IP Frags - Help!!!
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Thread: IP Frags - Help!!!

  1. #1
    Senior Member
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    IP Frags - Help!!!

    Hey guys, I need ur help on this one.
    Windows 2000 use as default the don't fragment bit to enabled and I'd like to disable it.

    Help!!!
    [shadow] SHARING KNOWLEDGE[/shadow]

  2. #2
    Just Another Geek
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    Found this in a knowledge base artikel (Q241985) about SNA. It talks about blackhole routers and disabling this 'feature'. It may help:

    1. Start Registry Editor (Regedt32.exe).

    2. Locate the following key in the registry:

    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\tcpip\parameters

    3. On the Edit menu, click Add Value, and then add the following registry
    value:

    Value Name: EnablePMTUBHDetect
    Data Type: REG_DWORD
    Value: 1

    4. Quit Registry Editor.

    MORE INFORMATION
    ================

    When a TCP/IP segment is sent from the host to the router with the "Don't
    Fragment" bit set, and the segment is too large for the router to handle, the
    router should notify the host with a "Destination Unreachable" message. However,
    sometimes the router does not send the notification to the host, and instead, it
    drops the segment. A router that simply drops packets in this manner is called a
    "Black Hole" router.

    Setting the registry parameter to 1 (True) enables TCP to try to detect "Black
    Hole" routers. With this feature enabled, TCP attempts to send segments without
    the "Don't Fragment" bit set if several retransmissions of a segment are
    unacknowledged. If the segment is acknowledged as a result, the TCP Maximum
    Segment Size (MSS) is decreased and the "Don't Fragment" bit is set in future
    packets on the connection.

    For additional information on this topic, click the article numbers below to view
    the articles in the Microsoft Knowledge Base:

    Q120642 TCP/IP and NBT Configuration Parameters for Windows 2000 or Windows
    NT

    Q136970 PMTU Black Hole Detection Algorithm Change for Windows NT 3.51

    Q159211 Diagnoses and Treatment of Black Hole Routers
    Oliver's Law:
    Experience is something you don't get until just after you need it.

  3. #3
    Senior Member
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    Post

    Thanx man, I found that as well in microsoft.com but my problem is a bit more complex.
    Any expertise are welcome .

    My goal is to have MTUdicovery enabled (and therefore there no black hole router) but in the mean time I'd like to set the don't fragment bit to disabled.
    In that case I don't think the EnablePMTUBHDetect will help!!!
    [shadow] SHARING KNOWLEDGE[/shadow]

  4. #4
    Just Another Geek
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    Originally posted here by Networker
    My goal is to have MTUdicovery enabled (and therefore there no black hole router) but in the mean time I'd like to set the don't fragment bit to disabled.
    Then you should take a look at Q120642, it lists all the tcp/ip settings.
    You should probably set EnablePMTUDiscovery to 1 (default setting).

    AFAIK there's no option to enable/disable the DF bit. Winblows
    Oliver's Law:
    Experience is something you don't get until just after you need it.

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