Let's begin by modeling our robot base. As we mentioned previously, ROS understands a robot in terms of URDF. URDF is a list of XML tags that contains all of the necessary information of the robot. Once the URDF for the robot base is created, we shall bring in the necessary connectors and wrappers around the code so that we can interact and communicate with a standalone physics simulator such as Gazebo. Let's see how the robot base is built step by step.