PChucks Network: Determining The MTU To A Single Server

If you have connectivity to the Internet in general, but not to some servers, some websites load slowly, or if problems come and go, you may have a problem with the size of the messages that pass between you and the web site in question. The Maximum Transmission Unit, aka MTU, is a setting on your computer, that controls the size of the messages.

Depending upon what networks are accessed, between your computer and a given web site, the MTU that you can use will vary. If you make your MTU exceedingly small, you'll be able to avoid an MTU related problem, but loading a complete web page will take a long time. If you make the MTU too large, you'll see a variety of symptoms.

Some web pages may load incompletely.
Some web pages will give you a white screen.
Some web pages may return the old familiar
404 Not Found
Some web pages may return another old favourite
The connection has timed out.
The server at xxxxxxx.com is taking too long to respond.

Determining your MTU will be a matter of patience and persistence, using the "ping" command repeatedly. You'll run "ping", as with all text based commands, in a Command Window.

Note here that if your computer is running Vista, you may also want to first try checking the setting for Receive Window AutoTuning.

I start by verifying that I have connectivity - period. I check the problem web site, in this example www.yahoo.com. You should check the web sites that you are having problems accessing, since different target servers may have differing MTU related problems.

C:\>ping www.yahoo.com

Pinging www.yahoo-ht3.akadns.net [209.131.36.158] with 32 bytes of data:

Reply from 209.131.36.158: bytes=32 time=13ms TTL=58
Reply from 209.131.36.158: bytes=32 time=14ms TTL=57
Reply from 209.131.36.158: bytes=32 time=14ms TTL=57
Reply from 209.131.36.158: bytes=32 time=14ms TTL=57

Ping statistics for 209.131.36.158:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 13ms, Maximum = 14ms, Average = 13ms

If I see anything different,

C:\>ping www.yahoo.com

Ping request could not find host www.yahoo.com. Please check the name and try again.

Then, I know that I have a different problem - maybe a DNS problem, or even LSP / Winsock / TCP/IP corruption.

Hoping that I'm seeing the first example above, I'll see what MTU I can use. I now know that the IP address, in this case, is 209.131.36.158. And I know that www.yahoo.com (in this case www.yahoo-ht3.akadns.net) will respond to me, with 32 bytes of data. But 32 data bytes is way too small.

So, I'll see what MTU I can use. The maximum that anybody can use is 1500, which includes a 28 byte header. You specify the amount of data, not including the header, in the ping command. Start with a data size of 1472.

And I'll ping using the IP address, so I don't involve DNS - deal with DNS problems, separately.

Now I ping with additional parameters, "-f" (lower case of "-F" = "Do not fragment"), and "-l" (lower case of "-L" = "Send buffer (data) of this size") options.

C:\>ping -f -l 1472 209.131.36.158

Pinging 209.131.36.158 with 1472 bytes of data:

Reply from 209.131.36.158: bytes=1472 time=83ms TTL=58
Reply from 209.131.36.158: bytes=1472 time=84ms TTL=57
Reply from 209.131.36.158: bytes=1472 time=87ms TTL=57
Reply from 209.131.36.158: bytes=1472 time=84ms TTL=57

Ping statistics for 209.131.36.158:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 83ms, Maximum = 87ms, Average = 84ms

That's a good return, and tells me that I don't have an MTU problem. If I had seen either of two other results

C:\>ping -f -l 1472 209.131.36.158

Pinging 209.131.36.158 with 1472 bytes of data:

Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 209.131.36.158:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

That's a fragmentation error from a router, or a server, telling me that my MTU is too large, and I set it to not fragment.

C:\>ping -f -l 1472 209.131.36.158

Pinging 209.131.36.158 with 1472 bytes of data:

Request timed out.
Request timed out.
Request timed out.
Request timed out.

Ping statistics for 209.131.36.158:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

That could be a normal "This server isn't online" message. But since I did my baseline connectivity test (above) I don't think that this is the case. In this case, a router or server is simply dropping my ping request, as above because my MTU is too large, and I set it to not fragment. This is called a "black hole" router or server.

Both a black hole error (router or server simply dropping my ping) and a fragmentation error (router or server telling me that I have a problem) tell me that I have to reduce my MTU. I'll start by reducing it by 20.

C:\>ping -f -l 1452 209.131.36.158

Pinging 209.131.36.158 with 1452 bytes of data:

Reply from 209.131.36.158: bytes=1452 time=83ms TTL=58
Reply from 209.131.36.158: bytes=1452 time=84ms TTL=57
Reply from 209.131.36.158: bytes=1452 time=87ms TTL=57
Reply from 209.131.36.158: bytes=1452 time=84ms TTL=57

Ping statistics for 209.131.36.158:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 83ms, Maximum = 87ms, Average = 84ms

So 1452 is good, but maybe too small. So now, I'll try making it bigger, in steps of 2.

C:\>ping -f -l 1454 209.131.36.158

Pinging 209.131.36.158 with 1454 bytes of data:

Reply from 209.131.36.158: bytes=1454 time=83ms TTL=58
Reply from 209.131.36.158: bytes=1454 time=84ms TTL=57
Reply from 209.131.36.158: bytes=1454 time=87ms TTL=57
Reply from 209.131.36.158: bytes=1454 time=84ms TTL=57

Ping statistics for 209.131.36.158:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 83ms, Maximum = 87ms, Average = 84ms

Again, too small. Keep going up by 2.

... (later)

C:\>ping -f -l 1466 209.131.36.158

Pinging 209.131.36.158 with 1466 bytes of data:

Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 209.131.36.158:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

OK, 1466 is too large. Back down by 2 to 1464. Add the 28 byte header, giving me an MTU of 1492. This is a common value for the MTU, for many who can't use 1500.

Do you see the technique used? I found the MTU, by bracketing it. I started from a value too large (1472 data bytes), went down by 20 (to 1452, though I might have gone progressively lower) to a value too small, then up by 2 (to 1466) again too large. Then back down to the largest value where no error was seen (1464, in this case), an even number.

There's nothing magical about the sequence "Down by 20, Up by 2". You can use any sequence that pleases you. Try "Down by 60, Up by 20, Down by 2". Or "Down by 80, Up by 8, Down by 2". It's your computer. The point is to find the largest data size that you can successfully ping the target server with, then add the 28 bytes for the header, giving you the maximum MTU that you can use.

Note that I said "maximum MTU". If you find out that "www.yahoo.com" is accessible with an MTU of 1492, but "www.google.com" isn't, you'll have to repeat the process, from the beginning, using "www.google.com" as the target. Always target the problem web site.

Once you know what MTU to use, if your computer is running Windows 2000 or XP, you can set it using the free tool DrTCP, from DSLReports. Download DrTCP, into any convenient folder, and run it from there. You'll be changing the "MaxMTU" or "MTU" value under Adapter Settings. If you have multiple network adapters, be sure to choose the one that provides the Internet service.

If your computer is running Windows Vista, you can set the MTU by using the "netsh" command. If your computer is running Vista, you may also want to first try checking the setting for Receive Window AutoTuning.

The MTU isn't an efficient tuning factor, but you have to use it. It's part of IP networking, and your access to the Internet.

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