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Reviving the thread.

On the design summit session dedicated to agent and plugin extensions [1]  
the following was stated for l2 agent extensions (I appreciate if someone  
checks me on the following though):

- current l2 agent extensions mechanism lacks insight into agent details  
like bridges or vlan maps;

- in some cases, we don’t care about extension portability across multiple  
agents, so it’s not of concern if some of them use implementation details  
like bridges to set specific flows, or to wire up some additional ports to  
them;

- that said, we still don’t want extensions to have unlimited access to  
agent details; the rationale for hard constraints on what is seen inside  
extensions is that we cannot support backwards compatibility for *all*  
possible internal attributes of an agent; instead, we should explicitly  
define where we can make an effort to provide stable API into agent  
details, and what’s, on contrary, beyond real life use cases and hence can  
be left to be broken/refactored as neutron developers see fit; this API can  
be agent specific though;

- agent details that are to be passed into extensions should be driven by  
actual use cases. There were several subprojects mentioned in the session  
that are assumed to lack enough access to agent attributes to do their job  
without patching core ovs agent files. Those are: BGP-VPN, SFC, (anything  
else?) Those subprojects that are interested in extending l2 agent  
extension framework are expected to come up with a list of things missing  
in current implementation, so that neutron developers can agree on proper  
abstractions to provide missing details to extensions. For that goal, I set  
up a new etherpad to collect feedback from subprojects [2].

Once we collect use cases there and agree on agent API for extensions (even  
if per agent type), we will implement it and define as stable API, then  
pass objects that implement the API into extensions thru extension manager.  
If extensions support multiple agent types, they can still distinguish  
between which API to use based on agent type string passed into extension  
manager.

I really hope we start to collect use cases early so that we have time to  
polish agent API and make it part of l2 extensions earlier in Mitaka cycle.

[1]: https://etherpad.openstack.org/p/mitaka-neutron-core-extensibility
[2]: https://etherpad.openstack.org/p/l2-agent-extensions-api-expansion

Ihar

Ihar Hrachyshka <ihrachys at redhat.com> wrote:

>> On 30 Sep 2015, at 12:53, Miguel Angel Ajo <mangelajo at redhat.com> wrote:
>>
>>
>>
>> Ihar Hrachyshka wrote:
>>>> On 30 Sep 2015, at 12:08, thomas.morin at orange.com wrote:
>>>>
>>>> Hi Ihar,
>>>>
>>>> Ihar Hrachyshka :
>>>>>> Miguel Angel Ajo :
>>>>>>> Do you have a rough idea of what operations you may need to do?
>>>>>> Right now, what bagpipe driver for networking-bgpvpn needs to  
>>>>>> interact with is:
>>>>>> - int_br OVSBridge (read-only)
>>>>>> - tun_br OVSBridge (add patch port, add flows)
>>>>>> - patch_int_ofport port number (read-only)
>>>>>> - local_vlan_map dict (read-only)
>>>>>> - setup_entry_for_arp_reply method (called to add static ARP entries)
>>>>> Sounds very tightly coupled to OVS agent.
>>>>>>> Please bear in mind, the extension interface will be available from  
>>>>>>> different agent types
>>>>>>> (OVS, SR-IOV, [eventually LB]), so this interface you're talking  
>>>>>>> about could also serve as
>>>>>>> a translation driver for the agents (where the translation is  
>>>>>>> possible), I totally understand
>>>>>>> that most extensions are specific agent bound, and we must be able  
>>>>>>> to identify
>>>>>>> the agent we're serving back exactly.
>>>>>> Yes, I do have this in mind, but what we've identified for now seems  
>>>>>> to be OVS specific.
>>>>> Indeed it does. Maybe you can try to define the needed pieces in high  
>>>>> level actions, not internal objects you need to access to. Like ‘-  
>>>>> connect endpoint X to Y’, ‘determine segmentation id for a network’  
>>>>> etc.
>>>> I've been thinking about this, but would tend to reach the conclusion  
>>>> that the things we need to interact with are pretty hard to abstract  
>>>> out into something that would be generic across different agents.   
>>>> Everything we need to do in our case relates to how the agents use  
>>>> bridges and represent networks internally: linuxbridge has one bridge  
>>>> per Network, while OVS has a limited number of bridges playing  
>>>> different roles for all networks with internal segmentation.
>>>>
>>>> To look at the two things you  mention:
>>>> - "connect endpoint X to Y" : what we need to do is redirect the  
>>>> traffic destinated to the gateway of a Neutron network, to the thing  
>>>> that will do the MPLS forwarding for the right BGP VPN context (called  
>>>> VRF), in our case br-mpls (that could be done with an OVS table too) ;  
>>>> that action might be abstracted out to hide the details specific to  
>>>> OVS, but I'm not sure on how to  name the destination in a way that  
>>>> would be agnostic to these details, and this is not really relevant to  
>>>> do until we have a relevant context in which the linuxbridge would  
>>>> pass packets to something doing MPLS forwarding (OVS is currently the  
>>>> only option we support for MPLS forwarding, and it does not really  
>>>> make sense to mix linuxbridge for Neutron L2/L3 and OVS for MPLS)
>>>> - "determine segmentation id for a network": this is something really  
>>>> OVS-agent-specific, the linuxbridge agent uses multiple linux bridges,  
>>>> and does not rely on internal segmentation
>>>>
>>>> Completely abstracting out packet forwarding pipelines in OVS and  
>>>> linuxbridge agents would possibly allow defining an interface that  
>>>> agent extension could use without to know about anything specific to  
>>>> OVS or the linuxbridge, but I believe this is a very significant taks  
>>>> to tackle.
>>>
>>> If you look for a clean way to integrate with reference agents, then  
>>> it’s something that we should try to achieve. I agree it’s not an easy  
>>> thing.
>>>
>>> Just an idea: can we have a resource for traffic forwarding, similar to  
>>> security groups? I know folks are not ok with extending security groups  
>>> API due to compatibility reasons, so maybe fwaas is the place to  
>>> experiment with it.
>>>
>>>> Hopefully it will be acceptable to create an interface, even it  
>>>> exposes a set of methods specific to the linuxbridge agent and a set  
>>>> of methods specific to the OVS agent.  That would mean that the agent  
>>>> extension that can work in both contexts (not our case yet) would  
>>>> check the agent type before using the first set or the second set.
>>>
>>> The assumption of the whole idea of l2 agent extensions is that they  
>>> are agent agnostic. In case of QoS, we implemented a common QoS  
>>> extension that can be plugged in any agent [1], and a set of backend  
>>> drivers (atm it’s just sr-iov [2] and ovs [3]) that are selected based  
>>> on the driver type argument passed into the extension manager [4][5].  
>>> Your extension could use similar approach to select the backend.
>>>
>>> [1]:  
>>> https://git.openstack.org/cgit/openstack/neutron/tree/neutron/agent/l2/extensions/qos.py#n169
>>> [2]:  
>>> https://git.openstack.org/cgit/openstack/neutron/tree/neutron/plugins/ml2/drivers/mech_sriov/agent/extension_drivers/qos_driver.py
>>> [3]:  
>>> https://git.openstack.org/cgit/openstack/neutron/tree/neutron/plugins/ml2/drivers/openvswitch/agent/extension_drivers/qos_driver.py
>>> [4]:  
>>> https://git.openstack.org/cgit/openstack/neutron/tree/neutron/plugins/ml2/drivers/openvswitch/agent/ovs_neutron_agent.py#n395
>>> [5]:  
>>> https://git.openstack.org/cgit/openstack/neutron/tree/neutron/plugins/ml2/drivers/mech_sriov/agent/sriov_nic_agent.py#n155
>>
>> I disagree on the agent-agnostic thing. QoS extension for SR-IOV is  
>> totally not agnostic for OVS or LB, in the QoS case, it's just
>> accidental that OVS & LB share common bridges now due to the OVS Hybrid  
>> implementation that leverages linux bridge
>> and iptables.
>
> Wait. The QoS extension has nothing agent backend specific. All it does  
> is it receives rpc updates for tracked resources and pass them into qos  
> drivers. Those latter are the bits that implement backend specific  
> operations. So I am not sure why you say the extension itself is agent  
> specific: any other amqp based agent in the wild can adopt the extension  
> as-is, only providing a new backend to load.
>
>> I agree on having a well defined interface, on which API is available to  
>> talking back to each agent, and it has to be common, where
>> it's possible to be common.
>>
>> It doesn't have to be easy, but it's the way if we want a world where  
>> those commonalities and reusability of extensions can
>> exist and not be just accidental, but it's not realistic in my opinion  
>> to AIM for it on every shot. I believe we should try where we can
>> but we should be open to agent specific extensions. The idea of the  
>> extensions is that you can extend specific agents without
>> being forced to have the main loop hijacked, or eventually having off  
>> tree code plugged into our agents.
>
> Partially, yes. The culprit here is how much the extension API should  
> know about an agent. We can probably make the extension API completely  
> extendable by allowing agents to pass any random kwargs into the  
> extension manager that will forward them into extensions. Note that it  
> breaks current API for extensions and technically breaks it (not that I  
> know of any external extensions that could be affected so far).
>
>> There we should add support to identify the type of agent the extension  
>> works with (compatibility, versioning, etc..)
>
> We already pass the type into extension manager, and that’s how we plug  
> in the proper backend driver in QoS.
>
>>>> Does this approach make sense ?
>>>>
>>>> -Thomas
>>>>
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>>> Note that you should really avoid putting that ^^ kind of signature  
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>>>
>>> Ihar
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