Signed-off-by: Ben Pfaff <b...@ovn.org>
---
FAQ.md | 42 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 42 insertions(+)
diff --git a/FAQ.md b/FAQ.md
index df6f225..2babb8f 100644
--- a/FAQ.md
+++ b/FAQ.md
@@ -2029,6 +2029,47 @@ A: OpenFlow actions are executed in the order specified.
Thus, the
ovs-ofctl add-flow br0 dl_vlan=123,actions=mod_vlan_vid:456,output:1
+### Q: The "learn" action can't learn the action I want, can you improve it?
+
+A: By itself, the "learn" action can only put two kinds of actions
+ into the flows that it creates: "load" and "output" actions. If
+ "learn" is used in isolation, these are severe limits.
+
+ Howver, "learn" is not meant to be used in isolation. It is a
+ primitive meant to be used together with other Open vSwitch
+ features to accomplish a task. Its existing features are enough to
+ accomplish most tasks.
+
+ Here is an outline of a typical pipeline structure that allows for
+ versatile behavior using "learn":
+
+ - Flows in table A contain a "learn" action, that populates flows
+ in table L, that use a "load" action to populate register R
+ with information about what was learned.
+
+ - Flows in table B contain two sequential resubmit actions: one
+ to table L and another one to table B+1.
+
+ - Flows in table B+1 match on register R and act differently
+ depending on what the flows in table L loaded into it.
+
+ This approach can be used to implement many "learn"-based features.
+ For example:
+
+ - Resubmit to a table selected based on learned information, e.g. see:
+ http://openvswitch.org/pipermail/discuss/2016-June/021694.html
+
+ - MAC learning in the middle of a pipeline, as described in
+ [Tutorial.md].
+
+ - TCP state based firewalling, by learning outgoing connections
+ based on SYN packets and matching them up with incoming
+ packets.
+
+ - At least some of the features described in T. A. Hoff,
+ "Extending Open vSwitch to Facilitate Creation of Stateful SDN
+ Applications".
+
Development
-----------
@@ -2095,3 +2136,4 @@ http://openvswitch.org/
[INSTALL.md]:INSTALL.md
[OPENFLOW-1.1+.md]:OPENFLOW-1.1+.md
[INSTALL.DPDK.md]:INSTALL.DPDK.md
+[Tutorial.md]:tutorial/Tutorial.md
--
2.1.3
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