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How Data Flow(or Routing) in Hierarchical or Layered Switches in SDN(Software Defined Networking)

Is it possible to send data packets from end to end nodes(Hosts) in SDN using OVS switches, where OVS switches formation is like Hierarchical or Layer structured and only the top or higher level OVS switches are physically connected with the controller , not the leaf or intermediate switches. The leaf or intermediate switches are logically connected or maybe not connected with the controller.

I'm create a Network Simulation in Mininet that look like following pic:


Here, I am trying to find information on how routing works in SDN networks. Consider a network with a client, 2 servers, one which hosts files and another one which hosts an SDN controller, and 2 switches that run in SDN mode. The network is connected like shown in the picture.

So now the Client wants to download a file from the server and therefore sends a message to it. When Switch 1 does not have a corresponding flow table entry it generates a packet-in message and sends it to the Server that hosts the controller.

Now what happens when the packet-in message arrives at Switch 2? Will it lookup what to do with the packet-in message? If so, and there is no flow table entry, I assume that Switch 2 will generate another packet-in message and sends it to the controller, waits for a flow-mod message and only then forwards the first packet-in message to the controller. Is that how it works? How likely is such a scenario in reality?

Please give some resource link also.

Thank You,    Chiranjit

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