libp2p: explode!

9 files changed, 3476 insertions, 0 deletions

diff --git a/src/transport/libp2p/behaviour.rs b/src/transport/libp2p/behaviour.rs
new file mode 100644
index 0000000..4ced88a
--- /dev/null
+++ b/src/transport/libp2p/behaviour.rs
@@ -0,0 +1,364 @@
+//! The libp2p behaviours this transport combines (`Behaviour`), hole punching with LAN
+//! candidates (`HolePunching`), and building the swarm (TCP and QUIC, or in memory).
+
+use super::*;
+
+/// The protocol version identify says (informational: nothing checks it).
+pub(super) const IDENTIFY_VERSION: &str = "/continuum/1";
+
+#[derive(::libp2p::swarm::NetworkBehaviour)]
+pub(super) struct Behaviour {
+    /// First: connections it denies need nothing from the others.
+    pub(super) ignoring: Ignoring,
+    pub(super) stream: libp2p_stream::Behaviour,
+    pub(super) identify: ::libp2p::identify::Behaviour,
+    pub(super) relay_client: ::libp2p::relay::client::Behaviour,
+    /// Only with `Libp2pConfig::relay`.
+    pub(super) relay: ::libp2p::swarm::behaviour::toggle::Toggle<::libp2p::relay::Behaviour>,
+    pub(super) dcutr: HolePunching,
+    /// Each connection's round trip (`Book::rtts`): of several to one peer, the fastest
+    /// is kept.
+    pub(super) ping: ::libp2p::ping::Behaviour,
+    pub(super) autonat: ::libp2p::autonat::Behaviour,
+    /// Only with `Libp2pConfig::mdns`.
+    pub(super) mdns: ::libp2p::swarm::behaviour::toggle::Toggle<::libp2p::mdns::tokio::Behaviour>,
+    /// Only with `Libp2pConfig::upnp`: maps our listen ports on the router, and confirms
+    /// the mapped addresses as ours (identify then tells them).
+    pub(super) upnp: ::libp2p::swarm::behaviour::toggle::Toggle<::libp2p::upnp::tokio::Behaviour>,
+    /// Only with `Libp2pConfig::public`: the IPFS DHT.
+    pub(super) kad: ::libp2p::swarm::behaviour::toggle::Toggle<
+        ::libp2p::kad::Behaviour<::libp2p::kad::store::MemoryStore>,
+    >,
+}
+
+/// libp2p's hole punching (DCUtR), offering our listen addresses too. DCUtR offers only
+/// where others saw us (external address candidates, from identify): seen by a relay
+/// out on the internet, two nodes on one LAN are both at their router's public address,
+/// and punching through it needs the router to hairpin, which many don't. Their LAN
+/// addresses work, the firewalls on the way being stateful. Only DCUtR is told:
+/// AutoNAT would test every candidate, and fail on all of these.
+pub(super) struct HolePunching {
+    pub(super) inner: ::libp2p::dcutr::Behaviour,
+    /// Where we listen (`offered`).
+    pub(super) listening: Vec<Multiaddr>,
+    /// `Libp2pConfig::ignore`: not offered.
+    pub(super) ignored: Vec<IpRange>,
+}
+
+/// Whether DCUtR offers our listen address `address` too: neither loopback nor
+/// link-local (as for publishing), nor through a relay.
+pub(super) fn offered(address: &Multiaddr) -> bool {
+    !is_local(address) && !is_relayed(address)
+}
+
+impl HolePunching {
+    pub(super) fn new(local: PeerId, ignored: Vec<IpRange>) -> Self {
+        Self {
+            inner: ::libp2p::dcutr::Behaviour::new(local),
+            listening: Vec::new(),
+            ignored,
+        }
+    }
+
+    /// Tells DCUtR where we listen, as if others had seen us there. Again after every
+    /// other candidate, as DCUtR keeps the latest 20.
+    pub(super) fn offer_listening(&mut self) {
+        use ::libp2p::swarm::{FromSwarm, NetworkBehaviour, NewExternalAddrCandidate};
+        for addr in &self.listening {
+            self.inner
+                .on_swarm_event(FromSwarm::NewExternalAddrCandidate(
+                    NewExternalAddrCandidate { addr },
+                ));
+        }
+    }
+}
+
+impl ::libp2p::swarm::NetworkBehaviour for HolePunching {
+    type ConnectionHandler =
+        <::libp2p::dcutr::Behaviour as ::libp2p::swarm::NetworkBehaviour>::ConnectionHandler;
+    type ToSwarm = ::libp2p::dcutr::Event;
+
+    fn handle_pending_inbound_connection(
+        &mut self,
+        connection_id: ConnectionId,
+        local_addr: &Multiaddr,
+        remote_addr: &Multiaddr,
+    ) -> Result<(), ::libp2p::swarm::ConnectionDenied> {
+        self.inner
+            .handle_pending_inbound_connection(connection_id, local_addr, remote_addr)
+    }
+
+    fn handle_established_inbound_connection(
+        &mut self,
+        connection_id: ConnectionId,
+        peer: PeerId,
+        local_addr: &Multiaddr,
+        remote_addr: &Multiaddr,
+    ) -> Result<::libp2p::swarm::THandler<Self>, ::libp2p::swarm::ConnectionDenied> {
+        if is_relayed(local_addr) {
+            // DCUtR offers its candidates on this connection: where others saw us, and
+            // these.
+            debug!(peer = %Addressed(SCHEME, &peer), listening = ?self.listening, "Reached through a relay; hole punching offers where we listen too");
+        }
+        self.inner.handle_established_inbound_connection(
+            connection_id,
+            peer,
+            local_addr,
+            remote_addr,
+        )
+    }
+
+    fn handle_pending_outbound_connection(
+        &mut self,
+        connection_id: ConnectionId,
+        maybe_peer: Option<PeerId>,
+        addresses: &[Multiaddr],
+        effective_role: ::libp2p::core::Endpoint,
+    ) -> Result<Vec<Multiaddr>, ::libp2p::swarm::ConnectionDenied> {
+        self.inner.handle_pending_outbound_connection(
+            connection_id,
+            maybe_peer,
+            addresses,
+            effective_role,
+        )
+    }
+
+    fn handle_established_outbound_connection(
+        &mut self,
+        connection_id: ConnectionId,
+        peer: PeerId,
+        addr: &Multiaddr,
+        role_override: ::libp2p::core::Endpoint,
+        port_use: ::libp2p::core::transport::PortUse,
+    ) -> Result<::libp2p::swarm::THandler<Self>, ::libp2p::swarm::ConnectionDenied> {
+        self.inner.handle_established_outbound_connection(
+            connection_id,
+            peer,
+            addr,
+            role_override,
+            port_use,
+        )
+    }
+
+    fn on_swarm_event(&mut self, event: ::libp2p::swarm::FromSwarm) {
+        use ::libp2p::swarm::FromSwarm;
+        self.inner.on_swarm_event(event);
+        match event {
+            FromSwarm::NewListenAddr(new)
+                if offered(new.addr) && !ignored(&self.ignored, new.addr) =>
+            {
+                if !self.listening.contains(new.addr) {
+                    debug!(address = %new.addr, "Offering where we listen for hole punching");
+                    self.listening.push(new.addr.clone());
+                }
+                self.offer_listening();
+            }
+            FromSwarm::ExpiredListenAddr(expired) => {
+                self.listening.retain(|address| address != expired.addr);
+            }
+            FromSwarm::NewExternalAddrCandidate(_) => self.offer_listening(),
+            _ => {}
+        }
+    }
+
+    fn on_connection_handler_event(
+        &mut self,
+        peer_id: PeerId,
+        connection_id: ConnectionId,
+        event: ::libp2p::swarm::THandlerOutEvent<Self>,
+    ) {
+        self.inner
+            .on_connection_handler_event(peer_id, connection_id, event)
+    }
+
+    fn poll(
+        &mut self,
+        cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<
+        ::libp2p::swarm::ToSwarm<Self::ToSwarm, ::libp2p::swarm::THandlerInEvent<Self>>,
+    > {
+        self.inner.poll(cx)
+    }
+}
+
+/// What the behaviour has besides what every node has.
+#[derive(Clone, Copy)]
+pub(super) struct Features {
+    pub(super) relay: bool,
+    pub(super) mdns: bool,
+    pub(super) upnp: bool,
+    pub(super) public: bool,
+}
+
+impl Features {
+    pub(super) fn of(config: &Libp2pConfig) -> Self {
+        Self {
+            relay: config.relay,
+            mdns: config.mdns,
+            upnp: config.upnp,
+            public: config.public,
+        }
+    }
+}
+
+/// The IPFS DHT's protocol, and its bootstrap nodes (all at `IPFS_BOOTSTRAP_ADDRESS`).
+pub(super) const IPFS_DHT: StreamProtocol = StreamProtocol::new("/ipfs/kad/1.0.0");
+
+pub(super) const IPFS_BOOTSTRAP_ADDRESS: &str = "/dnsaddr/bootstrap.libp2p.io";
+
+pub(super) const IPFS_BOOTSTRAP_PEERS: [&str; 4] = [
+    "QmNnooDu7bfjPFoTZYxMNLWUQJyrVwtbZg5gBMjTezGAJN",
+    "QmQCU2EcMqAqQPR2i9bChDtGNJchTbq5TbXJJ16u19uLTa",
+    "QmbLHAnMoJPWSCR5Zhtx6BHJX9KiKNN6tpvbUcqanj75Nb",
+    "QmcZf59bWwK5XFi76CZX8cbJ4BhTzzA3gU1ZjYZcYW3dwt",
+];
+
+/// How often a node on the DHT refreshes its routing table.
+pub(super) const DHT_BOOTSTRAP: Duration = Duration::from_secs(300);
+
+impl Behaviour {
+    pub(super) fn new(
+        key: &Keypair,
+        relay_client: ::libp2p::relay::client::Behaviour,
+        features: Features,
+        ignored: Vec<IpRange>,
+    ) -> std::io::Result<Self> {
+        let Features {
+            relay,
+            mdns,
+            upnp,
+            public,
+        } = features;
+        let peer_id = key.public().to_peer_id();
+        let kad = public.then(|| {
+            let mut dht = ::libp2p::kad::Behaviour::with_config(
+                peer_id,
+                ::libp2p::kad::store::MemoryStore::new(peer_id),
+                ::libp2p::kad::Config::new(IPFS_DHT),
+            );
+            let bootstrap: Multiaddr = IPFS_BOOTSTRAP_ADDRESS.parse().expect("a multiaddr");
+            for peer in IPFS_BOOTSTRAP_PEERS {
+                let peer: PeerId = peer.parse().expect("a peer ID");
+                let _ = dht.add_address(&peer, bootstrap.clone());
+            }
+            dht
+        });
+        let identify = ::libp2p::identify::Config::new(IDENTIFY_VERSION.to_owned(), key.public())
+            .with_agent_version(format!("continuum/{}", env!("CARGO_PKG_VERSION")));
+        let relay = relay.then(|| {
+            ::libp2p::relay::Behaviour::new(
+                peer_id,
+                ::libp2p::relay::Config {
+                    max_circuits: RELAY_CIRCUITS,
+                    max_circuits_per_peer: RELAY_CIRCUITS_PER_PEER,
+                    max_circuit_duration: RELAY_CIRCUIT_DURATION,
+                    max_circuit_bytes: RELAY_CIRCUIT_BYTES,
+                    reservation_rate_limiters: Vec::new(),
+                    circuit_src_rate_limiters: Vec::new(),
+                    ..Default::default()
+                },
+            )
+        });
+        let mdns = match mdns {
+            true => Some(::libp2p::mdns::tokio::Behaviour::new(
+                ::libp2p::mdns::Config::default(),
+                peer_id,
+            )?),
+            false => None,
+        };
+        Ok(Self {
+            ignoring: Ignoring {
+                ranges: ignored.clone(),
+            },
+            stream: libp2p_stream::Behaviour::new(),
+            identify: ::libp2p::identify::Behaviour::new(identify),
+            relay_client,
+            relay: relay.into(),
+            dcutr: HolePunching::new(peer_id, ignored),
+            ping: ::libp2p::ping::Behaviour::new(::libp2p::ping::Config::new()),
+            autonat: ::libp2p::autonat::Behaviour::new(
+                peer_id,
+                ::libp2p::autonat::Config {
+                    // Swarms on private networks (a LAN, WireGuard) count too.
+                    only_global_ips: false,
+                    ..Default::default()
+                },
+            ),
+            mdns: mdns.into(),
+            upnp: upnp.then(::libp2p::upnp::tokio::Behaviour::default).into(),
+            kad: kad.into(),
+        })
+    }
+}
+
+pub(super) fn network_swarm(
+    key: Keypair,
+    features: Features,
+    ignored: Vec<IpRange>,
+) -> Result<Swarm<Behaviour>> {
+    Ok(::libp2p::SwarmBuilder::with_existing_identity(key)
+        .with_tokio()
+        .with_tcp(
+            ::libp2p::tcp::Config::default(),
+            ::libp2p::noise::Config::new,
+            ::libp2p::yamux::Config::default,
+        )?
+        .with_quic()
+        .with_dns()?
+        .with_relay_client(
+            ::libp2p::noise::Config::new,
+            ::libp2p::yamux::Config::default,
+        )?
+        .with_behaviour(|key, relay_client| {
+            Behaviour::new(key, relay_client, features, ignored.clone())
+                .map_err(|error| Box::new(error) as Box<dyn std::error::Error + Send + Sync>)
+        })?
+        .with_swarm_config(|config| swarm_config(config, features))
+        .build())
+}
+
+/// The swarm's settings. Streams are opened without waiting for the other side to agree
+/// on the protocol (`V1Lazy`): the protocol and the first data go together, saving a
+/// round trip per message (one stream each, module docs). That is safe only towards
+/// peers sure to speak the protocol, so not with public libp2p infrastructure
+/// (`Libp2pConfig::public`), whose nodes speak only some: there a protocol they don't
+/// would fail late, and unclearly.
+pub(super) fn swarm_config(
+    config: ::libp2p::swarm::Config,
+    features: Features,
+) -> ::libp2p::swarm::Config {
+    let config = config
+        .with_idle_connection_timeout(IDLE_TIMEOUT)
+        .with_dial_concurrency_factor(DIAL_CONCURRENCY);
+    if features.public {
+        config
+    } else {
+        config.with_substream_upgrade_protocol_override(::libp2p::core::upgrade::Version::V1Lazy)
+    }
+}
+
+pub(super) fn memory_swarm(
+    key: Keypair,
+    features: Features,
+    ignored: Vec<IpRange>,
+) -> Result<Swarm<Behaviour>> {
+    use ::libp2p::Transport as _;
+    use ::libp2p::core::transport::{MemoryTransport, OrTransport};
+    use ::libp2p::core::upgrade::Version;
+
+    let peer_id = key.public().to_peer_id();
+    let (relay_transport, relay_client) = ::libp2p::relay::client::new(peer_id);
+    let transport = OrTransport::new(relay_transport, MemoryTransport::default())
+        .boxed()
+        .upgrade(Version::V1)
+        .authenticate(::libp2p::noise::Config::new(&key)?)
+        .multiplex(::libp2p::yamux::Config::default())
+        .timeout(CONNECT_TIMEOUT)
+        .boxed();
+    Ok(Swarm::new(
+        transport,
+        Behaviour::new(&key, relay_client, features, ignored)?,
+        peer_id,
+        swarm_config(::libp2p::swarm::Config::with_tokio_executor(), features),
+    ))
+}
diff --git a/src/transport/libp2p/book.rs b/src/transport/libp2p/book.rs
new file mode 100644
index 0000000..5658c56
--- /dev/null
+++ b/src/transport/libp2p/book.rs
@@ -0,0 +1,524 @@
+//! The book: who is where (peers' addresses, connections and their round trips), the
+//! relays and their slots, hole punching and upgrades, and the counts the figures show.
+
+use super::*;
+
+/// Who named a relay to pick, in the order they are picked.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub(super) enum Named {
+    /// `Libp2pConfig::relays`.
+    Configured,
+    /// The relays listed in the document (`Transport::learn_relays`).
+    Listed,
+}
+
+impl Named {
+    pub(super) fn as_str(self) -> &'static str {
+        match self {
+            Self::Configured => "configured",
+            Self::Listed => "listed",
+        }
+    }
+}
+
+/// A relay named to pick.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub(super) struct NamedRelay {
+    /// Where, with the relay's peer ID.
+    pub(super) at: Multiaddr,
+    /// As named: without the peer ID, it is found out again when the relay doesn't
+    /// grant a slot (it may have a new one).
+    pub(super) given: Multiaddr,
+    pub(super) named: Named,
+}
+
+/// Where peers are, and where we listen.
+#[derive(Default)]
+pub(super) struct Book {
+    pub(super) addresses: HashMap<PeerId, Vec<Multiaddr>>,
+    pub(super) listening: Vec<Multiaddr>,
+    /// Where the router forwards to our listen addresses (UPnP, `Libp2pConfig::upnp`).
+    pub(super) mapped: Vec<Multiaddr>,
+    /// `Libp2pConfig::announce`.
+    pub(super) announced: Vec<Multiaddr>,
+    /// `Libp2pConfig::ignore`: not published (but announced), not dialled.
+    pub(super) ignored: Vec<IpRange>,
+    /// `Libp2pConfig::relay`: this node relays, and says so.
+    pub(super) serving: bool,
+    /// Relays named to pick (configured, listed), by peer: picked first.
+    pub(super) named_relays: HashMap<PeerId, NamedRelay>,
+    /// Relays named without their peer ID, being found out (`Command::FindRelay`).
+    pub(super) finding: HashMap<Multiaddr, Named>,
+    /// The relays listed in the document, as last learned (`learn_listed`).
+    pub(super) listed: Vec<Multiaddr>,
+    /// Members that say they relay, and where (with the relay's peer ID).
+    pub(super) relays: HashMap<PeerId, Multiaddr>,
+    /// With `Libp2pConfig::public`: other peers that relay (identify says so), and
+    /// where.
+    pub(super) public_relays: HashMap<PeerId, Multiaddr>,
+    /// Relays not to be picked, until when (`RELAY_PAUSE`, `RELAY_RETRY`).
+    pub(super) failed_relays: HashMap<PeerId, tokio::time::Instant>,
+    /// Relays that granted us a slot we hold.
+    pub(super) slots: std::collections::HashSet<PeerId>,
+    /// Peers holding a slot on this node, as a relay.
+    pub(super) slots_here: std::collections::HashSet<PeerId>,
+    /// Peers connected only through a relay, dialled directly (`upgrade`): when to again
+    /// at the soonest, and how long to wait after that.
+    pub(super) upgrades: HashMap<PeerId, (tokio::time::Instant, Duration)>,
+    /// The shortest round trip measured on each open connection (libp2p's ping).
+    pub(super) rtts: HashMap<ConnectionId, Duration>,
+    /// Peers whose hole punching failed: how often in a row, and when to try again
+    /// (`PUNCH_RETRIES`).
+    pub(super) punching: HashMap<PeerId, (usize, tokio::time::Instant)>,
+    /// Open connections: to whom, whether through a relay, and the remote address.
+    pub(super) connections: HashMap<ConnectionId, (PeerId, bool, Multiaddr)>,
+    /// What AutoNAT says, and the address it confirmed while public.
+    pub(super) reachability: Reachability,
+    pub(super) public_address: Option<Multiaddr>,
+    /// Since start (`stats`).
+    pub(super) counts: Counts,
+    /// When each peer was last mentioned (`STALE_AFTER`).
+    pub(super) seen: HashMap<PeerId, tokio::time::Instant>,
+    /// Peers found on the LAN (mDNS) not known yet to speak continuum.
+    pub(super) lan: std::collections::HashSet<PeerId>,
+    /// Peers that speak continuum (identify says so).
+    pub(super) continuum: std::collections::HashSet<PeerId>,
+    /// Peers identify says don't speak continuum, and whether they relay: not sent to
+    /// (`send_encoded`), so that a seed that's no node says so.
+    pub(super) not_nodes: HashMap<PeerId, bool>,
+    /// Peers found on the LAN that speak continuum, with their address: seeds not
+    /// handed out yet (`discovered_seeds`).
+    pub(super) discovered: Vec<Multiaddr>,
+}
+
+/// What `Libp2pStats` counts since start.
+#[derive(Default)]
+pub(super) struct Counts {
+    pub(super) slots_granted: u64,
+    pub(super) slots_lost: u64,
+    pub(super) slots_not_granted: u64,
+    pub(super) hole_punches: HolePunches,
+    pub(super) dial_failures: DialFailures,
+}
+
+impl Book {
+    /// How we are connected to `peer`: directly if any connection is direct.
+    pub(super) fn connection(&self, peer: &PeerId) -> Option<Connection> {
+        book_connection(&self.connections, peer)
+    }
+
+    /// The connections to `peer` to close, of several: all but one, the oldest direct
+    /// one (the oldest relayed one if none is direct).
+    ///
+    /// None between a relay and a node with a slot on it, or reserving one (`reserving`):
+    /// a slot lives on one connection, which libp2p doesn't tell, and closing it would
+    /// end the slot, and with it every circuit to the node (the relay then answers
+    /// `NoReservation`).
+    pub(super) fn duplicates(&self, peer: &PeerId, reserving: bool) -> Vec<ConnectionId> {
+        if reserving || self.slots.contains(peer) || self.slots_here.contains(peer) {
+            return Vec::new();
+        }
+        self.ranked(peer).into_iter().skip(1).collect()
+    }
+
+    /// The connections to `peer`, the one to keep first: direct ones before relayed
+    /// ones, measured ones by their round trip (`rtts`: over the LAN before a VPN's hub,
+    /// whichever opened first), then the oldest.
+    pub(super) fn ranked(&self, peer: &PeerId) -> Vec<ConnectionId> {
+        let mut connections: Vec<(bool, Duration, ConnectionId)> = self
+            .connections
+            .iter()
+            .filter(|(_, (connected, _, _))| connected == peer)
+            .map(|(id, (_, relayed, _))| {
+                let rtt = self.rtts.get(id).copied().unwrap_or(Duration::MAX);
+                (*relayed, rtt, *id)
+            })
+            .collect();
+        connections.sort();
+        connections.into_iter().map(|(_, _, id)| id).collect()
+    }
+
+    /// The remote address of the connection to `peer` that is kept (`ranked`).
+    pub(super) fn location(&self, peer: &PeerId) -> Option<&Multiaddr> {
+        let kept = *self.ranked(peer).first()?;
+        self.connections.get(&kept).map(|(_, _, address)| address)
+    }
+
+    pub(super) fn stats(&self) -> Libp2pStats {
+        let mut connections = Connections::default();
+        for (_, relayed, _) in self.connections.values() {
+            if *relayed {
+                connections.relayed += 1;
+            } else {
+                connections.direct += 1;
+            }
+        }
+        Libp2pStats {
+            reachability: self.reachability,
+            public_address: self.public_address.as_ref().map(Multiaddr::to_string),
+            connections,
+            relay_slots: RelaySlots {
+                held: self.slots.len(),
+                paused: self
+                    .failed_relays
+                    .values()
+                    .filter(|until| tokio::time::Instant::now() < **until)
+                    .count(),
+                granted: self.counts.slots_granted,
+                lost: self.counts.slots_lost,
+                not_granted: self.counts.slots_not_granted,
+            },
+            hole_punches: self.counts.hole_punches,
+            dial_failures: self.counts.dial_failures,
+        }
+    }
+
+    /// `peer` was mentioned just now.
+    pub(super) fn touch(&mut self, peer: PeerId) {
+        self.seen.insert(peer, tokio::time::Instant::now());
+    }
+
+    /// Forgets the peers not mentioned within `STALE_AFTER` before `now`, but those
+    /// `connected`. Returns how many.
+    pub(super) fn prune(
+        &mut self,
+        now: tokio::time::Instant,
+        connected: impl Fn(&PeerId) -> bool,
+    ) -> usize {
+        let stale: Vec<PeerId> = self
+            .addresses
+            .keys()
+            .chain(self.relays.keys())
+            .chain(self.public_relays.keys())
+            .chain(self.continuum.iter())
+            .chain(self.lan.iter())
+            .filter(|peer| {
+                !connected(peer)
+                    && self
+                        .seen
+                        .get(peer)
+                        .is_none_or(|at| now.saturating_duration_since(*at) >= STALE_AFTER)
+            })
+            .copied()
+            .collect::<std::collections::HashSet<_>>()
+            .into_iter()
+            .collect();
+        for peer in &stale {
+            self.addresses.remove(peer);
+            self.relays.remove(peer);
+            self.public_relays.remove(peer);
+            self.continuum.remove(peer);
+            self.not_nodes.remove(peer);
+            self.lan.remove(peer);
+            self.failed_relays.remove(peer);
+            self.upgrades.remove(peer);
+            self.punching.remove(peer);
+            self.seen.remove(peer);
+        }
+        stale.len()
+    }
+
+    /// `peer`, found on the LAN, speaks continuum: a seed, at the address we know first.
+    pub(super) fn discover(&mut self, peer: PeerId) {
+        let Some(address) = self.addresses.get(&peer).and_then(|known| known.first()) else {
+            return;
+        };
+        if let Ok(address) = address.clone().with_p2p(peer)
+            && !self.discovered.contains(&address)
+            && self.discovered.len() < MAX_PEERS
+        {
+            self.discovered.push(address);
+        }
+    }
+
+    /// Where others could dial us directly: the announced addresses, or else those we
+    /// listen on (`published`), where the router forwards to them (`mapped`), and where
+    /// AutoNAT found others can dial us (`public_address`: a port forwarded by hand, say),
+    /// without the peer ID.
+    pub(super) fn direct(&self) -> Vec<Multiaddr> {
+        if !self.announced.is_empty() {
+            return self.announced.clone();
+        }
+        let direct: Vec<Multiaddr> = self
+            .listening
+            .iter()
+            .filter(|address| !is_relayed(address))
+            .cloned()
+            .collect();
+        let mut direct = published(&direct);
+        let confirmed = self.public_address.iter().filter_map(|address| {
+            let mut address = address.clone();
+            if let Some(Protocol::P2p(_)) = address.iter().last() {
+                address.pop();
+            }
+            (!address.is_empty() && !is_relayed(&address)).then_some(address)
+        });
+        // First: the member list keeps only so many (`members::MAX_ADDRESSES`), and a
+        // node listening everywhere has many.
+        let mut first: Vec<Multiaddr> = Vec::new();
+        for address in self.mapped.iter().cloned().chain(confirmed) {
+            if !first.contains(&address) {
+                first.push(address);
+            }
+        }
+        direct.retain(|address| !first.contains(address));
+        first.extend(direct);
+        first.retain(|address| !ignored(&self.ignored, address));
+        first
+    }
+}
+
+/// Whether `address` only reaches this host (or this link): no use to other hosts, and
+/// dialled by one, it reaches that host itself.
+pub(super) fn is_local(address: &Multiaddr) -> bool {
+    match address.iter().next() {
+        Some(Protocol::Ip4(ip)) => ip.is_loopback() || ip.is_link_local() || ip.is_unspecified(),
+        Some(Protocol::Ip6(ip)) => {
+            ip.is_loopback() || ip.is_unicast_link_local() || ip.is_unspecified()
+        }
+        _ => false,
+    }
+}
+
+/// The addresses of `all` worth publishing: all but local ones, unless they're all
+/// local (a test on one host).
+pub(super) fn published(all: &[Multiaddr]) -> Vec<Multiaddr> {
+    let reachable: Vec<Multiaddr> = all
+        .iter()
+        .filter(|address| !is_local(address))
+        .cloned()
+        .collect();
+    if reachable.is_empty() {
+        all.to_vec()
+    } else {
+        reachable
+    }
+}
+
+impl Book {
+    /// `named` names the relay at `given`; returns it if its peer is to be found out.
+    /// `local`: this node, never its own relay.
+    pub(super) fn name_relay(
+        &mut self,
+        given: Multiaddr,
+        named: Named,
+        local: PeerId,
+    ) -> Option<Multiaddr> {
+        match split_peer(&given) {
+            Some(peer) if peer == local => None,
+            Some(peer) => {
+                // Configured wins over listed.
+                if self
+                    .named_relays
+                    .get(&peer)
+                    .is_none_or(|known| known.named == Named::Listed || named == Named::Configured)
+                {
+                    self.named_relays.insert(
+                        peer,
+                        NamedRelay {
+                            at: given.clone(),
+                            given,
+                            named,
+                        },
+                    );
+                }
+                None
+            }
+            None => {
+                let known = self.finding.contains_key(&given)
+                    || self.named_relays.values().any(|relay| relay.given == given);
+                if known {
+                    return None;
+                }
+                self.finding.insert(given.clone(), named);
+                Some(given)
+            }
+        }
+    }
+
+    /// The relay named at `given` (without a peer ID) is `peer`: unless it is no longer
+    /// named (or is this node). Returns whether that's news.
+    pub(super) fn relay_found(&mut self, given: Multiaddr, peer: PeerId, local: PeerId) -> bool {
+        let Some(named) = self.finding.remove(&given) else {
+            return false;
+        };
+        let Ok(at) = given.clone().with_p2p(peer) else {
+            return false;
+        };
+        if peer == local
+            || (named == Named::Listed
+                && self
+                    .named_relays
+                    .get(&peer)
+                    .is_some_and(|known| known.named == Named::Configured))
+        {
+            return false;
+        }
+        self.named_relays
+            .insert(peer, NamedRelay { at, given, named });
+        true
+    }
+
+    /// `relay` didn't grant a slot: if it was named without its peer ID, its address is
+    /// to be found out again, returned.
+    pub(super) fn find_again(&mut self, relay: PeerId) -> Option<Multiaddr> {
+        if self
+            .named_relays
+            .get(&relay)
+            .is_none_or(|known| split_peer(&known.given).is_some())
+        {
+            return None;
+        }
+        let known = self.named_relays.remove(&relay)?;
+        self.finding.insert(known.given.clone(), known.named);
+        Some(known.given)
+    }
+
+    /// The relays listed in the document are `listed` now: returns those to find out,
+    /// None if nothing changed.
+    pub(super) fn learn_listed(
+        &mut self,
+        listed: Vec<Multiaddr>,
+        local: PeerId,
+    ) -> Option<Vec<Multiaddr>> {
+        if listed == self.listed {
+            return None;
+        }
+        self.named_relays
+            .retain(|_, relay| relay.named != Named::Listed || listed.contains(&relay.given));
+        self.finding
+            .retain(|given, named| *named != Named::Listed || listed.contains(given));
+        self.listed = listed.clone();
+        Some(
+            listed
+                .into_iter()
+                .filter_map(|given| self.name_relay(given, Named::Listed, local))
+                .collect(),
+        )
+    }
+
+    /// The named relays `named` names, by peer, where.
+    pub(super) fn named(&self, named: Named) -> HashMap<PeerId, Multiaddr> {
+        self.named_relays
+            .iter()
+            .filter(|(_, relay)| relay.named == named)
+            .map(|(peer, relay)| (*peer, relay.at.clone()))
+            .collect()
+    }
+
+    /// Where `peer` may be reached through the relays we know (named, then members that
+    /// relay): for a peer known by its ID only (a seed `/p2p/<peer ID>`), which may hold
+    /// a slot on one of them. `local`: this node, never its own relay.
+    pub(super) fn through_relays(&self, peer: PeerId, local: PeerId) -> Vec<Multiaddr> {
+        let mut relays: Vec<(PeerId, Multiaddr)> = self
+            .named_relays
+            .iter()
+            .map(|(relay, named)| (*relay, named.at.clone()))
+            .collect();
+        relays.sort_by_key(|(_, address)| address.to_string());
+        let mut members: Vec<(PeerId, Multiaddr)> = self
+            .relays
+            .iter()
+            .filter(|(relay, _)| !self.named_relays.contains_key(relay))
+            .map(|(relay, address)| (*relay, address.clone()))
+            .collect();
+        members.sort_by_key(|(_, address)| address.to_string());
+        relays
+            .into_iter()
+            .chain(members)
+            .filter(|(relay, _)| *relay != local && *relay != peer)
+            .map(|(_, address)| address.with(Protocol::P2pCircuit).with(Protocol::P2p(peer)))
+            .take(ADDRESSES_PER_PEER)
+            .collect()
+    }
+
+    /// Whether all of `addresses` are known of `peer` (`remember`).
+    pub(super) fn knows(&self, peer: PeerId, addresses: &[Multiaddr]) -> bool {
+        let Some(known) = self.addresses.get(&peer) else {
+            return false;
+        };
+        addresses.iter().all(|address| {
+            let mut address = address.clone();
+            if let Some(Protocol::P2p(_)) = address.iter().last() {
+                address.pop();
+            }
+            known.contains(&address)
+        })
+    }
+
+    /// `peer` is at `address` (without its `/p2p` part): kept first.
+    pub(super) fn remember(&mut self, peer: PeerId, mut address: Multiaddr) {
+        if let Some(Protocol::P2p(_)) = address.iter().last() {
+            address.pop();
+        }
+        if !self.addresses.contains_key(&peer) && self.addresses.len() >= MAX_PEERS {
+            return;
+        }
+        self.touch(peer);
+        let addresses = self.addresses.entry(peer).or_default();
+        addresses.retain(|known| *known != address);
+        addresses.insert(0, address);
+        addresses.truncate(ADDRESSES_PER_PEER);
+    }
+}
+
+/// Whether `address` goes through a relay.
+pub(super) fn is_relayed(address: &Multiaddr) -> bool {
+    address
+        .iter()
+        .any(|protocol| matches!(protocol, Protocol::P2pCircuit))
+}
+
+/// The relay `address` goes through, if it goes through one: the peer before
+/// `/p2p-circuit`.
+pub(super) fn relay_of(address: &Multiaddr) -> Option<PeerId> {
+    let mut relay = None;
+    for protocol in address.iter() {
+        match protocol {
+            Protocol::P2p(peer) => relay = Some(peer),
+            Protocol::P2pCircuit => return relay,
+            _ => {}
+        }
+    }
+    None
+}
+
+/// How `peer` is connected, of `connections` (`Book::connections`): directly if one is
+/// direct.
+pub(super) fn book_connection(
+    connections: &HashMap<ConnectionId, (PeerId, bool, Multiaddr)>,
+    peer: &PeerId,
+) -> Option<Connection> {
+    let mut found = None;
+    for (connected, relayed, _) in connections.values() {
+        if connected == peer {
+            if !relayed {
+                return Some(Connection::Direct);
+            }
+            found = Some(Connection::Relayed);
+        }
+    }
+    found
+}
+
+/// The addresses of `known` this node can dial: not those through itself as the relay
+/// (a relay reaches the nodes it relays for only while they are connected to it).
+pub(super) fn dialable(known: &[Multiaddr], local: PeerId) -> Vec<Multiaddr> {
+    known
+        .iter()
+        .filter(|address| relay_of(address) != Some(local))
+        .cloned()
+        .collect()
+}
+
+/// `address` with the peer ID it ends in, if it ends in one.
+pub(super) fn split_peer(address: &Multiaddr) -> Option<PeerId> {
+    match address.iter().last() {
+        Some(Protocol::P2p(peer)) => Some(peer),
+        _ => None,
+    }
+}
+
+pub(super) fn lock(book: &Mutex<Book>) -> MutexGuard<'_, Book> {
+    book.lock_or_recover()
+}
diff --git a/src/transport/libp2p/config.rs b/src/transport/libp2p/config.rs
new file mode 100644
index 0000000..012999d
--- /dev/null
+++ b/src/transport/libp2p/config.rs
@@ -0,0 +1,117 @@
+//! What the options make of the libp2p transport (`Libp2pConfig`), and their checks.
+
+use super::*;
+
+/// What a libp2p transport is made with.
+pub(crate) struct Libp2pConfig {
+    pub(crate) key: Keypair,
+    /// Multiaddrs to listen on.
+    pub(crate) listen: Vec<Multiaddr>,
+    /// Multiaddrs to publish instead of the listen addresses, if any.
+    pub(crate) announce: Vec<Multiaddr>,
+    /// Relays to reserve a slot on (multiaddrs; without the relay's peer ID, a connection
+    /// finds it out).
+    pub(crate) relays: Vec<Multiaddr>,
+    /// Relay for other nodes.
+    pub(crate) relay: bool,
+    /// Speak continuum (`PROTOCOL`): not on a node that only relays (`--relay-only`),
+    /// which nobody should take for a node.
+    pub(crate) continuum: bool,
+    /// Find peers on the LAN (mDNS).
+    pub(crate) mdns: bool,
+    /// Ask the router to forward our listen ports (UPnP), and publish where.
+    pub(crate) upnp: bool,
+    /// Address ranges neither published nor dialled, nor connected over (`Ignoring`).
+    pub(crate) ignore: Vec<IpRange>,
+    /// Use public infrastructure: the IPFS DHT, public relays.
+    pub(crate) public: bool,
+    /// libp2p's in-memory transport instead of TCP and QUIC: for tests.
+    pub(crate) memory: bool,
+}
+
+/// The multiaddrs of option `option`.
+pub(super) fn parse_addresses(option: &str, addresses: &[String]) -> Result<Vec<Multiaddr>> {
+    addresses
+        .iter()
+        .map(|address| {
+            address
+                .parse::<Multiaddr>()
+                .with_context(|| format!("{option} {address:?}"))
+        })
+        .collect()
+}
+
+/// The ranges of `libp2p-ignore`.
+pub(super) fn parse_ranges(ranges: &[String]) -> Result<Vec<IpRange>> {
+    ranges
+        .iter()
+        .map(|range| range.parse().context("libp2p-ignore"))
+        .collect()
+}
+
+/// This transport's options, checked when the configuration is loaded
+/// (`config::Settings::check`), so `--print-config` tells a wrong one too: the
+/// multiaddrs and ranges parse, and a relay that is no node (`--relay-only`) can be
+/// dialled and keeps its peer ID.
+pub(crate) fn check(settings: &crate::config::Settings) -> Result<()> {
+    parse_addresses("libp2p-listen", &settings.libp2p_listen)?;
+    parse_addresses("libp2p-announce", &settings.libp2p_announce)?;
+    parse_addresses("libp2p-relays", &settings.libp2p_relays)?;
+    parse_ranges(&settings.libp2p_ignore)?;
+    if settings.relay_only {
+        if settings.libp2p_listen.is_empty() {
+            bail!("relay-only needs libp2p-listen: where nodes reach the relay");
+        }
+        if settings.libp2p_key_file.is_none() {
+            bail!(
+                "relay-only needs libp2p-key-file: without it, the relay's peer ID changes with every start and nodes lose it"
+            );
+        }
+    }
+    Ok(())
+}
+
+impl Libp2pConfig {
+    /// The configuration `settings` give; `relay_only`: a relay that is no node
+    /// (`--relay-only`).
+    pub(crate) fn from_settings(
+        settings: &crate::config::Settings,
+        relay_only: bool,
+    ) -> Result<Self> {
+        let parse = parse_addresses;
+        Ok(Self {
+            key: Self::load_key(settings.libp2p_key_file.as_deref())?,
+            listen: parse("libp2p-listen", &settings.libp2p_listen)?,
+            announce: parse("libp2p-announce", &settings.libp2p_announce)?,
+            relays: parse("libp2p-relays", &settings.libp2p_relays)?,
+            relay: settings.libp2p_relay || relay_only,
+            continuum: !relay_only,
+            mdns: settings.libp2p_mdns,
+            upnp: settings.libp2p_upnp,
+            ignore: parse_ranges(&settings.libp2p_ignore)?,
+            public: settings.libp2p_public,
+            memory: false,
+        })
+    }
+
+    /// The key in `path` (32 raw bytes, an Ed25519 secret key), or a new one each start
+    /// without a file: the peer ID only says where a node is, so a new one costs nothing
+    /// but the WHO that finds it again (`transport::identity`).
+    pub(crate) fn load_key(path: Option<&Path>) -> Result<Keypair> {
+        let Some(path) = path else {
+            return Ok(Keypair::generate_ed25519());
+        };
+        let mut bytes = std::fs::read(path)
+            .with_context(|| format!("Reading the libp2p key from {}", path.display()))?;
+        crate::swarm::auth::warn_if_readable_by_others(path, "libp2p key");
+        if bytes.len() != 32 {
+            bail!(
+                "The libp2p key in {} has {} bytes; it has 32",
+                path.display(),
+                bytes.len()
+            );
+        }
+        Keypair::ed25519_from_bytes(&mut bytes)
+            .map_err(|error| anyhow!("The libp2p key in {}: {error}", path.display()))
+    }
+}
diff --git a/src/transport/libp2p/driver.rs b/src/transport/libp2p/driver.rs
new file mode 100644
index 0000000..efcf109
--- /dev/null
+++ b/src/transport/libp2p/driver.rs
@@ -0,0 +1,527 @@
+//! The swarm's task (`run`): it owns the swarm, takes `Command`s from the transport, and
+//! answers the swarm's events (connections, identify, relays, hole punching, AutoNAT,
+//! mDNS, UPnP, the DHT), keeping the book.
+
+use super::*;
+
+pub(super) enum Command {
+    /// Connect to `peer` (at `addresses`) unless connected.
+    Connect {
+        peer: PeerId,
+        addresses: Vec<Multiaddr>,
+        done: oneshot::Sender<Result<(), String>>,
+    },
+    /// Connect to `address` to find out which peer is there.
+    Identify {
+        address: Multiaddr,
+        done: oneshot::Sender<Result<PeerId, String>>,
+    },
+    /// See whether relays are needed now (one was just learned of).
+    CheckRelays,
+    /// Find out which peer relays at `address`, named without the relay's peer ID
+    /// (`Book::finding`).
+    FindRelay { address: Multiaddr },
+}
+
+/// Drives the swarm: dials what `commands` ask for and notes where we listen, until the
+/// transport is dropped. `again`: to schedule commands.
+pub(super) async fn run(
+    mut swarm: Swarm<Behaviour>,
+    mut commands: mpsc::UnboundedReceiver<Command>,
+    again: mpsc::UnboundedSender<Command>,
+    book: Arc<Mutex<Book>>,
+    relaying: Relaying,
+    stopping: watch::Sender<bool>,
+) {
+    let serving = relaying.serving;
+    let mut nat = ::libp2p::autonat::NatStatus::Unknown;
+    // Relays picked, by listener.
+    let mut picked: HashMap<::libp2p::core::transport::ListenerId, PeerId> = HashMap::new();
+    let mut check = tokio::time::interval(RELAY_CHECK);
+    check.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Delay);
+    let mut dht_bootstrap = tokio::time::interval(DHT_BOOTSTRAP);
+    dht_bootstrap.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Delay);
+    let mut connecting: HashMap<PeerId, Vec<oneshot::Sender<Result<(), String>>>> = HashMap::new();
+    let mut identifying: HashMap<
+        ConnectionId,
+        (Multiaddr, oneshot::Sender<Result<PeerId, String>>),
+    > = HashMap::new();
+    let mut prune = tokio::time::interval(PRUNE_EVERY);
+    prune.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Delay);
+    // Peers that may have several connections, and when to cut them to one. Only the
+    // side with the smaller peer ID does that, so the two never close different ones.
+    let mut dedupe_due: HashMap<PeerId, tokio::time::Instant> = HashMap::new();
+    // Connections closed as duplicates, until they are: their "Connection closed" says so.
+    let mut closing_duplicates: std::collections::HashSet<ConnectionId> =
+        std::collections::HashSet::new();
+    let mut dedupe = tokio::time::interval(DEDUPE_EVERY);
+    dedupe.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Delay);
+    loop {
+        tokio::select! {
+            _ = dedupe.tick(), if !dedupe_due.is_empty() => {
+                let now = tokio::time::Instant::now();
+                let due: Vec<PeerId> = dedupe_due
+                    .iter()
+                    .filter(|(_, at)| **at <= now)
+                    .map(|(peer, _)| *peer)
+                    .collect();
+                for peer in due {
+                    dedupe_due.remove(&peer);
+                    let reserving = picked.values().any(|relay| *relay == peer);
+                    let duplicates = lock(&book).duplicates(&peer, reserving);
+                    for connection in duplicates {
+                        if swarm.close_connection(connection) {
+                            closing_duplicates.insert(connection);
+                        }
+                    }
+                }
+            }
+            _ = prune.tick() => {
+                let forgotten = lock(&book).prune(tokio::time::Instant::now(), |peer| swarm.is_connected(peer));
+                if forgotten > 0 {
+                    debug!(forgotten, "Forgot peers nobody mentions any more");
+                }
+            }
+            _ = dht_bootstrap.tick(), if relaying.public => {
+                if let Some(dht) = swarm.behaviour_mut().kad.as_mut()
+                    && let Err(error) = dht.bootstrap()
+                {
+                    warn!(error = %error, "Couldn't join the IPFS DHT");
+                }
+            }
+            _ = check.tick() => {
+                pick_relays(&mut swarm, &book, &relaying, &nat, &mut picked);
+                retry_punches(&mut swarm, &book, relaying.local);
+            }
+            command = commands.recv() => match command {
+                None => break,
+                Some(Command::CheckRelays) => {
+                    pick_relays(&mut swarm, &book, &relaying, &nat, &mut picked);
+                }
+                Some(Command::Connect { peer, addresses, done }) => {
+                    if swarm.is_connected(&peer) {
+                        let _ = done.send(Ok(()));
+                        upgrade(&mut swarm, &book, peer, relaying.local);
+                        continue;
+                    }
+                    let waiting = connecting.entry(peer).or_default();
+                    waiting.push(done);
+                    if waiting.len() > 1 {
+                        continue; // Already dialing.
+                    }
+                    let mut opts = DialOpts::peer_id(peer)
+                        .addresses(addresses)
+                        .condition(PeerCondition::DisconnectedAndNotDialing);
+                    if relaying.public {
+                        // Addresses the DHT knows too.
+                        opts = opts.extend_addresses_through_behaviour();
+                    }
+                    if let Err(error) = swarm.dial(opts.build()) {
+                        count_dial_failure(&mut lock(&book).counts.dial_failures, &error);
+                        for done in connecting.remove(&peer).into_iter().flatten() {
+                            let _ = done.send(Err(error.to_string()));
+                        }
+                        find_on_dht(&mut swarm, peer);
+                    }
+                }
+                Some(Command::FindRelay { address }) => {
+                    let again = again.clone();
+                    let local = relaying.local;
+                    let book = Arc::clone(&book);
+                    tokio::spawn(async move {
+                        let (done, identified) = oneshot::channel();
+                        if again
+                            .send(Command::Identify { address: address.clone(), done })
+                            .is_err()
+                        {
+                            return;
+                        }
+                        match tokio::time::timeout(CONNECT_TIMEOUT, identified).await {
+                            Ok(Ok(Ok(peer))) => {
+                                if lock(&book).relay_found(address.clone(), peer, local) {
+                                    debug!(relay = %Addressed(SCHEME, &address), peer = %Addressed(SCHEME, &peer), "Found a named relay");
+                                    let _ = again.send(Command::CheckRelays);
+                                }
+                            }
+                            failed => {
+                                if !lock(&book).finding.contains_key(&address) {
+                                    return; // No longer named.
+                                }
+                                let error = match failed {
+                                    Ok(Ok(Err(error))) => error,
+                                    Ok(_) => "the libp2p swarm stopped".to_owned(),
+                                    Err(_) => format!("no answer within {CONNECT_TIMEOUT:?}"),
+                                };
+                                warn!(relay = %Addressed(SCHEME, &address), error = %error, retry = ?RELAY_RETRY, "Couldn't reach a named relay; trying again later");
+                                tokio::time::sleep(RELAY_RETRY).await;
+                                let _ = again.send(Command::FindRelay { address });
+                            }
+                        }
+                    });
+                }
+                Some(Command::Identify { address, done }) => {
+                    let opts = DialOpts::unknown_peer_id().address(address.clone()).build();
+                    let id = opts.connection_id();
+                    match swarm.dial(opts) {
+                        Ok(()) => {
+                            identifying.insert(id, (address, done));
+                        }
+                        Err(error) => {
+                            let _ = done.send(Err(error.to_string()));
+                        }
+                    }
+                }
+            },
+            event = swarm.select_next_some() => match event {
+                SwarmEvent::ConnectionEstablished { peer_id, connection_id, endpoint, .. } => {
+                    {
+                        let mut book = lock(&book);
+                        book.touch(peer_id);
+                        let relayed = endpoint.is_relayed();
+                        book.connections.insert(
+                            connection_id,
+                            (peer_id, relayed, endpoint.get_remote_address().clone()),
+                        );
+                        if !relayed {
+                            book.upgrades.remove(&peer_id);
+                            book.punching.remove(&peer_id);
+                        }
+                        if relaying.local < peer_id
+                            && book.connections.values().filter(|(connected, _, _)| *connected == peer_id).count() > 1
+                        {
+                            // Only through relays: maybe a punch under way.
+                            let grace = match book.connection(&peer_id) {
+                                Some(Connection::Relayed) => PUNCH_GRACE,
+                                _ => DUPLICATE_GRACE,
+                            };
+                            dedupe_due.insert(peer_id, tokio::time::Instant::now() + grace);
+                        }
+                        debug!(
+                            peer = %Addressed(SCHEME, &peer_id),
+                            connection = %connection_id,
+                            dialled = endpoint.is_dialer(),
+                            address = %endpoint.get_remote_address(),
+                            relayed,
+                            "Connection opened"
+                        );
+                    }
+                    for done in connecting.remove(&peer_id).into_iter().flatten() {
+                        let _ = done.send(Ok(()));
+                    }
+                    if let Some((address, done)) = identifying.remove(&connection_id) {
+                        lock(&book).remember(peer_id, address);
+                        let _ = done.send(Ok(peer_id));
+                    }
+                }
+                SwarmEvent::ConnectionClosed { peer_id, connection_id, cause, .. } => {
+                    {
+                        let mut book = lock(&book);
+                        book.connections.remove(&connection_id);
+                        book.rtts.remove(&connection_id);
+                    }
+                    let cause = match cause {
+                        _ if closing_duplicates.remove(&connection_id) => "a duplicate".to_owned(),
+                        // By us, or idle: libp2p doesn't tell which.
+                        None => "closed here".to_owned(),
+                        Some(error) => error.to_string(),
+                    };
+                    debug!(
+                        peer = %Addressed(SCHEME, &peer_id),
+                        connection = %connection_id,
+                        cause = %cause,
+                        "Connection closed"
+                    );
+                }
+                SwarmEvent::OutgoingConnectionError { connection_id, peer_id, error } => {
+                    count_dial_failure(&mut lock(&book).counts.dial_failures, &error);
+                    if let Some((_, done)) = identifying.remove(&connection_id) {
+                        let _ = done.send(Err(describe_dial_error(&error)));
+                    }
+                    if let Some(peer) = peer_id
+                        && !swarm.is_connected(&peer)
+                        && let Some(waiting) = connecting.remove(&peer)
+                    {
+                        for done in waiting {
+                            let _ = done.send(Err(describe_dial_error(&error)));
+                        }
+                        find_on_dht(&mut swarm, peer);
+                    }
+                }
+                SwarmEvent::NewListenAddr { address, .. } => {
+                    info!(address = %address, "Listening");
+                    // A relay tells those reserving where it is: its external addresses.
+                    if serving && !is_local(&address) && !is_relayed(&address) {
+                        swarm.add_external_address(address.clone());
+                    }
+                    let mut book = lock(&book);
+                    if !book.listening.contains(&address) {
+                        book.listening.push(address);
+                    }
+                }
+                SwarmEvent::ExpiredListenAddr { address, .. } => {
+                    info!(address = %address, "No longer listening");
+                    if serving {
+                        swarm.remove_external_address(&address);
+                    }
+                    lock(&book).listening.retain(|known| *known != address);
+                }
+                SwarmEvent::ListenerClosed { listener_id, addresses, reason } => {
+                    lock(&book)
+                        .listening
+                        .retain(|known| !addresses.contains(known));
+                    // A relay picked: the next check picks again, another one if this one
+                    // never granted the slot.
+                    if let Some(relay) = picked.remove(&listener_id) {
+                        let refused = match &reason {
+                            Ok(()) => false,
+                            Err(error) => relay_refused(&format!("{error:?}"), &innermost_causes(error)),
+                        };
+                        let (lost, pause) = {
+                            let mut book = lock(&book);
+                            let lost = book.slots.remove(&relay);
+                            let pause = match (refused, lost) {
+                                (true, _) => Some(RELAY_PAUSE),
+                                (false, true) => None,
+                                (false, false) => Some(RELAY_RETRY),
+                            };
+                            if lost {
+                                book.counts.slots_lost += 1;
+                            } else {
+                                book.counts.slots_not_granted += 1;
+                                // Named without its peer ID: maybe another one now.
+                                if let Some(address) = book.find_again(relay) {
+                                    let _ = again.send(Command::FindRelay { address });
+                                }
+                            }
+                            if let Some(pause) = pause {
+                                book.failed_relays.insert(relay, tokio::time::Instant::now() + pause);
+                            }
+                            (lost, pause)
+                        };
+                        // Again at once, or others meanwhile.
+                        let _ = again.send(Command::CheckRelays);
+                        let reason = relay_failure(&reason);
+                        match (lost, pause) {
+                            (true, None) => {
+                                info!(relay = %Addressed(SCHEME, &relay), reason = %reason, "Lost the slot on a relay; reserving again");
+                            }
+                            (true, Some(pause)) => {
+                                info!(relay = %Addressed(SCHEME, &relay), pause = ?pause, reason = %reason, "Lost the slot on a relay; picking others for a while");
+                            }
+                            (false, pause) => {
+                                info!(
+                                    relay = %Addressed(SCHEME, &relay),
+                                    pause = ?pause.unwrap_or_default(),
+                                    reason = %reason,
+                                    "A relay didn't grant a slot; picking others for a while"
+                                );
+                            }
+                        }
+                    }
+                }
+                SwarmEvent::ListenerError { error, .. } => {
+                    warn!(error = %error, "Listener failed");
+                }
+                SwarmEvent::Behaviour(BehaviourEvent::RelayClient(
+                    ::libp2p::relay::client::Event::ReservationReqAccepted { relay_peer_id, renewal, .. },
+                )) => {
+                    {
+                        let mut book = lock(&book);
+                        book.slots.insert(relay_peer_id);
+                        book.failed_relays.remove(&relay_peer_id);
+                        if !renewal {
+                            book.counts.slots_granted += 1;
+                        }
+                    }
+                    if !renewal {
+                        info!(relay = %Addressed(SCHEME, &relay_peer_id), "Reachable through a relay");
+                    }
+                }
+                SwarmEvent::Behaviour(BehaviourEvent::Autonat(
+                    ::libp2p::autonat::Event::StatusChanged { old, new },
+                )) => {
+                    info!(old = ?old, new = ?new, "Reachability (AutoNAT)");
+                    {
+                        use ::libp2p::autonat::NatStatus;
+                        let mut book = lock(&book);
+                        (book.reachability, book.public_address) = match &new {
+                            NatStatus::Unknown => (Reachability::Unknown, None),
+                            NatStatus::Private => (Reachability::Private, None),
+                            NatStatus::Public(address) => {
+                                (Reachability::Public, Some(address.clone()))
+                            }
+                        };
+                    }
+                    nat = new;
+                }
+                SwarmEvent::Behaviour(BehaviourEvent::Relay(event)) => {
+                    {
+                        use ::libp2p::relay::Event;
+                        let mut book = lock(&book);
+                        match &event {
+                            Event::ReservationReqAccepted { src_peer_id, .. } => {
+                                book.slots_here.insert(*src_peer_id);
+                            }
+                            Event::ReservationClosed { src_peer_id }
+                            | Event::ReservationTimedOut { src_peer_id } => {
+                                book.slots_here.remove(src_peer_id);
+                            }
+                            _ => {}
+                        }
+                    }
+                    debug!(event = ?event, "Relay event");
+                }
+                SwarmEvent::Behaviour(BehaviourEvent::Ping(::libp2p::ping::Event {
+                    peer,
+                    connection,
+                    result,
+                })) => match result {
+                    Ok(rtt) => {
+                        let mut book = lock(&book);
+                        if book.connections.contains_key(&connection) {
+                            let shortest = book.rtts.entry(connection).or_insert(rtt);
+                            *shortest = (*shortest).min(rtt);
+                        }
+                        trace!(peer = %Addressed(SCHEME, &peer), connection = %connection, rtt = ?rtt, "Connection's round trip");
+                    }
+                    Err(error) => {
+                        trace!(peer = %Addressed(SCHEME, &peer), connection = %connection, error = %error, "Connection's ping failed");
+                    }
+                },
+                SwarmEvent::Behaviour(BehaviourEvent::Upnp(event)) => match event {
+                    ::libp2p::upnp::Event::NewExternalAddr {
+                        local_addr,
+                        external_addr: address,
+                    } => {
+                        info!(address = %address, to = %local_addr, "The router forwards to us (UPnP)");
+                        let mut book = lock(&book);
+                        if !book.mapped.contains(&address) {
+                            book.mapped.push(address);
+                        }
+                    }
+                    ::libp2p::upnp::Event::ExpiredExternalAddr {
+                        local_addr,
+                        external_addr: address,
+                    } => {
+                        info!(address = %address, to = %local_addr, "The router no longer forwards to us (UPnP)");
+                        lock(&book).mapped.retain(|mapped| *mapped != address);
+                    }
+                    ::libp2p::upnp::Event::GatewayNotFound => {
+                        info!("No router that maps ports (UPnP) answered");
+                    }
+                    ::libp2p::upnp::Event::NonRoutableGateway => {
+                        warn!("The router that maps ports (UPnP) is behind another NAT itself; its mappings don't reach us from outside");
+                    }
+                },
+                SwarmEvent::Behaviour(BehaviourEvent::Dcutr(event)) => match event.result {
+                    Ok(_) => {
+                        let mut book = lock(&book);
+                        book.counts.hole_punches.succeeded += 1;
+                        book.punching.remove(&event.remote_peer_id);
+                        drop(book);
+                        info!(peer = %Addressed(SCHEME, &event.remote_peer_id), "Connected directly to a peer behind a relay");
+                    }
+                    Err(error) => {
+                        let retry = {
+                            let mut book = lock(&book);
+                            book.counts.hole_punches.failed += 1;
+                            let failed = book
+                                .punching
+                                .get(&event.remote_peer_id)
+                                .map_or(0, |(failed, _)| *failed);
+                            let retry = PUNCH_RETRIES[failed.min(PUNCH_RETRIES.len() - 1)];
+                            book.punching.insert(
+                                event.remote_peer_id,
+                                (failed + 1, tokio::time::Instant::now() + retry),
+                            );
+                            retry
+                        };
+                        debug!(peer = %Addressed(SCHEME, &event.remote_peer_id), error = %error, retry = ?retry, "Couldn't connect directly to a peer behind a relay; the relay carries on");
+                    }
+                },
+                SwarmEvent::Behaviour(BehaviourEvent::Identify(
+                    ::libp2p::identify::Event::Received { connection_id, peer_id, info },
+                )) => {
+                    {
+                        let mut book = lock(&book);
+                        let listen = published(&info.listen_addrs);
+                        // Every new connection identifies again: logged when it says something new.
+                        if book.knows(peer_id, &listen) {
+                            trace!(peer = %Addressed(SCHEME, &peer_id), connection = %connection_id, "Peer identified again");
+                        } else {
+                            debug!(
+                                peer = %Addressed(SCHEME, &peer_id),
+                                connection = %connection_id,
+                                listen = ?info.listen_addrs,
+                                observed = %info.observed_addr,
+                                agent = %info.agent_version,
+                                "Peer identified"
+                            );
+                        }
+                        // Last first: `remember` puts each in front.
+                        for address in listen.into_iter().rev() {
+                            book.remember(peer_id, address);
+                        }
+                        if relaying.public
+                            && info.protocols.contains(&::libp2p::relay::HOP_PROTOCOL_NAME)
+                            && (book.public_relays.len() < MAX_PEERS
+                                || book.public_relays.contains_key(&peer_id))
+                            && let Some(address) = book
+                                .addresses
+                                .get(&peer_id)
+                                .and_then(|known| known.first())
+                                .and_then(|address| address.clone().with_p2p(peer_id).ok())
+                        {
+                            book.public_relays.insert(peer_id, address);
+                        }
+                        if !info.protocols.contains(&PROTOCOL)
+                            && (book.not_nodes.len() < MAX_PEERS
+                                || book.not_nodes.contains_key(&peer_id))
+                        {
+                            let relays = info.protocols.contains(&::libp2p::relay::HOP_PROTOCOL_NAME);
+                            book.not_nodes.insert(peer_id, relays);
+                        }
+                        if info.protocols.contains(&PROTOCOL) {
+                            book.not_nodes.remove(&peer_id);
+                            book.continuum.insert(peer_id);
+                            if book.lan.remove(&peer_id) {
+                                book.discover(peer_id);
+                            }
+                        }
+                    }
+                    upgrade(&mut swarm, &book, peer_id, relaying.local);
+                }
+                SwarmEvent::Behaviour(BehaviourEvent::Mdns(
+                    ::libp2p::mdns::Event::Discovered(found),
+                )) => {
+                    for (peer, address) in found {
+                        if peer == relaying.local {
+                            continue;
+                        }
+                        debug!(peer = %Addressed(SCHEME, &peer), address = %address, "Peer on the LAN");
+                        let continuum = {
+                            let mut book = lock(&book);
+                            book.remember(peer, address.clone());
+                            book.continuum.contains(&peer)
+                        };
+                        upgrade(&mut swarm, &book, peer, relaying.local);
+                        if continuum {
+                            lock(&book).discover(peer);
+                        } else if lock(&book).lan.len() < MAX_PEERS {
+                            // Connected, identify says whether it speaks continuum.
+                            lock(&book).lan.insert(peer);
+                            let _ = swarm.dial(
+                                DialOpts::peer_id(peer)
+                                    .addresses(vec![address])
+                                    .condition(PeerCondition::DisconnectedAndNotDialing)
+                                    .build(),
+                            );
+                        }
+                    }
+                }
+                _ => {}
+            },
+        }
+    }
+    let _ = stopping.send(true);
+}
diff --git a/src/transport/libp2p/errors.rs b/src/transport/libp2p/errors.rs
new file mode 100644
index 0000000..7eb4551
--- /dev/null
+++ b/src/transport/libp2p/errors.rs
@@ -0,0 +1,128 @@
+//! What libp2p's errors say, for the log and the figures: dial failures, relay
+//! reservations refused or lost.
+
+use super::*;
+
+/// A dial error, shortly: for each address tried (without the peer ID the caller names
+/// already), the innermost cause, once. libp2p's own text nests each transport's errors
+/// in the next ("Multiple dial errors occurred: ..."), repeating the cause at each level.
+pub(super) fn describe_dial_error(error: &::libp2p::swarm::DialError) -> String {
+    use ::libp2p::swarm::DialError;
+    let DialError::Transport(errors) = error else {
+        return error.to_string();
+    };
+    let described: Vec<String> = errors
+        .iter()
+        .map(|(address, error)| {
+            let mut address = address.clone();
+            if let Some(Protocol::P2p(_)) = address.iter().last() {
+                address.pop();
+            }
+            format!("{address}: {}", innermost_causes(error))
+        })
+        .collect();
+    if described.is_empty() {
+        return error.to_string();
+    }
+    described.join("; ")
+}
+
+/// Why a slot on a relay ended, or was never granted, shortly: libp2p nests the
+/// relay's answer several types deep (`Left(Left(Left(Reservation(...))))`), and the
+/// common cases deserve a sentence.
+pub(super) fn relay_failure(reason: &std::io::Result<()>) -> String {
+    let Err(error) = reason else {
+        return "closed".to_owned();
+    };
+    relay_failure_text(&format!("{error:?}"), &innermost_causes(error))
+}
+
+/// `relay_failure` on the error's debug form, and its innermost cause as text.
+pub(super) fn relay_failure_text(debug: &str, innermost: &str) -> String {
+    let known = [
+        (
+            "NoAddressesInReservation",
+            "the relay gave no address to be reached through it",
+        ),
+        ("ResourceLimitExceeded", "the relay has no slot left"),
+        ("PermissionDenied", "the relay doesn't let us reserve"),
+        ("NoReservation", "the relay holds no slot for us"),
+        (
+            "connection gated",
+            "the relay refused the connection (connection gated)",
+        ),
+        (
+            "UnsupportedProtocol",
+            "the relay doesn't relay (no circuit relay protocol)",
+        ),
+        ("Timeout", "the relay didn't answer in time"),
+    ];
+    if let Some((_, said)) = known.iter().find(|(pattern, _)| debug.contains(pattern)) {
+        return (*said).to_owned();
+    }
+    // The protocol refused, but only after its first data (`swarm_config`).
+    if innermost == "Failed" {
+        return "the relay refused the circuit relay protocol (is it a relay?)".to_owned();
+    }
+    innermost.to_owned()
+}
+
+/// Whether a slot ended, or wasn't granted, because the relay refused it (its error's
+/// debug form and innermost cause, as for `relay_failure_text`), rather than because it
+/// didn't answer or the connection closed: only then is it worth not asking again for a
+/// while.
+pub(super) fn relay_refused(debug: &str, innermost: &str) -> bool {
+    let refusals = [
+        "NoAddressesInReservation",
+        "ResourceLimitExceeded",
+        "PermissionDenied",
+        "connection gated",
+        "UnsupportedProtocol",
+        "WrongPeerId",
+    ];
+    refusals.iter().any(|pattern| debug.contains(pattern)) || innermost == "Failed"
+}
+
+/// The innermost of `error`'s causes, each distinct one once: those libp2p lists in a
+/// "Multiple dial errors occurred:" text (one per line, ` - <cause>: <its cause>...`),
+/// or the last of the source chain.
+pub(super) fn innermost_causes(error: &(dyn std::error::Error + 'static)) -> String {
+    let mut innermost = error;
+    while let Some(source) = innermost.source() {
+        innermost = source;
+    }
+    let text = innermost.to_string();
+    let mut causes: Vec<&str> = Vec::new();
+    for line in text.lines() {
+        let Some(listed) = line.trim().strip_prefix("- ") else {
+            continue;
+        };
+        // A line ends in the next level's heading when that level lists more.
+        let cause = listed
+            .split(": ")
+            .filter(|part| !part.starts_with("Multiple dial errors"))
+            .last()
+            .unwrap_or(listed)
+            .trim();
+        if !cause.is_empty() && !causes.contains(&cause) {
+            causes.push(cause);
+        }
+    }
+    if causes.is_empty() {
+        let cause = text.rsplit(": ").next().unwrap_or(&text).trim();
+        return cause.to_owned();
+    }
+    causes.join(", ")
+}
+
+/// Counts a dial that failed with `error` (not one that wasn't needed).
+pub(super) fn count_dial_failure(failures: &mut DialFailures, error: &::libp2p::swarm::DialError) {
+    use ::libp2p::swarm::DialError;
+    match error {
+        DialError::DialPeerConditionFalse(_) => {}
+        DialError::Transport(_) => failures.unreachable += 1,
+        DialError::WrongPeerId { .. } => failures.wrong_peer_id += 1,
+        DialError::NoAddresses => failures.no_addresses += 1,
+        _ => failures.other += 1,
+    }
+}
diff --git a/src/transport/libp2p/ignore.rs b/src/transport/libp2p/ignore.rs
new file mode 100644
index 0000000..b14235e
--- /dev/null
+++ b/src/transport/libp2p/ignore.rs
@@ -0,0 +1,146 @@
+//! `--libp2p-ignore`: address ranges neither published nor dialled, nor connected over
+//! (`Ignoring`, a behaviour that denies connections there).
+
+use super::*;
+
+/// An address range (`10.244.0.0/16`, `42::/16`; a bare address is a range of one), for
+/// `Libp2pConfig::ignore`.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) struct IpRange {
+    pub(super) network: std::net::IpAddr,
+    pub(super) prefix: u8,
+}
+
+impl FromStr for IpRange {
+    type Err = anyhow::Error;
+
+    fn from_str(text: &str) -> Result<Self> {
+        let (address, prefix) = match text.split_once('/') {
+            Some((address, prefix)) => (address, Some(prefix)),
+            None => (text, None),
+        };
+        let network: std::net::IpAddr = address
+            .parse()
+            .with_context(|| format!("{text:?} is no address range (e.g. 10.244.0.0/16)"))?;
+        let bits = if network.is_ipv4() { 32 } else { 128 };
+        let prefix = match prefix {
+            Some(prefix) => prefix
+                .parse::<u8>()
+                .ok()
+                .filter(|prefix| *prefix <= bits)
+                .with_context(|| format!("{text:?}: the prefix is 0 to {bits}"))?,
+            None => bits,
+        };
+        Ok(Self { network, prefix })
+    }
+}
+
+impl IpRange {
+    pub(super) fn contains(&self, ip: std::net::IpAddr) -> bool {
+        use std::net::IpAddr;
+        match (self.network, ip) {
+            (IpAddr::V4(network), IpAddr::V4(ip)) => {
+                let mask = u32::MAX
+                    .checked_shl(32 - u32::from(self.prefix))
+                    .unwrap_or(0);
+                u32::from(network) & mask == u32::from(ip) & mask
+            }
+            (IpAddr::V6(network), IpAddr::V6(ip)) => {
+                let mask = u128::MAX
+                    .checked_shl(128 - u32::from(self.prefix))
+                    .unwrap_or(0);
+                u128::from(network) & mask == u128::from(ip) & mask
+            }
+            _ => false,
+        }
+    }
+}
+
+/// Whether `address` is in one of `ranges`: a direct address there (one through a relay
+/// is judged by the connection to the relay).
+pub(super) fn ignored(ranges: &[IpRange], address: &Multiaddr) -> bool {
+    if ranges.is_empty() || is_relayed(address) {
+        return false;
+    }
+    let ip = match address.iter().next() {
+        Some(Protocol::Ip4(ip)) => std::net::IpAddr::V4(ip),
+        Some(Protocol::Ip6(ip)) => std::net::IpAddr::V6(ip),
+        _ => return false,
+    };
+    ranges.iter().any(|range| range.contains(ip))
+}
+
+/// Keeps connections off the address ranges `libp2p-ignore` names, whoever dialled:
+/// hole punching and identify bring addresses there too, and a peer may dial us there.
+pub(super) struct Ignoring {
+    pub(super) ranges: Vec<IpRange>,
+}
+
+#[derive(Debug, thiserror::Error)]
+#[error("{0} is in a range left out (libp2p-ignore)")]
+pub(super) struct IgnoredAddress(Multiaddr);
+
+impl ::libp2p::swarm::NetworkBehaviour for Ignoring {
+    type ConnectionHandler = ::libp2p::swarm::dummy::ConnectionHandler;
+    type ToSwarm = std::convert::Infallible;
+
+    fn handle_pending_inbound_connection(
+        &mut self,
+        _connection_id: ConnectionId,
+        _local_addr: &Multiaddr,
+        remote_addr: &Multiaddr,
+    ) -> Result<(), ::libp2p::swarm::ConnectionDenied> {
+        if ignored(&self.ranges, remote_addr) {
+            return Err(::libp2p::swarm::ConnectionDenied::new(IgnoredAddress(
+                remote_addr.clone(),
+            )));
+        }
+        Ok(())
+    }
+
+    fn handle_established_inbound_connection(
+        &mut self,
+        _connection_id: ConnectionId,
+        _peer: PeerId,
+        _local_addr: &Multiaddr,
+        _remote_addr: &Multiaddr,
+    ) -> Result<::libp2p::swarm::THandler<Self>, ::libp2p::swarm::ConnectionDenied> {
+        Ok(::libp2p::swarm::dummy::ConnectionHandler)
+    }
+
+    fn handle_established_outbound_connection(
+        &mut self,
+        _connection_id: ConnectionId,
+        _peer: PeerId,
+        addr: &Multiaddr,
+        _role_override: ::libp2p::core::Endpoint,
+        _port_use: ::libp2p::core::transport::PortUse,
+    ) -> Result<::libp2p::swarm::THandler<Self>, ::libp2p::swarm::ConnectionDenied> {
+        if ignored(&self.ranges, addr) {
+            return Err(::libp2p::swarm::ConnectionDenied::new(IgnoredAddress(
+                addr.clone(),
+            )));
+        }
+        Ok(::libp2p::swarm::dummy::ConnectionHandler)
+    }
+
+    fn on_swarm_event(&mut self, _event: ::libp2p::swarm::FromSwarm) {}
+
+    fn on_connection_handler_event(
+        &mut self,
+        _peer_id: PeerId,
+        _connection_id: ConnectionId,
+        event: ::libp2p::swarm::THandlerOutEvent<Self>,
+    ) {
+        match event {}
+    }
+
+    fn poll(
+        &mut self,
+        _cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<
+        ::libp2p::swarm::ToSwarm<Self::ToSwarm, ::libp2p::swarm::THandlerInEvent<Self>>,
+    > {
+        std::task::Poll::Pending
+    }
+}
diff --git a/src/transport/libp2p/mod.rs b/src/transport/libp2p/mod.rs
new file mode 100644
index 0000000..86e7734
--- /dev/null
+++ b/src/transport/libp2p/mod.rs
@@ -0,0 +1,657 @@
+//! libp2p as a transport (`--transports libp2p`, in builds with the `libp2p` feature):
+//! direct connections over TCP (Noise, Yamux) and QUIC, to addresses this node knows.
+//!
+//! A node's transport identity is its libp2p peer ID; where it is, its listen addresses
+//! (multiaddrs ending in the peer ID, `locations`). Seeds are such multiaddrs, or ones
+//! without the peer ID, which a connection finds out. Messages go one per stream of the
+//! `/continuum/1` protocol: their length (4 bytes, big endian), then the message as
+//! `wire` encodes it. A send is done once the stream is written and closed; a connection
+//! that can't be made, or a stream that can't be written, refuses it. A stream's protocol
+//! goes with its first data, without waiting for the other side to agree
+//! (`swarm_config`): opening a stream costs no round trip.
+//!
+//! Connected peers tell each other where they listen (libp2p's identify protocol), so a
+//! node learns the addresses of peers that dialled it. A node publishes where it can be
+//! reached (`local_addresses`): the addresses it announces (`--libp2p-announce`, e.g. a
+//! public address forwarded to it), or else those it listens on, loopback left out.
+//!
+//! A node nobody can dial (behind NAT, or a closed port) can still be reached through a
+//! relay: a node of the swarm that relays (`--libp2p-relay`), or one configured
+//! (`--libp2p-relays`), on which it reserves a slot; its address through the relay
+//! (`<relay>/p2p-circuit/p2p/<it>`) is published with the others. Over a relayed connection, both sides then try to
+//! connect directly (libp2p's DCUtR, hole punching); the relay carries the traffic until
+//! that works, or for good if it doesn't (punched again now and then, `PUNCH_RETRIES`),
+//! within its limits (`RELAY_CIRCUIT_DURATION`,
+//! `RELAY_CIRCUIT_BYTES`; a circuit that ends is made again on the next send). Hole
+//! punching needs both sides to listen (`HolePunching`: at their LAN addresses too);
+//! where only one does, the other dials it directly once it knows where (`upgrade`).
+//!
+//! Of several connections to one peer (both dialled at once, or AutoNAT dialled back),
+//! all but one are closed a few seconds later (`DUPLICATE_GRACE`, `Book::duplicates`),
+//! by the side with the smaller peer ID: it keeps the direct one with the shortest round
+//! trip (libp2p's ping, on each connection), so a path over the LAN wins over one
+//! through a VPN, whichever opened first.
+//!
+//! A relay says so where it publishes its addresses: next to each, the same address
+//! ending in `/p2p-circuit` (where to reserve a slot), which other nodes find in the
+//! member list. A node that needs a relay picks up to `AUTO_RELAYS` relays
+//! (`relay_check`): those configured first, then those listed in the document
+//! (`learn_relays`), then members that relay. It needs one when
+//! it has no address others could dial, or when AutoNAT says it can't be dialled:
+//! AutoNAT asks connected peers (nodes of the swarm) to dial it back. Once AutoNAT says
+//! it can be dialled, it lets the relays it picked go.
+//!
+//! With `--libp2p-ignore`, address ranges are left out (VPNs): not published, not
+//! offered for hole punching, not dialled, and connections there refused (`Ignoring`).
+//!
+//! With `--libp2p-upnp`, a node asks its router to forward its listen ports (UPnP), and
+//! publishes the router's public address for them with its own (`Book::direct`): then
+//! it can be dialled from elsewhere, whatever the other side's NAT.
+//!
+//! With `--libp2p-mdns`, a node finds libp2p peers on its LAN (multicast DNS). Their
+//! LAN addresses are noted, and those that turn out to speak continuum (identify says
+//! so) are seeds while the node joins (`discovered_seeds`): nodes on one LAN find each
+//! other without configured seeds.
+//!
+//! It uses public libp2p infrastructure only with `--libp2p-public`: then it joins the
+//! IPFS DHT (Kademlia, through the IPFS bootstrap nodes), where it finds addresses of
+//! peers the member list doesn't have, and public peers that relay; those are picked
+//! after members that relay. Public relays keep circuits short (by default 2 minutes and
+//! 128 KiB): enough to punch holes through NATs, not to carry a swarm's traffic. Without
+//! it, relays are those configured, or members that say they relay.
+
+use std::collections::HashMap;
+use std::path::Path;
+use std::str::FromStr;
+use std::sync::{Arc, Mutex, MutexGuard};
+use std::time::Duration;
+
+use ::libp2p::multiaddr::Protocol;
+use ::libp2p::swarm::dial_opts::{DialOpts, PeerCondition};
+use ::libp2p::swarm::{ConnectionId, SwarmEvent};
+use ::libp2p::{Multiaddr, PeerId, StreamProtocol, Swarm, identity::Keypair};
+use anyhow::{Context, Result, anyhow, bail};
+use async_trait::async_trait;
+use futures::{AsyncReadExt, AsyncWriteExt, StreamExt};
+use tokio::sync::{Semaphore, mpsc, oneshot, watch};
+use tracing::{debug, info, trace, warn};
+
+use crate::locks::MutexExt;
+use crate::transport::stats::{
+    Connections, DialFailures, HolePunches, Libp2pStats, Reachability, RelaySlots, TransportStats,
+};
+use crate::transport::{
+    Addressed, Connection, EncodedMessage, HandlerFn, MessageHandler, ParsedMessage, Transport,
+    hand_on_probe, is_probe, wire,
+};
+
+mod behaviour;
+mod book;
+mod config;
+mod driver;
+mod errors;
+mod ignore;
+mod relays;
+
+use behaviour::*;
+use book::*;
+pub(crate) use config::{Libp2pConfig, check};
+use driver::*;
+use errors::*;
+use ignore::*;
+use relays::*;
+
+/// The scheme of this transport's addresses (`transport::paths`).
+pub(crate) const SCHEME: &str = "libp2p";
+
+const PROTOCOL: StreamProtocol = StreamProtocol::new("/continuum/1");
+/// Sends taking this long are logged with where the time went (at debug).
+const SLOW_SEND: Duration = Duration::from_millis(5);
+/// The largest message read: anyone who can connect can send anything, so this bounds
+/// what one stream costs. Sync messages above 16 chunks travel as blobs.
+const MAX_MESSAGE: usize = 128 << 20;
+/// How long a connection may take to come up.
+const CONNECT_TIMEOUT: Duration = Duration::from_secs(10);
+/// A connection nobody uses is closed after this; neighbours keep theirs busy.
+const IDLE_TIMEOUT: Duration = Duration::from_secs(300);
+/// Addresses of one dial tried at once (libp2p's default: 8). Hole punching (`HolePunching`)
+/// dials every address both sides offer, where they listen (LAN, VPNs, public IPv6) and
+/// where others saw them, often more than 8, while the other side punches towards all of
+/// them at the same moment: an address dialled only after others failed (each after
+/// QUIC's 5 s handshake timeout) misses its punch, and the LAN's is often among the
+/// last. Dials of the member list's addresses (`ADDRESSES_PER_PEER`) are unchanged.
+const DIAL_CONCURRENCY: std::num::NonZeroU8 = std::num::NonZeroU8::new(32).unwrap();
+/// How long a stream may take to open and to carry a message, before what its size adds
+/// (`stream_timeout`).
+const STREAM_TIMEOUT: Duration = Duration::from_secs(10);
+/// The slowest a message is expected to go, in bytes per second.
+const MIN_RATE: u64 = 1 << 20;
+/// Inbound streams read at once.
+const READS: usize = 32;
+/// Addresses kept per peer, and peers with addresses at most.
+const ADDRESSES_PER_PEER: usize = 8;
+const MAX_PEERS: usize = 4096;
+
+/// A relayed connection (circuit) through this node lasts at most this long and carries
+/// at most this much: generous, since the swarm's own relays may carry sync traffic for
+/// good where hole punching fails (libp2p's defaults, 2 minutes and 128 KiB, are for
+/// the punching only).
+const RELAY_CIRCUIT_DURATION: Duration = Duration::from_secs(3600);
+const RELAY_CIRCUIT_BYTES: u64 = 1 << 30;
+/// Circuits through this node at once, and per peer (either end: a node behind the
+/// relay is an end of every circuit to it). libp2p's rate limits on circuits and slots
+/// (a burst, then one every minute or two, per peer and per IP address: every node
+/// behind one NAT shares one) are left out: they're for public relays serving
+/// strangers, and refused circuits used them up too.
+const RELAY_CIRCUITS: usize = 1024;
+const RELAY_CIRCUITS_PER_PEER: usize = 64;
+/// A configured relay that couldn't be reached to find out its peer ID is tried again
+/// after this.
+const RELAY_RETRY: Duration = Duration::from_secs(30);
+/// Relays picked among the members, at most, and how often a node checks whether it
+/// needs them.
+const AUTO_RELAYS: usize = 2;
+const RELAY_CHECK: Duration = Duration::from_secs(30);
+/// A relay that refused a slot (`relay_refused`: none left, not allowed, not a relay,
+/// another peer at its address: a relay restarted without its libp2p key) isn't picked
+/// again for this long. One that didn't grant it for another reason (it doesn't answer,
+/// the connection closed) isn't for `RELAY_RETRY`; a slot lost is reserved again at
+/// once.
+const RELAY_PAUSE: Duration = Duration::from_secs(600);
+/// A peer connected only through a relay is dialled directly where it can be reached
+/// (`upgrade`): first at once, then after a failure no sooner than this, doubling up to
+/// `RELAY_PAUSE`.
+const UPGRADE_RETRY: Duration = Duration::from_secs(30);
+/// A peer nobody mentioned for this long (the member list, identify, mDNS, a
+/// connection) is forgotten, unless connected: mostly old peer IDs of nodes restarted
+/// without their libp2p key, whose new ones the member list has since. Members are
+/// mentioned every few seconds (`swarm::topology`). Checked every `PRUNE_EVERY`.
+const STALE_AFTER: Duration = Duration::from_secs(600);
+const PRUNE_EVERY: Duration = Duration::from_secs(60);
+/// Several connections to one peer (both dialled at once, or AutoNAT dialled back to
+/// see whether it can) are cut to one, the fastest (`Book::duplicates`), this long after
+/// the last one opened: long enough for what it was dialled for, and for each to be
+/// pinged (at once, as it opens). Looked for every `DEDUPE_EVERY`.
+const DUPLICATE_GRACE: Duration = Duration::from_secs(10);
+/// The same while a peer is connected only through relays: a new relayed connection is
+/// a hole punch under way (`PUNCH_RETRIES`), which takes its tries (up to a QUIC
+/// handshake's 5 s each, three times) on that connection.
+const PUNCH_GRACE: Duration = Duration::from_secs(60);
+/// After libp2p's hole punching gave up on a peer (three tries, as the relayed
+/// connection opened), it tries again after these, the last one over and over: what
+/// failed may have been passing (a NAT's mapping, a busy link). Each time through a new
+/// connection through the relay, on which the peer starts the punch (`retry_punches`).
+const PUNCH_RETRIES: [Duration; 3] = [
+    Duration::from_secs(60),
+    Duration::from_secs(300),
+    Duration::from_secs(1800),
+];
+const DEDUPE_EVERY: Duration = Duration::from_secs(5);
+
+/// How long a message of `bytes` may take on a stream.
+fn stream_timeout(bytes: usize) -> Duration {
+    STREAM_TIMEOUT + Duration::from_millis(bytes as u64 * 1000 / MIN_RATE)
+}
+
+pub(crate) struct Libp2pTransport {
+    local_id: String,
+    peer_id: PeerId,
+    control: libp2p_stream::Control,
+    commands: mpsc::UnboundedSender<Command>,
+    book: Arc<Mutex<Book>>,
+    incoming: tokio::sync::Mutex<Option<libp2p_stream::IncomingStreams>>,
+    stopped: watch::Receiver<bool>,
+}
+
+impl Libp2pTransport {
+    /// Builds the swarm, listens, and runs it in a task of its own.
+    pub(crate) fn start(config: Libp2pConfig) -> Result<Self> {
+        let peer_id = config.key.public().to_peer_id();
+        let features = Features::of(&config);
+        let mut swarm = if config.memory {
+            memory_swarm(config.key, features, config.ignore.clone())?
+        } else {
+            network_swarm(config.key, features, config.ignore.clone())?
+        };
+        for address in config.listen {
+            swarm
+                .listen_on(address.clone())
+                .with_context(|| format!("Listening on {address}"))?;
+        }
+        for address in &config.announce {
+            // Identify tells peers these.
+            swarm.add_external_address(address.clone());
+        }
+        let mut control = swarm.behaviour().stream.new_control();
+        let incoming = if config.continuum {
+            Some(
+                control
+                    .accept(PROTOCOL)
+                    .map_err(|error| anyhow!("Accepting {PROTOCOL}: {error}"))?,
+            )
+        } else {
+            None
+        };
+        let (commands, receiver) = mpsc::unbounded_channel();
+        let mut book = Book {
+            announced: config.announce,
+            ignored: config.ignore.clone(),
+            serving: config.relay,
+            ..Book::default()
+        };
+        for relay in config.relays {
+            if let Some(address) = book.name_relay(relay, Named::Configured, peer_id) {
+                let _ = commands.send(Command::FindRelay { address });
+            }
+        }
+        let book = Arc::new(Mutex::new(book));
+        let (stopping, stopped) = watch::channel(false);
+        tokio::spawn(run(
+            swarm,
+            receiver,
+            commands.clone(),
+            Arc::clone(&book),
+            Relaying {
+                serving: config.relay,
+                public: config.public,
+                local: peer_id,
+            },
+            stopping,
+        ));
+        info!(peer_id = %peer_id, "Transport started");
+        Ok(Self {
+            local_id: peer_id.to_string(),
+            peer_id,
+            control,
+            commands,
+            book,
+            incoming: tokio::sync::Mutex::new(incoming),
+            stopped,
+        })
+    }
+
+    /// Connected to `peer`, if it can be had in time.
+    async fn connect(&self, peer: PeerId) -> Result<()> {
+        let addresses = {
+            let book = lock(&self.book);
+            let mut known = book
+                .addresses
+                .get(&peer)
+                .map(|known| dialable(known, self.peer_id))
+                .unwrap_or_default();
+            known.retain(|address| !ignored(&book.ignored, address));
+            if known.is_empty() {
+                book.through_relays(peer, self.peer_id)
+            } else {
+                known
+            }
+        };
+        let (done, connected) = oneshot::channel();
+        self.commands
+            .send(Command::Connect {
+                peer,
+                addresses,
+                done,
+            })
+            .map_err(|_| anyhow!("the libp2p swarm stopped"))?;
+        match tokio::time::timeout(CONNECT_TIMEOUT, connected).await {
+            Ok(Ok(Ok(()))) => Ok(()),
+            Ok(Ok(Err(error))) => bail!("couldn't connect to {peer}: {error}"),
+            Ok(Err(_)) => bail!("the libp2p swarm stopped"),
+            Err(_) => bail!("connecting to {peer} took longer than {CONNECT_TIMEOUT:?}"),
+        }
+    }
+}
+
+/// One message from an inbound stream.
+async fn read_message(mut stream: ::libp2p::Stream) -> Result<Vec<u8>> {
+    let mut length = [0u8; 4];
+    tokio::time::timeout(STREAM_TIMEOUT, stream.read_exact(&mut length))
+        .await
+        .map_err(|_| anyhow!("no length within {STREAM_TIMEOUT:?}"))??;
+    let length = u32::from_be_bytes(length) as usize;
+    if length > MAX_MESSAGE {
+        bail!("a message of {length} bytes; at most {MAX_MESSAGE} are read");
+    }
+    let mut body = vec![0u8; length];
+    tokio::time::timeout(stream_timeout(length), stream.read_exact(&mut body))
+        .await
+        .map_err(|_| {
+            anyhow!(
+                "{length} bytes took longer than {:?}",
+                stream_timeout(length)
+            )
+        })??;
+    Ok(body)
+}
+
+#[async_trait]
+impl Transport for Libp2pTransport {
+    fn local_id(&self) -> &str {
+        &self.local_id
+    }
+
+    fn is_peer_id(&self, id: &str) -> bool {
+        PeerId::from_str(id).is_ok()
+    }
+
+    /// The longest a message may take on a stream.
+    fn delivery_limit(&self) -> Duration {
+        stream_timeout(MAX_MESSAGE)
+    }
+
+    /// Where we can be reached, each with our peer ID: the announced addresses, or else
+    /// those we listen on (`published`), and those through relays. A relay adds each
+    /// direct one again, ending in `/p2p-circuit`: where to reserve a slot.
+    fn local_addresses(&self) -> Vec<String> {
+        let book = lock(&self.book);
+        let direct: Vec<Multiaddr> = book
+            .direct()
+            .into_iter()
+            .filter_map(|address| address.with_p2p(self.peer_id).ok())
+            .collect();
+        let relayed = book
+            .listening
+            .iter()
+            .filter(|address| is_relayed(address))
+            .filter_map(|address| address.clone().with_p2p(self.peer_id).ok());
+        let offers: Vec<Multiaddr> = if book.serving {
+            direct
+                .iter()
+                .map(|address| address.clone().with(Protocol::P2pCircuit))
+                .collect()
+        } else {
+            Vec::new()
+        };
+        direct
+            .into_iter()
+            .chain(relayed)
+            .chain(offers)
+            .map(|address| address.to_string())
+            .collect()
+    }
+
+    /// A peer ID, or a multiaddr ending in one (noted as where that peer is).
+    fn locate(&self, target: &str) -> Option<String> {
+        if let Ok(peer) = PeerId::from_str(target) {
+            return Some(peer.to_string());
+        }
+        let mut address = Multiaddr::from_str(target).ok()?;
+        // A relay saying where to reserve a slot: `<address>/p2p/<relay>/p2p-circuit`.
+        if let Some(Protocol::P2pCircuit) = address.iter().last() {
+            address.pop();
+            let relay = split_peer(&address)?;
+            if relay != self.peer_id {
+                let new = {
+                    let mut book = lock(&self.book);
+                    let new = book.relays.get(&relay) != Some(&address);
+                    if new && (book.relays.len() < MAX_PEERS || book.relays.contains_key(&relay)) {
+                        book.relays.insert(relay, address.clone());
+                    }
+                    book.remember(relay, address);
+                    new
+                };
+                if new {
+                    let _ = self.commands.send(Command::CheckRelays);
+                }
+            }
+            return Some(relay.to_string());
+        }
+        let peer = split_peer(&address)?;
+        address.pop();
+        // A peer ID only says nothing about where.
+        if peer != self.peer_id && !address.is_empty() {
+            lock(&self.book).remember(peer, address);
+        }
+        Some(peer.to_string())
+    }
+
+    /// Multiaddrs, with or without the relay's peer ID; others are ignored.
+    fn learn_relays(&self, targets: &[String]) {
+        let listed = targets
+            .iter()
+            .filter_map(|target| Multiaddr::from_str(target).ok())
+            .collect();
+        let Some(found) = lock(&self.book).learn_listed(listed, self.peer_id) else {
+            return;
+        };
+        for address in found {
+            let _ = self.commands.send(Command::FindRelay { address });
+        }
+        let _ = self.commands.send(Command::CheckRelays);
+    }
+
+    fn connection(&self, target: &str) -> Option<Connection> {
+        let peer = PeerId::from_str(target).ok()?;
+        lock(&self.book).connection(&peer)
+    }
+
+    fn punching(&self, target: &str) -> Option<crate::transport::Punching> {
+        let peer = PeerId::from_str(target).ok()?;
+        let (failed, next) = *lock(&self.book).punching.get(&peer)?;
+        Some(crate::transport::Punching {
+            failed,
+            next_in_secs: next
+                .saturating_duration_since(tokio::time::Instant::now())
+                .as_secs(),
+        })
+    }
+
+    fn location(&self, target: &str) -> Option<String> {
+        let peer = PeerId::from_str(target).ok()?;
+        let address = lock(&self.book).location(&peer)?.clone();
+        Some(
+            address
+                .with_p2p(peer)
+                .unwrap_or_else(|address| address)
+                .to_string(),
+        )
+    }
+
+    fn stats(&self) -> TransportStats {
+        TransportStats {
+            libp2p: Some(lock(&self.book).stats()),
+            ..TransportStats::default()
+        }
+    }
+
+    /// Nodes found on the LAN (mDNS) that speak continuum.
+    fn discovered_seeds(&self) -> Vec<String> {
+        std::mem::take(&mut lock(&self.book).discovered)
+            .iter()
+            .map(Multiaddr::to_string)
+            .collect()
+    }
+
+    /// Where `target` is, each address with its peer ID.
+    fn locations(&self, target: &str) -> Vec<String> {
+        let Ok(peer) = PeerId::from_str(target) else {
+            return Vec::new();
+        };
+        lock(&self.book)
+            .addresses
+            .get(&peer)
+            .into_iter()
+            .flatten()
+            .filter_map(|address| address.clone().with_p2p(peer).ok())
+            .map(|address| address.to_string())
+            .collect()
+    }
+
+    /// A multiaddr ending in a peer ID is that peer; one without is connected to, to
+    /// find out. A bare peer ID resolves if this transport knows where it is.
+    async fn resolve_seed(&self, seed: &str) -> Result<String> {
+        let address = match PeerId::from_str(seed) {
+            Ok(peer) => Multiaddr::empty().with(Protocol::P2p(peer)),
+            Err(_) => {
+                Multiaddr::from_str(seed).with_context(|| format!("{seed} is no multiaddr"))?
+            }
+        };
+        // A peer ID only (`/p2p/<peer ID>`): reached where it's known, or through the
+        // relays we know.
+        if let [Protocol::P2p(peer)] = address.iter().collect::<Vec<_>>()[..] {
+            let book = lock(&self.book);
+            if book.addresses.contains_key(&peer)
+                || !book.through_relays(peer, self.peer_id).is_empty()
+            {
+                return Ok(peer.to_string());
+            }
+            bail!(
+                "no address known for libp2p peer {peer}, and no relay to reach it through; \
+                 give a multiaddr, or a relay (libp2p-relays)"
+            );
+        }
+        if let Some(peer) = self.locate(seed) {
+            return Ok(peer);
+        }
+        let (done, identified) = oneshot::channel();
+        self.commands
+            .send(Command::Identify { address, done })
+            .map_err(|_| anyhow!("the libp2p swarm stopped"))?;
+        match tokio::time::timeout(CONNECT_TIMEOUT, identified).await {
+            Ok(Ok(Ok(peer))) => Ok(peer.to_string()),
+            Ok(Ok(Err(error))) => bail!("couldn't connect to {seed}: {error}"),
+            Ok(Err(_)) => bail!("the libp2p swarm stopped"),
+            Err(_) => bail!("connecting to {seed} took longer than {CONNECT_TIMEOUT:?}"),
+        }
+    }
+
+    fn encode(&self, peer: &str, message: &ParsedMessage) -> Result<EncodedMessage> {
+        let body = wire::encode(message)?;
+        Ok(EncodedMessage {
+            peer: peer.to_owned(),
+            address: peer.to_owned(),
+            payload_len: body.len(),
+            body: body.into(),
+            kind: message.kind(),
+            message: None,
+        })
+    }
+
+    async fn send_encoded(&self, message: &EncodedMessage) -> Result<()> {
+        let peer = PeerId::from_str(&message.peer)
+            .with_context(|| format!("{} is no libp2p peer ID", message.peer))?;
+        let started = std::time::Instant::now();
+        self.connect(peer).await?;
+        // The protocol goes with the first data (`swarm_config`), so a peer that doesn't
+        // speak it seems to take the message: once identify has told, say so.
+        if let Some(relays) = lock(&self.book).not_nodes.get(&peer).copied() {
+            if relays {
+                bail!(
+                    "{peer} is a relay, not a node: give a node behind it as the seed \
+                     (<relay>/p2p-circuit/p2p/<peer ID>, or /p2p/<peer ID> with libp2p-relays)"
+                );
+            }
+            bail!("{peer} doesn't speak continuum");
+        }
+        let connected = started.elapsed();
+        let mut control = self.control.clone();
+        let body = message.body.clone();
+        let write = async move {
+            let mut stream = control.open_stream(peer, PROTOCOL).await?;
+            let opened = started.elapsed();
+            stream.write_all(&(body.len() as u32).to_be_bytes()).await?;
+            stream.write_all(&body).await?;
+            stream.close().await?;
+            // Kept until the peer has closed its side too. Over QUIC, dropping a stream
+            // stops its reading half (STOP_SENDING), and the peer's answer to the
+            // protocol (sent with the first data, `swarm_config`) then fails, and with it
+            // the stream, before the peer has read the message: identify, which reads
+            // its answers, works, while every message is lost. Yamux (TCP) doesn't mind.
+            // Off the send, which is done.
+            tokio::spawn(async move {
+                let mut rest = [0u8; 64];
+                let _ = tokio::time::timeout(STREAM_TIMEOUT, async {
+                    while let Ok(read) = stream.read(&mut rest).await {
+                        if read == 0 {
+                            break;
+                        }
+                    }
+                })
+                .await;
+            });
+            Ok::<_, anyhow::Error>(opened)
+        };
+        let timeout = stream_timeout(message.body.len());
+        let opened = tokio::time::timeout(timeout, write)
+            .await
+            .map_err(|_| anyhow!("sending to {peer} took longer than {timeout:?}"))?
+            .with_context(|| format!("Sending to {peer}"))?;
+        let took = started.elapsed();
+        if took >= SLOW_SEND {
+            // Where a send's time goes: the swarm task (connecting), the stream's protocol
+            // negotiation (opening), or the connection (writing).
+            debug!(
+                peer = %Addressed(SCHEME, &peer),
+                kind = ?message.kind,
+                bytes = message.body.len(),
+                connect_ms = connected.as_secs_f64() * 1000.0,
+                open_ms = (opened - connected).as_secs_f64() * 1000.0,
+                write_ms = (took - opened).as_secs_f64() * 1000.0,
+                "Slow send"
+            );
+        }
+        Ok(())
+    }
+
+    /// Reads inbound streams (`READS` at once) and handles their messages one at a
+    /// time, in the order they were read; PINGs and PONGs at once (`is_probe`).
+    async fn start_consumer(&self, handler: MessageHandler) -> Result<()> {
+        let Some(mut incoming) = self.incoming.lock().await.take() else {
+            bail!("libp2p consumer already started, or this node only relays");
+        };
+        let handler: Arc<HandlerFn> = Arc::from(handler);
+        let (sender, mut received) = mpsc::channel::<(PeerId, ParsedMessage, usize)>(READS);
+        let reads = Arc::new(Semaphore::new(READS));
+        let probes = Arc::clone(&handler);
+        tokio::spawn(async move {
+            while let Some((peer, stream)) = incoming.next().await {
+                let Ok(permit) = Arc::clone(&reads).acquire_owned().await else {
+                    break;
+                };
+                let sender = sender.clone();
+                let probes = Arc::clone(&probes);
+                tokio::spawn(async move {
+                    match read_message(stream).await {
+                        Ok(body) => match wire::decode(&body) {
+                            Ok(message) if is_probe(&message) => {
+                                hand_on_probe(
+                                    SCHEME,
+                                    &probes,
+                                    peer.to_string(),
+                                    message,
+                                    body.len(),
+                                );
+                            }
+                            Ok(message) => {
+                                let _ = sender.send((peer, message, body.len())).await;
+                            }
+                            Err(error) => {
+                                debug!(peer = %Addressed(SCHEME, &peer), error = %error, "Dropping a message that doesn't decode");
+                            }
+                        },
+                        Err(error) => {
+                            debug!(peer = %Addressed(SCHEME, &peer), error = %format!("{error:#}"), "Dropping an unreadable stream");
+                        }
+                    }
+                    drop(permit);
+                });
+            }
+        });
+        while let Some((peer, message, payload_len)) = received.recv().await {
+            if let Err(error) = handler(peer.to_string(), message, payload_len).await {
+                warn!(peer = %Addressed(SCHEME, &peer), error = %format!("{error:#}"), "Failed to process inbound message");
+            }
+        }
+        Ok(())
+    }
+
+    async fn wait_for_exit(&self) -> std::io::Result<std::process::ExitStatus> {
+        let mut stopped = self.stopped.clone();
+        let _ = stopped.wait_for(|stopped| *stopped).await;
+        Err(std::io::Error::other("the libp2p swarm stopped"))
+    }
+}
+
+#[cfg(test)]
+mod tests;
diff --git a/src/transport/libp2p/relays.rs b/src/transport/libp2p/relays.rs
new file mode 100644
index 0000000..3463408
--- /dev/null
+++ b/src/transport/libp2p/relays.rs
@@ -0,0 +1,234 @@
+//! Reaching peers past NATs: picking relays and keeping slots on them, upgrading
+//! relayed connections to direct ones, retrying hole punches, and finding peers on the
+//! DHT.
+
+use super::*;
+
+/// How a node relates to relays.
+pub(super) struct Relaying {
+    /// It relays for others.
+    pub(super) serving: bool,
+    /// Public relays may be picked, after members.
+    pub(super) public: bool,
+    pub(super) local: PeerId,
+}
+
+/// The relays to pick among `offers`, if this node needs relays, keeping `using`: a
+/// node needs them when it has no address others could dial (`direct`), or AutoNAT says
+/// it can't be dialled; up to `AUTO_RELAYS`, in an order of its own (the lowest hash of
+/// both peer IDs), so that not every node picks the same.
+pub(super) fn relay_check(
+    local: PeerId,
+    direct: bool,
+    nat: &::libp2p::autonat::NatStatus,
+    offers: &HashMap<PeerId, Multiaddr>,
+    using: &[PeerId],
+) -> Vec<(PeerId, Multiaddr)> {
+    use ::libp2p::autonat::NatStatus;
+    let needed = match nat {
+        NatStatus::Private => true,
+        NatStatus::Public(_) => false,
+        NatStatus::Unknown => !direct,
+    };
+    if !needed || using.len() >= AUTO_RELAYS {
+        return Vec::new();
+    }
+    let score = |relay: &PeerId| {
+        use std::hash::{Hash, Hasher};
+        let mut hasher = std::collections::hash_map::DefaultHasher::new();
+        (local, relay).hash(&mut hasher);
+        hasher.finish()
+    };
+    let mut candidates: Vec<(PeerId, Multiaddr)> = offers
+        .iter()
+        .filter(|(relay, _)| **relay != local && !using.contains(relay))
+        .map(|(relay, address)| (*relay, address.clone()))
+        .collect();
+    candidates.sort_by_key(|(relay, _)| score(relay));
+    candidates.truncate(AUTO_RELAYS - using.len());
+    candidates
+}
+
+/// Reserves slots on the relays `relay_check` picks (configured ones first, then listed
+/// ones, members, public ones), and lets those picked go once AutoNAT says this node can
+/// be dialled.
+pub(super) fn pick_relays(
+    swarm: &mut Swarm<Behaviour>,
+    book: &Mutex<Book>,
+    relaying: &Relaying,
+    nat: &::libp2p::autonat::NatStatus,
+    picked: &mut HashMap<::libp2p::core::transport::ListenerId, PeerId>,
+) {
+    if nat.is_public() && !picked.is_empty() {
+        info!("Reachable directly (AutoNAT); letting the relays picked go");
+        let mut book = lock(book);
+        for (listener, relay) in std::mem::take(picked) {
+            swarm.remove_listener(listener);
+            book.slots.remove(&relay);
+        }
+        return;
+    }
+    let (direct, configured, listed, members, public) = {
+        let mut book = lock(book);
+        let now = tokio::time::Instant::now();
+        book.failed_relays.retain(|_, until| now < *until);
+        let failed = book.failed_relays.clone();
+        let usable = |offers: &HashMap<PeerId, Multiaddr>| -> HashMap<PeerId, Multiaddr> {
+            offers
+                .iter()
+                .filter(|(relay, _)| !failed.contains_key(relay))
+                .map(|(relay, address)| (*relay, address.clone()))
+                .collect()
+        };
+        let public = if relaying.public {
+            usable(&book.public_relays)
+        } else {
+            HashMap::new()
+        };
+        (
+            !book.direct().is_empty(),
+            usable(&book.named(Named::Configured)),
+            usable(&book.named(Named::Listed)),
+            usable(&book.relays),
+            public,
+        )
+    };
+    let mut using: Vec<PeerId> = picked.values().copied().collect();
+    // Each kind for what the ones before left.
+    for (kind, offers) in [
+        (Named::Configured.as_str(), configured),
+        (Named::Listed.as_str(), listed),
+        ("member", members),
+        ("public", public),
+    ] {
+        for (relay, address) in relay_check(relaying.local, direct, nat, &offers, &using) {
+            using.push(relay);
+            match swarm.listen_on(address.clone().with(Protocol::P2pCircuit)) {
+                Ok(listener) => {
+                    info!(relay = %Addressed(SCHEME, &relay), address = %address, kind, "Picked a relay");
+                    picked.insert(listener, relay);
+                }
+                Err(error) => {
+                    debug!(relay = %Addressed(SCHEME, &relay), kind, error = %error, "Couldn't use a relay");
+                }
+            }
+        }
+    }
+}
+
+/// Dials `peer` directly if it's connected only through a relay and is known elsewhere
+/// too (the member list, identify, mDNS), since the first connection, made through a
+/// relay (a relayed seed), would otherwise be kept for good: a peer connected isn't
+/// dialled again, and hole punching needs both sides to listen. The direct connection,
+/// once open, wins over the relayed one (`Book::duplicates`).
+pub(super) fn upgrade(
+    swarm: &mut Swarm<Behaviour>,
+    book: &Mutex<Book>,
+    peer: PeerId,
+    local: PeerId,
+) {
+    let direct: Vec<Multiaddr> = {
+        let mut book = lock(book);
+        if book.connection(&peer) != Some(Connection::Relayed) {
+            return;
+        }
+        let direct: Vec<Multiaddr> = book
+            .addresses
+            .get(&peer)
+            .map(|known| dialable(known, local))
+            .unwrap_or_default()
+            .into_iter()
+            .filter(|address| !is_relayed(address) && !ignored(&book.ignored, address))
+            .collect();
+        if direct.is_empty() {
+            return;
+        }
+        let now = tokio::time::Instant::now();
+        let (due, wait) = book
+            .upgrades
+            .get(&peer)
+            .copied()
+            .unwrap_or((now, UPGRADE_RETRY));
+        if now < due {
+            return;
+        }
+        book.upgrades
+            .insert(peer, (now + wait, (wait * 2).min(RELAY_PAUSE)));
+        direct
+    };
+    debug!(peer = %Addressed(SCHEME, &peer), addresses = ?direct, "Connected only through a relay; dialling directly");
+    let _ = swarm.dial(
+        DialOpts::peer_id(peer)
+            .addresses(direct)
+            .condition(PeerCondition::NotDialing)
+            .build(),
+    );
+}
+
+/// Tries hole punching again with the peers it failed with that are due (`punching`).
+/// libp2p punches as a relayed connection opens, started by the side that was dialled
+/// through the relay; that side is where a punch's failure is told, so it is this one.
+/// A new relayed connection starts a new punch: we dial the peer through a relay if it
+/// has a slot on one; else we close our relayed connections to it, and its next message
+/// (a probe, within a minute) dials us through ours again. Peers connected directly by
+/// now are dropped; those not connected now are kept, failures and all.
+pub(super) fn retry_punches(swarm: &mut Swarm<Behaviour>, book: &Mutex<Book>, local: PeerId) {
+    let now = tokio::time::Instant::now();
+    let mut close: Vec<ConnectionId> = Vec::new();
+    let due: Vec<(PeerId, usize, Vec<Multiaddr>)> = {
+        let mut book = lock(book);
+        let book = &mut *book;
+        let mut due = Vec::new();
+        book.punching.retain(|peer, (failed, next)| {
+            match book_connection(&book.connections, peer) {
+                Some(Connection::Direct) => return false,
+                // Kept, failures and all: one we closed to have it dial anew comes back.
+                None => return true,
+                Some(Connection::Relayed) if now < *next => return true,
+                Some(Connection::Relayed) => {}
+            }
+            let through: Vec<Multiaddr> = book
+                .addresses
+                .get(peer)
+                .map(|known| dialable(known, local))
+                .unwrap_or_default()
+                .into_iter()
+                .filter(is_relayed)
+                .collect();
+            // Until its punch fails again (or the peer tries from its side).
+            *next = now + PUNCH_RETRIES[(*failed).min(PUNCH_RETRIES.len() - 1)];
+            if through.is_empty() {
+                close.extend(
+                    book.connections
+                        .iter()
+                        .filter(|(_, (connected, relayed, _))| connected == peer && *relayed)
+                        .map(|(id, _)| *id),
+                );
+            } else {
+                due.push((*peer, *failed, through));
+            }
+            true
+        });
+        due
+    };
+    for connection in close {
+        debug!(connection = %connection, "Hole punching again: closing a relayed connection, for the peer to dial anew");
+        swarm.close_connection(connection);
+    }
+    for (peer, failed, through) in due {
+        debug!(peer = %Addressed(SCHEME, &peer), failed, "Hole punching again, through a new relayed connection");
+        let _ = swarm.dial(
+            DialOpts::peer_id(peer)
+                .addresses(through)
+                .condition(PeerCondition::Always)
+                .build(),
+        );
+    }
+}
+
+/// Asks the DHT (with `--libp2p-public`) where `peer` is, for the next attempt.
+pub(super) fn find_on_dht(swarm: &mut Swarm<Behaviour>, peer: PeerId) {
+    if let Some(dht) = swarm.behaviour_mut().kad.as_mut() {
+        dht.get_closest_peers(peer);
+    }
+}
diff --git a/src/transport/libp2p/tests.rs b/src/transport/libp2p/tests.rs
new file mode 100644
index 0000000..f02668a
--- /dev/null
+++ b/src/transport/libp2p/tests.rs
@@ -0,0 +1,779 @@
+use std::sync::atomic::{AtomicU64, Ordering};
+
+use super::*;
+use crate::document::replica::Epoch;
+use crate::transport::{HeadsSummary, Status};
+
+/// Memory ports are process-wide.
+static NEXT_PORT: AtomicU64 = AtomicU64::new(40_000);
+
+/// A node on the in-memory transport, listening on a port of its own (or nowhere),
+/// and its address with its peer ID (or just its peer ID).
+fn memory_node_with(
+    listen: bool,
+    relays: Vec<Multiaddr>,
+    relay: bool,
+) -> (Libp2pTransport, Multiaddr) {
+    let port = NEXT_PORT.fetch_add(1, Ordering::Relaxed);
+    let key = Keypair::generate_ed25519();
+    let address: Multiaddr = format!("/memory/{port}").parse().unwrap();
+    let peer = key.public().to_peer_id();
+    let transport = Libp2pTransport::start(Libp2pConfig {
+        key,
+        listen: if listen {
+            vec![address.clone()]
+        } else {
+            Vec::new()
+        },
+        announce: Vec::new(),
+        relays,
+        relay,
+        continuum: true,
+        mdns: false,
+        upnp: false,
+        ignore: Vec::new(),
+        public: false,
+        memory: true,
+    })
+    .unwrap();
+    let address = if listen {
+        address.with_p2p(peer).unwrap()
+    } else {
+        Multiaddr::empty().with(Protocol::P2p(peer))
+    };
+    (transport, address)
+}
+
+fn memory_node() -> (Libp2pTransport, Multiaddr) {
+    memory_node_with(true, Vec::new(), false)
+}
+
+fn status() -> ParsedMessage {
+    ParsedMessage::Status(Status {
+        heads: HeadsSummary::of(Vec::new()),
+        epoch: Epoch::GENESIS,
+        epoch_seq: 3,
+        ready: true,
+        syncs_with_you: false,
+        leaving: false,
+    })
+}
+
+/// Starts `transport`'s consumer, forwarding what it receives.
+fn received(transport: Arc<Libp2pTransport>) -> mpsc::UnboundedReceiver<(String, ParsedMessage)> {
+    let (sender, receiver) = mpsc::unbounded_channel();
+    tokio::spawn(async move {
+        transport
+            .start_consumer(Box::new(move |from, message, _| {
+                let sender = sender.clone();
+                Box::pin(async move {
+                    let _ = sender.send((from, message));
+                    Ok(())
+                })
+            }))
+            .await
+    });
+    receiver
+}
+
+#[tokio::test]
+async fn messages_both_ways_over_one_connection() {
+    let (a, a_address) = memory_node();
+    let (b, _) = memory_node();
+    let (a, b) = (Arc::new(a), Arc::new(b));
+    let mut at_a = received(Arc::clone(&a));
+    let mut at_b = received(Arc::clone(&b));
+
+    assert_eq!(
+        b.resolve_seed(&a_address.to_string()).await.unwrap(),
+        a.local_id()
+    );
+    b.send_message(a.local_id(), &status()).await.unwrap();
+    let (from, message) = at_a.recv().await.unwrap();
+    assert_eq!(from, b.local_id());
+    assert!(matches!(message, ParsedMessage::Status(status) if status.epoch_seq == 3));
+
+    // a knows no address of b, but b's connection is there.
+    a.send_message(b.local_id(), &status()).await.unwrap();
+    let (from, _) = at_b.recv().await.unwrap();
+    assert_eq!(from, a.local_id());
+
+    assert_eq!(a.connection(b.local_id()), Some(Connection::Direct));
+    assert_eq!(b.connection(a.local_id()), Some(Connection::Direct));
+    let stats = a.stats().libp2p.unwrap();
+    assert!(stats.connections.direct >= 1);
+    assert_eq!(stats.connections.relayed, 0);
+}
+
+#[tokio::test]
+async fn a_seed_without_a_peer_id_is_connected_to_to_find_out() {
+    let (a, a_address) = memory_node();
+    let (b, _) = memory_node();
+    let mut bare = a_address.clone();
+    bare.pop();
+    assert_eq!(
+        b.resolve_seed(&bare.to_string()).await.unwrap(),
+        a.local_id()
+    );
+    // And b now knows where a is.
+    assert_eq!(b.locations(a.local_id()), [a_address.to_string()]);
+}
+
+#[tokio::test]
+async fn listen_addresses_carry_the_peer_id() {
+    let (a, a_address) = memory_node();
+    let deadline = tokio::time::Instant::now() + Duration::from_secs(5);
+    while a.local_addresses().is_empty() && tokio::time::Instant::now() < deadline {
+        tokio::time::sleep(Duration::from_millis(10)).await;
+    }
+    assert_eq!(a.local_addresses(), [a_address.to_string()]);
+    assert_eq!(
+        a.locate(&a_address.to_string()).as_deref(),
+        Some(a.local_id())
+    );
+}
+
+/// Waits up to 5 s for `condition`.
+async fn eventually(mut condition: impl FnMut() -> bool) -> bool {
+    let deadline = tokio::time::Instant::now() + Duration::from_secs(5);
+    while !condition() {
+        if tokio::time::Instant::now() >= deadline {
+            return false;
+        }
+        tokio::time::sleep(Duration::from_millis(10)).await;
+    }
+    true
+}
+
+/// A node learns where a peer that dialled it listens (identify), and can then
+/// pass it on.
+#[tokio::test]
+async fn a_dialled_node_learns_where_the_dialler_listens() {
+    let (a, a_address) = memory_node();
+    let (b, b_address) = memory_node();
+    let (a, b) = (Arc::new(a), Arc::new(b));
+    let _at_a = received(Arc::clone(&a));
+    b.resolve_seed(&a_address.to_string()).await.unwrap();
+    b.send_message(a.local_id(), &status()).await.unwrap();
+    let b_id = b.local_id().to_owned();
+    assert!(
+        eventually(|| a.locations(&b_id) == [b_address.to_string()]).await,
+        "a knows {:?} of b",
+        a.locations(&b_id)
+    );
+}
+
+#[tokio::test]
+async fn announced_addresses_are_published_instead() {
+    let key = Keypair::generate_ed25519();
+    let peer = key.public().to_peer_id();
+    let port = NEXT_PORT.fetch_add(1, Ordering::Relaxed);
+    let announced: Multiaddr = "/ip4/203.0.113.7/tcp/9661".parse().unwrap();
+    let transport = Libp2pTransport::start(Libp2pConfig {
+        key,
+        listen: vec![format!("/memory/{port}").parse().unwrap()],
+        announce: vec![announced.clone()],
+        relays: Vec::new(),
+        relay: false,
+        continuum: true,
+        mdns: false,
+        upnp: false,
+        ignore: Vec::new(),
+        public: false,
+        memory: true,
+    })
+    .unwrap();
+    assert_eq!(
+        transport.local_addresses(),
+        [announced.with_p2p(peer).unwrap().to_string()]
+    );
+}
+
+#[test]
+fn address_ranges() {
+    let range = |text: &str| text.parse::<IpRange>().unwrap();
+    let address = |text: &str| text.parse::<Multiaddr>().unwrap();
+    let ranges = [range("10.244.0.0/16"), range("42::/16"), range("192.0.2.7")];
+    for left_out in [
+        "/ip4/10.244.3.2/udp/9661/quic-v1",
+        "/ip6/42:1:a46e:be4a:cb58:e460:bf0:142c/tcp/9661",
+        "/ip4/192.0.2.7/tcp/1",
+    ] {
+        assert!(ignored(&ranges, &address(left_out)), "{left_out}");
+    }
+    for kept in [
+        "/ip4/10.23.1.144/udp/9661/quic-v1",
+        "/ip4/10.245.0.1/tcp/1",
+        "/ip6/2a0a:4cc0:c1:5eb0::1/udp/9661/quic-v1",
+        "/ip4/192.0.2.8/tcp/1",
+        "/dns4/example.org/tcp/1",
+    ] {
+        assert!(!ignored(&ranges, &address(kept)), "{kept}");
+    }
+    // Through a relay: judged by the connection to the relay, not here.
+    let relayed = format!("/ip4/10.244.0.1/tcp/1/p2p/{}/p2p-circuit", peer());
+    assert!(!ignored(&ranges, &address(&relayed)));
+    assert!(range("0.0.0.0/0").contains("203.0.113.1".parse().unwrap()));
+    assert!(!range("0.0.0.0/0").contains("::1".parse().unwrap()));
+    for wrong in ["10.244.0.0/33", "42::/129", "nowhere", "10.0.0.0/x"] {
+        assert!(wrong.parse::<IpRange>().is_err(), "{wrong}");
+    }
+}
+
+/// Ranges left out aren't published; announced addresses are, whatever they are.
+#[test]
+fn ignored_addresses_are_not_published() {
+    let parse = |text: &str| text.parse::<Multiaddr>().unwrap();
+    let lan = parse("/ip4/10.23.1.5/udp/9663/quic-v1");
+    let vpn = parse("/ip4/10.244.3.2/udp/9663/quic-v1");
+    let mut book = Book {
+        listening: vec![lan.clone(), vpn.clone()],
+        ignored: vec!["10.244.0.0/16".parse().unwrap()],
+        ..Book::default()
+    };
+    assert_eq!(book.direct(), [lan]);
+    book.announced = vec![vpn.clone()];
+    assert_eq!(book.direct(), [vpn]);
+}
+
+/// A port forwarded by hand: once AutoNAT found the node dialable at the router's
+/// address, that is published with the listen addresses; announced ones still win.
+#[test]
+fn the_address_autonat_confirmed_is_published() {
+    let parse = |text: &str| text.parse::<Multiaddr>().unwrap();
+    let lan = parse("/ip4/10.23.1.5/udp/9663/quic-v1");
+    let public = parse("/ip4/91.65.180.254/udp/9663/quic-v1");
+    let mut book = Book {
+        listening: vec![lan.clone()],
+        ..Book::default()
+    };
+    assert_eq!(book.direct(), [lan.clone()]);
+    book.public_address = Some(public.clone().with_p2p(peer()).unwrap());
+    assert_eq!(book.direct(), [public.clone(), lan.clone()]);
+    let announced = parse("/dns4/example.org/udp/9663/quic-v1");
+    book.announced = vec![announced.clone()];
+    assert_eq!(book.direct(), [announced]);
+}
+
+#[test]
+fn local_addresses_are_left_out_unless_there_are_only_local_ones() {
+    let parse = |text: &str| text.parse::<Multiaddr>().unwrap();
+    let loopback = parse("/ip4/127.0.0.1/tcp/1");
+    let link = parse("/ip6/fe80::1/tcp/1");
+    let lan = parse("/ip4/192.168.1.2/tcp/1");
+    let wireguard = parse("/ip6/fd00::2/tcp/1");
+    assert_eq!(
+        published(&[
+            loopback.clone(),
+            link.clone(),
+            lan.clone(),
+            wireguard.clone()
+        ]),
+        [lan, wireguard]
+    );
+    assert_eq!(
+        published(&[loopback.clone(), link.clone()]),
+        [loopback, link]
+    );
+}
+
+/// b listens nowhere but through the relay r; a reaches it there.
+#[tokio::test]
+async fn a_node_nobody_can_dial_is_reached_through_a_relay() {
+    let (_r, r_address) = memory_node_with(true, Vec::new(), true);
+    let (b, _) = memory_node_with(false, vec![r_address.clone()], false);
+    let b = Arc::new(b);
+    let mut at_b = received(Arc::clone(&b));
+    assert!(
+        eventually(|| !b.local_addresses().is_empty()).await,
+        "b got no slot on the relay"
+    );
+    let relayed = b.local_addresses()[0].clone();
+    assert!(
+        relayed.starts_with(&format!("{r_address}/p2p-circuit/")),
+        "{relayed}"
+    );
+
+    assert!(eventually(|| b.stats().libp2p.unwrap().relay_slots.held == 1).await);
+    let slots = b.stats().libp2p.unwrap().relay_slots;
+    assert_eq!((slots.granted, slots.not_granted), (1, 0));
+
+    let (a, _) = memory_node();
+    assert_eq!(a.resolve_seed(&relayed).await.unwrap(), b.local_id());
+    a.send_message(b.local_id(), &status()).await.unwrap();
+    let (from, _) = at_b.recv().await.unwrap();
+    assert_eq!(from, a.local_id());
+    assert!(a.connection(b.local_id()).is_some());
+}
+
+/// A seed that's a peer ID only is reached through the relays the node knows.
+#[tokio::test]
+async fn a_peer_id_seed_is_reached_through_a_relay() {
+    let (_r, r_address) = memory_node_with(true, Vec::new(), true);
+    let (b, _) = memory_node_with(false, vec![r_address.clone()], false);
+    let b = Arc::new(b);
+    let mut at_b = received(Arc::clone(&b));
+    assert!(
+        eventually(|| !b.local_addresses().is_empty()).await,
+        "b got no slot on the relay"
+    );
+    let (a, _) = memory_node_with(true, vec![r_address.clone()], false);
+    let seed = format!("/p2p/{}", b.local_id());
+    assert_eq!(a.resolve_seed(&seed).await.unwrap(), b.local_id());
+    assert_eq!(a.resolve_seed(b.local_id()).await.unwrap(), b.local_id());
+    // Nothing to dial is remembered.
+    assert!(!lock(&a.book).addresses.contains_key(&b.peer_id));
+    a.send_message(b.local_id(), &status()).await.unwrap();
+    let (from, _) = at_b.recv().await.unwrap();
+    assert_eq!(from, a.local_id());
+
+    // Without a relay, there's nowhere to look.
+    let (c, _) = memory_node();
+    assert!(c.resolve_seed(&seed).await.is_err());
+}
+
+/// b listens nowhere; a, which can be dialled, reaches it through the relay r: b
+/// then dials a directly (identify told it where), and both keep that.
+#[tokio::test]
+async fn a_relayed_connection_is_upgraded_where_one_side_can_be_dialled() {
+    let (_r, r_address) = memory_node_with(true, Vec::new(), true);
+    let (b, _) = memory_node_with(false, vec![r_address.clone()], false);
+    let b = Arc::new(b);
+    let mut at_b = received(Arc::clone(&b));
+    assert!(
+        eventually(|| !b.local_addresses().is_empty()).await,
+        "b got no slot on the relay"
+    );
+    let (a, _) = memory_node();
+    let relayed = b.local_addresses()[0].clone();
+    assert_eq!(a.resolve_seed(&relayed).await.unwrap(), b.local_id());
+    a.send_message(b.local_id(), &status()).await.unwrap();
+    at_b.recv().await.unwrap();
+    assert!(
+        eventually(|| b.connection(a.local_id()) == Some(Connection::Direct)).await,
+        "b didn't dial a directly"
+    );
+    assert_eq!(a.connection(b.local_id()), Some(Connection::Direct));
+    // The relayed one goes (`DUPLICATE_GRACE`); the direct one stays.
+    let deadline = tokio::time::Instant::now() + DUPLICATE_GRACE * 2;
+    while a.stats().libp2p.unwrap().connections.relayed > 0 {
+        assert!(
+            tokio::time::Instant::now() < deadline,
+            "the relayed connection stayed"
+        );
+        tokio::time::sleep(Duration::from_millis(100)).await;
+    }
+    b.send_message(a.local_id(), &status()).await.unwrap();
+    assert_eq!(a.connection(b.local_id()), Some(Connection::Direct));
+}
+
+/// A relay that is no node (`--relay-only`) relays, and takes no messages itself.
+#[tokio::test]
+async fn a_relay_that_is_no_node() {
+    let port = NEXT_PORT.fetch_add(1, Ordering::Relaxed);
+    let key = Keypair::generate_ed25519();
+    let r_peer = key.public().to_peer_id();
+    let r_listen: Multiaddr = format!("/memory/{port}").parse().unwrap();
+    let r = Libp2pTransport::start(Libp2pConfig {
+        key,
+        listen: vec![r_listen.clone()],
+        announce: Vec::new(),
+        relays: Vec::new(),
+        relay: true,
+        continuum: false,
+        mdns: false,
+        upnp: false,
+        ignore: Vec::new(),
+        public: false,
+        memory: true,
+    })
+    .unwrap();
+    let r_address = r_listen.with_p2p(r_peer).unwrap();
+    let (b, _) = memory_node_with(false, vec![r_address.clone()], false);
+    let b = Arc::new(b);
+    let mut at_b = received(Arc::clone(&b));
+    assert!(
+        eventually(|| !b.local_addresses().is_empty()).await,
+        "b got no slot on the relay"
+    );
+    let (a, _) = memory_node();
+    let relayed = b.local_addresses()[0].clone();
+    assert_eq!(a.resolve_seed(&relayed).await.unwrap(), b.local_id());
+    a.send_message(b.local_id(), &status()).await.unwrap();
+    let (from, _) = at_b.recv().await.unwrap();
+    assert_eq!(from, a.local_id());
+
+    // The relay itself doesn't speak continuum: it takes no messages, and a send to
+    // it is refused once identify has told (the relay's own refusal comes too late:
+    // the protocol goes with the first data, `swarm_config`), saying what to give
+    // as a seed instead.
+    assert_eq!(
+        a.resolve_seed(&r_address.to_string()).await.unwrap(),
+        r.local_id()
+    );
+    assert!(
+        eventually(|| lock(&a.book).not_nodes.get(&r.peer_id) == Some(&true)).await,
+        "identify didn't tell"
+    );
+    let refused = a.send_message(r.local_id(), &status()).await.unwrap_err();
+    assert!(
+        refused.to_string().contains("is a relay, not a node"),
+        "{refused:#}"
+    );
+    assert!(
+        r.start_consumer(Box::new(|_, _, _| Box::pin(async { Ok(()) })))
+            .await
+            .is_err()
+    );
+}
+
+/// A relay configured without its peer ID is found out, and used.
+#[tokio::test]
+async fn a_relay_configured_without_its_peer_id() {
+    let (_r, r_address) = memory_node_with(true, Vec::new(), true);
+    let mut bare = r_address.clone();
+    bare.pop();
+    let (b, _) = memory_node_with(false, vec![bare], false);
+    assert!(
+        eventually(|| !b.local_addresses().is_empty()).await,
+        "b got no slot on the relay"
+    );
+    assert!(b.local_addresses()[0].starts_with(&format!("{r_address}/p2p-circuit/")));
+}
+
+fn peer() -> PeerId {
+    Keypair::generate_ed25519().public().to_peer_id()
+}
+
+#[test]
+fn relays_are_picked_when_needed() {
+    use ::libp2p::autonat::NatStatus;
+    let local = peer();
+    let offers: HashMap<PeerId, Multiaddr> = (0..4)
+        .map(|port| (peer(), format!("/memory/{port}").parse().unwrap()))
+        .collect();
+    let check =
+        |direct, nat: &NatStatus, using: &[PeerId]| relay_check(local, direct, nat, &offers, using);
+    // Dialable, or not known yet with an address to dial: none.
+    assert!(check(true, &NatStatus::Public(Multiaddr::empty()), &[]).is_empty());
+    assert!(check(false, &NatStatus::Public(Multiaddr::empty()), &[]).is_empty());
+    assert!(check(true, &NatStatus::Unknown, &[]).is_empty());
+    // No address to dial, or AutoNAT says it can't be dialled: some.
+    assert_eq!(check(false, &NatStatus::Unknown, &[]).len(), AUTO_RELAYS);
+    let picked = check(true, &NatStatus::Private, &[]);
+    assert_eq!(picked.len(), AUTO_RELAYS);
+    // One in use: the next in the same order.
+    let more = check(true, &NatStatus::Private, &[picked[0].0]);
+    assert_eq!(more.len(), AUTO_RELAYS - 1);
+    assert_eq!(more[0].0, picked[1].0);
+    let all: Vec<PeerId> = picked.iter().map(|(relay, _)| *relay).collect();
+    assert!(check(true, &NatStatus::Private, &all).is_empty());
+    // Never itself.
+    let only_us = HashMap::from([(local, Multiaddr::empty())]);
+    assert!(relay_check(local, false, &NatStatus::Private, &only_us, &[]).is_empty());
+}
+
+/// A relay says so in its addresses; b, which nobody can dial, learns of it (as from
+/// the member list) and becomes reachable through it.
+#[tokio::test]
+async fn a_member_that_relays_is_picked() {
+    let (r, _) = memory_node_with(true, Vec::new(), true);
+    assert!(eventually(|| !r.local_addresses().is_empty()).await);
+    let offer = r
+        .local_addresses()
+        .into_iter()
+        .find(|address| address.ends_with("/p2p-circuit"))
+        .expect("the relay offers itself");
+    let (b, _) = memory_node_with(false, Vec::new(), false);
+    assert_eq!(b.locate(&offer).as_deref(), Some(r.local_id()));
+    assert!(
+        eventually(|| b
+            .local_addresses()
+            .iter()
+            .any(|address| address.contains("/p2p-circuit/")))
+        .await,
+        "b isn't reachable through r: {:?}",
+        b.local_addresses()
+    );
+}
+
+/// A peer on the LAN that speaks continuum becomes a seed once, at its address.
+#[test]
+fn lan_peers_become_seeds_once() {
+    let mut book = Book::default();
+    let lan = peer();
+    let address: Multiaddr = "/ip4/192.168.1.5/tcp/9661".parse().unwrap();
+    book.discover(lan); // No address known: nothing.
+    assert!(book.discovered.is_empty());
+    book.remember(lan, address.clone());
+    book.discover(lan);
+    book.discover(lan);
+    assert_eq!(book.discovered, [address.with_p2p(lan).unwrap()]);
+}
+
+#[test]
+fn named_relays() {
+    let (local, r1, r2) = (peer(), peer(), peer());
+    let at = |port: u16| -> Multiaddr { format!("/ip6/fd00::1/tcp/{port}").parse().unwrap() };
+    let mut book = Book::default();
+    // With a peer ID: known at once; without: to be found out, once.
+    assert_eq!(
+        book.name_relay(at(1).with_p2p(r1).unwrap(), Named::Configured, local),
+        None
+    );
+    assert_eq!(
+        book.name_relay(at(2), Named::Configured, local),
+        Some(at(2))
+    );
+    assert_eq!(book.name_relay(at(2), Named::Configured, local), None);
+    assert_eq!(
+        book.name_relay(at(3).with_p2p(local).unwrap(), Named::Configured, local),
+        None
+    );
+    assert!(book.relay_found(at(2), r2, local));
+    assert_eq!(book.named(Named::Configured).len(), 2);
+    // A relay named with its peer ID isn't found out again; one without is.
+    assert_eq!(book.find_again(r1), None);
+    assert_eq!(book.find_again(r2), Some(at(2)));
+    assert!(!book.named(Named::Configured).contains_key(&r2));
+
+    // Listed: configured wins; unlisted ones go, unless also configured.
+    let r3 = peer();
+    let listed = vec![
+        at(1).with_p2p(r1).unwrap(),
+        at(4).with_p2p(r3).unwrap(),
+        at(5),
+    ];
+    assert_eq!(book.learn_listed(listed.clone(), local), Some(vec![at(5)]));
+    assert_eq!(book.learn_listed(listed, local), None);
+    assert_eq!(book.named_relays[&r1].named, Named::Configured);
+    assert_eq!(book.named(Named::Listed).keys().collect::<Vec<_>>(), [&r3]);
+    assert_eq!(book.learn_listed(Vec::new(), local), Some(Vec::new()));
+    assert!(book.named(Named::Listed).is_empty());
+    assert!(!book.finding.contains_key(&at(5)));
+    assert!(!book.relay_found(at(5), peer(), local)); // No longer listed.
+    assert_eq!(book.named(Named::Configured).len(), 1);
+}
+
+#[test]
+fn known_addresses() {
+    let mut book = Book::default();
+    let (relay, us) = (peer(), peer());
+    let direct: Multiaddr = "/ip6/fd00::1/tcp/9661".parse().unwrap();
+    let relayed: Multiaddr = format!("/ip6/fd00::2/tcp/9661/p2p/{relay}/p2p-circuit/p2p/{us}")
+        .parse()
+        .unwrap();
+    assert!(!book.knows(us, &[direct.clone()]));
+    book.remember(us, relayed.clone());
+    book.remember(us, direct.clone());
+    // In any order, with or without the peer ID.
+    assert!(book.knows(us, &[relayed.clone(), direct.clone()]));
+    assert!(book.knows(us, &[direct.clone().with_p2p(us).unwrap()]));
+    assert!(!book.knows(us, &["/ip6/fd00::3/tcp/9661".parse().unwrap()]));
+}
+
+/// Peers nobody mentions any more are forgotten, but connected ones.
+#[test]
+fn stale_peers_are_forgotten() {
+    let (old, current, connected) = (peer(), peer(), peer());
+    let address: Multiaddr = "/ip6/fd00::1/tcp/9661".parse().unwrap();
+    let mut book = Book::default();
+    for peer in [old, current, connected] {
+        book.remember(peer, address.clone());
+    }
+    book.relays.insert(old, address.clone());
+    book.continuum.insert(old);
+    // From after the mentions above (taken at `Instant::now()`), however fast or
+    // slow the clock moved meanwhile.
+    let later = tokio::time::Instant::now() + STALE_AFTER;
+    // `current` was mentioned again since.
+    book.seen.insert(current, later);
+    assert_eq!(book.prune(later, |peer| *peer == connected), 1);
+    assert!(!book.addresses.contains_key(&old));
+    assert!(!book.relays.contains_key(&old) && !book.continuum.contains(&old));
+    assert!(book.addresses.contains_key(&current));
+    assert!(book.addresses.contains_key(&connected));
+    assert_eq!(book.prune(later, |_| false), 1); // `connected`, once it isn't.
+}
+
+#[test]
+fn relay_failures_are_short() {
+    assert_eq!(
+        relay_failure_text(
+            "Custom { kind: Other, error: Other(Left(Left(Left(Reservation(Protocol(NoAddressesInReservation)))))) }",
+            "x"
+        ),
+        "the relay gave no address to be reached through it"
+    );
+    assert_eq!(
+        relay_failure_text(
+            "Codec(Error(Custom { kind: Other, error: Failed }))",
+            "Failed"
+        ),
+        "the relay refused the circuit relay protocol (is it a relay?)"
+    );
+    assert_eq!(
+        relay_failure_text("Io(…)", "connection reset"),
+        "connection reset"
+    );
+    assert_eq!(relay_failure(&Ok(())), "closed");
+}
+
+/// Only refusals pause a relay; a slot that ended otherwise is asked for again.
+#[test]
+fn only_refusals_pause_relays() {
+    let refused = [
+        ("Reservation(Protocol(NoAddressesInReservation))", "x"),
+        ("Reservation(Status(ResourceLimitExceeded))", "x"),
+        ("Reservation(Status(PermissionDenied))", "x"),
+        ("Denied { cause: \"connection gated\" }", "x"),
+        ("Upgrade(UnsupportedProtocol)", "x"),
+        ("Dial(WrongPeerId { obtained: … })", "x"),
+        (
+            "Codec(Error(Custom { kind: Other, error: Failed }))",
+            "Failed",
+        ),
+    ];
+    for (debug, innermost) in refused {
+        assert!(relay_refused(debug, innermost), "{debug}");
+    }
+    let not = [
+        ("Io(Custom { kind: Other, error: Timeout })", "Timeout"),
+        (
+            "Io(Custom { kind: Other, error: Right(Closed) })",
+            "connection is closed",
+        ),
+        ("Io(…)", "connection reset"),
+    ];
+    for (debug, innermost) in not {
+        assert!(!relay_refused(debug, innermost), "{debug}");
+    }
+}
+
+#[test]
+fn listen_addresses_offered_for_hole_punching() {
+    let parse = |text: &str| text.parse::<Multiaddr>().unwrap();
+    assert!(offered(&parse("/ip4/192.168.1.2/udp/9661/quic-v1")));
+    assert!(offered(&parse("/ip6/fd00::2/tcp/9661")));
+    assert!(!offered(&parse("/ip4/127.0.0.1/udp/9661/quic-v1")));
+    assert!(!offered(&parse("/ip6/fe80::1/tcp/9661")));
+    let relay = PeerId::random();
+    assert!(!offered(&parse(&format!(
+        "/ip4/192.168.1.2/tcp/9661/p2p/{relay}/p2p-circuit"
+    ))));
+}
+
+/// libp2p's nested dial errors come out as each address's cause, once.
+#[test]
+fn dial_errors_are_short() {
+    use ::libp2p::core::transport::TransportError;
+    let peer = peer();
+    let address: Multiaddr = format!("/ip6/fd00::1/tcp/9661/p2p/{peer}").parse().unwrap();
+    let nested = std::io::Error::other(
+        "Multiple dial errors occurred:\n - Network is unreachable (os error 101): Multiple dial errors occurred:\n - Network is unreachable (os error 101): Network is unreachable (os error 101)",
+    );
+    let error =
+        ::libp2p::swarm::DialError::Transport(vec![(address, TransportError::Other(nested))]);
+    assert_eq!(
+        describe_dial_error(&error),
+        "/ip6/fd00::1/tcp/9661: Network is unreachable (os error 101)"
+    );
+    let refused = ::libp2p::swarm::DialError::Transport(vec![(
+        "/ip4/10.0.0.1/tcp/1".parse().unwrap(),
+        TransportError::Other(std::io::Error::from(std::io::ErrorKind::ConnectionRefused)),
+    )]);
+    assert_eq!(
+        describe_dial_error(&refused),
+        "/ip4/10.0.0.1/tcp/1: connection refused"
+    );
+}
+
+/// A relay doesn't dial the nodes it relays for through itself.
+#[test]
+fn no_circuits_through_ourselves() {
+    let (us, other_relay, target) = (peer(), peer(), peer());
+    let through = |relay: PeerId| -> Multiaddr {
+        format!("/ip6/fd00::1/tcp/9661/p2p/{relay}/p2p-circuit")
+            .parse()
+            .unwrap()
+    };
+    let direct: Multiaddr = "/ip6/fd00::2/tcp/9661".parse().unwrap();
+    assert_eq!(relay_of(&through(us)), Some(us));
+    assert_eq!(relay_of(&direct), None);
+    assert_eq!(relay_of(&direct.clone().with_p2p(target).unwrap()), None);
+    assert_eq!(
+        dialable(&[through(us), through(other_relay), direct.clone()], us),
+        [through(other_relay), direct]
+    );
+}
+
+#[tokio::test]
+async fn unknown_peers_are_refused() {
+    let (b, _) = memory_node();
+    let stranger = Keypair::generate_ed25519()
+        .public()
+        .to_peer_id()
+        .to_string();
+    assert!(b.send_message(&stranger, &status()).await.is_err());
+    assert!(b.resolve_seed(&stranger).await.is_err());
+    assert_eq!(b.connection(&stranger), None);
+}
+
+#[test]
+fn a_direct_connection_wins_over_a_relayed_one() {
+    let mut book = Book::default();
+    let (relayed_only, both) = (peer(), peer());
+    let mut id = 0;
+    let mut connect = |book: &mut Book, peer: PeerId, relayed: bool| {
+        id += 1;
+        let address: Multiaddr = format!("/ip4/10.0.0.{id}/tcp/1").parse().unwrap();
+        book.connections
+            .insert(ConnectionId::new_unchecked(id), (peer, relayed, address));
+    };
+    connect(&mut book, relayed_only, true);
+    connect(&mut book, both, true);
+    connect(&mut book, both, false);
+    assert_eq!(book.connection(&relayed_only), Some(Connection::Relayed));
+    assert_eq!(book.connection(&both), Some(Connection::Direct));
+    assert_eq!(book.connection(&peer()), None);
+    assert_eq!(
+        book.location(&both).map(|address| address.to_string()),
+        Some("/ip4/10.0.0.3/tcp/1".to_owned())
+    );
+    let connections = book.stats().connections;
+    assert_eq!((connections.direct, connections.relayed), (1, 2));
+    // Of several, the oldest direct one stays; a relayed one only without a direct.
+    connect(&mut book, both, false);
+    assert_eq!(
+        book.duplicates(&both, false),
+        [
+            ConnectionId::new_unchecked(4),
+            ConnectionId::new_unchecked(2)
+        ]
+    );
+    assert!(book.duplicates(&relayed_only, false).is_empty());
+    // Not while one of them carries a slot, ours or the peer's, or we reserve one.
+    assert!(book.duplicates(&both, true).is_empty());
+    book.slots.insert(both);
+    assert!(book.duplicates(&both, false).is_empty());
+    book.slots.clear();
+    book.slots_here.insert(both);
+    assert!(book.duplicates(&both, false).is_empty());
+    book.slots_here.clear();
+    // Once measured, the fastest direct one stays, however young.
+    book.rtts
+        .insert(ConnectionId::new_unchecked(3), Duration::from_millis(40));
+    book.rtts
+        .insert(ConnectionId::new_unchecked(4), Duration::from_millis(2));
+    assert_eq!(
+        book.duplicates(&both, false),
+        [
+            ConnectionId::new_unchecked(3),
+            ConnectionId::new_unchecked(2)
+        ]
+    );
+    assert_eq!(
+        book.location(&both).map(|address| address.to_string()),
+        Some("/ip4/10.0.0.4/tcp/1".to_owned())
+    );
+}