Path MTU discovery works on a path - something with an L3 router in the way
- where the outbound interface has a smaller MTU than the inbound one.
You're transmitting across an L2 network - no L3 routers present.  You send
a 1500 byte packet, the network fabric (which is not L3, has no address,
and therefore has no means to answer you) does all that it can do with that
packet - it drops it.  The sender retransmits, assuming congestion, but the
same thing happens.  Eventually the sender decides there's a network
problem and times out.

This is a common problem with Openstack deployments, although various
features of the virtual networking let you get away with it, with some
configs and not others.  OVS used to fake a PMTU exceeded message from the
destination if you tried to pass an overlarge packet - not in spec, but it
hid the problem nicely.  I have a suspicion that some implementations will
fragment the containing UDP packet, which is also not in spec and also
solves the problem (albeit with poor performance).

The right answer for you is to set the MTU in your machines to the same MTU
you've given the network, that is, 1450 bytes.  You can do this by setting
a DHCP option for MTU, providing your VMs support that option (search the
web for the solution, I don't have it offhand) or lower the MTU by hand or
by script when you start your VM.

The right answer for everyone is to properly determine and advertise the
network MTU to VMs (which, with provider networks, is not even consistent
from one network to the next) and that's the spec Kyle is referring to.
We'll be fixing this in Kilo.

On 27 October 2014 20:14, Li Tianqing <> wrote:

> --
> Best
>     Li Tianqing
> At 2014-10-27 17:42:41, "Ihar Hrachyshka" <> wrote:
> >Hash: SHA512
> >
> >On 27/10/14 02:18, Li Tianqing wrote:
> >> Hello, Right now, we test neutron under havana release. We
> >> configured network_device_mtu=1450 in neutron.conf, After create
> >> vm, we found the vm interface's mtu is 1500, the ping, ssh, is ok.
> >> But if we scp large file between vms then scp display 'stalled'.
> >> And iperf is also can not completed. If we configured vm's mtu to
> >> 1450, then iperf, scp all is ok. If we iperf with -M 1300, then the
> >> iperf is ok too. The vms path mtu discovery is set by default. I do
> >> not know why the vm whose mtu is 1500 can not send large file.
> >
> >There is a neutron spec currently in discussion for Kilo to finally
> >fix MTU issues due to tunneling, that also tries to propagate MTU
> >inside instances:
> The problem is i do not know why the vm with 1500 mtu can not send large file?
> I found the packet send out all with DF, and is it because the DF set default 
> by linux cause the packet
> be dropped? And the application do not handle the return back icmp packet 
> with the smaller mtu?
>  >
> >/Ihar
> >Version: GnuPG/MacGPG2 v2.0.22 (Darwin)
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