- * (In case you are familiar with OpenFlow, datapath flows are analogous
- * to OpenFlow flow matches. The most important difference is that
- * OpenFlow allows fields to be wildcarded and prioritized, whereas a
- * datapath's flow table is a hash table so every flow must be
- * exact-match, thus without priorities.)
+ * - A "mask" that, for each bit in the flow, specifies whether the datapath
+ * should consider the corresponding flow bit when deciding whether a
+ * given packet matches the flow entry. The original datapath design did
+ * not support matching: every flow entry was exact match. With the
+ * addition of a mask, the interface supports datapaths with a spectrum of
+ * wildcard matching capabilities, from those that only support exact
+ * matches to those that support bitwise wildcarding on the entire flow
+ * key, as well as datapaths with capabilities somewhere in between.
+ *
+ * Datapaths do not provide a way to query their wildcarding capabilities,
+ * nor is it expected that the client should attempt to probe for the
+ * details of their support. Instead, a client installs flows with masks
+ * that wildcard as many bits as acceptable. The datapath then actually
+ * wildcards as many of those bits as it can and changes the wildcard bits
+ * that it does not support into exact match bits. A datapath that can
+ * wildcard any bit, for example, would install the supplied mask, an
+ * exact-match only datapath would install an exact-match mask regardless
+ * of what mask the client supplied, and a datapath in the middle of the
+ * spectrum would selectively change some wildcard bits into exact match
+ * bits.
+ *
+ * Regardless of the requested or installed mask, the datapath retains the
+ * original flow supplied by the client. (It does not, for example, "zero
+ * out" the wildcarded bits.) This allows the client to unambiguously
+ * identify the flow entry in later flow table operations.
+ *
+ * The flow table does not have priorities; that is, all flow entries have
+ * equal priority. Detecting overlapping flow entries is expensive in
+ * general, so the datapath is not required to do it. It is primarily the
+ * client's responsibility not to install flow entries whose flow and mask
+ * combinations overlap.