Johnson believes RESTful APIs will be the standard on the northbound side while OpenFlow will be among several APIs used on the southbound side. In fact, one southbound API could be SNMP, he says, the network management polling protocol that's been in use for decades.
"There's a lot of legacy network infrastructure for which something else (besides OpenFlow) would be better, because you may not want to replace the control plane," Johnson says. "You may just want to leverage it."
Big Switch agrees with the need to be able to include legacy investments in SDN architectures and supports what it calls a hybrid approach to network virtualization, using both native OpenFlow and an overlay approach to support network resources that do not "sit on a hypervisor," says co-founder Kyle Forster.