machine[0], OP_EQ, NULL);
+  tt_ptr_op(relay_side->padding_machine[0], OP_EQ, NULL);
+  tt_ptr_op(relay_side->padding_info[0], OP_EQ, NULL);
+
+ done:
+  free_fake_origin_circuit(TO_ORIGIN_CIRCUIT(client_side));
+  free_fake_orcirc(relay_side);
+  circuitmux_detach_all_circuits(dummy_channel.cmux, NULL);
+  circuitmux_free(dummy_channel.cmux);
+  monotime_disable_test_mocking();
+  UNMOCK(node_get_by_id);
+  UNMOCK(circuit_package_relay_cell);
+  UNMOCK(circuitmux_attach_circuit);
+  nodes_free();
+}
+
+static void
+simulate_single_hop_extend(circuit_t *client, circuit_t *mid_relay,
+                           int padding)
+{
+  char whatevs_key[CPATH_KEY_MATERIAL_LEN];
+  char digest[DIGEST_LEN];
+  tor_addr_t addr;
+
+  // Pretend a non-padding cell was sent
+  circpad_cell_event_nonpadding_sent((circuit_t*)client);
+
+  // Receive extend cell at middle
+  circpad_cell_event_nonpadding_received((circuit_t*)mid_relay);
+
+  // Advance time a tiny bit so we can calculate an RTT
+  curr_mocked_time += 10 * NSEC_PER_MSEC;
+  monotime_coarse_set_mock_time_nsec(curr_mocked_time);
+  monotime_set_mock_time_nsec(curr_mocked_time);
+
+  // Receive extended cell at middle
+  circpad_cell_event_nonpadding_sent((circuit_t*)mid_relay);
+
+  // Receive extended cell at first hop
+  circpad_cell_event_nonpadding_received((circuit_t*)client);
+
+  // Add a hop to cpath
+  crypt_path_t *hop = tor_malloc_zero(sizeof(crypt_path_t));
+  onion_append_to_cpath(&TO_ORIGIN_CIRCUIT(client)->cpath, hop);
+
+  hop->magic = CRYPT_PATH_MAGIC;
+  hop->state = CPATH_STATE_OPEN;
+
+  // add an extend info to indicate if this node supports padding or not.
+  // (set the first byte of the digest for our mocked node_get_by_id)
+  digest[0] = padding;
+
+  hop->extend_info = extend_info_new(
+          padding ? "padding" : "non-padding",
+          digest, NULL, NULL, NULL,
+          &addr, padding);
+
+  circuit_init_cpath_crypto(hop, whatevs_key, sizeof(whatevs_key), 0, 0);
+
+  hop->package_window = circuit_initial_package_window();
+  hop->deliver_window = CIRCWINDOW_START;
+
+  // Signal that the hop was added
+  circpad_machine_event_circ_added_hop(TO_ORIGIN_CIRCUIT(client));
+}
+
+static circpad_machine_t *
+helper_create_conditional_machine(void)
+{
+  circpad_machine_t *ret = tor_malloc_zero(sizeof(circpad_machine_t));
+
+  /* Start, burst */
+  circpad_machine_states_init(ret, 2);
+
+  ret->states[CIRCPAD_STATE_START].
+      next_state[CIRCPAD_EVENT_PADDING_SENT] = CIRCPAD_STATE_BURST;
+
+  ret->states[CIRCPAD_STATE_BURST].
+      next_state[CIRCPAD_EVENT_PADDING_SENT] = CIRCPAD_STATE_BURST;
+
+  ret->states[CIRCPAD_STATE_BURST].
+      next_state[CIRCPAD_EVENT_LENGTH_COUNT] = CIRCPAD_STATE_END;
+
+  ret->states[CIRCPAD_STATE_BURST].token_removal =
+      CIRCPAD_TOKEN_REMOVAL_NONE;
+
+  ret->states[CIRCPAD_STATE_BURST].histogram_len = 3;
+  ret->states[CIRCPAD_STATE_BURST].start_usec = 0;
+  ret->states[CIRCPAD_STATE_BURST].range_usec = 1000000;
+  ret->states[CIRCPAD_STATE_BURST].histogram[0] = 6;
+  ret->states[CIRCPAD_STATE_BURST].histogram[1] = 0;
+  ret->states[CIRCPAD_STATE_BURST].histogram[1] = 0;
+  ret->states[CIRCPAD_STATE_BURST].histogram_total_tokens = 6;
+  ret->states[CIRCPAD_STATE_BURST].use_rtt_estimate = 0;
+  ret->states[CIRCPAD_STATE_BURST].length_includes_nonpadding = 1;
+
+  return ret;
+}
+
+static void
+helper_create_conditional_machines(void)
+{
+  circpad_machine_t *add = helper_create_conditional_machine();
+  origin_padding_machines = smartlist_new();
+  relay_padding_machines = smartlist_new();
+
+  add->machine_num = 2;
+  add->is_origin_side = 1;
+  add->should_negotiate_end = 1;
+  add->target_hopnum = 2;
+
+  /* Let's have this one end after 4 packets */
+  add->states[CIRCPAD_STATE_BURST].length_dist.type = CIRCPAD_DIST_UNIFORM;
+  add->states[CIRCPAD_STATE_BURST].length_dist.param1 = 4;
+  add->states[CIRCPAD_STATE_BURST].length_dist.param2 = 4;
+  add->states[CIRCPAD_STATE_BURST].max_length = 4;
+
+  add->conditions.requires_vanguards = 0;
+  add->conditions.min_hops = 2;
+  add->conditions.state_mask = CIRCPAD_CIRC_BUILDING|
+           CIRCPAD_CIRC_NO_STREAMS|CIRCPAD_CIRC_HAS_RELAY_EARLY;
+  add->conditions.purpose_mask = CIRCPAD_PURPOSE_ALL;
+
+  smartlist_add(origin_padding_machines, add);
+
+  add = helper_create_conditional_machine();
+  add->machine_num = 3;
+  add->is_origin_side = 1;
+  add->should_negotiate_end = 1;
+  add->target_hopnum = 2;
+
+  /* Let's have this one end after 4 packets */
+  add->states[CIRCPAD_STATE_BURST].length_dist.type = CIRCPAD_DIST_UNIFORM;
+  add->states[CIRCPAD_STATE_BURST].length_dist.param1 = 4;
+  add->states[CIRCPAD_STATE_BURST].length_dist.param2 = 4;
+  add->states[CIRCPAD_STATE_BURST].max_length = 4;
+
+  add->conditions.requires_vanguards = 1;
+  add->conditions.min_hops = 3;
+  add->conditions.state_mask = CIRCPAD_CIRC_OPENED|
+           CIRCPAD_CIRC_STREAMS|CIRCPAD_CIRC_HAS_NO_RELAY_EARLY;
+  add->conditions.purpose_mask = CIRCPAD_PURPOSE_ALL;
+  smartlist_add(origin_padding_machines, add);
+
+  add = helper_create_conditional_machine();
+  add->machine_num = 2;
+  smartlist_add(relay_padding_machines, add);
+
+  add = helper_create_conditional_machine();
+  add->machine_num = 3;
+  smartlist_add(relay_padding_machines, add);
+}
+
+void
+test_circuitpadding_conditions(void *arg)
+{
+  /**
+   * Test plan:
+   *  0. Make a few origin and client machines with diff conditions
+   *     * vanguards, purposes, has_opened circs, no relay early
+   *     * Client side should_negotiate_end
+   *     * Length limits
+   *  1. Test STATE_END transitions
+   *  2. Test new machine after end with same conditions
+   *  3. Test new machine due to changed conditions
+   *     * Esp: built event, no relay early, no streams
+   * XXX: Diff test:
+   *  1. Test STATE_END with pending timers
+   *  2. Test marking a circuit before padding callback fires
+   *  3. Test freeing a circuit before padding callback fires
+   */
+  (void)arg;
+  MOCK(circuitmux_attach_circuit, circuitmux_attach_circuit_mock);
+
+  nodes_init();
+  dummy_channel.cmux = circuitmux_alloc();
+  relay_side = (circuit_t *)new_fake_orcirc(&dummy_channel,
+                                            &dummy_channel);
+  client_side = (circuit_t *)origin_circuit_new();
+  relay_side->purpose = CIRCUIT_PURPOSE_OR;
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+
+  monotime_init();
+  monotime_enable_test_mocking();
+  monotime_set_mock_time_nsec(1*NSEC_PER_USEC);
+  monotime_coarse_set_mock_time_nsec(1*NSEC_PER_USEC);
+  curr_mocked_time = 1*NSEC_PER_USEC;
+
+  timers_initialize();
+  helper_create_conditional_machines();
+
+  MOCK(circuit_package_relay_cell,
+       circuit_package_relay_cell_mock);
+  MOCK(node_get_by_id,
+       node_get_by_id_mock);
+
+  /* Simulate extend. This should result in the original machine getting
+   * added, since the circuit is not built */
+  simulate_single_hop_extend(client_side, relay_side, 1);
+  simulate_single_hop_extend(client_side, relay_side, 1);
+
+  /* Verify that machine #2 is added */
+  tt_int_op(client_side->padding_machine[0]->machine_num, OP_EQ, 2);
+  tt_int_op(relay_side->padding_machine[0]->machine_num, OP_EQ, 2);
+
+  /* Deliver a padding cell to the client, to trigger burst state */
+  circpad_cell_event_padding_sent(client_side);
+
+  /* This should have trigger length shutdown condition on client.. */
+  tt_ptr_op(client_side->padding_info[0], OP_EQ, NULL);
+  tt_ptr_op(client_side->padding_machine[0], OP_EQ, NULL);
+
+  /* Verify machine is gone from both sides */
+  tt_ptr_op(relay_side->padding_info[0], OP_EQ, NULL);
+  tt_ptr_op(relay_side->padding_machine[0], OP_EQ, NULL);
+
+  /* Send another event.. verify machine gets re-added properly
+   * (test race with shutdown) */
+  simulate_single_hop_extend(client_side, relay_side, 1);
+  tt_int_op(client_side->padding_machine[0]->machine_num, OP_EQ, 2);
+  tt_int_op(relay_side->padding_machine[0]->machine_num, OP_EQ, 2);
+
+  TO_ORIGIN_CIRCUIT(client_side)->p_streams = 0;
+  circpad_machine_event_circ_has_no_streams(TO_ORIGIN_CIRCUIT(client_side));
+  tt_int_op(client_side->padding_machine[0]->machine_num, OP_EQ, 2);
+  tt_int_op(relay_side->padding_machine[0]->machine_num, OP_EQ, 2);
+
+  /* Now make the circuit opened and send built event */
+  TO_ORIGIN_CIRCUIT(client_side)->has_opened = 1;
+  circpad_machine_event_circ_built(TO_ORIGIN_CIRCUIT(client_side));
+  tt_int_op(client_side->padding_machine[0]->machine_num, OP_EQ, 2);
+  tt_int_op(relay_side->padding_machine[0]->machine_num, OP_EQ, 2);
+
+  TO_ORIGIN_CIRCUIT(client_side)->remaining_relay_early_cells = 0;
+  circpad_machine_event_circ_has_no_relay_early(
+          TO_ORIGIN_CIRCUIT(client_side));
+  tt_int_op(client_side->padding_machine[0]->machine_num, OP_EQ, 2);
+  tt_int_op(relay_side->padding_machine[0]->machine_num, OP_EQ, 2);
+
+  get_options_mutable()->HSLayer2Nodes = (void*)1;
+  TO_ORIGIN_CIRCUIT(client_side)->p_streams = (void*)1;
+  circpad_machine_event_circ_has_streams(TO_ORIGIN_CIRCUIT(client_side));
+
+  /* Verify different machine is added */
+  tt_int_op(client_side->padding_machine[0]->machine_num, OP_EQ, 3);
+  tt_int_op(relay_side->padding_machine[0]->machine_num, OP_EQ, 3);
+
+  /* Hold off on negotiated */
+  deliver_negotiated = 0;
+
+  /* Deliver a padding cell to the client, to trigger burst state */
+  circpad_cell_event_padding_sent(client_side);
+
+  /* This should have trigger length shutdown condition on client
+   * but not the response for the padding machine */
+  tt_ptr_op(client_side->padding_info[0], OP_EQ, NULL);
+  tt_ptr_op(client_side->padding_machine[0], OP_NE, NULL);
+
+  /* Verify machine is gone from the relay (but negotiated not back yet */
+  tt_ptr_op(relay_side->padding_info[0], OP_EQ, NULL);
+  tt_ptr_op(relay_side->padding_machine[0], OP_EQ, NULL);
+
+  /* Add another hop and verify it's back */
+  simulate_single_hop_extend(client_side, relay_side, 1);
+
+  tt_int_op(client_side->padding_machine[0]->machine_num, OP_EQ, 3);
+  tt_int_op(relay_side->padding_machine[0]->machine_num, OP_EQ, 3);
+
+  tt_ptr_op(client_side->padding_info[0], OP_NE, NULL);
+  tt_ptr_op(relay_side->padding_info[0], OP_NE, NULL);
+
+ done:
+  /* XXX: Free everything */
+  return;
+}
+
+void
+test_circuitpadding_circuitsetup_machine(void *arg)
+{
+  /**
+   * Test case plan:
+   *
+   * 1. Simulate a normal circuit setup pattern
+   *    a. Application traffic
+   *
+   * FIXME: This should focus more on exercising the machine
+   * features rather than actual traffic patterns. For example,
+   * test cancellation and bins empty/refill
+   */
+  (void)arg;
+
+  MOCK(circuitmux_attach_circuit, circuitmux_attach_circuit_mock);
+
+  dummy_channel.cmux = circuitmux_alloc();
+  client_side = TO_CIRCUIT(origin_circuit_new());
+  relay_side = TO_CIRCUIT(new_fake_orcirc(&dummy_channel, &dummy_channel));
+
+  relay_side->purpose = CIRCUIT_PURPOSE_OR;
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+
+  nodes_init();
+
+  monotime_init();
+  monotime_enable_test_mocking();
+  monotime_set_mock_time_nsec(1*NSEC_PER_USEC);
+  monotime_coarse_set_mock_time_nsec(1*NSEC_PER_USEC);
+  curr_mocked_time = 1*NSEC_PER_USEC;
+
+  timers_initialize();
+  circpad_machines_init();
+
+  MOCK(circuit_package_relay_cell,
+       circuit_package_relay_cell_mock);
+  MOCK(node_get_by_id,
+       node_get_by_id_mock);
+
+  /* Test case #1: Build a 3 hop circuit, then wait and let pad */
+  simulate_single_hop_extend(client_side, relay_side, 1);
+  simulate_single_hop_extend(client_side, relay_side, 1);
+  simulate_single_hop_extend(client_side, relay_side, 1);
+
+  tt_int_op(n_client_cells, OP_EQ, 1);
+  tt_int_op(n_relay_cells, OP_EQ, 1);
+  tt_int_op(client_side->padding_info[0]->current_state, OP_EQ,
+                CIRCPAD_STATE_BURST);
+  tt_int_op(relay_side->padding_info[0]->current_state, OP_EQ,
+          CIRCPAD_STATE_BURST);
+
+  tt_int_op(client_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_NE, 0);
+  tt_int_op(relay_side->padding_info[0]->is_padding_timer_scheduled,
+            OP_EQ, 0);
+  timers_advance_and_run(2000);
+  tt_int_op(n_client_cells, OP_EQ, 2);
+  tt_int_op(n_relay_cells, OP_EQ, 1);
+
+  tt_int_op(relay_side->padding_info[0]->current_state, OP_EQ,
+              CIRCPAD_STATE_GAP);
+
+  tt_int_op(client_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_EQ, 0);
+  tt_int_op(relay_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_NE, 0);
+  timers_advance_and_run(5000);
+  tt_int_op(n_client_cells, OP_EQ, 2);
+  tt_int_op(n_relay_cells, OP_EQ, 2);
+
+  tt_int_op(client_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_NE, 0);
+  tt_int_op(relay_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_EQ, 0);
+  timers_advance_and_run(2000);
+  tt_int_op(n_client_cells, OP_EQ, 3);
+  tt_int_op(n_relay_cells, OP_EQ, 2);
+
+  tt_int_op(client_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_EQ, 0);
+  tt_int_op(relay_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_NE, 0);
+  timers_advance_and_run(5000);
+  tt_int_op(n_client_cells, OP_EQ, 3);
+  tt_int_op(n_relay_cells, OP_EQ, 3);
+
+  tt_int_op(client_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_NE, 0);
+  tt_int_op(relay_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_EQ, 0);
+  timers_advance_and_run(2000);
+  tt_int_op(n_client_cells, OP_EQ, 4);
+  tt_int_op(n_relay_cells, OP_EQ, 3);
+
+  tt_int_op(client_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_EQ, 0);
+  tt_int_op(relay_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_NE, 0);
+  timers_advance_and_run(5000);
+  tt_int_op(n_client_cells, OP_EQ, 4);
+  tt_int_op(n_relay_cells, OP_EQ, 4);
+
+  tt_int_op(client_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_NE, 0);
+  tt_int_op(relay_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_EQ, 0);
+  timers_advance_and_run(2000);
+  tt_int_op(n_client_cells, OP_EQ, 5);
+  tt_int_op(n_relay_cells, OP_EQ, 4);
+
+  tt_int_op(client_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_EQ, 0);
+  tt_int_op(relay_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_NE, 0);
+  timers_advance_and_run(5000);
+  tt_int_op(n_client_cells, OP_EQ, 5);
+  tt_int_op(n_relay_cells, OP_EQ, 5);
+
+  tt_int_op(client_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_NE, 0);
+  tt_int_op(relay_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_EQ, 0);
+  timers_advance_and_run(2000);
+  tt_int_op(n_client_cells, OP_EQ, 6);
+  tt_int_op(n_relay_cells, OP_EQ, 5);
+
+  tt_int_op(client_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_EQ, 0);
+  tt_int_op(relay_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_NE, 0);
+  timers_advance_and_run(5000);
+  tt_int_op(n_client_cells, OP_EQ, 6);
+  tt_int_op(n_relay_cells, OP_EQ, 6);
+
+  tt_int_op(client_side->padding_info[0]->current_state,
+            OP_EQ, CIRCPAD_STATE_END);
+  tt_int_op(client_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_EQ, 0);
+  tt_int_op(relay_side->padding_info[0]->current_state,
+            OP_EQ, CIRCPAD_STATE_GAP);
+  tt_int_op(relay_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_EQ, 0);
+
+  /* Verify we can't schedule padding in END state */
+  circpad_decision_t ret =
+      circpad_machine_schedule_padding(client_side->padding_info[0]);
+  tt_int_op(ret, OP_EQ, CIRCPAD_STATE_UNCHANGED);
+
+  /* Simulate application traffic */
+  circpad_cell_event_nonpadding_sent(client_side);
+  circpad_deliver_unrecognized_cell_events(relay_side, CELL_DIRECTION_OUT);
+  circpad_deliver_unrecognized_cell_events(relay_side, CELL_DIRECTION_IN);
+  circpad_deliver_recognized_relay_cell_events(client_side, RELAY_COMMAND_DATA,
+                                  TO_ORIGIN_CIRCUIT(client_side)->cpath->next);
+
+  tt_ptr_op(client_side->padding_info[0], OP_EQ, NULL);
+  tt_ptr_op(client_side->padding_machine[0], OP_EQ, NULL);
+
+  tt_ptr_op(relay_side->padding_info[0], OP_EQ, NULL);
+  tt_ptr_op(relay_side->padding_machine[0], OP_EQ, NULL);
+  tt_int_op(n_client_cells, OP_EQ, 6);
+  tt_int_op(n_relay_cells, OP_EQ, 7);
+
+  // Test timer cancellation
+  simulate_single_hop_extend(client_side, relay_side, 1);
+  simulate_single_hop_extend(client_side, relay_side, 1);
+  timers_advance_and_run(5000);
+  circpad_cell_event_padding_received(client_side);
+
+  tt_int_op(client_side->padding_info[0]->current_state, OP_EQ,
+                CIRCPAD_STATE_BURST);
+  tt_int_op(relay_side->padding_info[0]->current_state, OP_EQ,
+          CIRCPAD_STATE_GAP);
+
+  tt_int_op(n_client_cells, OP_EQ, 8);
+  tt_int_op(n_relay_cells, OP_EQ, 8);
+  tt_int_op(client_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_NE, 0);
+  tt_int_op(relay_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_NE, 0);
+
+  /* Test timer cancel due to state rules */
+  circpad_cell_event_nonpadding_sent(client_side);
+  tt_int_op(client_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_EQ, 0);
+  circpad_cell_event_padding_received(client_side);
+  tt_int_op(client_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_NE, 0);
+
+  /* Simulate application traffic to cancel timer */
+  circpad_cell_event_nonpadding_sent(client_side);
+  circpad_deliver_unrecognized_cell_events(relay_side, CELL_DIRECTION_OUT);
+  circpad_deliver_unrecognized_cell_events(relay_side, CELL_DIRECTION_IN);
+  circpad_deliver_recognized_relay_cell_events(client_side, RELAY_COMMAND_DATA,
+                                  TO_ORIGIN_CIRCUIT(client_side)->cpath->next);
+
+  tt_ptr_op(client_side->padding_info[0], OP_EQ, NULL);
+  tt_ptr_op(client_side->padding_machine[0], OP_EQ, NULL);
+
+  tt_ptr_op(relay_side->padding_info[0], OP_EQ, NULL);
+  tt_ptr_op(relay_side->padding_machine[0], OP_EQ, NULL);
+
+  /* No cells sent, except negotiate end from relay */
+  tt_int_op(n_client_cells, OP_EQ, 8);
+  tt_int_op(n_relay_cells, OP_EQ, 9);
+
+  /* Test mark for close and free */
+  simulate_single_hop_extend(client_side, relay_side, 1);
+  simulate_single_hop_extend(client_side, relay_side, 1);
+  timers_advance_and_run(5000);
+  circpad_cell_event_padding_received(client_side);
+
+  tt_int_op(n_client_cells, OP_EQ, 10);
+  tt_int_op(n_relay_cells, OP_EQ, 10);
+
+  tt_int_op(client_side->padding_info[0]->current_state, OP_EQ,
+                CIRCPAD_STATE_BURST);
+  tt_int_op(relay_side->padding_info[0]->current_state, OP_EQ,
+          CIRCPAD_STATE_GAP);
+
+  tt_int_op(client_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_NE, 0);
+  tt_int_op(relay_side->padding_info[0]->padding_scheduled_at_usec,
+            OP_NE, 0);
+  circuit_mark_for_close(client_side, END_CIRC_REASON_FLAG_REMOTE);
+  free_fake_orcirc(relay_side);
+  timers_advance_and_run(5000);
+
+  /* No cells sent */
+  tt_int_op(n_client_cells, OP_EQ, 10);
+  tt_int_op(n_relay_cells, OP_EQ, 10);
+
+ done:
+  free_fake_origin_circuit(TO_ORIGIN_CIRCUIT(client_side));
+
+  circuitmux_detach_all_circuits(dummy_channel.cmux, NULL);
+  circuitmux_free(dummy_channel.cmux);
+  timers_shutdown();
+  monotime_disable_test_mocking();
+  UNMOCK(circuit_package_relay_cell);
+  UNMOCK(circuitmux_attach_circuit);
+
+  return;
+}
+
+/** Helper function: Initializes a padding machine where every state uses the
+ *  uniform probability distribution.  */
+static void
+helper_circpad_circ_distribution_machine_setup(int min, int max)
+{
+  circpad_machine_states_init(&circ_client_machine, 7);
+
+  circpad_state_t *zero_st = &circ_client_machine.states[0];
+  zero_st->next_state[CIRCPAD_EVENT_NONPADDING_RECV] = 1;
+  zero_st->iat_dist.type = CIRCPAD_DIST_UNIFORM;
+  zero_st->iat_dist.param1 = min;
+  zero_st->iat_dist.param2 = max;
+  zero_st->start_usec = min;
+  zero_st->range_usec = max;
+
+  circpad_state_t *first_st = &circ_client_machine.states[1];
+  first_st->next_state[CIRCPAD_EVENT_NONPADDING_RECV] = 2;
+  first_st->iat_dist.type = CIRCPAD_DIST_LOGISTIC;
+  first_st->iat_dist.param1 = min;
+  first_st->iat_dist.param2 = max;
+  first_st->start_usec = min;
+  first_st->range_usec = max;
+
+  circpad_state_t *second_st = &circ_client_machine.states[2];
+  second_st->next_state[CIRCPAD_EVENT_NONPADDING_RECV] = 3;
+  second_st->iat_dist.type = CIRCPAD_DIST_LOG_LOGISTIC;
+  second_st->iat_dist.param1 = min;
+  second_st->iat_dist.param2 = max;
+  second_st->start_usec = min;
+  second_st->range_usec = max;
+
+  circpad_state_t *third_st = &circ_client_machine.states[3];
+  third_st->next_state[CIRCPAD_EVENT_NONPADDING_RECV] = 4;
+  third_st->iat_dist.type = CIRCPAD_DIST_GEOMETRIC;
+  third_st->iat_dist.param1 = min;
+  third_st->iat_dist.param2 = max;
+  third_st->start_usec = min;
+  third_st->range_usec = max;
+
+  circpad_state_t *fourth_st = &circ_client_machine.states[4];
+  fourth_st->next_state[CIRCPAD_EVENT_NONPADDING_RECV] = 5;
+  fourth_st->iat_dist.type = CIRCPAD_DIST_WEIBULL;
+  fourth_st->iat_dist.param1 = min;
+  fourth_st->iat_dist.param2 = max;
+  fourth_st->start_usec = min;
+  fourth_st->range_usec = max;
+
+  circpad_state_t *fifth_st = &circ_client_machine.states[5];
+  fifth_st->next_state[CIRCPAD_EVENT_NONPADDING_RECV] = 6;
+  fifth_st->iat_dist.type = CIRCPAD_DIST_PARETO;
+  fifth_st->iat_dist.param1 = min;
+  fifth_st->iat_dist.param2 = max;
+  fifth_st->start_usec = min;
+  fifth_st->range_usec = max;
+}
+
+/** Simple test that the padding delays sampled from a uniform distribution
+ *  actually faill within the uniform distribution range. */
+/* TODO: Upgrade this test so that each state tests a different prob
+ * distribution */
+static void
+test_circuitpadding_sample_distribution(void *arg)
+{
+  circpad_machineinfo_t *mi;
+  int n_samples;
+  int n_states;
+
+  (void) arg;
+
+  /* mock this function so that we dont actually schedule any padding */
+  MOCK(circpad_machine_schedule_padding,
+       circpad_machine_schedule_padding_mock);
+
+  /* Initialize a machine with multiple probability distributions that should
+   * return values between 0 and 5 */
+  circpad_machines_init();
+  helper_circpad_circ_distribution_machine_setup(0, 10);
+
+  /* Initialize machine and circuits */
+  client_side = TO_CIRCUIT(origin_circuit_new());
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+  client_side->padding_machine[0] = &circ_client_machine;
+  client_side->padding_info[0] =
+    circpad_circuit_machineinfo_new(client_side, 0);
+  mi = client_side->padding_info[0];
+
+  /* For every state, sample a bunch of values from the distribution and ensure
+   * they fall within range. */
+  for (n_states = 0 ; n_states < 6; n_states++) {
+    /* Make sure we in the right state */
+    tt_int_op(client_side->padding_info[0]->current_state, OP_EQ, n_states);
+
+    for (n_samples = 0; n_samples < 100; n_samples++) {
+      circpad_delay_t delay = circpad_machine_sample_delay(mi);
+      tt_int_op(delay, OP_GE, 0);
+      tt_int_op(delay, OP_LE, 10);
+    }
+
+    /* send a non-padding cell to move to the next machine state */
+    circpad_cell_event_nonpadding_received((circuit_t*)client_side);
+  }
+
+ done:
+  free_fake_origin_circuit(TO_ORIGIN_CIRCUIT(client_side));
+  UNMOCK(circpad_machine_schedule_padding);
+}
+
+static circpad_decision_t
+circpad_machine_transition_mock(circpad_machineinfo_t *mi,
+                                circpad_event_t event)
+{
+  (void) mi;
+  (void) event;
+
+  return CIRCPAD_STATE_UNCHANGED;
+}
+
+/* Test per-machine padding rate limits */
+static void
+test_circuitpadding_machine_rate_limiting(void *arg)
+{
+  (void) arg;
+  bool retval;
+  circpad_machineinfo_t *mi;
+  int i;
+
+  /* Ignore machine transitions for the purposes of this function, we only
+   * really care about padding counts */
+  MOCK(circpad_machine_transition, circpad_machine_transition_mock);
+  MOCK(circpad_send_command_to_hop, circpad_send_command_to_hop_mock);
+
+  /* Setup machine and circuits */
+  client_side = TO_CIRCUIT(origin_circuit_new());
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+  helper_create_basic_machine();
+  client_side->padding_machine[0] = &circ_client_machine;
+  client_side->padding_info[0] =
+    circpad_circuit_machineinfo_new(client_side, 0);
+  mi = client_side->padding_info[0];
+  /* Set up the machine info so that we can get through the basic functions */
+  mi->state_length = CIRCPAD_STATE_LENGTH_INFINITE;
+
+  /* First we are going to test the per-machine rate limits */
+  circ_client_machine.max_padding_percent = 50;
+  circ_client_machine.allowed_padding_count = 100;
+
+  /* Check padding limit, should be fine since we haven't sent anything yet. */
+  retval = circpad_machine_reached_padding_limit(mi);
+  tt_int_op(retval, OP_EQ, 0);
+
+  /* Send 99 padding cells which is below circpad_global_allowed_cells=100, so
+   * the rate limit will not trigger */
+  for (i=0;i<99;i++) {
+    circpad_send_padding_cell_for_callback(mi);
+  }
+  retval = circpad_machine_reached_padding_limit(mi);
+  tt_int_op(retval, OP_EQ, 0);
+
+  /* Now send another padding cell to pass circpad_global_allowed_cells=100,
+     and see that the limit will trigger */
+  circpad_send_padding_cell_for_callback(mi);
+  retval = circpad_machine_reached_padding_limit(mi);
+  tt_int_op(retval, OP_EQ, 1);
+
+  retval = circpad_machine_schedule_padding(mi);
+  tt_int_op(retval, OP_EQ, CIRCPAD_STATE_UNCHANGED);
+
+  /* Cover wrap */
+  for (;i<UINT16_MAX;i++) {
+    circpad_send_padding_cell_for_callback(mi);
+  }
+  tt_int_op(mi->padding_sent, OP_EQ, UINT16_MAX/2+1);
+
+  tt_ptr_op(client_side->padding_info[0], OP_EQ, mi);
+  for (i=0;i<UINT16_MAX;i++) {
+    circpad_cell_event_nonpadding_sent(client_side);
+  }
+
+  tt_int_op(mi->nonpadding_sent, OP_EQ, UINT16_MAX/2);
+  tt_int_op(mi->padding_sent, OP_EQ, UINT16_MAX/4+1);
+
+ done:
+  free_fake_origin_circuit(TO_ORIGIN_CIRCUIT(client_side));
+}
+
+/* Test global padding rate limits */
+static void
+test_circuitpadding_global_rate_limiting(void *arg)
+{
+  (void) arg;
+  bool retval;
+  circpad_machineinfo_t *mi;
+  int i;
+
+  /* Ignore machine transitions for the purposes of this function, we only
+   * really care about padding counts */
+  MOCK(circpad_machine_transition, circpad_machine_transition_mock);
+  MOCK(circuitmux_attach_circuit, circuitmux_attach_circuit_mock);
+  MOCK(circuit_package_relay_cell,
+       circuit_package_relay_cell_mock);
+  MOCK(monotime_absolute_usec, mock_monotime_absolute_usec);
+
+  monotime_init();
+  monotime_enable_test_mocking();
+  monotime_set_mock_time_nsec(1*TOR_NSEC_PER_USEC);
+  monotime_coarse_set_mock_time_nsec(1*TOR_NSEC_PER_USEC);
+  curr_mocked_time = 1*TOR_NSEC_PER_USEC;
+  timers_initialize();
+
+  client_side = (circuit_t *)origin_circuit_new();
+  client_side->purpose = CIRCUIT_PURPOSE_C_GENERAL;
+  dummy_channel.cmux = circuitmux_alloc();
+
+  /* Setup machine and circuits */
+  relay_side = (circuit_t *)new_fake_orcirc(&dummy_channel, &dummy_channel);
+  relay_side->purpose = CIRCUIT_PURPOSE_OR;
+  helper_create_basic_machine();
+  relay_side->padding_machine[0] = &circ_client_machine;
+  relay_side->padding_info[0] =
+    circpad_circuit_machineinfo_new(relay_side, 0);
+  mi = relay_side->padding_info[0];
+  /* Set up the machine info so that we can get through the basic functions */
+  mi->state_length = CIRCPAD_STATE_LENGTH_INFINITE;
+
+  simulate_single_hop_extend(client_side, relay_side, 1);
+  simulate_single_hop_extend(client_side, relay_side, 1);
+
+  /* Now test the global limits by setting up the consensus */
+  networkstatus_t vote1;
+  vote1.net_params = smartlist_new();
+  smartlist_split_string(vote1.net_params,
+         "circpad_global_allowed_cells=100 circpad_global_max_padding_pct=50",
+                         NULL, 0, 0);
+  /* Register global limits with the padding subsystem */
+  circpad_new_consensus_params(&vote1);
+
+  /* Check padding limit, should be fine since we haven't sent anything yet. */
+  retval = circpad_machine_reached_padding_limit(mi);
+  tt_int_op(retval, OP_EQ, 0);
+
+  /* Send 99 padding cells which is below circpad_global_allowed_cells=100, so
+   * the rate limit will not trigger */
+  for (i=0;i<99;i++) {
+    circpad_send_padding_cell_for_callback(mi);
+  }
+  retval = circpad_machine_reached_padding_limit(mi);
+  tt_int_op(retval, OP_EQ, 0);
+
+  /* Now send another padding cell to pass circpad_global_allowed_cells=100,
+     and see that the limit will trigger */
+  circpad_send_padding_cell_for_callback(mi);
+  retval = circpad_machine_reached_padding_limit(mi);
+  tt_int_op(retval, OP_EQ, 1);
+
+  retval = circpad_machine_schedule_padding(mi);
+  tt_int_op(retval, OP_EQ, CIRCPAD_STATE_UNCHANGED);
+
+  /* Now send 92 non-padding cells to get near the
+   * circpad_global_max_padding_pct=50 limit; in particular with 96 non-padding
+   * cells, the padding traffic is still 51% of total traffic so limit should
+   * trigger */
+  for (i=0;i<92;i++) {
+    circpad_cell_event_nonpadding_sent(relay_side);
+  }
+  retval = circpad_machine_reached_padding_limit(mi);
+  tt_int_op(retval, OP_EQ, 1);
+
+  /* Send another non-padding cell to bring the padding traffic to 50% of total
+   * traffic and get past the limit */
+  circpad_cell_event_nonpadding_sent(relay_side);
+  retval = circpad_machine_reached_padding_limit(mi);
+  tt_int_op(retval, OP_EQ, 0);
+
+ done:
+  free_fake_orcirc(relay_side);
+  circuitmux_detach_all_circuits(dummy_channel.cmux, NULL);
+  circuitmux_free(dummy_channel.cmux);
+  SMARTLIST_FOREACH(vote1.net_params, char *, cp, tor_free(cp));
+  smartlist_free(vote1.net_params);
+}
+
+#define TEST_CIRCUITPADDING(name, flags) \
+    { #name, test_##name, (flags), NULL, NULL }
+
+struct testcase_t circuitpadding_tests[] = {
+  //TEST_CIRCUITPADDING(circuitpadding_circuitsetup_machine, 0),
+  TEST_CIRCUITPADDING(circuitpadding_tokens, TT_FORK),
+  TEST_CIRCUITPADDING(circuitpadding_negotiation, TT_FORK),
+  TEST_CIRCUITPADDING(circuitpadding_wronghop, TT_FORK),
+  TEST_CIRCUITPADDING(circuitpadding_circuitsetup_machine, TT_FORK),
+  TEST_CIRCUITPADDING(circuitpadding_conditions, TT_FORK),
+  TEST_CIRCUITPADDING(circuitpadding_rtt, TT_FORK),
+  TEST_CIRCUITPADDING(circuitpadding_sample_distribution, TT_FORK),
+  TEST_CIRCUITPADDING(circuitpadding_machine_rate_limiting, TT_FORK),
+  TEST_CIRCUITPADDING(circuitpadding_global_rate_limiting, TT_FORK),
+  TEST_CIRCUITPADDING(circuitpadding_token_removal_lower, TT_FORK),
+  TEST_CIRCUITPADDING(circuitpadding_token_removal_higher, TT_FORK),
+  TEST_CIRCUITPADDING(circuitpadding_closest_token_removal, TT_FORK),
+  TEST_CIRCUITPADDING(circuitpadding_closest_token_removal_usec, TT_FORK),
+  TEST_CIRCUITPADDING(circuitpadding_token_removal_exact, TT_FORK),
+  END_OF_TESTCASES
+};
+
diff --git a/src/test/test_containers.c b/src/test/test_containers.c
index 717eb0892..ad0edf4aa 100644
--- a/src/test/test_containers.c
+++ b/src/test/test_containers.c
@@ -96,6 +96,30 @@ test_container_smartlist_basic(void *arg)
   tor_free(v555);
 }
 
+/** Test SMARTLIST_FOREACH_REVERSE_BEGIN loop macro */
+static void
+test_container_smartlist_foreach_reverse(void *arg)
+{
+  smartlist_t *sl = smartlist_new();
+  int i;
+
+  (void) arg;
+
+  /* Add integers to smartlist in increasing order */
+  for (i=0;i<100;i++) {
+    smartlist_add(sl, (void*)(uintptr_t)i);
+  }
+
+  /* Pop them out in reverse and test their value */
+  SMARTLIST_FOREACH_REVERSE_BEGIN(sl, void*, k) {
+    i--;
+    tt_ptr_op(k, OP_EQ, (void*)(uintptr_t)i);
+  } SMARTLIST_FOREACH_END(k);
+
+ done:
+  smartlist_free(sl);
+}
+
 /** Run unit tests for smartlist-of-strings functionality. */
 static void
 test_container_smartlist_strings(void *arg)
@@ -1281,6 +1305,7 @@ test_container_smartlist_strings_eq(void *arg)
 struct testcase_t container_tests[] = {
   CONTAINER_LEGACY(smartlist_basic),
   CONTAINER_LEGACY(smartlist_strings),
+  CONTAINER_LEGACY(smartlist_foreach_reverse),
   CONTAINER_LEGACY(smartlist_overlap),
   CONTAINER_LEGACY(smartlist_digests),
   CONTAINER_LEGACY(smartlist_join),



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