TxD

TxD is commonly used to denote a transmit descriptor, a data structure used by network interface controllers (NICs) and device drivers to manage outbound data. In DMA-based NICs, a transmit descriptor ring resides in host memory and contains entries that describe packet buffers to be transmitted. Each descriptor typically includes a buffer address or a chain of addresses, the length of the data to send, and command and status fields. Additional flags may indicate offload features such as checksum offload, TCP segmentation offload, VLAN tagging, or timestamping.

The driver populates descriptors with pointers to packet data and appropriate control bits, then hands them to the NIC by updating ownership and related registers. Hardware DMA engines read the descriptor, fetch the packet data from memory, and transmit it over the network. When transmission completes or an error occurs, the NIC updates the descriptor’s status, often generating an interrupt or signaling completion to the driver. The software then reclaims buffers and refills or recycles descriptors for subsequent transmissions.

Key design considerations include ring size, memory alignment, and cache efficiency, since larger or poorly aligned rings can affect latency and throughput. Many NICs support multiple transmit queues to improve parallelism and reduce contention. Descriptor formats vary by vendor and model, with common families including Intel, Broadcom, and Realtek devices; drivers abstract these differences to present a consistent interface to upper software layers.

Other uses of the acronym TxD exist in electronics and serial communications, where it can denote a transmit data signal, but in networking contexts TxD most often refers to a transmit descriptor.