TCP segmentation offloading
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TCP segmentation offload or TSO is a technology for reducing CPU overhead of TCP/IP on fast networks. TSO is also referred to as large segment offload or LSO.
When large chunks of data are to be sent over a computer network, they need to be first broken down to 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. Segmentation is often done by the TCP protocol in the host computer. Offloading this work to the network card is called TCP segmentation offload (TSO).
For example, a unit of 64KB Bytes of data is usually segmented to 44 segments of 1500 bytes each before it is sent over the network through the network interface controller (NIC). With some intelligence in the NIC, the host CPU can hand over the 64 KB of data to the NIC in a single packet, and the NIC can break that packet down into smaller segments of 1500 bytes, add the TCP and IP protocol headers according to a template provided by the TCP stack to each segment, and send them over the network. This significantly reduces the work done by the CPU. Many new NICs in the market today 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 theirselves, such as UDP. This is not as useful, though, as other protocols aren't commonly used for transmitting large amounts of data.
[edit] See also
- TCP Offload Engine (TOE)
