For some reason that has escaped me Path MTU Discovery in Windows just doesn’t seem to figure out the MTU for a given path (something to do with routers being poorly configured to not respond to ICMP requests). So Windows uses the default. For the most part this doesn’t affect anyone, however if it dos affect you, it really annoys you. Failure of PMTUD will result in some websites not loading correctly, having trouble connecting to normally reliable online services and general Internet weirdness.

The resolution is to set your default MTU to one lower than the Ethernet default of 1500. Here is how:

Step 1: Find your MTU
From an elevated CMD Shell enter the following command:

netsh interface ipv4 show subinterfaces

You should get something like this

MTU         MediaSenseState  Bytes In    Bytes Out  Interface
----------  ---------------  ---------   ---------  -------------
4294967295  1                0           13487914   Loopback Pseudo-Interface 1
1500        1                3734493902  282497358  Local Area Connection

If you are using Ethernet cable you will be looking for “Local Area Connection” or “Local Area Connection 2″ (if you happened to plug into the second network port). If you are using Wireless you will be looking for “Wireless Network Connection”. The MTU is in the first column.

Step 2: Find out what it should be

In the CMD shell type:

ping www.cantreachthissite.com -f -l 1472

The host name should be a site you can not reach, -f marks the packet as one that should not be fragmented the -l 1472 sets the size of the packet (1472 = Ethernet Default MTU – Packet Header, where the Ethernet Default MTU is 1500 and the Packet Header is 28 bytes)

If the packet can’t be sent because it would need to be fragmented you will get something similar to this:

Packet needs to be fragmented but DF set.

Keep trying lower packet sizes by 10 (i.e. -l 1460, 1450, 1440, etc.) until you get a successful ping request. Raise your packet sizes by one until you get a “Packet needs to be fragmented but DF set.”. The last successful value plus 28 will be your MTU value.

In my case a packet size of 1430 succeeds but 1431 fails, so 1430 + 28 = 1458.

Step 3: Set your MTU

Now you have identified the interface you need to change and the ideal MTU for you, now it is time to make the change. Again from an elevated CMD Shell type the following replacing my MTU of 1458 with your own value:

netsh interface ipv4 set subinterface "Local Area Connection" mtu=1458 store=persistent

Or if you are using a Wireless connection:

netsh interface ipv4 set subinterface "Wireless Network Connection" mtu=1458 store=persistent

If all has gone well you should have a perfectly working internet connection.

I have been running the Beta since it was first available to the general public, and I have had Windows 7 Business at work for a couple of weeks now. Bought Windows 7 Home Premium for about £45 from Tesco. One copy for the main PC which I will probably re-build at Christmas and one copy for the laptop which is re-built now. Scott Hanselman described Vista as an operating system that stabs you in the eye a thousand different ways, I would have to agree with that. Windows 7 is different, things work how you think they should and don’t seem to randomly break when you least expect it.

As for the title of this blog post, I have mostly been installing Windows 7 to Vampire Weekend which Kev introduced me to. The track M79 seems to have taken some inspiration from the Sailor’s Hornpipe better known as the Blue Peter theme tune, seems to cut off just at the moment I am expecting it to carry on into the main part of the music. Maybe there is a Mike Oldfield link in addition to the lyrical Peter Gabriel link.

If I am moved to over the course of the next couple of week I shall try and point out some of the new features available in Windows 7 that are cool. For the time being try the following hold down Alt, while still holding down Alt press “Alt Gr” (The other side of the space bar) then press Tab. For some reason you get the old XP style Alt-Tab window. Utterly pointless?