Let’s make address scopes a first class thing in Neutron. With all of the overlapping and arbitrary use of the both IPv4 and IPv6 addresses it is impossible for Neutron to know what can be routed where. Address scopes will be used by Neutron to address this problem.
This proposal has no relation to RFC 4007 IPv6 address scopes .
Neutron allows tenants to bring their own addresses and allows overlapping IP addresses on any networks. Before subnet pools, a subnet could be created with any cidr the user wanted to supply. It didn’t matter if the address overlapped with another subnet in the same project or another project. Neutron provides isolation at the L2 level to ensure this can work.
Subnet Pools  added in Kilo are a step in the right direction. They add overlap protection and allow an admin to define a pool of addresses from which subnets can be allocated by multiple projects. Now we can coordinate addresses. This is good.
Neutron L3 provides a limited routing solution to isolate between projects. As long as routers only connect one project’s internal networks, floating ips will work it out between projects. This can only go so far.
With IPv6, we want to forget floating IPs and route straight in to tenant networks. We can’t just turn on routing assuming that everything will just work. How can we tell the difference between a valid, unique, and routable subnet and one that someone just made up while filling out the subnet create details? Even with IPv4, some clouds in small to medium size organizations may want to just dispense with floating IPs and use some private addressing directly.
Also, a full integration of MPLS/BGP VPN in to Neutron can benefit from this down the road. An analog to address scopes in the L3 VPN world are route distinguishers. With the addition of address scopes, we can easily map to route distinguishers. How address scopes relate to route distinguishers and the somewhat related route targets is out of scope for this blueprint. Other blueprints like this one  will address this more completely.
Add a new object called AddressScope. An AddressScope can be associated with multiple SubnetPool objects in a one-to-many relationship. This will allow delegating parts of an address scope to different tenants. The SubnetPools under an AddressScope must not overlap. They must also be from the same address family. Only an admin can associate an AddressScope and SubnetPool from different tenants. Once a subnet pool has been associated, it can only be updated by the address scope owner. If this owner is not the subnet pool owner then it must be an admin.
A SubnetPool which is not associated with an AddressScope will be treated the same as subnets which were not created from any pool from an address scope point of view. The reason for this is to preserve the behavior introduced in Kilo where address scopes do not exist.
In the reference implementation of the L3 plugin, Neutron routers will be aware of address scopes. It will use multiple routing tables and policy routing – features found in Linux and managed by iproute2 – to implement connecting to networks with different address scopes. The incoming device (iif) will be specified with each rule.
Subnets on a network will be constrained to the same subnet pool . Even if there are multiple pools in a scope, a network will be constrained to only one pool per address family. This work has already been completed and backported to Kilo.
NAT will be automatically included between an internal network and an “external” network to which a router is connected as a gateway unless the two networks have subnets from subnet pools in the same address scope. It will provide 1-1 NAT for floating IPs and many-1 for default SNAT just like today. This will support existing use cases. We don’t intend to implement any more NAT use cases than this at this time. An addition (post-Liberty) may be a more general ipv4 NAT functionality but we want to gauge the demand for that and explore other options too, like providing something as part of LBaaS or something like that.
Routers will route between networks of the same scope without NAT. For example, if a tenant has a subnet pool and the admin includes that subnet pool in the same address scope as the external network then that tenant’s subnets will be routed directly to the external network.
You might be wondering here if a tenant could add any addresses he desires to the subnet pool after the admin linked it to a scope. Once a subnet pool has been associated with an address pool, only the owner of the address scope will be able to update the pool.
BGP dynamic routing will use the concept of address scopes. It will attach metadata in order to know how to advertise routes in the address scopes. That will be worked out as part of late work on the BGP spec .
Basically, we add CRUD semantics for the new address scope object.
We need API to associate address scopes with subnet pools. This will be done using a list field on the address scope. Note that the list will be updated with overwrite semantics, not patch semantics. If a subnet pool being added to the address pool is associated with a different address scope, the operation will fail.
Only an admin can associate a subnet pool with a different owner than the address scope. Once a subnet pool has been associated, only the address scope owner can update the addresses in the subnet pool.
The subnet pool will be enhanced with a read-only attribute to return the ID of the address scope to which it belongs.
See the performance impact. A change in an address scope could have a big impact on the control plane.
Notifications to routers on...
The addition, deletion, or update or AddressScopes could result in a lot of L3 agent activity. This is because any change in address pools can change the scope of a subnet. Any changes involving more than one project will be an admin only operation which should limit the concern. In any case, it shouldn’t have any bigger impact than deploying a software upgrade which would restart L3 agents.
When an L3 agent requests information about a router, the address scope of each of the subnets to which the router is connected must be looked up. This can be done without adding any new queries. Existing queries will need to join the address scope table on the subnet pool id to retrieve the address scope id.
This will work for IPv4 and IPv6 equally as well. Some might think that we don’t need this for IPv6 because everyone will have unique addresses. But, they’re missing the point because this isn’t just about solving the problem of overlapping IPv4 addresses. It also recognizes that we have tenants choosing their own IPv6 addresses too. They could overlap or not be routable for other reasons. We’re giving Neutron the ability to know how to handle them.
The community will love it. ;)
Seriously though, this allows Neutron L3 to know where stuff can be routed. This is important especially with IPv6 where we want to just route through to project’s networks instead of using floating IPs to traverse the address scope boundary.
It is also an important piece to integrating MPLS/VPN which has been requested countless times from the community. This work will be integrated with the BGP work after that work has matured a bit. BGP and other dynamic routing protocols will be address scope aware so that only addresses from the right scope are advertised to a given peer or on a given network.
There are alternatives to using multiple routing tables and policy routing. For example, iptables rules could be attempted but may not be able to address all cases, especially if addresses overlap. Address scopes map very nicely on to multiple routing tables and routing policy.
There is also work to introduce VRF capability in to the Linux kernel directly . This work could provide richer capability. However, it is very new and isn’t ready to be used for this purpose.
Functional testing of the L3 agent enhancements to the routing tables.
Full API test coverage of the new API.
This feature deserves some new user documentation for how/why to use the new API.
New API to be documented.