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This issue is most often caused by Path MTU Discovery being turned Off, which tends to decrease your upload speeds, sometimes considerably.

Download DrTCP021.exe (for Windows OS's only) from this page to your desktop. Open it and set Path MTU Discovery to either "Default" or "Yes" (same thing). Click Save, then reboot and retest.


This is a false issue on the Mac; the TweakTester deducts 12 bytes (the timestamp segment) from the MTU. The actual MTU is whatever is shown plus 12.

Feedback received on this FAQ entry:
  • I turned Path MTU Discovery on as suggested yet still get the indication that the upload packet is bigger than your MTU. Any other suggestions. Normal DSL connection on Win98SE.

    2012-06-19 18:33:50 (MikeM01 See Profile)

  • You will get this if you are on Vista/7 and you have the nagle algorithm turned off (NoTCPDelay on) .

    2012-06-04 11:43:00 (darkwish See Profile)

  • This did not fix my problem. PPPoE & DSL Router with WinXP.

    2012-06-03 16:30:21 (OriginalDrM See Profile)

  • Same as the user Dsaunder. Ditto: I did the suggestion and it still gives me the upload MTU warning. Does any avail FAQ offer a suggestion if DR TCP does not change result?

    2011-09-28 03:02:01

  • Suggestion did not fix MTU warning for me.

    2011-02-24 18:34:02 (steffey0 See Profile)

  • For Windows 7 you can tweek MTU by opening cmd as Administrator. Then using following commands, Netsh interface ipv4 set subinterface "local Area Connection" MTU=1500 store=persistant you can test the setting by typing show interface. hope thats helpful, tho it did not change the error reported by dslreports but it is now most certainly set to 1500. and i have noticed a difference in page loads.

    2011-02-20 18:15:47 (klickinc See Profile)

  • What does this error mean for Vista and Windows 7? Don't think this can be tweaked but getting this warning/FAQ on my upload results.

    2011-01-25 19:29:42 (drshock See Profile)

  • did not fix this problem

    2009-08-31 22:43:45

  • In addition to my earlier comment about looking at the HTTP header (to see what platform the browser claims) and the user's response in the OS selection query box, Section 1 (Your Tweakable Settings) says "RFC1323 Time Stamping: ON" so the system KNOWS that there is a 12 byte timestamp segment but does not allow for it when reporting the MTU. BAD DESIGN in my opinion.

    2009-02-11 13:04:15 (RARPSL See Profile)

  • The comment about Mac also applies to Linux.

    2009-01-31 22:02:28

  • I use Mac Leopard 10.5.5 and MacPilot (<-program) in place of DrTCP. Under the 'network' heading you can change all your set up configurations to speed both your network connections and your computer. It doubled my connection speeds for both upload and download and reduced my latency (ping time) from and average of 67ms to average of 47ms = big performance boost when watching videos or large pics on the internet.

    2008-09-29 15:03:32 (Mosley See Profile)

  • Ditto: I did the suggestion and it still gives me the upload MTU warning. Does any avail FAQ offer a suggestion if DR TCP does not change result?

    2008-08-26 12:34:53 (dsaunder See Profile)

  • slackware 12.1 linux, raspppoe, mtu 1492 for ppp0 Tester show 1480 issue same as Mac...

    2008-08-05 18:36:41

  • Seems I was wrong as to why, but my advice was good. Linux(including Ubuntu)/UNiX/BSD/Mac OS X do all use timestamps, which I briefly forgot about. They can be turned off via your sysctl.conf file, but I wouldn't recommend it. Consider yourself ok.

    2008-05-01 11:36:42 (Selenia See Profile)

  • Reply to other feedback: The TCP stack in Ubuntu Linux closer resembles the Mac than it does Windows. It can be set to parameters, but it is an autotuning stack. It is also UNiX based, like the Mac. I would assume that could be the issue and it's safe to ignore the warning, as long as data stream is good. My upload/download comes close to my ISP's claimed max, which generally does require a properly tuned stack in this area on Roadrunner.

    2008-05-01 11:33:11 (Selenia See Profile)

  • I did the suggestion and it still gives me the upload MTU warning.

    2008-04-24 18:42:11

  • In addition to the suggestion of checking the HTTP Headers, you can also do the "Add 12" if the user STATES that they are a Mac (or Linux). You have been provided with the platform in the query - Why not use it as you do with the provisioned speed?

    2008-01-13 03:11:01 (RARPSL See Profile)

  • The note for Mac is also true for Linux and FreeBSD. Perhaps the test can determine the host OS (from the HTTP headers?) and automatically add 12, so as not to report an erroneous value?

    2008-01-04 14:43:16 (pflog See Profile)

  • What about Ubuntu Linux?

    2007-11-08 19:01:38

Expand got feedback?

by Pinan See Profile edited by JMGullett See Profile
last modified: 2007-02-23 17:00:41