diff --git a/readme.md b/readme.md
new file mode 100644
index 0000000..e38f349
--- /dev/null
+++ b/readme.md
@@ -0,0 +1,25 @@
+System of monitoring and controlling heating.
+
+Modules (called units) send their data via UDP.
+Each datagram contains JSON object of following form:
+{
+	"id": "<ID>",
+	"ip": "<IP>",
+	"v1": "123",
+	...
+}
+
+Where ID is unit's identifier (MAC address usually),
+IP is unit's current IP address (for server to be able
+to contact it).
+Other values in object are names and values of sensors 
+available on the unit.
+
+Server could send datagram with some commands to unit.
+For simplicity datagram is simple string with command and
+optional arguments, everything separated with character 0x01.
+
+Possible commands are:
+
+1. ping - no arguments. Informs unit that server is still alive
+and able to communicate
\ No newline at end of file
diff --git a/thermo_sensor/thermo_sensor.ino b/thermo_sensor/thermo_sensor.ino
index aefa276..b7044aa 100644
--- a/thermo_sensor/thermo_sensor.ino
+++ b/thermo_sensor/thermo_sensor.ino
@@ -6,12 +6,15 @@
 #define DHTPIN 5
 #define DHTTYPE DHT22
 #define READ_INTERVAL 1000
-
+#define UDP_PORT 7000
+#define PING_INTERVAL 60000 // one minute seems to be ok for us
 
 WiFiUDP udp;
 DHT dht(DHTPIN, DHTTYPE);
 char id[64];
 
+long last_ping;
+
 void setup() 
 {
 	Serial.begin(115200);
@@ -34,6 +37,9 @@ void setup()
 	WiFi.macAddress(macAddr);
 	sprintf(id, "%02x:%02x:%02x:%02x:%02x:%02x", macAddr[0], macAddr[1], macAddr[2], macAddr[3], macAddr[4], macAddr[5]);
 	Serial.printf("Connected, mac address: %s\n", id);
+	udp.begin(UDP_PORT);
+
+	last_ping = millis();
 }
 
 char payload[1024];
@@ -73,6 +79,28 @@ void ReadAndSend()
 	udp.endPacket();
 }
 
+
+
+char incoming_packet[4096];
+
+void ParseIncomingPacket()
+{
+	int len = udp.read(incoming_packet, sizeof(incoming_packet));
+	if (len == 0) return;
+	// find command
+	int pos = 0;
+	while (pos < len && incoming_packet[pos] != 0x01) pos++;
+	if (strncmp(incoming_packet, "ping", pos) == 0)
+	{
+		Serial.println("Ping command received");
+	}
+	else
+	{
+		// unknown command
+		Serial.println("Unknown command received");
+	}
+}
+
 unsigned long last_read = 0;
 
 void loop() 
@@ -83,4 +111,15 @@ void loop()
 		ReadAndSend();
 		last_read = current;
 	}
+	int packetSize = udp.parsePacket();
+	if (packetSize)
+	{
+		ParseIncomingPacket();
+	}
+	if (current < last_ping || current - last_ping >= PING_INTERVAL)
+	{
+		// no pings for long time, reboot
+		Serial.println("No pings for long time, reboot");
+		ESP.restart();
+	}
 }