A First Look At San Diego @Home
Phil Karn karn@qualcomm.com
May 10, 1997; revised May 13, 1997

On May 10 1997, Cox Cable and @Home put on a public demonstration of their San Diego cable modem service at the Bookstar store in Point Loma. Although I'm not in Cox territory (I live in Time Warner/Southwestern territory and subscribe to Road Runner) I was curious to see how the two services compare.

This writeup is based on my observations at the demo as well as a phone conversation I had on May 12 with Milo Medin, @Home's Vice President-Networks. I've known Milo since his days as NASA's network administrator. He and several other key technical people in @Home have considerable experience in the Internet community; they're long-time participants in the IETF, for example. I don't know of anyone within Time Warner/Road Runner of comparable technical stature. I think this helps explain many of Road Runner's difficulties.

I emphasize that both @Home and Road Runner use different technologies in the different cities in which they operate, so my discussion here is valid only for the San Diego incarnations of these two systems.

The Similarities

There are some important similarities between the two services:

The Differences

Number of Computers Supported

Both Road Runner and @Home are primarily intended for single computers. Road Runner currently allows multiple computers to be attached to a single modem through a hub, but they have announced their intention to charge an extra $5/mo for each extra computer.

Each @Home modem currently supports only a single computer. @Home is looking into supporting additional computers; no additional information was publicly available.

Price

Installation

Road Runner: $99.95; @Home: $149.95. Both include an Ethernet card for the PC. If you already have an Ethernet card, the @Home installation drops to $99.95. Roadrunner does not have a discount for those with Ethernet cards.

Monthly Charge

Road Runner: $44.95/mo; @Home: $39.95/mo. Both prices assume you are at least a basic cable TV subscriber. If not, the monthly price rises to $49.95/mo for both Road Runner and @Home.

Login Programs and IP Address Assignments

Road Runner requires that the user computer continuously execute a special "login program" to gain packet-level access to the Internet. Supported versions of the login program exist only for Windows 95 and for the Mac, though functional clones are proliferating for UNIX-like systems. @Home does not require any such program.

Both Road Runner and @Home use DHCP to assign IP addresses to users. Road Runner issues leases with a duration of 3600 sec (1 hr). Assigned addresses are wholly temporary; the RR system does not make any attempt to assign any particular IP address (or domain name) to any particular user (though an address, once allocated, will not change if the user leaves his computer on continuously.)

According to Milo Medin, @Home's DHCP server issues leases of much longer duration, e.g., 1 week. Additionally, the @Home DHCP server is keyed to a client ID field in the DHCP request; in Windows 95, this is derived from the host name entered in the network setup. The intent is to always give you the same IP address even if your machine has been off for an extended period.

@Home does reserve the right to change individual IP addresses when necessary (e.g., when the address space allocated to a particular cable router nears exhaustion and they have to shuffle address blocks around to free up address space). If it does become necessary to change a user's IP address, they will update the DNS so that the users' corresponding domain name does not change. This is good news for those running servers at home whose domain names are widely published.

Proxy Servers

Both @Home and Road Runner provide caching web proxy servers. Road Runner uses Sun Microsystems Ultra Sparcs; @Home uses Silicon Graphics machines. @Home does not make their use mandatory. Road Runner forces its users to use the proxies by blocking upstream traffic to TCP port 80 (HTTP).

Modem Performance

Because both Road Runner and @Home use the same cable modem (the Motorola CyberSURFR) one would expect comparable performance. But according to the @Home people at the demo, their cable router -- at least the one supporting the demo -- is running a newer version of the Motorola firmware. The older version currently running on most of the Road Runner cable routers has some nasty bugs in the polling algorithm that seriously affect reverse channel latency. Road Runner is currently in the process of updating their routers, but they do not expect to finish until the end of May.

Ping tests on Road Runner to the local router typically show a wide variance in round trip delay, from less than ten milliseconds up to nearly two seconds, with an average of a few hundred milliseconds.

Ping tests on the @Home demo network with the upgraded cable router firmware showed consistently lower delays in the range 10 to 30 milliseconds, except for the first packet. The Motorola modems are known to use an adaptive polling scheme where active users are polled continuously while inactive users are polled relatively infrequently. This explains the increased latency for the first packet.

Hopefully, once Road Runner finishes upgrading its cable routers their modems will perform as well as those on @Home.

Performance

The @Home advertising copy at the demo claimed that three megabyte files could be transferred in 7 seconds, which is 428 kilobytes/s. Their web page claims 2 megabytes in less than two seconds, which is more than 1 megabyte/sec. This latter figure is apparently derived from the speed of a 10 megabite/sec Ethernet.

These claims are somewhat exaggerated. A file transfer from Qualcomm's public FTP server over @Home went at about 85 kilobytes/sec, about the same as Road Runner on a good day. Of course, throughput depends on many factors, only some of which are under the control of the cable company.

According to Milo Medin, the customized version of Netscape bundled with the @Home service increases the default Windows 95 TCP receive window size to 48K to improve performance. I didn't have a chance to verify the actual value during the demo.

External Internet Connectivity

Road Runner relies entirely on MCI for its external Internet connectivity, while @Home has built its own virtual Internet backbone on top of Sprint's ATM service. This means @Home runs its own IP backbone routers, while MCI runs those used by Road Runner.

The access link between the San Diego Road Runner head end and the MCI POP in Los Angeles is currently 4 T-1 lines. The access link from the San Diego @Home head end and the @Home backbone is currently a DS3 (28 T-1s, seven times as much as Road Runner).

Both MCI and @Home are Tier 1 backbone networks. Both peer with each other and with CERFnet at MAE-West. This means traffic from @Home customers to CERFnet sites like Qualcomm and UCSD must travel up to the San Francisco Bay area and back -- just as with Road Runner. Neither service has a direct local connection to CERFnet yet.

No packets were lost in a ping test of a few hundred packets from @Home to Qualcomm. Neither were any packets lost in a simultaneous test from Road Runner to Qualcomm -- but then again, it was saturday afternoon, not a workday afternoon when the Internet is usually at maximum load.

There's an interesting oddity in the routing between @Home and Road Runner. Packets from @Home to Road Runner travel over @Home's network all the way to MAE-East in Washington DC, where they enter MCI's network and return to the west coast! I believe this particular route was deliberately chosen by @Home some months ago because of MCI's chronically overloaded router at MAE-West. (Milo points out that it may not be a coincidence that Netscape is a major MCI customer.) This was the same problem that caused the high packet loss rate for packets from CERFnet to MCI and Road Runner, as CERFnet has not tried to avoid the overloaded MCI router at MAE-West.

One can still hope that someday within our lifetimes, @Home, Road Runner and CERFnet will all peer locally within San Diego.