Swarm engineering is a systematic application of scientific and technical knowledge to specify requirements, model, design, realize, verify, validate, operate and maintain a swarm intelligence system. In swarm robotics, there is not a well-structured methodology until today for developing robotic swarm systems. Several researchers have developed steps to design swarm robots but these steps are still incomplete. In this paper, we focus on the functional architecture of the swarm robots where we propose a top-down approach to ensure consistency and continuity from requirement level to behavioral level up to the functional and structural levels. This approach is based on the Model-Based Systems Engineering method (MBSE) using the Systems Modeling Language (SysML) where we present the allocations between the functions of each swarm member and the overall swarm behaviors. Then, we will be interested in the architecture of Robot Operating System (ROS) of a swarm behavior where we identify the allocations between the component and the functions of a robot.