From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-x22a.google.com (mail-oi0-x22a.google.com [IPv6:2607:f8b0:4003:c06::22a]) (using TLSv1 with cipher RC4-SHA (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority G2" (verified OK)) by huchra.bufferbloat.net (Postfix) with ESMTPS id 580E521F52F for ; Fri, 5 Jun 2015 18:58:56 -0700 (PDT) Received: by oigz2 with SMTP id z2so1639942oig.1 for ; Fri, 05 Jun 2015 18:58:56 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; bh=xuRCwkvQ6iJyslc0hXhlV2QLEClq333dqNEncJW6I24=; b=v4X/CDBSkMEiO8kW3n0b5i6pNRzBfL+Z4WaO5RkS2B893HoyWMMGUfLf18gUtq6hIH B+ltWPatJO58vHXymsiih6BN/hDHcmGsJ0cIPub6mzPkAea2Mmzp4IyyLExkUvZVrt82 +2G+0pmSGA66du+/BKOLdG8YfkT6u4KULxMboRoUEAwojVXrER4hr5xqJLxeELH1Fbry eqXFYSHpycwCN1P/ZWk1QV/Rnqk+z/3OBKgj6IAh1vzdEv0Wb35uFf/AEbOjzGRTuEKG E7c/m98zWlh2c9qma2AjcINcSx1u8+nZA3wwjiRSPPM8B6CJWUgH/Nrm29BmD1WoxsJP GTHQ== MIME-Version: 1.0 X-Received: by 10.202.216.138 with SMTP id p132mr5076334oig.133.1433555936183; Fri, 05 Jun 2015 18:58:56 -0700 (PDT) Received: by 10.202.105.129 with HTTP; Fri, 5 Jun 2015 18:58:56 -0700 (PDT) In-Reply-To: References: Date: Fri, 5 Jun 2015 18:58:56 -0700 Message-ID: From: Dave Taht To: David Lang Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable 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 01:59:25 -0000 On Fri, Jun 5, 2015 at 6:02 PM, David Lang wrote: > 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 o= f > buffering per RTT? Flight. I also disagree with this statement: "It is always wrong to send smaller packets more often, because the constraint may be packet pro- cessing, not bits." because I believe (but would have to go look at some data to make sure) that we're good with packet sizes down into the 300 byte range on most hardware, and thus could (and SHOULD) also reduce the mss in these cases to keep the "signal strength" up at a sustainable levels. I do recall that bittorrent used to reduce the mss and were asked to stop (a decade ago), when they got as far down as 600 bytes or so. but the rest I quite liked. > > Two packets in flight per RTT would make sense as a minimum, but two pack= ets > 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 betwe= en > the source and the destination will not change X. Flight. > If the requirement is that there are always at least two packets in fligh= t > 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 promisi= ng > 7ms Los Angeles to Los Vegas. For the vast majority of that 7ms the packe= ts > are not in the buffers of the routers, but exist only as light in the fib= er > (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? Flight, not buffering. Redefining the goal of an aqm to keep packets in flight rather than achieve a fixed queuing delay is what this is about, and improving tcps to also keep packets in flight with subpacket windows is part of his answer. I like getting away from a target constant for delay (why 5ms when 5us is doable) and this is an interesting way to think about it from both ends. And I was nattering about how I didn't like delayed acks just a few hours a= go. > > 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 T=C3=A4ht >> 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 --=20 Dave T=C3=A4ht What will it take to vastly improve wifi for everyone? https://plus.google.com/u/0/explore/makewififast