PMTU = 1518 over local network at 1500 MTU

[Guus Sliepen]

On Tue, Feb 25, 2014 at 01:53:51PM +0100, Florent Bautista wrote:

> I have two nodes, connected to a switch, using Tinc 1.1 from git.
> 
> They connect each other with sptps, and to other nodes in the Internet
> with old protocol because they have Tinc 1.0.
> 
> There is no problem with remote nodes, but between my 2 local nodes,
> they see 1518 PMTU. But local network is 1500 MTU !!! So nodes can ping
> each other but larger data does not go.

That is strange, when the network has a MTU of 1500, tinc should detect a PMTU
of 1451. I cannot reproduce the problem you have. Could it be that those two
nodes are running inside a VM or that you have jumbo frames enabled on the
local network?

> I did -d5 in Tinc daemon but I see nothing interesting:
> 
> root at test2:~# cat /var/log/tinc-coppint.log | grep PMTU
> 2014-02-25 13:31:18 tinc.coppint[413140]: Increase in PMTU to sllm1
> (192.168.0.200 port 655) detected, restarting PMTU discovery

Hm, that is somewhat interesting, normally you would not expect the MTU of the
local network to change.

> Host config contains only Address, Cipher, Digest and MACLength; but I
> think that 3 last are rewritten by sptps.

That's correct.

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