Just adding specific info from the RFC 3588 sec 8.
When a Diameter server receives an Origin-State-Id that is greater
than the Origin-State-Id previously received from the same issuer, it
may assume that the issuer has lost state since the previous message
and that all sessions that were active under the lower Origin-State-
Id have been terminated. The Diameter server MAY clean up all
session state associated with such lost sessions, and MAY also issues
STRs for all such lost sessions that were authorized on upstream
servers, to allow session state to be cleaned up globally.
The Origin-State-Id AVP also present in Device-Watchdog Answer sent in response to Device-Watchdog Request.
If the value is ZERO means no reboot happened or u can say no restart happened.