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RMR: Check the read-the-docs page: https://docs.o-ran-sc.org/projects/o-ran-sc-ric-plt-lib-rmr/en/latest/rel-notes.html Additionally you might read RIC Message Router (RMR)
Generally check the read-the-docs pages: https://docs.o-ran-sc.org/en/latest/projects.html#near-realtime-ran-intelligent-controller-ric
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SDL API: https://docs.o-ran-sc.org/projects/o-ran-sc-ric-plt-sdl/en/latest/user-guide.html
Xapp onboarding: old link (likely outdated): https://docs.o-ran-sc.org/projects/o-ran-sc-it-dev/en/latest/developer-guide.html