Open Stack

Hi,
Pl find my reply .

Regards,
keshava

From: Ian Wells [mailto:ijw.ubuntu at cack.org.uk]
Sent: Tuesday, October 28, 2014 1:19 PM
To: OpenStack Development Mailing List (not for usage questions)
Subject: Re: [openstack-dev] [neutron] vm can not transport large file under neutron ml2 + linux bridge + vxlan

On 28 October 2014 00:18, A, Keshava <keshava.a at hp.com<mailto:keshava.a at hp.com>> wrote:
Hi,

Currently OpenStack have any framework to notify the Tennant/Service-VM for such kind of notification based on VM’s interest ?

It's possible to use DHCP or RA to notify a VM of the MTU but there are limitations (RAs don't let you increase the MTU, only decrease it, and obviously VMs must support the MTU element of DHCP) and Openstack doesn't currently use it.  You can statically configure the DHCP MTU number that DHCP transmits; this is useful to work around problems but not really the right answer to the problem.

VM may be very much interested for such kind of notification like

1.       Path MTU.
This will be correctly discovered from the ICMP PMTU exceeded message, and Neutron routers should certainly be expected to send that.  (In fact the namespace implementation of routers would do this if the router ever had different MTUs on its ports; it's in the kernel network stack.)  There's no requirement for a special notification, and indeed you couldn't do it that way anyway.

In the network interface/router going down is  common scenario. In that case  the packet will take different path which may have different MTU.
In that case the PATH MTU calculated at the source may be different and should be notified dynamically to VM. So that  VM can originate the packet with requirement MTU size .
If there is no notification mechanism ( as per this reply):
If there is no such dynamic PATH MTU change notification to VM,  how VM  can change the  packet size  ?
Or
Do we expect ICMP too big message reaches all the way  to VM ?
Or
VM itself the run the PATH MTU ?


2.       Based on specific incoming Tennant traffic, block/Allow  particular traffic flow at infrastructure level itself, instead of at VM.
I don't see the relevance; and you appear to be describing security groups.

This may require OpenStack infrastructure notification support to Tenant/Service VM.
Not particularly, as MTU doesn't generally change, and I think we would forbid changing the MTU of a network after creation.  It's only an initial configuration thing, therefore.  It might involve better cloud-init support for network configuration, something that gets discussed periodically.

--
Ian.

                …
Thanks & regards,
Keshava

From: Ian Wells [mailto:ijw.ubuntu at cack.org.uk<mailto:ijw.ubuntu at cack.org.uk>]
Sent: Tuesday, October 28, 2014 11:40 AM
To: OpenStack Development Mailing List (not for usage questions)
Subject: Re: [openstack-dev] [neutron] vm can not transport large file under neutron ml2 + linux bridge + vxlan

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.
--
Ian.

On 27 October 2014 20:14, Li Tianqing <jazeltq at 163.com<mailto:jazeltq at 163.com>> wrote:



--
Best
    Li Tianqing


At 2014-10-27 17:42:41, "Ihar Hrachyshka" <ihrachys at redhat.com<mailto:ihrachys at redhat.com>> wrote:

>-----BEGIN PGP SIGNED MESSAGE-----

>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: https://review.openstack.org/#/c/105989/



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

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