NCSC-TG-011: Trusted Network Interpretation Environments Guideline – Guidance for Applying the TNI, 1 August 1990. (Red Book)
You can download it from the following link: https://packetstormsecurity.com/files/download/14002/NCSC-TG-011.html
Source: https://packetstormsecurity.com/files/14002/NCSC-TG-011.html.html