LTE modem where I can control the MTU

Phil Lavin phil.lavin at
Fri May 1 15:01:26 UTC 2020

> We have VZ wireless in the data center as a backup to our core infrastructure. We have an issue where if packets have a large MTU they seem to die. Does anyone know of a good 4G modem where I can set the MTU on the cellular connection?

I suspect it's a bit more complex than just changing LTE MTU. The time when MTU matters in this situation (larger packets getting lost) is when DF bit is set on the packet - that's the case for all TCP data packets and it often crops up during TLS negotiations. Is that what you're seeing?

Consider the following scenario:

Your Server-------Switch-------LTE Router-------T'Internet-------Another Network-------Other Server

If both "Your Server" and "Other Server" are connected to their relevant local networks at 1500 MTU then they will negotiate a TCP MSS slightly below 1500 bytes (probably 1460 bytes???) because they have no concept of the path MTU. If the MTU between LTE Router and the Internet is below 1500 then the LTE Router will drop larger packets because it's not allowed to fragment them.

The solutions are:

:: Have the LTE Router reduce the MSS of TCP negotiation packets as they flow through it. This is the approach normally taken by any cheap DSL router so I'd think your current LTE router should be able to do this also
:: Have the LTE Router strip the DF bit from packets and fragment them anyway. I don't have any particular experience/opinions either way on this one so I'll leave it to others to comment/berate 
:: Implement path MTU discovery so your devices are aware of the path MTU and so set their MSS accordingly

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