"Problems only arise when a PC fires bigger packets than the hole in the router allows to pass".

Yup, understood. Prior to starting this thread, my Windows would have been using 1500 and my router was using 1460. (Both figures include the 28). But now they're both 1500.

Just out of interest, are there any of you using WinXP who are not using the default MTU of 1500? If so, which value are you using, and does it definitely cater for all the websites you use?

The Netgear MTU setting is 1500

XP Home, MTU setting using TCP Optimiser is 1430

Tcp WindowSize=51480

Seems to be OK with the websites I've visited so far.


Edit >>> I'm using a fixed speed 2Mbps connection.

Edited by Apprentice (Fri 20-Nov-09 17:19:11)

In reply to a post by meditator:

Just out of interest, are there any of you using WinXP who are not using the default MTU of 1500? If so, which value are you using, and does it definitely cater for all the websites you use?

Yes, as I indicated earlier, I am using MTU=1454 calculated into the Registry by TCPOptimizer. I never have any problem with any communication with any Internet destination (it's not just Browsers that it applies to but all Net programs).

My router is set to MTU=1492, probably a hang-over from when Win XP was tuned for my previous USB Modem, when TCPOptimizer calculated MTU=1492 for XP. Looks like I never bothered to reset the MTU in the router.

EDIT: Oh, BTW, my TCPWindowSize as calculated by TCPOptimizer is the largish 520352.

Edited by XRaySpeX (Fri 20-Nov-09 17:33:51)

OK - so now both router and PC are running at 1500 - try doing your unfragmented ping test again and see if you can pass 1500 byte packets without fragmentation.

I suspect the answer will be no.

e.g. something like this.....

H:\>ping -f -l 1500 www.thinkbroadband.com

Pinging www.thinkbroadband.com [80.249.99.130] with 1500 bytes of data:

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

Ping statistics for 80.249.99.130:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

I suspect the answer will be no.

of course it will, because to test a 1500 MTU you need to ping with a 1472 packet !

Indeed so. My bad, been a long week.

I'm just interested to see what results the OP gets now everything is aligned.

In reply to a post by meditator:

"Problems only arise when a PC fires bigger packets than the hole in the router allows to pass".

Yup, understood. Prior to starting this thread, my Windows would have been using 1500 and my router was using 1460. (Both figures include the 28). But now they're both 1500.

So, basically you are back to where you were before you started fiddling with it. You should try more rewarding hobbies, like building the Taj Mahal out of match sticks

I've already done that, GeeTee, and it does pass all the packets unfragmented.

As yarwell points out, the ping value for that is 1472.

No, you've got it wrong, I'm not back to where I started at all. I started with the router MTU being 1458. An initial tweak to 1460 made no difference and, at that point, I began this thread. The router's MTU is now 1500 (which includes the 28 overhead).

something I should add here is that some routers will clamp the mtu. eg. on the billion I think its called tcp clamping.

Whenever this kind of feature is available on a router I would advise to turn it off, just keep it at a unclamped 1500 setting and any adjustment (if required) to be done on the pc.