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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.

[burakkucat]

It is certainly annoying -- even I can see why -- but I am unable to assist. 

[Weaver]

I am talking to AA about it. It seems there is a chance that some SIMs or accounts or whatever are not showing the reduced MTU as AA’s logs show that Janet’s iPad (they looked at the wrong SIM) which uses an AA 4G SIM was connecting and an MTU of 1500 was reported by PPP.

I have emailed them with more details and they have not had chance to reply yet.

[Weaver]

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.

[Weaver]

Reviving an old topic.

I repeated some MTU tests, using that buggy http://www.letmecheck.it/mtu-test.php (or whatever) MTU test website and then again using loads of pings by hand sent by my Firebrick router itself to endpoints within the AA network.

(Health warning regarding that nutty website: see https://forum.kitz.co.uk/index.php/topic,22654.msg385835.html#msg385835)

Firstly I tried the AA 4G/3G service into my iPad - with an AA-supplied SIM. Recall that this AA service is based on the Three network with ‘AQL’ as an intermediate carrier, who seems to be an interconnection service provider who nails the whole thing together. Using the MTU test website with IPv4 pings, this time I get a different result from before - a ping max of 1472 so 1472+28 bytes of ping overhead = 1500 bytes IPv4 PDU MTU. So at some point in the last few months the whole setup of this entire end-to-end AA service itself has been sorted out so it can carry 1500 byte IPv4 PDUs now in the case of the iPad with AA SIM.

I could not try IPv6 using this tool because the website is completely broken in that it falls over if you even have an IPv6 address iirc, and goes nutty in that case even if you merely select the IPv4 test never mind trying the IPv6 one. Am I remembering correctly here?

Then I retested the combination of my AA-supplied 3G Huawei ‘dongle’ USB NIC in my Firebrick with the AA/Three service. I’m assuming that if the results should turn out to be different from the first iPad-direct test then the 3G USB NIC would have to be guilty. Using manual ping tests from the Firebrick’s diagnostic tools facility to bottomless.aa.net.uk using IPv4 only, this time I got a weird result of 1468 bytes. So it seems that the 3G USB NIC is indeed restricting MTU. Damn.

I can’t recall with certainty but I thought the result for this test before was not the same, at 1440. The fact that there is this 28 byte discrepancy is more than a tad suspicious and make me think that I either screwed things up last time or this time. If this result is correct, how does one get an IP PDU MTU of 1468?

[Weaver]

The Firebrick has a show dongle command in the CLI and a dongle status report in the web UI. These list the interface as having MTU 1440, and I’m assuming that means either IP PDU MTU or PPP PDU MTU, one or the other.

I don’t know where the 1468 came from. I will have to recheck it.

[Tim]

I have the same problem.  Huawei usb modem, aaisp, MTU won't go above 1440.  I'm using a Linux machine as my router, so I'm sure of the numbers.

I'm at early stages of sussing it out.

[Weaver]

Hi Tim, welcome to the forum. Good to know it’s not just me misreading things.

Would you mind telling me what model your usb modem is? Can it do 4G without all the NAT nonsense?

[phr]

Recent iOS versions fall back to MTU 1450, if the mobile network doesn't provide explicit MTU value during initial handshake in Protocol configuration options - see:

https://nickvsnetworking.com/mtu-in-lte-5g-transmission-networks-part-1/
https://nickvsnetworking.com/mtu-in-lte-5g-transmission-networks-part-2/

In such case, all iPhones and iPads are limited to 1450 bytes 

Active MTU can be checked by some iOS apps - e.g.

https://apps.apple.com/us/app/it-tools-network-analyzer/id324054954