An issue that I’ve encountered fairly often are complaints of slow network performance, especially when transferring large files. Although there are many issues that can affect network throughput, the most common issue is related to Large Send Offload.
Large Send Offload (also known as Large Segmentation Offload, and LSO for short) is a feature that allows the operating system TCP\IP network stack to build a large TCP message of up to 64KB in length before sending to the Ethernet adapter. Then the hardware on the Ethernet adapter — what I’ll call the LSO engine — segments it into smaller data packets (known as “frames” in Ethernet terminology) that can be sent over the wire. This is up to 1500 bytes for standard Ethernet frames and up to 9000 bytes for jumbo Ethernet frames. (The actual sizes are bit larger to accommodate the overhead – header and frame check sequence – in the packet). This is designed to free up the CPU on the server from having to handle segmenting large TCP messages into smaller packets required by the frame size. Sounds like a good deal. What could possibly go wrong?