Kitz ADSL Broadband Information
adsl spacer  
Support this site
Home Broadband ISPs Tech Routers Wiki Forum
 
     
   Compare ISP   Rate your ISP
   Glossary   Glossary
 
Please login or register.

Login with username, password and session length
Advanced search  

News:

Author Topic: A & A 4G / 3G data SIMs - MTU  (Read 480 times)

Weaver

  • Addicted Kitizen
  • *****
  • Posts: 7116
  • Retd sw dev; A&A; 4 × 7km ADSL2; IPv6; Firebrick
A & A 4G / 3G data SIMs - MTU
« on: October 30, 2018, 07:35:28 AM »

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?
Logged

chenks

  • Reg Member
  • ***
  • Posts: 657
Re: A & A 4G / 3G data SIMs - MTU
« Reply #1 on: October 30, 2018, 08:02:43 AM »

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?
Logged

Weaver

  • Addicted Kitizen
  • *****
  • Posts: 7116
  • Retd sw dev; A&A; 4 × 7km ADSL2; IPv6; Firebrick
Re: A & A 4G / 3G data SIMs - MTU
« Reply #2 on: October 30, 2018, 08:32:52 AM »

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 ?
Logged

chenks

  • Reg Member
  • ***
  • Posts: 657
Re: A & A 4G / 3G data SIMs - MTU
« Reply #3 on: October 30, 2018, 08:35:45 AM »

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)."
Logged

Weaver

  • Addicted Kitizen
  • *****
  • Posts: 7116
  • Retd sw dev; A&A; 4 × 7km ADSL2; IPv6; Firebrick
Re: A & A 4G / 3G data SIMs - MTU
« Reply #4 on: October 30, 2018, 07:22:26 PM »

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.
Logged

burakkucat

  • Global Moderator
  • Senior Kitizen
  • *
  • Posts: 26172
  • Over the Rainbow Bridge
    • The ELRepo Project
Re: A & A 4G / 3G data SIMs - MTU
« Reply #5 on: October 30, 2018, 08:56:43 PM »

It is certainly annoying -- even I can see why -- but I am unable to assist.  :(
Logged
:cat:  100% Linux and, previously, Unix. Co-founder of the ELRepo Project.

Please consider making a donation to support the running of this site.

Weaver

  • Addicted Kitizen
  • *****
  • Posts: 7116
  • Retd sw dev; A&A; 4 × 7km ADSL2; IPv6; Firebrick
Re: A & A 4G / 3G data SIMs - MTU
« Reply #6 on: October 31, 2018, 12:48:56 AM »

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.
Logged

Weaver

  • Addicted Kitizen
  • *****
  • Posts: 7116
  • Retd sw dev; A&A; 4 × 7km ADSL2; IPv6; Firebrick
Re: A & A 4G / 3G data SIMs - MTU
« Reply #7 on: November 02, 2018, 02:53:39 PM »

This is the reply I got from support

Quote
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.
Logged