- * A few "mf_field"s don't correspond to NXM fields. Those have 0 and
- * NULL for the following members, respectively. */
- uint32_t nxm_header; /* An NXM_* constant (a few fields have 0). */
- const char *nxm_name; /* The "NXM_*" constant's name. */
-
- /* OXM properties */
- uint32_t oxm_header; /* Field id in the OXM basic class,
- * an OXM_* constant.
- * Ignored if oxm_name is NULL */
- const char *oxm_name; /* The OXM_* constant's name,
- * NULL if the field is not present
- * in the OXM basic class */
-
+ * There are the following possibilities for these members for a given
+ * mf_field:
+ *
+ * - Neither NXM nor OXM defines such a field: these members will all be
+ * zero or NULL.
+ *
+ * - NXM and OXM both define such a field: nxm_header and oxm_header will
+ * both be nonzero and different, similarly for nxm_name and oxm_name.
+ *
+ * - Only NXM or only OXM defines such a field: nxm_header and oxm_header
+ * will both have the same value (either an OXM_* or NXM_* value) and
+ * similarly for nxm_name and oxm_name.
+ *
+ * Thus, 'nxm_header' is the appropriate header to use when outputting an
+ * NXM formatted match, since it will be an NXM_* constant when possible
+ * for compatibility with OpenFlow implementations that expect that, with
+ * OXM_* constants used for fields that OXM adds. Conversely, 'oxm_header'
+ * is the header to use when outputting an OXM formatted match. */
+ uint32_t nxm_header; /* An NXM_* (or OXM_*) constant. */
+ const char *nxm_name; /* The nxm_header constant's name. */
+ uint32_t oxm_header; /* An OXM_* (or NXM_*) constant. */
+ const char *oxm_name; /* The oxm_header constant's name */