Our ISP has recently enacted data caps. The first time they threatened to disable our account for going over, I pointed out to them that:
- My firewall has logged tens of gigabytes of dropped/rejected traffic.
- That if I ran a TCPDump on our WAN interface, could see management and broadcast traffic for neighboring CPEs (modem).
- That I know for a fact (I used to work for them in a previous iteration) that their network isn't built in a way that would allow them differentiate their management and monitoring traffic from individual customer traffic.
- And finally, that the Layer 2 & 3 statistics of that WAN interface were nowhere close to their own statistics for the account.
They have yet to respond to that complaint, despite going to the FCC. I really wonder what would happen if one were to take their ISP to court with the proper documentation.
I plugged my cable modem into a dumb switch. It had a link, but there was nothing else attached; there was no router to act as a DHCP client, no frames being sent to the cable modem at all. In six days, my usage meter incremented by 0.5 GB. Plugging in a protocol analyzer revealed a constant stream of ARP requests for other subscribers' IPs. These were apparently counted against my cap, and presumably they were counted against everyone's caps, making 250 GB/month 1% smaller than advertised.
Comcast sent me to a document describing their metering methodology:
http://www.netforecast.com/documents/NFR5101_Comcast_Usage_M...
Their reps defended the practice of counting Comcast-generated broadcast traffic against everyone, even though there's no way for subscribers to prevent it.