Re: [PATCH net v2] L2TP:Adjust intf MTU,factor underlay L3,overlay L2

[R. Parameswaran]

Hi James,

On Thu, 29 Sep 2016, James Chapman wrote:

> On 22/09/16 21:52, R. Parameswaran wrote:
> > From ed585bdd6d3d2b3dec58d414f514cd764d89159d Mon Sep 17 00:00:00 2001
> > From: "R. Parameswaran" <rparames@xxxxxxxxxxx>
> > Date: Thu, 22 Sep 2016 13:19:25 -0700
> > Subject: [PATCH] L2TP:Adjust intf MTU,factor underlay L3,overlay L2
> >
> > 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: R. Parameswaran <rparames@xxxxxxxxxxx>
> >
> > Reviewed-by: "N. Prachanda" <nprachan@xxxxxxxxxxx>,
> > Reviewed-by: "R. Shearman" <rshearma@xxxxxxxxxxx>,
> > Reviewed-by: "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));
> What about options in the IP header? If certain options are set on the
> socket, the IP header may be larger.
> 

Thanks for the reply - It looks like IP options can only be 
enabled through setsockopt on an application's socket (if there's any 
other way to turn on IP options, please let me know - didn't see any 
sysctl setting for transmit). This scenario would come 
into picture when an application opens a raw IP or UDP socket such that it 
routes into the L2TP logical interface.

If you take the case of a plain IP (ethernet) interface, even if an
application opened a socket turning on IP options, it would not change
the MTU of the underlying interface, and it would not affect other 
applications transacting packets on the same interface. I know its not an 
exact parallel to this case, but since the IP option control is per 
application, we probably should not factor it into the L2TP logical interface?
We cannot affect other applications/processes running on the same L2TP 
tunnel. Also, since the application  using IP options knows that it has turned 
on IP options, maybe we can count on it to factor the size of the options 
into the size of the payload it sends into the socket, or set the mtu on the 
L2TP interface through config? 

Other than this, I don't see keepalives or anything else in which the 
kernel will source its own packet into the L2TP interface, outside of 
an application injected packet - if there is something like that, please
let me know. The user space L2TP daemon would probably fall in the 
category of applications.

thanks,

Ramkumar 


> > +	/* 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;
> 
> 
> 
>