What is a minimum MTU?

[Rob Townley]

On Wed, Oct 13, 2010 at 2:21 PM, Rob Townley <rob.townley at gmail.com> wrote:
> On Wed, Oct 13, 2010 at 10:25 AM, Guus Sliepen <guus at tinc-vpn.org> wrote:
>> On Tue, Oct 12, 2010 at 05:44:47PM -0500, Rob Townley wrote:
>>
>>> I know MTU is Maximum Transmission Unit, but what is a "minimum
>>> Maximum Transmission Unit"?
>>> Shows up in tinc.log as "Packet for PC5 (ip.x.y.z port 655) larger
>>> than minimum MTU, forwarding via TCP".
>>
>> Tinc tries to discover the actual MTU of the path between two nodes by sending
>> probe packets of various sizes.  It can detect a lower bound and upper bound
>> for the true MTU. The lower bound (minimum MTU) is the biggest probe that has
>> been succesfully sent and received back, the upper bound (maximum MTU) is
>> decreased whenever a RFC compliant router sends an ICMP Packet Too Big message
>> in response to a probe that is larger than the true MTU.

So maximum MTU is the theoretical maximum packet size which may not
include all headers needed along the route.
The following terminology could substitute for "minimum MTU":
  - "true MTU"
  - "empirical MTU"
  - "actual MTU"
  - "known MTU"
  - "tested MTU"



>> Tinc will only send UDP packets that are smaller than the minimum MTU to the
>> destination, because there is no guarantee that larger packets will arrive.

Makes perfect sense.



>> If UDP communication is not possible, no probes will succeed, so the minimum
>> MTU will be 0 and the maximum MTU will be 1514 by default, and all packets are
>> thus forwarded via TCP, as it should.
i notice MYSELF (the Linux sever) has a minimum MTU of 0 at times and
will watch for
a corresponding increase in latency.  Not sure why it would change,
but it does.


>> Tinc keeps sending probes after the real MTU has been determined (by default
>> once a minute) to check whether UDP communication is still possible.

If those are the PING PONG probes, i do not see them once a minute.
What setting is determines the 1 minute interval?



>> the MTU probes to fail, resulting in tinc falling back to TCP.  It should
>> normally try to reestablish UDP connections after an hour.
>
> Is there some way to ask tinc to reestablish UDP connections much more
> frequently?
> The Tablets (including wireless card) may go into a low power mode on
> battery, but the network itself is never down.


Guus, after printing out your email and reading while on walk outside
in beautiful weather here in Omaha, NE, i realized it made perfect
sense and that you wrote it extremely well.  Thank You.