Flexible networks need flexible addressing systems. Since libp2p is designed to work across a wide variety of networks, we need a way to work with a lot of different addressing schemes in a consistent way.
multiaddress (often abbreviated
multiaddr), is a convention for encoding multiple layers of addressing information into a single “future-proof” path structure. It defines human-readable and machine-optimized encodings of common transport and overlay protocols and allows many layers of addressing to be combined and used together.
/ip4/127.0.0.1/udp/1234 encodes two protocols along with their essential addressing information. The
/ip4/127.0.0.1 informs us that we want the
127.0.0.1 loopback address of the IPv4 protocol, and
/udp/1234 tells us we want to send UDP packets to port
Things get more interesting as we compose further. For example, the multiaddr
/p2p/QmYyQSo1c1Ym7orWxLYvCrM2EmxFTANf8wXmmE7DWjhx5N uniquely identifies my local IPFS node, using libp2p’s registered protocol id
/p2p/ and the multihash of my IPFS node’s public key.
For more on peer identity and its relation to public key cryptography, see Peer Identity.
Let’s say that I have the peer id
QmYyQSo1c1Ym7orWxLYvCrM2EmxFTANf8wXmmE7DWjhx5N as above, and my public ip is
188.8.131.52. I start my libp2p application and listen for connections on TCP port
Now I can start handing out multiaddrs to all my friends, of the form
/ip4/184.108.40.206/tcp/4242/p2p/QmYyQSo1c1Ym7orWxLYvCrM2EmxFTANf8wXmmE7DWjhx5N. Combining my “location multiaddr” (my IP and port) with my “identity multiaddr” (my libp2p
PeerId), produces a new multiaddr containing both key pieces of information.
Now not only do my friends know where to find me, anyone they give that address to can verify that the machine on the other side is really me, or at least, that they control the private key for my
PeerId. They also know (by virtue of the
/p2p/ protocol id) that I’m likely to support common libp2p interactions like opening connections and negotiating what application protocols we can use to communicate. That’s not bad!
This can be extended to account for multiple layers of addressing and abstraction. For example, the addresses used for circuit relay combine transport addresses with multiple peer identities to form an address that describes a “relay circuit”:
For more detail, see the multiaddr spec, which has links to many implementations.