On the server side:
tcpdump -n -i interface -n "src host client-host.com and dst port server-port"
You can get the client host using lsof:
lsof -itcp -a -p server_pid
This will print all active tcp connections to the server app with the specific pid.
On the client side:
tcpdump -n -i interface -n "dst host server-host.com and dst port server-port"
interface can be taken via ip addr