FYI: Set your MTU to 1478 for best performance on ADSL

Discussion in 'NZ Computing' started by Mark C, May 8, 2005.

  1. Mark C

    Mark C Guest

    I've recently googled and run some tests, and my ADSL (PPPoA)
    connection runs fastest if I set MTU to 1478 bytes.
    Its faster by about 1.7% compared to the standard MTU of 1500 (yeah,
    I know "whoop-de-doo..." ;-)


    Check your MTU here:

    Set your MTU using DRTCP:

    Also set your RWIN (TCP Receive Window Size).
    "For best results, the TCPWindow should be a (even) multiple of MSS
    (Maximum Segment Size). MSS is generally MTU - 40" RWIN

    On my 256kbps Bitstream connection, assuming a max latency of 500ms,
    I have set RWIN to 17256 bytes (=12 * MSS)

    Having a too high an RWIN can slow down downloads when high packet
    loss occurs (such as Orcon bitstream with packet loss).

    Mark C, May 8, 2005
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  2. Mark C

    nil_einne1 Guest

    Hmm interesting. I came across this issue before but at the time, there
    was only Lawrence Baldwin's post which is only right for PPPoE on DSL
    (possibly not even right for that). I asked him but he said he switched
    over from DSL to cable and never completed the new calculations. I did
    some more searching and did my own calculations and can't remember
    whether I reached the same conclusions but it looks like it since I
    think I just did the same thing as the other person did. I believe I
    posted something about this to the NZ-Dsl list. In any case, it didn't
    matter much at the time since it was the days when we we're speed
    limited at the IP level (or whatever it's called) which meant MTU
    issues related to the ATM shouldn't apply. Now that we're limited at
    the ATM/line level, it should make a difference though I suspect but as
    you say, the difference is so small despite our miniscule speeds it's
    of little importance to most of us :)
    nil_einne1, May 12, 2005
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