MTU-Speed Reference

UPDATE:  September 1,2000 PC Magazine has an article on all of this.  According to this article, "Although all versions of Windows struggle with the Receive Window, they excel at MTU processing... the default MTU value provides adequate performance for most broadband connections."  They do recommend changing the Receive Window.  Do this with REGEDIT.EXE.   Navigate to HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\MSTCP.  Once there look for the DefaultRcvWindow setting.  If you don't have one (I didn't) then right click in the right hand pane and select new string.  Name it DefaultRcvWindow (spell it exactly this way) and set the value to 32767.  (They say NT and 2000 use a key named "TcpWindowSize" instead.)  Reboot your computer.  I did this and the effective throughput of my connection went from 135kb to 155kb.  Wow!  I wonder if I got the performance below because MTUSpeed messed with the Recieve Window too?

For those of you who want to read my original notes on MTU Speed anyway, here they are...

I found this referenced in several Windows magazines, but I didn't believe it would work. Then one idle night I downloaded this utility and found that it works! I monitor modem transmission speeds with Net.Medic. It shows that actual throughput went up about 30% after I used this stuff.

As I understand it, when your browser connects to a site, your PC opens a TCP/IP connection.  In doing so, your PC tells the remote host what maximum transmission unit (size) to use for each packet. By default Windows uses a transmission frame size that is bigger than the maximum frame size that most ISP's routers are set to use. (The Windows default works well over a LAN.) In this situation the remote host sends packets to your PC that have to be split into smaller packets by the various routers on the internet and then reassembled before they get to your PC.  This really hurts throughput. The solution is to set the maximum transmission unit (MTU) to be the minimum of the transmission sizes of any router between your PC and the host you're connecting to.  According to the documentation, 576 is the internet standard MTU.

A confusion in the documentation relates to ISP MTU vs. internet MTU.  In many cases your ISP will use an MTU of 1500. This means your PC can also use an MTU of 1500 when connecting to ISP resources (email, newsgroups, homepages on your ISP's network, etc).  If your browser connects to a site that is hosted on another ISP and their Network MTU is also 1500, then your PC can again use 1500 with no reduction in speed. In both these cases, if your PC MTU is set less than 1500 you will be a little bit slower than a full 1500, but not much.  However, when you connect to a web site that is on a network that has a smaller MTU, then you get a big 30% reduction in speed.

This problem occurs over any modem speed, and fixing it will improve the speed of whatever modem you are using. As I see it, the best bet is to set the MTU on your PC to 576, the internet standard.  In this setting your email and newsgroups will come down to your PC a little bit slower, but your speed when browsing most web sites will be a whopping 30% faster. I think this is an OK tradeoff to make.

I fixed my system with the shareware utility  MTU-Speed. The software comes with some rather confusing documentation.

What I did: 

  1. Backup the registry (as explained at both the above sites)
  2. Run the Utility
  3. Select Dial-Up Adapter, Enable RWIN setting; make sure Apply to All TCP Keys is not set.
  4. Click Optimum Settings, Click Update Registry, and then reboot. Your MTU should now be set to 576.
  5. Connect to your ISP and watch your modem fly!

I'd be interested in hearing how this works for you.  The MTU-Speed author claims that it works for NT4.0 as well.