How to reference a technical document (like an RFC) in APA style?
The APA Style blog walks you through citations with missing info. There is also how to cite a book with no author or editor. You seem to be using DARPA as the author, but from the title page, it seems mote likely that ISI/USC is the institutional author. If you dig deeper, the linked document lists Postel as the editor. In that case you just do a book without an author, but with an editor: https://owl.purdue.edu/owl/research_and_citation/apa_style/apa_formatting_and_style_guide/reference_list_books.html

Edited Book, No Author
Duncan, G. J., &amp; Brooks-Gunn, J. (Eds.). (1997). Consequences of growing up poor. New York, NY: Russell Sage Foundation.