diff --git a/src/yggdrasil/admin.go b/src/yggdrasil/admin.go
index 3270365..75561bd 100644
--- a/src/yggdrasil/admin.go
+++ b/src/yggdrasil/admin.go
@@ -280,6 +280,17 @@ func (a *admin) startTunWithMTU(ifname string, iftapmode bool, ifmtu int) error
 		if err != nil {
 			return err
 		}
+		// If we have open sessions then we need to notify them
+		// that our MTU has now changed
+		for _, sinfo := range a.core.sessions.sinfos {
+			if ifname == "none" {
+				sinfo.myMTU = 0
+			} else {
+				sinfo.myMTU = uint16(ifmtu)
+			}
+			a.core.sessions.sendPingPong(sinfo, false)
+		}
+		// Aaaaand... go!
 		go a.core.tun.read()
 	}
 	go a.core.tun.write()
@@ -374,6 +385,9 @@ func (a *admin) getData_getSessions() []admin_nodeInfo {
 				{"IP", net.IP(sinfo.theirAddr[:]).String()},
 				{"coords", fmt.Sprint(sinfo.coords)},
 				{"MTU", fmt.Sprint(sinfo.getMTU())},
+				{"wasMTUFixed", fmt.Sprint(sinfo.wasMTUFixed)},
+				{"bytesSent", fmt.Sprint(sinfo.bytesSent)},
+				{"bytesRecvd", fmt.Sprint(sinfo.bytesRecvd)},
 			}
 			infos = append(infos, info)
 		}
diff --git a/src/yggdrasil/router.go b/src/yggdrasil/router.go
index 78bfa36..ad11d6e 100644
--- a/src/yggdrasil/router.go
+++ b/src/yggdrasil/router.go
@@ -164,6 +164,31 @@ func (r *router) sendPacket(bs []byte) {
 		}
 		fallthrough // Also send the packet
 	default:
+		// Drop packets if the session MTU is 0 - this means that one or other
+		// side probably has their TUN adapter disabled
+		if sinfo.getMTU() == 0 {
+			// Get the size of the oversized payload, up to a max of 900 bytes
+			window := 900
+			if len(bs) < window {
+				window = len(bs)
+			}
+
+			// Create the Destination Unreachable response
+			ptb := &icmp.DstUnreach{
+				Data: bs[:window],
+			}
+
+			// Create the ICMPv6 response from it
+			icmpv6Buf, err := r.core.tun.icmpv6.create_icmpv6_tun(
+				bs[8:24], bs[24:40],
+				ipv6.ICMPTypeDestinationUnreachable, 1, ptb)
+			if err == nil {
+				r.recv <- icmpv6Buf
+			}
+
+			// Don't continue - drop the packet
+			return
+		}
 		// Generate an ICMPv6 Packet Too Big for packets larger than session MTU
 		if len(bs) > int(sinfo.getMTU()) {
 			// Get the size of the oversized payload, up to a max of 900 bytes
diff --git a/src/yggdrasil/session.go b/src/yggdrasil/session.go
index f413174..090c973 100644
--- a/src/yggdrasil/session.go
+++ b/src/yggdrasil/session.go
@@ -21,6 +21,7 @@ type sessionInfo struct {
 	myNonce      boxNonce
 	theirMTU     uint16
 	myMTU        uint16
+	wasMTUFixed  bool      // Was the MTU fixed by a receive error?
 	time         time.Time // Time we last received a packet
 	coords       []byte    // coords of destination
 	packet       []byte    // a buffered packet, sent immediately on ping/pong
@@ -32,6 +33,8 @@ type sessionInfo struct {
 	mtuTime      time.Time // time myMTU was last changed
 	pingTime     time.Time // time the first ping was sent since the last received packet
 	pingSend     time.Time // time the last ping was sent
+	bytesSent    uint64    // Bytes of real traffic sent in this session
+	bytesRecvd   uint64    // Bytes of real traffic received in this session
 }
 
 type sessionPing struct {
@@ -62,7 +65,7 @@ func (s *sessionInfo) update(p *sessionPing) bool {
 		s.theirNonce = boxNonce{}
 		s.nonceMask = 0
 	}
-	if p.mtu >= 1280 {
+	if p.mtu >= 1280 || p.mtu == 0 {
 		s.theirMTU = p.mtu
 	}
 	s.coords = append([]byte{}, p.coords...)
@@ -310,6 +313,9 @@ func (n *boxNonce) minus(m *boxNonce) int64 {
 }
 
 func (sinfo *sessionInfo) getMTU() uint16 {
+	if sinfo.theirMTU == 0 || sinfo.myMTU == 0 {
+		return 0
+	}
 	if sinfo.theirMTU < sinfo.myMTU {
 		return sinfo.theirMTU
 	}
@@ -384,6 +390,7 @@ func (sinfo *sessionInfo) doSend(bs []byte) {
 		payload: payload,
 	}
 	packet := p.encode()
+	sinfo.bytesSent += uint64(len(bs))
 	sinfo.core.router.out(packet)
 }
 
@@ -411,6 +418,7 @@ func (sinfo *sessionInfo) doRecv(p *wire_trafficPacket) {
 				//sinfo.core.log.Println("DEBUG set MTU to:", sinfo.myMTU)
 				sinfo.core.sessions.sendPingPong(sinfo, false)
 				sinfo.mtuTime = time.Now()
+				sinfo.wasMTUFixed = true
 			}
 		}
 		go func() { sinfo.core.router.admin <- fixSessionMTU }()
@@ -427,5 +435,6 @@ func (sinfo *sessionInfo) doRecv(p *wire_trafficPacket) {
 	go func() { sinfo.core.router.admin <- fixSessionMTU }()
 	sinfo.updateNonce(&p.nonce)
 	sinfo.time = time.Now()
+	sinfo.bytesRecvd += uint64(len(bs))
 	sinfo.core.router.recvPacket(bs, &sinfo.theirAddr, &sinfo.theirSubnet)
 }