diff --git a/CHANGELOG b/CHANGELOG
index 9e7e1e57..3a2b17d1 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -19,6 +19,10 @@ HISTORY
 
   ++ New features:
 
+  2010-01-27: Simon Goldschmidt
+  * opt.h, tcp.h, init.c, api_msg.c: Added TCP_SNDQUEUELOWAT corresponding
+    to TCP_SNDLOWAT and added tcp_sndqueuelen() - this fixes bug #28605
+
   2010-01-26: Simon Goldschmidt
   * snmp: Use memp pools for snmp instead of the heap; added 4 new pools.
 
diff --git a/src/api/api_msg.c b/src/api/api_msg.c
index cddd6f93..c796769d 100644
--- a/src/api/api_msg.c
+++ b/src/api/api_msg.c
@@ -292,7 +292,10 @@ sent_tcp(void *arg, struct tcp_pcb *pcb, u16_t len)
   }
 
   if (conn) {
-    if ((conn->pcb.tcp != NULL) && (tcp_sndbuf(conn->pcb.tcp) > TCP_SNDLOWAT)) {
+    /* If the queued byte- or pbuf-count drops below the configured low-water limit,
+       let select mark this pcb as writable again. */
+    if ((conn->pcb.tcp != NULL) && (tcp_sndbuf(conn->pcb.tcp) > TCP_SNDLOWAT) &&
+      (tcp_sndqueuelen(conn->pcb.tcp) < TCP_SNDQUEUELOWAT)) {
       API_EVENT(conn, NETCONN_EVT_SENDPLUS, len);
     }
   }
@@ -1101,7 +1104,10 @@ do_writemore(struct netconn *conn)
       conn->state = NETCONN_NONE;
     }
     err = tcp_output_nagle(conn->pcb.tcp);
-    if ((err == ERR_OK) && (tcp_sndbuf(conn->pcb.tcp) <= TCP_SNDLOWAT)) {
+    /* If the queued byte- or pbuf-count exceeds the configured low-water limit,
+       let select mark this pcb as non-writable. */
+    if ((err == ERR_OK) && (tcp_sndbuf(conn->pcb.tcp) <= TCP_SNDLOWAT) ||
+        (tcp_sndqueuelen(conn->pcb.tcp) >= TCP_SNDQUEUELOWAT)) {
       API_EVENT(conn, NETCONN_EVT_SENDMINUS, len);
     }
   } else if (err == ERR_MEM) {
diff --git a/src/core/init.c b/src/core/init.c
index 944ffb54..2af563cf 100644
--- a/src/core/init.c
+++ b/src/core/init.c
@@ -219,8 +219,10 @@ lwip_sanity_check(void)
     LWIP_PLATFORM_DIAG(("lwip_sanity_check: WARNING: TCP_SND_BUF must be at least as much as (2 * TCP_MSS) for things to work smoothly\n"));
   if (TCP_SND_QUEUELEN < (2 * (TCP_SND_BUF/TCP_MSS)))
     LWIP_PLATFORM_DIAG(("lwip_sanity_check: WARNING: TCP_SND_QUEUELEN must be at least as much as (2 * TCP_SND_BUF/TCP_MSS) for things to work\n"));
-  if (TCP_SNDLOWAT > TCP_SND_BUF)
-    LWIP_PLATFORM_DIAG(("lwip_sanity_check: WARNING: TCP_SNDLOWAT must be less than or equal to TCP_SND_BUF.\n"));
+  if (TCP_SNDLOWAT >= TCP_SND_BUF)
+    LWIP_PLATFORM_DIAG(("lwip_sanity_check: WARNING: TCP_SNDLOWAT must be less than TCP_SND_BUF.\n"));
+  if (TCP_SNDQUEUELOWAT >= TCP_SND_QUEUELEN)
+    LWIP_PLATFORM_DIAG(("lwip_sanity_check: WARNING: TCP_SNDQUEUELOWAT must be less than TCP_SND_QUEUELEN.\n"));
   if (TCP_WND > (PBUF_POOL_SIZE*PBUF_POOL_BUFSIZE))
     LWIP_PLATFORM_DIAG(("lwip_sanity_check: WARNING: TCP_WND is larger than space provided by PBUF_POOL_SIZE*PBUF_POOL_BUFSIZE\n"));
   if (TCP_WND < TCP_MSS)
diff --git a/src/include/lwip/opt.h b/src/include/lwip/opt.h
index cb65964c..73dbc269 100644
--- a/src/include/lwip/opt.h
+++ b/src/include/lwip/opt.h
@@ -901,14 +901,23 @@
 #endif
 
 /**
- * TCP_SNDLOWAT: TCP writable space (bytes). This must be less than or equal
- * to TCP_SND_BUF. It is the amount of space which must be available in the
- * TCP snd_buf for select to return writable.
+ * TCP_SNDLOWAT: TCP writable space (bytes). This must be less than
+ * TCP_SND_BUF. It is the amount of space which must be available in the
+ * TCP snd_buf for select to return writable (combined with TCP_SNDQUEUELOWAT).
  */
 #ifndef TCP_SNDLOWAT
 #define TCP_SNDLOWAT                    ((TCP_SND_BUF)/2)
 #endif
 
+/**
+ * TCP_SNDQUEUELOWAT: TCP writable bufs (pbuf count). This must be grater
+ * than TCP_SND_QUEUELEN. If the number of pbufs queued on a pcb drops below
+ * this number, select returns writable (combined with TCP_SNDLOWAT).
+ */
+#ifndef TCP_SNDQUEUELOWAT
+#define TCP_SNDQUEUELOWAT               ((TCP_SND_QUEUELEN)/2)
+#endif
+
 /**
  * TCP_LISTEN_BACKLOG: Enable the backlog option for tcp listen pcb.
  */
diff --git a/src/include/lwip/tcp.h b/src/include/lwip/tcp.h
index dc392ac9..1a9b2629 100644
--- a/src/include/lwip/tcp.h
+++ b/src/include/lwip/tcp.h
@@ -142,11 +142,12 @@ void             tcp_sent    (struct tcp_pcb *pcb, tcp_sent_fn sent);
 void             tcp_poll    (struct tcp_pcb *pcb, tcp_poll_fn poll, u8_t interval);
 void             tcp_err     (struct tcp_pcb *pcb, tcp_err_fn err);
 
-#define          tcp_mss(pcb)      ((pcb)->mss)
-#define          tcp_sndbuf(pcb)   ((pcb)->snd_buf)
-#define          tcp_nagle_disable(pcb)  ((pcb)->flags |= TF_NODELAY)
-#define          tcp_nagle_enable(pcb) ((pcb)->flags &= ~TF_NODELAY)
-#define          tcp_nagle_disabled(pcb) (((pcb)->flags & TF_NODELAY) != 0)
+#define          tcp_mss(pcb)             ((pcb)->mss)
+#define          tcp_sndbuf(pcb)          ((pcb)->snd_buf)
+#define          tcp_sndqueuelen(pcb)     ((pcb)->snd_queuelen)
+#define          tcp_nagle_disable(pcb)   ((pcb)->flags |= TF_NODELAY)
+#define          tcp_nagle_enable(pcb)    ((pcb)->flags &= ~TF_NODELAY)
+#define          tcp_nagle_disabled(pcb)  (((pcb)->flags & TF_NODELAY) != 0)
 
 #if TCP_LISTEN_BACKLOG
 #define          tcp_accepted(pcb) (((struct tcp_pcb_listen *)(pcb))->accepts_pending--)