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Thread: strange MTU behavior!

  1. #1

    Question strange MTU behavior!

    Something is stragne to me; maybe someone can explain it to me!
    Here are the results from the analyzer:
    ________________________________________________ TCP Analyzer Results
    Tested on: 02.23.2007 06:24
    IP address:

    TCP options string: 020405ac01010402
    MSS: 1452
    MTU: 1492
    TCP Window: 33396 (multiple of MSS)
    RWIN Scaling: 0
    Unscaled RWIN : 33396
    Reccomended RWINs: 63888, 127776, 255552, 511104
    BDP limit (200ms): 1336kbps (167KBytes/s)
    BDP limit (500ms): 534kbps (67KBytes/s)
    MTU Discovery: ON
    TTL: 46
    Timestamps: OFF
    SACKs: ON
    IP ToS: 00000000 (0)
    Ok! Thing I dont understand is when I try to ping with ping -f -l 1492 I get the hint that the packet has to be fragmented but df-flag is set....
    I have to lower the the value till 1464 until I get packages back!
    But according to the analyzer it should be 1492.

    Why do this happen ???
    Thanks in advance!

    best regards

  2. #2
    Elite Member trogers's Avatar
    Join Date
    Jan 2005
    Bangkok, Thailand
    When you use the DOS command for ping -f -l, you have to chose a packet size which is 28 bytes less than your MTU.

    Thus if your MTU is 1500, you can ping with a packet size not greater than 1472. If greater, you get fragmentation.

    For MTU 1492, the largest packet size is 1464.

  3. #3
    OH my god!
    i am dumb!
    thanks a lot for your answer and explanation!

    best reagards

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