Well, I've no concrete proof that WinXP automatically adjusts. It's just that it kinda sticks in my mind from way back. You could well be right.

Thanks for that link to the Microsoft KB article on MTU. I notice, though, that that KB article applies only to the Pro version of WinXP.

Zebedee,

Googling, I've found this:

http://setiathome.berkeley.edu/forum_thread.php?id=2...

It dates back to 2005 but nevertheless it appears to say that Windows XP does work out and set its own MTU. If you change the Windows MTU value, Windows will ignore that change, anyway. (I do vaguely remember Win2K reacting like that, when I was using Win2K some years ago. I ended up getting in a bit of a mess, trying to use Dr TCP to change it, only to find that the Dr TCP value was always ignored). So unless WinXP has undergone changes in that dept since then, it does look as though I was correct.

Well, maybe they're right - who knows ?

One good piece of advice in there anyway - turn MTU path discovery ON.

Yes, that'd be fine, but there's no facility in my WinXP's Registry for doing that. I've tried looking in a number of different likely places for that setting in the Registry but, in WinXP Home, an MTU Discovery folder of any sort does not seem to exist. That could mean that the Windows MTU is indeed fixed at 1500 (and therefore there should never be a need to alter it) but, equally, it could mean that the Windows MTU does change dynamically (and so, again, there's no point in manually changing it). Possibly, things are done differently in WinXP Pro.

As I've pointed out, I'm not actually wanting to change the Windows MTU, I'd just like to know what current value it's using on my machine, so that I can then judge the corresponding value to configure in the router.

You rarely see the divisor of 48 in discussion about IP MTU because this value is entirely specific to IP data encapsulated in a PPPoA session being carried on an ATM transport layer. I'd hoped my explanation made the reason for using that divisor clear.

Most MTU discussions are based on information gleaned from articles found littering the tinterweb which primarily revolve around optimising MTU for IP transit over Ethernet.

Based on the fact you supplied that you experience fragmentation with an MTU anything over 1430, why would you be considering trying values greater than that?

It is basic maths that determines that 1390 is the highest IP MTU you can run IF you want to fully utilise the DSL part of your end to end connection at the same time as avoiding fragmentation. No theory there, just simple numbers.

The theoretical aspect refers to whether trying to optimise the ATM transport in this way will result in any real world benefit in terms of increased throughput. In theory it should. In my own experience it does. Will it for everyone - most likely not as other factors come into play.

I think you are also heading down a deadend looking for what XP has your MTU set to currently in order to determine what to set your router too. ~correction here~> XP automatically configures its RWIN value not its MTU so unless you have configured something different it will be at its default of 1500.

edit - for correction

Edited by deleted (Wed 18-Nov-09 18:48:02)

Path MTU Discovery is fundamentally broken on the internet as a whole due to the number of core routers and devices that either ignore it or actively drop the ICMP traffic related to it. It's worse than useless.

The MTU limitation you have is going to be on your PPP connection to your ISP. Worry about not fragmenting over that section of your connection and everything else will be fine.

In reply to a post by meditator:

Yes, that'd be fine, but there's no facility in my WinXP's Registry for doing that.

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

EnablePMTUDiscovery ....... set to 1. (default value)

Which version of Windows are you using, zebedee? In my version of WinXP, there is no such parameter as EnablePMTUDiscovery in that location.

It's present in Windows Media Centre Edition, built upon XP Pro; can't speak for XP Home.

Tho' I've never seen my Win's MTU value changed from its TCPOptimizer-set value of 1454.

Edited by XRaySpeX (Thu 19-Nov-09 00:33:23)