Computers & Hardware > Networking

A & A 4G / 3G data SIMs - MTU

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Weaver:
As mentioned in another thread, I have discovered that the MTU for IPv4 through two of my AA 4G / 3G data SIMs is only 1440 bytes, yet the service is advertised as offering a full 1500 MTU.

I have one 4G SIM in an iPad and a web-based test tool reports a low MTU. I also have a Huawei 3G dongle provided by AA containing one of their SIMs and this dongle is plugged into a Firebrick router for use as a backup / failover device which will take over in the event that all the DSL links go down. The Firebrick shows MTU 1440 for this dongle too.

So what is going on? Has anyone else seen this behaviour? Or seen a full 1500 byte MTU?

chenks:
are you sure the wording says it "offers" full 1500 MTU or simply supports "up to" 1550 MTU ?
i suspect it'll be the latter.

what did AA say when you asked them?

Weaver:
https://aa.net.uk/telecoms-mobile-data.html Says "This data SIM provides fixed IPv4 address mobile data with full 1500 byte MTU" towards the end of the page. In fact it says 1500 byte MTU three times I think.

I emailed AA support yesterday but I haven’t had a reply yet apart from the usual automatic acknowledgement.

My iPad with a 3G SIM in it suggests to me an MTU of 1450. It fails with an IPv4 ping larger than 1422, so is it 1422 + 20 + 8 ?

chenks:
the bit i picked out from their text was

"Full 1500 byte MTU is supported."

to me, that's not the same as "1500 MTU provided", only that it's supported.
do your devices support 1500 MTU?

also from their text - "Note that a reduced MTU is required unless you have direct baby-jumbo links to us in London (e.g. via datahop)."

Weaver:
I see your point. I see the distinction. You are wise to be cynical.

It could be that the iPad is being weird when it reports an MTU of 1450.

In the case of the Firebrick though, it’s a dongle that AA supplied in a Firebrick that they supplied, so it would be very dishonest and misleading sophistry to claim an MTU of 1500 is only ‘potentially’ available rather than actually supplied in a real service in a real common use-case where it is all their kit. This is from an officially and literally ‘We have a policy of no bullsh*t’ company.

The Firebrick is reporting the low MTU, which doesn’t match that obtained from the iPad - dongle is quoted as 1440 and the iPad seems to be 1450.

Has any other dongle user seen this? Or Firebrick user?

I should ask other Apple iPad users about it too.

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