[Open Discussion] neutron can create port from network which has no subnet

[LIU Yulong]

# openstack network create test-network
+---------------------------+--------------------------------------+
| Field | Value |
+---------------------------+--------------------------------------+
| admin_state_up | UP |
| availability_zone_hints | |
| availability_zones | |
| created_at | 2023-04-13T07:31:16Z |
| description | |
| dns_domain | None |
| id | d595860d-e576-4899-882a-8428cfe601f2 |
| ipv4_address_scope | None |
| ipv6_address_scope | None |
| is_default | False |
| is_vlan_transparent | None |
| mtu | 1450 |
| name | test-network |
| port_security_enabled | True |
| project_id | c675870e8b8e48e7a339907b5bcf7c8c |
| provider:network_type | vxlan |
| provider:physical_network | None |
| provider:segmentation_id | 99941 |
| qos_policy_id | None |
| revision_number | 1 |
| router:external | Internal |
| segments | None |
| shared | False |
| status | ACTIVE |
| subnets | |
| tags | |
| updated_at | 2023-04-13T07:31:16Z |
+---------------------------+--------------------------------------+

# openstack port create --network d595860d-e576-4899-882a-8428cfe601f2 test-create-port-net-has-no-subnet
+-----------------------+--------------------------------------+
| Field | Value |
+-----------------------+--------------------------------------+
| admin_state_up | UP |
| allowed_address_pairs | |
| binding_host_id | |
| binding_profile | |
| binding_vif_details | |
| binding_vif_type | unbound |
| binding_vnic_type | normal |
| created_at | 2023-04-13T07:35:46Z |
| data_plane_status | None |
| description | |
| device_id | |
| device_owner | |
| dns_assignment | None |
| dns_domain | None |
| dns_name | None |
| extra_dhcp_opts | |
| fixed_ips | |
| id | 1f2e55bc-9e8f-45ca-b16d-d8e32c1c4bb5 |
| mac_address | fa:16:3e:04:18:10 |
| name | test-create-port-net-has-no-subnet |
| network_id | d595860d-e576-4899-882a-8428cfe601f2 |
| port_security_enabled | True |
| project_id | c675870e8b8e48e7a339907b5bcf7c8c |
| qos_policy_id | None |
| revision_number | 2 |
| security_group_ids | a3d879a7-2e72-4b68-960e-cfa20569c2b0 |
| status | DOWN |
| tags | |
| trunk_details | None |
| updated_at | 2023-04-13T07:35:46Z |
+-----------------------+--------------------------------------+

Port has no fixed ips.

Is such behavior is useful? or neutron should disable such creation?

[Lajos Katona (lajos-katona)]

Shall we bring this to drivers meeting?

[Bence Romsics (bence-romsics)]

IIRC creating a port on a network without a subnet was considered a feature at some point in the past. Which was admittedly rarely used, so at some point it got broken. But I believe there was a bug report to restore the ability to do this. I will try to find it.

Most use cases I can imagine are telco-related (I don't know if anybody uses neutron like this today):

* using something other than IP
* using IP, but with so many IP addresses behind one port, that it's a pain to manage it with allowed address pairs.

Maybe Sean was involved with this earlier and may remember more.

update: I found a spec even: https://specs.openstack.org/openstack/neutron-specs/specs/newton/unaddressed-port.html
update2: example for the non-IP use case: I have a vague memory from many years before that somebody wanted to put a telco app on neutron that used TIPC directly over ethernet (https://docs.kernel.org/networking/tipc.html)

[Rodolfo Alonso (rodolfo-alonso-hernandez)]

This topic will be discussed next Friday April 21st during the Neutron drivers meeting [1].

[1]https://wiki.openstack.org/wiki/Meetings/NeutronDrivers#Agenda

[Rodolfo Alonso (rodolfo-alonso-hernandez)]

This proposal has been declined in the Neutron drivers meeting (see meeting logs). The change of the current behaviour of the API is not needed to fix LP#2016198 and could be in use now.

The proposal to fix LP#2016198 is to limit the creation of the "L3HARouterNetwork" DB resource [1][2]. The HA network is created per project. Only one network should be created at once. If we limit in the DB engine the number of "L3HARouterNetwork" per project, by making the "project_id" unique, we'll prevent the race condition.

[1]https://github.com/openstack/neutron/blob/47d070c71e795e41e698cdb278d99dcfb3448bde/neutron/objects/l3_hamode.py#L58
[2]https://github.com/openstack/neutron/blob/master/neutron/db/models/l3ha.py#L62