Aspects relate to methods, devices and manufacturing relating to routing networks including a method for routing data units. The data units are individually routable through a routing network. A reservation request data unit is received over the routing network and from a sender. At least one speculative data unit associated with the reservation request data unit from the sender is received. The at least one speculative data unit is dropped in response to the at least one speculative data unit being blocked within the routing network. The sender is provided, over the routing network, a negative acknowledgement data unit that indicates the dropping of the at least one speculative data unit. The sender is provided a grant data unit indicating a start time. After the start time, at least one non-speculative data unit corresponding to the reservation request from the sender is received.