[openstack-dev] [neutron] How could an L2 agent extension access agent methods ?

Ihar Hrachyshka ihrachys at redhat.com
Thu Dec 3 15:46:36 UTC 2015


Small update on the RFE. It was approved for Mitaka, assuming we come up  
with proper details upfront thru neutron-specs process.

In the meantime, we have found more use cases for flow management among  
features in development: QoS DSCP, also the new OF based firewall driver.  
Both authors for those new features independently realized that agent does  
not currently play nice with flows set by external code due to its graceful  
restart behaviour when rules with unknown cookies are cleaned up. [The  
agent uses a random session uuid() to mark rules that belong to its current  

Before I proceed, full disclosure: I know almost nothing about OpenFlow  
capabilities, so some pieces below may make no sense. I tried to come up  
with high level model first and then try to map it to available OF  
features. Please don’t hesitate to comment, I like to learn new stuff! ;)

I am thinking lately on the use cases we collected so far. One common need  
for all features that were seen to be interested in proper integration with  
Open vSwitch agent is to be able to manage feature specific flows on br-int  
and br-tun. There are other things that projects may need, like patch  
ports, though I am still struggling with the question of whether it may be  
postponed or avoided for phase 1.

There are several specific operation 'kinds' that we should cover for the  
- managing flows that modify frames in-place;
- managing flows that redirect frames.

There are some things that should be considered to make features cooperate  
with the agent and other extensions:
- feature flows should have proper priorities based on their ‘kind’ (f.e.  
in-place modification probably go before redirections);
- feature flows should survive flow reset that may be triggered by the agent;
- feature flows should survive flow reset without data plane disruption  
(=they should support graceful restart:  

With that in mind, I see the following high level design for the flow tables:

- table 0 serves as a dispatcher for specific features;
- each feature gets one or more tables, one per flow ‘kind’ needed;
- for each feature table, a new flow entry is added to table 0 that would  
redirect to feature specific table; the rule will be triggered only if OF  
metadata is not updated inside the feature table (see the next bullet); the  
rule will have priority that is defined for the ‘kind’ of the operation  
that is implemented by the table it redirects to;
-  each feature table will have default actions that will 1) mark OF  
metadata for the frame as processed by the feature; 2) redirect back to  
table 0;
- all feature specific flow rules (except dispatcher rules) belong to  
feature tables;

Now, the workflow for extensions that are interested in setting flows would  
- on initialize() call, extension defines feature tables it will need; it  
passes the name of the feature table and the ‘kind’ of the actions it will  
execute; with that, the following is initialized by the agent: 1) table 0  
dispatcher entry to redirect frames into feature table; the entry has the  
priority according to the ‘kind’ of the table; 2) the actual feature table  
with two default rules (update metadata and push back to table 0);
- whenever extension needs to add a new flow rule, it passes the following  
into the agent: 1) table name; 2) flow specific parameters (actions,  
priority, ...)

Since the agent will manage setting flows for extensions, it will be able  
to use the active agent cookie for all feature flows; next time the agent  
is restarted, it should be able to respin extension flows with no data  
plane disruption. [Note: we should make sure that on agent restart, we call  
to extensions *before* we clean up stale flow rules.]

That design will hopefully allow us to abstract interaction with flows from  
extensions into management code inside the agent. It should guarantee  
extensions cooperate properly assuming they properly define their  
priorities thru ‘kinds’ of tables they have.

It is also assumed that existing flow based features integrated into the  
agent (dvr? anti-spoofing?) will eventually move to the new flow table  
management model.

I understand that the model does not reflect how do feature processing for  
existing OF based features in the agent. It may require some smart  
workarounds to allow non-disruptive migration to new flow table setup.

It would be great to see the design bashed hard before I start to put it  
into spec format. Especially if it’s not sane. :)


Mathieu Rohon <mathieu.rohon at gmail.com> wrote:

> Thanks ihar!
> On Thu, Nov 19, 2015 at 2:32 PM, Ihar Hrachyshka <ihrachys at redhat.com>  
> wrote:
> UPD: now that we have some understanding what’s needed from l2 agent  
> extension mechanism to cater for interested subprojects (and now that we  
> see that probably the agent in focus right now is OVS only), we may move  
> to RFE step. I reported the following RFE for the feature:
> https://bugs.launchpad.net/neutron/+bug/1517903
> It may require BP if drivers team will request one.
> Cheers,
> Ihar
> Ihar Hrachyshka <ihrachys at redhat.com> wrote:
> 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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> Ihar
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