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MTU Problems

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MTU Problems

Any time I setup or reinstall a new OS on any of my PCs I always need to change the MTU to 1492 from the 1500 default in linux and windows. If I don't, then a fair number of sites don't show up, while other sites show up quickly as normal.

I remember an issue a long time ago about MTU, and how if you were on BT's network you'd need to use 1492, but I thought that was all resolved and that 1500 would worked fine.

It doesn't bother me having to do this but I'm just curious to know why.

Has anyone got an explanation for this?

Thanks
5 REPLIES
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MTU

Hello caruth,

OK! I've just read through your posting, now as far as I can remember (and I'm really digging down into the back of my brain) the reason the MTU value had to be changed was due to continuing problems with connections & network traffic which was passing through or being handled by BT's RAS server's(remote access servers).

If I remember correctly these RAS servers slowed down or even delayed traffic because they couldnt cope with packets of the an incompatible size which meant packets fagmented or failed to arrive in tact or in some cases never arrived at all. It was later found that if users on F9, PN & FOL changed the MTU value this helped solve the problem and we've all being tweaking the MTU since apart from which it does seem to improve speed & performace over ADSL.

**Although F9 and many people recommend a utility Dr.TCP for changing the MTU, I found what I consider to be a better tool on a website http://www.speedguide.com which is called TCPOptimiser its a freedownload (freeware) and it adjusts the MTU for your specific connection & conditions. It does require a live BB connection too do this though & a quick reboot afterwards to enable the new value, but it can be done very quickly and my connections performance was greatly imrpoved bye it useage. So I highly recommend TCPOptimiser to you.

**Yes! you can adjust the MTU value yourself if your very clever, & familiar with editing the windows registry but its not easy the value is a REG_DWORD & so you cannot just write in the value in normal alphanumerics (the MTU value is written in either HEX or Decimal looks abit like 0x00000DFG).

Ivan


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F9 FOL Forum Moderator
F9 Broadband Premier 2MB User
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MTU Problems

Thanks for that Ivan, was just curious about the reasons.

I normally just modify the reg & do an ifconfig eth0 mtu 1492 for the linux box in the startup scripts, but I do remember using a program called TweakDUN back in the days when everyone was trying to squeeze every possible byte of speed out of their 56k (v.92 of course) modems...

What suprised me was the sudden difference in performance when the MTU is dropped only 8 bytes to 1492. This seems to work for all sites with a negligable degredation in performance over lan.

Thanks again
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MTU Problems

cyteck, i know i have probably asked you this before but what are your MTU settings? when i use the optimiser it gets stuck at 1465 bytes so i cant get a final reading.

I have found where I can chage the MTU (and two others) in the router settings (only taken me a year Wink) so I could change it there... mind you you still get stupid fragmentation if I do it there.... :?

Jarv
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MTU

Hi jarvis,

Hope the following might make some sense to you, dont know how helpful this will be for you though as each persons settings will be different and you cannot compare one MTU value by another. (well thats my personal opinion) dont forget I'm on a 2MB service.

My Computer\Hkey_Local_Machine\system\controlset002\services\tcp/ip\parameters\MTU Reg_Dword = 0x0000ffd (4093)

And
My Computer\Hkey_Local_Machine\system\controlset002\services\tcp/ip\parameters\interfaces{78hjbvb;kb;kv bvjk\dvkjnvkjv} MTU Reg_Dword = 0x00005dc (1500)

Ivan
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F9 FOL Forum Moderator
F9 Broadband Premier 2MB User
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MTU Problems

thanks again cyteck!