On Cisco routers there is an option to do port mirroring only on Rx or Tx side, by using this command:
monitor session session_number source interface interface-id [, | -] [both | rx | tx]
(Optional) Specify the direction of traffic to monitor. If you do not specify a traffic direction, the source interface sends both sent and received traffic.
both - Monitor both received and sent traffic
rx - Monitor received traffic
tx - Monitor sent traffic.
Is there a similar configuration on a Juniper MX960?