Intel PCI User Manual

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1. Even though CSO and CSS are in units of bytes, the checksum calculation typically works on 

16-bit words. Hardware does not enforce even byte alignment.

2. Hardware does not add the 802.1Q EtherType or the VLAN field following the 802.1Q Ether-

Type to the checksum. So for VLAN packets, software can compute the values to back out 
only on the encapsulated packet rather than on the added fields. 

3. Although the Ethernet controller can be programmed to calculate and insert TCP checksum 

using the legacy descriptor format as described above, it is recommended that software use the 
newer TCP/IP Context Transmit Descriptor Format. This newer descriptor format allows the 
hardware to calculate both the IP and TCP checksums for outgoing packets. See 

 

for more information about how the new descriptor format can be used to accomplish this task.

CMD

Command field
See 

 for a detailed field description.

STA

Status field
See 

 for a detailed field description.

RSV

Reserved
Should be written with 0b for future compatibility.

CSS

Checksum Start Field
The Checksum start field (TDESC.CSS) indicates where to begin computing 
the checksum. The software must compute this offset to back out the bytes 
that should not be included in the TCP checksum. CSS is provided in units 
of bytes and must be in the range of data provided to the Ethernet controller 
in the descriptor (CSS < length). For short packets that ar padded by the 
software, CSS must be in the range of the unpadded data length. A value of 
0b corresponds to the first byte in the packet.
CSS must be set in the first descriptor of the packet.

Special

Special Field
See the notes that follow this table for a detailed field description.