Here is the Cap'n Proto RPC protocol:

https://github.com/capnproto/capnproto/blob/v2/c%2B%2B/src/c...

That's just the RPC state machine -- the serialization is specified elsewhere, and the state machine is actually schema-agnostic. (Schemas are applied at the edges, when messages are actually received from the app or delivered to it.)

This is the Cap'n Web protocol, including serialization details:

https://github.com/cloudflare/capnweb/blob/main/protocol.md

Now, to be fair, Cap'n Proto has a lot of features that Cap'n Web doesn't have yet. But Cap'n Web's high-level design is actually a lot simpler.

Among other things, I merged the concepts of call-return and promise-resolve. (Which, admittedly, CapTP was doing it that way before I even designed Cap'n Proto. It was a complete mistake on my part to turn them into two separate concepts in Cap'n Proto, but it seemed to make sense at the time.)

What I'd like to do is go back and revise the Cap'n Proto protocol to use a similar design under the hood. This would make no visible difference to applications (they'd still use schemas), but the state machine would be much simpler, and easier to port to more languages.

I was trying to port Cap'n Proto to modern C# as a side project when I was unemployed, since the current implementation years old and new C# features have been released that would make it much nicer to use.

I love the no-copy serialization and object capabilities, but wow, the RPC protocol is incredibly complex, it took me a while to wrap my head around it, and I often had to refer to the C++ implementation to really get it.