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Thread: MTU ...something strange, please advice for tweak

  1. #1
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    Question MTU ...something strange, please advice for tweak

    Good day,

    there is something i am not quite understanding....

    TCP options string: 0204057801010402
    MSS: 1400
    MTU: 1440
    TCP Window: 51800 (multiple of MSS)
    RWIN Scaling: 0
    Unscaled RWIN : 51800
    Reccomended RWINs: 64400, 128800, 257600, 515200
    BDP limit (200ms): 2072kbps (259KBytes/s)
    BDP limit (500ms): 829kbps (104KBytes/s)
    MTU Discovery: ON
    TTL: 46
    Timestamps: OFF
    SACKs: ON
    IP ToS: 00000000 (0)


    as you can see the MTU is 1440, im on a PPPoE connection, the thing is....when i put in the router 1492 and reboot the router (saved) the Analyzer still reports 1440, i have also tweaked the MTU in the registry to 1492 under my ethernet adapter and same result.

    the strange thing is that i can ping any website with a payload of 1464 without problems and websites open correctly.

    eg. ping speedguide.net -f -l 1464 i get results fine, if it was really 1440 MTU then the max payload would've been 1412 (ping speedguiden.net -f -l 1412) would give results, 1413 wouldn't.

    so which to believe, the analyzer or the ping command?

    i have tried also a USB modem and another Ethernet modem (different brands than my main modem) and tweaked the MTU in the router and in the USB modem to 1492 but analyzer still says 1440 MTU.
    it's obvious not the pc nor the modems since i tried 3 and all of them different brands (netgear, D-link, Billion).

    im not having issues with pings, surfing or speed, but i am puzzled which to believe so i can properely tweak my RWIN and stuff.

    thank you.

  2. #2
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    something interesting.....i tweaked on pc the MTU from 1492 to 1430 and now this reads :

    TCP options string: 0204056e01010402
    MSS: 1390
    MTU: 1430

    so the analyzer is reading anything below an MTU 1440 but not anything above it.

    i'm not on a PROXY.

    Router set to 1492, pc set to 1430 = analyzer reads 1430 mtu

    Router set to 1492, pc set to 1492 = analyzer reads 1440 mtu

    advice please?

  3. #3
    Elite Member trogers's Avatar
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    Check to see if your router is using the latest firmware suitable for your country.

  4. #4
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    Quote Originally Posted by trogers View Post
    Check to see if your router is using the latest firmware suitable for your country.
    i have tried 3 different routers of 3 different brands and all with latest firmware, same thing.

    any more ideas please?

  5. #5
    Elite Member trogers's Avatar
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    Quote Originally Posted by legumo View Post
    i have tried 3 different routers of 3 different brands and all with latest firmware, same thing.

    any more ideas please?
    Which country are you in and who is your ISP?

    Sometimes, it is the ISP who is limiting MTU values. Give them a call to find out.

  6. #6
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    Quote Originally Posted by trogers View Post
    Which country are you in and who is your ISP?

    Sometimes, it is the ISP who is limiting MTU values. Give them a call to find out.
    i am from Egypt (although i'm british actually), i called the ISP today and their MTU is actually 1440 and my connection is PPPoE.

    even more puzzling now....setting MTU to 1440 things work ok and max speed is 106kb/s steady and browsing no problem, setting MTU to 1400 and adjusting the Rwin accordingly gained me 2kb/s ...aka 108kb/s and browsing no difference.

    i thought MTU should be as high as possible?

    seems as if the MTU 1400 is saving some hassle and (i think) is avoiding the PathMTU fragmenting anything.

    it seems most servers accept an MTU of 1400 easily without any fragmentation as proven by using MTUROUTE.EXE ...this is surprising.

    i set the MTU 1400 and i'll leave it like this the Analyzer sees it 1400 MTU and MSS 1360 fine and speed is good.

    bloody internet mysteries lol

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