Reliability of Tinc under load

[Dominik George]

Hi,

> Couldn't this be a PMTU issue?  My guess is that Tinc's UDP packets are
> being silently dropped on the path when they reach a certain size.  From
> my experience, Tinc is generally clever about figuring out the optimal
> packet size, but you never know.

I do not really know how to find out if that is (part of) the issue, I
did my best like so:

# ip link show tap0
7: tap0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
state UP mode DEFAULT group default qlen 100
    link/ether be:12:4a:bd:4b:40 brd ff:ff:ff:ff:ff:ff

# ping -M do -s 1492
terra                                                                  
PING terra.lan.teckids.org (172.30.254.254) 1492(1520) bytes of data.
ping: local error: Message too long, mtu=1500
ping: local error: Message too long, mtu=1500
ping: local error: Message too long, mtu=1500

# ping -M do -s 1400
terra                                                                
PING terra.lan.teckids.org (172.30.254.254) 1400(1428) bytes of data.
From terra.lan.teckids.org (172.30.254.254) icmp_seq=1 Frag needed and
DF set (mtu = 1381)
ping: local error: Message too long, mtu=1381
ping: local error: Message too long, mtu=1381

# ping -M do -s 1200
terra                                                                
PING terra.lan.teckids.org (172.30.254.254) 1200(1228) bytes of data.
1208 bytes from terra.lan.teckids.org (172.30.254.254): icmp_seq=1
ttl=64 time=44.0 ms


That looks strange to me. Where is that final MTU of 1381 enforced? Why
does it change over time?

Could it be possible that Tinc did its PMTU discovery magic in that moment?

I remember that the issues I described magically vanish if a ping is
running at the same time ...


Cheers,
Nik

-- 
Dominik George (Vorstandsvorsitzender, Pädagogischer Leiter)
Teckids e.V. - Erkunden, Entdecken, Erfinden.
https://www.teckids.org


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