Which layer of the SDN architecture orchestrates how the applications are given the resources available in the network?

The southbound API is only the communication interface between the control layer and the infrastructure (data plane) devices; it carries instructions downward but does not itself perform resource orchestration decisions.

The northbound API is only the communication interface between the application layer and the control layer; it carries application requests upward to the controller but does not perform orchestration.

An orchestration layer, when present, typically manages coordination across multiple SDN controllers or administrative domains at a higher abstraction level, rather than directly mediating between individual applications and network device resources.

The SDN control layer houses the SDN controller, which translates application requirements received via the northbound API into concrete network resource allocation decisions, then programs network devices via the southbound API to enforce those decisions. It is the decision-making plane that determines which network resources each application receives, acting as the central orchestrating intelligence between applications and the physical network. This function - mediating between application demand and infrastructure availability - is the defining role of the control layer.