[e2e] What's wrong with this picture?

David P. Reed dpreed at reed.com
Tue Sep 8 07:05:54 PDT 2009

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).

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.

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.

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.

It's fascinating to me that people REALLY WANT to call this "measurement 
error". As opposed to *operator* misconfiguration (or 

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?

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?

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.

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