diff --git a/DBusInterface.md b/DBusInterface.md
index 39a7615..a201966 100644
--- a/DBusInterface.md
+++ b/DBusInterface.md
@@ -336,12 +336,17 @@ files). The JSON objects are "drawing" (the root object), "strokes",
 class Drawing {
         version: uint32
         devicename: string
+        sessionid: string            // used for debugging
         dimensions: [uint32, uint32] // x/y physical dimensions in µm
         timestamp: uint64
         strokes: [ Stroke, Stroke, ...]
 }
 ```
 
+A session id is a random string that identifies a Tuhi session. This is
+debugging information only, it makes it possible to associate a JSON file
+with the corresponding sequence in the log. Do not use in clients.
+
 The **strokes** list contains all strokes of a single drawing, each stroke
 consisting of a number of **points**.
 
@@ -367,6 +372,7 @@ An expanded file looks like this:
 {
    "version" : 1,                       // JSON file format version number
    "devicename":  "Wacom Bamboo Spark", 
+   "sessionid": "somerandomstring-1",
    "dimensions": [ 100000, 200000],      // width/height in µm
    "timestamp" : 12345,
    "strokes" : [
diff --git a/tuhi/drawing.py b/tuhi/drawing.py
index 04adb49..28821ed 100644
--- a/tuhi/drawing.py
+++ b/tuhi/drawing.py
@@ -97,6 +97,7 @@ class Drawing(GObject.Object):
         self.timestamp = timestamp  # unix seconds
         self.strokes = []
         self._current_stroke = -1
+        self.session_id = 'unset'
 
     def seal(self):
         # Drop empty strokes
@@ -130,6 +131,7 @@ class Drawing(GObject.Object):
         json_data = {
             'version': self.JSON_FILE_FORMAT_VERSION,
             'devicename': self.name,
+            'sessionid': self.session_id,
             'dimensions': list(self.dimensions),
             'timestamp': self.timestamp,
             'strokes': [s.to_dict() for s in self.strokes]
diff --git a/tuhi/wacom.py b/tuhi/wacom.py
index 8b6db19..82cc6a1 100644
--- a/tuhi/wacom.py
+++ b/tuhi/wacom.py
@@ -40,6 +40,20 @@ SYSEVENT_NOTIFICATION_SERVICE_UUID = '3a340720-c572-11e5-86c5-0002a5d5c51b'  # N
 SYSEVENT_NOTIFICATION_CHRC_UUID    = '3a340721-c572-11e5-86c5-0002a5d5c51b'  # NOQA
 
 
+class IDGenerator(object):
+    _session = uuid.uuid4().hex
+    _instance = 0
+
+    @classmethod
+    def current(cls):
+        return f'{cls._session}-{cls._instance}'
+
+    @classmethod
+    def next(cls):
+        cls._instance += 1
+        return cls.current()
+
+
 @enum.unique
 class DeviceMode(enum.Enum):
     REGISTER = 1
@@ -198,6 +212,9 @@ class DataLogger(object):
         path = Path(self.logdir, fname)
         self.logfile = open(path, 'w+')
 
+        session_id = IDGenerator.next()
+        self.logger.debug(f'sessionid: {session_id}')
+        self.logfile.write(f'sessionid: {session_id}\n')
         self.logfile.write(f'name: {self.device.name}\n')
         self.logfile.write(f'bluetooth: {self.btaddr}\n')
         self.logfile.write(f'time: {timestamp} # host time: {time.strftime("%Y-%m-%d %H:%M:%S")}\n')
@@ -661,6 +678,7 @@ class WacomProtocolBase(WacomProtocolLowLevelComm):
 
         f = StrokeFile(data)
         drawing = Drawing(self.device.name, (self.width, self.height), timestamp)
+        drawing.session_id = IDGenerator.current()
         ps = self.point_size
 
         def normalize(p):