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Receive and Transmit Description
Software Developer’s Manual 37
Notes:
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
EtherType 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 Section 3.3.5
for more information about how the new descriptor format can be used to accomplish this task.
CMD
Command field
See Section 3.3.3.1 for a detailed field description.
STA
Status field
See Section 3.3.3.2 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.
Transmit Descriptor
Legacy
Description