Remote-Link Details Matter: Gatorlink Vs. LanRovers
By Bruce Schneier
Both Shiva Corp. and Cayman Systems Inc. are readying multiport Ethernet remote-access products for shipment sometime this fall. At the Boston Macworld Expo in August, Cayman announced GatorLink and Shiva demonstrated LanRover/E. Shiva's LanRover/L, a single-port LocalTalk remote-access product, has been shipping since April. Both the LanRover/E and the GatorLink are hardware devices that connect AppleTalk Remote Access users directly into the network without the need for a dedicated Mac.
One interesting difference between the products already has been brought to light by the vendors: the way in which they connect users to the network. GatorLink will be a bridge. LanRover/E also will be a bridge, but users also will be able to configure it as a router.
Bridge over Troubled Router?
LanRover/E and GatorLink appear similar on the surface. Both will allow remote users to dial into their Ethernet networks, access files, run applications, and send and receive electronic mail. Both will provide modem links for Mac users through AppleTalk Remote Access (ARA). Both will have multiple serial ports for modems: the LanRover/E will have four ports and the GatorLink will have three. The difference is in the connectivity.
Cayman calls the GatorLink a "forwarding end node" between the remote user and the network. A forwarding end node is a kind of bridge, one that passes only traffic addressed to nodes on the other side. Remote users will be treated as the logical equivalents to nodes on the same Ethernet network as the GatorLink rather than as nodes on a separate but connected network.
Cayman's decision to configure the GatorLink as a forwarding end node was based on its experience in router design.
"We have been in the router business for a long time and could not find any benefit in making GatorLink a router," said Vicky Risk, product manager for GatorLink at Cayman Systems of Cambridge, Mass. "There is nothing it could do better, faster or more reliably if it were a router."
Routers add complexity to the network; they must pass routing information among themselves frequently. The latest version of that information must be stored by the router in memory. And, every new AppleTalk router adds more information for the other routers to maintain.
"Routers perform critical network functions. As a result, they can require a lot of attention and maintenance from the person maintaining the network," Risk said.
The Local Route
Shiva's LanRover/L is not implemented as a bridge but as a router. According to Michael Feinstein, Macintosh business-unit manager for Shiva, also in Cambridge, this is because the LanRover/L is a LocalTalk product. Every time a packet comes across LocalTalk, every Macintosh on the network is interrupted, no matter which machine that packet is destined for. According to Feinstein, a LocalTalk bridge would forward all traffic and treat the remote user's Mac exactly as if it were on the local network, subject to all the delays of local traffic.
Router implementations on LocalTalk do not require interrupting the remote Mac for every packet, because only the packets destined for the Mac cross the router; all other packets are filtered out. Also, a router allows dial-in users to be placed in a separate zone, minimizing the AppleTalk broadcast traffic sent across the remote-access link.
Shiva believed that performance benefits justified configuring the LanRover/L as a router. "If your dial-in users are on a different network than your server [and connected by a router], then you don't have to take every packet the server sees and send it over the phone wire. On LocalTalk, a router gives you much better performance than a bridge," Feinstein said.
When Shiva set out to design the LanRover/E, it realized that the router solution would not necessarily be best. "Ethernet interfaces are more intelligent; the [Mac's main] processor doesn't get interrupted for every packet on the network. You get equivalent performance when the product is configured as a bridge," Feinstein said.
The LanRover/E has a default configuration as a bridge, like the GatorLink, but it also can be reconfigured as a router.
Shiva says a router may provide better performance on a busy Ethernet backbone by isolating the remote users from the rest of the network. A router also gives the network manager the opportunity to create a new zone for dial-in users, which may be desirable for security purposes.
These considerations prompted Shiva to give its LanRover/E users the choice. "A bridge provides better performance on smaller networks where the routing overheard would be significant," Feinstein said.
Although Feinstein could not quote any performance characteristics for LanRover/E in its two configurations, he promised benchmarks when the product is released. "If there is a difference on average, it will be relatively small," he said.
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