ofagent: port monitoring w/o ovsdb accesses

Change-Id: Id3ca698c84a4df5dd8900029fbf3e669eb87f615

specs/juno/ofagent-port-monitor.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+===========================================
+ofagent: port monitoring w/o ovsdb accesses
+===========================================
+
+https://blueprints.launchpad.net/neutron/+spec/ofagent-port-monitor
+
+Problem description
+===================
+
+ofagent currently scans ovsdb (via ovs-vsctl command) to get a list of ports.
+ovsdb is not likely available for other openflow switch implementations.
+
+Proposed change
+===============
+
+* implement the functionality using OFPMP_PORT_DESC instead of ovsdb.
+
+* (optional) use OFPT_PORT_STATUS asynchronous messages to
+  avoid periodic polling.
+
+* as there is no pure openflow equivalent for port external-ids,
+  we plan to use port name to identify devices. (as linuxbridge does)
+
+Alternatives
+------------
+
+* implement switch-specific methods for every switch implementations
+
+* use of-config
+  at a glance of the spec, it doesn't seem usable at this point, though.
+
+Data model impact
+-----------------
+
+none
+
+REST API impact
+---------------
+
+none
+
+Security impact
+---------------
+
+none
+
+Notifications impact
+--------------------
+
+none
+
+Other end user impact
+---------------------
+
+none
+
+Performance Impact
+------------------
+
+in case asyncronous messages are used, performance might be improved.
+
+Other deployer impact
+---------------------
+
+none
+
+Developer impact
+----------------
+
+none
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  yamamoto
+
+Other contributors:
+  kakuma
+
+Work Items
+----------
+
+see "Proposed change" section.
+
+Dependencies
+============
+
+none
+
+but some of bug fixes need to be merged to make this useful.
+for example, https://review.openstack.org/#/c/88224/
+
+Testing
+=======
+
+ryu/ofagent third party testing would find regressions.
+
+Documentation Impact
+====================
+
+none
+
+References
+==========
+
+* https://www.opennetworking.org/sdn-resources/onf-specifications/openflow
+* OpenFlow 1.3.3 7.3.5.7 Port Description
+* OpenFlow 1.3.3 7.4.3 Port Status Message
+