diff --git a/CHANGELOG b/CHANGELOG
index 8c5377a1..b8e5a51b 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -19,6 +19,11 @@ HISTORY
 
   ++ New features:
 
+  2007-11-21 Simon Goldschmidt
+  * tcp.h, opt.h, tcp.c, tcp_in.c: implemented calculating the effective send-mss
+    based on the MTU of the netif used to send. Enabled by default. Disable by
+    setting LWIP_CALCULATE_EFF_SEND_MSS to 0. This fixes bug #21535.
+
   2007-11-19 Frédéric Bernon
   * api_msg.c, dns.h, dns.c: Implement DNS_DOES_NAME_CHECK option (check if name
     received match the name query), implement DNS_USES_STATIC_BUF (the place where
diff --git a/src/core/tcp.c b/src/core/tcp.c
index 422f0eb3..34e460f2 100644
--- a/src/core/tcp.c
+++ b/src/core/tcp.c
@@ -493,6 +493,9 @@ tcp_connect(struct tcp_pcb *pcb, struct ip_addr *ipaddr, u16_t port,
   pcb->snd_wnd = TCP_WND;
   /* The send MSS is updated when an MSS option is received. */
   pcb->mss = (TCP_MSS > 536) ? 536 : TCP_MSS;
+#if LWIP_CALCULATE_EFF_SEND_MSS
+  pcb->mss = tcp_eff_send_mss(pcb->mss, ipaddr);
+#endif /* LWIP_CALCULATE_EFF_SEND_MSS */
   pcb->cwnd = 1;
   pcb->ssthresh = pcb->mss * 10;
   pcb->state = SYN_SENT;
@@ -1151,6 +1154,30 @@ tcp_next_iss(void)
   return iss;
 }
 
+#if LWIP_CALCULATE_EFF_SEND_MSS
+/**
+ * Calcluates the effective send mss that can be used for a specific IP address
+ * by using ip_route to determin the netif used to send to the address and
+ * calculating the minimum of TCP_MSS and that netif's mtu (if set).
+ */
+u16_t
+tcp_eff_send_mss(u16_t sendmss, struct ip_addr *addr)
+{
+  u16_t mss_s;
+  struct netif *outif;
+
+  outif = ip_route(addr);
+  if ((outif != NULL) && (outif->mtu != 0)) {
+    mss_s = outif->mtu - IP_HLEN - TCP_HLEN;
+    /* RFC 1122, chap 4.2.2.6:
+     * Eff.snd.MSS = min(SendMSS+20, MMS_S) - TCPhdrsize - IPoptionsize
+     * but we only send options with SYN and that is never filled with data! */
+    sendmss = LWIP_MIN(sendmss, mss_s);
+  }
+  return sendmss;
+}
+#endif /* LWIP_CALCULATE_EFF_SEND_MSS */
+
 #if TCP_DEBUG || TCP_INPUT_DEBUG || TCP_OUTPUT_DEBUG
 /**
  * Print a tcp header for debugging purposes.
diff --git a/src/core/tcp_in.c b/src/core/tcp_in.c
index 7dbe1f93..fac1f8cf 100644
--- a/src/core/tcp_in.c
+++ b/src/core/tcp_in.c
@@ -412,6 +412,9 @@ tcp_listen_input(struct tcp_pcb_listen *pcb)
 
     /* Parse any options in the SYN. */
     tcp_parseopt(npcb);
+#if LWIP_CALCULATE_EFF_SEND_MSS
+    npcb->mss = tcp_eff_send_mss(npcb->mss, &(npcb->remote_ip));
+#endif /* LWIP_CALCULATE_EFF_SEND_MSS */
 
     snmp_inc_tcppassiveopens();
 
@@ -519,6 +522,9 @@ tcp_process(struct tcp_pcb *pcb)
       /* Parse any options in the SYNACK before using pcb->mss since that
        * can be changed by the received options! */
       tcp_parseopt(pcb);
+#if LWIP_CALCULATE_EFF_SEND_MSS
+      pcb->mss = tcp_eff_send_mss(pcb->mss, &(pcb->remote_ip));
+#endif /* LWIP_CALCULATE_EFF_SEND_MSS */
 
       /* Set ssthresh again after changing pcb->mss (already set in tcp_connect
        * but for the default value of pcb->mss) */
diff --git a/src/include/lwip/opt.h b/src/include/lwip/opt.h
index 2c9579ad..c79aec9f 100644
--- a/src/include/lwip/opt.h
+++ b/src/include/lwip/opt.h
@@ -636,11 +636,27 @@
 /**
  * TCP_MSS: TCP Maximum segment size. (default is 128, a *very*
  * conservative default.)
+ * For the receive side, this MSS is advertised to the remote side
+ * when opening a connection. For the transmit size, this MSS sets
+ * an upper limit on the MSS advertised by the remote host.
  */
 #ifndef TCP_MSS
 #define TCP_MSS                         128
 #endif
 
+/**
+ * LWIP_CALCULATE_EFF_SEND_MSS: "The maximum size of a segment that TCP really
+ * sends, the 'effective send MSS,' MUST be the smaller of the send MSS (which
+ * reflects the available reassembly buffer size at the remote host) and the
+ * largest size permitted by the IP layer" (RFC 1122)
+ * Setting this to 1 enables code that checks TCP_MSS against the MTU of the
+ * netif used for a connection and limits the MSS it would be too big otherwise.
+ */
+#ifndef LWIP_CALCULATE_EFF_SEND_MSS
+#define LWIP_CALCULATE_EFF_SEND_MSS     1
+#endif
+
+
 /**
  * TCP_SND_BUF: TCP sender buffer space (bytes). 
  */
diff --git a/src/include/lwip/tcp.h b/src/include/lwip/tcp.h
index ecf5f778..6130d196 100644
--- a/src/include/lwip/tcp.h
+++ b/src/include/lwip/tcp.h
@@ -508,6 +508,10 @@ u32_t tcp_next_iss(void);
 
 void tcp_keepalive(struct tcp_pcb *pcb);
 
+#if LWIP_CALCULATE_EFF_SEND_MSS
+u16_t tcp_eff_send_mss(u16_t sendmss, struct ip_addr *addr);
+#endif /* LWIP_CALCULATE_EFF_SEND_MSS*/
+
 extern struct tcp_pcb *tcp_input_pcb;
 extern u32_t tcp_ticks;