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As part of the routed networks work [1], the DHCP agent and scheduling
needs to be segment aware.  Right now, the dhcpagentscheduler extension
exposes API resources to manage networks:

- List networks hosted by an agent
	- GET /agents/{agent_id}/dhcp-networks
	- Response Body: {"networks": [{...}]}

- List agents hosting a network - GET /network
	- GET /networks/{network_id}/dhcp-agents
	- Response Body: {"agents": [{...}]}

- Add a network to an agent
	- POST /agents/{agent_id}/dhcp-networks
	- Request Body: {"network_id": "NETWORK_UUID"}

- Remove a network from an agent
	- DELETE /agents/{agent_id}/dhcp-networks/{network_id}

This same functionality needs to also be exposed for working with
segments.  We need some opinions on the best way to do this.  The
options are:

A) Expose new resources for segment dhcp agent manipulation
	- GET /agents/{agent_id}/dhcp-segments
	- GET /segments/{segment_id}/dhcp-agents
	- POST /agents/{agent_id}/dhcp-segments
	- DELETE /agents/{agent_id}/dhcp-segments/{segment_id}

B) Allow segment info gathering and manipulation via the current network
dhcp agent API resources. No new API resources.

C) Some combination of A and B.

My current opinion is that option C shouldn't even be an option but I
just put it on here just in case someone has a strong argument.  If
we're going to add new resources, we may as well do it all the way,
which is what C implies would happen.

Option B would be great to use if segment support could easily be added
in while maintaining backwards compatibility.  I'm not sure if that is
going to be possible in a clean way.  Regardless, an extension will have
to be created for this.

Option A is the cleanest strategy IMHO.  It may not be the most user
friendly though because some networks may have multiple segments while
others may not.  If a network is made up of just a single segment then
the current network dhcp agent calls will be fine.  However, once a
network is made up of multiple segments, it wouldn't make sense for the
current network dhcp agent calls to be valid, they'd need to be made to
the new segment resources.  This same line of thinking would probably
have to be considered with Option B as well, so it may be a problem for
both.

Anyway, I'd like to gather suggestions and opinions on this.  If there's
some other option that I somehow missed please suggest it.

Thanks,
Brandon

[1]
https://specs.openstack.org/openstack/neutron-specs/specs/newton/routed-networks.html#dhcp-scheduling