Multiple transport support with fallbacks

[jsamol]

Hi all! 👋 I've been playing around with rust-libp2p recently, I wrote a simple application that discovers peers in the local network with mdns and then sends a simple JSON message using request-response. Technically, everything works fine, but I want to support multiple transports and this is the part that complicates it for me.

When my peer listens on multiple multiaddresses, it naturally sends all of them during the mDNS discovery phase. This means that in the mdns::Event::Discovered I get a list containing multiple entries for the same peer id. I then propagate these with Swarm::add_peer_address to my behaviours, so later I can call request_response::Behaviour::send_request to send my message to the selected peer. send_request takes only a peer id as an argument, so I can't specify which transport I want to use for this operation, which results in the message being sent multiple times to all the multiaddresses stored for the peer id.

Ideally, I'd like to specify an order of transports to try for communication, so only one is tried at a time and the next is used only as a fallback in case the first one didn't work. Is it somehow supported and I just missed it? Is it specific to this protocol only? Or should I rather handle the dialing myself to make sure only one transport is used to establish a connection with the peer?