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Date:      Mon, 12 Aug 2002 13:06:27 -0700
From:      "Henderson, Thomas R" <thomas.r.henderson@boeing.com>
To:        "'Oleg Polyakov'" <opolyakov@yahoo.com>
Cc:        freebsd-net@freebsd.org
Subject:   RE: Initial congestion window increase
Message-ID:  <00EBC850E752CC46B8509DAB4D0D2CB910695B@xch-nw-29.nw.nos.boeing.com>
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> By the way looking into tcpdump revealed we have initial window 
> = 2 * mss in case of slowstart_flightsize = 1. I've seen it on
> FreeBSD 4.5 and 4.6 .
> What is this - an ancient bug in BSD stack?
> 

This used to be a bug/feature of the Net/3 TCP code (ack of syn caused
cwnd increment).  The snippet below seems to be aimed at preventing this
in current FreeBSD, but perhaps something else is incrementing initial
window to be 2 (not that this is a bad thing).

from tcp_input.c:1795
                /*
                 * If no data (only SYN) was ACK'd,
                 *    skip rest of ACK processing.
                 */
                if (acked == 0)
                        goto step6;


Tom

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