I've read a number of times over the years about how it's possible to communicate between cable modems in the same subsection of the network without it counting towards your data cap by virtue of it being a "direct" connection that doesn't have to go through the ISP's main routers, or something. Does anyone know anything about this? I'm struggling to find anything about it on the net because I can't think of the right search terms.
How would it break the TOS? I was under the impression that it is simply a consequence of how the network operates. For instance, if I do a traceroute to a different gateway than mine, it goes directly from my router to that gateway. Surely then any traffic to and from there (and any modems using that gateway) will not count towards data caps?
The TCL cable system does not allow customers to ARP directly for each other, so all traffic goes through the virtual interface on the BRAS. That virtual interface is the point at which packets are counted and that count is the basis of the bill.
Some systems do allow direct communication. If you look up proxy-ARP and DOCSIS you'll get some more information.
Note that 'direct' communication is a bit of a misnomer here. All DOCSIS traffic is *from* CMTS *to* cable modem or *from* cable modem *to* CMTS. The way DOCSIS works, direct cable modem to cable modem traffic is physically not possible. The DOCSIS CMTS is frequently also a layer-3 device (a router). Otherwise, it tends to hand off transparently to the real router, a BRAS. In the first case, all traffic is always going to pass through the router interface and in the second case it is desirable to force all traffic to go to the BRAS because you can get some weird and hard to diagnose network errors otherwise. Plus, when they DoS you, it's a lot easier for the service provider to spot it :)
The way a BRAS is set up, for both cable and xDSL, is as a router with many thousands of (virtual) interfaces---one per customer. This is quite different from having a shared LAN with a gateway to the Internet on it, which is the way most people probably expect it to be, given their experience with home and office LANs.