From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from bifrost.lang.hm (mail.lang.hm [64.81.33.126]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by huchra.bufferbloat.net (Postfix) with ESMTPS id 23EE221F60A for ; Fri, 5 Jun 2015 18:02:58 -0700 (PDT) Received: from asgard.lang.hm (asgard.lang.hm [10.0.0.100]) by bifrost.lang.hm (8.13.4/8.13.4/Debian-3) with ESMTP id t5612tX8012412; Fri, 5 Jun 2015 18:02:55 -0700 Date: Fri, 5 Jun 2015 18:02:55 -0700 (PDT) From: David Lang X-X-Sender: dlang@asgard.lang.hm To: Dave Taht In-Reply-To: Message-ID: References: User-Agent: Alpine 2.02 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: MULTIPART/MIXED; BOUNDARY="680960-1855027388-1433552575=:25770" Cc: cake@lists.bufferbloat.net Subject: Re: [Cake] lower bounds for latency X-BeenThere: cake@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list List-Id: Cake - FQ_codel the next generation List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 06 Jun 2015 01:03:27 -0000 This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. --680960-1855027388-1433552575=:25770 Content-Type: TEXT/PLAIN; charset=utf-8; format=flowed Content-Transfer-Encoding: 8BIT On Fri, 5 Jun 2015, Dave Taht wrote: > bob's been up to good stuff lately.. > > http://bobbriscoe.net/projects/latency/sub-mss-w.pdf one thing that looks wrong to me. He talks about how TCP implementations cannot operate at less than two packets per RTT. It's not clear what he means. Does he mean two packets in flight per RTT? or two packets worth of buffering per RTT? Two packets in flight per RTT would make sense as a minimum, but two packets worth of buffering on N devices in the path doesn't. using the example of a 6ms RTT. Depending on the equipment involved, this could have from one to several devices handling the packets between the source and the destination. Saying that each device in the path must have two packets worth of buffering doesn't make sense. At a given line speed and data rate, you will have X packets in flight. the number of devices between the source and the destination will not change X. If the requirement is that there are always at least two packets in flight in a RTT, it doesn't then follow that both packets are going to be in the buffer of the same device at the same time. I spoke with a vendor promising 7ms Los Angeles to Los Vegas. For the vast majority of that 7ms the packets are not in the buffers of the routers, but exist only as light in the fiber (I guess you could view the fiber acting as a buffer in such conditions) where is the disconnect between my understanding and what Bob is talking about? David Lang > It was weird, only last night I was thinking upon the real lower > bounds on what was needed to keep a flow going in tcp at X,Y,Z rtts > (in the context of being dissatisified with the stanford result, and > not "quite" in the context of "buffering"), and he nails that in the > first paragraph. > > Have to work through his prescription though.... > > -- > Dave Täht > What will it take to vastly improve wifi for everyone? > https://plus.google.com/u/0/explore/makewififast > _______________________________________________ > Cake mailing list > Cake@lists.bufferbloat.net > https://lists.bufferbloat.net/listinfo/cake --680960-1855027388-1433552575=:25770--