On 07/11/2016 07:45 AM, Sam Yaple wrote:
Hello,
There was alot of work to get MTU advertisement working well in Mitaka.
With the work that was done we can finally have 1500 mtu networks for
tunneled networks if the underlying infrastructure supports jumbo frames.
Its fantastic for people who have 1500 mtu networks and want to use
vxlan, no more hacks to get the instance to use 1450 mtu. Its fantastic
for people who want to use 1500+ networks and get the instances setup
with 9000 mtu interfaces. Its is not good for people who want consistent
mtu no matter the network type. But thats fine, since mtu advertisement
_could_ be disabled. Its a fantastic default to have it turned on.
With a recent patchset [1] the ability to turn off MTU advertisements
was deprecated in Newton. In the review it was stated there is no valid
use case for it. I disagree.
The scenario is the infrastructure has jumbo frames enabled, but I do
not want the instances to be using jumbo frames, but I want them to be
using the default 1500 mtu that the rest of the world operates on. This
would still setup all of the virtual switching infrastructure to the
correct MTUs, but not try to adjust the instances MTUs. In this way the
instances are only communicating at 1500 mtu, but never having to
fragment/drop inside of the SDN when communicating with other networks
even if it is a VXLAN or other tunneled network.
Without the option to disable mtu advertisement, inside the same
environment flat/vlan and gre/vxlan network will always have different
mtu, even if the backend supports jumbo frames.
My ask is we keep the advertise_mtu option, and keep it enabled by
default. This would allow for the default, common 1500 mtu across
networks of different types.
This scenario would be very similiar to having a computer with 1500 mtu
attached to a switch which supports jumbo frames. Just because the
switch will accept and process a 9000 mtu frame, doesnt mean the
computer has to send a 9000 mtu frame. A very common scenario in the
real world.
[1] https://review.openstack.org/#/c/310448/
Hi Sam,
Out of curiosity, in what scenarios is it better to limit the instance's
MTU to a value lower than that of the maximum path MTU of the
infrastructure? In other words, if the infrastructure supports jumbo
frames, why artificially limit an instance's MTU to 1500 if that
instance could theoretically communicate with other instances in the
same infrastructure via a higher MTU?
Sorry if my question is poorly worded. I'm no networking expert and am
genuinely curious here. :)
Best,
-jay
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