Ask HN: Which protocols should every developer know?

Hey HN! I'm looking to compile a list of interesting and useful protocols that would be good to be aware of as software developer.

Secondary goal is to find ideas for clients, and proofs of concepts to implement on niche languages. For example some that I have added to the list already

- DNS - SMTP - Scalar - Redis Protocol - gRPC

What else is out there that is interesting or could be fun to implement from scratch?