Hi,
Please review the following threads:
https://forums.juniper.net/t5/Routing/IP-SLA-equilvilent-for-Junos/td-p/297703
https://forums.juniper.net/t5/IOS-to-Junos-I2J-Tips/ip-sla-ip-monitoring/td-p/140513
https://forums.juniper.net/t5/Ethernet-Switching/services-ip-monitoring-Feature-On-EX/td-p/274823
Note: "services-ip-monitoring" for specfic to SRX, but there are way around for that in thread.