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Guru
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Message 1 of 14

BT Broadband and MTU

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Hi folks,

 

To those forum users who are comfortable with tweeking their line connection, and have the knowledge to do so....I thought it worth a mention about MTU.

 

I know a lot of members tune MTU already, so if so, ignore this post, it's intended for those who don't know.Smiley Wink

 

MTU is nothing new, it's always been around, and it's the Maximum Transmission Unit.

The MTU runs hand in hand with another system variable called RWIN, the Windows receive page file segment setup configuration.

MTU can be fine tuned, but RWIN cannot be fine tuned in Vista, it sets it dynamically, but it can be set in XP.

 

In broad terms MTU determines the "best" packet size for data transfer across a WAN, wide area network, and thus between you, your ISP and the net in general, depending on your latency.

Windows and router manufacturers often by default made any MTU setting the value of 1500.

Any Netgear routers that were set to run on Sky broadband and other ISPs, were given the same setup configuration, however when I came back to BT from Tiscali (TalkTalk), for which my connection was tuned, I noticed a higher degree of latency on the BT network than I'd previously had on Tiscali, for a similar throughput.

 

It was only when I started to look at tuning my connection again, that I found that the BT network ran significantly smoother using an MTU of 1458, rather than 1500.

I also set my XP machine to run with same, and a RWIN of 65535, the maximum size "potentially."

 

After making these changes my throughput increased from 94% to 100% testing on DSLreports.com, with very few packet acknowledgements and clearly a more spritely performance.

The 1458 MTU value has long been a BT recommendation for data transfer in the past, but whether it is a present recommendation in BTw's config.papers I don't know, frankly I couldn't be bothered to try and find if there were any "new" present day recommendations for IP datastream products.

 

It follows that the difference in throughput between 94% and 100%, is a good improvement, and would benefit those with "slower connections" and long lines....incidently my router has seen less CRC errors since doing it also, but I have no reason to suspect there's a correlation unless FEC is not as efficient as it should be.

 

It doesn't necessarily follow IT WILL BE THE SAME RESULT FOR YOU,  but if you feel confident in trying it, for those that can, it was worth it for me.Smiley Tongue

 

Feel free to share comments, I know a lot of folk on here have the skills and do line tweeks, but there is a majority that don't, that might like to try.....BUT

 

 I TAKE NO RESPONSIBILITY FOR ANY CHANGES YOU MAKE TO YOUR SYSTEM, AND YOU GET IT WRONG, AND THEY CAUSE YOU GRIEF, AND NEITHER WILL BT, but for those that have the skills, feel free to try.  Smiley Wink

 

You can set MTU in your WAN setup, depending on router.

and you need a programme called DR.TCP to set the RWIN in XP, it's freely available, Google is your friend.

 

For those with home hubs there is an external link here:

 

USE IT AT YOUR OWN RISK, IF YOU ARE NOT SKILLED LEAVE IT ALONE.

 

http://www.stevelarkins.freeuk.com/bthomehub_softwareupgrade.htm

 

I CAN NOT VOUCH FOR THE FILES ON THIS SITE, AND NEITHER DO I KNOW WHETHER THE LATEST HUB IS SUPPORTED BY THEM, I DON'T USE ONE, YOU NEED TO CHECK YOUR OWN HUB AND READ THE INFORMATION SUPPLIED ON THE SITE, AND AGAIN, IT NEEDS SKILL, IF YOU DON'T HAVE THE SKILLSET DON'T GO THERE.

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utopian
Contributor
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Message 2 of 14

Re: BT Broadband and MTU

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Hi

 

i don't see any MTU in the HHv2... is it changeable ?

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Guru
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Message 3 of 14

Re: BT Broadband and MTU

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I can't comment either way whether its adjustable on the latest version of software, you'd have to check yourself.
I don't use a hub myself, but there is a link to update the hubs, but again I can't comment on the status of any upgrades, the string is merely about MTU, which is a configurable in most router setups....( but maybe not the HH2 )

 

My homehub is gathering dust, somewhere in the cupboard..

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TheWire
Beginner
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Message 4 of 14

Re: BT Broadband and MTU

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You change it from within Windows.

 

if you want to find out what MTU value is ok(no packet fragmentation), or be like some and set it to as close as to near max. as possible, open a command window and type, obviously the MTU value set will affect the result so if you've already changed it from 1500 to 1458, and try an MTU value of 1458 it will show fragmented packets 'packet needs to be fragmented'. You may want to do this first just to try out the value, some get it as near as possible but 1458 is pretty fine for most people here.

 

ping www.bt.com -f -l {MTU value}

 

 

For Windows 7, you can look ar what your existing MTU is opening a command window(might save time for next command and open an elavated command window) and typing:-

 

netsh interface ipv4 show subinterfaces

 

The standard MTU for Windows 7(and as I remember Vista) is 1500, you will get a list consisting of, Loopback Pseudo-Interface then, Local Area Connection if you are using an ethernet cable or Wireless Network if you are using wireless. If like me you have a few ports/devices you can easily tell which interface is being used by the amount of data it will show under the Bytes In/Bytes Out.

 

If you wish, type:-

 

ipconfig /all 

 

for a more descriptive list(well in depth).

 

Then you can set your MTU by opening an elevated command window or if you've already opened one type. The command we will be using is netsh, like so(I take it's obvious what parameters represent the necessary information in this exercise).

 

netsh interface ipv4 set subinterface "Local Area Connection" mtu=1458 store=persistant

 

or ir could be

 

netsh interface ipv4 set subinterface "Wireless Network Connection" mtu store=persistant

 

If you want to find out what this all means then it's very googable and pretty easy to understand, some go to the trouble of complex registery alterations, I do not know why but maybe there is some reason I am unaware of, this method seems to work perfectly and easily. It's a dead easy fix and results are good.

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utopian
Contributor
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Message 5 of 14

Re: BT Broadband and MTU

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thanks for that...

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PheeragHfre
Recognised Expert
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Message 6 of 14

Re: BT Broadband and MTU

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A couple of useful links for setting MTU values.

 

Tweaking your MTU settings. (2000/XP)

Changing MTU in Windows Vista

 

VPN entails further encapsulation, values such as 1430(or lower) instead of the BT recommendation of 1458 may help in such cases.

 

"I have this awful feeling someone is watching every move I make (one of my pet hates is router location tagging)." Marvin (A paranoid Android)
Jarviser
Aspiring Expert
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Message 7 of 14

Re: BT Broadband and MTU

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@utopian wrote:

Hi

 

i don't see any MTU in the HHv2... is it changeable ?


 

 

The Stevelarkins article refers to early firmwares on V1.0 hubs and is a little dated now. The user name is all wrong, for example.

The V1.0 config used to have MTU set at 1500 in the ip ini section

[ ip.ini ]
ifadd intf=LocalNetwork dest=bridge
ifconfig intf=loop mtu=65535 group=local
ifconfig intf=LocalNetwork mtu=1500 group=lan linksensing=disabled primary=enabled
ifattach intf=LocalNetwork...etc

 

This could be changed only on 6.2.2.6 or earlier firmwares where the config was user-saveable, but the 6.2.6.x firmware soon put a stop to that for "ordinary" users like me who don't want to get the soldering iron out.

 

Whether changing MTU in windows where the hub is fixed at 1500 would make any difference was always a question, but which Roger et al seems to have answered now, as there appears to be some improvement in throughput. Maybe the V2.0 is set at 1458 now? Anyone with a soldering iron?

_________________________________________


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Jarviser
Aspiring Expert
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Message 8 of 14

Re: BT Broadband and MTU

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Actually to answer myself, ADSLNation.com has the following explanation...

 

As the BT ADSL-line uses PPPoA with L2TP encapsulation the maximum value is 1458. 'Normal' Ethernet MTU is 1500 bytes, but after encapsulating in PPPoA you have only 1458 bytes left. The encapsulation itself effectively steals 42 bytes from your frames.

Setting the MTU value manually to 1458 will overcome these issues.

 

I assume the above ip.ini refers to ethernet setting. 

 


_________________________________________


..... Jarviser's Home Hub Index
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Guru
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Message 9 of 14

Re: BT Broadband and MTU

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Thanks Jarviser for the embellishment on the subject...and PheeragHfre for the links  Smiley Wink

 

Clearly some will benefit, others maybe not, it's one of those unknowns, you don't know till you try...

But if it does it for you...it can be well worth it.

 

 

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PheeragHfre
Recognised Expert
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Message 10 of 14

Re: BT Broadband and MTU

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When using VPN/SSL extensively an MTU value of around 1430 might be more advisable?

 

There is I believe another layer of encapsulation which would increase the fragmentation level.

"I have this awful feeling someone is watching every move I make (one of my pet hates is router location tagging)." Marvin (A paranoid Android)