ACL Handling optimizations in networking-ovn

ACL Handling optimizations in networking-ovn

This document presents the current problem with ACLs and the design changes proposed to core OVN as well as the necessary modifications to be made to networking-ovn to improve their usage.

Problem description

There is basically two problems being addressed in this spec:

1. While in Neutron, a Security Group Rule is tied to a Security Group, in OVN ACLs are created per port. Therefore, we’ll typically have many more ACLs than Security Group Rules, resulting in a performance hit as the number of ports grows.

2. An ACL in OVN is applied to a Logical Switch. As a result, networking-ovn has to figure out which Logical Switches to apply the generated ACLs per each Security Rule.

Let’s highlight both problems with an example:

  • Neutron Networks: NA, NB, NC
  • Neutron Security Group: SG1
  • Number of Neutron Security Group Rules in SG1: 10
  • Neutron Ports in NA: 100
  • Neutron Ports in NB: 100
  • Neutron Ports in NC: 100
  • All ports belong to SG1

When we implement the above scenario in OVN, this is what we’ll get:

  • OVN Logical Switches: NA, NB, NC
  • Number of ACL rows in Northbound DB ACL table: 3000 (10 rules * 100 ports * 3 networks)
  • Number of elements in acl column on each Logical_Switch row: 1000 (10 rules * 100 ports).

And this is how, for example, the ACL match fields for the default Neutron Security Group would look like:

outport == <port1_uuid> && ip4 && ip4.src == $as_ip4_<sg1_uuid>
outport == <port2_uuid> && ip4 && ip4.src == $as_ip4_<sg1_uuid>
outport == <port3_uuid> && ip4 && ip4.src == $as_ip4_<sg1_uuid>
outport == <port300_uuid> && ip4 && ip4.src == $as_ip4_<sg1_uuid>

As you can see, all of them look the same except for the outport field which is clearly redundant and makes the NB database grow a lot at scale. Also, networking-ovn had to figure out for each rule in SG1 which Logical Switches it had to apply the ACLs on (NA, NB and NC). This can be really costly when the number of networks and port grows.

Proposed optimization

In the OpenStack context, we’ll be facing this scenario most of the time where the majority of the ACLs will look the same except for the outport/inport fields in the match column. It would make sense to be able to substitute all those ACLs by a single one which references all the ports affected by that SG rule:

outport == @port_group1 && ip4 && ip4.src == $port_group1_ip4

Implementation Details

Core OVN

There’s a series of patches in Core OVN that will enable us to achieve this optimization:

In summary, these patches are:

  • Adding a new entity called Port_Group which will hold a list of weak references to the Logical Switch ports that belong to it.
  • Automatically creating/updating two Address Sets (_ip4 and _ip6) in Southbound database every time a new port is added to the group.
  • Support adding a list of ACLs to a Port Group. As the SG rules may span across different Logical Switches, we used to insert the ACLs in all the Logical Switches where we have ports in within a SG. Figuring this out is expensive and this new feature is a huge gain in terms of performance when creating/deleting ports.


In the OpenStack integration driver, the following changes are required to accomplish this optimization:

  • When a Neutron Security Group is created, create the equivalent Port Group in OVN (pg-<security_group_id>), instead of creating a pair of Adress Sets for IPv4 and IPv6. This Port Group will reference Neutron SG id in its external_ids column.
  • When a Neutron Port is created, the equivalent Logical Port in OVN will be added to those Port Groups associated to the Neutron Security Groups this port belongs to.
  • When a Neutron Port is deleted, we’ll delete the associated Logical Port in OVN. Since the schema includes a weak reference to the port, when the LSP gets deleted, it will also be automatically deleted from any Port Group entry where it was previously present.
  • Instead of handling SG rules per port, we now need to handle them per SG referencing the associated Port Group in the outport/inport fields. This will be the biggest gain in terms of processing since we don’t need to iterate through all the ports anymore. For example:
-def acl_direction(r, port):
+def acl_direction(r):
    if r['direction'] == 'ingress':
        portdir = 'outport'
        portdir = 'inport'
-   return '%s == "%s"' % (portdir, port['id'])
+   return '%s == "@%s"' % (portdir, utils.ovn_name(r['security_group_id'])
  • Every time a SG rule is created, instead of figuring out the ports affected by its SG and inserting an ACL row which will be referrenced by different Logical Switches, we will just reference it from the associated Port Group.
  • For Neutron remote security groups, we just need to reference the automatically created Address_Set for that Port Group.

As a bonus, we are tackling the race conditions that could happen in Address_Sets right now when we’re deleting and creating a port at the same time. This is thanks to the fact that the Address_Sets in the SB table are generated automatically by ovn-northd from the Port_Group contents and Port Group is referencing actual Logical Switch Ports. More info at:

Backwards compatibility considerations

  • If the schema doesn’t include the Port_Group table, keep the old behavior(Address Sets) for backwards compatibility.
  • If the schema supports Port Groups, then a migration task will be performed from an OvnWorker. This way we’ll ensure that it’ll happen only once across the cloud thanks the OVSDB lock. This will be done right at the beginning of the ovn_db_sync process to make sure that when neutron-server starts, everything is in place to work with Port Groups. This migration process will perform the following steps:
    • Create the default drop Port Group and add all ports with port security enabled to it.
    • Create a Port Group for every existing Neutron Security Group and add all its Security Group Rules as ACLs to that Port Group.
    • Delete all existing Address Sets in NorthBound database which correspond to a Neutron Security Group.
    • Delete all the ACLs in every Logical Switch (Neutron network).

We should eventually remove the backwards compatibility and migration path. At that point we should require OVS >= 2.10 from networking-ovn.

Special cases

Ports with no security groups

When a port doesn’t belong to any Security Group and port security is enabled, we, by default, drop all the traffic to/from that port. In order to implement this through Port Groups, we’ll create a special Port Group with a fixed name (neutron_pg_drop) which holds the ACLs to drop all the traffic.

This PG will be created automatically when we first need it, avoiding the need to create it beforehand or during deployment.

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