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RFC 0003 - Peer Routing Records

Abstract

This RFC proposes a method for distributing peer routing records, which contain a peer's publicly reachable listen addresses, and may be extended in the future to contain additional metadata relevant to routing. This serves a similar purpose to Ethereum Node Records. Like ENR records, libp2p routing records should be extensible, so that we can add information relevant to as-yet unknown use cases.

The record described here does not include a signature, but it is expected to be serialized and wrapped in a signed envelope, which will prove the identity of the issuing peer. The dialer can then prioritize self-certified addresses over addresses from an unknown origin.

Problem Statement

All libp2p peers keep a "peer store", which maps peer ids to a set of known addresses for each peer. When the application layer wants to contact a peer, the dialer will pull addresses from the peer store and try to initiate a connection on one or more addresses.

Addresses for a peer can come from a variety of sources. If we have already made a connection to a peer, the libp2p identify protocol will inform us of other addresses that they are listening on. We may also discover their address by querying the DHT, checking a fixed "bootstrap list", or perhaps through a pubsub message or an application-specific protocol.

In the case of the identify protocol, we can be fairly certain that the addresses originate from the peer we're speaking to, assuming that we're using a secure, authenticated communication channel. However, more "ambient" discovery methods such as DHT traversal and pubsub depend on potentially untrustworthy third parties to relay address information.

Even in the case of receiving addresses via the identify protocol, our confidence that the address came directly from the peer is not actionable, because the peer store does not track the origin of an address. Once added to the peer store, all addresses are considered equally valid, regardless of their source.

We would like to have a means of distributing verifiable address records, which we can prove originated from the addressed peer itself. We also need a way to track the "provenance" of an address within libp2p's internal components such as the peer store. Once those pieces are in place, we will also need a way to prioritize addresses based on their authenticity, with the most strict strategy being to only dial certified addresses.

Complications

While producing a signed record is fairly trivial, there are a few aspects to this problem that complicate things.

  1. Addresses are not static. A given peer may have several addresses at any given time, and the set of addresses can change at arbitrary times.
  2. Peers may not know their own addresses. It's often impossible to automatically infer one's own public address, and peers may need to rely on third party peers to inform them of their observed public addresses.
  3. A peer may inadvertently or maliciously sign an address that they do not control. In other words, a signature isn't a guarantee that a given address is valid.
  4. Some addresses may be ambiguous. For example, addresses on a private subnet are valid within that subnet but are useless on the public internet.

The first point can be addressed by having records contain a sequence number that increases monotonically when new records are issued, and by having newer records replace older ones.

The other points, while worth thinking about, are out of scope for this RFC. However, we can take care to make our records extensible so that we can add additional metadata in the future. Some thoughts along these lines are in the Future Work section below.

Address Record Format

Here's a protobuf that might work:

syntax = "proto3";

package peer.pb;

// PeerRecord messages contain information that is useful to share with other peers.
// Currently, a PeerRecord contains the public listen addresses for a peer, but this
// is expected to expand to include other information in the future.
//
// PeerRecords are designed to be serialized to bytes and placed inside of
// SignedEnvelopes before sharing with other peers.
message PeerRecord {

  // AddressInfo is a wrapper around a binary multiaddr. It is defined as a
  // separate message to allow us to add per-address metadata in the future.
  message AddressInfo {
    bytes multiaddr = 1;
  }

  // peer_id contains a libp2p peer id in its binary representation.
  bytes peer_id = 1;

  // seq contains a monotonically-increasing sequence counter to order PeerRecords in time.
  uint64 seq = 2;

  // addresses is a list of public listen addresses for the peer.
  repeated AddressInfo addresses = 3;
}

The AddressInfo wrapper message is used instead of a bare multiaddr to allow us to extend addresses with additional metadata in the future.

The seq field contains a sequence number that MUST increase monotonically as new records are created. Newer records MUST have a higher seq value than older records. To avoid persisting state across restarts, implementations MAY use unix epoch time as the seq value, however they MUST NOT attempt to interpret a seq value from another peer as a valid timestamp.

Example

  {
    peer_id: "QmAlice...",
    seq: 1570215229,
    addresses: [
      {
        multiaddr: "/ip4/1.2.3.4/tcp/42/p2p/QmAlice",
      },
      {
        multiaddr: "/ip4/10.0.1.2/tcp/42/p2p/QmAlice",
      }
    ]
  }

A peer SHOULD only include addresses that it believes are routable via the public internet, ideally having confirmed that this is the case via some external mechanism such as a successful AutoNAT dial-back.

In some cases we may want to include localhost or LAN-local address; for example, when testing the DHT using many processes on a single machine. To support this, implementations may use a global runtime configuration flag or environment variable to control whether local addresses will be included.

Certification / Verification

This structure can be serialized and contained in a signed envelope, which lets us issue "self-certified" address records that are signed by the peer that the addresses belong to.

To produce a "self-certified" address, a peer will construct a RoutingState containing their listen addresses and serialize it to a byte array using a protobuf encoder. The serialized records will then be wrapped in a signed envelope, which is signed with the libp2p peer's private host key. The corresponding public key MUST be included in the envelope's public_key field.

When receiving a RoutingState wrapped in a signed envelope, a peer MUST validate the signature before deserializing the RoutingState record. If the signature is invalid, the envelope MUST be discarded without deserializing the envelope payload.

Once the signature has been verified and the RoutingState has been deserialized, the receiving peer MUST verify that the peer_id contained in the RoutingState matches the public_key from the envelope. If the public key in the envelope cannot derive the peer id contained in the routing state record, the RoutingState MUST be discarded.

Signed Envelope Domain

Signed envelopes require a "domain separation" string that defines the scope or purpose of a signature.

When wrapping a RoutingState in a signed envelope, the domain string MUST be libp2p-routing-state.

Signed Envelope Payload Type

Signed envelopes contain a payload_type field that indicates how to interpret the contents of the envelope.

Ideally, we should define a new multicodec for routing records, so that we can identify them in a few bytes. While we're still spec'ing and working on the initial implementation, we can use the UTF-8 string "/libp2p/routing-state-record" as the payload_type value.

Peer Store APIs

We will need to add a few methods to the peer store:

  • AddCertifiedAddrs(envelope) -> Maybe<Error>

    • Add a self-certified address, wrapped in a signed envelope. This should validate the envelope signature & store the envelope for future reference. If any certified addresses already exist for the peer, only accept the new envelope if it has a greater seq value than existing envelopes.

  • CertifiedAddrs(peer_id) -> Set<Multiaddr>

    • return the set of self-certified addresses for the given peer id

  • SignedRoutingState(peer_id) -> Maybe<SignedEnvelope>

    • retrieve the signed envelope that was most recently added to the peerstore for the given peer, if any exists.

And possibly:

  • IsCertified(peer_id, multiaddr) -> Boolean
    • has a particular address been self-certified by the given peer?

We'll also need a method that constructs a new RoutingState containing our listen addresses and wraps it in a signed envelope. This may belong on the Host instead of the peer store, since it needs access to the private signing key.

When adding records to the peerstore, a receiving peer MUST keep track of the latest seq value received for each peer and reject incoming RoutingState messages unless they contain a greater seq value than the last received.

After integrating the information from the RoutingState into the peerstore, implementations SHOULD retain the original signed envelope. This will allow other libp2p systems to share signed RoutingState records with other peers in the network, preserving the signature of the issuing peer. The Exchanging Records section section lists some systems that would need to retrieve the original signed record from the peerstore.

Dialing Strategies

Once self-certified addresses are available via the peer store, we can update the dialer to prefer using them when possible. Some systems may want to only dial self-certified addresses, so we should include some configuration options to control whether non-certified addresses are acceptable.

Exchanging Records

We currently have several systems in libp2p that deal with peer addressing and which could be updated to use signed routing records:

Of these, the highest priority for updating seems to be the DHT, since it's actively used by several deployed systems and is vulnerable to routing attacks by malicious peers. We should work on extending the FIND_NODE, ADD_PROVIDER, and GET_PROVIDERS RPC messages to support returning signed records in addition to the current unsigned address information they currently support.

We should also either define a new "secure peer routing" interface or extend the existing peer routing interfaces to support signed records, so that we don't end up with a bunch of similar but incompatible APIs for exchanging signed address records.

Future Work

Some things that were originally considered in this RFC were trimmed so that we can focus on delivering a basic self-certified record, which is a pressing need.

This includes a notion of "routability", which could be used to communicate whether a given address is global (reachable via the public internet), LAN-local, etc. We may also want to include some kind of confidence score or priority ranking, so that peers can communicate which addresses they would prefer other peers to use.

To allow these fields to be added in the future, we wrap multiaddrs in the AddressInfo message instead of having the addresses field be a list of "raw" multiaddrs.

Another potentially useful extension would be a compact protocol table or bloom filter that could be used to test whether a peer supports a given protocol before interacting with them directly. This could be added as a new field in the RoutingState message.