https://www.juniper.net/documentation/us/en/software/junos/cli-reference/topics/ref/statement/source-udp-port-edit-services-rpm-rfc2544-benchmarking.html
Specify the UDP port of the source to be used in the UDP header for the generated frames. If you do not specify the UDP port, the default value of 4041 is used.
udp portjunos osjuniper networkssourcerfc