[PATCH] L2TP:Adjust intf MTU, add underlay L3, overlay L2

[R. Parameswaran]

Take into account all of the tunnel encapsulation headers when setting
up the MTU on the L2TP logical interface device. Otherwise, packets
created by the applications on top of the L2TP layer are larger
than they ought to be, relative to the underlay MTU, leading to
needless fragmentation once the outer IP encap is added.

Specifically, take into account the (outer, underlay) IP header
imposed on the encapsulated L2TP packet, and the Layer 2 header
imposed on the inner IP packet prior to L2TP encapsulation.

Do not assume an Ethernet (non-jumbo) underlay. Use the PMTU mechanism
and the dst entry in the L2TP tunnel socket to directly pull up
the underlay MTU (as the baseline number on top of which the
encapsulation headers are factored in).  Fall back to Ethernet MTU
if this fails.

Signed-off-by: Ramkumar Parameswaran <rparames@xxxxxxxxxxx>

Reviewed-by: N. Prachanda <nprachan@xxxxxxxxxxx>,
             R. Shearman  <rshearma@xxxxxxxxxxx>,
             D. Fawcus    <dfawcus@xxxxxxxxxxx>


---
 net/l2tp/l2tp_eth.c | 48 ++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 44 insertions(+), 4 deletions(-)

diff --git a/net/l2tp/l2tp_eth.c b/net/l2tp/l2tp_eth.c
index 57fc5a4..dbcd6bd 100644
--- a/net/l2tp/l2tp_eth.c
+++ b/net/l2tp/l2tp_eth.c
@@ -30,6 +30,9 @@
 #include <net/xfrm.h>
 #include <net/net_namespace.h>
 #include <net/netns/generic.h>
+#include <linux/ip.h>
+#include <linux/ipv6.h>
+#include <linux/udp.h>

 #include "l2tp_core.h"

@@ -206,6 +209,46 @@ static void l2tp_eth_show(struct seq_file *m, void 
*arg)
 }
 #endif

+static void l2tp_eth_adjust_mtu(struct l2tp_tunnel *tunnel,
+				struct l2tp_session *session,
+				struct net_device *dev)
+{
+	unsigned int overhead = 0;
+	struct dst_entry *dst;
+
+	if (session->mtu != 0) {
+		dev->mtu = session->mtu;
+		dev->needed_headroom += session->hdr_len;
+		if (tunnel->encap == L2TP_ENCAPTYPE_UDP)
+			dev->needed_headroom += sizeof(struct udphdr);
+		return;
+	}
+	overhead = session->hdr_len;
+	/* Adjust MTU, factor overhead - underlay L3 hdr, overlay L2 hdr*/
+	if (tunnel->sock->sk_family == AF_INET)
+		overhead += (ETH_HLEN + sizeof(struct iphdr));
+	else if (tunnel->sock->sk_family == AF_INET6)
+		overhead += (ETH_HLEN + sizeof(struct ipv6hdr));
+	/* Additionally, if the encap is UDP, account for UDP header size */
+	if (tunnel->encap == L2TP_ENCAPTYPE_UDP)
+		overhead += sizeof(struct udphdr);
+	/* If PMTU discovery was enabled, use discovered MTU on L2TP device */
+	dst = sk_dst_get(tunnel->sock);
+	if (dst) {
+		u32 pmtu = dst_mtu(dst);
+
+		if (pmtu != 0)
+			dev->mtu = pmtu;
+		dst_release(dst);
+	}
+	/* else (no PMTUD) L2TP dev MTU defaulted to Ethernet MTU in caller */
+	session->mtu = dev->mtu - overhead;
+	dev->mtu = session->mtu;
+	dev->needed_headroom += session->hdr_len;
+	if (tunnel->encap == L2TP_ENCAPTYPE_UDP)
+		dev->needed_headroom += sizeof(struct udphdr);
+}
+
 static int l2tp_eth_create(struct net *net, u32 tunnel_id, u32 
session_id, u32 peer_session_id, struct l2tp_session_cfg *cfg)
 {
 	struct net_device *dev;
@@ -255,11 +298,8 @@ static int l2tp_eth_create(struct net *net, u32 
tunnel_id, u32 session_id, u32 p
 	}

 	dev_net_set(dev, net);
-	if (session->mtu == 0)
-		session->mtu = dev->mtu - session->hdr_len;
-	dev->mtu = session->mtu;
-	dev->needed_headroom += session->hdr_len;

+	l2tp_eth_adjust_mtu(tunnel, session, dev);
 	priv = netdev_priv(dev);
 	priv->dev = dev;
 	priv->session = session;
--
2.1.4