FreeRadius : https://freeradius.org/rfc/rfc2865.html

RADIUS Attributes : https://www.cisco.com/c/en/us/td/docs/routers/asr9000/software/asr9k_r4-2/bng/configuration/guide/b_bng_cg42asr9k/b_bng_cg42asr9k_appendix_01000.pdf

ASR9k VSA : https://supportforums.cisco.com/t5/service-providers-documents/asr9000-xr-bng-vsa-s-vendor-specific-attributes-and-services/ta-p/3141601