" Unacceptably high cost of ownership due to bandwidth over-provisioning

But the latency of proprietary server adaptors and standard Ethernet is only one hindrance to achieving the performance necessary for a wider exploitation of Ethernet in HPC environments. Legacy Ethernet switches traditionally have not been conducive to exploitation at large scale given that:

" Underlying standards have not supported loss-less transmission; the main intent of TCP is to support packet re-transmission and best efforts

" Heavyweight algorithms such as (STP) to avoid deadlock have encumbered the use of flat networks, and therefore have often required complex tiered Layer 2 and 3 switches to support scale-out architectures; this imposes significant latency penalties when in operation and also necessitates significant bandwidth over-subscription

" Individual switch silicon has imposed severe latency on individual packets and when compared to the latency at the server side, adds significantly to overall round-trips in large systems