This book is irrevocably open source.
For text, we apply the Creative Commons Attribution 4.0 license, known as CC BY 4.0. See https://creativecommons.org/licenses/by/4.0/. This is also the license used by the IETF Trust for text.
For any associated code, we preferably apply the Revised BSD License (a.k.a the BSD 3-clause license). This is also the license used by the IETF Trust for code.