diff --git a/src/core/tcp.c b/src/core/tcp.c
index f20b44a9..8d4af978 100644
--- a/src/core/tcp.c
+++ b/src/core/tcp.c
@@ -960,7 +960,7 @@ tcp_connect(struct tcp_pcb *pcb, const ip_addr_t *ipaddr, u16_t port,
      The send MSS is updated when an MSS option is received. */
   pcb->mss = INITIAL_MSS;
 #if TCP_CALCULATE_EFF_SEND_MSS
-  pcb->mss = tcp_eff_send_mss(pcb->mss, &pcb->local_ip, &pcb->remote_ip);
+  pcb->mss = tcp_eff_send_mss_netif(pcb->mss, netif, &pcb->remote_ip);
 #endif /* TCP_CALCULATE_EFF_SEND_MSS */
   pcb->cwnd = 1;
   pcb->ssthresh = TCP_WND;
@@ -1903,21 +1903,15 @@ tcp_next_iss(struct tcp_pcb *pcb)
 #if TCP_CALCULATE_EFF_SEND_MSS
 /**
  * Calculates the effective send mss that can be used for a specific IP address
- * by using ip_route to determine the netif used to send to the address and
- * calculating the minimum of TCP_MSS and that netif's mtu (if set).
+ * by calculating the minimum of TCP_MSS and the mtu (if set) of the target
+ * netif (if not NULL).
  */
 u16_t
-tcp_eff_send_mss_impl(u16_t sendmss, const ip_addr_t *dest
-#if LWIP_IPV6 || LWIP_IPV4_SRC_ROUTING
-                     , const ip_addr_t *src
-#endif /* LWIP_IPV6 || LWIP_IPV4_SRC_ROUTING */
-                     )
+tcp_eff_send_mss_netif(u16_t sendmss, struct netif *outif, const ip_addr_t *dest)
 {
   u16_t mss_s;
-  struct netif *outif;
   s16_t mtu;
 
-  outif = ip_route(src, dest);
 #if LWIP_IPV6
 #if LWIP_IPV4
   if (IP_IS_V6(dest))
diff --git a/src/core/tcp_out.c b/src/core/tcp_out.c
index 108028b2..940fc9fa 100644
--- a/src/core/tcp_out.c
+++ b/src/core/tcp_out.c
@@ -1216,7 +1216,7 @@ tcp_output_segment(struct tcp_seg *seg, struct tcp_pcb *pcb, struct netif *netif
   if (seg->flags & TF_SEG_OPTS_MSS) {
     u16_t mss;
 #if TCP_CALCULATE_EFF_SEND_MSS
-    mss = tcp_eff_send_mss(TCP_MSS, &pcb->local_ip, &pcb->remote_ip);
+    mss = tcp_eff_send_mss_netif(TCP_MSS, netif, &pcb->remote_ip);
 #else /* TCP_CALCULATE_EFF_SEND_MSS */
     mss = TCP_MSS;
 #endif /* TCP_CALCULATE_EFF_SEND_MSS */
diff --git a/src/include/lwip/priv/tcp_priv.h b/src/include/lwip/priv/tcp_priv.h
index 03ebcb63..ffffb977 100644
--- a/src/include/lwip/priv/tcp_priv.h
+++ b/src/include/lwip/priv/tcp_priv.h
@@ -459,16 +459,10 @@ err_t tcp_zero_window_probe(struct tcp_pcb *pcb);
 void  tcp_trigger_input_pcb_close(void);
 
 #if TCP_CALCULATE_EFF_SEND_MSS
-u16_t tcp_eff_send_mss_impl(u16_t sendmss, const ip_addr_t *dest
-#if LWIP_IPV6 || LWIP_IPV4_SRC_ROUTING
-                           , const ip_addr_t *src
-#endif /* LWIP_IPV6 || LWIP_IPV4_SRC_ROUTING */
-                           );
-#if LWIP_IPV6 || LWIP_IPV4_SRC_ROUTING
-#define tcp_eff_send_mss(sendmss, src, dest) tcp_eff_send_mss_impl(sendmss, dest, src)
-#else /* LWIP_IPV6 || LWIP_IPV4_SRC_ROUTING */
-#define tcp_eff_send_mss(sendmss, src, dest) tcp_eff_send_mss_impl(sendmss, dest)
-#endif /* LWIP_IPV6 || LWIP_IPV4_SRC_ROUTING */
+u16_t tcp_eff_send_mss_netif(u16_t sendmss, struct netif *outif,
+                             const ip_addr_t *dest);
+#define tcp_eff_send_mss(sendmss, src, dest) \
+    tcp_eff_send_mss_netif(sendmss, ip_route(src, dest), dest)
 #endif /* TCP_CALCULATE_EFF_SEND_MSS */
 
 #if LWIP_CALLBACK_API