Re: xfrm: is pmtu broken with ESP tunneling?

[Hannes Frederic Sowa]

On Tue, Feb 11, 2014 at 09:20:40PM +0100, Ortwin GlÃck wrote:
> On 02/11/2014 03:32 AM, Hannes Frederic Sowa wrote:
> >>net.ipv4.ip_no_pmtu_disc=1.
> >
> >This setting will shrink the path mtu to min_pmtu when a frag needed icmp 
> >is
> >received.
> 
> The UDP+ESP encapsulation adds 60 bytes to the original packet size.
> 
> ifconfig wla0 shows an mtu of 1500.
> 
> The size of the first big packet on the interface:
> net.ipv4.ip_no_pmtu_disc=1: packet length is 1300
> net.ipv4.ip_no_pmtu_disc=0: packet length is 1500
> 
> Length is without the ESP wrapper and UDP encapsulation. The packets are so 
> big that they can't even leave the wireless interface and never show up on 
> the router. So no ICMP packets are received. PMTU can't work with initial 
> packets of that size.
> 
> dump question: which layer discard these packets? qdisc? why no 
> notification to the sender?

Could you try either dropwatch or perf script net_dropmonitor and flood the
network with the problematic packets. From the traces we could see where the
packets get dropped without notification in the kernel.

> When I increase the mtu of the interface to 2000 with ifconfig, then I 
> start seeing ICMP fragmentation needed from the next hop, indicating 1500 
> as the mtu as response to a 1560 byte UDP[ESP] packet.
> 
> The next UDP[ESP] packet is shorter: 1360 bytes. It gets hard to see what's 
> going on after that, but the connection is still not working.
> 
> So, instead of somehow losing these packets on the way out of the interface 
> should the kernel not start with a lower mtu in the first place? Now it 
> seems it is trying with the maximum of the interface and expecting to scale 
> down with pmtu - which can ever happen.
> 
> >Can you send a ip route get <ip> to the problematic target to see how
> >far off the calculated value is?
> 
> That command doesn't return anything useful. No hint on the mtu here.
> 
> BTW, instead of disabling pmtu, setting mtu explicitly also helps:
> ip route add 10.6.6.0/24 via ${localip} mtu 1300

Strange that the problem disappears if you enable no_pmtu_disc then.

Thanks,

  Hannes
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