What is a minimum MTU?

[Guus Sliepen]

On Wed, Oct 13, 2010 at 05:00:46PM -0500, Rob Townley wrote:

> >> 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.

"MYSELF" should always have a minimum MTU of 0, since a node does not send
packets to itself, and hence never performs PMTU discovery on itself. Whatever
the value is, it should not have any effect on packets to other nodes.

> >> 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 "PING PONG probes" are sent to test whether the TCP connections are still
alive.  Both TCP and UDP probes are sent with PingInterval seconds inbetween.

> 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.

Fresh air is always good :)

-- 
Met vriendelijke groet / with kind regards,
     Guus Sliepen <guus at tinc-vpn.org>
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