On Tue, Dec 02, 2014 at 10:12:04AM -0800, Jesse Gross wrote: > On Tue, Dec 2, 2014 at 9:41 AM, Thomas Graf <tg...@suug.ch> wrote: > > On 12/02/14 at 07:34pm, Michael S. Tsirkin wrote: > >> On Tue, Dec 02, 2014 at 05:09:27PM +0000, Thomas Graf wrote: > >> > On 12/02/14 at 01:48pm, Flavio Leitner wrote: > >> > > What about containers or any other virtualization environment that > >> > > doesn't use Virtio? > >> > > >> > The host can dictate the MTU in that case for both veth or OVS > >> > internal which would be primary container plumbing techniques. > >> > >> It typically can't do this easily for VMs with emulated devices: > >> real ethernet uses a fixed MTU. > >> > >> IMHO it's confusing to suggest MTU as a fix for this bug, it's > >> an unrelated optimization. > >> ICMP_DEST_UNREACH/ICMP_FRAG_NEEDED is the right fix here. > > > > PMTU discovery only resolves the issue if an actual IP stack is > > running inside the VM. This may not be the case at all. > > It's also only really a correct thing to do if the ICMP packet is > coming from an L3 node. If you are doing straight bridging then you > have to resort to hacks like OVS had before, which I agree are not > particularly desirable.
The issue seems to be that fundamentally, this is bridging interfaces with variable MTUs (even if MTU values on devices don't let us figure this out)- that is already not straight bridging, and I would argue sending ICMPs back is the right thing to do. > > I agree that exposing an MTU towards the guest is not applicable > > in all situations, in particular because it is difficult to decide > > what MTU to expose. It is a relatively elegant solution in a lot > > of virtualization host cases hooked up to an orchestration system > > though. > > I also think this is the right thing to do as a common case > optimization and I know other platforms (such as Hyper-V) do it. It's > not a complete solution so we still need the original patch in this > thread to handle things transparently. Well, as I believe David (and independently Jason) is saying, it looks like the ICMPs we are sending back after applying the original patch have the wrong MTU. And if I understand what David is saying here, IP is also the wrong place to do it. -- MST _______________________________________________ dev mailing list dev@openvswitch.org http://openvswitch.org/mailman/listinfo/dev
