<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; color: rgb(0, 0, 0); font-size: 14px; font-family: Calibri, sans-serif;">
<div>FYI piggybacked requests in DOCSIS eliminate the possibility of request collisions (and resulting backoff/retry). In DOCSIS 3.0 they will result in lower latency only as a result of eliminating these events. In a lightly loaded DOCSIS 3.0 network (few
neighbors contending for bandwidth) the collision probability will be low anyway, so it won't make much difference. In a highly utilized network (especially in the case where a lot of neighbors are sending at rates below the piggybacking threshold) it can
make a bigger difference.</div>
<div><br>
</div>
<div>Packet rates above ~200pps will be sufficient to ensure piggybacking.</div>
<div><br>
</div>
<div>I usually consider ~6ms to be the nominal upstream MAC latency, and ~0.7ms to be the equivalent on downstream, making the RTT on the DOCSIS 3.0 link close to 7ms (depending on configuration). </div>
<div><br>
</div>
<div>-Greg</div>
<div><br>
</div>
<span id="OLK_SRC_BODY_SECTION">
<div style="font-family:Calibri; font-size:11pt; text-align:left; color:black; BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0in; PADDING-LEFT: 0in; PADDING-RIGHT: 0in; BORDER-TOP: #b5c4df 1pt solid; BORDER-RIGHT: medium none; PADDING-TOP: 3pt">
<span style="font-weight:bold">From: </span>Jim Gettys <<a href="mailto:jg@freedesktop.org">jg@freedesktop.org</a>><br>
<span style="font-weight:bold">Date: </span>Tuesday, April 29, 2014 at 11:07 AM<br>
<span style="font-weight:bold">To: </span>Toke Høiland-Jørgensen <<a href="mailto:toke@toke.dk">toke@toke.dk</a>><br>
<span style="font-weight:bold">Cc: </span>bloat <<a href="mailto:bloat@lists.bufferbloat.net">bloat@lists.bufferbloat.net</a>>, "Fred Baker (fred)" <<a href="mailto:fred@cisco.com">fred@cisco.com</a>>, "<a href="mailto:aqm@ietf.org">aqm@ietf.org</a>" <<a href="mailto:aqm@ietf.org">aqm@ietf.org</a>>,
"<a href="mailto:cerowrt-devel@lists.bufferbloat.net">cerowrt-devel@lists.bufferbloat.net</a>" <<a href="mailto:cerowrt-devel@lists.bufferbloat.net">cerowrt-devel@lists.bufferbloat.net</a>>, Mikael Abrahamsson <<a href="mailto:swmike@swm.pp.se">swmike@swm.pp.se</a>><br>
<span style="font-weight:bold">Subject: </span>Re: [aqm] [Bloat] the side effects of 330ms lag in the real world<br>
</div>
<div><br>
</div>
<div>
<div>
<div dir="ltr">
<div class="gmail_default" style="font-size:small"><br>
</div>
<div class="gmail_extra"><br>
<br>
<div class="gmail_quote">On Tue, Apr 29, 2014 at 1:01 PM, Toke Høiland-Jørgensen <span dir="ltr">
<<a href="mailto:toke@toke.dk" target="_blank">toke@toke.dk</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div class="">Jim Gettys <<a href="mailto:jg@freedesktop.org">jg@freedesktop.org</a>> writes:<br>
<br>
> Now, if someone gives me real fiber to the home, with a real switch fabric<br>
> upstream, rather than gpon life might be somewhat better (if the switches aren't<br>
> themselves overbuffered.... But so far, it isn't.<br>
<br>
</div>
As a data point for this, I have fibre to my apartment building and<br>
ethernet into the apartment. I get .5 ms to my upstream gateway and<br>
about 6 ms to Google. Still measured up to ~20 ms of bufferbloat while<br>
running at 100 Mbps...<br>
<br>
<a href="http://files.toke.dk/bufferbloat/data/karlstad/cdf_comparison.png" target="_blank">http://files.toke.dk/bufferbloat/data/karlstad/cdf_comparison.png</a><br>
<br>
However, as that graph shows, it is quite possible to completely avoid<br>
bufferbloat by deploying the right shaping</blockquote>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
And in that case fibre<br>
*does* have a significant latency advantage. The best latency I've seen<br>
to the upstream gateway on DSL has been ~12 ms.<br>
</blockquote>
<div><br>
</div>
<div class="gmail_default" style="font-size:small">Media access is a killer on Cable too, putting the latency floor at around 8ms on my Docsis 3.0 Comcast service, though you can sometimes get lucky and piggyback. to somewhat lower latency, IIRC conversations
with Greg White about how cable works.</div>
<div class="gmail_default" style="font-size:small"> - Jim</div>
<div class="gmail_default" style="font-size:small"><br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<span class="HOEnZb"><font color="#888888"><br>
-Toke<br>
</font></span></blockquote>
</div>
<br>
</div>
</div>
</div>
</div>
</span>
</body>
</html>