In  2 Repeatable Requests change An  unsafe request  is a non-idempotent request; that is, a request which has the potential to change the service each time it is executed. In 10.2 Client Conformance change If a client wants to safely repeat a request, a client: MUST specify the  Repeatability-Request-ID  header in an unsafe request (section  3.1.1 ) MUST specify the  Repeatability-First-Sent  header in a an unsafe request (section  3.1.2 )