This FAQ was written pretty quickly - if there is anything major missing let us know.
1. What is MTU?
It stands for Maximum Transmittable Unit - and defines the size of the packets sent. Situations can arise where your computer is sending a higher size packet than a network can cope with, lowering your transimission size, the MTU can help. In normal situations this should be an automatic process with routers/computers negotiating this - but this can break, examples are some routers block all ICMP traffic, as do some software firewalls like ZoneAlarm.
2. What are good values?
Well the default in Windows is 1500. This can vary from provider to provider, a list to try for Talk Talk are below:
1432 - LLU users should try this setting first
1458 - IPStream customers try this one first
3. How do I change it?
There are many ways, most are fairly simple, look at the following links, and if it still worries you, make a post and ask any questions you have. I recommend while experimenting to change it on your computer, ensuring you reboot each time. Changing the setting on your router only can leave the PC running at a higher value increasing the load on your router, and slowing things down.
http://www.adslguide.org.uk/guide/mtu.asp ADSLGuide page on changing it, perhaps a bit techie for lots of people. Does give links for MAC OSX
http://www.kitz.co.uk/adsl/tweak2.htm A good page written by Kitz who users this BBS. People may want to use the MTU values suggested above.
http://www.dslreports.com/drtcp the DrTCP tool, good tool for changing the MTU on your Windows machines.It is a very small download, so even if stuck with dialup access it will download quickly
http://www.dslreports.com/tweaks a US based speed tester, that will report what MTU is visible to the outside world. Handing for checking what you have before playing around, and checking the results.
4. I have a USB ADSL modem, I dont see it in the DrTCP list?
In which case the USB modems use the Windows Dial up Networking interface, so enter the new MTU in the Dial Up RAS MTU field
5. I've changed the MTU but it is not doing anything?
Did you remember to reboot the PC/router to ensure the network interface is using the new value.
6. Do I change it on just my router or my PC?
In theory you can just change it on the router if it has a MTU field. If you cannot find a MTU field in your router, just change it on the computers. I actually recommend changing the PC's anyway even if the router you use will let you change it, since if the PC's are sending larger packets and the router has to do more work to handle them it can slow your connection down.
7. Lots of sites talk about changing the RWIN value too?
Initially I would only worry about the MTU, and once it looks like this has improved things, you can try the calculations to find the optimum RWIN value.
8. How do I reset to the default values?
In Windows you simply can specify 1500 which is the default, or alternatively just blank out the MTU field in DrTCP and this should make the PC use its defaults.
9. I've broke my PC now?
Sorry, but any hints and tips here are very much try and see. We cannot be held responsible for things. Please feel free to post and hopefully someone will suggest some helpful things.
10. This is all too confusing?
In which case sit tight, watch the forums and you may start to understand what to do, or whatever is causing the problem will be fixed.
11. What happens if the ISP say they've fixed the issue?
You can carry on using the MTU you've decided upon, or revert to your previous values if you want. It is down to you.
Edited by seb (Fri 22-Aug-08 19:58:45)