Distributedmultimedia applicationsrequire performanceguarantees from the underlying network subsystem. Ethernet has been the dominant local area network architecture in the last decade, and we believe that it will remain popular because of its cost-eectiveness and the availability of higherbandwidth Ethernets. We present the design, implementation and evaluation of a software-based timed-token protocol called RETHER that provides real-time performance guarantees to multimedia applications without requiring any modications to existing Ethernet hardware. RETHER features a hybrid mode of operation to reduce the performance impact on non-real-time network trac, a race-condition-free distributed admission control mechanism, and an ecient token-passing scheme that protects the network against token loss due to node failures or otherwise. To our knowledge, this is the rst software implementation of a real-time protocol over existing Ethernet hardware. Performance measurements from exper...