1 files changed, 21 insertions, 3 deletions
diff --git a/src/swarm/peer/inbound.rs b/src/swarm/peer/inbound.rs
index f43a45a..c75f66b 100644
--- a/src/swarm/peer/inbound.rs
+++ b/src/swarm/peer/inbound.rs
@@ -38,8 +38,26 @@ pub(crate) async fn handle_message(
         note_alive(state, &peer, &message).await;
         return Ok(());
     }
-    // WHO and ME end at the identity layer (`transport::identity`).
-    let Some((epoch, epoch_seq)) = message.epoch_stamp() else {
+    // WHO and ME end at the identity layer (`transport::identity`). A sync message names
+    // its epoch by a tag (`EpochTag`): ours, or the sender's as we know it; one naming
+    // neither is of an epoch we don't sync on, and says nothing we could record.
+    let stamp = match &message {
+        transport::ParsedMessage::Sync(envelope) => {
+            let known = match state.get_peer_state(&peer).await {
+                Some(peer_state) => peer_state.lock().await.epoch,
+                None => None,
+            };
+            let epoch = envelope
+                .epoch
+                .resolve(std::iter::once(state.local_epoch()).chain(known));
+            if epoch.is_none() {
+                debug!(peer = %peer, epoch = %envelope.epoch, "Dropping a sync message of an epoch we don't know");
+            }
+            epoch.map(|epoch| (epoch, envelope.epoch_seq))
+        }
+        _ => message.epoch_stamp(),
+    };
+    let Some((epoch, epoch_seq)) = stamp else {
         return Ok(());
     };
 
@@ -145,7 +163,7 @@ pub(crate) async fn handle_message(
                 ensure_handshake(state, &peer).await;
             }
             if !same_epoch {
-                debug!(peer = %peer, epoch = %envelope.epoch, "Dropping sync message from another epoch");
+                debug!(peer = %peer, epoch = %epoch, "Dropping sync message from another epoch");
                 return Ok(());
             }
             crate::swarm::source::synced_from(state, &peer);