diff --git a/src/runtime2/inbox.rs b/src/runtime2/inbox.rs
index 2d64dd23404f8dea01d3441b5adc99902eaba789..022ab16496f3aa2ab43511267f73f022916e8654 100644
--- a/src/runtime2/inbox.rs
+++ b/src/runtime2/inbox.rs
@@ -42,6 +42,29 @@ pub struct DataMessage {
     pub message: ValueGroup,
 }
 
+pub enum SyncBranchConstraint {
+    SilentPort(PortIdLocal),
+    BranchNumber(u32),
+    PortMapping(PortIdLocal, u32),
+}
+
+pub struct SyncConnectorSolution {
+    connector_id: ConnectorId,
+    terminating_branch_id: BranchId,
+    execution_branch_ids: Vec<BranchId>, // ends with terminating branch ID
+}
+
+pub struct SyncConnectorConstraints {
+    connector_id: ConnectorId,
+    constraints: Vec<SyncBranchConstraint>,
+}
+
+pub struct SyncMessage {
+    connector_solutions: Vec<SyncConnectorSolution>,
+    connector_constraints: Vec<SyncConnectorConstraints>,
+    connectors_to_visit: Vec<u32>,
+}
+
 /// A control message. These might be sent by the scheduler to notify eachother
 /// of asynchronous state changes.
 pub struct ControlMessage {
@@ -59,8 +82,10 @@ pub enum ControlMessageVariant {
 /// out and handles all control message and potential routing). The correctly
 /// addressed `Data` variants will end up at the connector.
 pub enum Message {
-    Data(DataMessage),
-    Control(ControlMessage),
+    Data(DataMessage),          // data message, handled by connector
+    Sync(SyncMessage),          // sync message, handled by both connector/scheduler
+    Control(ControlMessage),    // control message, handled by scheduler
+    Ping,                       // ping message, intentionally waking up a connector (used for native connectors)
 }
 
 /// The public inbox of a connector. The thread running the connector that owns