Error in cwin estimation?

From: Bogdan Ghita (b.ghita@jack.see.plym.ac.uk)
Date: 04/09/01

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    Message-ID: <3AD19898.B1698CAF@jack.see.plym.ac.uk>
Date: Mon, 09 Apr 2001 12:10:16 +0100
From: Bogdan Ghita <b.ghita@jack.see.plym.ac.uk>
Subject: Error in cwin estimation?

    Dear all

    I am using tcptrace to analyse a TCP trace and I found something wrong
    in the output: the cwin estimation. Here is a grep-ed output:
         max cwin: 108 bytes max cwin: 2920
    bytes
         min cwin: 108 bytes min cwin: 940
    bytes
         avg cwin: 19 bytes avg cwin: 1051
    bytes

         max cwin: 297 bytes max cwin: 441
    bytes
         min cwin: 297 bytes min cwin: 441
    bytes
         avg cwin: 59 bytes avg cwin: 220
    bytes

         max cwin: 196 bytes max cwin: 1826
    bytes
         min cwin: 196 bytes min cwin: 1460
    bytes
         avg cwin: 65 bytes avg cwin: 1095
    bytes

    As it can be seen, always the average cwin is lower than the minimum
    value. I believe the difference comes from the way avg cwin is
    determined:

    pab->ack_pkts==0?0:pab->cwin_tot/pab->ack_pkts

    The number of ACKs is higher than the number of cwin calculations, at
    least if we consider the retransmissions and the SYN and FIN sequences.
    Should there be another variable (yet another one :-) ), such as
    cwin_count, that is incremented each time cwin_tot is updated?

    Best regards
    Bogdan Ghita
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