Add known gaps between ML2/OVS and OVN

These were listed in the spec, but let's add them to the
documentation so they are easier to find.

Change-Id: I48a38460cf6e74dffa7ecb780d2f90b4403c62bd
Related-bug: #1861032

doc/source/ovn/gaps.rst

@@ -0,0 +1,72 @@
+.. _ovn_gaps:
+
+Gaps from ML2/OVS
+=================
+
+This is a list of some of the currently known gaps between ML2/OVS and OVN.
+It is not a complete list, but is enough to be used as a starting point for
+implementors working on closing these gaps. A TODO list for OVN is located
+at [1]_.
+
+* Port forwarding
+
+  Currently ML2/OVS supports Port Forwarding in the North/South plane.
+  Specific L4 Ports of the Floating IP can be directed to a specific
+  FixedIP:PortNumber of a VM, so that different services running in a VM
+  can be isolated, and can communicate with external networks easily.
+
+  This is a relatively new extension, support would need to be added to OVN.
+
+  One possible way would be to use the OVN native load balancing feature.
+  An OVN load balancer is expressed in the OVN northbound load_balancer
+  table. Normally the VIP and its members are expressed as [2]_:
+
+  .. code-block:: console
+
+     VIP:PORT = MEMBER1:MPORT1, MEMBER2:MPORT2
+
+     The same could be extended for port forwarding as:
+
+     FIP:PORT = PRIVATE_IP:PRIV_PORT
+
+* Security Groups logging API
+
+  Currently ML2/OVS, with the OpenvSwitch firewall, supports a log file where
+  security groups events are logged to be consumed by a security entity. This
+  allows users to have a way to check if an instance is trying to execute
+  restricted operations, or access restricted ports in remote servers.
+
+  This is a relatively new extension, support would need to be added to OVN.
+
+* QoS DSCP support
+
+  Currently ML2/OVS supports QoS DSCP tagging and egress bandwidth limiting.
+  Those are basic QoS features that while integrated in the OVS/OVN C core
+  are not integrated (or fully tested) in the neutron OVN mechanism driver.
+
+* QoS for Layer 3 IPs
+
+  Currently the Neutron L3-agent supports floating IP and gateway IP bandwidth
+  limiting based on Linux TC. Networking-ovn L3 had a prototype
+  implementation [3]_ based on the meter of openvswitch [4]_ utility that
+  has been abandoned. This is supported in user space datapath only, or
+  kernel versions 4.15+ [5]_.
+
+* QoS Minimum Bandwidth support
+
+  Currently ML2/OVS supports QoS Minimum Bandwidth limiting, but it is
+  not supported in OVN.
+
+* BGP support
+
+  Currently ML2/OVS supports making a tenant subnet routable via BGP, and
+  can announce host routes for both floating and fixed IP addresses.
+
+References
+----------
+
+.. [1] https://github.com/ovn-org/ovn/blob/master/TODO.rst
+.. [2] https://github.com/ovn-org/ovn/blob/master/ovn-nb.ovsschema#L160
+.. [3] https://review.opendev.org/#/c/539826/
+.. [4] https://github.com/openvswitch/ovs/commit/66d89287269ca7e2f7593af0920e910d7f9bcc38
+.. [5] https://github.com/torvalds/linux/blob/master/net/openvswitch/meter.h

doc/source/ovn/index.rst

@@ -9,4 +9,5 @@ OVN Driver
    :maxdepth: 1
 
    migration.rst
+   gaps.rst
    faq/index.rst