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Why was network MTU lowered from 1000 to 512?

At one point MTU was raised to 1024 (mentioned in 4.9 release notes: https://forums.unrealengine.com/showthread.php?82266-Unreal-Engine-4-9-Released!&highlight=mtu )

But then it was lowered again to 512:

Engine/Source/Runtime/CoreUObject/Public/UObject/CoreNet.h

Here is the git commit where it happened:

https://github.com/EpicGames/UnrealEngine/commit/6ab7fcd2462da40fb63e4454788013741526affa

It says:

“Added comment to enums in CoreNet, changed MTU back to 512 until discussion on this topic has finished”

What were the downsides? Any further conclusions on this? I’m sending some small unreliable video data and benefit from a bigger MTU. I’ve set it to 1024 for now, but what are the ways that this could bite me?

Funny enough, not far down from the 512->1024 change there is:

“Changed MTU back to 512 from 1000 as discussions of MTU changes are still happening.”

My guess is there was something bad happening, they reverted to see if it helped.

Now in 4.19 it seems to be back… maybe from the system call pressure from spectre/meltdown on Fortnite servers? Or just Fortnite networking in general?