outbound peer eviction

crates/p2p/Cargo.toml

@@ -43,6 +43,7 @@ rayon = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
 sha2 = "0.10.7"
+sha3 = { workspace = true }
 tokio = { version = "1.32.0", features = ["macros", "rt-multi-thread", "sync"] }
 tracing = "0.1.37"
 tracing-subscriber = { version = "0.3.17", features = ["env-filter"] }

crates/p2p/src/behaviour.rs

@@ -41,6 +41,12 @@ pub fn kademlia_protocol_name(chain_id: ChainId) -> String {
 pub struct Behaviour {
     cfg: Config,
     peers: PeerSet,
+    swarm: crate::Client,
+    /// Secret value used in the peer eviction process.
+    ///
+    /// This value is used to pick the peer to be evicted in a deterministic, but
+    /// unpredictable way.
+    eviction_secret: [u8; 32],
     inner: Inner,
 }
 
@@ -332,6 +338,9 @@ impl NetworkBehaviour for Behaviour {
             if effective_role.is_dialer() {
                 // This really is an outbound connection, and not a connection that requires
                 // hole-punching.
+
+                self.evict_outbound_peer()?;
+
                 self.peers.upsert(
                     peer_id,
                     |peer| {
@@ -365,6 +374,7 @@ impl Behaviour {
     pub fn new(
         identity: &identity::Keypair,
         chain_id: ChainId,
+        swarm: crate::Client,
         cfg: Config,
     ) -> (Self, relay::client::Transport) {
         const PROVIDER_PUBLICATION_INTERVAL: Duration = Duration::from_secs(600);
@@ -414,6 +424,8 @@ impl Behaviour {
             Self {
                 peers: PeerSet::new(cfg.eviction_timeout),
                 cfg,
+                swarm,
+                eviction_secret: identity.derive_secret(b"eviction").unwrap(),
                 inner: Inner {
                     relay,
                     autonat: autonat::Behaviour::new(peer_id, Default::default()),
@@ -468,6 +480,58 @@ impl Behaviour {
         Ok(())
     }
 
+    /// Evict an outbound peer if the maximum number of outbound peers has been reached.
+    fn evict_outbound_peer(&mut self) -> Result<(), ConnectionDenied> {
+        let mut candidates: Vec<_> = self.outbound_peers().collect();
+
+        // Only evict a peer if the maximum number of outbound peers has been reached.
+        if candidates.len() < self.cfg.max_outbound_peers {
+            return Ok(());
+        }
+
+        // Only peers which are flagged as not useful are considered for eviction.
+        candidates.retain(|(_, peer)| !peer.useful);
+
+        // The peer to be evicted is the one with the highest
+        candidates.sort_by_key(|(peer_id, _)| {
+            use sha3::{Digest, Sha3_256};
+            let mut hasher = Sha3_256::default();
+            hasher.update(self.eviction_secret);
+            hasher.update(peer_id.to_bytes());
+            hasher.finalize()
+        });
+        let (peer_id, _) = candidates.pop().ok_or_else(|| {
+            ConnectionDenied::new(anyhow!(
+                "outbound peer limit reached and no peers could be evicted"
+            ))
+        })?;
+        drop(candidates);
+        self.peers.update(peer_id, |peer| {
+            peer.connectivity = Connectivity::Disconnecting {
+                connected_at: peer.connected_at(),
+            };
+            peer.evicted = true;
+        });
+
+        // Disconnect the evicted peer.
+        tokio::spawn({
+            let swarm = self.swarm.clone();
+            async move {
+                if let Err(e) = swarm.disconnect(peer_id).await {
+                    tracing::debug!(%peer_id, %e, "Failed to disconnect evicted peer");
+                }
+            }
+        });
+
+        Ok(())
+    }
+
+    pub fn not_useful(&mut self, peer_id: PeerId) {
+        self.peers.update(peer_id, |peer| {
+            peer.useful = false;
+        });
+    }
+
     pub fn kademlia_mut(&mut self) -> &mut kad::Behaviour<MemoryStore> {
         &mut self.inner.kademlia
     }
@@ -500,26 +564,26 @@ impl Behaviour {
         self.peers.iter()
     }
 
-    pub fn num_outbound_peers(&self) -> usize {
+    /// Outbound peers connected to us.
+    pub fn outbound_peers(&self) -> impl Iterator<Item = (PeerId, &Peer)> {
         self.peers
             .iter()
-            .filter(|(_, peer)| peer.is_outbound())
-            .count()
+            .filter(|(_, peer)| peer.is_connected() && peer.is_outbound())
     }
 
-    /// Number of inbound non-relayed peers.
+    /// Inbound non-relayed peers connected to us.
     fn num_inbound_direct_peers(&self) -> usize {
         self.peers
             .iter()
-            .filter(|(_, peer)| peer.is_inbound() && !peer.is_relayed())
+            .filter(|(_, peer)| peer.is_connected() && peer.is_inbound() && !peer.is_relayed())
             .count()
     }
 
-    /// Number of inbound relayed peers.
+    /// Inbound relayed peers connected to us.
     fn num_inbound_relayed_peers(&self) -> usize {
         self.peers
             .iter()
-            .filter(|(_, peer)| peer.is_inbound() && peer.is_relayed())
+            .filter(|(_, peer)| peer.is_connected() && peer.is_inbound() && peer.is_relayed())
             .count()
     }
 }

crates/p2p/src/client/peer_aware.rs

@@ -179,6 +179,18 @@ impl Client {
         receiver.await.expect("Sender not to be dropped")
     }
 
+    /// Mark a peer as not useful.
+    ///
+    /// These peers will be candidates for outbound peer eviction.
+    pub async fn not_useful(&self, peer_id: PeerId) {
+        let (sender, receiver) = oneshot::channel();
+        self.sender
+            .send(Command::NotUseful { peer_id, sender })
+            .await
+            .expect("Command receiver not to be dropped");
+        receiver.await.expect("Sender not to be dropped")
+    }
+
     #[cfg(test)]
     pub(crate) fn for_test(&self) -> test_utils::Client {
         test_utils::Client::new(self.sender.clone())

crates/p2p/src/lib.rs

@@ -43,7 +43,11 @@ pub use behaviour::{kademlia_protocol_name, IDENTIFY_PROTOCOL_NAME};
 pub fn new(keypair: Keypair, cfg: Config, chain_id: ChainId) -> (Client, EventReceiver, MainLoop) {
     let local_peer_id = keypair.public().to_peer_id();
 
-    let (behaviour, relay_transport) = behaviour::Behaviour::new(&keypair, chain_id, cfg.clone());
+    let (command_sender, command_receiver) = mpsc::channel(1);
+    let client = Client::new(command_sender, local_peer_id);
+
+    let (behaviour, relay_transport) =
+        behaviour::Behaviour::new(&keypair, chain_id, client.clone(), cfg.clone());
 
     let swarm = Swarm::new(
         transport::create(&keypair, relay_transport),
@@ -61,11 +65,10 @@ pub fn new(keypair: Keypair, cfg: Config, chain_id: ChainId) -> (Client, EventRe
         swarm::Config::with_tokio_executor().with_idle_connection_timeout(Duration::MAX),
     );
 
-    let (command_sender, command_receiver) = mpsc::channel(1);
     let (event_sender, event_receiver) = mpsc::channel(1);
 
     (
-        Client::new(command_sender, local_peer_id),
+        client,
         event_receiver,
         MainLoop::new(swarm, command_receiver, event_sender, cfg, chain_id),
     )
@@ -78,10 +81,12 @@ pub struct Config {
     pub direct_connection_timeout: Duration,
     /// A relayed peer can only connect once in this period.
     pub relay_connection_timeout: Duration,
-    /// Maximum number of direct (non-relayed) peers.
+    /// Maximum number of direct (non-relayed) inbound peers.
     pub max_inbound_direct_peers: usize,
-    /// Maximum number of relayed peers.
+    /// Maximum number of relayed inbound peers.
     pub max_inbound_relayed_peers: usize,
+    /// Maximum number of outbound peers.
+    pub max_outbound_peers: usize,
     /// The minimum number of peers to maintain. If the number of outbound peers drops below this
     /// number, the node will attempt to connect to more peers.
     pub low_watermark: usize,
@@ -175,6 +180,10 @@ enum Command {
         new_block: NewBlock,
         sender: EmptyResultSender,
     },
+    NotUseful {
+        peer_id: PeerId,
+        sender: oneshot::Sender<()>,
+    },
     /// For testing purposes only
     _Test(TestCommand),
 }

crates/p2p/src/main_loop.rs

@@ -172,7 +172,7 @@ impl MainLoop {
                             _ => self.ongoing_bootstrap = None,
                         }
                     }
-                    if self.swarm.behaviour_mut().num_outbound_peers() < self.cfg.low_watermark {
+                    if self.swarm.behaviour_mut().outbound_peers().count() < self.cfg.low_watermark {
                         if let Ok(query_id) = self.swarm.behaviour_mut().kademlia_mut().bootstrap() {
                             self.ongoing_bootstrap = Some(query_id);
                         }
@@ -857,6 +857,10 @@ impl MainLoop {
                 let result = self.publish_data(topic, &data);
                 let _ = sender.send(result);
             }
+            Command::NotUseful { peer_id, sender } => {
+                self.swarm.behaviour_mut().not_useful(peer_id);
+                let _ = sender.send(());
+            }
             Command::_Test(command) => self.handle_test_command(command).await,
         };
     }

crates/p2p/src/peers.rs

@@ -48,7 +48,8 @@ impl Peer {
 
     pub fn connected_at(&self) -> Option<Instant> {
         match self.connectivity {
-            Connectivity::Connected { connected_at } => Some(connected_at),
+            Connectivity::Connected { connected_at, .. } => Some(connected_at),
+            Connectivity::Disconnecting { connected_at, .. } => connected_at,
             Connectivity::Disconnected { connected_at, .. } => connected_at,
             Connectivity::Dialing => None,
         }
@@ -62,6 +63,10 @@ pub enum Connectivity {
         /// When the peer was connected.
         connected_at: Instant,
     },
+    Disconnecting {
+        /// When the peer was connected, if he was connected.
+        connected_at: Option<Instant>,
+    },
     Disconnected {
         /// When the peer was connected, if he was connected.
         connected_at: Option<Instant>,