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~~