Tales from Comcast’s data cap nation: Can the meter be trusted?


“We know that our meter is right… with our meter, we give you
a guarantee that it is perfect,” one Comcast customer service
representative insisted to another subscriber who disputed
data charges a few months ago. That subscriber, Chris from
Georgia, had been measuring Internet usage on his own router
and found big discrepancies between his own measurements and
Comcast’s. We’ll have more on Chris and other customers later
in this article. First, let’s tell Brad’s story.

Karl Auerbach

One of the sources of discrepancies comes about because different systems count the size of packets in different ways.

For example, some systems count the ethernet frame header.  That adds at least 14 bytes to each packet.  For big packets that’s not a big incremental percentage, but for VoIP it can be a lot.  This difference in counting method has, alone, been a source of inter-carrier disputes for decades.  (It first surfaced on long haul circuits from Australia to the US.)

A lot of people get confused by the commonly used IPERF program.  That one tends to cause confusion because it generally only reports the amount of transport-level content that has been transferred rather than all of the protocol framing and protocol handshaking packets.

And big-iron routers/switches often have difficulty that come from the asynchronous updates of a centralized counter store.

And then there is just plain stupid code.  My business at InterWorking Labs is testing internet protocols, among them is the ubiquitous SNMP.  For many years one of the largest companies had problems in which some of the counters in its MIB (Management Information Base) that handled packet sizes would go backwards.

Some of that comes from “JPOSC” – Just Plain Old Stupid Code, such as when a programmer doesn’t bother to specify the number of bits in an integer variable or, worse, forgets to mark that integer as “unsigned”.  (This same kind of stupid programming also has been found in voting machines.)


Bob Frankston

Caps again … they still do that?   Microsoft’s updates now will, like Bit Torrent, gather the bits from any available sources like adjacent machines. This means that if you have a gigabyte update you might see a full gigabyte going past Comcast’s metering point or zero bytes. You can’t know beforehand. This is yet one more example of how fanciful the measure of bytes is. It’s all based on naïve model of the Internet as a series or water pipes carrying flows. It is an arbitrary and perverse measure. Why doesn’t Comcast return to plan A and charge you a monthly for fee each computer attached to the network just like they charge a monthly fee for each TV you happen to have. Oh – then they would have to charge a monthly fee for each IP light bulb and the absurdity would be too obvious. So better to pretend that bytes are meaningful measures like gallons and gills.


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