diff --git a/src/runtime2/inbox.rs b/src/runtime2/inbox.rs
index f5bc869b78ce3cc081b673b240a2d3fb2847d24f..6e5854c85dd7b27d7967b48bc2baa1f472bc0aeb 100644
--- a/src/runtime2/inbox.rs
+++ b/src/runtime2/inbox.rs
@@ -94,6 +94,10 @@ pub(crate) enum SyncPortContent {
     NotificationWave,
 }
 
+/// A sync message intended for the consensus algorithm. This message does not
+/// go to a component, but through a channel (and results in potential
+/// rerouting) because we're not sure about the ID of the component that holds
+/// the other end of the channel.
 #[derive(Debug)]
 pub(crate) struct SyncPortMessage {
     pub sync_header: SyncHeader,
@@ -102,6 +106,24 @@ pub(crate) struct SyncPortMessage {
     pub content: SyncPortContent,
 }
 
+#[derive(Debug)]
+pub(crate) enum SyncControlContent {
+    ChannelIsClosed(PortIdLocal), // contains port that is owned by the recipient of the message
+}
+
+/// A sync control message: originating from the scheduler, but intended for the
+/// current sync round of the recipient. Every kind of consensus algorithm must
+/// be able to handle such a message.
+#[derive(Debug)]
+pub(crate) struct SyncControlMessage {
+    // For now these control messages are only aimed at components. Might change
+    // in the future. But for now we respond to messages from components that
+    // have, because of that message, published their ID.
+    pub in_response_to_sync_round: u32,
+    pub target_component_id: ConnectorId,
+    pub content: SyncControlContent,
+}
+
 /// A control message is a message intended for the scheduler that is executing
 /// a component.
 #[derive(Debug)]
@@ -125,6 +147,7 @@ pub(crate) enum Message {
     Data(DataMessage),
     SyncComp(SyncCompMessage),
     SyncPort(SyncPortMessage),
+    SyncControl(SyncControlMessage),
     Control(ControlMessage),
 }
 
@@ -137,6 +160,7 @@ impl Message {
             Message::Data(message) => return Some(message.data_header.sending_port),
             Message::SyncPort(message) => return Some(message.source_port),
             Message::SyncComp(_) => return None,
+            Message::SyncControl(_) => return None,
             Message::Control(_) => return None,
         }
     }