diff --git a/cmd/vail/book.go b/cmd/vail/book.go
index 15c0823..6f8bafa 100644
--- a/cmd/vail/book.go
+++ b/cmd/vail/book.go
@@ -29,7 +29,7 @@ type bookEvent struct {
 	eventType bookEventType
 	name      string
 	w         io.Writer
-	p         []byte
+	m         Message
 }
 
 func (b Book) Join(name string, w io.Writer) {
@@ -48,11 +48,11 @@ func (b Book) Part(name string, w io.Writer) {
 	}
 }
 
-func (b Book) Send(name string, p []byte) {
+func (b Book) Send(name string, m Message) {
 	b.events <- bookEvent{
 		eventType: sendEvent,
 		name:      name,
-		p:         p,
+		m:         m,
 	}
 }
 
@@ -87,6 +87,6 @@ func (b Book) loop() {
 			log.Println("WARN: Sending to an empty channel:", event.name)
 			break
 		}
-		repeater.Send(event.p)
+		repeater.Send(event.m)
 	}
 }
diff --git a/cmd/vail/book_test.go b/cmd/vail/book_test.go
index 0eabb9f..fd59482 100644
--- a/cmd/vail/book_test.go
+++ b/cmd/vail/book_test.go
@@ -7,53 +7,63 @@ import (
 
 func TestBook(t *testing.T) {
 	b := NewBook()
+	m := TestMessage{Message{1, 2, []uint8{3, 4}}}
 
 	buf1 := bytes.NewBufferString("buf1")
+	buf1Expect := bytes.NewBufferString("buf1")
 	b.Join("moo", buf1)
+	m.Clients = 1
 	b.loop()
 	if len(b.entries) != 1 {
 		t.Error("Wrong number of entries")
 	}
 
 	// Send to an empty channel
-	b.Send("merf", []byte("goober"))
+	b.Send("merf", m.Message)
 	b.loop()
-	if buf1.String() != "buf1" {
+	if buf1.String() != buf1Expect.String() {
 		t.Error("Sending to empty channel sent to non-empty channel")
 	}
 
 	// Send to a non-empty channel!
-	b.Send("moo", []byte("goober"))
+	b.Send("moo", m.Message)
 	b.loop()
-	if buf1.String() != "buf1goober" {
+	buf1Expect.Write(m.bytes())
+	if buf1.String() != buf1Expect.String() {
 		t.Error("Sending didn't work")
 	}
 
 	// Join another client
 	buf2 := bytes.NewBufferString("buf2")
+	buf2Expect := bytes.NewBufferString("buf2")
 	b.Join("moo", buf2)
+	m.Clients = 2
 	b.loop()
 
 	// Send to both
-	b.Send("moo", []byte("snerk"))
+	b.Send("moo", m.Message)
 	b.loop()
-	if buf1.String() != "buf1goobersnerk" {
+	buf1Expect.Write(m.bytes())
+	buf2Expect.Write(m.bytes())
+	if buf1.String() != buf1Expect.String() {
 		t.Error("Send to 2-member channel busted", buf1)
 	}
-	if buf2.String() != "buf2snerk" {
+	if buf2.String() != buf2Expect.String() {
 		t.Error("Send to 2-member channel busted", buf2)
 	}
 
 	// Part a client
 	b.Part("moo", buf1)
 	b.loop()
+	m.Clients = 1
 
-	b.Send("moo", []byte("peanut"))
+	b.Send("moo", m.Message)
 	b.loop()
-	if buf1.String() != "buf1goobersnerk" {
+	buf2Expect.Write(m.bytes())
+	if buf1.String() != buf1Expect.String() {
 		t.Error("Parted channel but still getting messages", buf1)
 	}
-	if buf2.String() != "buf2snerkpeanut" {
+	if buf2.String() != buf2Expect.String() {
 		t.Error("Someone else parting somehow messed up sends", buf2)
 	}
 }
diff --git a/cmd/vail/main.go b/cmd/vail/main.go
index b602fa7..8e305ca 100644
--- a/cmd/vail/main.go
+++ b/cmd/vail/main.go
@@ -2,10 +2,11 @@ package main
 
 import (
 	"fmt"
-	"time"
-	"os"
 	"log"
 	"net/http"
+	"os"
+	"time"
+
 	"golang.org/x/net/websocket"
 )
 
@@ -16,12 +17,13 @@ type Client struct {
 }
 
 func (c Client) Handle(ws *websocket.Conn) {
-	ws.MaxPayloadBytes = 500
+	nowMilli := time.Now().UnixMilli()
+	ws.MaxPayloadBytes = 50
 	book.Join(c.repeaterName, ws)
 	defer book.Part(c.repeaterName, ws)
-	
+
 	// Tell the client what time we think it is
-	fmt.Fprintf(ws, "[%d]", time.Now().UnixNano() / time.Millisecond.Nanoseconds())
+	fmt.Fprintf(ws, "[%d]", time.Now().UnixNano()/time.Millisecond.Nanoseconds())
 
 	for {
 		buf := make([]byte, ws.MaxPayloadBytes)
@@ -29,15 +31,34 @@ func (c Client) Handle(ws *websocket.Conn) {
 		if n, err := ws.Read(buf); err != nil {
 			break
 		} else {
-		  buf = buf[:n]
+			buf = buf[:n]
 		}
-		
-		book.Send(c.repeaterName, buf)
+
+		// Decode into a Message
+		var m Message
+		if err := m.UnmarshalBinary(buf); err != nil {
+			fmt.Fprintln(ws, err)
+			ws.Close()
+			return
+		}
+
+		// If it's wildly out of time, reject it
+		timeDelta := (nowMilli - m.Timestamp)
+		if timeDelta < 0 {
+			timeDelta = -timeDelta
+		}
+		if timeDelta > 9999 {
+			fmt.Fprintln(ws, "Bad timestamp")
+			ws.Close()
+			return
+		}
+
+		book.Send(c.repeaterName, m)
 	}
 }
 
 func ChatHandler(w http.ResponseWriter, r *http.Request) {
-	c := Client {
+	c := Client{
 		repeaterName: r.FormValue("repeater"),
 	}
 
@@ -57,7 +78,7 @@ func main() {
 		port = "8080"
 	}
 	log.Println("Listening on port", port)
-	err := http.ListenAndServe(":" + port, nil)
+	err := http.ListenAndServe(":"+port, nil)
 	if err != nil {
 		log.Fatal(err.Error())
 	}
diff --git a/cmd/vail/message.go b/cmd/vail/message.go
index c9d80db..5cadb7f 100644
--- a/cmd/vail/message.go
+++ b/cmd/vail/message.go
@@ -8,11 +8,12 @@ import (
 
 // VailMessage is a single Vail message.
 type Message struct {
-	// Relative time in ms of this message.
-	// These timestamps need to be consistent, but the offset can be anything.
-	// ECMAScript `performance.now()` is ideal.
+	// Timestamp of this message. Milliseconds since epoch.
 	Timestamp int64
-	
+
+	// Number of connected clients
+	Clients uint16
+
 	// Message timing in ms.
 	// Timings alternate between tone and silence.
 	// For example, `A` could be sent as [80, 80, 240]
@@ -22,13 +23,13 @@ type Message struct {
 func NewMessage(ts time.Time, durations []time.Duration) Message {
 	msg := Message{
 		Timestamp: ts.UnixNano() / time.Millisecond.Nanoseconds(),
-		Duration: make([]uint8, len(durations)),
+		Duration:  make([]uint8, len(durations)),
 	}
 	for i, dns := range durations {
 		ms := dns.Milliseconds()
-		if (ms > 255) {
+		if ms > 255 {
 			ms = 255
-		} else if (ms < 0) {
+		} else if ms < 0 {
 			ms = 0
 		}
 		msg.Duration[i] = uint8(ms)
@@ -42,6 +43,9 @@ func (m Message) MarshalBinary() ([]byte, error) {
 	if err := binary.Write(&w, binary.BigEndian, m.Timestamp); err != nil {
 		return nil, err
 	}
+	if err := binary.Write(&w, binary.BigEndian, m.Clients); err != nil {
+		return nil, err
+	}
 	if err := binary.Write(&w, binary.BigEndian, m.Duration); err != nil {
 		return nil, err
 	}
@@ -54,6 +58,9 @@ func (m *Message) UnmarshalBinary(data []byte) error {
 	if err := binary.Read(r, binary.BigEndian, &m.Timestamp); err != nil {
 		return err
 	}
+	if err := binary.Read(r, binary.BigEndian, &m.Clients); err != nil {
+		return err
+	}
 	dlen := r.Len()
 	m.Duration = make([]uint8, dlen)
 	if err := binary.Read(r, binary.BigEndian, &m.Duration); err != nil {
@@ -66,16 +73,16 @@ func (m Message) Equal(m2 Message) bool {
 	if m.Timestamp != m2.Timestamp {
 		return false
 	}
-	
+
 	if len(m.Duration) != len(m2.Duration) {
 		return false
 	}
-	
+
 	for i := range m.Duration {
 		if m.Duration[i] != m2.Duration[i] {
 			return false
 		}
 	}
-	
+
 	return true
 }
diff --git a/cmd/vail/message_test.go b/cmd/vail/message_test.go
index 94354c6..70ff0ec 100644
--- a/cmd/vail/message_test.go
+++ b/cmd/vail/message_test.go
@@ -6,9 +6,9 @@ import (
 	"time"
 )
 
-func TestMessage(t *testing.T) {
-	m := Message{0x1122334455, []uint8{0xaa, 0xbb, 0xcc}}
-	m2 := Message{12, []uint8{1}}
+func TestMessageStruct(t *testing.T) {
+	m := Message{0x1122334455, 0, []uint8{0xaa, 0xbb, 0xcc}}
+	m2 := Message{12, 0, []uint8{1}}
 
 	if !m.Equal(m) {
 		t.Error("Equal messages did not compare equal")
@@ -16,7 +16,7 @@ func TestMessage(t *testing.T) {
 	if m.Equal(m2) {
 		t.Error("Unequal messages compared equal")
 	}
-	if m.Equal(Message{m.Timestamp, []uint8{1, 2, 3}}) {
+	if m.Equal(Message{m.Timestamp, 0, []uint8{1, 2, 3}}) {
 		t.Error("Messages with different payloads compared equal")
 	}
 
@@ -24,7 +24,7 @@ func TestMessage(t *testing.T) {
 	if err != nil {
 		t.Error(err)
 	}
-	if !bytes.Equal(bm, []byte("\x00\x00\x00\x11\x22\x33\x44\x55\xaa\xbb\xcc")) {
+	if !bytes.Equal(bm, []byte("\x00\x00\x00\x11\x22\x33\x44\x55\x00\x00\xaa\xbb\xcc")) {
 		t.Error("Encoded wrong:", bm)
 	}
 
diff --git a/cmd/vail/repeater.go b/cmd/vail/repeater.go
index c0366f5..1fbd0b9 100644
--- a/cmd/vail/repeater.go
+++ b/cmd/vail/repeater.go
@@ -2,6 +2,7 @@ package main
 
 import (
 	"io"
+	"log"
 )
 
 // A Repeater is just a list of Writers.
@@ -29,9 +30,14 @@ func (r *Repeater) Part(w io.Writer) {
 	}
 }
 
-func (r *Repeater) Send(p []byte) {
+func (r *Repeater) Send(m Message) {
+	m.Clients = uint16(r.Listeners())
+	buf, err := m.MarshalBinary()
+	if err != nil {
+		log.Fatal(err)
+	}
 	for _, s := range r.writers {
-		s.Write(p)
+		s.Write(buf)
 	}
 }
 
diff --git a/cmd/vail/repeater_test.go b/cmd/vail/repeater_test.go
index c09d5f0..6d52b0c 100644
--- a/cmd/vail/repeater_test.go
+++ b/cmd/vail/repeater_test.go
@@ -5,35 +5,54 @@ import (
 	"testing"
 )
 
+type TestMessage struct {
+	Message
+}
+
+func (m TestMessage) bytes() []byte {
+	b, _ := m.MarshalBinary()
+	return b
+}
+
 func TestRepeater(t *testing.T) {
 	r := NewRepeater()
+	m := TestMessage{Message{1, 3, []uint8{3, 4}}}
 
 	buf1 := bytes.NewBufferString("buf1")
+	buf1Expect := bytes.NewBufferString("buf1")
 	r.Join(buf1)
 	if r.Listeners() != 1 {
 		t.Error("Joining did nothing")
 	}
-	r.Send([]byte("moo"))
-	if buf1.String() != "buf1moo" {
+	r.Send(m.Message)
+	m.Clients = 1
+	buf1Expect.Write(m.bytes())
+	if buf1.String() != buf1Expect.String() {
 		t.Error("Client 1 not repeating", buf1)
 	}
 
 	buf2 := bytes.NewBufferString("buf2")
+	buf2Expect := bytes.NewBufferString("buf2")
 	r.Join(buf2)
-	r.Send([]byte("bar"))
-	if buf1.String() != "buf1moobar" {
-		t.Error("Client 1 not repeating", buf1)
+	r.Send(m.Message)
+	m.Clients = 2
+	buf1Expect.Write(m.bytes())
+	buf2Expect.Write(m.bytes())
+	if buf1.String() != buf1Expect.String() {
+		t.Errorf("Client 1 not repeating %#v %#v", buf1, buf1Expect)
 	}
-	if buf2.String() != "buf2bar" {
+	if buf2.String() != buf2Expect.String() {
 		t.Error("Client 2 not repeating", buf2)
 	}
 
 	r.Part(buf1)
-	r.Send([]byte("baz"))
-	if buf1.String() != "buf1moobar" {
+	r.Send(m.Message)
+	m.Clients = 1
+	buf2Expect.Write(m.bytes())
+	if buf1.String() != buf1Expect.String() {
 		t.Error("Client 1 still getting data after part", buf1)
 	}
-	if buf2.String() != "buf2barbaz" {
+	if buf2.String() != buf2Expect.String() {
 		t.Error("Client 2 not getting data after part", buf2)
 	}
 }