diff --git a/modules/network/SMSTCPIP/api_lib.c b/modules/network/SMSTCPIP/api_lib.c
index 22010ee6c..1299b72d2 100644
--- a/modules/network/SMSTCPIP/api_lib.c
+++ b/modules/network/SMSTCPIP/api_lib.c
@@ -38,6 +38,7 @@
 #include "lwip/api_msg.h"
 #include "lwip/memp.h"
 
+#include <intrman.h>
 #include <thsemap.h>
 #include <sysclib.h>
 
@@ -72,17 +73,22 @@ void netbuf_delete(struct netbuf *buf)
 
 } /* end netbuf_delete */
 
-void netbuf_ref(struct netbuf *buf, void *dataptr, u16_t size)
+err_t netbuf_ref(struct netbuf *buf, void *dataptr, u16_t size)
 {
 
     if (buf->p)
         pbuf_free(buf->p);
 
     buf->p = pbuf_alloc(PBUF_TRANSPORT, 0, PBUF_REF);
+    if (buf->p == NULL) {
+        buf->ptr = NULL;
+        return ERR_MEM;
+    }
     buf->p->payload = dataptr;
     buf->p->len = buf->p->tot_len = size;
     buf->ptr = buf->p;
 
+    return ERR_OK;
 } /* end netbuf_ref */
 
 void netbuf_copy_partial(
@@ -434,7 +440,7 @@ netconn_recv(struct netconn *conn)
 
         if (p != NULL) {
             len = p->tot_len;
-            conn->recv_avail -= len;
+            SYS_ARCH_DEC(conn->recv_avail, len);
         } else
             len = 0;
 
@@ -474,7 +480,7 @@ netconn_recv(struct netconn *conn)
         memp_free(MEMP_API_MSG, msg);
     } else {
         sys_mbox_fetch(conn->recvmbox, (void **)&buf);
-        conn->recv_avail -= buf->p->tot_len;
+        SYS_ARCH_DEC(conn->recv_avail, buf->p->tot_len);
         /* Register event with callback */
         if (conn->callback)
             (*conn->callback)(conn, NETCONN_EVT_RCVMINUS, buf->p->tot_len);
@@ -512,7 +518,7 @@ err_t netconn_send(struct netconn *conn, struct netbuf *buf)
 err_t netconn_write(struct netconn *conn, void *dataptr, u16_t size, u8_t copy)
 {
     struct api_msg *msg;
-    u16_t len;
+    u16_t len, sndbuf;
 
     if (conn == NULL) {
         return ERR_VAL;
@@ -535,16 +541,16 @@ err_t netconn_write(struct netconn *conn, void *dataptr, u16_t size, u8_t copy)
         msg->msg.msg.w.copy = copy;
 
         if (conn->type == NETCONN_TCP) {
-            if (tcp_sndbuf(conn->pcb.tcp) == 0) {
+            while ((sndbuf = tcp_sndbuf(conn->pcb.tcp)) == 0) {
                 sys_sem_wait(conn->sem);
                 if (conn->err != ERR_OK) {
                     goto ret;
                 }
             }
-            if (size > tcp_sndbuf(conn->pcb.tcp)) {
+            if (size > sndbuf) {
                 /* We cannot send more than one send buffer's worth of data at a
      time. */
-                len = tcp_sndbuf(conn->pcb.tcp);
+                len = sndbuf;
             } else {
                 len = size;
             }
diff --git a/modules/network/SMSTCPIP/api_msg.c b/modules/network/SMSTCPIP/api_msg.c
index cb5be00b7..c0712d60e 100644
--- a/modules/network/SMSTCPIP/api_msg.c
+++ b/modules/network/SMSTCPIP/api_msg.c
@@ -37,6 +37,7 @@
 #include "lwip/sys.h"
 #include "lwip/tcpip.h"
 
+#include <intrman.h>
 #include <thsemap.h>
 
 #if LWIP_RAW
@@ -61,7 +62,7 @@ recv_raw(void *arg, struct raw_pcb *pcb, struct pbuf *p,
         buf->fromaddr = addr;
         buf->fromport = pcb->protocol;
 
-        conn->recv_avail += p->tot_len;
+        SYS_ARCH_INC(conn->recv_avail, p->tot_len);
         /* Register event with callback */
         if (conn->callback)
             (*conn->callback)(conn, NETCONN_EVT_RCVPLUS, p->tot_len);
@@ -97,7 +98,7 @@ recv_udp(void *arg, struct udp_pcb *pcb, struct pbuf *p,
             buf->fromport = port;
         }
 
-        conn->recv_avail += p->tot_len;
+        SYS_ARCH_INC(conn->recv_avail, p->tot_len);
         /* Register event with callback */
         if (conn->callback)
             (*conn->callback)(conn, NETCONN_EVT_RCVPLUS, p->tot_len);
@@ -123,7 +124,7 @@ static err_t recv_tcp(void *arg, struct tcp_pcb *pcb, struct pbuf *p, err_t err)
 
             if (p) {
                 len = p->tot_len;
-                conn->recv_avail += len;
+                SYS_ARCH_INC(conn->recv_avail, len);
             } else
                 len = 0;
 
@@ -283,7 +284,7 @@ do_newconn(struct api_msg_msg *msg)
         /* This "new" connection already has a PCB allocated. */
         /* Is this an error condition? Should it be deleted? 
       We currently just are happy and return. */
-        sys_mbox_post(msg->conn->mbox, NULL);
+        TCPIP_APIMSG_ACK(msg->conn->mbox);
         return;
     }
 
@@ -340,7 +341,7 @@ do_newconn(struct api_msg_msg *msg)
     }
 
 
-    sys_mbox_post(msg->conn->mbox, NULL);
+    TCPIP_APIMSG_ACK(msg->conn->mbox);
 }
 
 
@@ -392,7 +393,7 @@ do_delconn(struct api_msg_msg *msg)
     }
 
     if (msg->conn->mbox != SYS_MBOX_NULL) {
-        sys_mbox_post(msg->conn->mbox, NULL);
+        TCPIP_APIMSG_ACK(msg->conn->mbox);
     }
 }
 
@@ -455,7 +456,7 @@ do_bind(struct api_msg_msg *msg)
         default:
             break;
     }
-    sys_mbox_post(msg->conn->mbox, NULL);
+    TCPIP_APIMSG_ACK(msg->conn->mbox);
 }
 #if LWIP_TCP
 
@@ -495,7 +496,7 @@ do_connect(struct api_msg_msg *msg)
                 msg->conn->pcb.udp = udp_new();
                 if (msg->conn->pcb.udp == NULL) {
                     msg->conn->err = ERR_MEM;
-                    sys_mbox_post(msg->conn->mbox, NULL);
+                    TCPIP_APIMSG_ACK(msg->conn->mbox);
                     return;
                 }
                 udp_setflags(msg->conn->pcb.udp, UDP_FLAGS_UDPLITE);
@@ -505,7 +506,7 @@ do_connect(struct api_msg_msg *msg)
                 msg->conn->pcb.udp = udp_new();
                 if (msg->conn->pcb.udp == NULL) {
                     msg->conn->err = ERR_MEM;
-                    sys_mbox_post(msg->conn->mbox, NULL);
+                    TCPIP_APIMSG_ACK(msg->conn->mbox);
                     return;
                 }
                 udp_setflags(msg->conn->pcb.udp, UDP_FLAGS_NOCHKSUM);
@@ -515,7 +516,7 @@ do_connect(struct api_msg_msg *msg)
                 msg->conn->pcb.udp = udp_new();
                 if (msg->conn->pcb.udp == NULL) {
                     msg->conn->err = ERR_MEM;
-                    sys_mbox_post(msg->conn->mbox, NULL);
+                    TCPIP_APIMSG_ACK(msg->conn->mbox);
                     return;
                 }
                 udp_recv(msg->conn->pcb.udp, recv_udp, msg->conn);
@@ -526,7 +527,7 @@ do_connect(struct api_msg_msg *msg)
                 msg->conn->pcb.tcp = tcp_new();
                 if (msg->conn->pcb.tcp == NULL) {
                     msg->conn->err = ERR_MEM;
-                    sys_mbox_post(msg->conn->mbox, NULL);
+                    TCPIP_APIMSG_ACK(msg->conn->mbox);
                     return;
                 }
 #endif
@@ -538,7 +539,7 @@ do_connect(struct api_msg_msg *msg)
 #if LWIP_RAW
         case NETCONN_RAW:
             raw_connect(msg->conn->pcb.raw, msg->msg.bc.ipaddr);
-            sys_mbox_post(msg->conn->mbox, NULL);
+            TCPIP_APIMSG_ACK(msg->conn->mbox);
             break;
 #endif
 #if LWIP_UDP
@@ -548,7 +549,7 @@ do_connect(struct api_msg_msg *msg)
         /* FALLTHROUGH */
         case NETCONN_UDP:
             udp_connect(msg->conn->pcb.udp, msg->msg.bc.ipaddr, msg->msg.bc.port);
-            sys_mbox_post(msg->conn->mbox, NULL);
+            TCPIP_APIMSG_ACK(msg->conn->mbox);
             break;
 #endif
 #if LWIP_TCP
@@ -589,7 +590,7 @@ do_disconnect(struct api_msg_msg *msg)
         default:
             break;
     }
-    sys_mbox_post(msg->conn->mbox, NULL);
+    TCPIP_APIMSG_ACK(msg->conn->mbox);
 }
 
 
@@ -633,7 +634,7 @@ do_listen(struct api_msg_msg *msg)
                 break;
         }
     }
-    sys_mbox_post(msg->conn->mbox, NULL);
+    TCPIP_APIMSG_ACK(msg->conn->mbox);
 }
 
 static void
@@ -688,7 +689,7 @@ do_send(struct api_msg_msg *msg)
                 break;
         }
     }
-    sys_mbox_post(msg->conn->mbox, NULL);
+    TCPIP_APIMSG_ACK(msg->conn->mbox);
 }
 
 static void do_recv(struct api_msg_msg *msg)
@@ -733,7 +734,7 @@ do_write(struct api_msg_msg *msg)
    segments when new outgoing data arrives from the user if any
    previously transmitted data on the connection remains
    unacknowledged. */
-                if (err == ERR_OK && (msg->conn->pcb.tcp->unacked == NULL || (msg->conn->pcb.tcp->flags & TF_NODELAY))) {
+                if (err == ERR_OK && (msg->conn->pcb.tcp->unacked == NULL || (msg->conn->pcb.tcp->flags & TF_NODELAY) || ( msg->conn->pcb.tcp->snd_queuelen) > 1)) {
                     tcp_output(msg->conn->pcb.tcp);
                 }
                 msg->conn->err = err;
@@ -747,7 +748,7 @@ do_write(struct api_msg_msg *msg)
                 break;
         }
     }
-    sys_mbox_post(msg->conn->mbox, NULL);
+    TCPIP_APIMSG_ACK(msg->conn->mbox);
 }
 
 static void
@@ -782,7 +783,7 @@ do_close(struct api_msg_msg *msg)
                 break;
         }
     }
-    sys_mbox_post(msg->conn->mbox, NULL);
+    TCPIP_APIMSG_ACK(msg->conn->mbox);
 }
 
 api_msg_decode __decode__[API_MSG_MAX] = {
diff --git a/modules/network/SMSTCPIP/include/lwip/api.h b/modules/network/SMSTCPIP/include/lwip/api.h
index a450e282b..32f654dd0 100644
--- a/modules/network/SMSTCPIP/include/lwip/api.h
+++ b/modules/network/SMSTCPIP/include/lwip/api.h
@@ -76,7 +76,6 @@ struct netbuf
     struct pbuf *p, *ptr;
     struct ip_addr *fromaddr;
     u16_t fromport;
-    err_t err;
 };
 
 struct netconn
@@ -95,7 +94,7 @@ struct netconn
     sys_mbox_t acceptmbox;
     sys_sem_t sem;
     int socket;
-    u16_t recv_avail;
+    s16_t recv_avail;
     void (*callback)(struct netconn *, enum netconn_evt, u16_t len);
 };
 
@@ -104,7 +103,7 @@ struct netbuf *netbuf_new(void);
 void netbuf_delete(struct netbuf *buf);
 void *netbuf_alloc(struct netbuf *buf, u16_t size);
 void netbuf_free(struct netbuf *buf);
-void netbuf_ref(struct netbuf *buf,
+err_t netbuf_ref(struct netbuf *buf,
                 void *dataptr, u16_t size);
 void netbuf_chain(struct netbuf *head,
                   struct netbuf *tail);
diff --git a/modules/network/SMSTCPIP/include/lwip/sys.h b/modules/network/SMSTCPIP/include/lwip/sys.h
index d48efc4bd..6899c8fe1 100644
--- a/modules/network/SMSTCPIP/include/lwip/sys.h
+++ b/modules/network/SMSTCPIP/include/lwip/sys.h
@@ -151,4 +151,40 @@ unsigned long sys_now(void);
 
 #endif /* SYS_ARCH_PROTECT */
 
+#ifndef SYS_ARCH_INC
+#define SYS_ARCH_INC(var, val) do { \
+                                SYS_ARCH_DECL_PROTECT(old_level); \
+                                SYS_ARCH_PROTECT(old_level); \
+                                var += val; \
+                                SYS_ARCH_UNPROTECT(old_level); \
+                              } while(0)
+#endif /* SYS_ARCH_INC */
+
+#ifndef SYS_ARCH_DEC
+#define SYS_ARCH_DEC(var, val) do { \
+                                SYS_ARCH_DECL_PROTECT(old_level); \
+                                SYS_ARCH_PROTECT(old_level); \
+                                var -= val; \
+                                SYS_ARCH_UNPROTECT(old_level); \
+                              } while(0)
+#endif /* SYS_ARCH_DEC */
+
+#ifndef SYS_ARCH_GET
+#define SYS_ARCH_GET(var, ret) do { \
+                                SYS_ARCH_DECL_PROTECT(old_level); \
+                                SYS_ARCH_PROTECT(old_level); \
+                                ret = var; \
+                                SYS_ARCH_UNPROTECT(old_level); \
+                              } while(0)
+#endif /* SYS_ARCH_GET */
+
+#ifndef SYS_ARCH_SET
+#define SYS_ARCH_SET(var, val) do { \
+                                SYS_ARCH_DECL_PROTECT(old_level); \
+                                SYS_ARCH_PROTECT(old_level); \
+                                var = val; \
+                                SYS_ARCH_UNPROTECT(old_level); \
+                              } while(0)
+#endif /* SYS_ARCH_SET */
+
 #endif /* __LWIP_SYS_H__ */
diff --git a/modules/network/SMSTCPIP/include/lwip/tcpip.h b/modules/network/SMSTCPIP/include/lwip/tcpip.h
index ff25dcbe8..4f93c2cf4 100644
--- a/modules/network/SMSTCPIP/include/lwip/tcpip.h
+++ b/modules/network/SMSTCPIP/include/lwip/tcpip.h
@@ -43,9 +43,11 @@ extern sys_sem_t lock_tcpip_core;
 #define LOCK_TCPIP_CORE()     sys_sem_wait(lock_tcpip_core)
 /** Unlock lwIP core mutex (needs @ref LWIP_TCPIP_CORE_LOCKING 1) */
 #define UNLOCK_TCPIP_CORE()   sys_sem_signal(lock_tcpip_core)
+#define TCPIP_APIMSG_ACK(m)
 #else /* LWIP_TCPIP_CORE_LOCKING */
 #define LOCK_TCPIP_CORE()
 #define UNLOCK_TCPIP_CORE()
+#define TCPIP_APIMSG_ACK(m)   sys_mbox_post(m, NULL)
 #endif /* LWIP_TCPIP_CORE_LOCKING */
 
 void tcpip_init(void (*tcpip_init_done)(void *), void *arg);
diff --git a/modules/network/SMSTCPIP/sockets.c b/modules/network/SMSTCPIP/sockets.c
index 36012cc40..d75c32c3f 100644
--- a/modules/network/SMSTCPIP/sockets.c
+++ b/modules/network/SMSTCPIP/sockets.c
@@ -39,6 +39,7 @@
 
 #include "lwip/sockets.h"
 
+#include <intrman.h>
 #include <thsemap.h>
 #include <sysclib.h>
 #include <errno.h>
@@ -52,7 +53,7 @@ struct lwip_socket
     struct netconn *conn;
     struct netbuf *lastdata;
     u16_t lastoffset;
-    u16_t rcvevent;
+    s16_t rcvevent;
     u16_t sendevent;
     u16_t flags;
     int err;
@@ -363,7 +364,7 @@ int lwip_recvfrom(int s, void *header, int index, void *payload, int plen, unsig
         buf = sock->lastdata;
     } else {
         /* If this is non-blocking call, then check first */
-        if (((flags & MSG_DONTWAIT) || (sock->flags & O_NONBLOCK)) && !sock->rcvevent) {
+        if (((flags & MSG_DONTWAIT) || (sock->flags & O_NONBLOCK)) && (sock->rcvevent <= 0)) {
             LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_recvfrom(%d): returning EWOULDBLOCK\n", s));
             sock_set_errno(sock, EWOULDBLOCK);
             return -1;
@@ -497,10 +498,10 @@ int lwip_send(int s, void *data, int size, unsigned int flags)
 
             /* make the buffer point to the data that should
        be sent */
-            netbuf_ref(buf, data, size);
-
-            /* send the data */
-            err = netconn_send(sock->conn, buf);
+            if ((err = netbuf_ref(buf, data, size))==ERR_OK) {
+                /* send the data */
+                err = netconn_send(sock->conn, buf);
+            }
 
             /* deallocated the buffer */
             netbuf_delete(buf);
@@ -627,7 +628,7 @@ lwip_selscan(int maxfdp1, fd_set *readset, fd_set *writeset, fd_set *exceptset)
         if (FD_ISSET(i, readset)) {
             /* See if netconn of this socket is ready for read */
             p_sock = get_socket(i);
-            if (p_sock && (p_sock->lastdata || p_sock->rcvevent)) {
+            if (p_sock && (p_sock->lastdata || (p_sock->rcvevent > 0))) {
                 FD_SET(i, &lreadset);
                 LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_selscan: fd=%d ready for reading\n", i));
                 nready++;
@@ -744,7 +745,7 @@ int lwip_select(int maxfdp1, fd_set *readset, fd_set *writeset, fd_set *exceptse
         sys_sem_signal(selectsem);
 
         sys_sem_free(select_cb.sem);
-        if (i == 0) /* Timeout */
+        if (i == SYS_ARCH_TIMEOUT) /* Timeout */
         {
             if (readset)
                 FD_ZERO(readset);
@@ -852,7 +853,7 @@ event_callback(struct netconn *conn, enum netconn_evt evt, u16_t len)
             if (scb->sem_signalled == 0) {
                 /* Test this select call for our socket */
                 if (scb->readset && FD_ISSET(s, scb->readset))
-                    if (sock->rcvevent)
+                    if (sock->rcvevent > 0)
                         break;
                 if (scb->writeset && FD_ISSET(s, scb->writeset))
                     if (sock->sendevent)
@@ -861,8 +862,8 @@ event_callback(struct netconn *conn, enum netconn_evt evt, u16_t len)
         }
         if (scb) {
             scb->sem_signalled = 1;
-            sys_sem_signal(selectsem);
             sys_sem_signal(scb->sem);
+            sys_sem_signal(selectsem);
         } else {
             sys_sem_signal(selectsem);
             break;
@@ -1348,7 +1349,7 @@ int lwip_ioctl(int s, long cmd, void *argp)
                 return -1;
             }
 
-            *((u16_t *)argp) = sock->conn->recv_avail;
+            SYS_ARCH_GET(sock->conn->recv_avail, *((u16_t*)argp));
 
             LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_ioctl(%d, FIONREAD, %p) = %u\n", s, argp, *((u16_t *)argp)));
             sock_set_errno(sock, 0);