An interesting topic is how to fight Hyrum's law. A possibility is to add randomness in things you don't want people to rely on. If I remember well, this is what the QUIC protocol does. Some fields are unused in the current version, but required by the specification to be set to random values, not null bytes, so that routers don't start relying on them to identify the packets.

EDIT.

I think I found the source: https://www.rfc-editor.org/rfc/rfc9000#section-17.2.1

> The value in the Unused field is set to an arbitrary value by the server. Clients MUST ignore the value of this field. [...] Note that other versions of QUIC might not make a similar recommendation.

I think they call it "greasing", to prevent "ossification".

> I think they call it "greasing"

This is a reference to RFC 8701, which coined the acronym GREASE ("Generate Random Extensions And Sustain Extensibility"), first in the context of TLS.

https://www.rfc-editor.org/rfc/rfc8701.html

(The earliest draft of the RFC dates back to mid-2016, which is likely the first public mention of the term: https://datatracker.ietf.org/doc/html/draft-davidben-tls-gre...)

This is wonderful. I’m quite familiar with QUIC but hadn’t heard about this.

Nothing like waking up after 10 years, realize you now really need those bits, and 20 different routers from 10 brands have decided that those bits must be a certain way.

Bonus points for checksums/crypto that breaks on the other end if the bits have been messed with. Curse those middle-boxes and their “clever hacks”.

Interesting thanks! Might indeed be valuable to add to one's toolbox.