The Echo example is like a UDP echo server. It does nothing more than send back whatever packets are sent to it. No control plane software is required. The UDP port is set up to listen to port 0x1234, packets with a different destination port remain unmodified.
The Echo example is the only example design that uses an input metadata file, traffic_in.meta. The UDP destination port is set in traffic_in.meta as follows:
% Packet #1
metadata.udp_dst_port=1234 ;