  The most demanding tenants of shared clouds require complete isolation from
their neighbors, in order to guarantee that their application performance is
not affected by other tenants. Unfortunately, while shared clouds can offer an
option whereby tenants obtain dedicated servers, they do not offer any network
provisioning service, which would shield these tenants from network
interference. In this paper, we introduce Links as a Service, a new abstraction
for cloud service that provides physical isolation of network links. Each
tenant gets an exclusive set of links forming a virtual fat tree, and is
guaranteed to receive the exact same bandwidth and delay as if it were alone in
the shared cloud. Under simple assumptions, we derive theoretical conditions
for enabling LaaS without capacity over-provisioning in fat-trees. New tenants
are only admitted in the network when they can be allocated hosts and links
that maintain these conditions. Using experiments on real clusters as well as
simulations with real-life tenant sizes, we show that LaaS completely avoids
the performance degradation caused by traffic from concurrent tenants on shared
links. Compared to mere host isolation, LaaS can improve the application
performance by up to 200%, at the cost of a 10% reduction in the cloud
utilization.
