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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.

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.

Does this approach make sense ?

-Thomas

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