diff --git a/lib/gtp/xact.h b/lib/gtp/xact.h
index 455e8372a..3a3bfd3c0 100644
--- a/lib/gtp/xact.h
+++ b/lib/gtp/xact.h
@@ -96,16 +96,16 @@ typedef struct ogs_gtp_xact_s {
 #define OGS_GTP_MODIFY_QOS_UPDATE ((uint64_t)1<<1)
     uint64_t        update_flags;
 
-#define OGS_GTP_DELETE_NO_ACTION                              1
-#define OGS_GTP_DELETE_SEND_AUTHENTICATION_REQUEST            2
-#define OGS_GTP_DELETE_SEND_DETACH_ACCEPT                     3
-#define OGS_GTP_DELETE_SEND_DEACTIVATE_BEARER_CONTEXT_REQUEST 4
-#define OGS_GTP_DELETE_SEND_UE_CONTEXT_RELEASE_COMMAND        5
-#define OGS_GTP_DELETE_SEND_S1_REMOVE_AND_UNLINK              6
-#define OGS_GTP_DELETE_HANDLE_PDN_CONNECTIVITY_REQUEST        7
-#define OGS_GTP_DELETE_UE_CONTEXT_REMOVE                      8
-#define OGS_GTP_DELETE_UE_CONTEXT_COMPLETE_REMOVE             9
-#define OGS_GTP_DELETE_IN_PATH_SWITCH_REQUEST                10
+#define OGS_GTP_DELETE_NO_ACTION                                1
+#define OGS_GTP_DELETE_SEND_AUTHENTICATION_REQUEST              2
+#define OGS_GTP_DELETE_SEND_DETACH_ACCEPT                       3
+#define OGS_GTP_DELETE_SEND_DEACTIVATE_BEARER_CONTEXT_REQUEST   4
+#define OGS_GTP_DELETE_SEND_RELEASE_WITH_UE_CONTEXT_REMOVE      5
+#define OGS_GTP_DELETE_SEND_RELEASE_WITH_S1_REMOVE_AND_UNLINK   6
+#define OGS_GTP_DELETE_HANDLE_PDN_CONNECTIVITY_REQUEST          7
+#define OGS_GTP_DELETE_UE_CONTEXT_REMOVE_PARTIAL                8
+#define OGS_GTP_DELETE_UE_CONTEXT_REMOVE_ALL                    9
+#define OGS_GTP_DELETE_IN_PATH_SWITCH_REQUEST                   10
 
     int             delete_action;
 
diff --git a/src/mme/mme-context.c b/src/mme/mme-context.c
index 5368f1f2e..ef41b5edc 100644
--- a/src/mme/mme-context.c
+++ b/src/mme/mme-context.c
@@ -2698,7 +2698,7 @@ int mme_ue_set_imsi(mme_ue_t *mme_ue, char *imsi_bcd)
             if (SESSION_CONTEXT_IS_AVAILABLE(old_mme_ue)) {
                 ogs_warn("[%s] Trigger OLD Session Remove", mme_ue->imsi_bcd);
                 mme_gtp_send_delete_all_sessions(old_mme_ue,
-                        OGS_GTP_DELETE_UE_CONTEXT_REMOVE);
+                        OGS_GTP_DELETE_UE_CONTEXT_REMOVE_PARTIAL);
             }
         }
     }
diff --git a/src/mme/mme-path.c b/src/mme/mme-path.c
index 211465a22..c1c88e3ad 100644
--- a/src/mme/mme-path.c
+++ b/src/mme/mme-path.c
@@ -41,9 +41,16 @@ void mme_send_delete_session_or_detach(mme_ue_t *mme_ue)
 
     /* MME Explicit Detach, ie: O&M Procedures */
     case MME_DETACH_TYPE_MME_EXPLICIT:
+        ogs_fatal("Not Implemented : MME_DETACH_TYPE_MME_EXPLICIT");
+        ogs_assert_if_reached();
         break;
 
-    /* HSS Explicit Detach, ie: Subscription Withdrawl Cancel Location */
+    /* HSS Explicit Detach, ie: Subscription Withdrawl Cancel Location
+     *
+     * TS23.401 - V16.10.0
+     * Ch 5.3.8 Detach procedure
+     * Ch 5.3.8.4 HSS-initiated Detach procedure
+     */
     case MME_DETACH_TYPE_HSS_EXPLICIT:
         ogs_debug("Explicit HSS Detach");
         if (SESSION_CONTEXT_IS_AVAILABLE(mme_ue)) {
@@ -53,24 +60,39 @@ void mme_send_delete_session_or_detach(mme_ue_t *mme_ue)
 
     /* MME Implicit Detach, ie: Lost Communication */
     case MME_DETACH_TYPE_MME_IMPLICIT:
+        ogs_fatal("Not Implemented : MME_DETACH_TYPE_MME_IMPLICIT");
+        ogs_assert_if_reached();
         break;
 
-    /* HSS Implicit Detach, ie: MME-UPDATE-PROCEDURE */
+    /* HSS Implicit Detach, ie: MME-UPDATE-PROCEDURE
+     *
+     * TS23.401 - V16.10.0
+     * Ch 5.3.2 Attach procedure
+     * Ch 5.3.2.1 E-UTRAN Initial Attach
+     *
+     * 9. The HSS sends Cancel Location (IMSI, Cancellation Type)
+     * to the old MME. The old MME acknowledges with Cancel Location Ack (IMSI)
+     * and removes the MM and bearer contexts. If the ULR-Flags indicates
+     * "Initial-Attach-Indicator" and the HSS has the SGSN registration,
+     * then the HSS sends Cancel Location (IMSI, Cancellation Type)
+     * to the old SGSN. The Cancellation Type indicates the old MME/SGSN
+     * to release the old Serving GW resource.
+     */
     case MME_DETACH_TYPE_HSS_IMPLICIT:
         ogs_debug("Implicit HSS Detach");
         if (SESSION_CONTEXT_IS_AVAILABLE(mme_ue)) {
             if (ECM_IDLE(mme_ue)) {
                 mme_gtp_send_delete_all_sessions(mme_ue,
-                    OGS_GTP_DELETE_UE_CONTEXT_COMPLETE_REMOVE);
+                    OGS_GTP_DELETE_UE_CONTEXT_REMOVE_ALL);
             } else {
                 mme_gtp_send_delete_all_sessions(mme_ue,
-                    OGS_GTP_DELETE_SEND_UE_CONTEXT_RELEASE_COMMAND);
+                    OGS_GTP_DELETE_SEND_RELEASE_WITH_UE_CONTEXT_REMOVE);
             }
         }
         break;
 
     default:
-        ogs_fatal("    Invalid OGS_NAS_EPS TYPE[%d]", mme_ue->nas_eps.type);
+        ogs_fatal("    Invalid OGS_NAS_EPS TYPE[%d]", mme_ue->detach_type);
         ogs_assert_if_reached();
     }
 }
@@ -81,7 +103,7 @@ void mme_send_delete_session_or_mme_ue_context_release(mme_ue_t *mme_ue)
 
     if (SESSION_CONTEXT_IS_AVAILABLE(mme_ue)) {
         mme_gtp_send_delete_all_sessions(mme_ue,
-                OGS_GTP_DELETE_SEND_UE_CONTEXT_RELEASE_COMMAND);
+                OGS_GTP_DELETE_SEND_RELEASE_WITH_UE_CONTEXT_REMOVE);
     } else {
         enb_ue_t *enb_ue = enb_ue_cycle(mme_ue->enb_ue);
         if (enb_ue) {
diff --git a/src/mme/mme-s11-handler.c b/src/mme/mme-s11-handler.c
index 6d2497d2d..1869d0f7a 100644
--- a/src/mme/mme-s11-handler.c
+++ b/src/mme/mme-s11-handler.c
@@ -592,7 +592,7 @@ void mme_s11_handle_delete_session_response(
         CLEAR_SGW_S1U_PATH(sess);
         return;
 
-    } else if (action == OGS_GTP_DELETE_SEND_UE_CONTEXT_RELEASE_COMMAND) {
+    } else if (action == OGS_GTP_DELETE_SEND_RELEASE_WITH_UE_CONTEXT_REMOVE) {
         if (mme_sess_count(mme_ue) == 1) /* Last Session */ {
             enb_ue_t *enb_ue = NULL;
 
@@ -606,7 +606,8 @@ void mme_s11_handle_delete_session_response(
                 ogs_error("ENB-S1 Context has already been removed");
         }
 
-    } else if (action == OGS_GTP_DELETE_SEND_S1_REMOVE_AND_UNLINK) {
+    } else if (action ==
+            OGS_GTP_DELETE_SEND_RELEASE_WITH_S1_REMOVE_AND_UNLINK) {
         if (mme_sess_count(mme_ue) == 1) /* Last Session */ {
             enb_ue_t *enb_ue = NULL;
 
@@ -633,13 +634,13 @@ void mme_s11_handle_delete_session_response(
             }
         }
 
-    } else if (action == OGS_GTP_DELETE_UE_CONTEXT_REMOVE) {
+    } else if (action == OGS_GTP_DELETE_UE_CONTEXT_REMOVE_PARTIAL) {
 
         /* Remove MME-UE Context with Session Context since IMSI duplicated */
         mme_ue_remove(mme_ue);
         return;
 
-    } else if (action == OGS_GTP_DELETE_UE_CONTEXT_COMPLETE_REMOVE) {
+    } else if (action == OGS_GTP_DELETE_UE_CONTEXT_REMOVE_ALL) {
         /* Remove MME-UE Context and hash after Implicit Detach */
         mme_ue_hash_remove(mme_ue);
         mme_ue_remove(mme_ue);
diff --git a/src/mme/mme-s6a-handler.c b/src/mme/mme-s6a-handler.c
index 78183f882..ba6ae6789 100644
--- a/src/mme/mme-s6a-handler.c
+++ b/src/mme/mme-s6a-handler.c
@@ -202,12 +202,7 @@ void mme_s6a_handle_clr(mme_ue_t *mme_ue, ogs_diam_s6a_message_t *s6a_message)
 
     ogs_debug("    OGS_NAS_EPS TYPE[%d]", mme_ue->nas_eps.type);
 
-    if (OGS_FSM_CHECK(&mme_ue->sm, emm_state_de_registered)) {
-        /* Remove all trace of subscriber even when detached. */
-        mme_ue_hash_remove(mme_ue);
-        mme_ue_remove(mme_ue);
-        return;
-    } else if (clr_message->cancellation_type ==
+    if (clr_message->cancellation_type ==
             OGS_DIAM_S6A_CT_MME_UPDATE_PROCEDURE) {
         mme_ue->detach_type = MME_DETACH_TYPE_HSS_IMPLICIT;
         if (MME_P_TMSI_IS_AVAILABLE(mme_ue)) {