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Thread: Finding mtu size

  1. #1
    Junior Member
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    Finding mtu size

    Hello,

    i don't understand this at all.
    [TCP Optimization for lower pings]

    I have modem ube evm3230 and when i tried ping google for example with mtu 1500, it writes:"packets needs to be fragmented".

    I wasn't able find any information about mtu size of this modem and i tested it without router.

    And it is weird, i thought for ethernet is 1500, so why should has this modem problems with it today times, i mean this is old a while.

    To tutorial: when i found mtu size, which doesn't need to be fragmented, they write you have to add header to this value +28.

    It makes no sense, by this http://www.dcgaming.org/wp-content/u...1/fragment.jpg should be header already included in mtu.

    So why i should add it, in addition, if i add it, it starts again writes:"packets needs to be fragmented".

  2. #2
    Administrator Philip's Avatar
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    The 1500 bytes packet/MTU includes TCP/IP headers. TCP and IP headers are 20 bytes each, so, in essence you have:
    TCP/IP traffic = 1460 bytes payload + 20 bytes IP header + 20 bytes TCP header = 1500 bytes max.

    These 40 bytes headers can differ as well (if you add 1323 TCPOptions timestamps, for example it adds another 12 bytes of headers further reducing the payload/useful data).

    With that in mind, when you do a "ping", you sending out ICMP, not TCP/IP packets. ICMP adds 8 bytes header, and your packets still have the 20 bytes IP header, so, then:
    ICMP packet = 1472 bytes data + 20 bytes IP header + 8 bytes ICMP header = 1500 bytes max.

    That is why you have +28 bytes of headers with ICMP pings.

    I don't know about the Ubee evm3230 specifically, but some ISPs and devices may add other headers (like ADSL PPPoE, for example), reducing that 1500 bytes MTU. In some cases they can be configured to increase the MTU.

    I hope this helps.

  3. #3
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    Than my mtu size is okay, i can ping google with 1472 bytes, without message: packets needs to be fragmented.
    Thanks for perfect explanation.

  4. #4
    Administrator Philip's Avatar
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    Yes, if you can ping up to 1472 bytes without fragmentation, your MTU is 1500. The TCP Optimizer has that ping test built-in as well.

    Glad I could help.

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