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<div class=Section1>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>David,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Another possible explanation would be that
the IAP equipment (or DOCSIS modem) provides some level of packet differentiation
and queuing/prioritization (e.g., real-time v/s non-real-time) and during your
test, there was a lot of real-time traffic, reducing the amount of bandwidth
available to TCP-traffic. Even if queues are “properly” sized based
on access link speed, multiple queues and prioritization can create havoc with
packet delays.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>To some extent, we are all to blame, since
we (the IETF community) have not really offered any strong guidelines on how to
size router buffers. There are plenty of papers on the subject. The most we
seem to suggest is that it should be some fraction of the bandwidth-delay
product, with little agreement on the fraction value.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Some critical questions remain unanswered –<o:p></o:p></span></font></p>
<ol style='margin-top:0in' start=1 type=1>
<li class=MsoNormal style='color:navy;mso-list:l1 level1 lfo4'><font size=2
color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial'>What
does bandwidth-delay product mean for a poor access router, probably pre-configured
at the factory? Is the bandwidth based on its own access link speed? What
delay value should be used? - delay can vary dynamically across a wide
range.<o:p></o:p></span></font></li>
<li class=MsoNormal style='color:navy;mso-list:l1 level1 lfo4'><font size=2
color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial'>How
about links with variable speeds (wireless, satellite)? Should buffer size
be computed dynamically.<o:p></o:p></span></font></li>
<li class=MsoNormal style='color:navy;mso-list:l1 level1 lfo4'><font size=2
color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial'>What
happens with use of Differentiated services and queues? The bandwidth
available to a specific queue can dynamically vary across a wide range. Best
Effort queue probably suffers the most.<o:p></o:p></span></font></li>
<li class=MsoNormal style='color:navy;mso-list:l1 level1 lfo4'><font size=2
color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial'>Should
queues be sized based on maximum queuing delay instead of a fixed/computed
amount of buffer space?<o:p></o:p></span></font></li>
<li class=MsoNormal style='color:navy;mso-list:l1 level1 lfo4'><font size=2
color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial'>Similar
questions apply to use of RED/ECN; how do we compute (all) RED parameters
for such links and queues?<o:p></o:p></span></font></li>
</ol>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Anil<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<div>
<p class=MsoNormal><em><i><font size=2 color=black face=Arial><span
style='font-size:10.0pt;font-family:Arial'>Anil Agarwal</span></font></i></em><o:p></o:p></p>
<p class=MsoNormal><em><i><font size=2 color=black face=Arial><span
style='font-size:10.0pt;font-family:Arial'>ViaSat Inc.</span></font></i></em><o:p></o:p></p>
<p class=MsoNormal><em><i><font size=2 color=black face=Arial><span
style='font-size:10.0pt;font-family:Arial'>20511 Seneca <st1:Street w:st="on"><st1:address
w:st="on">Meadows Parkway</st1:address></st1:Street></span></font></i></em><o:p></o:p></p>
<p class=MsoNormal><st1:place w:st="on"><st1:City w:st="on"><em><i><font
size=2 color=black face=Arial><span style='font-size:10.0pt;font-family:Arial'>Germantown</span></font></i></em></st1:City><em><i><font
size=2 face=Arial><span style='font-size:10.0pt;font-family:Arial'>, <st1:State
w:st="on">MD</st1:State> <st1:PostalCode w:st="on">20876</st1:PostalCode></span></font></i></em></st1:place><o:p></o:p></p>
<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
style='font-size:12.0pt'> </span><o:p></o:p></font></p>
</div>
<div>
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color=black face="Times New Roman"><span style='font-size:12.0pt;color:windowtext'>
<hr size=2 width="100%" align=center tabindex=-1>
</span></font></div>
<p class=MsoNormal><b><font size=2 color=black face=Tahoma><span
style='font-size:10.0pt;font-family:Tahoma;color:windowtext;font-weight:bold'>From:</span></font></b><font
size=2 color=black face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma;
color:windowtext'> end2end-interest-bounces@postel.org
[mailto:end2end-interest-bounces@postel.org] <b><span style='font-weight:bold'>On
Behalf Of </span></b>David P. Reed<br>
<b><span style='font-weight:bold'>Sent:</span></b> Tuesday, September 08, 2009
10:06 AM<br>
<b><span style='font-weight:bold'>To:</span></b> Jim Gettys<br>
<b><span style='font-weight:bold'>Cc:</span></b> jeroen@unfix.org; sthaug@nethelp.no;
end2end-interest@postel.org<br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: [e2e] What's wrong
with this picture?</span></font><font color=black><span style='color:windowtext'><o:p></o:p></span></font></p>
</div>
<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
style='font-size:12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><font size=3 color=black
face=Helvetica><span style='font-size:12.0pt;font-family:Helvetica'>Jim - I
suspect your Comcast support person was partly right. ICMP
*echoing* is sidelined. However, IP packets that contain ICMP messages
destined farther down the line are NOT dropped by routers and switches.
That would be dumb, though I'm sure some networks that don't want to monitor
their own congestion might be so dumb as to imagine that ICMP mice will somehow
overload a network. I don't think such people are members of NANOG).<br>
<br>
It turns out that Comcast's problem (extensively investigated by technologists
rather than their PR dept., only after the Harvard FCC hearing) was that DOCSIS
modems they had bought actually had multiple-seconds worth of buffering on
their upstream-facing interfaces, and did not under any circumstances drop
packets in a way that would allow TCP to know enough to slow down the AI part
of AIMD.<br>
<br>
Given the sidelining of *echoing* yes, pinging a router might not give much
info about that router. But pinging the next, unloaded router down the
route will tell you a lot.<br>
<br>
In any case, it's easy to open up a TCP connection and carry out an end-to-end
ping without ever using ICMP. Just wait a few seconds after a sync, send
a few bytes, and have a responder echo them. If you use TCPNODELAY
option, you will get a reliable result. I have a python program on
my server that handles such things. In this particular measurement,
the data from this "TCP ping" gave consistent RTT's with the ICMP
ping.<br>
<br>
It's fascinating to me that people REALLY WANT to call this "measurement
error". </span></font>As opposed to *operator*
misconfiguration (or router-designer-error).<br>
<br>
Perhaps someone might actually be able to guess what manufacturer sells the
equipment that routinely buffers 8 seconds of outgoing packets on a link
without a hint of backpressure that would allow TCP's congestion control to
kick in?<br>
<br>
I just want to see it fixed before Sandvine sells some more TCP-RST-injectors
and DPI spies to that vendor, and starts accusing people with some very cool
handsets of "attacking the network". Maybe the handset vendor
would be interested in having interactions take less than 8-20 seconds between
gesture and response from a server? <br>
<br>
One thing that is clear: the spate of news stories about "spectrum
shortage" has missed a fundamental technical problem that has NOTHING to
do with spectrum.<br>
<br>
<o:p></o:p></p>
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