[openstack-dev] [Neutron][L3] Representing a networks connected by routers

Ian Wells ijw.ubuntu at cack.org.uk
Mon Jul 20 22:24:37 UTC 2015

There are two routed network models:

- I give my VM an address that bears no relation to its location and ensure
the routed fabric routes packets there - this is very much the routing
protocol method for doing things where I have injected a route into the
network and it needs to propagate.  It's also pretty useless because there
are too many host routes in any reasonable sized cloud.

- I give my VM an address that is based on its location, which only becomes
apparent at binding time.  This means that the semantics of a port changes
- a port has no address of any meaning until binding, because its location
is related to what it does - and it leaves open questions about what to do
when you migrate.

Now, you seem to generally be thinking in terms of the latter model,
particularly since the provider network model you're talking about fits
there.  But then you say:

On 20 July 2015 at 10:33, Carl Baldwin <carl at ecbaldwin.net> wrote:

> When creating a
> port, the binding information would be sent to the IPAM system and the
> system would choose an appropriate address block for the allocation.

No, it wouldn't, because creating and binding a port are separate
operations.  I can't give the port a location-specific address on creation
- not until it's bound, in fact, which happens much later.

On proposal 1: consider the cost of adding a datamodel to Neutron.  It has
to be respected by all developers, it frequently has to be deployed by all
operators, and every future change has to align with it.  Plus either it
has to be generic or optional, and if optional it's a burden to some
proportion of Neutron developers and users.  I accept proposal 1 is easy,
but it's not universally applicable.  It doesn't work with Neil Jerram's
plans, it doesn't work with multiple interfaces per host, and it doesn't
work with the IPv6 routed-network model I worked on.

Given that, I wonder whether proposal 2 could be rephrased.

1: some network types don't allow unbound ports to have addresses, they
just get placeholder addresses for each subnet until they're bound
2: 'subnets' on these networks are more special than subnets on other
networks.  (More accurately, they dont use subnets.  It's a shame subnets
are core Neutron, because they're pretty horrible and yet hard to replace.)
3: there's an independent (in an extension?  In another API endpoint?)
datamodel that the network points to and that IPAM refers to to find a port
an address.  Bonus, people who aren't using funky network types can disable
this extension.
4: when the port is bound, the IPAM is referred to, and it's told the
binding information of the port.
5: when binding the port, once IPAM has returned its address, the network
controller probably does stuff with that address when it completes the
binding (like initialising routing).
6: live migration either has to renumber a port or forward old traffic to
the new address via route injection.  This is an open question now, so I'm
mentioning it rather than solving it.

In fact, adding that hook to IPAM at binding plus setting aside a 'not set'
IP address might be all you need to do to make it possible.  The IPAM needs
data to work out what an address is, but that doesn't have to take the form
of existing Neutron constructs.
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