core/: Concurrent connection attempts - aka. happy eyeball

[quininer]

I noticed that libp2p currently only tries next address when one address fails, which causes some users to wait a long time when trying to connect to an unavailable ipv6 address.

Maybe we can try to use a fast fallback strategy like happy eyeballs v2 to quickly connect to available addresses.

I don't know if there are some special reasons that limit connection strategy of libp2p, if not, I will be happy to implement one.

[quininer]

I checked the implementation of the connection pool, which may be a lot of work.

At present, we can use dial_addr to try concurrent connections.

[mxinden]

I am re-opening this issue. While complex for sure, it would greatly benefit hole punching.

When two peers coordinate a hole punch via the Direct Connection Upgrade through Relay protocol, each peer sends the remote a list of protocols to dial. Given that both sides need to simultaneously dial an address in order for a hole punch to suceed, one can not expect a high hole punch success rate when trying each address sequentially.

Given that rust-libp2p does not support parallel dialing today the rust-libp2p DCUtR implementation only attempts to dial the first address provided by the remote peer. Though it could instead emit multiple DialAddress requests at once.

[mxinden]

Implemented in #2248.