Open Stack

On 11 July 2016 at 11:12, Chris Friesen <chris.friesen at windriver.com> wrote:

> On 07/11/2016 10:39 AM, Jay Pipes wrote:
>
> 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?
>>
>
> It is my understanding that using the max path MTU is ideal, but that not
> all software does path MTU discovery.  Also, some badly-designed security
> devices can mess up PMTUD.


Ignoring PMTUD cases, which are routed (the original question was L2
transit), there are several use cases for specific networks having MTUs
other than 9000.  Maybe you're talking to a device on a provider network
that has a 1500 MTU not under your control, for instance.  It's also
reasonable to have a cloud with a high internal MTU and a low external
network MTU - maybe you have control over your own domain but not the whole
network in which you're situated.

That is *not*, in fact, the same as having no MTU advertisement (but it
seems to address the use case originally mentioned; if you could be
selective about the MTU you used and the advertisements were corrected
accordingly you could simply choose 1500).  There are also ports that need
an MTU - router ports, DHCP ports - that are not receiving it via MTU
advertisement, so  turning advertisement off and letting nature take its
course doesn't work for everything.

The original MTU spec [1] - which never got fully implemented - detailed
the intent, which was that OpenStack would choose a sensible default MTU
value and advertise it, and that you could override that value for your own
purposes.  I still think we need to implement the missing bits.
-- 
Ian.

[1]
https://specs.openstack.org/openstack/neutron-specs/specs/kilo/mtu-selection-and-advertisement.html
- the bit that wasn't completed was 'the tenant can request a specific MTU
on a network' - which, incidentally, is not 'the network will not pass
packets bigger than X', but simply 'OpenStack and the tenant will agree
that X is the MTU that ports shall use on that network; packets that size
or less are guaranteed to pass over the L2 domain unmolested, and where an
MTU is set on a port or advertised to it that will be the one'.  If you
want a lecture on cloud MTUs I can give one.  But you really, really don't.
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