MTU and video, why doesn't it show video on an incorrectly set mtu value?

Question

Computer networks

Mtu is set incorrectly, sites are loading, dns resolves, video on YouTube does not show
Set the value correctly, the video worked

Answer 1

All this, as a rule, does not affect the DNS, tk. the DNS packet size is usually below the MTU. The operation of TCP connections with an incorrectly set MTU depends on the set TCP MSS parameter on the server side (many large services deliberately lower it, because an incorrectly configured MTU is a very common problem) and the functioning of blackhole router detection algorithms (PMTU) in the client and server systems Black Hole Detection / PMTU probing) which are enabled by default in Windows and are available on almost all systems. Therefore, you can not notice problems with MTU for a long time or notice them only in certain situations, for example, when submitting a form with a large amount of data or uploading a file to the server.
If the problem surfaced on YouTube, most likely this means that the default MSS is used there and the PMTU Black Hole Detection algorithms for video streams are not used, possibly due to the use of some special hardware with reduced TCP support.