Implement new "learn" action.

There are a few loose ends here.  First, learning actions cause too much
flow revalidation.  Upcoming commits will fix that problem.  The following
additional issues have not yet been addressed:

    * Resource limits: nothing yet limits the maximum number of flows that
      can be learned.  It is possible to exhaust all system memory.

    * Age reporting: there is no way to find out how soon a learned table
      entry is due to be evicted.

To try this action out, here's a recipe for a very simple-minded MAC
learning switch.  It uses a 10-second MAC expiration time to make it easier
to see what's going on:

ovs-vsctl del-controller br0
ovs-ofctl del-flows br0
ovs-ofctl add-flow br0 "table=0 actions=learn(table=1, hard_timeout=10, \
	NXM_OF_VLAN_TCI[0..11], NXM_OF_ETH_DST[]=NXM_OF_ETH_SRC[], \
	output:NXM_OF_IN_PORT[]), resubmit(,1)"
ovs-ofctl add-flow br0 "table=1 priority=0 actions=flood"

You can then dump the MAC learning table with:

ovs-ofctl dump-flows br0 table=1

1 files changed, 38 insertions, 0 deletions

diff --git a/lib/learn.h b/lib/learn.h
new file mode 100644
index 00000000..19a9089c
--- /dev/null
+++ b/lib/learn.h
@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2011 Nicira Networks.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef LEARN_H
+#define LEARN_H 1
+
+struct ds;
+struct flow;
+struct ofpbuf;
+struct ofputil_flow_mod;
+struct nx_action_learn;
+
+/* NXAST_LEARN helper functions.
+ *
+ * See include/openflow/nicira-ext.h for NXAST_LEARN specification.
+ */
+
+int learn_check(const struct nx_action_learn *, const struct flow *);
+void learn_execute(const struct nx_action_learn *, const struct flow *,
+                   struct ofputil_flow_mod *);
+
+void learn_parse(struct ofpbuf *, char *, const struct flow *);
+void learn_format(const struct nx_action_learn *, struct ds *);
+
+#endif /* learn.h */