diff --git a/pkg/eap/eap.go b/pkg/eap/eap.go
index 0e2bde7..5ba7a0a 100644
--- a/pkg/eap/eap.go
+++ b/pkg/eap/eap.go
@@ -22,11 +22,13 @@ const (
 	CodeSuccess
 	CodeFailure
 
-	SubTypeAkaChallenge = 1
+	SubTypeAkaChallenge              = 1
+	SubTypeAkaSynchronizationFailure = 4
 
 	AT_RAND      AttributeType = 1
 	AT_AUTN      AttributeType = 2
 	AT_RES       AttributeType = 3
+	AT_AUTS      AttributeType = 4
 	AT_CUSTOM_IV AttributeType = 126
 )
 
@@ -54,9 +56,10 @@ type EapAkaChallenge struct {
 	amf  uint16
 	op   []byte
 
-	podIV []byte
-	pdmIV []byte
-	Sqn   uint64
+	podIV   []byte
+	pdmIV   []byte
+	Sqn     uint64 // the controller's, read from AUTN
+	lastSqn uint64 // the last one the pod accepted
 
 	identifier byte
 }
@@ -107,6 +110,10 @@ func Unmarshal(data []byte) (*EapAka, error) {
 
 			}
 			data = data[2:10] // 8 bytes
+		case AT_AUTS:
+			if len != 16 {
+				return nil, fmt.Errorf("invalid len received for attribute: %d -- %d", aType, len)
+			}
 		case AT_CUSTOM_IV:
 			if len != 8 {
 				return nil, fmt.Errorf("invalid len received for attribute: %d -- %d", len, aType)
@@ -162,6 +169,10 @@ func (e *EapAka) Marshal() ([]byte, error) {
 			buf.WriteByte(0)
 			buf.WriteByte(64) // RES len in bits
 			dataLen = 8
+		case AT_AUTS:
+			len = 4 // 4 * 4 = 16 bytes, no reserved bytes
+			buf.WriteByte(len)
+			dataLen = 14
 		case AT_CUSTOM_IV:
 			len = 2
 			buf.WriteByte(len)
@@ -180,17 +191,75 @@ func (e *EapAka) Marshal() ([]byte, error) {
 	return ret, nil
 }
 
-func NewEapAkaChallenge(k []byte, sqn uint64) *EapAkaChallenge {
+func NewEapAkaChallenge(k []byte, lastSqn uint64) *EapAkaChallenge {
 	op, _ := hex.DecodeString("cdc202d5123e20f62b6d676ac72cb318")
 	// amf, _ := hex.DecodeString("b9b9")
-	log.Debugf("Starting EAP-AKA session with SQN(after incrementing SQN): %d", sqn+1)
+	log.Debugf("Starting EAP-AKA session; last accepted SQN: %d", lastSqn)
 	return &EapAkaChallenge{
-		k:     k,
-		op:    op,
-		Sqn:   sqn + 1,
-		amf:   47545,                      // b9b9
-		podIV: []byte{0xa, 0xa, 0xa, 0xa}, // constant for now, easier to debug. TODO
+		k:       k,
+		op:      op,
+		lastSqn: lastSqn,
+		amf:     47545,                      // b9b9
+		podIV:   []byte{0xa, 0xa, 0xa, 0xa}, // constant for now, easier to debug. TODO
+	}
+}
+
+// CheckSqn reads the controller's SQN from AUTN (SQN xor AK || AMF || MAC-A) and
+// verifies MAC-A. It returns false when the SQN is not past the last one accepted,
+// which a pod answers with a synchronization failure.
+func (e *EapAkaChallenge) CheckSqn() (bool, error) {
+	if len(e.autn) != 16 || len(e.rand) != 16 {
+		return false, fmt.Errorf("missing RAND or AUTN")
+	}
+	_, _, _, ak, err := milenage.New(e.k, e.op, e.rand, 0, 0).F2345()
+	if err != nil {
+		return false, err
 	}
+	var sqn uint64
+	for i := 0; i < 6; i++ {
+		sqn = sqn<<8 | uint64(e.autn[i]^ak[i])
+	}
+	amf := uint16(e.autn[6])<<8 | uint16(e.autn[7])
+	mac, err := milenage.New(e.k, e.op, e.rand, sqn, amf).F1()
+	if err != nil {
+		return false, err
+	}
+	if !bytes.Equal(mac, e.autn[8:16]) {
+		return false, fmt.Errorf("AUTN MAC-A mismatch for SQN %d", sqn)
+	}
+	e.Sqn = sqn
+	return sqn > e.lastSqn, nil
+}
+
+// GenerateSynchronizationFailure answers a stale SQN with AUTS = SQN_ms xor AK* || MAC-S,
+// SQN_ms being the last SQN the pod accepted.
+func (e *EapAkaChallenge) GenerateSynchronizationFailure() (*message.Message, error) {
+	mil := milenage.New(e.k, e.op, e.rand, e.lastSqn, 0)
+	aks, err := mil.F5Star()
+	if err != nil {
+		return nil, err
+	}
+	macS, err := mil.F1Star(mil.SQN, []byte{0, 0})
+	if err != nil {
+		return nil, err
+	}
+	auts := make([]byte, 0, 14)
+	for i := 0; i < 6; i++ {
+		auts = append(auts, mil.SQN[i]^aks[i])
+	}
+	auts = append(auts, macS...)
+
+	eap := &EapAka{Code: CodeResponse,
+		Attributes: map[AttributeType]*Attribute{AT_AUTS: {Data: auts}},
+		SubType:    SubTypeAkaSynchronizationFailure,
+		Identifier: e.identifier,
+	}
+	ret := message.NewMessage(message.MessageTypeSessionEstablishment, e.podID, e.pdmID)
+	ret.Payload, err = eap.Marshal()
+	if err != nil {
+		return nil, err
+	}
+	return ret, nil
 }
 
 func (e *EapAkaChallenge) ParseChallenge(msg *message.Message) error {
diff --git a/pkg/eap/eap_test.go b/pkg/eap/eap_test.go
index 15f4742..79c199e 100644
--- a/pkg/eap/eap_test.go
+++ b/pkg/eap/eap_test.go
@@ -54,6 +54,18 @@ func TestEapAka_MarshalUnmarshal(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "synchronization failure",
+			eap: &EapAka{
+				Code:    CodeResponse,
+				SubType: SubTypeAkaSynchronizationFailure,
+				Attributes: map[AttributeType]*Attribute{
+					AT_AUTS: {
+						Data: make([]byte, 14),
+					},
+				},
+			},
+		},
 	}
 
 	for _, tt := range tests {
diff --git a/pkg/pod/pod.go b/pkg/pod/pod.go
index a50d1c8..69659ca 100644
--- a/pkg/pod/pod.go
+++ b/pkg/pod/pod.go
@@ -183,7 +183,7 @@ func (p *Pod) StartActivation() {
 		log.Fatalf("pkg pod; could not get LTK %s", err)
 	}
 	log.Infof("pkg pod; LTK %x", p.state.LTK)
-	p.state.EapAkaSeq = 1
+	p.state.EapAkaSeq = 0 // the controller's first session uses SQN 1
 	p.state.Save()
 
 	p.EapAka()
@@ -191,19 +191,36 @@ func (p *Pod) StartActivation() {
 
 func (p *Pod) EapAka() {
 
-	session := eap.NewEapAkaChallenge(p.state.LTK, p.state.EapAkaSeq)
-
-	msg, ok := p.readSetup()
-	if !ok {
-		return
-	}
-	err := session.ParseChallenge(msg)
-	if err != nil {
-		p.startOver("error parsing the EAP-AKA challenge: %s", err)
-		return
+	var session *eap.EapAkaChallenge
+	var msg *message.Message
+	var ok bool
+	for {
+		session = eap.NewEapAkaChallenge(p.state.LTK, p.state.EapAkaSeq)
+		if msg, ok = p.readSetup(); !ok {
+			return
+		}
+		if err := session.ParseChallenge(msg); err != nil {
+			p.startOver("error parsing the EAP-AKA challenge: %s", err)
+			return
+		}
+		fresh, err := session.CheckSqn()
+		if err != nil {
+			p.startOver("error checking the EAP-AKA challenge: %s", err)
+			return
+		}
+		if fresh {
+			break
+		}
+		// Another controller has had a session since this one's last: tell it our SQN.
+		// It retries on the same connection with the next one.
+		log.Infof("pkg pod; EAP-AKA SQN %d is not past %d; sending a synchronization failure", session.Sqn, p.state.EapAkaSeq)
+		if msg, err = session.GenerateSynchronizationFailure(); err != nil {
+			log.Fatalf("pkg pod; error generating the eap-aka synchronization failure: %s", err)
+		}
+		p.ble.WriteMessage(msg)
 	}
 
-	msg, err = session.GenerateChallengeResponse()
+	msg, err := session.GenerateChallengeResponse()
 	if err != nil {
 		log.Fatalf("pkg pod; error generating the eap-aka challenge response")
 	}
diff --git a/pkg/pod/pod_test.go b/pkg/pod/pod_test.go
index 274033a..6ed83ee 100644
--- a/pkg/pod/pod_test.go
+++ b/pkg/pod/pod_test.go
@@ -1,13 +1,17 @@
 package pod
 
 import (
+	"bytes"
+	"encoding/hex"
 	"path/filepath"
 	"sync"
 	"testing"
 	"time"
 
 	"github.com/avereha/pod/pkg/bluetooth"
+	"github.com/avereha/pod/pkg/eap"
 	"github.com/avereha/pod/pkg/message"
+	"github.com/wmnsk/milenage"
 )
 
 // radio carries one side's notifications to the other side, and records
@@ -83,3 +87,141 @@ func TestAGarbledSessionMessageHangsUp(t *testing.T) {
 		t.Fatal("the pod did not hang up")
 	}
 }
+
+var milenageOP, _ = hex.DecodeString("cdc202d5123e20f62b6d676ac72cb318")
+
+// challenge is the controller's EAP-AKA challenge: AUTN = SQN xor AK || AMF || MAC-A.
+func challenge(t *testing.T, ltk, rand []byte, sqn uint64) *message.Message {
+	m := milenage.New(ltk, milenageOP, rand, sqn, 0xb9b9)
+	_, _, _, ak, err := m.F2345()
+	if err != nil {
+		t.Fatal(err)
+	}
+	mac, err := m.F1()
+	if err != nil {
+		t.Fatal(err)
+	}
+	autn := make([]byte, 0, 16)
+	for i := 0; i < 6; i++ {
+		autn = append(autn, m.SQN[i]^ak[i])
+	}
+	autn = append(append(autn, 0xb9, 0xb9), mac...)
+	msg := message.NewMessage(message.MessageTypeSessionEstablishment, []byte{1, 2, 3, 4}, []byte{5, 6, 7, 8})
+	msg.Payload, err = (&eap.EapAka{Code: eap.CodeRequest, SubType: eap.SubTypeAkaChallenge,
+		Attributes: map[eap.AttributeType]*eap.Attribute{
+			eap.AT_RAND:      {Data: rand},
+			eap.AT_AUTN:      {Data: autn},
+			eap.AT_CUSTOM_IV: {Data: []byte{1, 2, 3, 4}},
+		}}).Marshal()
+	if err != nil {
+		t.Fatal(err)
+	}
+	return msg
+}
+
+func readEap(t *testing.T, phone *bluetooth.Ble) *eap.EapAka {
+	msg, didTimeout, _ := phone.ReadMessageWithTimeout(2 * time.Second)
+	if didTimeout || msg == nil {
+		t.Fatal("no answer from the pod")
+	}
+	e, err := eap.Unmarshal(msg.Payload)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return e
+}
+
+// A controller whose SQN is behind gets the pod's in AUTS, as OmnipodKit reads it, and its
+// retry with the next SQN on the same connection establishes the session.
+func TestAStaleSqnGetsASynchronizationFailure(t *testing.T) {
+	toPhone, toPod := &radio{hungUp: make(chan bool, 1)}, &radio{}
+	podBle, phone := newBle(t, toPhone), newBle(t, toPod)
+	toPod.peer = podBle
+	toPhone.peer = phone
+
+	ltk := bytes.Repeat([]byte{0x5a}, 16)
+	state := NewFreshState(filepath.Join(t.TempDir(), "state.toml"))
+	state.LTK = ltk
+	state.EapAkaSeq = 40
+	p := New(podBle, state)
+
+	podBle.Connected()
+	go p.StartAcceptingCommands()
+	podBle.CmdWritten(bluetooth.Packet{6, 1, 4})
+	phone.Connected()
+	phone.StartMessageLoop()
+
+	rand := bytes.Repeat([]byte{0x11}, 16)
+	phone.WriteMessage(challenge(t, ltk, rand, 37))
+	answer := readEap(t, phone)
+	if answer.SubType != eap.SubTypeAkaSynchronizationFailure || answer.Attributes[eap.AT_AUTS] == nil {
+		t.Fatalf("expected a synchronization failure, got %+v", answer)
+	}
+	auts := answer.Attributes[eap.AT_AUTS].Data
+	m := milenage.New(ltk, milenageOP, rand, 0, 0)
+	aks, err := m.F5Star()
+	if err != nil {
+		t.Fatal(err)
+	}
+	sqnMs := make([]byte, 6)
+	for i := range sqnMs {
+		sqnMs[i] = auts[i] ^ aks[i]
+	}
+	if !bytes.Equal(sqnMs, []byte{0, 0, 0, 0, 0, 40}) {
+		t.Fatalf("AUTS carries SQN %x, want 40", sqnMs)
+	}
+	macS, err := m.F1Star(sqnMs, []byte{0, 0})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(macS, auts[6:]) {
+		t.Fatalf("MAC-S %x, want %x", auts[6:], macS)
+	}
+
+	phone.WriteMessage(challenge(t, ltk, bytes.Repeat([]byte{0x22}, 16), 41))
+	if answer = readEap(t, phone); answer.SubType != eap.SubTypeAkaChallenge || answer.Attributes[eap.AT_RES] == nil {
+		t.Fatalf("expected a challenge response, got %+v", answer)
+	}
+	success := message.NewMessage(message.MessageTypeSessionEstablishment, []byte{1, 2, 3, 4}, []byte{5, 6, 7, 8})
+	success.Payload, _ = (&eap.EapAka{Code: eap.CodeSuccess}).Marshal()
+	phone.WriteMessage(success)
+
+	deadline := time.Now().Add(2 * time.Second)
+	for {
+		p.mtx.Lock()
+		sqn := p.state.EapAkaSeq
+		p.mtx.Unlock()
+		if sqn == 41 {
+			break
+		}
+		if time.Now().After(deadline) {
+			t.Fatalf("pod SQN %d after the session, want 41", sqn)
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+}
+
+// An AUTN whose MAC-A doesn't verify ends the connection.
+func TestABadAutnHangsUp(t *testing.T) {
+	toPhone, toPod := &radio{hungUp: make(chan bool, 1)}, &radio{}
+	podBle, phone := newBle(t, toPhone), newBle(t, toPod)
+	toPod.peer = podBle
+	toPhone.peer = phone
+
+	state := NewFreshState(filepath.Join(t.TempDir(), "state.toml"))
+	state.LTK = bytes.Repeat([]byte{0x5a}, 16)
+	p := New(podBle, state)
+
+	podBle.Connected()
+	go p.StartAcceptingCommands()
+	podBle.CmdWritten(bluetooth.Packet{6, 1, 4})
+	phone.Connected()
+	phone.StartMessageLoop()
+
+	phone.WriteMessage(challenge(t, bytes.Repeat([]byte{0x77}, 16), bytes.Repeat([]byte{0x11}, 16), 1))
+	select {
+	case <-toPhone.hungUp:
+	case <-time.After(2 * time.Second):
+		t.Fatal("the pod did not hang up")
+	}
+}