Introduction

Programmability of virtual switches in hypervisor combined with ability to control data path flows with OpenFlow opens up different possibilities where L2-L4 services typically provided by virtual/physical appliances are pushed on to edge switches in hypervisors. In the current VPC model in CloudStack VPC VR provides many L3-L7 services. One of the services provided by VPC VR is to route inter-tier traffic. Entire VPC's inter-tier traffic has to get routed by VPC VR. As the size of VPC increases, VPC VR can easily become choke-point. VPC VR is also a single point-of-failure in current VPC model. There is also traffic trombone [1] problem where routing by VPC VR can become in-efficient if the source and destination VM's are placed far (in different pod/zone for e.g) from the VPC VR. Traffic trombone could become serious problem in case of region-level VPC [2].

Current network services network ACL and routing support by CloudStack for east-west traffic (inter-tier traffic) can be orchestrated to be provided by virtual switches in hypervisors. Goal of this proposal to add distributed routing and firewall functionality to native SDN controller that leverages OpenVswith capabilities to provide inter-tier routing and network ACL's at hypervisor level in distributed fashion. This would enable a scale-out model and VPC VR being choke point is avoided. Also traffic trombone problem is eliminated as traffic gets routed directly to destination hypervisor from source hypervisor.

References

[1] http://blog.ipspace.net/2011/02/traffic-trombone-what-it-is-and-how-you.html

[2]https://cwiki.apache.org/confluence/display/CLOUDSTACK/Region+level+VPC+and+guest+network+spanning+multiple+zones

Glossory & Conventions

Bridge: bridge in this document refers to a OpenVswith bridge

Conceptual model 

This section will describe conceptually how distributed routing and network ACL's are achieved in an example VPC deployment with three tiers with VM's spanning three hosts. Further sections builds on the concepts/design principles introduced in this section to elaborate the architecture and design on how CloudStack and OVS plug-in can orchestrate setting up VPC's with distributed routing and network ACL's. 

Here is an example VPC deployment with three tiers, with VM's spanning 3 hypervisor hosts. VPC VR still needed to be deployed for north-south traffic. In this example VPC VR is deployed on host 3. A logical router which is nothing but a OVS bridge is provisioned on the rest of the hosts (excluding the host running VPC VR) in which VPC spans. On the host on which VPC VR is running there is no need for a logical router (bridge). Irrespective of weather a host has VM's belonging to a tier or not, a bridge is setup on each host for each tier on the all of the hosts on which VPC spans. For e.g. host 1, does not have any tier 2 VM's still a bridge is created and is in full-mesh topology with the bridges created for tier 2 on host 2 and 3. 

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