Finding the correct value for path MTU might come in handy if you experience problems with VPN connections, video conferences or port knocking. I stumbled over this around 2014 when I've been a business customer at Kabel Deutschland/Vodafone.
I assume you are using Linux. On debian and it's derivates just install tracepath via apt.
apt -y install iputils-tracepath tracepath www.google.de
1?: [LOCALHOST] pmtu 1500 1: pfSense.xxxxxx.xx 1.515ms 1: pfSense.xxxxxx.xx 0.803ms 2: fritz.box 1.284ms 3: 192.0.0.2 1.505ms pmtu 1460 3: ip53a9b731.static.kabel-deutschland.de 11.703ms 4: ip5886edbb.static.kabel-deutschland.de 19.727ms 5: 145.254.3.60 16.172ms 6: 145.254.2.175 15.456ms asymm 7 7: 145.254.2.175 16.805ms 8: no reply Too many hops: pmtu 1460 Resume: pmtu 1460
As you can see, the MTU drops to 1460 at hop 3. That's because Kabel Deutschland/Vodafone is still running DS-Lite, an IPv6 transition mechanism, in 2021. That causes your IPv4 Packets to get wrapped into an IPv6 packet which causes fragmentation.
There is another command which can be used to find the path MTU. It's traceroute:
traceroute --mtu www.google.de
traceroute to www.google.de (172.217.22.195), 30 hops max, 65000 byte packets 1 pfSense.xxxxxx.xx (10.10.50.1) 0.954 ms F=1500 0.646 ms 0.707 ms 2 fritz.box (192.168.2.1) 1.123 ms 1.054 ms 1.086 ms 3 83-169-183-49-isp.superkabel.de (83.169.183.49) 12.923 ms F=1460 12.524 ms 11.722 ms 4 ip5886edbb.static.kabel-deutschland.de (88.134.237.187) 14.579 ms 14.417 ms 16.797 ms 5 145.254.3.60 (145.254.3.60) 13.371 ms 13.320 ms 13.748 ms 6 145.254.2.175 (145.254.2.175) 17.586 ms 14.530 ms 17.386 ms 7 145.254.2.175 (145.254.2.175) 27.975 ms 20.617 ms 13.480 ms 8 72.14.195.160 (72.14.195.160) 24.337 ms 18.965 ms 21.535 ms 9 108.170.247.113 (108.170.247.113) 21.433 ms 15.377 ms 108.170.247.97 (108.170.247.97) 16.248 ms 10 108.170.234.217 (108.170.234.217) 17.219 ms 209.85.142.129 (209.85.142.129) 16.759 ms 17.982 ms 11 muc11s01-in-f3.1e100.net (172.217.22.195) 15.696 ms 15.307 ms 17.975 ms
At hop 3 you will see “F=1460” which indicates the mtu dropping to 1460.
~~DISCUSSION~~