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Thread: 1492 or 1480 question

  1. #1

    1492 or 1480 question

    I have pppoe with windows xp does one mtu value have more advantages over the other? 1492 and 1480.

  2. #2
    Second Most EVIL YARDofSTUF's Avatar
    Join Date
    Nov 2000
    ya 1492 offers best results for dsl, 1480 doesnt

    Short for Maximum Transmission Unit, the largest physical packet size, measured in bytes, that a network can transmit. Any messages larger than the MTU are divided into smaller packets before being sent.

    Every network has a different MTU, which is set by the network administrator. On Windows 95, you can also set the MTU of your machine. This defines the maximum size of the packets sent from your computer onto the network. Ideally, you want the MTU to be the same as the smallest MTU of all the networks between your machine and a message's final destination. Otherwise, if your messages are larger than one of the intervening MTUs, they will get broken up (fragmented), which slows down transmission speeds.

  3. #3

    help needed please

    ive used the TCPOptimizer and restarted the pc but its defaulting to a MTU = 1500.can you help me please to get the best settings for my using xp prp adsl 2000/256 pppoe

    TCP options string = 020405b401010402
    MTU = 1500
    MTU is fully optimized for broadband.
    MSS = 1460
    Maximum useful data in each packet = 1460, which equals MSS.
    Default Receive Window (RWIN) = 64240
    RWIN Scaling (RFC1323) = 0 bits
    Unscaled Receive Window = 64240

    RWIN is a multiple of MSS
    Other values for RWIN that might work well with your current MTU/MSS:
    513920 (MSS x 44 * scale factor of 8)
    256960 (MSS x 44 * scale factor of 4)
    128480 (MSS x 44 * scale factor of 2)
    bandwidth * delay product (Note this is not a speed test):

    Your RcvWindow limits you to: 2569.6 kbps (321.2 KBytes/s) @ 200ms
    Your RcvWindow limits you to: 1027.84 kbps (128.48 KBytes/s) @ 500ms
    MTU Discovery (RFC1191) = ON
    Time to live left = 125 hops

    TTL value is ok.
    Timestamps (RFC1323) = OFF
    Selective Acknowledgements (RFC2018) = ON
    IP type of service field (RFC1349) = 00000000

  4. #4
    SG Enthusiast
    Join Date
    Mar 2002

    RE : 1492 or 1480 as MTU doubts

    1492 bits
    is default MTU value for protocols : PPPoE and or PPPoA
    (for both direction transfers ISP to customers)

    1480 bits
    is default MTU value for native client PPPoE in wXP
    (one can ask MS why)

    importance of above 12 bits is increased with connection's speed
    ~0,8% from total data stream (12/1492=0,0080428954423592493297587131367292)
    so only connection faster than 10 MBps
    and with good the Internet weather
    can be noted as differentiation in speed test results

    BTW also 'Time Stamp' option is 12 bits worth (from MSS size 'useful data')
    so one can compare MTU=1492 speed result with 'Time Stamp' on vs. off
    OTOH mentioned TS is much more important in same connections
    e.g. for retransmissions

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