From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-02-ewr.dyndns.com (mxout-035-ewr.mailhop.org [216.146.33.35]) by lists.bufferbloat.net (Postfix) with ESMTP id 98FF82E04A9 for ; Tue, 15 Mar 2011 23:26:42 -0700 (PDT) Received: from scan-01-ewr.mailhop.org (scanner [10.0.141.223]) by mail-02-ewr.dyndns.com (Postfix) with ESMTP id 0CC4873DC5A for ; Wed, 16 Mar 2011 06:26:42 +0000 (UTC) X-Spam-Score: -1.0 (-) X-Mail-Handler: MailHop by DynDNS X-Originating-IP: 74.125.82.171 Received: from mail-wy0-f171.google.com (mail-wy0-f171.google.com [74.125.82.171]) by mail-02-ewr.dyndns.com (Postfix) with ESMTP id BC7A573DC36 for ; Wed, 16 Mar 2011 06:26:41 +0000 (UTC) Received: by wyb32 with SMTP id 32so1599492wyb.16 for ; Tue, 15 Mar 2011 23:26:41 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=domainkey-signature:subject:mime-version:content-type:from :in-reply-to:date:cc:content-transfer-encoding:message-id:references :to:x-mailer; bh=WekjmiQT6MUDTVJ1H1hWjQB6nU2NqGlvAXwHzV6Tgsk=; b=Stlq1Up2Kp+yPAEDJahj8a/P74J7Fu8PY/vOhiSka155ppqrYwLF5oeTCBjkdp/h9B peMC1+cIwwR0YlVBUFnqpKaC4Qnjr2VlsUF4CYUleS83i1iEjt2xlhrZ55OXIK8oQCmi kKpnSLThOsVgOrZg7IYXo+PoOqSosvGscNhfY= DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=subject:mime-version:content-type:from:in-reply-to:date:cc :content-transfer-encoding:message-id:references:to:x-mailer; b=eeevuRoo79g27rO1gpnYPpg144CyFTBJVPqU3vRNMA2VzexRBIRb7rlhiVS7KBPTWh 0+wVVSZlVDM7RcSWI47BcfqEMk1DaV3R1Jvc+WhEalMTn8tHiE6htxN2NwqLgf2d0L1T /JwWvxW6F6fcvzQTOcjD4jg20XyNTvViz4rBQ= Received: by 10.227.202.73 with SMTP id fd9mr386119wbb.72.1300256801003; Tue, 15 Mar 2011 23:26:41 -0700 (PDT) Received: from [192.168.239.42] (xdsl-83-150-84-172.nebulazone.fi [83.150.84.172]) by mx.google.com with ESMTPS id x1sm735743wbh.14.2011.03.15.23.26.40 (version=TLSv1/SSLv3 cipher=OTHER); Tue, 15 Mar 2011 23:26:40 -0700 (PDT) Mime-Version: 1.0 (Apple Message framework v1082) Content-Type: text/plain; charset=us-ascii From: Jonathan Morton In-Reply-To: <2927A265-BB9F-4466-BA03-1522AFF989C3@cisco.com> Date: Wed, 16 Mar 2011 08:26:39 +0200 Content-Transfer-Encoding: quoted-printable Message-Id: References: <4D7F4121.40307@freedesktop.org> <2927A265-BB9F-4466-BA03-1522AFF989C3@cisco.com> To: Fred Baker X-Mailer: Apple Mail (2.1082) Cc: bloat@lists.bufferbloat.net Subject: Re: [Bloat] Random idea in reaction to all the discussion of TCP flavours - timestamps? X-BeenThere: bloat@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list List-Id: General list for discussing Bufferbloat List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 16 Mar 2011 06:26:43 -0000 On 16 Mar, 2011, at 7:41 am, Fred Baker wrote: > The question isn't "what is the magic mean queue depth for = min-threshold to be set to"; it's "what mark/drop rate is sufficient to = keep the queue somewhat shallow most of the time". And that's what BLUE (and SFB) tries to do. If the queue grows beyond = some limit, it increments the marking rate. If it becomes empty, it = decrements it. The result is an I-type control loop which has a = reasonable chance of finding a steady state, if there is one. SFB does = it on a per-flow basis. I also thought of an analogy just now, as I was playing with my train = simulator - many people like car analogies, I prefer railway ones. A = router is like a bunch of railways meeting at a grand-union flying = junction (typically implemented as a cloverleaf in the real world). The = more expensive kinds are built with wider curves that let trains go fast = even in the junction. You can have lightweight, fast passenger trains, running loaded in both = directions, and these are your VoIP traffic. Among them you might have = heavy, slow freight trains, which just happen to weigh about 1500 tons = each, but which run empty from the power station back to the mines. You = don't want to be on a passenger train stuck behind a freight, so = railways build extra tracks, either at intervals or continuously, to = keep freight trains in and allow passenger trains past. But a railway can only carry one train on each track at a time, and = tracks are expensive. So sometimes a train still has to wait for = another one. They can simply wait one behind the other at signals, or = the railways might decide to put a marshalling yard in at the junction, = so that many trains can be stored and rearranged for efficient = prioritisation. Meanwhile, a wireless network is more like a bunch of railways which = meet at the cheapest, skinniest single-track junction the builders could = devise - only one train at a time can use it, and sometimes they even = fall off the rails and have to be crowbarred back on. It's a bit of a = mess, but the junction is up in the mountains so it's very difficult to = improve it. The problem is that the railway company doesn't like to admit that the = trains are slow and unreliable at this junction, so it employs lots of = men with crowbars, and tries to avoid the subject when trains arrive = several hours late. Yet people *do* notice, especially during the = holiday season when *everyone* is travelling and the freight trains are = chock-full of parcels - and the snow is just starting to fall in = earnest, which freezes the points. - Jonathan