diff --git a/src/upf/gtp-path.c b/src/upf/gtp-path.c
index d1f8c4873..c859bd6ee 100644
--- a/src/upf/gtp-path.c
+++ b/src/upf/gtp-path.c
@@ -466,6 +466,36 @@ static void _gtpv1_u_recv_cb(short when, ogs_socket_t fd, void *data)
              */
             if (far->dst_if != OGS_PFCP_INTERFACE_ACCESS) {
 
+/*
+ * Discussion #1776 was raised,
+ * but we decided not to allow unspecified addresses
+ * because Open5GS has already sent interface identifiers
+ * in the registgration/attach process.
+ *
+ *
+ * RFC4861
+ * 4.  Message Formats
+ * 4.1.  Router Solicitation Message Format
+ * IP Fields:
+ *    Source Address
+ *                  An IP address assigned to the sending interface, or
+ *                  the unspecified address if no address is assigned
+ *                  to the sending interface.
+ *
+ * 6.1.  Message Validation
+ * 6.1.1.  Validation of Router Solicitation Messages
+ *  Hosts MUST silently discard any received Router Solicitation
+ *  Messages.
+ *
+ *  A router MUST silently discard any received Router Solicitation
+ *  messages that do not satisfy all of the following validity checks:
+ *
+ *  ..
+ *  ..
+ *
+ *  - If the IP source address is the unspecified address, there is no
+ *    source link-layer address option in the message.
+ */
                 if (IN6_IS_ADDR_LINKLOCAL((struct in6_addr *)src_addr) &&
                     src_addr[2] == sess->ipv6->addr[2] &&
                     src_addr[3] == sess->ipv6->addr[3]) {
diff --git a/tests/attach/simple-test.c b/tests/attach/simple-test.c
index cf30174c4..d86462aff 100644
--- a/tests/attach/simple-test.c
+++ b/tests/attach/simple-test.c
@@ -209,6 +209,47 @@ static void test1_func(abts_case *tc, void *data)
     ABTS_PTR_NOTNULL(tc, recvbuf);
     testgtpu_recv(test_ue, recvbuf);
 
+/*
+ * Discussion #1776 was raised,
+ * but we decided not to allow unspecified addresses
+ * because Open5GS has already sent interface identifiers
+ * in the registgration/attach process.
+ *
+ *
+ * RFC4861
+ * 4.  Message Formats
+ * 4.1.  Router Solicitation Message Format
+ * IP Fields:
+ *    Source Address
+ *                  An IP address assigned to the sending interface, or
+ *                  the unspecified address if no address is assigned
+ *                  to the sending interface.
+ *
+ * 6.1.  Message Validation
+ * 6.1.1.  Validation of Router Solicitation Messages
+ *  Hosts MUST silently discard any received Router Solicitation
+ *  Messages.
+ *
+ *  A router MUST silently discard any received Router Solicitation
+ *  messages that do not satisfy all of the following validity checks:
+ *
+ *  ..
+ *  ..
+ *
+ *  - If the IP source address is the unspecified address, there is no
+ *    source link-layer address option in the message.
+ */
+#if 0
+    /* Send GTP-U Router Solicitation with an unspecified source address */
+    rv = test_gtpu_send_slacc_rs_with_unspecified_source_address(gtpu, bearer);
+    ABTS_INT_EQUAL(tc, OGS_OK, rv);
+
+    /* Receive GTP-U Router Solicitation */
+    recvbuf = test_gtpu_read(gtpu);
+    ABTS_PTR_NOTNULL(tc, recvbuf);
+    testgtpu_recv(test_ue, recvbuf);
+#endif
+
     /* Send GTP-U ICMP Packet */
     rv = test_gtpu_send_ping(gtpu, bearer, TEST_PING_IPV4);
     ABTS_INT_EQUAL(tc, OGS_OK, rv);
diff --git a/tests/common/gtpu.c b/tests/common/gtpu.c
index bb2264eb6..b692a43de 100644
--- a/tests/common/gtpu.c
+++ b/tests/common/gtpu.c
@@ -409,6 +409,68 @@ int test_gtpu_send_slacc_rs(ogs_socknode_t *node, test_bearer_t *bearer)
     return test_gtpu_send(node, bearer, &gtp_hdesc, &ext_hdesc, pkbuf);
 }
 
+int test_gtpu_send_slacc_rs_with_unspecified_source_address(
+        ogs_socknode_t *node, test_bearer_t *bearer)
+{
+    test_sess_t *sess = NULL;
+
+    ogs_gtp2_header_t gtp_hdesc;
+    ogs_gtp2_extension_header_t ext_hdesc;
+
+    ogs_pkbuf_t *pkbuf = NULL;
+    struct ip6_hdr *ip6_h = NULL;
+    uint8_t *src_addr = NULL;
+
+    const char *payload =
+        "60000000"
+        "00103afffe800000 0000000074ee25ff fee4b579ff020000 0000000000000000"
+        "000000028500da95 00000000010176ee 25e4b579";
+    unsigned char tmp[OGS_MAX_SDU_LEN];
+    int payload_len = 56;
+
+    ogs_assert(bearer);
+    sess = bearer->sess;
+    ogs_assert(sess);
+
+    pkbuf = ogs_pkbuf_alloc(
+            NULL, 200 /* enough for ICMP; use smaller buffer */);
+    ogs_assert(pkbuf);
+    ogs_pkbuf_reserve(pkbuf, OGS_GTPV1U_5GC_HEADER_LEN);
+    ogs_pkbuf_put(pkbuf, 200-OGS_GTPV1U_5GC_HEADER_LEN);
+    memset(pkbuf->data, 0, pkbuf->len);
+
+    OGS_HEX(payload, strlen(payload), tmp);
+    memcpy(pkbuf->data, tmp, payload_len);
+
+    ogs_pkbuf_trim(pkbuf, payload_len);
+
+    memset(&gtp_hdesc, 0, sizeof(gtp_hdesc));
+    memset(&ext_hdesc, 0, sizeof(ext_hdesc));
+
+    gtp_hdesc.type = OGS_GTPU_MSGTYPE_GPDU;
+    gtp_hdesc.flags = OGS_GTPU_FLAGS_S;
+
+    if (bearer->qfi) {
+        gtp_hdesc.teid = sess->upf_n3_teid;
+
+/*
+ * Discussion #1506
+ * Router Soliciation should include QFI in 5G Core
+ */
+        ext_hdesc.qos_flow_identifier = bearer->qfi;
+
+    } else if (bearer->ebi) {
+        gtp_hdesc.teid = bearer->sgw_s1u_teid;
+
+    } else {
+        ogs_fatal("No QFI[%d] and EBI[%d]", bearer->qfi, bearer->ebi);
+        ogs_assert_if_reached();
+    }
+
+    return test_gtpu_send(node, bearer, &gtp_hdesc, &ext_hdesc, pkbuf);
+}
+
+
 int test_gtpu_send_error_indication(
         ogs_socknode_t *node, test_bearer_t *bearer)
 {
diff --git a/tests/common/gtpu.h b/tests/common/gtpu.h
index 512264f1b..6db7b95b9 100644
--- a/tests/common/gtpu.h
+++ b/tests/common/gtpu.h
@@ -40,6 +40,8 @@ int test_gtpu_send(
 int test_gtpu_send_ping(
         ogs_socknode_t *node, test_bearer_t *bearer, const char *dst_ip);
 int test_gtpu_send_slacc_rs(ogs_socknode_t *node, test_bearer_t *bearer);
+int test_gtpu_send_slacc_rs_with_unspecified_source_address(
+        ogs_socknode_t *node, test_bearer_t *bearer);
 int test_gtpu_send_error_indication(
         ogs_socknode_t *node, test_bearer_t *bearer);
 int test_gtpu_send_indirect_data_forwarding(