How to Know the port number after the topic are subscribed

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I'm running cartographer_node (at a server, computer) which subscribed to rplidar_node(at a client, jetson nano) at /scan topic using ROSUDP, and I want to get the specific udp port of the cartographer_node of the server and rplidar_node of the client.

from the wireshark, I saw at first the rplidar would tell the master its ip:port(but it is TCPROS)

Afterwards, the cartographer_node(at server) would request a topic and passing its ip:port of the /scan topic to the client (I'm new here so I can't upload photos...) but then it just connected and sends topic data

I'm wonder if there is a sign to it?

I know there would be ephemeral ports after the publish-topic-subscribe are established, but is there anyways to know the ports of the specific topic of two ends after the socket are established?

I looked into codes of roscomm, but hard to understand

any assistance of yours would be greatly appreciated.