Communication channel for distributed network control system

For a first controller for managing managed forwarding elements that forward data in a network, a method for computing forwarding state to send to a second controller, the method comprising:

• generating a set of outputs that includes the forwarding state that defines forwarding behavior of a set of managed forwarding elements;
• sending the outputs to the second controller as the outputs are generated;
• and
• sending an indicator that indicates end of the set of outputs to the second controller so that the second controller starts processing the outputs only after receiving the indicator.

For a recipient controller for managing a set of managed forwarding elements that forward data in a network, a method for computing forwarding state using several sets of inputs from the several source controllers, the method comprising:

• receiving a first set of inputs from a first source controller, the first set of inputs comprising a group of inputs that is to be separately from another group of inputs;
• receiving a second set of inputs from a second source controller, the second set of inputs comprising a group of inputs that is to be processed separately from another group of inputs;
• combining the first and second set of inputs to form a single group of inputs that is to be processed separately from another group of inputs;
• and
• computing forwarding state using the single set of inputs, the forwarding state defining forwarding behaviors of managed forwarding elements.