diff --git a/mqttpacket.cpp b/mqttpacket.cpp
index b8080ef..00f0849 100644
--- a/mqttpacket.cpp
+++ b/mqttpacket.cpp
@@ -54,9 +54,8 @@ MqttPacket::MqttPacket(const ConnAck &connAck) :
     char first_byte = static_cast<char>(packetType) << 4;
     writeByte(first_byte);
     writeByte(2); // length is always 2.
-    writeByte(0); // all connect-ack flags are 0, except session-present, but we don't have that yet. TODO: make that
+    writeByte(connAck.session_present & 0b00000001); // all connect-ack flags are 0, except session-present. [MQTT-3.2.2.1]
     writeByte(static_cast<char>(connAck.return_code));
-
 }
 
 MqttPacket::MqttPacket(const SubAck &subAck) :
@@ -155,6 +154,8 @@ void MqttPacket::handleConnect()
 
     GlobalSettings *settings = GlobalSettings::getInstance();
 
+    std::shared_ptr<SubscriptionStore> subscriptionStore = sender->getThreadData()->getSubscriptionStore();
+
     uint16_t variable_header_length = readTwoBytesToUInt16();
 
     if (variable_header_length == 4 || variable_header_length == 6)
@@ -278,17 +279,18 @@ void MqttPacket::handleConnect()
 
         if (sender->getThreadData()->authPlugin.unPwdCheck(username, password) == AuthResult::success)
         {
-            sender->getThreadData()->getSubscriptionStore()->registerClientAndKickExistingOne(sender);
+            bool sessionPresent = protocolVersion >= ProtocolVersion::Mqtt311 && !clean_session && subscriptionStore->sessionPresent(client_id);
+            subscriptionStore->registerClientAndKickExistingOne(sender);
 
             sender->setAuthenticated(true);
-            ConnAck connAck(ConnAckReturnCodes::Accepted);
+            ConnAck connAck(ConnAckReturnCodes::Accepted, sessionPresent);
             MqttPacket response(connAck);
             sender->writeMqttPacket(response);
             logger->logf(LOG_NOTICE, "User '%s' logged in successfully", username.c_str());
         }
         else
         {
-            ConnAck connDeny(ConnAckReturnCodes::NotAuthorized);
+            ConnAck connDeny(ConnAckReturnCodes::NotAuthorized, false);
             MqttPacket response(connDeny);
             sender->setDisconnectReason("Access denied");
             sender->setReadyForDisconnect();
diff --git a/subscriptionstore.cpp b/subscriptionstore.cpp
index f067df1..bec9118 100644
--- a/subscriptionstore.cpp
+++ b/subscriptionstore.cpp
@@ -121,6 +121,22 @@ void SubscriptionStore::registerClientAndKickExistingOne(Client_p &client)
     session->sendPendingQosMessages();
 }
 
+bool SubscriptionStore::sessionPresent(const std::string &clientid)
+{
+    RWLockGuard lock_guard(&subscriptionsRwlock);
+    lock_guard.rdlock();
+
+    bool result = false;
+
+    auto it = sessionsByIdConst.find(clientid);
+    if (it != sessionsByIdConst.end())
+    {
+        it->second->touch(); // Touching to avoid a race condition between using the session after this, and it expiring.
+        result = true;
+    }
+    return result;
+}
+
 // TODO: should I implement cache, this needs to be changed to returning a list of clients.
 void SubscriptionStore::publishNonRecursively(const MqttPacket &packet, const std::vector<Subscription> &subscribers) const
 {
diff --git a/subscriptionstore.h b/subscriptionstore.h
index 1732574..958a809 100644
--- a/subscriptionstore.h
+++ b/subscriptionstore.h
@@ -70,6 +70,7 @@ public:
 
     void addSubscription(Client_p &client, const std::string &topic, char qos);
     void registerClientAndKickExistingOne(Client_p &client);
+    bool sessionPresent(const std::string &clientid);
 
     void queuePacketAtSubscribers(const std::string &topic, const MqttPacket &packet, const Client_p &sender);
     void giveClientRetainedMessages(const std::shared_ptr<Session> &ses, const std::string &subscribe_topic, char max_qos);
diff --git a/types.cpp b/types.cpp
index 96fc5b5..973820e 100644
--- a/types.cpp
+++ b/types.cpp
@@ -1,9 +1,12 @@
 #include "types.h"
 
-ConnAck::ConnAck(ConnAckReturnCodes return_code) :
-    return_code(return_code)
+ConnAck::ConnAck(ConnAckReturnCodes return_code, bool session_present) :
+    return_code(return_code),
+    session_present(session_present)
 {
-
+    // [MQTT-3.2.2-4]
+    if (return_code > ConnAckReturnCodes::Accepted)
+        session_present = false;
 }
 
 SubAck::SubAck(uint16_t packet_id, const std::list<char> &subs_qos_reponses) :
diff --git a/types.h b/types.h
index 8523907..c6d3352 100644
--- a/types.h
+++ b/types.h
@@ -46,8 +46,9 @@ enum class ConnAckReturnCodes
 class ConnAck
 {
 public:
-    ConnAck(ConnAckReturnCodes return_code);
+    ConnAck(ConnAckReturnCodes return_code, bool session_present=false);
     ConnAckReturnCodes return_code;
+    bool session_present = false;
     size_t getLengthWithoutFixedHeader() const { return 2;} // size of connack is always the same
 };