diff --git a/README.md b/README.md
index 699e0e6..1ea3768 100644
--- a/README.md
+++ b/README.md
@@ -37,8 +37,21 @@ sudo ip link set eip0 up
 sudo ip addr add 192.0.2.1/30 dev eip0
 ```
 
+MTUを省略した場合、Overlay MTUはUnderlayの経路MTUからIPv6、EtherIP、
+Ethernetヘッダーの合計56 byteを差し引いた値に自動調整されます。例えば
+Underlay MTUが1500の場合、Overlay MTUは1444になり、通常の送信では外側IPv6
+フラグメントを生成しません。
+
+明示的に大きなMTUを設定することもできます。その場合は、カプセル化後の
+パケットが実際のUnderlay経路MTUを超えたときだけ送信元でIPv6フラグメントを
+生成します。
+
+```sh
+sudo ip link set eip0 mtu 1500
+```
+
 ## トンネルの削除
 
 ```sh
 sudo etherip-client delete -d eip0
-```
\ No newline at end of file
+```
diff --git a/etherip6.c b/etherip6.c
index f3d4d24..8747e7c 100644
--- a/etherip6.c
+++ b/etherip6.c
@@ -32,6 +32,8 @@
 #define ETHERIP6_HLEN 2
 #define ETHERIP6_DEFAULT_HOP_LIMIT 64
 #define ETHERIP6_MAX_MTU 9000
+#define ETHERIP6_ENCAP_HLEN (sizeof(struct ipv6hdr) + ETHERIP6_HLEN)
+#define ETHERIP6_MTU_OVERHEAD (ETHERIP6_ENCAP_HLEN + ETH_HLEN)
 
 struct etherip6_tunnel {
 	struct list_head list;
@@ -140,6 +142,35 @@ static struct etherip6_tunnel *etherip6_lookup_rx(struct net *net,
 	return etherip6_lookup_unique_rx(net, local, remote, iif, false, false);
 }
 
+static unsigned int etherip6_underlay_mtu(struct net *net,
+					  const struct etherip6_tunnel *tun)
+{
+	struct dst_entry *dst;
+	struct flowi6 fl6 = {};
+	unsigned int mtu = 0;
+
+	fl6.flowi6_proto = IPPROTO_ETHERIP;
+	fl6.flowi6_oif = tun->link;
+	fl6.daddr = tun->remote;
+	fl6.saddr = tun->local;
+
+	dst = ip6_route_output(net, NULL, &fl6);
+	if (!dst->error)
+		mtu = dst_mtu(dst);
+	dst_release(dst);
+
+	return mtu;
+}
+
+static unsigned int etherip6_overlay_mtu(unsigned int underlay_mtu)
+{
+	if (underlay_mtu <= ETHERIP6_MTU_OVERHEAD + ETH_MIN_MTU)
+		return ETH_MIN_MTU;
+
+	return min_t(unsigned int, underlay_mtu - ETHERIP6_MTU_OVERHEAD,
+		     ETHERIP6_MAX_MTU);
+}
+
 static netdev_tx_t etherip6_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct etherip6_tunnel *tun = netdev_priv(dev);
@@ -191,7 +222,8 @@ static netdev_tx_t etherip6_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	skb->protocol = htons(ETH_P_IPV6);
 	skb->dev = dst->dev;
-	skb->ignore_df = 1;
+	/* IPv6 source fragmentation is needed only above the underlay PMTU. */
+	skb->ignore_df = skb->len > dst_mtu(dst);
 	skb_dst_set(skb, dst);
 
 	stats = this_cpu_ptr(tun->stats);
@@ -252,6 +284,7 @@ static void etherip6_setup(struct net_device *dev)
 	dev->vlan_features = 0;
 	dev->min_mtu = ETH_MIN_MTU;
 	dev->max_mtu = ETHERIP6_MAX_MTU;
+	dev->needed_headroom = ETHERIP6_ENCAP_HLEN;
 	eth_hw_addr_random(dev);
 }
 
@@ -318,6 +351,15 @@ static int etherip6_newlink(struct net_device *dev,
 		return -EINVAL;
 	}
 
+	/* Keep the default overlay packets below the current underlay PMTU. */
+	if (!params->tb[IFLA_MTU]) {
+		unsigned int underlay_mtu;
+
+		underlay_mtu = etherip6_underlay_mtu(dev_net(dev), tun);
+		if (underlay_mtu)
+			dev->mtu = etherip6_overlay_mtu(underlay_mtu);
+	}
+
 	tun->dev = dev;
 	atomic_long_set(&tun->tx_dropped, 0);
 	atomic_long_set(&tun->rx_dropped, 0);