diff --git a/apps/sst_b2b/SSTB2B.cpp b/apps/sst_b2b/SSTB2B.cpp
index 2e046935..01df81be 100644
--- a/apps/sst_b2b/SSTB2B.cpp
+++ b/apps/sst_b2b/SSTB2B.cpp
@@ -69,6 +69,8 @@ int SSTB2BFactory::onLoad()
 
 AmSession* SSTB2BFactory::onInvite(const AmSipRequest& req)
 {
+  if (!session_timer_fact->onInvite(req, cfg))
+    return NULL;
 
   SSTB2BDialog* b2b_dlg = new SSTB2BDialog();
   AmSessionEventHandler* h = session_timer_fact->getHandler(b2b_dlg);
@@ -76,6 +78,7 @@ AmSession* SSTB2BFactory::onInvite(const AmSipRequest& req)
     ERROR("could not get a session timer event handler\n");
     throw AmSession::Exception(500,"Server internal error");
   }
+
   if(h->configure(cfg)){
     ERROR("Could not configure the session timer: disabling session timers.\n");
     delete h;
diff --git a/core/AmApi.cpp b/core/AmApi.cpp
index d15510cc..ac9b6df5 100644
--- a/core/AmApi.cpp
+++ b/core/AmApi.cpp
@@ -94,9 +94,10 @@ AmSessionEventHandlerFactory::AmSessionEventHandlerFactory(const string& name)
 }
 
 bool AmSessionEventHandlerFactory::onInvite(const AmSipRequest& req, 
-						  AmArg& session_params) {
+					    AmArg& session_params,
+					    AmConfigReader& cfg) {
   WARN("discarding session parameters for new session.\n");
-  return onInvite(req);
+  return onInvite(req, cfg);
 }
 
 // AmSIPEventHandler::AmSIPEventHandler(const string& name) 
diff --git a/core/AmApi.h b/core/AmApi.h
index f9e76e98..dafbab2f 100644
--- a/core/AmApi.h
+++ b/core/AmApi.h
@@ -110,10 +110,10 @@ class AmSessionEventHandlerFactory: public AmPluginFactory
   virtual AmSessionEventHandler* getHandler(AmSession*)=0;
 
   /**
-   * @return true if session creation should be stopped
+   * @return false if session creation should be stopped
    */
-  virtual bool onInvite(const AmSipRequest& req)=0;
-  virtual bool onInvite(const AmSipRequest& req, AmArg& session_params);
+  virtual bool onInvite(const AmSipRequest& req, AmConfigReader& cfg)=0;
+  virtual bool onInvite(const AmSipRequest& req, AmArg& session_params, AmConfigReader& cfg);
 };
 
 /** \brief Interface for plugins to create sessions */
diff --git a/core/AmSession.h b/core/AmSession.h
index 4e0e5cda..20c6d1f8 100644
--- a/core/AmSession.h
+++ b/core/AmSession.h
@@ -195,7 +195,8 @@ public:
   struct Exception {
     int code;
     string reason;
-    Exception(int c, string r) : code(c), reason(r) {}
+    string hdrs;
+    Exception(int c, string r, string h="") : code(c), reason(r), hdrs(h) {}
   };
 
   /** 
diff --git a/core/AmSessionContainer.cpp b/core/AmSessionContainer.cpp
index 0da7022b..5378f927 100644
--- a/core/AmSessionContainer.cpp
+++ b/core/AmSessionContainer.cpp
@@ -307,8 +307,8 @@ void AmSessionContainer::startSessionUAS(AmSipRequest& req)
       }
   } 
   catch(const AmSession::Exception& e){
-    ERROR("%i %s\n",e.code,e.reason.c_str());
-    AmSipDialog::reply_error(req,e.code,e.reason);
+    ERROR("%i %s %s\n",e.code,e.reason.c_str(), e.hdrs.c_str());
+    AmSipDialog::reply_error(req,e.code,e.reason, e.hdrs);
   }
   catch(const string& err){
     ERROR("startSession: %s\n",err.c_str());
diff --git a/core/plug-in/echo/Echo.cpp b/core/plug-in/echo/Echo.cpp
index a90e051a..7a39c088 100644
--- a/core/plug-in/echo/Echo.cpp
+++ b/core/plug-in/echo/Echo.cpp
@@ -76,10 +76,18 @@ int EchoFactory::onLoad()
 
 AmSession* EchoFactory::onInvite(const AmSipRequest& req)
 {
+  if (NULL != session_timer_f) {
+    if (!session_timer_f->onInvite(req, conf))
+      return NULL;
+  }
+
   AmSession* s = new EchoDialog();
   
   if (NULL != session_timer_f) {
+
     AmSessionEventHandler* h = session_timer_f->getHandler(s);
+    if (NULL == h)
+      return NULL;
     
     if(h->configure(conf)){
       ERROR("Could not configure the session timer: disabling session timers.\n");
diff --git a/core/plug-in/session_timer/SessionTimer.cpp b/core/plug-in/session_timer/SessionTimer.cpp
index 06e1c16c..0926d38c 100644
--- a/core/plug-in/session_timer/SessionTimer.cpp
+++ b/core/plug-in/session_timer/SessionTimer.cpp
@@ -37,11 +37,9 @@ int SessionTimerFactory::onLoad()
   return 0;
 }
 
-bool SessionTimerFactory::onInvite(const AmSipRequest& req)
+bool SessionTimerFactory::onInvite(const AmSipRequest& req, AmConfigReader& cfg)
 {
-  if(!checkSessionExpires(req))
-    return true;
-  return false;
+  return checkSessionExpires(req, cfg);
 }
 
 
@@ -127,11 +125,9 @@ bool SessionTimer::onSendReply(const AmSipRequest& req,
   return false;
 }
 
-
-/* Session Timer: -ssa */
-int  SessionTimer::configure(AmConfigReader& conf) 
+int SessionTimer::configure(AmConfigReader& conf)
 {
-  if(session_timer_conf.readFromConfig(conf))
+  if(!session_timer_conf.readFromConfig(conf))
     return -1;
 
   session_interval = session_timer_conf.getSessionExpires();
@@ -152,22 +148,29 @@ int  SessionTimer::configure(AmConfigReader& conf)
  *   (<locally configured Min-SE)                  
  * Throws SessionIntervalTooSmallException if too low
  */
-bool SessionTimerFactory::checkSessionExpires(const AmSipRequest& req) 
+bool SessionTimerFactory::checkSessionExpires(const AmSipRequest& req, AmConfigReader& cfg)
 {
+  AmSessionTimerConfig sst_cfg;
+  if (sst_cfg.readFromConfig(cfg)) {
+    return false;
+  }
+
   string session_expires = getHeader(req.hdrs, SIP_HDR_SESSION_EXPIRES,
 				     SIP_HDR_SESSION_EXPIRES_COMPACT, true);
 
   if (session_expires.length()) {
     unsigned int i_se;
     if (!str2i(strip_header_params(session_expires), i_se)) {
-
-      //if (i_se < session_timer_conf.getMinimumTimer()) {
-      //TODO: reply 422...
-      //}
-    } else
-      throw AmSession::Exception(500,"internal error"); // malformed request?
+      if (i_se < sst_cfg.getMinimumTimer()) {
+	throw AmSession::Exception(422, "Session Interval Too Small",
+				   SIP_HDR_COLSP(SIP_HDR_MIN_SE)+
+				   int2str(sst_cfg.getMinimumTimer())+CRLF);
+      }
+    } else {
+      WARN("parsing session expires '%s' failed\n", session_expires.c_str());
+      throw AmSession::Exception(400,"Bad Request");
+    }
   }
-  //}
 
   return true;
 }
diff --git a/core/plug-in/session_timer/SessionTimer.h b/core/plug-in/session_timer/SessionTimer.h
index 533a7ed4..1ca75f95 100644
--- a/core/plug-in/session_timer/SessionTimer.h
+++ b/core/plug-in/session_timer/SessionTimer.h
@@ -49,20 +49,20 @@ class AmTimeoutEvent;
 /** \brief Factory of the session timer event handler */
 class SessionTimerFactory: public AmSessionEventHandlerFactory
 {
-  bool checkSessionExpires(const AmSipRequest& req);
+  bool checkSessionExpires(const AmSipRequest& req, AmConfigReader& cfg);
 
  public:
   SessionTimerFactory(const string& name)
     : AmSessionEventHandlerFactory(name) {}
 
   int onLoad();
-  bool onInvite(const AmSipRequest&);
+  bool onInvite(const AmSipRequest& req, AmConfigReader& cfg);
 
   AmSessionEventHandler* getHandler(AmSession* s);
 };
 
 /** \brief config for the session timer */
-class AmSessionTimerConfig 
+class AmSessionTimerConfig
 {
 
   /** Session Timer: enable? */
diff --git a/core/plug-in/uac_auth/UACAuth.cpp b/core/plug-in/uac_auth/UACAuth.cpp
index 16664efa..a98d55e7 100644
--- a/core/plug-in/uac_auth/UACAuth.cpp
+++ b/core/plug-in/uac_auth/UACAuth.cpp
@@ -74,9 +74,9 @@ int UACAuthFactory::onLoad()
   return 0;
 }
 
-bool UACAuthFactory::onInvite(const AmSipRequest& req)
+bool UACAuthFactory::onInvite(const AmSipRequest& req, AmConfigReader& conf)
 {
-  return false;
+  return true;
 }
 
 AmSessionEventHandler* UACAuthFactory::getHandler(AmSession* s)
diff --git a/core/plug-in/uac_auth/UACAuth.h b/core/plug-in/uac_auth/UACAuth.h
index 01f20d1d..93af41d5 100644
--- a/core/plug-in/uac_auth/UACAuth.h
+++ b/core/plug-in/uac_auth/UACAuth.h
@@ -72,7 +72,7 @@ class UACAuthFactory
 
   // SessionEventHandler API
   AmSessionEventHandler* getHandler(AmSession* s);
-  bool onInvite(const AmSipRequest&);
+  bool onInvite(const AmSipRequest& req, AmConfigReader& conf);
 
   static UACAuthFactory* instance();
   AmDynInvoke* getInstance() { return instance(); }
diff --git a/doc/Readme.echo.txt b/doc/Readme.echo.txt
index 7e0980ce..df2941e3 100644
--- a/doc/Readme.echo.txt
+++ b/doc/Readme.echo.txt
@@ -12,3 +12,6 @@ star key pressed on the phone switches between adaptive playout
 buffer and fifo playout buffer. Thus the effect of the adaptive 
 playout buffering can easily be verified.
 
+The echo application is also a test application for RFC4028 SIP
+Session Timers. For this, enable_session_timer=yes needs to be
+set in echo.conf