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Author Topic: Best MTU?  (Read 6594 times)

hake

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Best MTU?
« on: July 04, 2008, 06:56:59 PM »

What MTU to use.  Do you choose a lower value and more or less eliminate packet fragmentation or do you reduce the overhead with a slightly larger value and accept occasional fragmentation?

I'll start the ball rolling.  I am a PlusNet customer and find that setting MTU to 1430 eliminates packet fragmentation (checked using TCPOptimizer.exe).  I ensure that MSS and RWIN are calculated to suit the MTU value exactly.

What do others find?

 :hmm:
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kitz

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Re: Best MTU?
« Reply #1 on: July 04, 2008, 08:10:57 PM »

Ive used MTU of 1430 for years - I was one of the "original tweakers" and found when combined with the correct RWIN its given me the best speeds.

I'd rather set at 1430 rather than run the risk of packet fragmentation.

When on a line syncing at 8128 I found the best setting for my line was
MTU 1430
RWIN 166800

I used that since I got maxed and have recommended it since, as it also included an inbuilt allowance 
I believe Plusnet also now recommend the same setting.  (DaveT did a fair bit of testing and came to the same conclusion).

If you sync at lower speeds then theres some more recommendations here.

ATM Im using some really weird settings, which dont seem to follow the rules which was  a result of an accidental figure input...
and  works very well on my 24Mb line.   
I havent had chance to dig deep to find out why it works  - hence why I wont publish yet.

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hake

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Re: Best MTU?
« Reply #2 on: July 05, 2008, 12:29:06 PM »

Thanks Kitz.   I viewed the page with the lists of recommendations.  One or two RWIN values were not exact multiples of MSS.  Does it matter?

Above RWIN = 65535, Window Scaling should be enabled, I think.  Should Time Stamping also be enabled?
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mr_chris

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Re: Best MTU?
« Reply #3 on: July 05, 2008, 05:52:00 PM »

I know that when kitz compiled the list, the recommendations came directly from forum users (mostly on ThinkBroadband forums) who posted the values that seemed to work best for them.

RWIN should be an exact multiple of MSS, and I think the TCP/IP stack in the operating system adjusts the RWIN to be a multiple no matter what you put in. It certainly doesn't make any sense not to set the RWIN to an exact multiple of MSS, as that would decrease efficiency of transmission, almost certainly negating the effects of any tweaking.

You're right about Window Scaling - above 65535, Window scaling has to be enabled for it to work.

I'm not sure about Time Stamping, and there doesn't seem to be a definitive answer. It might be worth having a read of the DrTCP Tweaking FAQ page which briefly explains what each of the settings do.

Also, when tweaking and testing, try to do it at a non-busy time (early hours of the morning is good), so that exchange / ISP contention has less of a chance of affecting your test results.

Have fun! :)
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mr_chris

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Re: Best MTU?
« Reply #4 on: July 05, 2008, 06:02:42 PM »

Having just read your initial question (!) it all depends on your net usage. A lot of people (me included) have found the best overall results using an MTU of 1430, simply because some quite major sites don't actually work at all if the packets are getting fragmented. Not sure if this is still the case, and you may find that different values work better for you!
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hake

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Re: Best MTU?
« Reply #5 on: July 06, 2008, 09:24:41 AM »

Thank you Mr Chris.  RFC1323 is a relevant if very hairy subject area, possibly the most interesting RFC of all, if only I could understand it.   ???

I quote from RFC1323 the following text, which can be found straddling the boundary of pages 7 and 8 : -

"A question may be raised about the bandwidth and processing overhead for TCP options.  Those options that occur on SYN segments are not likely to cause a performance concern.  Opening a TCP connection requires execution of significant special-case code, and the processing of options is unlikely to increase that cost significantly.

On the other hand, a Timestamps option may appear in any data or  ACK segment, adding 12 bytes to the 20-byte TCP header.  We believe that the bandwidth saved by reducing unnecessary retransmissions will more than pay for the extra header bandwidth."

Now there's encouragement to experiment.  I have hunch that it might help to mitigate the effects of router congestion.
« Last Edit: July 06, 2008, 04:48:09 PM by hake »
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kitz

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Re: Best MTU?
« Reply #6 on: July 06, 2008, 10:05:00 PM »

>> if only I could understand it. 


You me both then :D

I seemed to muddle through by trying various things and reporting on what worked best for my lines.  The original tweaking stuff goes way back to 2003 when there were a few of us on AG experimenting and reporting back what worked best.  Back then speeds were less likely to fluctuate (I always got a rock steady 478 kbps) so it was very easy to immediately see what effects any tweaking had.
Since then Ive had to resort to tweaking during wee small hours when variance will be less to make sure of any improvements :/

Theres an interesting post here about windows scaling, congestion and timestamping..  after scanning the RFC and reading the post then it certainly seems like a good idea to have timestamping enabled

http://osdir.com/ml/openbsd.tech/2003-02/msg00108.html
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hake

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Re: Best MTU?
« Reply #7 on: July 07, 2008, 07:55:06 AM »

So it does appear that time stamping combined with window scaling could be an effective response to network congestion.  I remember seeing an animation of the effect of the van Jacobson retransmit algorithm.  Such aids are essential for such as me who are not quite able to visualise these phenomena straight off the page.

I wonder if those who do make use of the RFC1323 tweaks gain an advantage over those who do not when busy routers are dropping packets like they are going out of fashion.
« Last Edit: July 07, 2008, 07:57:16 AM by hake »
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kitz

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Re: Best MTU?
« Reply #8 on: July 07, 2008, 01:20:07 PM »

Indeed it does, and in view of that Im going to change the image on the example MTU settings.
As said the additional overhead should be negligible and should be used with Windows scaling anyhow. 
I've changed mine and although I haven't noticed any real speed improvements, I certainly haven't noticed a decline.
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hake

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Re: Best MTU?
« Reply #9 on: July 07, 2008, 08:29:21 PM »

It might help us to keep our end up against the iPlayer mob.  I don't stream very much, hardly at all in fact.  My Internet use is very bursty.  My TV reception is through an aerial.

Steve Gibson has an entertaining talk about network congestion to download from https://www.grc.com/securitynow.htm

It's in episode #139.

I find Steve's enthusiasm quite infectious.  It's a very different culture in the USA.

BTW, my Internet is working very well with RWIN set to 111200 and using windows scaling with time stamping.  Stuff seems to flow more freely and smoothly.
« Last Edit: July 07, 2008, 10:14:40 PM by hake »
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kitz

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Re: Best MTU?
« Reply #10 on: July 08, 2008, 07:33:07 AM »

Thanks for the linky hake..  I listened to it last night whilst doing something else.
Ive never heard his securitynow reports before the weekend.. and its weird that Ive listened to 2 within a couple of days.

Some good info in there and he seems to make things easy for the average person to understand. 
I also had a bit of a chuckle at some of the things he talked about simply because it so much reminded me of some of the things that PN came out with a few years back when they first started talking about traffic shaping. :D

>> and using windows scaling with time stamping

Mine still seems to be working fine..  things are running smoothly but I still cant say Ive noticed anything definite. 
Then again that may be a good thing, because I normally notice more when "things arent just right".
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hake

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Re: Best MTU?
« Reply #11 on: July 09, 2008, 11:19:38 AM »

Gotcha!  I found the animations I mentioned previously that illustrate some of what goes on in TCP/IP networks.  It's pretty abstruse stuff but less hard on the grey matter than trying to visualise it from the theory.

The link is :  http://www.cs.utk.edu/~dunigan/tcptour/javis/

You will need a recent JRE as the animations are expressed as Java applets.
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