IP SLA - Responder Functionality
When the ip sla responder command is issued on a target device, it enables the device to reply to all incoming IP SLA packets by default. The intelligence and configuration details are primarily managed by the IP SLA sending device.
The responder is necessary in order to measure metrics including jitter, one way delay, and packet loss. For other metrics they can be measured without responder, but not as accurately.
Before any Service Level Agreements (SLAs) are sent, the sender dispatches a control message to the responder, providing it with all necessary information to ensure accurate responses. This setup allows the responder to function correctly without requiring additional configuration.
Links
https://forum.networklessons.com/t/ip-sla-service-level-agreement-on-cisco-ios/789/51?u=lagapidis
https://www.cisco.com/c/en/us/td/docs/ios/ipsla/command/reference/sla_book/sla_02.html#wp1055041