From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Edgar E. Iglesias" Subject: Re: [PATCH] make _minimum_ TCP retransmission timeout configurable Date: Thu, 30 Aug 2007 00:53:00 +0200 Message-ID: <20070829225300.GA15652@edgar.underground.se.axis.com> References: <5640c7e00708291432q6acde704od52247647a6b453@mail.gmail.com> <20070829.144656.104048365.davem@davemloft.net> <46D5F32F.2070502@hp.com> <20070829.153503.18295527.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: rick.jones2@hp.com, ian.mcdonald@jandi.co.nz, netdev@vger.kernel.org To: David Miller Return-path: Received: from miranda.se.axis.com ([193.13.178.8]:57096 "EHLO miranda.se.axis.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754881AbXH2XFN (ORCPT ); Wed, 29 Aug 2007 19:05:13 -0400 Received: from axis.com (edgar.se.axis.com [10.92.151.1]) by miranda.se.axis.com (8.13.4/8.13.4/Debian-3sarge3) with ESMTP id l7TMr1Du031166 for ; Thu, 30 Aug 2007 00:53:01 +0200 Content-Disposition: inline In-Reply-To: <20070829.153503.18295527.davem@davemloft.net> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Wed, Aug 29, 2007 at 03:35:03PM -0700, David Miller wrote: > From: Rick Jones > Date: Wed, 29 Aug 2007 15:29:03 -0700 > > > David Miller wrote: > > > None of the research folks want to commit to saying a lower value is > > > OK, even though it's quite clear that on a local 10 gigabit link a > > > minimum value of even 200 is absolutely and positively absurd. > > > > > > So what do these cellphone network people want to do, increate the > > > minimum RTO or increase it? Exactly how does it help them? > > > > They want to increase it. The folks who triggered this want to make it > > 3 seconds to avoid spurrious RTOs. Their experience the "other > > platform" they widh to replace suggests that 3 seconds is a good value > > for their network. > > > > > If the issue is wireless loss, algorithms like FRTO might help them, > > > because FRTO tries to make a distinction between capacity losses > > > (which should adjust cwnd) and radio losses (which are not capacity > > > based and therefore should not affect cwnd). > > > > I was looking at that. FRTO seems only to affect the cwnd calculations, > > and not the RTO calculation, so it seems to "deal with" spurrious RTOs > > rather than preclude them. There is a strong desire here to not have > > spurrious RTO's in the first place. Each spurrious retransmission will > > increase a user's charges. > > All of this seems to suggest that the RTO calculation is wrong. > > It seems that packets in this network can be delayed several orders of > magnitude longer than the usual round trip as measured by TCP. > > What exactly causes such a huge delay? What is the TCP measured RTO > in these circumstances where spurious RTOs happen and a 3 second > minimum RTO makes things better? I don't know what they are doing, but it reminds me of what happens when you run TCP over a reliable medium. You don't see loss, instead the RTT starts to jitter alot. IIRC FRTO does help avoid unnecessary retransmits (although the RTO still hits). Best regards -- Programmer Edgar E. Iglesias 46.46.272.1946