  Optical interconnects are already the dominant technology in large-scale data
center networks. However, the high optical loss of many optical components
coupled with the low efficiency of laser sources result in high aggregate power
requirements for the thousands of optical transceivers used by these networks.
As optical interconnects stay always on even as traffic demands ebb and flow,
most of this power is wasted. We present LC/DC, a data center network system
architecture in which the operating system, the switch, and the optical
components are co-designed to achieve energy proportionality.
  LC/DC capitalizes on the path divergence of data center networks to turn on
and off redundant paths according to traffic demand, while maintaining full
connectivity. Turning off redundant paths allows the optical transceivers and
their electronic drivers to power down and save energy. Maintaining full
connectivity hides the laser turn-on delay. At the node layer, intercepting
send requests within the OS allows for the NIC's laser turn-on delay to be
fully overlapped with TCP/IP packet processing, and thus egress links can
remain powered off until needed with zero performance penalty.
  We demonstrate the feasibility of LC/DC by i) implementing the necessary
modifications in the Linux kernel and device drivers, ii) implementing a
10Gbit/s FPGA switch, and iii) performing physical experiments with optical
devices and circuit simulations. Our results on university data center traces
and models of Facebook and Microsoft data center traffic show that LC/DC saves
on average 60% of the optical transceivers power (68% max) at the cost of 6%
higher packet delay.
