4G / 3G data MTU

My Firebrick says that I have an MTU of only 1440 over AA 3G when using an AA-supplied Huawei dongle. Tests using IPv4 seem to confirm this. I think AA�s website says the MTU is 1500 for AA mobile data SIMs?

Also, one of my iPads has an AA 4G SIM fitted and IPv4 ping tests seem to indicate an MTU of 1450.

Is anyone else seeing this?

Reduced MTU is a pain. I am talking to AA support about this, but have not had a useful reply yet, could be they are still looking into it

This is the reply I got from support

It's a hard question to answer, our web site may be incorrect.

It depends on what native MTU is being used on the mobile back-haul network, from the radio equipment up to the GGSN, where the tunnel is decapsulated. If the back-haul MTU is at least 1512 bytes, the MTU can be a regular 1500 bytes.

However some devices deliberately lower the MTU announced to the mobile equipment to 1488 or even lower so that the back-haul's MTU doesn't need to be changed.

Slower connections trying to handle a high MTU can cause a bottleneck and lead to less efficiency than greater though. That's when fragmentation occurs.

So smaller MTU rates may not necessarily be a bad thing.

If I get time today I will try and replicate the setup here and compare results.