openflow_input: add discriminator fields
diff --git a/openflow_input/oxm-1.2 b/openflow_input/oxm-1.2
index e75d218..5006cff 100644
--- a/openflow_input/oxm-1.2
+++ b/openflow_input/oxm-1.2
@@ -33,7 +33,7 @@
 #version 4
 
 struct of_oxm {
-    uint32_t type_len;
+    uint32_t type_len == ?;
 };
 
 struct of_oxm_arp_op : of_oxm {