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How generic routing encapsulation works in Windows Server 2012 R2

Generic routing encapsulation can help Windows Server 2012 R2 admins with performance enhancements.

Windows Server 2012 R2 supports network virtualization with generic routing encapsulation. GRE-encapsulated packets can directly support NIC adapters with performance-enhancing features including large send offload, receive-side scaling and VM queue. Consider deploying GRE-enabled NIC adapters, such as products from Mellanox or Emulex, for the benefits each feature provides.

First, data waiting for transmission onto a network is typically queued in memory. Communication drivers use interrupts that prompt the CPU to take small portions of the queue and encapsulate the little data segments into packets that can be sent. Large send offload, or LSO, in Windows Server 2012 R2 improves this performance by queuing the data directly on the NIC adapter rather than on a buffer in main memory, then allowing the NIC adapter to packetize the data on board without generating interrupts to the CPU -- this improves CPU (and system) performance because the NIC adapter offloads this segmentation of data.

Second, receiving data from the network traditionally is also CPU-intensive because the CPU must stop, strip the frame from each incoming data packet, then move the remaining data segment to a buffer in the server's memory. Heavy incoming traffic can cripple a CPU's performance. Receive-side scaling, or RSS, works by spreading out the processing tasks needed to receive network packets across multiple CPUs in the server rather than leaving all the receive work to a single CPU.

Finally, virtual machine queuing (VMQ) is a virtualization technology that allows incoming packets for a VM to queue up in buffer space allocated for the VM on the physical NIC; then direct memory access, or DMA, moves all the data from the queue buffer to the virtual network adapter servicing the destination VM. This requires a host OS such as Windows 2012 R2, but queue space must be allocated sparingly for greatest benefit. For example, use VMQ for VMs with the greatest incoming traffic.

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