diff --git a/components/lwip/api/api_lib.c b/components/lwip/api/api_lib.c
index fd2ca16772..e93a2a96ab 100755
--- a/components/lwip/api/api_lib.c
+++ b/components/lwip/api/api_lib.c
@@ -407,9 +407,9 @@ netconn_accept(struct netconn *conn, struct netconn **new_conn)
 #if TCP_LISTEN_BACKLOG
   /* Let the stack know that we have accepted the connection. */
   API_MSG_VAR_ALLOC_DONTFAIL(msg);
-  API_MSG_VAR_REF(msg).msg.conn = conn;
-  /* don't care for the return value of lwip_netconn_do_recv */
-  TCPIP_APIMSG_NOERR(&API_MSG_VAR_REF(msg), lwip_netconn_do_recv);
+  API_MSG_VAR_REF(msg).msg.conn = newconn;
+  /* don't care for the return value of lwip_netconn_do_accepted */
+  TCPIP_APIMSG_NOERR(&API_MSG_VAR_REF(msg), lwip_netconn_do_accepted);
   API_MSG_VAR_FREE(msg);
 #endif /* TCP_LISTEN_BACKLOG */
 
diff --git a/components/lwip/api/api_msg.c b/components/lwip/api/api_msg.c
index 2bc02c91e6..443adb192e 100755
--- a/components/lwip/api/api_msg.c
+++ b/components/lwip/api/api_msg.c
@@ -536,6 +536,9 @@ accept_function(void *arg, struct tcp_pcb *newpcb, err_t err)
      to the application thread */
   newconn->last_err = err;
 
+  /* handle backlog counter */
+  tcp_backlog_delayed(newpcb);
+
   if (sys_mbox_trypost(&conn->acceptmbox, newconn) != ERR_OK) {
     ESP_STATS_DROP_INC(esp.acceptmbox_post_fail);
     /* When returning != ERR_OK, the pcb is aborted in tcp_process(),
@@ -1503,24 +1506,38 @@ lwip_netconn_do_recv(void *m)
   msg->err = ERR_OK;
   if (msg->conn->pcb.tcp != NULL) {
     if (NETCONNTYPE_GROUP(msg->conn->type) == NETCONN_TCP) {
-#if TCP_LISTEN_BACKLOG
-      if (msg->conn->pcb.tcp->state == LISTEN) {
-        tcp_accepted(msg->conn->pcb.tcp);
-      } else
-#endif /* TCP_LISTEN_BACKLOG */
-      {
-        u32_t remaining = msg->msg.r.len;
-        do {
-          u16_t recved = (remaining > 0xffff) ? 0xffff : (u16_t)remaining;
-          tcp_recved(msg->conn->pcb.tcp, recved);
-          remaining -= recved;
-        } while (remaining != 0);
-      }
+      u32_t remaining = msg->msg.r.len;
+      do {
+        u16_t recved = (remaining > 0xffff) ? 0xffff : (u16_t)remaining;
+        tcp_recved(msg->conn->pcb.tcp, recved);
+        remaining -= recved;
+      } while (remaining != 0);
     }
   }
   TCPIP_APIMSG_ACK(msg);
 }
 
+#if TCP_LISTEN_BACKLOG
+/** Indicate that a TCP pcb has been accepted
+ * Called from netconn_accept
+ *
+ * @param m the api_msg pointing to the connection
+ */
+void
+lwip_netconn_do_accepted(void *m)
+{
+  struct api_msg_msg *msg = (struct api_msg_msg *)m;
+
+  msg->err = ERR_OK;
+  if (msg->conn->pcb.tcp != NULL) {
+    if (NETCONNTYPE_GROUP(msg->conn->type) == NETCONN_TCP) {
+      tcp_backlog_accepted(msg->conn->pcb.tcp);
+    }
+  }
+  TCPIP_APIMSG_ACK(msg);
+}
+#endif /* TCP_LISTEN_BACKLOG */
+
 /**
  * See if more data needs to be written from a previous call to netconn_write.
  * Called initially from lwip_netconn_do_write. If the first call can't send all data
diff --git a/components/lwip/core/tcp.c b/components/lwip/core/tcp.c
index 96e35d0dfc..596f3d4343 100755
--- a/components/lwip/core/tcp.c
+++ b/components/lwip/core/tcp.c
@@ -149,6 +149,22 @@ tcp_tmr(void)
   }
 }
 
+#if LWIP_CALLBACK_API || TCP_LISTEN_BACKLOG
+/** Called when a listen pcb is closed. Iterates one pcb list and removes the
+ * closed listener pcb from pcb->listener if matching.
+ */
+static void
+tcp_remove_listener(struct tcp_pcb *list, struct tcp_pcb_listen *lpcb)
+{
+  struct tcp_pcb *pcb;
+  for (pcb = list; pcb != NULL; pcb = pcb->next) {
+    if (pcb->listener == lpcb) {
+      pcb->listener = NULL;
+    }
+  }
+}
+#endif
+
 void
 tcp_set_fin_wait_1(struct tcp_pcb *pcb)
 {
@@ -158,6 +174,70 @@ tcp_set_fin_wait_1(struct tcp_pcb *pcb)
 #endif
 }
 
+/** Called when a listen pcb is closed. Iterates all pcb lists and removes the
+ * closed listener pcb from pcb->listener if matching.
+ */
+static void
+tcp_listen_closed(struct tcp_pcb *pcb)
+{
+#if LWIP_CALLBACK_API || TCP_LISTEN_BACKLOG
+  size_t i;
+  LWIP_ASSERT("pcb != NULL", pcb != NULL);
+  LWIP_ASSERT("pcb->state == LISTEN", pcb->state == LISTEN);
+  for (i = 1; i < LWIP_ARRAYSIZE(tcp_pcb_lists); i++) {
+    tcp_remove_listener(*tcp_pcb_lists[i], (struct tcp_pcb_listen *)pcb);
+  }
+#endif
+  LWIP_UNUSED_ARG(pcb);
+}
+
+#if TCP_LISTEN_BACKLOG
+/** @ingroup tcp_raw
+ * Delay accepting a connection in respect to the listen backlog:
+ * the number of outstanding connections is increased until
+ * tcp_backlog_accepted() is called.
+ *
+ * ATTENTION: the caller is responsible for calling tcp_backlog_accepted()
+ * or else the backlog feature will get out of sync!
+ *
+ * @param pcb the connection pcb which is not fully accepted yet
+ */
+void
+tcp_backlog_delayed(struct tcp_pcb *pcb)
+{
+  LWIP_ASSERT("pcb != NULL", pcb != NULL);
+  if ((pcb->flags & TF_BACKLOGPEND) == 0) { 
+    if (pcb->listener != NULL) {
+      pcb->listener->accepts_pending++;
+      LWIP_ASSERT("accepts_pending != 0", pcb->listener->accepts_pending != 0);
+      pcb->flags |= TF_BACKLOGPEND;
+    }    
+  }
+}
+
+/** @ingroup tcp_raw
+ * A delayed-accept a connection is accepted (or closed/aborted): decreases
+ * the number of outstanding connections after calling tcp_backlog_delayed().
+ *
+ * ATTENTION: the caller is responsible for calling tcp_backlog_accepted()
+ * or else the backlog feature will get out of sync!
+ *
+ * @param pcb the connection pcb which is now fully accepted (or closed/aborted)
+ */
+void
+tcp_backlog_accepted(struct tcp_pcb *pcb)
+{
+  LWIP_ASSERT("pcb != NULL", pcb != NULL);
+  if ((pcb->flags & TF_BACKLOGPEND) != 0) {
+    if (pcb->listener != NULL) {
+      LWIP_ASSERT("accepts_pending != 0", pcb->listener->accepts_pending != 0);
+      pcb->listener->accepts_pending--;
+      pcb->flags &= ~TF_BACKLOGPEND;
+    }
+  }
+}
+#endif /* TCP_LISTEN_BACKLOG */
+
 /**
  * Closes the TX side of a connection held by the PCB.
  * For tcp_close(), a RST is sent if the application didn't receive all data
@@ -227,6 +307,7 @@ tcp_close_shutdown(struct tcp_pcb *pcb, u8_t rst_on_unacked_data)
     break;
   case LISTEN:
     err = ERR_OK;
+    tcp_listen_closed(pcb);
     tcp_pcb_remove(&tcp_listen_pcbs.pcbs, pcb);
     memp_free(MEMP_TCP_PCB_LISTEN, pcb);
     pcb = NULL;
@@ -241,6 +322,7 @@ tcp_close_shutdown(struct tcp_pcb *pcb, u8_t rst_on_unacked_data)
   case SYN_RCVD:
     err = tcp_send_fin(pcb);
     if (err == ERR_OK) {
+      tcp_backlog_accepted(pcb);
       MIB2_STATS_INC(mib2.tcpattemptfails);
       tcp_set_fin_wait_1(pcb);
     }
@@ -408,6 +490,7 @@ tcp_abandon(struct tcp_pcb *pcb, int reset)
       pcb->ooseq = NULL;
     }
 #endif /* TCP_QUEUE_OOSEQ */
+    tcp_backlog_accepted(pcb);
     if (send_rst) {
       LWIP_DEBUGF(TCP_RST_DEBUG, ("tcp_abandon: sending RST\n"));
       tcp_rst(seqno, ackno, &pcb->local_ip, &pcb->remote_ip, local_port, pcb->remote_port);
@@ -1747,39 +1830,7 @@ tcp_pcb_purge(struct tcp_pcb *pcb)
 
     LWIP_DEBUGF(TCP_DEBUG, ("tcp_pcb_purge\n"));
 
-#if TCP_LISTEN_BACKLOG
-    if (pcb->state == SYN_RCVD) {
-      /* Need to find the corresponding listen_pcb and decrease its accepts_pending */
-      struct tcp_pcb_listen *lpcb;
-
-      /*
-       * The official LWIP will assert the system if tcp_listen_pcbs.listen_pcbs is NULL, it's a bug.
-       *
-       * Considering following scenario:
-       * 1. On TCP server side, one of TCP pcb is in SYNC_RECV state and is waiting for TCP ACK from TCP client.
-       * 2. The TCP server is closed by application, which causes the tcp_listen_pcbs.listen_pcbs to become NULL.
-       * 3. When SYNC_RECV state timeout (20s by default), tcp_pcb_purge() is called in tcp_slowtmr(), it asserts
-       *    the system.
-       * This is a normal scenario, should NOT assert the system. So just remove it.
-       * 
-       */
-      if (tcp_listen_pcbs.listen_pcbs) {
-        for (lpcb = tcp_listen_pcbs.listen_pcbs; lpcb != NULL; lpcb = lpcb->next) {
-          if ((lpcb->local_port == pcb->local_port) &&
-              (IP_IS_V6_VAL(pcb->local_ip) == IP_IS_V6_VAL(lpcb->local_ip)) &&
-              (ip_addr_isany(&lpcb->local_ip) ||
-               ip_addr_cmp(&pcb->local_ip, &lpcb->local_ip))) {
-              /* port and address of the listen pcb match the timed-out pcb */
-              LWIP_ASSERT("tcp_pcb_purge: listen pcb does not have accepts pending",
-                lpcb->accepts_pending > 0);
-              lpcb->accepts_pending--;
-              break;
-          }
-        }
-      }
-    }
-#endif /* TCP_LISTEN_BACKLOG */
-
+    tcp_backlog_accepted(pcb);
 
     if (pcb->refused_data != NULL) {
       LWIP_DEBUGF(TCP_DEBUG, ("tcp_pcb_purge: data left on ->refused_data\n"));
diff --git a/components/lwip/core/tcp_in.c b/components/lwip/core/tcp_in.c
index 23dc5ae1c1..a6f2946299 100755
--- a/components/lwip/core/tcp_in.c
+++ b/components/lwip/core/tcp_in.c
@@ -568,6 +568,7 @@ tcp_listen_input(struct tcp_pcb_listen *pcb)
     }
 #if TCP_LISTEN_BACKLOG
     pcb->accepts_pending++;
+    npcb->flags |= TF_BACKLOGPEND;
 #endif /* TCP_LISTEN_BACKLOG */
     /* Set up the new PCB. */
     ip_addr_copy(npcb->local_ip, *ip_current_dest_addr());
@@ -582,6 +583,10 @@ tcp_listen_input(struct tcp_pcb_listen *pcb)
 #if LWIP_CALLBACK_API
     npcb->accept = pcb->accept;
 #endif /* LWIP_CALLBACK_API */
+
+#if LWIP_CALLBACK_API || TCP_LISTEN_BACKLOG
+    npcb->listener = pcb;
+#endif
     /* inherit socket options */
     npcb->so_options = pcb->so_options & SOF_INHERITED;
     /* Register the new PCB so that we can begin receiving segments
@@ -785,11 +790,19 @@ tcp_process(struct tcp_pcb *pcb)
       if (TCP_SEQ_BETWEEN(ackno, pcb->lastack+1, pcb->snd_nxt)) {
         pcb->state = ESTABLISHED;
         LWIP_DEBUGF(TCP_DEBUG, ("TCP connection established %"U16_F" -> %"U16_F".\n", inseg.tcphdr->src, inseg.tcphdr->dest));
+#if LWIP_CALLBACK_API || TCP_LISTEN_BACKLOG
 #if LWIP_CALLBACK_API
         LWIP_ASSERT("pcb->accept != NULL", pcb->accept != NULL);
 #endif
-        /* Call the accept function. */
-        TCP_EVENT_ACCEPT(pcb, ERR_OK, err);
+
+        if (pcb->listener == NULL) {
+          err = ERR_VAL;
+        } else {
+#endif
+          tcp_backlog_accepted(pcb);
+          /* Call the accept function. */
+          TCP_EVENT_ACCEPT(pcb, ERR_OK, err);
+        }
         if (err != ERR_OK) {
           /* If the accept function returns with an error, we abort
            * the connection. */
diff --git a/components/lwip/include/lwip/lwip/priv/api_msg.h b/components/lwip/include/lwip/lwip/priv/api_msg.h
index 02d191a53c..a786b2ee44 100755
--- a/components/lwip/include/lwip/lwip/priv/api_msg.h
+++ b/components/lwip/include/lwip/lwip/priv/api_msg.h
@@ -232,6 +232,9 @@ void lwip_netconn_do_disconnect      (void *m);
 void lwip_netconn_do_listen          (void *m);
 void lwip_netconn_do_send            (void *m);
 void lwip_netconn_do_recv            (void *m);
+#if TCP_LISTEN_BACKLOG
+void lwip_netconn_do_accepted        (void *m);
+#endif
 void lwip_netconn_do_write           (void *m);
 void lwip_netconn_do_getaddr         (void *m);
 void lwip_netconn_do_close           (void *m);
diff --git a/components/lwip/include/lwip/lwip/tcp.h b/components/lwip/include/lwip/lwip/tcp.h
index f7a46b2e83..269b4e3d39 100755
--- a/components/lwip/include/lwip/lwip/tcp.h
+++ b/components/lwip/include/lwip/lwip/tcp.h
@@ -138,7 +138,7 @@ typedef u16_t tcpflags_t;
 #define TCPWND16(x)             (x)
 #define TCP_WND_MAX(pcb)        TCP_WND(pcb)
 typedef u16_t tcpwnd_size_t;
-typedef u8_t tcpflags_t;
+typedef u16_t tcpflags_t;
 #endif
 
 enum tcp_state {
@@ -203,6 +203,9 @@ struct tcp_pcb {
 #define TF_NAGLEMEMERR 0x80U   /* nagle enabled, memerr, try to output to prevent delayed ACK to happen */
 #if LWIP_WND_SCALE
 #define TF_WND_SCALE   0x0100U /* Window Scale option enabled */
+#endif
+#if TCP_LISTEN_BACKLOG
+#define TF_BACKLOGPEND 0x0200U /* If this is set, a connection pcb has increased the backlog on its listener */
 #endif
 
   /* the rest of the fields are in host byte order
@@ -311,6 +314,10 @@ struct tcp_pcb {
   u8_t rcv_scale;
 #endif
 
+#if TCP_LISTEN_BACKLOG
+  struct tcp_pcb_listen *listener;
+#endif
+
 #if ESP_STATS_TCP
 #define ESP_STATS_TCP_ARRAY_SIZE 20
   u16_t retry_cnt[TCP_MAXRTX];
@@ -383,16 +390,17 @@ void             tcp_err     (struct tcp_pcb *pcb, tcp_err_fn err);
 #define          tcp_nagle_disabled(pcb)  (((pcb)->flags & TF_NODELAY) != 0)
 
 #if TCP_LISTEN_BACKLOG
-#define          tcp_accepted(pcb) do { \
-  LWIP_ASSERT("pcb->state == LISTEN (called for wrong pcb?)", pcb->state == LISTEN); \
-  (((struct tcp_pcb_listen *)(pcb))->accepts_pending--); } while(0)
 #define          tcp_backlog_set(pcb, new_backlog) do { \
   LWIP_ASSERT("pcb->state == LISTEN (called for wrong pcb?)", (pcb)->state == LISTEN); \
   ((struct tcp_pcb_listen *)(pcb))->backlog = ((new_backlog) ? (new_backlog) : 1); } while(0)
+void             tcp_backlog_delayed(struct tcp_pcb* pcb);
+void             tcp_backlog_accepted(struct tcp_pcb* pcb);
 #else  /* TCP_LISTEN_BACKLOG */
-#define          tcp_accepted(pcb) LWIP_ASSERT("pcb->state == LISTEN (called for wrong pcb?)", \
-                                               (pcb)->state == LISTEN)
+#define          tcp_backlog_set(pcb, new_backlog)
+#define          tcp_backlog_delayed(pcb)
+#define          tcp_backlog_accepted(pcb)
 #endif /* TCP_LISTEN_BACKLOG */
+#define          tcp_accepted(pcb) do { LWIP_UNUSED_ARG(pcb); } while(0) /* compatibility define, not needed any more */
 
 void             tcp_recved  (struct tcp_pcb *pcb, u16_t len);
 err_t            tcp_bind    (struct tcp_pcb *pcb, const ip_addr_t *ipaddr,