Intel PCI User Manual

17

The Ethernet controller provides the ability to offload IP, TCP, and UDP checksum for transmit. 

The functionality provided by these features can significantly reduce processor utilization by 
shifting the burden of the functions from the driver to the hardware.

The checksum offloading feature is briefly outlined in the following sections. More detail about all 

of the hardware acceleration capabilities is provided in 

.

The Ethernet controller provides the ability to offload the IP, TCP, and UDP checksum require-

ments from the software device driver. For common frame types, the hardware automatically 

calculates, inserts, and checks the appropriate checksum values normally handled by software.

For transmits, every Ethernet packet might have two checksums calculated and inserted by the 
Ethernet controller. Typically, these would be the IP checksum, and either the TCP or UDP 

checksum. The software device driver specifies which portions of the packet are included in the 

checksum calculations, and where the calculated values are inserted via descriptors (refer to 

 for details).

For receives, the hardware recognizes the packet type and performs the checksum calculations and 

error checking automatically. Checksum and error information is provided to software through the 

receive descriptors (refer to 

 for details).

The Ethernet controller implements a TCP segmentation capability for transmits that allows the 

software device driver to offload packet segmentation and encapsulation to the hardware. The 
software device driver can send the Ethernet controller the entire IP, TCP or UDP message sent 

down by the Network Operating System (NOS) for transmission. The Ethernet controller segments 

the packet into legal Ethernet frames and transmit them on the wire. By handling the segmentation 
tasks, the hardware alleviates the software from handling some of the framing responsibilities. This 

reduces the overhead on the CPU for the transmission process thus reducing overall CPU 
utilization. See 

 for details.

Software allocates the transmit and receive buffers, and also forms the descriptors that contain 

pointers to, and the status of, those buffers. A conceptual ownership boundary exists between the 

driver software and the hardware of the buffers and descriptors. The software gives the hardware 
ownership of a queue of buffers for receives. These receive buffers store data that the software then 

owns once a valid packet arrives.

For transmits, the software maintains a queue of buffers. The driver software owns a buffer until it 

is ready to transmit. The software then commits the buffer to the hardware; the hardware then owns 
the buffer until the data is loaded or transmitted in the transmit FIFO.