topic Re: MTU in Archived Posts

MTU

LeeM87 — Thu, 20 May 2021 21:33:56 GMT

I have the EE home router 2, model number b625-261 And the MTU is lower than the my Xbox series s would like. How can I increase the MTU, I'm struggling to find the setting. Thanks

Re: MTU

Captgeneralmark — Thu, 20 May 2021 21:45:18 GMT

@LeeM87 On my Brightbox 2, the MTU settings are on the advanced/broadband settings page. The maximum value allowed is 1492, Which is what mine is set to.

Re: MTU

XRaySpeX — Thu, 20 May 2021 22:05:44 GMT

You're unlikely to find an MTU setting on an EE mobile router. They are dumbed-down cousins of real routers.

Have you looked amongst its WAN settings?

Re: MTU

mikeliuk — Fri, 21 May 2021 06:39:02 GMT

In general, do not change the MTU on any network interface or vlan (i.e. on switch) which must send and receive packets from the internet as the default of MTU 1500 would be the most appropriate setting.

I wouldn't care about a few bytes off that which may be to account for various packet headers. If the default lower value is correct and you increase it, you may get packet fragmentation and worse performance, or see some traffic dropped entirely.

For internal vlans and interfaces which never need to send or receive packets from the internet, MTU 9000 on switches is fine but you might want interfaces with slightly lower MTUs to account for headers (e.g. TCP, vxlan, vlan).

Edit: your best bet is to lower the MTU on you device to match the service provider's switch/router as the default value is likely to be correct and will work and any higher value could cause a hard to debug problem.

Re: MTU

mikeliuk — Fri, 21 May 2021 06:45:42 GMT

Hi @Captgeneralmark ,

A search online for sizes of TCP, vlan, and vxlan headers shows that 8 bytes is the size of a vxlan header so that default value has likely been chosen to allow the use of vxlan (or similar) on the service provider's network. 😃

Re: MTU

XRaySpeX — Fri, 21 May 2021 14:51:25 GMT

The MTU size for BTw-based BB is 1492. Dunno what it is for mobile networks.

Re: MTU

LeeM87 — Fri, 21 May 2021 19:02:37 GMT

Mine is 1320 and the Xbox needs 1380 I believe.

Re: MTU

LeeM87 — Fri, 21 May 2021 19:05:29 GMT

I don't seem to have a WAN settings. Will EE likely change the MTU for me? It's there anything I can do hardware or software to change this?

Re: MTU

mikeliuk — Fri, 21 May 2021 19:37:50 GMT

The MTU of interfaces on devices should be less than or equal to the value on routers and switches.

I can't think of any good reason why a service provider would supply a router misconfigured but 1320 feels lower than might be reasonably justifiable. I see some online references suggesting the Linux tracepath command can detect MTU changes on a route but it's probably better to wait for someone who knows how high the router MTU could be set to.

If you wanted to experiment, you could set the router to 1492 and tracepath to some known destination and lower the router MTU to the lowest MTU found on the route. Packets which are less than the MTU will be transmitted just fine so you're unlikely to break anything.

Having just done a test, MTU 1450 (Correction: actually full MTU 1500 as below) traffic passes for 4G EE SIM in a Netgear M1.

This is based on Red Hat documentation https://access.redhat.com/solutions/2440411

ping -M do -s 1422 domain.com # MTU is 28 bytes higher

Edit: it's disappointing you don't have the option to change that as I get the feeling someone slightly incompetent took a guess and has negatively impacted a lot of people with no current way to fix.

Edit2: correction, I can get a full MTU 1500 ping to pass. The previously mentioned limit was due to the target.

ping -M do -s 1472 domain.com # passes

Re: MTU

XRaySpeX — Fri, 21 May 2021 22:48:35 GMT

For Windows you would use:

ping -n 1 -f -l size bbc.co.uk

If size was too big it would respond:

"Packet needs to be fragmented but DF set.".

You would use a loop counting down to discover the largest packet size that didn't fragment, viz:

for /L %i in (1472,-1,1440) DO ping -n 1 -f -l %i bbc.co.uk

The 1st one that doesn't "Packet needs to be fragmented" is the largest packet size wanted (For BTw-based BB it's 1464).

Then add 28 to get the MTU for the network (For BTw-based BB it's 1492).

Re: MTU

XRaySpeX — Fri, 21 May 2021 22:47:55 GMT

@mikeliuk wrote:

Edit: it's disappointing you don't have the option to change that as I get the feeling someone slightly incompetent took a guess and has negatively impacted a lot of people with no current way to fix.

Don't understand. What does that mean? What is disappointing?

Re: MTU

XRaySpeX — Sat, 22 May 2021 11:49:51 GMT

By the same means the MTU for the O2 network is 1500. I would expect EE mobile network to be the same.

(Fixed BB is 8 bytes lower for the PPPoE/A overhead).

Re: MTU

LeeM87 — Sat, 22 May 2021 12:10:12 GMT

I have called EE and the guy said he would change MTU, I personally don't think he knows what it was. He said he changed some settings to boost my connection and started talking about an antenna.

However, since he has 'boosted' my speeds I have gone from 40-120mb download and 20mb upload to 5-10mb on both.

I asked him to revert the changes and he said that there are problems in my area. I said the connection other than the MTU was fine before this call. He said that he has raised a ticket about the area fault.

How has it gone from an MTU query to an area fault within one call?

Guy couldn't get off the phone quick enough.

Now I'm stuck with low speeds and low MTU 😞

Re: MTU

XRaySpeX — Sat, 22 May 2021 12:26:46 GMT

Can you check what your MTU actually is using my method with the loop:

for /L %i in (1500,-1,1460) DO ping -n 1 -f -l %i bbc.co.uk ?

I can't believe it's as low as 1320.

Re: MTU

mikeliuk — Sat, 22 May 2021 13:16:18 GMT

Hi @LeeM87 ,

I think that's the most impressive thing I've heard all month! 😂

The MTU setting which needed increasing is likely a setting on your router and as you saw no option to change it, I didn't expect it could be done remotely.

I always remind my colleagues that things go wrong and disasters happen because someone was trying to improve something and make it better. No one starts out their day planning to cause some global issue.

As the engineer was unable to revert the change, either it was an amazing coincidence or it should be ruled out that the change didn't exacerbate or cause the area issue.

The best you can do is use the hints on this thread to probe what the measured MTU is. Hopefully it will be unchanged at the low value you quoted. Maybe it has increased to near 1492 or 1500. Unlikely but it might have dropped lower than your quoted low value which would have taken quite some incompetence to have accomplished.

Best not to panic and take the service provider which shall not be named's word that it is an area issue which the engineer had run away to solve. Presumably if you register your location on the coverage query page, you will be notified when the area issue is fixed.