From owner-freebsd-hackers@FreeBSD.ORG  Wed Sep  8 16:43:39 2004
Return-Path: <owner-freebsd-hackers@FreeBSD.ORG>
Delivered-To: freebsd-hackers@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP id B04C416A4CE
	for <freebsd-hackers@freebsd.org>;
	Wed,  8 Sep 2004 16:43:39 +0000 (GMT)
Received: from mproxy.gmail.com (mproxy.gmail.com [216.239.56.240])
	by mx1.FreeBSD.org (Postfix) with ESMTP id 9C0BD43D45
	for <freebsd-hackers@freebsd.org>;
	Wed,  8 Sep 2004 16:43:39 +0000 (GMT)
	(envelope-from shchoi@gmail.com)
Received: by mproxy.gmail.com with SMTP id w67so97456cwb
	for <freebsd-hackers@freebsd.org>;
	Wed, 08 Sep 2004 09:43:36 -0700 (PDT)
Received: by 10.11.119.70 with SMTP id r70mr58105cwc;
        Wed, 08 Sep 2004 09:43:36 -0700 (PDT)
Received: by 10.11.117.28 with HTTP; Wed, 8 Sep 2004 09:43:36 -0700 (PDT)
Message-ID: <34b425c504090809435a949a03@mail.gmail.com>
Date: Wed, 8 Sep 2004 17:43:36 +0100
From: Soo-Hyun Choi <shchoi@gmail.com>
To: freebsd-hackers@freebsd.org
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Subject: TCP RTO
X-BeenThere: freebsd-hackers@freebsd.org
X-Mailman-Version: 2.1.1
Precedence: list
Reply-To: Soo-Hyun Choi <shchoi@gmail.com>
List-Id: Technical Discussions relating to FreeBSD
	<freebsd-hackers.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>,
	<mailto:freebsd-hackers-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-hackers>
List-Post: <mailto:freebsd-hackers@freebsd.org>
List-Help: <mailto:freebsd-hackers-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>,
	<mailto:freebsd-hackers-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Wed, 08 Sep 2004 16:43:39 -0000

Hi,

I'm not sure if this list is appropriate to ask about the FreeBSD kernel
source or not. If not, could somebody direct me in an appropriate
list?

My curiosity is if we see the tcp.cc code inside, there are two
different version of srtt (smoothed rtt) and rttvar (smoothed mean
deviation estimator). The one is simply 'srtt' and 'rttvar' and the
other is 't_srtt' and 't_rttvar'. The unit of t_srtt is 'ticks * 8'
and the unit of t_rttvar is 'ticks * 4'.

These variables are used to calculate the TCP RTO. But why do they
have the two different version of variables? The interesting thing is
the 't_' variables are a fixed-point integer, and the original
variables are just floating-point values. I assume the reason why they
have is to avoid the floating-point arithmetic in the kernel. Is this
really only reason for being two different version of those?

Cheers,
Soo-Hyun