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Actually, I note that that document is Juno and there doesn't seem to be
anything at all in the Liberty guide now, so the answer is probably to add
settings for path_mtu and segment_mtu in the recommended Neutron
configuration.

On 24 January 2016 at 22:26, Ian Wells <ijw.ubuntu at cack.org.uk> wrote:

> On 24 January 2016 at 22:12, Kevin Benton <blak111 at gmail.com> wrote:
>
>> >The reason for that was in the other half of the thread - it's not
>> possible to magically discover these things from within Openstack's own
>> code because the relevant settings span more than just one server
>>
>> IMO it's better to have a default of 1500 rather than let VMs
>> automatically default to 1500 because at least we will deduct the encap
>> header length when necessary in the dhcp/ra advertised value so overlays
>> work on standard 1500 MTU networks.
>>
>> In other words, our current empty default is realistically a terrible
>> default of 1500 that doesn't account for network segmentation overhead.
>>
> It's pretty clear that, while the current setup is precisely the old
> behaviour (backward compatibility, y'know?), it's not very useful.  Problem
> is, anyone using the 1550+hacks and other methods of today will find their
> system changes behaviour if we started setting that specific default.
>
> Regardless, we need to take that documentation and update it.  It was a
> nasty hack back in the day and not remotely a good idea now.
>
>
>
>> On Jan 24, 2016 23:00, "Ian Wells" <ijw.ubuntu at cack.org.uk> wrote:
>>
>>> On 24 January 2016 at 20:18, Kevin Benton <blak111 at gmail.com> wrote:
>>>
>>>> I believe the issue is that the default is unspecified, which leads to
>>>> nothing being advertised to VMs via dhcp/ra. So VMs end up using 1500,
>>>> which leads to a catastrophe when running on an overlay on a 1500 underlay.
>>>>
>>> That's not quite the point I was making here, but to answer that: looks
>>> to me like (for the LB or OVS drivers to appropriately set the network MTU
>>> for the virtual network, at which point it will be advertised because
>>> advertise_mtu defaults to True in the code) you *must* set one or more of
>>> path_mtu (for L3 overlays), segment_mtu (for L2 overlays) or physnet_mtu
>>> (for L2 overlays with differing MTUs on different physical networks).
>>> That's a statement of faith - I suspect if we try it we'll find a few
>>> niggling problems - but I can find the code, at least.
>>>
>>> The reason for that was in the other half of the thread - it's not
>>> possible to magically discover these things from within Openstack's own
>>> code because the relevant settings span more than just one server.  They
>>> have to line up with both your MTU settings for the interfaces in use, and
>>> the MTU settings for the other equipment within and neighbouring the cloud
>>> - switches, routers, nexthops.  So they have to be provided by the operator
>>> - then everything you want should kick in.
>>>
>>> If all of that is true, it really is just a documentation problem - we
>>> have the idea in place, we're just not telling people how to make use of
>>> it.  We can also include a checklist or a check script with that
>>> documentation - you might not be able to deduce the MTU values, but you can
>>> certainly run some checks to see if the values you have been given are
>>> obviously wrong.
>>>
>>> In the meantime, Matt K, you said you hadn't set path_mtu in your tests,
>>> but [1] says you have to ([1] is far from end-user consumable
>>> documentation, which again illustrates our problem).
>>>
>>> Can you set both path_mtu and segment_mtu to whatever value your switch
>>> MTU is (1500 or 9000), confirm your outbound interface MTU is the same
>>> (1500 or 9000), and see if that changes things?  At this point, you should
>>> find that your networks get appropriate 1500/9000 MTUs on VLAN based
>>> networks and 1450/8950 MTUs on VXLAN networks, that they're advertised to
>>> your VMs via DHCP and RA, and that your routers even know that different
>>> interfaces have different MTUs in a mixed environment, at least if
>>> everything is working as intended.
>>> --
>>> Ian.
>>>
>>> [1]
>>> https://github.com/openstack/neutron/blob/544ff57bcac00720f54a75eb34916218cb248213/releasenotes/notes/advertise_mtu_by_default-d8b0b056a74517b8.yaml#L5
>>>
>>>
>>>> On Jan 24, 2016 20:48, "Ian Wells" <ijw.ubuntu at cack.org.uk> wrote:
>>>>
>>>>> On 23 January 2016 at 11:27, Adam Lawson <alawson at aqorn.com> wrote:
>>>>>
>>>>>> For the sake of over-simplification, is there ever a reason to NOT
>>>>>> enable jumbo frames in a cloud/SDN context where most of the traffic is
>>>>>> between virtual elements that all support it? I understand that some
>>>>>> switches do not support it and traffic form the web doesn't support it
>>>>>> either but besides that, seems like a default "jumboframes = 1" concept
>>>>>> would work just fine to me.
>>>>>>
>>>>>
>>>>> Offhand:
>>>>>
>>>>> 1. you don't want the latency increase that comes with 9000 byte
>>>>> packets, even if it's tiny (bearing in mind that in a link shared between
>>>>> tenants it affects everyone when one packet holds the line for 6 times
>>>>> longer)
>>>>> 2. not every switch in the world is going to (a) be configurable or
>>>>> (b) pass 9000 byte packets
>>>>> 3. not every VM has a configurable MTU that you can set on boot, or
>>>>> supports jumbo frames, and someone somewhere will try and run one of those
>>>>> VMs
>>>>> 4. when you're using provider networks, not every device attached to
>>>>> the cloud has a 9000 MTU (and this one's interesting, in fact, because it
>>>>> points to the other element the MTU spec was addressing, that *not all
>>>>> networks, even in Neutron, will have the same MTU*).
>>>>> 5. similarly, if you have an external network in Openstack, and you're
>>>>> using VXLAN, the MTU of the external network is almost certainly 50 bytes
>>>>> bigger than that of the inside of the VXLAN overlays, so no one number can
>>>>> ever be right for every network in Neutron.
>>>>>
>>>>> Also, I say 9000, but why is 9000 even the right number?  We need a
>>>>> number... and 'jumbo' is not a number.  I know devices that will let you
>>>>> transmit 9200 byte packets.  Conversely, if the native L2 is 9000 bytes,
>>>>> then the MTU in a Neutron virtual network is less than 9000 - so what MTU
>>>>> do you want to offer your applications?  If your apps don't care, why not
>>>>> tell them what MTU they're getting (e.g. 1450) and be done with it?
>>>>> (Memory says that the old problem with that was that github had problems
>>>>> with PMTUD in that circumstance, but I don't know if that's still true, and
>>>>> even if it is it's not technically our problem.)
>>>>>
>>>>> Per the spec, I would like to see us do the remaining fixes to make
>>>>> that work as intended - largely 'tell the VMs what they're getting' - and
>>>>> then, as others have said, lay out simple options for deployments, be they
>>>>> jumbo frame or otherwise.
>>>>>
>>>>> If you're seeing MTU related problems at this point, can you file bugs
>>>>> on them and/or report back the bugs here, so that we can see what we're
>>>>> actually facing?
>>>>> --
>>>>> Ian.
>>>>>
>>>>>
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>>>>>
>>>>
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