+/* Immutable standard messages.
+ *
+ * The OpenFlow standard promises to preserve these messages and their numbers
+ * in future versions, so we mark them as <all>, which covers every OpenFlow
+ * version numbered 0x01...0xff, rather than as OF1.0+, which covers only
+ * OpenFlow versions that we otherwise implement.
+ *
+ * Without <all> here, then we would fail to decode "hello" messages that
+ * announce a version higher than we understand, even though there still could
+ * be a version in common with the peer that we do understand. The <all>
+ * keyword is less useful for the other messages, because our OpenFlow channels
+ * accept only OpenFlow messages with a previously negotiated version.
+ */