diff --git a/gossipsub.go b/gossipsub.go
index d34bfff..a399f88 100644
--- a/gossipsub.go
+++ b/gossipsub.go
@@ -25,7 +25,7 @@ func NewGossipSub(ctx context.Context, h host.Host, opts ...Option) (*PubSub, er
 		peers:  make(map[peer.ID]protocol.ID),
 		mesh:   make(map[string]map[peer.ID]struct{}),
 		fanout: make(map[string]map[peer.ID]struct{}),
-		mcache: NewMessageCache(5),
+		mcache: NewMessageCache(3, 5),
 	}
 	return NewPubSub(ctx, h, rt, opts...)
 }
@@ -172,7 +172,7 @@ func (gs *GossipSubRouter) handlePrune(p peer.ID, ctl *pb.ControlMessage) {
 }
 
 func (gs *GossipSubRouter) Publish(from peer.ID, msg *pb.Message) {
-	gs.mcache.Add(msg)
+	gs.mcache.Put(msg)
 
 	tosend := make(map[peer.ID]struct{})
 	for _, topic := range msg.GetTopicIDs() {
@@ -274,10 +274,10 @@ func (gs *GossipSubRouter) sendMessage(p peer.ID, out *RPC) {
 }
 
 func (gs *GossipSubRouter) sendRPC(p peer.ID, out *RPC, ctl bool) {
-	// TODO control messages and gossip piggyback
+	// TODO control message reliability and gossip piggyback
 	// - control messages (GRAFT/PRUNE) must be reliable and should
 	//   be scheduled for piggyback or retry if the queue is full
-	// - gossip should be piggybacked on messages oppurtinistcally
+	// - gossip (IHAVE) should be piggybacked on messages oppurtinistcally
 
 	mch, ok := gs.p.peers[p]
 	if !ok {
@@ -328,7 +328,70 @@ func (gs *GossipSubRouter) heartbeatTimer() {
 }
 
 func (gs *GossipSubRouter) heartbeat() {
-	// TODO
+	gs.mcache.Shift()
+
+	tograft := make(map[peer.ID][]string)
+	toprune := make(map[peer.ID][]string)
+
+	for topic, peers := range gs.mesh {
+
+		if len(peers) < GossipSubDlo {
+			ineed := GossipSubD - len(peers)
+			plst := gs.getPeers(topic, func(p peer.ID) bool {
+				_, ok := peers[p]
+				return !ok
+			})
+
+			for _, p := range plst[:ineed] {
+				peers[p] = struct{}{}
+				topics := tograft[p]
+				tograft[p] = append(topics, topic)
+			}
+		}
+
+		if len(peers) > GossipSubDhi {
+			idontneed := len(peers) - GossipSubD
+			plst := peerMapToList(peers)
+			shufflePeers(plst)
+			for _, p := range plst[:idontneed] {
+				delete(peers, p)
+				topics := toprune[p]
+				toprune[p] = append(topics, topic)
+			}
+		}
+
+		// TODO gossip
+	}
+
+	for p, topics := range tograft {
+		graft := make([]*pb.ControlGraft, 0, len(topics))
+		for _, topic := range topics {
+			graft = append(graft, &pb.ControlGraft{TopicID: &topic})
+		}
+
+		var prune []*pb.ControlPrune
+		pruning, ok := toprune[p]
+		if ok {
+			delete(toprune, p)
+			prune = make([]*pb.ControlPrune, 0, len(pruning))
+			for _, topic := range pruning {
+				prune = append(prune, &pb.ControlPrune{TopicID: &topic})
+			}
+		}
+
+		out := rpcWithControl(nil, nil, nil, graft, prune)
+		gs.sendControl(p, out)
+	}
+
+	for p, topics := range toprune {
+		prune := make([]*pb.ControlPrune, 0, len(topics))
+		for _, topic := range topics {
+			prune = append(prune, &pb.ControlPrune{TopicID: &topic})
+		}
+
+		out := rpcWithControl(nil, nil, nil, nil, prune)
+		gs.sendControl(p, out)
+	}
 }
 
 func peerListToMap(peers []peer.ID) map[peer.ID]struct{} {
@@ -339,6 +402,14 @@ func peerListToMap(peers []peer.ID) map[peer.ID]struct{} {
 	return pmap
 }
 
+func peerMapToList(peers map[peer.ID]struct{}) []peer.ID {
+	plst := make([]peer.ID, 0, len(peers))
+	for p := range peers {
+		plst = append(plst, p)
+	}
+	return plst
+}
+
 func shufflePeers(peers []peer.ID) {
 	// TODO
 }
diff --git a/mcache.go b/mcache.go
index 01d78f6..0b79892 100644
--- a/mcache.go
+++ b/mcache.go
@@ -4,14 +4,14 @@ import (
 	pb "github.com/libp2p/go-floodsub/pb"
 )
 
-func NewMessageCache(win int) *MessageCache {
+func NewMessageCache(gossip, history int) *MessageCache {
 	return &MessageCache{}
 }
 
 type MessageCache struct {
 }
 
-func (mc *MessageCache) Add(msg *pb.Message) {
+func (mc *MessageCache) Put(msg *pb.Message) {
 	// TODO
 }
 
@@ -19,3 +19,7 @@ func (mc *MessageCache) Get(mid string) (*pb.Message, bool) {
 	// TODO
 	return nil, false
 }
+
+func (mc *MessageCache) Shift() {
+	// TODO
+}