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.la...@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 > > >
