[openstack-dev] [Neutron] About ports backing floating IPs

Salvatore Orlando sorlando at nicira.com
Tue Jan 14 23:50:36 UTC 2014

I have been looking back at the API and found out that it's a bit weird how
floating IPs are mapped to ports. This might or might not be an issue, and
several things can be done about it.
The rest of this post is a boring description of the problem and a possibly
even more boring list of potential solutions.

Floating IPs are backed by ports on the external network where they are
implemented; while there are good reason for doing so, this has some
seemingly weird side effects, which are usually not visible to tenants as
only admins are allowed (by default) to view the ports backing the floating

Assigning an external port to a floating IP is an easy way for ensuring the
IP address used for the floating IP is then not reused for other allocation
purposes on the external network; indeed admin users might start VMs on
external networks as well. Conceptually, it is also an example of
port-level insertion for a network service (DNAT/SNAT).

However these are the tricky aspects:
- IP Address changes: The API allows IP address updates for a floating IP
port. However as it might be expected, the IP of the floating IP entities
does not change, as well as the actual floating IP implemented in the
backend (l3 agent or whatever the plugin uses).
- operational status: It is always down at least for plugins based on
OVS/LB agents. This is because there is no actual VIF backing a floating
IP, so there is nothing to wire.
- admin status: updating it just has no effect at all
- Security groups and  allowed address pairs: The API allows for updating
them, but it is not clear whether something actually happens in the
backend, and I'm even not entirely sure this makes sense at all.

Why these things happen, whether it's intended behaviour, and whether it's
the right behaviour it's debatable.

>From my perspective, this leads to inconsistent state, as:
- the address reported in the floating IP entity might differ from the one
on the port backing the floating IP
- operational status is wrongly represented as down
- expectations concerning operations on the port are not met (eg: admin
status update)
And I reckon state inconsistencies should always be avoided.

Considering the situation described above, there are few possible options.

1- don't do anything, since the port backing the floating IP is hidden from
the tenant.
This might be ok provided that a compelling reason for ignoring entities
not visible to tenants is provided.
However it has to be noted that Neutron authZ logic, which is based on
openstack.common would allow deployers to change that (*)

2- remove the need for a floating IP to be backed from a port
While this might seem simple, this has non-trivial implications as IPAM
logic would need to become aware of floating IPs, and should  be discussed

3- leverage policy-based APIs, and transform floating IPs in a "remote
access policy"
In this way the floating IP will become a policy to apply to a port; it
will be easier to solve conflicts with security policies and it will be
possible to just use IPs (or addressing policies) configured on the port.
However, this will be hardly backward compatible, and its feasibility
depends on the outcome of the more general discussions on policy-based APIs
for neutron.

4- Document the current behaviour
This is something which is probably worth doing anyway until a solution is
agreed upon

Summarising, since all the 'technical' options sounds not feasible for the
upcoming Icehouse release, it seems worth at least documenting the current
behaviour, and start a discussion on whether we should do something about
this and, if yes, what.

Regards and apologies for the long post,

(*) As an interesting corollary, the flexibility of making authZ policies
super-configurable causes the API to be non-portable. However, this is a
subject for a different discussion.
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