Publish UDP packet using rospy to topic

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New to ROS and networking. I am asked to write a python script that sends UDP packets to a ROS topic and the packets will be received by the ground station which receives the UDP ROS packets from the topic and sends back TCP packets written in non-ROS, pure python code. I am able to send UDP packets through ports, but not sure how to publish them to a topic. I would really appreciate if I could get some help on how to send UDP packets to a topic.