mysql unable to start with semi sync parameter

Im using mysql 5.7 and it has 2 slaves with GTID. I tried to enable semi-sync and I have done with set global variable in mysql prompt. It was running fine. Then I did a restart, after that mysql unable to start and it says unknown variable 'rpl-semi-sync-master-enabled=1' ## mysql log 2018-11-03T08:28:35.453089Z 5 [Note] Start binlog_dump to master_thread_id(5) slave_server(2), pos(, 4) 2018-11-03T08:28:35.453119Z 5 [Note] Start asynchronous binlog_dump to slave (server_id: 2), pos(, 4) 2018-11-03T08:28:35.793103Z 6 [Note] Start binlog_dump to master_thread_id(6) slave_server(3), pos(, 4) 2018-11-03T08:28:35.793141Z 6 [Note] Start semi-sync binlog_dump to slave (server_id: 3), pos(, 4) 2018-11-03T08:29:25.656749Z 0 [Note] Giving 5 client threads a chance to die gracefully 2018-11-03T08:29:25.656793Z 0 [Note] Shutting down slave threads 2018-11-03T08:29:33.658167Z 5 [Note] Stop asynchronous binlog_dump to slave (server_id: 2) 2018-11-03T08:29:33.658360Z 6 [Note] Stop semi-sync binlog_dump to slave (server_id: 3) 2018-11-03T08:29:35.658275Z 0 [Note] Forcefully disconnecting 0 remaining clients 2018-11-03T08:29:35.658368Z 0 [Note] Event Scheduler: Purging the queue. 0 events 2018-11-03T08:29:35.658532Z 0 [Note] Binlog end 2018-11-03T08:29:35.662977Z 0 [Note] Shutting down plugin 'rpl_semi_sync_slave' 2018-11-03T08:29:35.663025Z 0 [Note] Shutting down plugin 'rpl_semi_sync_master' 2018-11-03T08:29:35.663059Z 0 [Note] unregister_replicator OK 2018-11-03T08:29:35.663062Z 0 [Note] Shutting down plugin 'auth_socket' ## syslog: Nov 3 04:11:17 mysql-systemd-start: ERROR: Unable to start MySQL server: Nov 3 04:11:17 mysql-systemd-start: 2018-11-03T04:11:17.961175Z 0 [ERROR] unknown variable 'rpl-semi-sync-master-enabled=1' Nov 3 04:11:17 mysql-systemd-start: 2018-11-03T04:11:17.964899Z 0 [ERROR] Aborting ## Update: I used the lines then it worked. But what is the use of these parameters? loose-rpl_semi_sync_master_enabled = 1 loose-rpl_semi_sync_slave_enabled = 1