diff --git a/src/runtime2/communication.rs b/src/runtime2/communication.rs
index 50bd8692519941c659dcfbaff9bb0e3287631a12..1f5cbaca826bf937a6facc7a952e517388a04e74 100644
--- a/src/runtime2/communication.rs
+++ b/src/runtime2/communication.rs
@@ -2,10 +2,13 @@ use crate::protocol::eval::*;
 use super::runtime::*;
 use super::component::*;
 
+// -----------------------------------------------------------------------------
+// Generic types
+// -----------------------------------------------------------------------------
+
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub struct PortId(pub u32);
 
-
 impl PortId {
     /// This value is not significant, it is chosen to make debugging easier: a
     /// very large port number is more likely to shine a light on bugs.
@@ -20,7 +23,7 @@ pub struct Peer {
     pub(crate) handle: CompHandle,
 }
 
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
 pub enum PortKind {
     Putter,
     Getter,
@@ -46,6 +49,10 @@ pub struct Channel {
     pub getter_id: PortId,
 }
 
+// -----------------------------------------------------------------------------
+// Data messages
+// -----------------------------------------------------------------------------
+
 #[derive(Debug)]
 pub struct DataMessage {
     pub data_header: MessageDataHeader,
@@ -53,19 +60,78 @@ pub struct DataMessage {
     pub content: ValueGroup,
 }
 
-#[derive(Debug)]
-pub struct MessageSyncHeader {
-    pub sync_round: u32,
-}
-
 #[derive(Debug)]
 pub struct MessageDataHeader {
-    pub expected_mapping: Vec<(PortId, u32)>,
+    pub expected_mapping: Vec<(PortId, Option<u32>)>,
     pub new_mapping: u32,
     pub source_port: PortId,
     pub target_port: PortId,
 }
 
+// -----------------------------------------------------------------------------
+// Sync messages
+// -----------------------------------------------------------------------------
+
+#[derive(Debug)]
+pub struct SyncMessage {
+    pub sync_header: MessageSyncHeader,
+    pub content: SyncMessageContent,
+}
+
+pub struct SyncLocalSolutionEntry {
+    pub self_port_id: PortId,
+    pub peer_comp_id: CompId,
+    pub peer_port_id: PortId,
+    pub mapping: u32,
+    pub port_kind: PortKind,
+}
+
+pub type SyncLocalSolution = Vec<SyncLocalSolutionEntry>;
+
+pub struct SyncSolutionPort {
+    pub self_comp_id: CompId,
+    pub self_port_id: PortId,
+    pub peer_comp_id: CompId,
+    pub peer_port_id: PortId,
+    pub mapping: u32,
+    pub port_kind: PortKind,
+}
+
+pub struct SyncSolutionChannel {
+    pub putter: Option<SyncSolutionPort>,
+    pub getter: Option<SyncSolutionPort>,
+}
+
+#[derive(Copy, Clone)]
+pub enum RoundDecision {
+    None,
+    Solution,
+    Failure,
+}
+
+impl RoundDecision {
+    fn is_some(&self)
+}
+
+pub struct SyncPartialSolution {
+    pub submissions_by: Vec<(CompId, bool)>,
+    pub channel_mapping: Vec<SyncSolutionChannel>,
+    pub decision: SyncRoundDecision,
+}
+
+#[derive(Debug, Clone)]
+pub enum SyncMessageContent {
+    NotificationOfLeader,
+    LocalSolution(CompId, SyncLocalSolution), // local solution of the specified component
+    PartialSolution(SyncPartialSolution), // partial solution of multiple components
+    GlobalSolution,
+    GlobalFailure,
+}
+
+// -----------------------------------------------------------------------------
+// Control messages
+// -----------------------------------------------------------------------------
+
 #[derive(Debug)]
 pub struct ControlMessage {
     pub(crate) id: ControlId,
@@ -84,9 +150,21 @@ pub enum ControlMessageContent {
     PortPeerChangedUnblock(PortId, CompId),
 }
 
+// -----------------------------------------------------------------------------
+// Messages (generic)
+// -----------------------------------------------------------------------------
+
+#[derive(Debug)]
+pub struct MessageSyncHeader {
+    pub sync_round: u32,
+    pub sending_id: CompId,
+    pub highest_id: CompId,
+}
+
 #[derive(Debug)]
 pub enum Message {
     Data(DataMessage),
+    Sync(SyncMessage),
     Control(ControlMessage),
 }
 
@@ -97,6 +175,8 @@ impl Message {
                 return Some(v.data_header.target_port),
             Message::Control(v) =>
                 return v.target_port_id,
+            Message::Sync(_) =>
+                return None,
         }
     }
 }