diff --git a/changes/bug23816 b/changes/bug23816
new file mode 100644
index 000000000..6139dec9e
--- /dev/null
+++ b/changes/bug23816
@@ -0,0 +1,6 @@
+ o Minor bugfixes (directory client):
+ - On failure to download directory information, delay retry attempts
+ by a random amount based on the "decorrelated jitter" algorithm.
+ Our previous delay algorithm tended to produce extra-long delays too
+ easily. Fixes bug 23816; bugfix on 0.2.9.1-alpha.
+
diff --git a/src/or/directory.c b/src/or/directory.c
index 7a1364bd7..b2e0a96a2 100644
--- a/src/or/directory.c
+++ b/src/or/directory.c
@@ -5421,54 +5421,68 @@ find_dl_min_and_max_delay(download_status_t *dls, const or_options_t *options,
*max = INT_MAX;
}
-/** Advance one delay step. The algorithm is to use the previous delay to
- * compute an increment, we construct a value uniformly at random between
- * delay+1 and (delay*(DIR_DEFAULT_RANDOM_MULTIPLIER+1))+1 (or
- * DIR_TEST_NET_RANDOM_MULTIPLIER in test networks).
+/** As next_random_exponential_delay() below, but does not compute a random
+ * value. Instead, compute the range of values that
+ * next_random_exponential_delay() should use when computing its random value.
+ * Store the low bound into *low_bound_out, and the high bound into
+ * *high_bound_out. Guarantees that the low bound is strictly less
+ * than the high bound. */
+STATIC void
+next_random_exponential_delay_range(int *low_bound_out,
+ int *high_bound_out,
+ int delay,
+ int base_delay)
+{
+ // This is the "decorrelated jitter" approach, from
+ // https://www.awsarchitectureblog.com/2015/03/backoff.html
+ // The formula is
+ // sleep = min(cap, random_between(base, sleep * 3))
+
+ const int delay_times_3 = delay < INT_MAX/3 ? delay * 3 : INT_MAX;
+ *low_bound_out = base_delay;
+ if (delay_times_3 > base_delay) {
+ *high_bound_out = delay_times_3;
+ } else {
+ *high_bound_out = base_delay+1;
+ }
+}
+
+/** Advance one delay step. The algorithm will generate a random delay,
+ * such that each failure is possibly (random) longer than the ones before.
+ *
* We then clamp that value to be no larger than max_delay, and return it.
*
+ * The base_delay parameter is lowest possible delay time (can't be
+ * zero); the backoff_position parameter is the number of times we've
+ * generated a delay; and the delay argument is the most recently used
+ * delay.
+ *
* Requires that delay is less than INT_MAX, and delay is in [0,max_delay].
*/
STATIC int
-next_random_exponential_delay(int delay, int max_delay)
+next_random_exponential_delay(int delay,
+ int base_delay,
+ int max_delay)
{
/* Check preconditions */
if (BUG(max_delay < 0))
max_delay = 0;
if (BUG(delay > max_delay))
delay = max_delay;
- if (delay == INT_MAX)
- return INT_MAX; /* prevent overflow */
if (BUG(delay < 0))
delay = 0;
- /* How much are we willing to add to the delay? */
- int max_increment;
- int multiplier = DIR_DEFAULT_RANDOM_MULTIPLIER;
- if (get_options()->TestingTorNetwork) {
- /* Decrease the multiplier in testing networks. This reduces the variance,
- * so that bootstrap is more reliable. */
- multiplier = DIR_TEST_NET_RANDOM_MULTIPLIER;
- }
+ if (base_delay < 1)
+ base_delay = 1;
- if (delay && delay < (INT_MAX-1) / multiplier) {
- max_increment = delay * multiplier;
- } else if (delay) {
- max_increment = INT_MAX-1;
- } else {
- max_increment = 1;
- }
+ int low_bound=0, high_bound=max_delay;
- if (BUG(max_increment < 1))
- max_increment = 1;
+ next_random_exponential_delay_range(&low_bound, &high_bound,
+ delay, base_delay);
- /* the + 1 here is so that we always wait longer than last time. */
- int increment = crypto_rand_int(max_increment)+1;
+ int rand_delay = crypto_rand_int_range(low_bound, high_bound);
- if (increment < max_delay - delay)
- return delay + increment;
- else
- return max_delay;
+ return MIN(rand_delay, max_delay);
}
/** Find the current delay for dls based on schedule or min_delay/
@@ -5517,7 +5531,7 @@ download_status_schedule_get_delay(download_status_t *dls,
while (dls->last_backoff_position < dls_schedule_position) {
/* Do one increment step */
- delay = next_random_exponential_delay(delay, max_delay);
+ delay = next_random_exponential_delay(delay, min_delay, max_delay);
/* Update our position */
++(dls->last_backoff_position);
}
diff --git a/src/or/directory.h b/src/or/directory.h
index 79984be32..d26d83537 100644
--- a/src/or/directory.h
+++ b/src/or/directory.h
@@ -226,7 +226,15 @@ STATIC const smartlist_t *find_dl_schedule(const download_status_t *dls,
STATIC void find_dl_min_and_max_delay(download_status_t *dls,
const or_options_t *options,
int *min, int *max);
-STATIC int next_random_exponential_delay(int delay, int max_delay);
+
+STATIC int next_random_exponential_delay(int delay,
+ int base_delay,
+ int max_delay);
+
+STATIC void next_random_exponential_delay_range(int *low_bound_out,
+ int *high_bound_out,
+ int delay,
+ int base_delay);
STATIC int parse_hs_version_from_post(const char *url, const char *prefix,
const char **end_pos);
diff --git a/src/test/test_dir.c b/src/test/test_dir.c
index 87b86c38b..ee4a9780b 100644
--- a/src/test/test_dir.c
+++ b/src/test/test_dir.c
@@ -4126,9 +4126,9 @@ download_status_random_backoff_helper(int min_delay, int max_delay)
int increment = -1;
int old_increment = -1;
time_t current_time = time(NULL);
- const int exponent = DIR_DEFAULT_RANDOM_MULTIPLIER + 1;
/* Check the random backoff cases */
+ int n_attempts = 0;
do {
increment = download_status_schedule_get_delay(&dls_random,
NULL,
@@ -4148,40 +4148,16 @@ download_status_random_backoff_helper(int min_delay, int max_delay)
/* Test */
tt_int_op(increment, OP_GE, min_delay);
tt_int_op(increment, OP_LE, max_delay);
- if (dls_random.last_backoff_position == 0) {
- /* regression tests for 17750
- * Always use the minimum delay for the first increment */
- tt_int_op(increment, OP_EQ, min_delay);
- } else {
- /* It's times like these I'd love a good saturating arithmetic
- * implementation */
- int min_inc = INT_MAX;
- if (old_increment <= INT_MAX - 1) {
- min_inc = old_increment + 1;
- }
-
- int max_inc = INT_MAX;
- if (old_increment <= (INT_MAX - 1)/exponent) {
- max_inc = (exponent * old_increment) + 1;
- }
-
- /* Regression test for 20534 and friends:
- * increment must always increase after the first */
- tt_int_op(increment, OP_GE, min_inc);
- /* We at most quadruple, and always add one */
- tt_int_op(increment, OP_LE, max_inc);
- }
/* Advance */
- ++(dls_random.n_download_attempts);
- ++(dls_random.n_download_failures);
+ if (dls_random.n_download_attempts < IMPOSSIBLE_TO_DOWNLOAD - 1) {
+ ++(dls_random.n_download_attempts);
+ ++(dls_random.n_download_failures);
+ }
/* Try another maybe */
old_increment = increment;
- if (increment >= max_delay)
- current_time += increment;
-
- } while (increment < max_delay);
+ } while (increment < max_delay && ++n_attempts < 1000);
done:
return;
@@ -4208,6 +4184,38 @@ test_dir_download_status_random_backoff(void *arg)
download_status_random_backoff_helper(INT_MAX/2, INT_MAX);
}
+static void
+test_dir_download_status_random_backoff_ranges(void *arg)
+{
+ (void)arg;
+ int lo, hi;
+ next_random_exponential_delay_range(&lo, &hi, 0, 10);
+ tt_int_op(lo, OP_EQ, 10);
+ tt_int_op(hi, OP_EQ, 11);
+
+ next_random_exponential_delay_range(&lo, &hi, 6, 10);
+ tt_int_op(lo, OP_EQ, 10);
+ tt_int_op(hi, OP_EQ, 6*3);
+
+ next_random_exponential_delay_range(&lo, &hi, 13, 10);
+ tt_int_op(lo, OP_EQ, 10);
+ tt_int_op(hi, OP_EQ, 13 * 3);
+
+ next_random_exponential_delay_range(&lo, &hi, 37, 10);
+ tt_int_op(lo, OP_EQ, 10);
+ tt_int_op(hi, OP_EQ, 111);
+
+ next_random_exponential_delay_range(&lo, &hi, 123, 10);
+ tt_int_op(lo, OP_EQ, 10);
+ tt_int_op(hi, OP_EQ, 369);
+
+ next_random_exponential_delay_range(&lo, &hi, INT_MAX-5, 10);
+ tt_int_op(lo, OP_EQ, 10);
+ tt_int_op(hi, OP_EQ, INT_MAX);
+ done:
+ ;
+}
+
static void
test_dir_download_status_increment(void *arg)
{
@@ -6208,6 +6216,7 @@ struct testcase_t dir_tests[] = {
DIR(packages, 0),
DIR(download_status_schedule, 0),
DIR(download_status_random_backoff, 0),
+ DIR(download_status_random_backoff_ranges, 0),
DIR(download_status_increment, 0),
DIR(authdir_type_to_string, 0),
DIR(conn_purpose_to_string, 0),