From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Date: Fri, 13 Apr 2007 19:33:56 +0000 Subject: Re: Sensitivity of TFRC throughput equation wrt to changes of RTT Message-Id: <20070413.123356.21601263.davem@davemloft.net> List-Id: References: <200704131303.03072@strip-the-willow> In-Reply-To: <200704131303.03072@strip-the-willow> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: dccp@vger.kernel.org From: Gerrit Renker Date: Fri, 13 Apr 2007 13:03:02 +0100 > RFC 3448 gives in section 8 the following alternative format > of the throughput equation (which is directly responsible for > the alllowed sending rate X): > > s > X = -------- > R * f(p) > > This shows that the dependence is reciprocal. Thus using an RTT > which differs by a factor of 10 to account for in-stack processing > results an a throughput reduction of factor 10. > > In other words, 90 Mbits/sec becomes 9 Mbits/sec. What I'd like to know in all this is why the RTT influences the sending rate at all in such a manner. Please teach me :) If I have a 10gbit pipe all the way to the planet mars I should still be feeding that pipe at a rate of 10gbit. :) TCP doesn't have any of these problems, and we use incredibly coarse timestamping for RTTs. We get jiffies granularity at best, with many in-stack delays, and we still send at full line rate over large RTTs.