Add some defines of use to low level developers.

include/ZeroTierOne.h

@@ -92,6 +92,21 @@ extern "C" {
  */
 #define ZT_MAX_MTU 10000
 
+/**
+ * Maximum physical UDP payload
+ */
+#define ZT_MAX_PHYSPAYLOAD 10100
+
+/**
+ * Headroom for max physical MTU
+ */
+#define ZT_MAX_HEADROOM 224
+
+/**
+ * Maximum physical MTU
+ */
+#define ZT_MAX_PHYSMTU (ZT_MAX_PHYSPAYLOAD + ZT_MAX_HEADROOM)
+
 /**
  * Maximum size of a remote trace message's serialized Dictionary
  */