Ad hoc networks (AHNs) include mobile nodes with limited communication resources on which the path selection is usually made according to a best effort strategy. Today, AHNs are facing a new challenge: a QoS routing based on bandwidth and delay requirements. The importance of this challenge makes this issue focused by several researchers. Consequently, we are interested in this paper by a routing QoS oriented in AHNs. One of the major contributions of this paper concerns the proposition of a novel layered QoS architecture and a contention resolution mechanism allowing a QoS routing definition. Our proposition is based on traffic classification and requires metrics definition allowing the expression of the QoS requirements in order to provide differentiated routing according to traffic classes.