OS X 10.4 Interface
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Tiger 10.4 includes support for link aggregation or IEEE 802.3ad. If your switch supports it, this lets you marry multiple network connections on one virtual connection with a single IP address for added throughput and failover. The switch I used didn't support 802.3ad, but I bound the two NICs in my Xserve into failover mode. When I unplugged the primary network adapter, the secondary NIC took over in 10 to 15 seconds--kind of slow, but I wasn't using an 802.3ad-enabled switch.
Tiger 10.4 maintains session states during the transition, but some existing connections may time out. In my tests, an FTP transfer from my Tiger 10.4 laptop continued after the failover occurred, but the Windows FTP program did not.
Apple has added several open-source packages for new services. All services are configured through ServerAdmin, an OS X program that can be run on the local server or remotely from another Mac. System-configuration tools are Mac-only, but I could change configuration settings and view log files, connections and graphs for almost every service.