Large segment offload

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Dialogue box showing offload TCP segmentation settings for an Intel Pro 1000 NIC

In computer networking, large segment offload (LSO) is a technique for increasing outbound throughput of high-bandwidth network connections by reducing CPU overhead. It works by queuing up large buffers and letting the network interface card (NIC) split them into separate packets. The technique is also called TCP segmentation offload (TSO) when applied to TCP, or generic segmentation offload (GSO).

The inbound counterpart of large segment offload is large receive offload (LRO).

Operation[edit]

When a system needs to send large chunks of data out over a computer network, the chunks first need breaking down into smaller segments that can pass through all the network elements like routers and switches between the source and destination computers. This process is referred to as segmentation. Often the TCP protocol in the host computer performs this segmentation. Offloading this work to the NIC is called TCP segmentation offload (TSO).

For example, a unit of 64kB (65,536 bytes) of data is usually segmented to 46 segments of 1448 bytes each before it is sent through the NIC and over the network. With some intelligence in the NIC, the host CPU can hand over the 64 KB of data to the NIC in a single transmit-request, the NIC can break that data down into smaller segments of 1448 bytes, add the TCP, IP, and data link layer protocol headers - according to a template provided by the host's TCP/IP stack - to each segment, and send the resulting frames over the network. This significantly reduces the work done by the CPU. As of 2014 many new NICs on the market support TSO.

Some network cards implement TSO generically enough that it can be used for offloading fragmentation of other transport layer protocols, or by doing IP fragmentation for protocols that don't support fragmentation by themselves, such as UDP.

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