Re: LTE modem where I can control the MTU

[Dovid Bender <dovid () telecurve com>]

I currently have an airlink that is connected directly to a raritan console
server. The public IP sits on the raritan. The airlink does not seem to
have any MTU options. Ideally I would change the MTU on the interface of
the LTE modem wich would force the raritan to send all data < 1400 bytes
per packet. I never thought about the reverse so we may need something that
would tinker with the MSS as well.


On Fri, May 1, 2020 at 11:01 AM Phil Lavin <phil.lavin () cloudcall com> wrote:

> > 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