diff --git a/README.md b/README.md
index a416d59..d3b5133 100644
--- a/README.md
+++ b/README.md
@@ -912,6 +912,17 @@ How it works:
   | `month` | 6 hours | hourly |
   | `year` | 1 month | monthly |
 
+- **Site-to-site availability:** `site_to_site_up` of the same answer divides the view into slots of its own, `slot`, and gives for each slot and connection the percentage of the samples in which the connection was up, 0 to 100, as `points` of `{"time", "connections": {name: percent}}`:
+
+  | Range | Slot | Source |
+  |---|---|---|
+  | `hour` | 5 minutes | details |
+  | `day` | 1 hour | details |
+  | `month` | 1 day | hourly |
+  | `year` | 1 month | monthly |
+
+  A sample is taken every minute, so an interruption shorter than a minute may be missed; a slot without samples has no value, and the year view, like the others of its level, has the complete months only.
+
 ### Scraping
 
 `GET /metrics` of the metrics service serves the last sample in the Prometheus text format, written by `prometheus_client`.
diff --git a/debian-package/debian/changelog b/debian-package/debian/changelog
index e42cdfd..0fcdbcc 100644
--- a/debian-package/debian/changelog
+++ b/debian-package/debian/changelog
@@ -1,3 +1,14 @@
+vpn-router (0.19.1-1) unstable; urgency=medium
+
+  fix: /metrics/history gives the availability of each site-to-site connection as the percentage of time it was up in slots of the view.
+
+  * site_to_site_up of /metrics/history divides the view into slots of 5
+    minutes for an hour, 1 hour for a day, 1 day for a month and 1 month
+    for a year, and gives for each slot and connection the percentage of
+    the samples in which the connection was up.
+
+ -- Slawomir Koszewski <slawek@koszewscy.waw.pl>  Mon, 26 Oct 2026 21:00:00 +0100
+
 vpn-router (0.19.0-1) unstable; urgency=medium
 
   feature: service accounts have a display name and a description, which the router sets for its internal ones.
diff --git a/debian-package/src/usr/lib/vpn-router/vpnrouter_metrics.py b/debian-package/src/usr/lib/vpn-router/vpnrouter_metrics.py
index 2bc733f..fdbb051 100644
--- a/debian-package/src/usr/lib/vpn-router/vpnrouter_metrics.py
+++ b/debian-package/src/usr/lib/vpn-router/vpnrouter_metrics.py
@@ -44,6 +44,15 @@ LEVELS = (
 #: The kinds of VPN clients of metrics_vpn.
 VPN_KINDS = ('p2s', 'openvpn', 'wireguard')
 
+#: The time slots of the site-to-site availability of each view of history(): the summary level read (None: the
+#: details), the slot as PostgreSQL starts it from time, and its length.
+UP_SLOTS = {
+    'hour': (None, "date_bin('5 minutes', time, timestamptz '2000-01-01 00:00:00+00')", '5 minutes'),
+    'day': (None, "date_trunc('hour', time, 'UTC')", '1 hour'),
+    'month': ('hourly', "date_trunc('day', time, 'UTC')", '1 day'),
+    'year': ('monthly', "date_trunc('month', time, 'UTC')", '1 month'),
+}
+
 #: The views of history(): the summary level read (None: the details), the bucket and the span.
 RANGES = {
     'hour': (None, '1 minute', '1 hour'),
@@ -148,8 +157,13 @@ def history(connection, view):
     each interface to its rx and tx, clients, which maps each kind of VPN client to its clients,
     and site_to_site, which maps each site-to-site connection to the values of
     metrics_site_to_site; a value without data is None.
+
+    site_to_site_up is the availability of each site-to-site connection in the time slots of UP_SLOTS, oldest first:
+    for each slot its start in Unix seconds and, per connection, the percentage of its samples in which the connection
+    was up, 0 to 100; the summaries' averages of up give it for the month and the year.
     """
     level, bucket, span = RANGES[view]
+    up_level, up_start, up_slot = UP_SLOTS[view]
 
     def rows(table):
         keys, values = TABLES[table]
@@ -182,4 +196,12 @@ def history(connection, view):
     for row in rows('metrics_site_to_site'):
         point(row[0])['site_to_site'][row[1]] = pairs(row[2:], TABLES['metrics_site_to_site'][1])
     empty = {value: {'avg': None, 'max': None} for value in TABLES['metrics_system'][1]}
-    return {'range': view, 'bucket': bucket, 'points': [{**empty, **points[start]} for start in sorted(points)]}
+
+    slots = {}
+    for start, name, percent in connection.execute(
+            f"SELECT extract(epoch FROM {up_start})::bigint AS slot, connection, avg({'up' if up_level is None else 'up_avg'}) * 100 "
+            f"FROM {'metrics_site_to_site' if up_level is None else f'metrics_site_to_site_{up_level}'} "
+            f"WHERE time >= now() - interval '{span}' GROUP BY slot, connection ORDER BY slot").fetchall():
+        slots.setdefault(start, {'time': start, 'connections': {}})['connections'][name] = percent
+    return {'range': view, 'bucket': bucket, 'points': [{**empty, **points[start]} for start in sorted(points)],
+            'site_to_site_up': {'slot': up_slot, 'points': [slots[start] for start in sorted(slots)]}}