--- openflow-2012-07-06.h	2015-07-23 09:54:17.180479363 -0700
+++ openflow-2012-07-05.h	2015-07-23 09:54:17.184479363 -0700
@@ -1062,7 +1062,9 @@
     OFPIT_EXPERIMENTER = 0xFFFF  /* Experimenter instruction */
 };
 
-/* Generic ofp_instruction structure */
+/* Instruction header that is common to all instructions.  The length includes
+ * the header and any padding used to make the instruction 64-bit aligned.
+ * NB: The length of an instruction *must* always be a multiple of eight. */
 struct ofp_instruction {
     uint16_t type;                /* Instruction type */
     uint16_t len;                 /* Length of this struct in bytes. */