diff --git a/config/config_defaults.go b/config/config_defaults.go
index 956ae1073..07297ee60 100755
--- a/config/config_defaults.go
+++ b/config/config_defaults.go
@@ -418,7 +418,8 @@ const CGRATES_CFG_JSON = `
 	"session_indexes": [],					// index sessions based on these fields for GetActiveSessions API
 	"client_protocol": 1.0,					// version of protocol to use when acting as JSON-PRC client <"0","1.0">
 	"channel_sync_interval": "0",			// sync channels to detect stale sessions (0 to disable)
-	"terminate_attempts": 5					// attempts to get the session before terminating it
+	"terminate_attempts": 5,				// attempts to get the session before terminating it
+	"alterable_fields": [],					// the session fields that can be updated
 },
 
 
diff --git a/config/config_it_test.go b/config/config_it_test.go
index 4983daab2..03afc8403 100644
--- a/config/config_it_test.go
+++ b/config/config_it_test.go
@@ -394,6 +394,7 @@ func testCGRConfigReloadSessionS(t *testing.T) {
 		SessionIndexes:    utils.NewStringMap(),
 		ClientProtocol:    1,
 		TerminateAttempts: 5,
+		AlterableFields:   utils.NewStringSet([]string{}),
 	}
 	if !reflect.DeepEqual(expAttr, cfg.SessionSCfg()) {
 		t.Errorf("Expected %s , received: %s ", utils.ToJSON(expAttr), utils.ToJSON(cfg.SessionSCfg()))
@@ -953,6 +954,7 @@ func testCGRConfigReloadConfigFromJSONSessionS(t *testing.T) {
 		SessionIndexes:    utils.NewStringMap(),
 		ClientProtocol:    1,
 		TerminateAttempts: 5,
+		AlterableFields:   utils.NewStringSet([]string{}),
 	}
 	if !reflect.DeepEqual(expAttr, cfg.SessionSCfg()) {
 		t.Errorf("Expected %s , received: %s ", utils.ToJSON(expAttr), utils.ToJSON(cfg.SessionSCfg()))
@@ -993,6 +995,7 @@ func testCGRConfigReloadAll(t *testing.T) {
 		SessionIndexes:    utils.NewStringMap(),
 		ClientProtocol:    1,
 		TerminateAttempts: 5,
+		AlterableFields:   utils.NewStringSet([]string{}),
 	}
 	if !reflect.DeepEqual(expAttr, cfg.SessionSCfg()) {
 		t.Errorf("Expected %s , received: %s ", utils.ToJSON(expAttr), utils.ToJSON(cfg.SessionSCfg()))
diff --git a/config/config_json_test.go b/config/config_json_test.go
index 46bdfc90d..5a6e59128 100755
--- a/config/config_json_test.go
+++ b/config/config_json_test.go
@@ -648,6 +648,7 @@ func TestSmgJsonCfg(t *testing.T) {
 		Client_protocol:       utils.Float64Pointer(1.0),
 		Channel_sync_interval: utils.StringPointer("0"),
 		Terminate_attempts:    utils.IntPointer(5),
+		Alterable_fields:      &[]string{},
 	}
 	if cfg, err := dfCgrJsonCfg.SessionSJsonCfg(); err != nil {
 		t.Error(err)
diff --git a/config/config_test.go b/config/config_test.go
index 64ecf6832..17cbce757 100755
--- a/config/config_test.go
+++ b/config/config_test.go
@@ -646,6 +646,7 @@ func TestCgrCfgJSONDefaultsSMGenericCfg(t *testing.T) {
 		ClientProtocol:      1.0,
 		ChannelSyncInterval: 0,
 		TerminateAttempts:   5,
+		AlterableFields:     utils.NewStringSet([]string{}),
 	}
 	if !reflect.DeepEqual(eSessionSCfg, cgrCfg.sessionSCfg) {
 		t.Errorf("expecting: %s, received: %s",
diff --git a/config/configsanity.go b/config/configsanity.go
index 19d61a50e..575ecf9b5 100644
--- a/config/configsanity.go
+++ b/config/configsanity.go
@@ -215,6 +215,11 @@ func (cfg *CGRConfig) checkConfigSanity() error {
 		if cfg.cacheCfg[utils.CacheClosedSessions].Limit == 0 {
 			return fmt.Errorf("<%s> %s needs to be != 0, received: %d", utils.CacheS, utils.CacheClosedSessions, cfg.cacheCfg[utils.CacheClosedSessions].Limit)
 		}
+		for alfld := range cfg.sessionSCfg.AlterableFields.Data() {
+			if utils.ProtectedSFlds.Has(alfld) {
+				return fmt.Errorf("<%s> The following protected field can't be altered by session: <%s>", utils.SessionS, alfld)
+			}
+		}
 	}
 
 	// FreeSWITCHAgent checks
diff --git a/config/libconfig_json.go b/config/libconfig_json.go
index 45ce60b29..5072de192 100755
--- a/config/libconfig_json.go
+++ b/config/libconfig_json.go
@@ -235,6 +235,7 @@ type SessionSJsonCfg struct {
 	Client_protocol       *float64
 	Channel_sync_interval *string
 	Terminate_attempts    *int
+	Alterable_fields      *[]string
 }
 
 // FreeSWITCHAgent config section
diff --git a/config/smconfig.go b/config/smconfig.go
index 07fe388d0..f6bb0358b 100644
--- a/config/smconfig.go
+++ b/config/smconfig.go
@@ -87,6 +87,7 @@ type SessionSCfg struct {
 	ClientProtocol      float64
 	ChannelSyncInterval time.Duration
 	TerminateAttempts   int
+	AlterableFields     *utils.StringSet
 }
 
 func (scfg *SessionSCfg) loadFromJsonCfg(jsnCfg *SessionSJsonCfg) (err error) {
@@ -249,6 +250,9 @@ func (scfg *SessionSCfg) loadFromJsonCfg(jsnCfg *SessionSJsonCfg) (err error) {
 	if jsnCfg.Terminate_attempts != nil {
 		scfg.TerminateAttempts = *jsnCfg.Terminate_attempts
 	}
+	if jsnCfg.Alterable_fields != nil {
+		scfg.AlterableFields = utils.NewStringSet(*jsnCfg.Alterable_fields)
+	}
 	return nil
 }
 
diff --git a/data/conf/samples/smg/cgrates.json b/data/conf/samples/smg/cgrates.json
index 16a5680f8..8fd88f43b 100644
--- a/data/conf/samples/smg/cgrates.json
+++ b/data/conf/samples/smg/cgrates.json
@@ -59,6 +59,7 @@
 	"rals_conns": ["*localhost"],
 	"cdrs_conns": ["*localhost"],
 	"chargers_conns": ["*internal"],
+	"alterable_fields": ["Extra1"],			
 },
 
 
diff --git a/data/conf/samples/smg_gob/cgrates.json b/data/conf/samples/smg_gob/cgrates.json
index 79cdfddcc..dc44bdb68 100644
--- a/data/conf/samples/smg_gob/cgrates.json
+++ b/data/conf/samples/smg_gob/cgrates.json
@@ -66,6 +66,7 @@
 	"rals_conns": ["conn1"],
 	"cdrs_conns": ["conn1"],
 	"chargers_conns": ["*internal"],
+	"alterable_fields": ["Extra1"],			
 },
 
 
diff --git a/sessions/libsessions.go b/sessions/libsessions.go
index 019ba4702..b88061048 100644
--- a/sessions/libsessions.go
+++ b/sessions/libsessions.go
@@ -27,13 +27,6 @@ import (
 	"github.com/cgrates/cgrates/utils"
 )
 
-var protectedSFlds = engine.MapEvent{
-	utils.CGRID:      struct{}{},
-	utils.OriginHost: struct{}{},
-	utils.OriginID:   struct{}{},
-	utils.Usage:      struct{}{},
-}
-
 var unratedReqs = engine.MapEvent{
 	utils.META_POSTPAID:      struct{}{},
 	utils.META_PSEUDOPREPAID: struct{}{},
diff --git a/sessions/session.go b/sessions/session.go
index 582f60b25..a5c7c5f03 100644
--- a/sessions/session.go
+++ b/sessions/session.go
@@ -305,3 +305,28 @@ func (sr *SRun) debitReserve(dur time.Duration, lastUsage *time.Duration) (rDur
 	}
 	return
 }
+
+// updateSRuns updates the SRuns event with the alterable fields (is not thread safe)
+func (s *Session) updateSRuns(updEv engine.MapEvent, alterableFields *utils.StringSet) {
+	if alterableFields.Size() == 0 {
+		return
+	}
+	for k, v := range updEv {
+		if !alterableFields.Has(k) {
+			continue
+		}
+		for _, sr := range s.SRuns {
+			sr.Event[k] = v
+		}
+	}
+}
+
+// UpdateSRuns updates the SRuns event with the alterable fields (is thread safe)
+func (s *Session) UpdateSRuns(updEv engine.MapEvent, alterableFields *utils.StringSet) {
+	if alterableFields.Size() == 0 { // do not lock if we can't update any field
+		return
+	}
+	s.Lock()
+	s.updateSRuns(updEv, alterableFields)
+	s.Unlock()
+}
diff --git a/sessions/sessions.go b/sessions/sessions.go
index 86fb262b0..30b4dff4e 100644
--- a/sessions/sessions.go
+++ b/sessions/sessions.go
@@ -1352,11 +1352,12 @@ func (sS *SessionS) updateSession(s *Session, updtEv engine.MapEvent, isMsg bool
 
 		// update fields from new event
 		for k, v := range updtEv {
-			if protectedSFlds.HasField(k) {
+			if utils.ProtectedSFlds.Has(k) {
 				continue
 			}
 			s.EventStart[k] = v // update previoius field with new one
 		}
+		s.updateSRuns(updtEv, sS.cgrCfg.SessionSCfg().AlterableFields)
 		sS.setSTerminator(s) // reset the terminator
 	}
 	//init has no updtEv
@@ -2475,6 +2476,7 @@ func (sS *SessionS) BiRPCv1TerminateSession(clnt rpcclient.ClientConnector,
 			}
 
 		}
+		s.UpdateSRuns(ev, sS.cgrCfg.SessionSCfg().AlterableFields)
 		if err = sS.terminateSession(s,
 			ev.GetDurationPtrIgnoreErrors(utils.Usage),
 			ev.GetDurationPtrIgnoreErrors(utils.LastUsed),
@@ -2584,14 +2586,8 @@ func (sS *SessionS) BiRPCv1ProcessCDR(clnt rpcclient.ClientConnector,
 	}
 
 	// Use previously stored Session to generate CDRs
-	// update stored event with fields out of CDR
-	for k, v := range ev {
-		if protectedSFlds.HasField(k) {
-			continue
-		}
-		s.EventStart[k] = v // update previoius field with new one
-	}
-	// create one CGREvent for each session run plus *raw one
+	s.updateSRuns(ev, sS.cgrCfg.SessionSCfg().AlterableFields)
+	// create one CGREvent for each session run
 	var cgrEvs []*utils.CGREvent
 	if cgrEvs, err = s.asCGREvents(); err != nil {
 		return utils.NewErrServerError(err)
diff --git a/sessions/sessions_data_it_test.go b/sessions/sessions_data_it_test.go
index bf9559c12..fc1b9cbb1 100644
--- a/sessions/sessions_data_it_test.go
+++ b/sessions/sessions_data_it_test.go
@@ -321,6 +321,7 @@ func TestSessionsDataLastUsedMultipleUpdates(t *testing.T) {
 				utils.AnswerTime:  time.Date(2016, time.January, 5, 18, 31, 05, 0, time.UTC),
 				utils.Usage:       "8192", // 8 MB
 				utils.LastUsed:    "7168",
+				"Extra1":          "other",
 			},
 		},
 	}
@@ -344,6 +345,8 @@ func TestSessionsDataLastUsedMultipleUpdates(t *testing.T) {
 	} else if len(aSessions) != 1 ||
 		aSessions[0].Usage != time.Duration(15360) {
 		t.Errorf("wrong active sessions: %v", aSessions[0].Usage)
+	} else if aSessions[0].ExtraFields["Extra1"] != "other" {
+		t.Errorf("Expected: \"other\", received: %v", aSessions[0].ExtraFields["Extra1"])
 	}
 
 	usage = int64(1024)
@@ -366,6 +369,8 @@ func TestSessionsDataLastUsedMultipleUpdates(t *testing.T) {
 				utils.AnswerTime:  time.Date(2016, time.January, 5, 18, 31, 05, 0, time.UTC),
 				utils.Usage:       "1024", // 8 MB
 				utils.LastUsed:    "5120", // 5 MB
+				"Extra1":          "other2",
+				"Extra2":          "other",
 			},
 		},
 	}
@@ -388,6 +393,10 @@ func TestSessionsDataLastUsedMultipleUpdates(t *testing.T) {
 	} else if len(aSessions) != 1 ||
 		aSessions[0].Usage != time.Duration(13312) { // 14MB in used, 2MB extra reserved
 		t.Errorf("wrong active sessions: %+v", aSessions[0].Usage)
+	} else if aSessions[0].ExtraFields["Extra1"] != "other2" {
+		t.Errorf("Expected: \"other2\", received: %v", aSessions[0].ExtraFields["Extra1"])
+	} else if _, has := aSessions[0].ExtraFields["Extra"]; has {
+		t.Errorf("Expected: no Extra2, received: %v", aSessions[0].ExtraFields["Extra2"])
 	}
 
 	usage = int64(1024)
@@ -451,6 +460,7 @@ func TestSessionsDataLastUsedMultipleUpdates(t *testing.T) {
 				utils.SetupTime:   time.Date(2016, time.January, 5, 18, 30, 49, 0, time.UTC),
 				utils.AnswerTime:  time.Date(2016, time.January, 5, 18, 31, 05, 0, time.UTC),
 				utils.LastUsed:    "0", // refund 1024 (extra used) + 1024 (extra reserved)
+				"Extra1":          "done",
 			},
 		},
 	}
@@ -489,6 +499,9 @@ func TestSessionsDataLastUsedMultipleUpdates(t *testing.T) {
 		if cdrs[0].Usage != "13312" {
 			t.Errorf("Unexpected CDR Usage received, cdr: %v %+v ", cdrs[0].Usage, cdrs[0])
 		}
+		if cdrs[0].ExtraFields["Extra1"] != "done" {
+			t.Errorf("Expected: \"done\", received: %v", cdrs[0].ExtraFields["Extra1"])
+		}
 	}
 }
 
diff --git a/utils/consts.go b/utils/consts.go
index f6523d2a3..e7d327d7a 100755
--- a/utils/consts.go
+++ b/utils/consts.go
@@ -131,6 +131,9 @@ var (
 		CacheDispatcherProfiles, CacheDispatcherHosts, CacheResourceFilterIndexes, CacheStatFilterIndexes,
 		CacheThresholdFilterIndexes, CacheSupplierFilterIndexes, CacheAttributeFilterIndexes,
 		CacheChargerFilterIndexes, CacheDispatcherFilterIndexes, CacheLoadIDs, CacheAccounts})
+
+	// ProtectedSFlds are the fields that sessions should not alter
+	ProtectedSFlds = NewStringSet([]string{CGRID, OriginHost, OriginID, Usage})
 )
 
 const (
diff --git a/utils/set.go b/utils/set.go
index bb4ea260d..b7f2a688a 100644
--- a/utils/set.go
+++ b/utils/set.go
@@ -62,3 +62,11 @@ func (s *StringSet) AsSlice() []string {
 func (s *StringSet) Data() map[string]struct{} {
 	return s.data
 }
+
+// Size returns the size of the set
+func (s *StringSet) Size() int {
+	if s == nil || s.data == nil {
+		return 0
+	}
+	return len(s.data)
+}