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Date: Mon, 10 Jan 2005 17:37:51 -0800
From: Girish Rayas <grayas@gmail.com>
To: freebsd-net@freebsd.org
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Subject: Bug in TCP window update?
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In tcp_input.c, window is updated when below condition is true,

if ((thflags & TH_ACK) &&
(SEQ_LT(tp->snd_wl1, th->th_seq) ||
(tp->snd_wl1 == th->th_seq && (SEQ_LT(tp->snd_wl2, th->th_ack) ||
(tp->snd_wl2 == th->th_ack && tiwin > tp->snd_wnd)))))

This check is to prevent old segments from affecting the send window.
But, left trim logic that was executed earlier in tcp_input.c sets the
th->th_seq to
tp->rcv_nxt for old segments. In many scenarios this effectively causes
snd_wl1 < th_seq and results in incorrect window update by old
segments.

Using actual sequence number of received segment in the above if
statement will fix the problem. Any comments?