Bhushan, A., Braden, B., Crowther, W., Harslem, E., Heafner, J., McKenzie, A., Melvin, J., Sundberg, B., Watson, D., and J. White, "The File Transfer Protocol", RFC 172, DOI 10.17487/RFC0172, June 1971, <https://www.rfc-editor.org/info/rfc172>.