Using custom Transport Protocol (Cap'n Proto RPC)

[Tanguille]

I want to use libp2p with Cap'n Proto. I can't seem to find any documentation in how you implement a protocol that isn't supported by libp2p so I hope someone here can help me.

I tried to compose a multiaddr with this custom format and it gave a compilation error:

swarm.listen_on("/ip4/0.0.0.0/capnp-rpc/0".parse()?)?;

Here you can find my sandbox project where I'm testing this out if that would help to show what I mean.
Thanks in advance.

[thomaseizinger]

You seem to be conflating several things. I can recommend reading the general docs first: https://docs.libp2p.io/concepts/introduction/overview/

[Tanguille]

Hey, thanks for responding. I have read everything until PubSub. What part are you referring to that describes how you can implement a custom protocol?

[thomaseizinger]

Cool, thanks for reading through it! :)

As you know now, a transport in libp2p is typically multiplexed, authenticated and encrypted. Captn proto provides none of these things.

But what I think you should be able to use is its serialization and data format. If you just want to do request-response style messages, then our request_response module might be of help.

If you want other message patterns, then you'd have to implement a custom ConnectionHandler and NetworkBehaviour. In that case though, I'd suggest to think about your problem differently. In rust-libp2p, we have modules that implement NetworkBehaviour. They are essentially plugins with a certain responsibility like managing a DHT, pub-sub, etc.

Captn Proto is a serialisation format. It doesn't really make sense IMO to ask: "How can I write a plugin for that?". You should be thinking about your usecase, which messages are to be exchanged and when and then perhaps use captn proto as an implementation detail for your serialisation, the same way you could just use cbor, json or anything else.

Hope that helps :)

[Tanguille]

This is certainly helpful, thanks. Might be that I totally misunderstood this but isn't RPC also a transport protocol? Cap'n Proto has an RPC implementation that has several benefits. That is what I wanna use and why I formulated my question that way. What I want is that the Switch makes a secure channel with multiplexing using this RPC. Is that something that would make sense to you?

[thomaseizinger]

I think you should be able to just use captn proto on top of an existing stream established by e.g. yamux layered on top of noise. Or a QUIC stream.

If I understand the above article correctly, captn proto just emits bytes that you still need to send over a socket. You can just send that over a stream instead :)