In this paper, we are interested in real-time flows requiring quantitative and deterministic Quality of Service (QoS) guarantees. We focus more particularly on two QoS parameters: the worst case end-to-end response time and jitter. We consider a non-preemptive scheduling of flows, called FP/FIFO, based on fixed priorities. On each node, packets are scheduled according to their fixed priority, if several packets share the same one, they are scheduled according to their arrival time on the node considered. The fixed priority associated with a flow denotes the importance of the flow from the user point of view. The scheduling FP/FIFO is the most common implementation of FP. We show how to take into account the FIFO arbitration in the worst case analysis, based on the trajectory approach, allowing to establish a bound on the worst case end-to-end response time of any flow in the network. Finally, we present an example illustrating our results.