lkml.org 
[lkml]   [2014]   [Feb]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: xfrm: is pmtu broken with ESP tunneling?
On Tue, Feb 11, 2014 at 09:20:40PM +0100, Ortwin Glück 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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2014-02-13 01:41    [W:0.093 / U:0.084 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site