Below are some interesting examples of RFS, you can browse all of them at the link here: https://www.rfc-editor.org/
UDP
https://www.rfc-editor.org/rfc/rfc768.txt
TCP
https://www.ietf.org/rfc/rfc793.txt
Endianness:
https://www.rfc-editor.org/ien/ien137.txt
Port Assignments
https://www.rfc-editor.org/rfc/rfc1700.txt
Domains
https://www.rfc-editor.org/rfc/rfc799.txt
SMTP
https://www.rfc-editor.org/rfc/rfc788.txt
FTL Communication
https://www.rfc-editor.org/rfc/rfc6921.txt
IP over AC
https://www.rfc-editor.org/rfc/rfc1149.txt
US Domain
https://www.rfc-editor.org/rfc/rfc1386.txt
Misc
https://www.rfc-editor.org/rfc/rfc204.txt
https://www.rfc-editor.org/rfc/rfc425.txt