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DF bit change in Windows Server 2008 R2 RRS feed

  • Question

  • Can we change DF bit in windows server 2008 R2? 

    What i understand is that we cannot change DF in windows server 2008 R2, The DF bit is set by default on all tcp traffic, for icmp we can use "-f"

    wht is difference between MTU and DF bit



    • Edited by Risingflight Friday, February 19, 2016 7:14 PM e
    Friday, February 19, 2016 6:28 PM

Answers

All replies

  • MTU is the maximum transmission unit (the size of the box in which packets travel). DF (Don't Fragment) instructs the router to drop packets instead of breaking them into smaller pieces when a given packet is bigger than the MTU allows.

    http://blog.ine.com/2008/11/05/dealing-with-fragmented-traffic/

    In Windows Vista/2008 and later, MTU is managed automatically, but I believe you can set EnablePMTUBHDetect to 1 which will set the DF bit.

    https://msdn.microsoft.com/en-us/library/cc558565(v=bts.10).aspx

    More background:

    https://blogs.technet.microsoft.com/askpfeplat/2014/12/01/psa-incorrect-mtu-size-causes-connectivity-issues-with-windows-server-2012-and-windows-server-2012-r2/


    Mike Crowley | MVP
    My Blog -- Baseline Technologies


    Saturday, February 20, 2016 12:20 AM
  • In a MTU of 1500 i need to set DF for icmp how can i do. i have a server by name testsrv.mydomain.com. how do i do for this.

    in 1500 bytes how many bytes will for IP header and ICMP datagram.

    for one of our server, the router is dropping packes which are larger then 1500 bytes, server is set to 1514 bytes. so i need to disable DF bit.

    Saturday, February 20, 2016 6:46 AM
  • Hi Risingfligh,

    >server is set to 1514 bytes.

    What does the 1514 represent?

    Basically, the Ethernet standard limits the size of an Ethernet frame to 1514 bytes, with 14 byte Ethernet header and 1500 bytes data. In another word, at Layer3- network layer, packets are 1500 bytes limit, at layer2- data link layer, frames are 1514 limit (1500 + 14).

    Best Regards,

    Anne


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    Wednesday, February 24, 2016 8:47 AM
  • Hi Risingfligh,

    >server is set to 1514 bytes.

    What does the 1514 represent?

    Basically, the Ethernet standard limits the size of an Ethernet frame to 1514 bytes, with 14 byte Ethernet header and 1500 bytes data. In another word, at Layer3- network layer, packets are 1500 bytes limit, at layer2- data link layer, frames are 1514 limit (1500 + 14).

    Best Regards,

    Anne


    Actually, that's not quite correct.  The 1500 byte limit applies only to 802.3 Ethernet, which has the length field in the header, not Ethernet II, which has an Ethertype field.  IP uses Ethernet II.  It is entirely possible to have a 9K byte jumbo frame carrying IP, though most ISPs will not pass anything beyond 1500.  Ethernet II frames have nothing to indicate a length, other than the packet ending.

    This link shows the difference between Ethernet II and 802.3.

    https://en.wikipedia.org/wiki/Ethernet_frame

    BTW, jumbo frames are often used in data centres.

    Tuesday, November 14, 2017 10:25 PM