1. A data processing system, comprising: a system bus; a host bridge connected to the system bus; at least one input/output unit connected to the host bridge; at least one of a traffic class mechanism in conjunction with virtual channel resources and a relaxed ordering mechanism for differentiating input/output unit data flow operations; and functionality in the host bridge for controlling the input/output unit data flow operations using the at least one of the traffic class mechanism in conjunction with virtual channel resources and the relaxed ordering mechanism.