diff --git a/controlstream.go b/controlstream.go
index 94a7386..64eea4a 100644
--- a/controlstream.go
+++ b/controlstream.go
@@ -30,12 +30,9 @@ type controlStream struct {
 }
 
 func (s *controlStream) sendPktLogin() error {
-	s.common.pkt0.sendSeqLock()
-	defer s.common.pkt0.sendSeqUnlock()
-
 	// The reply to the auth packet will contain a 6 bytes long auth ID with the first 2 bytes set to our ID.
 	authStartID := []byte{0x63, 0x00}
-	p := []byte{0x80, 0x00, 0x00, 0x00, 0x00, 0x00, byte(s.common.pkt0.sendSeq), byte(s.common.pkt0.sendSeq >> 8),
+	p := []byte{0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		byte(s.common.localSID >> 24), byte(s.common.localSID >> 16), byte(s.common.localSID >> 8), byte(s.common.localSID),
 		byte(s.common.remoteSID >> 24), byte(s.common.remoteSID >> 16), byte(s.common.remoteSID >> 8), byte(s.common.remoteSID),
 		0x00, 0x00, 0x00, 0x70, 0x01, 0x00, 0x00, byte(s.authInnerSendSeq),
@@ -52,14 +49,10 @@ func (s *controlStream) sendPktLogin() error {
 		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}
-	if err := s.common.send(p); err != nil {
-		return err
-	}
-	if err := s.common.send(p); err != nil {
+	if err := s.common.pkt0.sendTrackedPacket(&s.common, p); err != nil {
 		return err
 	}
 
-	s.common.pkt0.sendSeq++
 	s.authInnerSendSeq++
 	return nil
 }
@@ -82,10 +75,7 @@ func (s *controlStream) sendPktAuth(magic byte) error {
 	//                           0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 	//                           0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
 
-	s.common.pkt0.sendSeqLock()
-	defer s.common.pkt0.sendSeqUnlock()
-
-	p := []byte{0x40, 0x00, 0x00, 0x00, 0x00, 0x00, byte(s.common.pkt0.sendSeq), byte(s.common.pkt0.sendSeq >> 8),
+	p := []byte{0x40, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		byte(s.common.localSID >> 24), byte(s.common.localSID >> 16), byte(s.common.localSID >> 8), byte(s.common.localSID),
 		byte(s.common.remoteSID >> 24), byte(s.common.remoteSID >> 16), byte(s.common.remoteSID >> 8), byte(s.common.remoteSID),
 		0x00, 0x00, 0x00, 0x30, 0x01, magic, 0x00, byte(s.authInnerSendSeq),
@@ -94,23 +84,16 @@ func (s *controlStream) sendPktAuth(magic byte) error {
 		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}
-	if err := s.common.send(p); err != nil {
+	if err := s.common.pkt0.sendTrackedPacket(&s.common, p); err != nil {
 		return err
 	}
-	if err := s.common.send(p); err != nil {
-		return err
-	}
-	s.common.pkt0.sendSeq++
 	s.authInnerSendSeq++
 	return nil
 }
 
 func (s *controlStream) sendRequestSerialAndAudio() error {
-	s.common.pkt0.sendSeqLock()
-	defer s.common.pkt0.sendSeqUnlock()
-
 	log.Print("requesting serial and audio stream")
-	p := []byte{0x90, 0x00, 0x00, 0x00, 0x00, 0x00, byte(s.common.pkt0.sendSeq), byte(s.common.pkt0.sendSeq >> 8),
+	p := []byte{0x90, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		byte(s.common.localSID >> 24), byte(s.common.localSID >> 16), byte(s.common.localSID >> 8), byte(s.common.localSID),
 		byte(s.common.remoteSID >> 24), byte(s.common.remoteSID >> 16), byte(s.common.remoteSID >> 8), byte(s.common.remoteSID),
 		0x00, 0x00, 0x00, 0x80, 0x01, 0x03, 0x00, byte(s.authInnerSendSeq),
@@ -129,14 +112,10 @@ func (s *controlStream) sendRequestSerialAndAudio() error {
 		0x00, 0x00, 0xbb, 0x80, 0x00, 0x00, 0xc3, 0x52,
 		0x00, 0x00, 0xc3, 0x53, 0x00, 0x00, 0x00, 0xa0,
 		0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}
-	if err := s.common.send(p); err != nil {
-		return err
-	}
-	if err := s.common.send(p); err != nil {
+	if err := s.common.pkt0.sendTrackedPacket(&s.common, p); err != nil {
 		return err
 	}
 
-	s.common.pkt0.sendSeq++
 	s.authInnerSendSeq++
 
 	return nil
@@ -244,8 +223,6 @@ func (s *controlStream) handleRead(r []byte) error {
 func (s *controlStream) loop() {
 	startTime := time.Now()
 
-	s.common.pkt0.startPeriodicSend(&s.common)
-
 	s.secondAuthTimer = time.NewTimer(time.Second)
 	reauthTicker := time.NewTicker(60 * time.Second)
 	statusLogTicker := time.NewTicker(3 * time.Second)
@@ -299,7 +276,8 @@ func (s *controlStream) start() error {
 		return err
 	}
 
-	s.common.pkt0.sendSeq = 1
+	s.common.pkt0.startPeriodicSend(&s.common)
+
 	if err := s.sendPktLogin(); err != nil {
 		return err
 	}
diff --git a/pkt0.go b/pkt0.go
index 099e2ab..9e6353a 100644
--- a/pkt0.go
+++ b/pkt0.go
@@ -1,8 +1,13 @@
 package main
 
 import (
+	"bytes"
+	"encoding/binary"
+	"math"
 	"sync"
 	"time"
+
+	"github.com/nonoo/kappanhang/log"
 )
 
 type pkt0Type struct {
@@ -11,6 +16,8 @@ type pkt0Type struct {
 
 	sendTicker *time.Ticker
 
+	txSeqBuf txSeqBufStruct
+
 	periodicStopNeededChan   chan bool
 	periodicStopFinishedChan chan bool
 }
@@ -23,13 +30,81 @@ func (p *pkt0Type) sendSeqUnlock() {
 	p.mutex.Unlock()
 }
 
-func (p *pkt0Type) send(s *streamCommon) error {
+func (p *pkt0Type) retransmitRange(s *streamCommon, start, end uint16) error {
+	if int(math.Abs(float64(start)-float64(end))) > len(p.txSeqBuf.entries) {
+		log.Debug(s.name+"/can't retransmit #", start, "-", end, " - range too big")
+		return nil
+	}
+
+	log.Debug("got retransmit request for #", start, "-", end)
+	for {
+		d := p.txSeqBuf.get(seqNum(start))
+		if d != nil {
+			log.Debug(s.name+"/retransmitting #", start)
+			if err := s.send(d); err != nil {
+				return err
+			}
+		} else {
+			log.Debug(s.name+"/can't retransmit #", start, " - not found")
+		}
+
+		if start == end {
+			break
+		}
+		start++
+	}
+	return nil
+}
+
+func (p *pkt0Type) handle(s *streamCommon, r []byte) error {
+	if len(r) < 16 {
+		return nil
+	}
+
+	if bytes.Equal(r[:6], []byte{0x10, 0x00, 0x00, 0x00, 0x01, 0x00}) {
+		seq := binary.LittleEndian.Uint16(r[6:8])
+		d := p.txSeqBuf.get(seqNum(seq))
+		if d != nil {
+			log.Debug(s.name+"/retransmitting #", seq)
+			if err := s.send(d); err != nil {
+				return err
+			}
+		} else {
+			log.Debug(s.name+"/can't retransmit #", seq, " - not found")
+		}
+	} else if bytes.Equal(r[:6], []byte{0x18, 0x00, 0x00, 0x00, 0x01, 0x00}) {
+		r = r[16:]
+		for len(r) >= 4 {
+			start := binary.LittleEndian.Uint16(r[0:2])
+			end := binary.LittleEndian.Uint16(r[2:4])
+			if err := p.retransmitRange(s, start, end); err != nil {
+				return err
+			}
+			r = r[4:]
+		}
+	}
+	return nil
+}
+
+func (p *pkt0Type) isIdlePkt0(r []byte) bool {
+	return len(r) == 16 && bytes.Equal(r[:6], []byte{0x10, 0x00, 0x00, 0x00, 0x00, 0x00})
+}
+
+func (p *pkt0Type) isPkt0(r []byte) bool {
+	return len(r) >= 16 && (p.isIdlePkt0(r) ||
+		bytes.Equal(r[:6], []byte{0x10, 0x00, 0x00, 0x00, 0x01, 0x00}) || // Retransmit request for 1 packet.
+		bytes.Equal(r[:6], []byte{0x18, 0x00, 0x00, 0x00, 0x01, 0x00})) // Retransmit request for ranges.
+}
+
+// The radio can request retransmit for tracked packets. If there are no tracked packets to send, idle pkt0
+// packets are periodically sent.
+func (p *pkt0Type) sendTrackedPacket(s *streamCommon, d []byte) error {
 	p.sendSeqLock()
 	defer p.sendSeqUnlock()
 
-	d := []byte{0x10, 0x00, 0x00, 0x00, 0x00, 0x00, byte(p.sendSeq), byte(p.sendSeq >> 8),
-		byte(s.localSID >> 24), byte(s.localSID >> 16), byte(s.localSID >> 8), byte(s.localSID),
-		byte(s.remoteSID >> 24), byte(s.remoteSID >> 16), byte(s.remoteSID >> 8), byte(s.remoteSID)}
+	d[6] = byte(p.sendSeq)
+	d[7] = byte(p.sendSeq >> 8)
+	p.txSeqBuf.add(seqNum(p.sendSeq), d)
 	if err := s.send(d); err != nil {
 		return err
 	}
@@ -37,11 +112,18 @@ func (p *pkt0Type) send(s *streamCommon) error {
 	return nil
 }
 
+func (p *pkt0Type) sendIdle(s *streamCommon) error {
+	d := []byte{0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		byte(s.localSID >> 24), byte(s.localSID >> 16), byte(s.localSID >> 8), byte(s.localSID),
+		byte(s.remoteSID >> 24), byte(s.remoteSID >> 16), byte(s.remoteSID >> 8), byte(s.remoteSID)}
+	return p.sendTrackedPacket(s, d)
+}
+
 func (p *pkt0Type) loop(s *streamCommon) {
 	for {
 		select {
 		case <-p.sendTicker.C:
-			if err := p.send(s); err != nil {
+			if err := p.sendIdle(s); err != nil {
 				reportError(err)
 			}
 		case <-p.periodicStopNeededChan:
@@ -52,6 +134,7 @@ func (p *pkt0Type) loop(s *streamCommon) {
 }
 
 func (p *pkt0Type) startPeriodicSend(s *streamCommon) {
+	p.sendSeq = 1
 	p.sendTicker = time.NewTicker(100 * time.Millisecond)
 
 	p.periodicStopNeededChan = make(chan bool)
diff --git a/serialstream.go b/serialstream.go
index 369efef..3fd5d88 100644
--- a/serialstream.go
+++ b/serialstream.go
@@ -36,26 +36,19 @@ type serialStream struct {
 }
 
 func (s *serialStream) send(d []byte) error {
-	s.common.pkt0.sendSeqLock()
-	defer s.common.pkt0.sendSeqUnlock()
-
 	l := byte(len(d))
-	p := append([]byte{0x15 + l, 0x00, 0x00, 0x00, 0x00, 0x00, byte(s.common.pkt0.sendSeq), byte(s.common.pkt0.sendSeq >> 8),
+	p := append([]byte{0x15 + l, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		byte(s.common.localSID >> 24), byte(s.common.localSID >> 16), byte(s.common.localSID >> 8), byte(s.common.localSID),
 		byte(s.common.remoteSID >> 24), byte(s.common.remoteSID >> 16), byte(s.common.remoteSID >> 8), byte(s.common.remoteSID),
 		0xc1, l, 0x00, byte(s.sendSeq >> 8), byte(s.sendSeq)}, d...)
-	if err := s.common.send(p); err != nil {
+	if err := s.common.pkt0.sendTrackedPacket(&s.common, p); err != nil {
 		return err
 	}
-	s.common.pkt0.sendSeq++
 	s.sendSeq++
 	return nil
 }
 
 func (s *serialStream) sendOpenClose(close bool) error {
-	s.common.pkt0.sendSeqLock()
-	defer s.common.pkt0.sendSeqUnlock()
-
 	var magic byte
 	if close {
 		magic = 0x00
@@ -63,14 +56,13 @@ func (s *serialStream) sendOpenClose(close bool) error {
 		magic = 0x05
 	}
 
-	p := []byte{0x16, 0x00, 0x00, 0x00, 0x00, 0x00, byte(s.common.pkt0.sendSeq), byte(s.common.pkt0.sendSeq >> 8),
+	p := []byte{0x16, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 		byte(s.common.localSID >> 24), byte(s.common.localSID >> 16), byte(s.common.localSID >> 8), byte(s.common.localSID),
 		byte(s.common.remoteSID >> 24), byte(s.common.remoteSID >> 16), byte(s.common.remoteSID >> 8), byte(s.common.remoteSID),
 		0xc0, 0x01, 0x00, byte(s.sendSeq >> 8), byte(s.sendSeq), magic}
-	if err := s.common.send(p); err != nil {
+	if err := s.common.pkt0.sendTrackedPacket(&s.common, p); err != nil {
 		return err
 	}
-	s.common.pkt0.sendSeq++
 	s.sendSeq++
 	return nil
 }
@@ -92,7 +84,7 @@ func (s *serialStream) handleRxSeqBufEntry(e seqBufEntry) {
 	s.lastReceivedSeq = gotSeq
 	s.receivedSerialData = true
 
-	if len(e.data) == 16 { // Do not send pkt0s.
+	if s.common.pkt0.isPkt0(e.data) {
 		return
 	}
 
@@ -118,9 +110,8 @@ func (s *serialStream) handleSerialPacket(r []byte) error {
 }
 
 func (s *serialStream) handleRead(r []byte) error {
-	// We add both serial data and pkt0 to the seqbuf.
-	if (len(r) == 16 && bytes.Equal(r[:6], []byte{0x10, 0x00, 0x00, 0x00, 0x00, 0x00})) || // Pkt0?
-		(len(r) >= 22 && r[16] == 0xc1 && r[0]-0x15 == r[17]) { // Serial data?
+	// We add both idle pkt0 and serial data to the seqbuf.
+	if s.common.pkt0.isIdlePkt0(r) || (len(r) >= 22 && r[16] == 0xc1 && r[0]-0x15 == r[17]) {
 		return s.handleSerialPacket(r)
 	}
 	return nil
diff --git a/streamcommon.go b/streamcommon.go
index 5d43b60..fe801f2 100644
--- a/streamcommon.go
+++ b/streamcommon.go
@@ -51,7 +51,12 @@ func (s *streamCommon) reader() {
 			if err := s.pkt7.handle(s, r); err != nil {
 				reportError(err)
 			}
+			// Don't let pkt7 packets further downstream.
 			continue
+		} else if s.pkt0.isPkt0(r) {
+			if err := s.pkt0.handle(s, r); err != nil {
+				reportError(err)
+			}
 		}
 
 		select {
diff --git a/txseqbuf.go b/txseqbuf.go
new file mode 100644
index 0000000..31b146e
--- /dev/null
+++ b/txseqbuf.go
@@ -0,0 +1,38 @@
+package main
+
+import "time"
+
+type txSeqBufStruct struct {
+	entries []seqBufEntry
+}
+
+func (s *txSeqBufStruct) add(seq seqNum, p []byte) {
+	s.entries = append(s.entries, seqBufEntry{
+		seq:     seq,
+		data:    p,
+		addedAt: time.Now(),
+	})
+	s.purgeOldEntries()
+}
+
+func (s *txSeqBufStruct) purgeOldEntries() {
+	for len(s.entries) > 0 && time.Since(s.entries[0].addedAt) > time.Second {
+		s.entries = s.entries[1:]
+	}
+}
+
+func (s *txSeqBufStruct) get(seq seqNum) (d []byte) {
+	if len(s.entries) == 0 {
+		return nil
+	}
+
+	// Searching from backwards, as we expect most queries for latest entries.
+	for i := len(s.entries) - 1; i >= 0; i-- {
+		if s.entries[i].seq == seq {
+			d = s.entries[i].data
+			break
+		}
+	}
+	s.purgeOldEntries()
+	return d
+}