diff --git a/callbacks.go b/callbacks.go
index f9ba131b13..1aeafbfc14 100644
--- a/callbacks.go
+++ b/callbacks.go
@@ -11,10 +11,14 @@ import (
 type Callbacks struct {
 	// Called after a peer connection completes the BitTorrent handshake. The Client lock is not
 	// held.
-	CompletedHandshake    func(*PeerConn, InfoHash)
-	ReadMessage           func(*PeerConn, *pp.Message)
+	CompletedHandshake func(*PeerConn, InfoHash)
+	ReadMessage        func(*PeerConn, *pp.Message)
+	// This can be folded into the general case below.
 	ReadExtendedHandshake func(*PeerConn, *pp.ExtendedHandshakeMessage)
 	PeerConnClosed        func(*PeerConn)
+	// BEP 10 message. Not sure if I should call this Ltep universally. Each handler here is called
+	// in order.
+	PeerConnReadExtensionMessage []func(PeerConnReadExtensionMessageEvent)
 
 	// Provides secret keys to be tried against incoming encrypted connections.
 	ReceiveEncryptedHandshakeSkeys mse.SecretKeyIter
@@ -38,3 +42,12 @@ type PeerRequestEvent struct {
 	Peer *Peer
 	Request
 }
+
+type PeerConnReadExtensionMessageEvent struct {
+	PeerConn *PeerConn
+	// Whether the client has builtin support for this extension.
+	BuiltinHandler bool
+	// You can look up what protocol this corresponds to using the PeerConn.LocalLtepProtocolMap.
+	ExtensionNumber pp.ExtensionNumber
+	Payload         []byte
+}
diff --git a/peer_protocol/extended.go b/peer_protocol/extended.go
index 8bc5181633..019590e40a 100644
--- a/peer_protocol/extended.go
+++ b/peer_protocol/extended.go
@@ -24,7 +24,7 @@ type (
 	}
 
 	ExtensionName   string
-	ExtensionNumber int
+	ExtensionNumber uint8
 )
 
 const (
diff --git a/peerconn.go b/peerconn.go
index e2d944ff26..03500be1ff 100644
--- a/peerconn.go
+++ b/peerconn.go
@@ -44,6 +44,8 @@ type PeerConn struct {
 	PeerID             PeerID
 	PeerExtensionBytes pp.PeerExtensionBits
 	PeerListenPort     int
+	// 1-based mapping from extension number to extension name.
+	LocalLtepProtocolMap []pp.ExtensionName
 
 	// The actual Conn, used for closing, and setting socket options. Do not use methods on this
 	// while holding any mutexes.
@@ -55,6 +57,7 @@ type PeerConn struct {
 
 	messageWriter peerConnMsgWriter
 
+	// The peer's extension map, as sent in their extended handshake.
 	PeerExtensionIDs map[pp.ExtensionName]pp.ExtensionNumber
 	PeerClientName   atomic.Value
 	uploadTimer      *time.Timer
@@ -854,8 +857,20 @@ func (c *PeerConn) onReadExtendedMsg(id pp.ExtensionNumber, payload []byte) (err
 	}()
 	t := c.t
 	cl := t.cl
-	switch id {
-	case pp.HandshakeExtendedID:
+	{
+		event := PeerConnReadExtensionMessageEvent{
+			PeerConn: c,
+			// Add one for the handshake ID. This isn't quite right yet, we don't actually have a
+			// way to differentiate builtin handlers.
+			BuiltinHandler:  int(id) < len(c.LocalLtepProtocolMap)+1,
+			ExtensionNumber: id,
+			Payload:         payload,
+		}
+		for _, cb := range c.callbacks.PeerConnReadExtensionMessage {
+			cb(event)
+		}
+	}
+	if id == pp.HandshakeExtendedID {
 		var d pp.ExtendedHandshakeMessage
 		if err := bencode.Unmarshal(payload, &d); err != nil {
 			c.logger.Printf("error parsing extended handshake message %q: %s", payload, err)
@@ -864,7 +879,6 @@ func (c *PeerConn) onReadExtendedMsg(id pp.ExtensionNumber, payload []byte) (err
 		if cb := c.callbacks.ReadExtendedHandshake; cb != nil {
 			cb(c, &d)
 		}
-		// c.logger.WithDefaultLevel(log.Debug).Printf("received extended handshake message:\n%s", spew.Sdump(d))
 		if d.Reqq != 0 {
 			c.PeerMaxRequests = d.Reqq
 		}
@@ -896,13 +910,19 @@ func (c *PeerConn) onReadExtendedMsg(id pp.ExtensionNumber, payload []byte) (err
 			c.pex.Init(c)
 		}
 		return nil
-	case metadataExtendedId:
+	}
+	// Zero was taken care of above.
+	if int(id) >= len(c.LocalLtepProtocolMap)+1 {
+		return fmt.Errorf("unexpected extended message ID: %v", id)
+	}
+	switch c.LocalLtepProtocolMap[id-1] {
+	case pp.ExtensionNameMetadata:
 		err := cl.gotMetadataExtensionMsg(payload, t, c)
 		if err != nil {
 			return fmt.Errorf("handling metadata extension message: %w", err)
 		}
 		return nil
-	case pexExtendedId:
+	case pp.ExtensionNamePex:
 		if !c.pex.IsEnabled() {
 			return nil // or hang-up maybe?
 		}
@@ -911,7 +931,7 @@ func (c *PeerConn) onReadExtendedMsg(id pp.ExtensionNumber, payload []byte) (err
 			err = fmt.Errorf("receiving pex message: %w", err)
 		}
 		return
-	case utHolepunchExtendedId:
+	case utHolepunch.ExtensionName:
 		var msg utHolepunch.Msg
 		err = msg.UnmarshalBinary(payload)
 		if err != nil {
@@ -921,7 +941,9 @@ func (c *PeerConn) onReadExtendedMsg(id pp.ExtensionNumber, payload []byte) (err
 		err = c.t.handleReceivedUtHolepunchMsg(msg, c)
 		return
 	default:
-		return fmt.Errorf("unexpected extended message ID: %v", id)
+		// This should have been a user configured protocol, and handled by a callback. How can they
+		// propagate errors?
+		return nil
 	}
 }