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Author Topic: MTU tweak problem?  (Read 4487 times)

smudge

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MTU tweak problem?
« on: July 05, 2008, 01:36:05 AM »

hello

im a bit confused with the MTU tweak page...
it says

Working out your maximum MTU

The best way of finding out your optimum MTU is by pinging your ISP with packets of data to see what size you can send without the data being fragmented.

The ping command you will need to use is: ping -f -l [packet size] [www.bbc.co.uk]
where [packet size] is the size of the packet
start at say 1472, and work your way down until you get a responsive ping from the highest number you can.


if i start at 1472 and work my way DOWN i can keep going and going so it seems..?!?!?

is the instructions perhaps worded a bit wrong??
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kitz

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Re: MTU tweak problem?
« Reply #1 on: July 05, 2008, 11:46:29 AM »

>> if i start at 1472 and work my way DOWN i can keep going and going so it seems

Its correct..  you start at 1472 - if its unresponsive at 1472 then try a lower figure if it still unresponsive try another lower figure until you find the point when the ping responds.

How far down have you tried?
In some cases it may be as low as 1372 ( for MTU of 1400)

Try going down in 10s until you get a response..  and then go up in 1s until it becomes unresponsive again. 
Youre looking for the highest responsive figure - Then add on 28 (the overhead) to find the optimum MTU size.

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smudge

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Re: MTU tweak problem?
« Reply #2 on: July 05, 2008, 11:50:14 AM »

sorry i was confused with it - its says DOWN then HIGHEST NUMBER - confused me a bit lol (simple haha)

i get response at 1472 and all the way down as far as i go but 1473 onwards no response so does that mean my MAX is 1500?
i tried 1430 sometime ago from your suggestions page and im sure it sped up browsing but just want to be sure to have on right one

i use VISTA and DG834Gv3

thanks kitz
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kitz

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Re: MTU tweak problem?
« Reply #3 on: July 05, 2008, 12:00:25 PM »

>> i get response at 1472 ... /snip/ .. so does that mean my MAX is 1500

Yep :)

>> i tried 1430 sometime ago

I continued to use 1430 all the time I was on Maxdsl (and prior to that)  iirc my "optimum" figure was around 1458, but I tended to find that 1430 just worked best (as do quite a few others).  You must never go above the max figure though.


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smudge

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Re: MTU tweak problem?
« Reply #4 on: July 05, 2008, 12:20:31 PM »

so what is best? the highest figure of 1500 or the lower figure of 1430?
 im on the BE* network with o2#

offtopic: do you know the telnet command for a thomson 585v6 to change DNS servers?
« Last Edit: July 05, 2008, 12:25:34 PM by smudge »
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kitz

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Re: MTU tweak problem?
« Reply #5 on: July 05, 2008, 12:44:09 PM »

>> so what is best? the highest figure of 1500 or the lower figure of 1430?

Hard to say - depends on your own system, ISP routing, if its via BTw and your speed.
Try it and see.

All I will maintain is that when on Maxdsl 1430 worked best for me.

Now Im with Be* Im using 1500 MTU .. but you must ensure that you set a high RWIN to get optimum speed.


I spent a fair bit of time tweaking in the wee small hours when my speed was fairly static to be able to notice any increases best.  For my own speed (I can sync at 24Mb) I need a high RWIN and therefore I calculated one based around the multiplier that previously worked best for me and adjusted it for the higher speeds.

Im currently using 513920 RWIN.  Although I can never be certain, I think I got a speed increase of around 600-700 kbps by changing the RWIN.

You can see my results from dslreports
http://www.dslreports.com/tweakr/block:59168f5?verbose=1&service=dsl&speed=24000&os=winXP&via=routerpppoe

As you can see it looks pretty healthy.
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smudge

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Re: MTU tweak problem?
« Reply #6 on: July 05, 2008, 12:50:54 PM »

thanks kitz....

I have vista though and RCWIN cant be set..... i have AUTOTUNING enabled and ECN enabled
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kitz

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Re: MTU tweak problem?
« Reply #7 on: July 06, 2008, 08:49:43 PM »

>> I have vista though and RCWIN cant be set.

:(

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hake

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Re: MTU tweak problem?
« Reply #8 on: July 08, 2008, 07:16:03 PM »

You can go to www.speedguide.net and download TCPOptimize.exe

I'm back in the stone age using Windows XP SP3 so I cannot say if Vista will play ball (I hate when I can't tweak). 

TCPOPtimize.exe allows the user to quickly home in on a MTU value which will eliminate fragmentation with almost all hosts.  You have to try the program with a number of hosts, the further away and in the more remote places the better.  It won't guarantee freedom from fragmentation but it will reduce it to negligible instances.
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kitz

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Re: MTU tweak problem?
« Reply #9 on: July 08, 2008, 09:29:09 PM »

>> I cannot say if Vista will play ball

It doesn't.  Well you can tweak the MTU, but wont allow you to do much more - there's no registry variable for the TCP Window size, so youre stuck with how the o/s thinks it should perform. :(

http://www.kitz.co.uk/adsl/vistaMTU.htm
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