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The answers to these questions are, alas, quite technical and vary greatly between one cable system and another. Some of today's cable modem networks are quite secure, while others are wide open.

Cable modem systems are roughly based on the Ethernet technology developed at Xerox PARC in the 1970s and popularized in the late '80s and early '90s. To the first approximation, a typical neighborhood cable network looks like a big local area network.

This structure also lowers the cost of deployment. Making a neighborhood network look like one big Ethernet saved a bundle of money for the cable modem companies, because it let them use off-the-shelf hardware and software. Practically every operating system in use today can speak the Internet's TCP/IP protocols over Ethernet. By making the cable modems look like little Ethernet hubs, vendors could avoid writing network drivers for Windows, MacOS, Linux -- and every other operating system, for that matter. Mimicking Ethernet also let cable companies use off-the-shelf Ethernet cards in desktop and laptop computers, further cutting costs. Overall, going with Ethernet was the right decision.

Unfortunately, Ethernet has a problem that makes it somewhat unsuited to a neighborhood environment: It's a broadcast network. On a typical office LAN, for example, you can run a program called a "network analyzer" and see the packets of every other machine. The computer underground has written programs called "packet sniffers," which exploit this feature to capture a victim's password as it is typed.

When the first cable modem networks were deployed in the Boston area, I loaded my favorite packet sniffer onto a floppy disk and headed over to my friend Richard's house. Richard had already told me that he could see other Macintosh computers in his neighborhood, using his Mac's "Chooser" program. Sometimes people even accidentally printed files on his printer. With packet sniffer in hand, I was prepared to capture the passwords and Web traffic of everybody else in the neighborhood. Then I planned to write an article about the total lack of privacy on cable modem networks

But when I got to Richard's house and set up the sniffer, something went wrong: I could see only a tiny fraction of the network's traffic. The engineers at the company that had built my friend's cable modem weren't bozos after all. Each cable modem on the network had been programmed to filter out packets destined for other subscribers. To use the jargon of Ethernet, the cable modems weren't acting as Ethernet hubs, but rather as "bridges" or "switches." Richard could still use his Mac's Chooser to see the other computers because AppleTalk's network protocol uses broadcast Ethernet packets to let the machines on the network find each other.

When a computer on an Ethernet network sends a broadcast packet, that packet is automatically received by every other host on the network. Microsoft Windows uses broadcast packets to find the hosts for the "Network Neighborhood" window. The Internet's "ARP" protocol uses Ethernet broadcast to determine which computer on a local area network has a particular address.

Cable modems transmitting Ethernet broadcast packets to every subscriber on the neighborhood are a significant vulnerability, easily exploited by a technically savvy attacker. For example, using a freely available program called "arpwatch," I can scan for the ARP packets and detect how many subscribers are on my cable segment. Since MediaOne has assigned host names that look a lot like user names (e.g. sjones.ne.mediaone.net), I can learn the names of my cyber-neighbors. I can also learn when the ARP packets are sent, and establish when my neighbors are using their computers -- and when they are at work.

Cable modems have another security risk. But that risk doesn't have anything to do with the modems themselves; rather, it arises from the Windows operating system. That vulnerability is called file sharing. If you have two computers in the same house, you can turn on file sharing to let one computer access the other's hard drive. The danger here is that many people turn on file sharing but don't set up a password. Put a computer configured this way on the Internet, and a hundred million people can look at any of the files on your machine.

Different cable companies have tried different approaches to address these security issues. MediaOne, for example, blocks the particular TCP/IP ports used by the Microsoft file sharing protocol. (The company can unblock the ports on a subscriber-by-subscriber basis, if you call them up and make the request.) Other companies leave the ports open, and warn their customers to disable file sharing. I prefer MediaOne's approach.

The ARP problem, meanwhile, will be solved by the next-generation cable modems that implement the so-called DOCSIS 1.1 protocol. Instead of broadcasting ARP packets over the entire cable segment, DOCSIS 1.1 makes sure that each customer will only see the ARP messages intended for his or her machine. As an added protection, DOCSIS 1.1 is capable of encrypting all information sent over the cable itself, with a separate encryption key for each customer. This security measure prevents an attacker from splicing their own cable modem into the backbone, the way that some people used to hook up unauthorized cable decoders to get free cable TV service.

DSL users have pretty much the same set of security concerns as cable modem customers. That's because DSL modems can be set up as routers or bridges. At my Boston office, the Concentric DSL modem is set up as a router. Concentric has given us 6 IP addresses for our own use, and we can't see anybody else's traffic. The Bell Atlantic DSL modem, on the other hand, is configured as a bridge, with potentially 253 other customers sharing the same network -- in this case, a VLAN, or Virtual Local Area Network.

Neither Concentric nor Bell Atlantic filter their DSL connections, potentially leaving customers open to file-sharing attacks. The service technician who installed the Concentric modem told me that I could have it configured as a firewall, if I wanted. Bell Atlantic, on the other hand, simply gave me a small pamphlet that explained the dangers of the Internet and suggested that I turn off file sharing on my Windows computer. As for the problem with broadcast ARPs, Bell Atlantic has its own, somewhat inelegant solution. Instead of filtering the broadcast packets, Bell Atlantic has simply programmed its computers to make it impossible for me to exchange packets with any of the 253 other subscribers on my local area network. Most of the time this shouldn't cause any problem, but if two customers want to play Doom with each other over the network, and they happen to be on the same VLAN, they're out of luck.

To the future

Whether or not cable modems will be more or less reliable than DSL is an open question. When it comes to delivering a consistently reliable service, cable companies certainly do not have a good history. This may be because they've never been forced to deliver a consistently high-quality service: Although it might be an inconvenience to go without cable for a few nights, nobody will die if they can't get their fill of "Star Trek" and "Gilligan's Island." Telephone companies, however, have been required for years to deliver highly reliable dial tone, since 911 is the basis of police, fire and ambulance services. As cable companies begin to provide their own dial tone, they will fall under the same regulations.

With that said, my experience has been the reverse. During the past year, my cable modem has been down only a single day. Meanwhile, I have lost dial tone on my primary phone line on no less than six occasions. Bell Atlantic says this is because I live in an old neighborhood where the wires aren't in very good shape; on the other hand, the cable was installed relatively recently.

If the history of technology is any guide, however, it's unlikely that the battle between cable modems and DSL will be won on technical merits. It's exceedingly difficult to find a single case within the past 50 years where a better technology won out in the marketplace against an inferior one. Beta lost to VHS, after all, and the sleek "RISC" microprocessors from companies like Sun Microsystems and MIPS lost out to Intel's technologically inferior Pentium chips. Ultimately, this battle will be won and lost on mundane issues like price and quality of service.

It's for these reasons, in fact, that I firmly believe companies like MediaOne should be forced to open up their cable networks to other providers. Gargantuan companies like Bell Atlantic ultimately will be able to compete against the cable modem providers: They'll just spend a lot of money to make their DSL offerings competitive with the cable systems. But given all of the natural advantages that cable enjoys, small-to-medium-sized ISPs really don't stand a chance. Unless these networks are opened for all to use, consumers may soon have just two choices for an Internet provider: their cable company or their Baby Bell.
salon.com | Sept. 23, 1999

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