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MTU Tweak

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MTU Tweak

Been eavesdropping over at ADSLGuide.
BT have recently carried out upgrades so that MTU
of 1500 can be used allowing for maximum throughput.
However 'performance enhancements' can be gained by
using an MTU of 1478. Packets still fragment above this value.
Have been testing this out over the past week (after switching
back to 1500) and whilst there are no significant speed gains
web browsing is much faster, pages don't take ages to load,
especially long distance sites, a general gain in stability.
Due to the higher than usual latencies especially on US servers
I've settled on RWIN of 17,256 for my 512K line, for 1Meg try 31,636
fore 2 Meg try 63,272. If you wish to experiment with RWIN try multiples
of MSS (143Cool.
So if your currently using MTU of 1430 or 1458 you'll get some more
throughput with 1478 and if you've gone back to 1500 or haven't
tweaked, try it and please post if you notice / don't notice any gains.
If you haven't tweaked (MTU Smiley ) before a simple program is DrTCP
Save the file to hard disk open and set MTU & Dial up MTU RAS
to 1478 , set RWIN (TCP Receive) to 17,256 (512K) + - 1438.
Leave / set all boxes to default, Max dup acks x 2 ( or leave blank),
TTL 64 ( or leave blank) save exit and re - boot.
To reverse clear boxes (as before) save exit and re - boot.
DrTCp will look for a network adaptor but if the box is blank,
leave it so and make the above changes. Smiley
Edit : RWIN values firmed up.
151 REPLIES
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Re: MTU Tweak

Quote
whilst there are no significant speed gains
web browsing is much faster, pages don't take ages to load,
especially long distance sites, a general gain in stability.


As well as the normal speed test, ADSL Guide, Speakeasy, McAfee etc, I use www.numion.com as it loads website logo's and is a little more indicative of "real world" browsing. However, Numion results do depend on the routing and traffic for each test, so take an average of many tests before making a decision. Fortunately, if you have coookies enabled, Numion keeps a log of your previous tests.
holdtight
Grafter
Posts: 1,634
Registered: 15-06-2007

MTU Tweak

Seems good ! i got Drtcp and configured exactly as yours then went and did a few speed tests, its the first time i have ever tipped over the 500k mark on the +net test with a speed of 500.1, best yet ! it might just have the wind behind it tonight so ill do a few more tests tonight and in the morning and let you know how it goes, great tip thanks aleca.
Eserim
Rising Star
Posts: 376
Thanks: 17
Registered: 01-08-2007

MTU Tweak

20348 BPS to 30062 BPS done at 4 AM so I doubt if many others contended on my exchange so a result Cheesy

Cheers

CHawke
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MTU Tweak

Thanks for the link cqg4uzg, a very usefull test.
Bang goes my theory about very subtle improvements,
with those results from holdtight and chawke1. Lol
keithcalder
Grafter
Posts: 182
Registered: 03-08-2007

MTU Tweak

Just got this results from dsl reports:

Example 146000 byte download
Actual data bytes sent: 146000
Actual data packets: 100
Max packet sent (MTU): 1500
Max packet recd (MTU): 1500
Retransmitted data packets: 0
sacks you sent: 0
pushed data pkts: 2
data transmit time: 2.021 secs
our max idletime: 156.3 ms
transfer rate: 43555 bytes/sec
transfer rate: 348 kbits/sec
This is not a speed test!
transfer efficiency: 100%


Notes and recommendations:
Good data stream (no/few rexmits)

Dont think I need change a thing, getting constant speeds when downloading, 58-60KB no problems. Great going Plusnet!!!!
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MTU Tweak

Hi keithcalder.
You'd pretty much get identical results to those you
have posted using any MTU below 1500.
The dslreports tweak test will come up with the basic
'looking good' and smiley faces, it's inconclusive.
At the bottom of the results page is a link 'more details'
On the right hand column there are values for min, average
and max latency (RTT). Over a few days of testing different MTU
I found that 1478 gave the overall lowest latency,1500 the highest.
It's not just about taking different speed and tweak tests. I found
that for general day to day use this setting gives me the best,
not very scientific I know Smiley
I can understand your point of view that if your line is
working fine, you don't need to make changes.
Ianwild
Grafter
Posts: 3,835
Registered: 05-04-2007

MTU Tweak

I am going to make this post sticky. I would be interested in everyone's results from Tweaking and I would like to eventually build a tutorial about this, so everyones results and feedback would be gratefully accepted. ADSLGuide's forum has been the ruler in this area for too long!

Just for the record, BT did have a problem with packet fragmentation on their network causing the routers to develop processing overheads. They fixed this by setting all their routers on the network to a nominal MTU of 1900, so
no packets were getting fragmented and as such the problem with their
routers running hot has now been solved. That doesn't mean though that
there are not other issues in relation to how packets are being handled over the BT network. Tweaking your MTU for the best performance is still a good idea for those of you who want every last drop from your connection. That said, remember the old expression - If it ain't broke, don't fix it!

Regards,

Ian
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MTU Tweak

My MTU is set at 1458 with an RWIN of 12762, have found this to be optimal for packet loss.
But with the Sar 110 route it has a fixed MTU of 1452 so in theory no amount of tweaking will make much difference.
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MTU Tweak

The 'Plusnet forum' is definitely the place to come for anybody wanting to tweak their connection to the max Smiley
There are quite a few registry tweaks that can be carried out
but due to the obvious risks in messing around with the registry,
an easier option is to use a dedicated program.
I've mentioned DrTCP above and another very advanced
programme is 'Cablenut'. It's set up for PPPOE but can be
easily adapted for PPPOA so you will need to use DrTCP aswell
if you want to change MTU. So for those who want to go one stage further:
Download and install Cablenut
Download and save to hard disk Cablenutupdate
I'll describe procedure for XP 512K ADSL. Setting RWIN 17256
MTU 1478 MSS 1438.
First open DrTCP and set MTU& MTU RAS to 1478, The rest can be left blank/default . Save exit re-boot.
In Cablenut Update open folder Win XP 2KDSL.
Open file DSL PPPOE 512.256.
In the right hand column change TcpRecvSegmentSize
andTcpSendSegmentSize to your value of MSS
in this case 1438.
In both the right and left hand columns change TcpWindowSize
and GlobalMaxTcpWindowSize to your value of RWIN in this case
17,256. Then save to registry exit and re-boot.
Similarly for a 1meg line open the PPPOE file for 1000.256
change MSS as above 1438 change RWIN to 31636.
The nearest for 2Meg is 1500.256 change MSS as above 1438
change RWIN 63272 . Top left column increase
DefaultReceiveWindow to 262144.
For other values of MTU use MSS as (MTU-40) and
RWIN in multiples of MSS.
Note : to return to windows defaults simply clear all
boxes save to registry exit and re-boot.

Edit: typing error
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MTU Tweak

Reset, my advice would be an attempt to match the MTU of the router or lower.

This may possibly account for your higher than average ping times to your router beleive it or not.

When a packet MTU exceeds the size of that defined by the router, it fragments it. Your MTU exceeds it by a very tiny margin, but is enough to fragment the packet.

In the larger BT network, the process of fragmenting the packets was bogging the routers down. Under your own network this isn't so bad, however, may still account for decreased performance.
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MTU Tweak

Reset, this sounds very strange why would the router
be set to 1452.
Your saying that you've set MTU to 1458 in DrTCP.
When you take the tweak test
do your results show that packets are being transmited at 1458 or 1452.
If you run the command "ping -f -l 1430 www.plus.com" ( minus exclamation marks) do you get the message packets were fragmented ?
(1458 -28 ICMP & IP headers =1430)
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MTU Tweak

Packets are transfered at 1452, no fragmented packets.

Will try MTU at 1452 and see how it goes.
Reset MTU and RWIN, twak test comes up fine and transfer is up from 97 to 99%.
+net speed tester gives me 425 but it's been erratic for weeks now.
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MTU Tweak

Just tried the www.mirror.ac.uk test and it comes up with "server not found" crazy or what..
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MTU Tweak

Reset it looks like your router is set to 1452 I've looked
on Solwise website, whilst I couldn't find no specific details
the author of the set-up guide recommends an MTU below
1500 and also states that his/her own personal router is set
at 1200.
Your line seems to be suffering from similar issues to mine
and it's difficult to gain significant results with things like packet
loss occuring, you can't really tweak that away.
For anybody sceptical about tweaking I've got acces to
a neighbours 512k connection which has no problems.
I know for a fact that this line has achieved a maximum of
507 Kbps (estimated line speed) on adslguide speedtest.
A pretty good result and has been consistent for the past year
or so, but has never gone above 507.
I just simply changed MTU 1478 and RWIN 17256 with
DrTCP (previously 1500 and 17520, windows defaults).
Here is the first result and
here is the second.
Incase the links fail, 517kbps and 516kbps respectively
a small jump in speed at 6pm on Friday a very busy time,
the third and fourth results the same.
Back to windows defaults and yes,not quite 507
but 506Kbps.
Sixth and seven results 506 /503 Kbps upload 261/249/Kbps,
so pretty consistent.