diff --git a/clatd.c b/clatd.c
index 3ab2371..a89c588 100644
--- a/clatd.c
+++ b/clatd.c
@@ -51,6 +51,9 @@
 #define DEVICENAME6 "clat"
 #define DEVICENAME4 "clat4"
 
+/* 40 bytes IPv6 header - 20 bytes IPv4 header + 8 bytes fragment header */
+#define MTU_DELTA 28
+
 int forwarding_fd = -1;
 volatile sig_atomic_t running = 1;
 
@@ -271,8 +274,9 @@
     Global_Clatd_Config.mtu = 1280;
   }
 
-  if(Global_Clatd_Config.ipv4mtu <= 0 || (Global_Clatd_Config.ipv4mtu > Global_Clatd_Config.mtu - 20)) {
-    Global_Clatd_Config.ipv4mtu = Global_Clatd_Config.mtu-20;
+  if(Global_Clatd_Config.ipv4mtu <= 0 ||
+     Global_Clatd_Config.ipv4mtu > Global_Clatd_Config.mtu - MTU_DELTA) {
+    Global_Clatd_Config.ipv4mtu = Global_Clatd_Config.mtu - MTU_DELTA;
     logmsg(ANDROID_LOG_WARN,"ipv4mtu now set to = %d",Global_Clatd_Config.ipv4mtu);
   }