@scanlime And as is tradition, as soon as I ask, I figure it out for myself:
https://www.rfc-editor.org/ien/ien2.txt
https://www.rfc-editor.org/ien/ien26.pdf (WHERE IS PAGE 3?!)
https://www.rfc-editor.org/ien/ien28.pdf (secion 2.3)
... and I am sure it continues as the evolution of the protocol got to v4)