Now activated with IDNET but with one small problem (MTU)...

[Scorpio]

Hi again,

I have just put in my details into my router and over the moon to see my download speeds double from just under 1.5mb to just under 3mb!!!! Excellent.

With my previous ISP, my router had an MTU setting of 1458 so I called IdNet's support line to see what they recommended for their service. I was told to put it at 1500. The problem that I am having is that I am unable to view some sites and download some files from yousendit.

I know this is related to the MTU setting so please could someone tell me what setting I should have in my router?

Also will my speeds get any better over the next few days from these stats?

Data rate     
4096    768
      
Noise margin    
8.9    7.0
         
Output power    
19.4    12.4
         
Attenuation    
54.0    29.5

Thanks again........

[sallyandjames]

Rather than make a hash of explaining myself, I'll refer you to a 'real' expert.

Kitz knows her stuff (ie more than most ISP's do) there's not much you can tell her about ADSL!

http://www.kitz.co.uk/adsl/MTU2.htm is a pretty good explanation of MTU

[Scorpio]

Hi there sallyandjames,

Guess what, that link you posted..

I CANT VIEW IT!!!

Help........

[sallyandjames]

Is this any better?

Reproduced - original source http://www.kitz.co.uk/adsl/MTU2.htm with thanks to Kitz

Introduction

Before being able to explain MTU and RWIN, it helps if we have a basic understanding of how data is transmitted.

Networks use certain protocols to send and receive data - TCP (Transmission Control Protocol) is one of the core protocols in the Internet suite as it provides a means of reliably sending data between two connections. How its able to guarantee a reliable connection is through its use of acknowledgements (ACK) from the receiving system.
When someone sends you data, this is broken up into small packets, the host will send a packet of data which then requires an acknowledgement from the receiving system. Once the sender has received an ACK, it will continue to send the next packet of data. If for any reason an ACK is not received, the host will wait for a short period of time before trying to retransmit. This constant "chatter" of stop-start between the 2 connections ensures that all packets are received in the correct order.


Maximum Transmission Unit
- MTU

The MTU is the maximum amount of data (packet size) that can be transmitted in one go before it has to be cut up into smaller chunks. Each network has a maximum limit of data size it can cope with before data is fragmented into smaller packets. This figure may vary from network to network.

TCP Receive Window
- RWIN

RWIN is the amount of received data that you can accept (buffer) without having to send an acknowledgement to the remote host. With TCP it's the stop start action whilst waiting for an ACK during the sending/receiving of data that can slow things down. Now since the default RWIN is often too small for higher speed broadband connections, you can increase the size of your RWIN thereby allowing more data to be transferred at once, in turn reducing the pauses and thus speeding up your connection.

Maximum Segment Size
- MSS

MSS is the maximum size of useful data that can traverse the network before it has to be fragmented. Data Packets also have to include something called headers which is a small amount of additional data that include instructions such as where the data is being sent to. TCP and IP headers increase the packet size by 40 bytes. Therefore MSS = MTU - 40
If it helps think of these headers as being the brown paper or envelope on which you write the address if you are sending a parcel by post.

Hopefully by this stage you will see where this is heading.. in other words you may be thinking hey... I know I'll increase all by settings to make them bigger. Unfortunately, its not quite so simple, as has already been said each network on the internet may have a different MTU and therefore if you increase things too much it will have an adverse effect and slow things down.

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.

More information about the PING command



"Packet needs to be fragmented but DF not set" indicates that the packet size is too large.
In the example above it shows that my highest responsive ping is 1402.
From this we need to add on 28 to get the maximum MTU figure.
(28 being the header size for IP + ICMP)

Do not go above 1472. (1472 + 28 = 1500 MTU) since 1500 is the maximum MTU.


Working out your RWIN

You calculate your RWIN using the following formula:

RWIN = n * (MTU - 40)
where n is a whole number.

n can be 2, 4, 6, 8 or 16 etc (8 is the Win9x default)

Therefore if you choose an MTU of 1430

(1430 - 40 = 1390)
20 * 1390 = 27800
so RWIN = 27800



Default settings

Default MTU
1500 - Windows machines

Default RWINs
8192 - Windows 95/98/98SE/NT
16384 - Windows ME/2k
Windows XP dynamically calculates the RWIN

[Scorpio]

Thanks for putting the info up.

Seems like a lot to take in right now so I will look at it later.

Any recommended MTU setting to use for the time being and should this setting be placed in the router, within Windows itself or both??

Sorry to be a pain...

[stevelondon]

I used to leave mine at 1500 i used to get the fastest speed on that too but for some strange reason my MTU changed on both my PC's and will not go any higher than 1460 even though my registry settings are 1500 so the best setting for me is 1430 its the best setting for me since its been stuck on 1460.
Router 1500 ive never changed it from that.
PC 1430

[sallyandjames]

So long as the router is higher than your computer, you never need to change it.

If its lower than your computer, you'll have problems methinks.

Mine are just whatever the defaults are - never even looked!

[Nerval]

Some people love changing MTUs and the like, but I've never found any benefit at all.  I just leave the router to sort it out and it's fine. Whenever I've tried to change it, performance has slumped.

And there's no magic MTU figure one can give you - the best one for you varies according to loads of factors.

Just had a look and mine is 1500 on the router, though it must have done that on its own. Can't seem to find one in windows.
If you want to change yours, try 1500.

[Inactive]

Mine is on 1400, never touched it, just left it to sort itself.

[stevelondon]

Mine was always defaulted to 1500 but now it wont go higher than 1460 and i only noticed that because my speeds was slower it is stuck on that and aint got the foggiest why.

Nerval
I also have never touched mine before till i noticed my speeds slowed down then noticed it was at 1460 and got the speed back up when i set the pc to 1430 but is still considerably slower than when it was on 1500 would love to know why its stuck on 1460 as toyah said "Itth a mythetery

[Nerval]

I think itth Riks fault as he's always messing with MTUs.

[stevelondon]

Do u think that he sneaked in and messed with mine? 

[Nerval]

I'm reading all about Wifi hacking  at the moment, so I'll let you know 

[stevelondon]

LOL

[Scorpio]

Maybe I should locate my default Belkin router MTU setting then and try that right??

[Nerval]

return it to factory default settings should do it I suppose.
Or restore the configuration you saved in case of emergency 

[Scorpio]

I sorted the problem. I configured my TCP/IP settings with my own IP address and dns servers. I beleive the problem was caused by the old dns server settings in that configuration.

[stevelondon]

Glad u got it sorted 

[MoHux]

Mine was always defaulted to 1500 but now it wont go higher than 1460 and i only noticed that because my speeds was slower it is stuck on that and aint got the foggiest why.

Nerval
I also have never touched mine before till i noticed my speeds slowed down then noticed it was at 1460 and got the speed back up when i set the pc to 1430 but is still considerably slower than when it was on 1500 would love to know why its stuck on 1460 as toyah said "Itth a mythetery

Steve, I was re-reading an old favourite web-site about MTU and came across this paragraph or three;

                            Why is Path MTU Discovery not working?

Path MTU Discovery uses the ICMP protocol to check the maximum packet size. One of the other uses of ICMP is something called the ICMP PING. Many security sites recommend that you make your router/PC unpingable, so when you turn of pings on your router or firewall quite often you are blocking the ICMP protocol completely and therefore Path MTU Discovery stops working.

Despite what some security sites say, there is no harm in making yourself pingable. So what if someone can ping you and check if you are online - it really makes no difference particulary these days when more and more users are using routers which are online 24/7.
Making yourself pingable can cure many of the problems associated of being unable to view certain websites due to MTU problems.

Could it account for the stuck 1460 MTU do you think?

In case it helps others, this is where I got it from; click here

HTH 

[RobMc]

for what it's worth, I use MTU=1430 and RWin=115818 (I sync at 8128) I "made up" these figures as I seemed to remember (sometime I remember stuff and sometimes my memory makes things up) hearing someone suggesting an MTU of 1430 with idnet and I calculated the RWin using http://www.dslreports.com/tweaks.

This seemed to make a big difference compared to the settings I'd been using for normal 2Mbit adsl.

Rob.

[cavillas]

Glad u got it sorted 

How did you know Scorpios name was Glad? 

[Rik]

I hate to mention this, but Christmas was a month ago - today, it's happy Burns' Night!

Oh, and MTU 1458 works best for me, but I leave the router at 1500.

[Inactive]

I hate to mention this, but Christmas was a month ago -

Nah, that looks like a January Sail to me .. groan.. 

[Nerval]

oh no, you're not trawling again............

[Rik]

It all seems very sloopy to me.